Rogers Et Al(Hebecourt Econ Geol)2014

8/13/2019 Rogers Et Al(Hebecourt Econ Geol)2014

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©2014 Society of Economic Geologists, Inc.Economic Geology, v. 109, pp. 61–88

Using Physical Volcanology, Chemical Stratigraphy, and Pyrite Geochemisfor Volcanogenic Massive Sulde Exploration:

An Example from the Blake River Group, Abitibi Greenstone Belt*,**RUSSELL ROGERS,1 PIERRE-SIMON ROSS,1,† JEAN GOUTIER,2 AND PATRICK MERCIER -LANGEVIN3

1Institut national de la recherche scientique, centre Eau Terre Environnement, 490 rue de la Couronne, Québec, QC, Canada G1K 9A9 2Ministère des Ressources naturelles et de la Faune (Québec), 70 avenue Québec, Rouyn-Noranda, QC, Canada J9X 6R1

3Geological Survey of Canada, 490 rue de la Couronne, Québec, QC, Canada G1K 9A9

AbstractAn innovative approach to enhance volcanogenic massive sulde (VMS) exploration in regions outside of

mining camps is to rst use physical volcanology with litho- and chemostratigraphy to establish the locationof effusive centers in the volcanic units and use pyrite geochemistry in sulde-bearing stratied intervals with whole-rock geochemistry in the underlying volcanic units to identify hydrothermal upow zones.

This methodology is illustrated by this study in the Archean Blake River Group within the Abitibi greenstonebelt of Quebec and Ontario. The Blake River Group contains numerous VMS deposits, yet large segmentsremain underexplored, including the Hébécourt Formation which contains four tholeiitic units that range frombasalt to rhyolite in composition. Effusive centers are located for three felsic units and subunits: (1) low Ti(porphyritic) subunit of the main rhyolite, (2) high Ti (aphyric) subunit of the main rhyolite, and (3) the upperrhyolite, and for a basaltic andesite unit. Stringer and disseminated Zn-Cu mineralization occurs within theank breccia of the low Ti rhyolite dome. An inferred vent area for an overlying basaltic andesitic unit has alsobeen identied in this area, illustrating the coincidence of hydrothermal upow zones with volcanic vents.

LA-ICP-MS analysis of pyrite grains from several sulde-bearing stratied intervals indicates two broad areasof higher Cu, Zn, Au, and Ag contents. The eastern region corresponds to known volcanic vents and mineraliza-tion. The western region also indicates upow of Cu-bearing hydrothermal uids and corresponds to a possibleeffusive center for the high Ti subunit. The western region does not contain known mineralization at lowerstratigraphic positions, but it has not been thoroughly explored.

Introduction V OLCANOGENIC massive sulde (VMS) deposits typically formpolymetallic sulde lenses at or near the sea oor in subma-rine volcanic successions (Franklin et al., 2005; Galley et al.,2007). Worldwide, VMS deposits are major sources of Zn, Cu,Pb, Ag, and Au, and signicant sources for Co, Sn, Se, Mn,Cd, In, Bi, Te, Ga, and Ge (Galley et al., 2007). At a regionalscale, the exploration for VMS deposits uses criteria such asthe presence of submarine volcanic rocks, large synvolcanicintrusions (thought to drive hydrothermal convection), andbimodal volcanic sequences (thought to indicate extension).At a more local scale, features such as texturally destructivealteration of volcanic rocks to chlorite and sericite (indicatingintense water-rock interaction in greenschist facies terranes),broader Na-depletion haloes (again supporting hydrothermalalteration), synvolcanic faults (to provide uid conduits), thepresence of rhyolite domes and dome alignments, and exha-lites (indicating a hiatus in volcanism and venting of hydro-thermal uids at the sea oor; see nomenclature below) areconsidered favorable (Franklin et al., 2005; Galley et al.,2007).

Within a given volcanic succession, VMS deposits tend tobe associated with specic stratigraphic levels (e.g., Gibsonand Watkinson, 1990; Piché et al., 1993). Therefore, relatively

detailed geologic mapping, core logging, and chemical strraphy can help VMS exploration, especially in the absentraceable exhalites (e.g., Mercier-Langevin et al., 2009).

Where exhalites, iron formations, tuftes, or metal-sediments, such as argillites are present, approaches thatbe useful in providing a vector to ores in VMS and SEDsystems include geochemical ratios derived from whole-analyses (e.g., Scott et al., 1983; Liaghat and MacLean, Spry et al., 2000; Peter, 2003; Barrie et al., 2005), analyssulde separates (e.g., Hannington et al., 1999), and elemtal analysis of specic minerals such as chlorites or sul(e.g., Kalogeropoulos and Scott, 1989; Peter et al., 2003Chapman et al., 2008). So far the literature describing theof such techniques seems to be limited to districts wheredeposits have already been found (e.g., Liaghat and MacL1992; Peter et al., 2003a, b; Barrie et al., 2005); in other w

they are not applied outside of mining camps (at least nothe literature).The study area, representing a very small portion of

Archean Abitibi greenstone belt, does not contain a kn VMS deposit and is located outside of a mining camp. Hever, it displays many of the favorable features descri within the VMS model: bimodal submarine volcanism ining rhyolite domes; Zn-Cu mineralization and hydrotheralteration zones; and metal-bearing stratied intervals. have used a combination of techniques to improve the unstanding of the volcanological and hydrothermal evolutithis area, and to facilitate future VMS exploration. This cbination of eld-based and advanced laboratory techniq

0361-0128/14/4180/61-28 61

†Corresponding author: e-mail, [email protected]*Ministère des Ressources naturelles et de la Faune, Québec, Contribu-

tion 8439-2011-2012-5.**Geological Survey of Canada, Contribution 20110204.



62 ROGERS ET AL.

provides an example of a methodology applicable to otherprospective areas, including those outside of established min-ing camps, in Archean or younger volcanic successions.

The objectives of this paper are (1) to present a chemicalstratigraphy framework for the volcanic rocks in the studyarea (Figs. 1−3), integrating data from this study with dataobtained in previous regional studies; (2) to document the vol-canic facies within each unit including lateral and vertical vari-ations in order to determine the vent areas; (3) to use the traceelement geochemistry of pyrite, from sulde-bearing strati-ed intervals, in combination with geochemical and mineral-ogical studies of alteration in the underlying volcanic rocks, inorder to identify high-temperature, VMS-related hydrother-mal upow zones. We also make preliminary comments onthe possible petrogenesis and tectonic settings of the studiedrocks using a uniformitarian approach.

Geologic Context and Nomenclature

Regional geologyThe Archean Abitibi greenstone belt is the world’s largest

greenstone belt (Goodwin, 1982; Fig. 1). The Blake RiverGroup is the youngest subalkaline volcanic succession in thebelt (Thurston et al., 2008) and is dominated by submarinemac to intermediate lavas with lesser felsic volcanic rocks andmac to intermediate volcaniclastic rocks (Mercier-Langevinet al., 2008; Goutier et al., 2009, 2012; McNicoll et al., 2014,and references therein). The Blake River Group is intrudedby felsic to mac synvolcanic and syntectonic dikes, sills, andplutons (Piercey et al., 2008; Pearson and Daigneault, 2009;Kuiper, 2011, Kuiper et al., unpub. data, 2012). It is struc-turally juxtaposed against and overlain stratigraphically byArchean to Proterozoic sedimentary units (Dimroth et al.,1982; Péloquin et al., 1990).

The Blake River Group is well known for its mineral depos-its, particularly VMS deposits (e.g., Spence and de Rosen-Spence, 1975; Gibson and Watkinson, 1990; Kerr and Gibson,1993; Gibson et al., 2001; Dubé et al., 2007a, b; Gibson andGalley, 2007; Mercier-Langevin et al., 2007a, b, 2011). In theupper (i.e., younger) part of the Blake River Group, VMSdeposits are mostly found in two areas: (1) Cu-Zn deposits inthe Noranda mining camp north of Rouyn-Noranda (Gibsonand Watkinson, 1990; Goutier et al., 2012; McNicoll et al.,2014); and (2) Au-rich VMS deposits of the Doyon-Bousquet-LaRonde mining camp, east of Rouyn-Noranda (Lafrance etal., 2003; Dubé et al., 2007a; Mercier-Langevin et al., 2007a,b, 2011; Wright-Holfeld et al., 2010).

However, the spatially extensive lower Blake River Groupstratigraphy is now considered highly prospective since newU-Pb zircon ages for volcanic rocks that host the Horne andQuemont Au-rich VMS deposits indicate they are lowerBlake River Group in age (Goutier et al., 2012; McNicoll etal., 2014). Improved geologic understanding of the geologyand hydrothermal history of the lower Blake River Group,including the study area, is needed to provide explorationfocus. This area (Fig. 2) consists of the largely tholeiitic basal-tic Hébécourt Formation (e.g., Lafrance et al., 2003; Legaultet al., 2005), and the overlying basaltic to rhyolitic Reneault-Dufresnoy formation (e.g., Goutier, 1997; Laèche et al.,1992; Lafrance and Dion, 2004) of variable magmatic afnity.

In general, lavas dominate the two formations, and volcaclastic rocks are more abundant in the Reneault-Dufresnformation. The Hébécourt Formation (Goutier, 1997) is cosidered a formal stratigraphic unit in the Blake River Grou whereas the Reneault-Dufresnoy formation is not yet a formunit.

Geologic summary of the study area andchemostratigraphic methodsIn the study area, the Hébécourt and Reneault-Dufresno

formations form a steeply south dipping, south youngihomocline (Figs. 2, 3). The stratigraphy illustrated in Figu2 and 3 and discussed below is based on a compilation of p vious studies (including an unpublished map from CogitResources, Inc.), new mapping, and core logging by the senauthor over two eld seasons, and over 100 new geochecal analyses of major and trace elements. Chemostratigrapunits are dened based on rock classications diagrams suas those of Winchester and Floyd (1977), magmatic afndiagrams as reviewed by Ross and Bédard (2009), extentrace element plots using only immobile elements followJenner (1996), and additional binary diagrams in some caThe chemostratigraphic units were checked for spatial cosistency on downhole plots for the seven drill holes studias well as on cross sections and on the surface map. Onthese units were dened, volcanic facies and thickness vations were compiled and interpreted for each unit in ordto determine emplacement mechanisms and locate volcan vent areas. Without the mapping, core logging, and detaichemostratigraphy, the volcanological interpretations wonot have been possible.

In summary, the Hébécourt Formation contains four tholeitic volcanic units (Rogers, 2010; Rogers et al., 2010a, b; F2, 3, Table 1). The oldest unit is a voluminous basalt, interlated with variably variolitic basaltic andesite. The main munit is overlain by a≤ 495 m thick rhyolite. This “main rhyolite” is locally overlain by a thin basalt and a≤ 210 m thick unitof basaltic andesite in the eastern part only. The 45 to 75thick, “upper” rhyolite overlies the basaltic andesite. U-zircon ages for the two rhyolite units are 2703.0 ± 0.9 Mathe main rhyolite and 2702.0 ± 1.0 Ma for the upper rhyo(Fig. 2; McNicoll et al, 2014). There are minor intercalatioof extrusive calc-alkaline volcanic rocks, occurring abovemain rhyolite unit and within the youngest basaltic andite unit in the Hébécourt Formation (Fig. 3). The mac intermediate lavas at the base of the Reneault-Dufresnoy fmation conformably overlie the Hébécourt Formation. T

chemical stratigraphy and physical volcanology of all throcks will be described in detail below. Nomenclature

In this paper, volcaniclastic nomenclature follows Whand Houghton (2006) and references therein. Straticatithickness in bedded rocks (e.g., “thinly laminated”) is afIngram (1954). For Archean volcanic rocks that have expenced metamorphism and hydrothermal alteration, magmaafnities cannot be reliably determined using major elemenTherefore, ratios of immobile trace elements such as Zr/Y, Yb, or Th/Yb are used to assign magmatic afnities (PearceNorry, 1979; Barrett and MacLean, 1999; Ross and Béda



VMS EXPLORATION: BLAKE RIVER GROUP, ABITIBI GREENSTONE BELT 63

50 km

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Kirkland Lake

ARCHEAN

Fig 1c

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Undifferentiated sedimentary rocks

ARCHEAN ( not in stratigraphic order )

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Blake River Group volcanic rocks (dioriteand gabbro not shown)

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66

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Fig 2

A B

Rouyn-Noranda

FIG. 1. Location maps showing (A) the location of the Abitibi greenstone belt within eastern Canada, (B) the location ofthe Blake River Group on a map of the Abitibi greenstone belt (courtesy of Marc Legault), and (C) a simplied geologic mapof the Blake River Group showing the location of the study area.



64 ROGERS ET AL.

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Reneault-Dufresnoyformation

Intrusions

Basalt

Hébécourt Formation

Basaltic andesite

Main rhyolite (low-Ti)

Main rhyolite (high-Ti)

Upper rhyolite

Legend

Field station

No verticalexaggeration100 m

A

B Additional legend

Sulfide-bearingstratified interval

Undifferentiated rhyoliteCalc-alkaline intercalationsUn maficsdifferentiated


FIG. 3. A. Vertical cross section looking WSW, though the trace of diamond drill hole HEB-02. B. Stratigraphic correlationpanel showing most of the diamond drill holes studied. The location of sulde-bearing stratied intervals is highlighted. A andA' are two intervals found approximately at the top of the Hébécourt Formation.



66 ROGERS ET AL.

TABLE 1. Stratigraphic and Volcanological Summary of the Study Area

Stratigraphic Distribution of unitunit Thickness in the formation Facies Geochemistry 1 Interpretation

Hébécourt FormationBasalt >3 km2 Dominant Mostly pillowed and Four new samples (3 from upper 770 m Submarine l massive lavas (Table 2), of mac units below main rhyolite, plain probably forming 1 from above main rhyolite); laterally extensive ows; Basalt to basaltic andesite, tholeiitic; Minor volcaniclastic Extended multielement diagrams show at facies3 proles, small negative Ti anomalies, and Th, Nb, and Ta are slightly depleted

relative to other elements; somewhatMORB-like or BABB-like

Basaltic At least four Several occurrences, Massive lava, pillow lava, 20 new samples (5 from surface, 15 from Part andesite units 50− mostly toward pillow breccia and core); two were taken below main lava plain 700 m thick4 the top hyaloclastite (Table 2; rhyolite, remainder above; Figs. 4, 5) Basaltic andesite to andesite, tholeiitic; Extended multielement diagram:

like the basalt, but more enrichedMain rhyolite ≤ 495 m5 One occurrence Massive or lobate 29 new samples; See subunits toward the top (locally ow-banded) to Rhyolite, tholeiitic (± transitional); volcaniclastic (Table 3; Separated in two subunits based on Ti Figs. 8, 9) contents, Zr/Y ratios, and quartz phenocrysts; Extended multielement plots for two subunits have relatively at proles, with signicant negative Ti anomaliesLow Ti subunit ≤ 490 m5 Domal feature Massive core and base, TiO2 ≤ 0.16%, mostly tholeiitic Submarine lava volcaniclastic top and Zr/Y (with quartz phenocrysts) dome anksHigh Ti subunit Unknown High Ti subunit is Two massive zones TiO2 ≥ 0.16%, mostly transitional Submarine lavas thinner where low surrounded by Zr/Y (no quartz phenocrysts) or domes Ti subunit is thickest volcaniclastic rocksUpper rhyolite ≤ 75 m5 Uppermost unit Massive to volcaniclastic 19 new samples; Submarine (Table 3; Fig. 12); Rhyolite (± rhyodacite), tholeiitic domes or lava

Contains intercalations (± transitional); including of nely laminated mac Richer in TiO2 than main rhyolite, but remobilized tuffs and argillite similar extended trace element pattern debris

Calc-alkaline 37 m in total Three occurrences, Massive to volcaniclastic Five samples; Mixture ointercalations— (HEB-03) between main and (Table 4) Dacite/rhyodacite, calc-alkaline; submarinerhyodacite 27 m in upper rhyolite, in Extended multielement proles show lavas and de HEB-015 DDH HEB-03; one steeper slopes than those of the tholeiitic current occurrence in HEB-01 rhyolites, with higher Th, Nb-Ta troughs, deposits Zr-Hf plateaus and negative Ti anomaliesCalc-alkaline 12 m One occurrence in Pillow breccia One sample;intercalation— HEB-03 Similar to Group 2 in theandesite Reneault-Dufresnoy formation Fragmental lava

Reneault-Dufresnoy formationReneault- The rst Not applicable Mostly pillowed and 28 new samples; Submarine lDufresnoy 300 m were massive lavas, minor Basalt to andesite, tholeiitic to calc-alkaline; possibly formation studied volcaniclastic facies Divided into two geochemical groups; with shield vol (Table 4) group 1 older than group 2; Group 1: tholeiitic to transitional; on

extended trace element diagrams: gentle tomoderate overall slopes, shallow negativeNb-Ta anomalies, small Zr-Hf plateaus, small

negative (sometime positive) Ti anomalies; Group 2: transitional to calc-alkaline; trace

element proles are more steeply inclined,have pronounced Nb-Ta troughs, morepronounced Zr-Hf plateaus and pronouncednegative Ti anomalies

1 See Figures 6, 7, 10, and 11 for details; the summary in this table is based on new analyses only 2 The Hébécourt basalt has not been mapped in detail; the thickness listed here assumes no folding, no intrusions, and incorporates intercalat

basaltic andesite3 For example, diamond drill hole HEB-01 intersected 36 m of basaltic pillow breccia above the main rhyolite4 Intrusions not removed from thickness calculation5 Intrusions removed from thickness calculation




2009). ”Transitional” is employed to describe rocks that plotbetween tholeiitic and calc-alkaline elds. The term “exha-lite” (Ridler, 1971) is commonly used in VMS exploration andresearch. True exhalites must have an exhalative component(e.g., Knuckey et al., 1982; Kalogeropoulos and Scott, 1989)and this is typically difcult to prove since postdepositionaluid circulation in a sediment or tuff can introduce compo-nents such as silica and suldes. Herein, we use the descrip-tive expression “sulde-bearing stratied intervals,” which hasno genetic connotation, instead of exhalites.

Mac Tholeiitic Rocks in the Hébécourt Formation

DescriptionIn the study area (Fig. 2), the Hébécourt Formation is

divided into two intercalated aphyric units: (1) the volu-metrically dominant Hébécourt basalt, and (2) the variably variolitic Hébécourt basaltic andesite. The volcanological andgeochemical characteristics of these units are summarized inTables 1 and 2. Figure 4 shows the typical facies, Figure 5shows the facies variation in one well-studied basaltic andesiteunit, and geochemistry is presented in Figures 6 and 7. Fur-ther details are available from Rogers (2010), and Rogers etal. (2010a, b).

Mapping focused on a 132 m thick basaltic andesite interval located between the main and upper rhyolites (Figs. 4,5). Based on eld and core observations in four separate drillholes, this unit thins westward, from 132 to 56 m, until iteventually pinches beyond hole HEB-08 (Fig. 5). The pillowfacies dominates, with average pillow size decreasing from 90to 100 cm in the east (DDHs HEB-01, HEB-02, and HEB-03) to 40 cm in the west (DDH HEB-08). The proportion of volcaniclastic rocks within the unit increases westward, from

0% in DDH HEB-03 to 30% in DDH HEB-08 (Fig. 5). Tmassive facies, by contrast, is absent in DDH HEB-08 anthickest in DDH HEB-03.Interpretation

The Hébécourt basalt and the Hébécourt basaltic andesconsist of submarine mac lavas displaying the facies tyof such ows (e.g., Dimroth et al., 1978; Gibson et al., 1In the equivalent lower Blake River strata of Ontario, theborne magnetic patterns suggest laterally extensive subu which may represent individual ows or packages of possibly with intercalated mac sills. The Hébécourt Fortion has been interpreted a submarine lava plain (Dimrotal., 1982).

A comparison of our observed facies variations with tyfacies variations in mac to intermediate lavas suggeststhe vent area for the uppermost basaltic andesite is locain the eastern part of the unit, near DDH HEB-03. Masive rocks are more abundant in HEB-03 and from here unit thins westward, pillow size decreases westward, andproportion of volcaniclastic rocks within the unit incre westward.

Geochemically, data from the Hébécourt basalt and Hébécourt basaltic andesite plot along the same trendsbinary diagrams displaying incompatible elements (e.g.,6D); further, their extended trace element proles are vsimilar (Fig. 7). This geochemical similarity and their sgraphic relationships suggest that the two units are commatic where the Hébécourt basaltic andesite may be a sligdifferentiated version of the Hébécourt basalt.

The trace elements patterns for the Hébécourt basalt, wdepletions of Th, Nb, and Ta relative to the rest of the proare similar to those of modern back-arc basin basalts (BA

TABLE 2. Summary Facies Descriptions of the Mac Tholeiitic Rocks in the Hébécourt Formation

Thickness/ EmplacementStratigraphic unit Facies distribution Characteristics mechanisms

Hébécourt basalt Massive Not mapped Coherent volcanic rock, ne-grained to aphanitic, medium gray in Submar fresh surface, generally nonvesicular

Pillowed Not mapped Pillows are ne grained, medium to dark gray in fresh surface and Submarin 60−80 cm across on average, with 2−5 mm thick chilled margins;

quartz amygdales≤ 2 mm; interstitial hyaloclastite shards≤ 1 cm;rare pale-colored varioles≤ 1 cm

Hébécourt Massive Figure 5: minor, Coherent volcanic rock, ne-grained, aphyric, medium gray to Submaribasaltic andesite mostly near the green in fresh surface with variable abundances (1−2% but up to base of unit 20% locally) of 2−3 mm quartz, chlorite, or calcite amygdales;

locally 10 to 90% varioles, circular and 5 mm to 1 cm across(Fig. 4a) Pillowed Figure 5: Pillows are aphyric, ne grained and medium to dark gray in fresh Submarin

dominant surface (Fig. 4b, c); 1−2%, 1−2 mm, quartz, calcite or chloriteamygdales, which increase in abundance (≤ 5%) toward themargins;≤ 90% varioles at margins which decrease in size andabundance inward; interstitial chloritized hyaloclastite clasts,1−2 cm, with common jigsaw t texture

Pillow breccia Figure 5 Monomictic breccia contains round to uidal pillow clasts, Submarin 5−10 cm diam, max 25 cm, locally variolitic; chilled margins in

some cases; also contains angular hyaloclastite clasts, 2−3 cmacross, with common jigsaw t texture, between the pillowfragments (Fig. 4d)

Pillow-free Figure 5 Chloritized angular (former glass) shards≤ 4 cm, jigsaw-t aspect, Submarine lavas hyaloclastite quartz cement (Fig. 4e-f)



68 ROGERS ET AL.

Fig. 7F) or to a lesser extent, normal mid-ocean ridge basalts(N-MORB, Fig. 7E). In a tectonic classication diagram, theHébécourt basalts and basaltic andesites plot in the MORBeld but not far from the island-arc basalt (IAB) eld (Fig.7G). Finally, in the Th/Yb versus Nb/Yb diagram, the samplesplot near the present-day MORB-OIB array, indicating onlyminor contributions of a subduction component or limitedcrustal contamination (Pearce, 2008; Fig. 7H).

Felsic Tholeiitic Rocks in the Hébécourt Formation

DescriptionThere are two main tholeiitic felsic units in the Hébécou

Formation within the study area: the main rhyolite (separatinto two subunits) and the thinner upper rhyolite. Their volnological and geochemical characteristics are summarizedTables 1 and 3. Figure 8 shows the map distribution of fac

8 cm

Hy Var

15 cm

Var

A

C

F

B

E

D

FIG. 4. The uppermost occurrence of the Hébécourt basaltic andesite. A. Massive facies displaying a high concentrationof varioles. Scale is graduated centimeters. B. Triple junction in the pillow facies with interstitial hyaloclastite (Hy). Varioles(Var) are abundant in the pillow margins. C. A complete pillow with an increase in concentration of varioles and vesiclestoward the margin. D. Pillow breccia facies in drill core, comprising in situ fragmented hyaloclastite and larger clasts mostlikely representing pillow fragments. The ruler is graduated in centimeters and millimeters. E. and F. Hyaloclastite facies indrill core, displaying in situ fragmentation and small, chloritized clasts. In both cases the scale is a 15 cm plastic ruler.




in the main rhyolite, whereas Figure 9 illustrates typical tex-tures in these submarine felsic rocks. Figures 10 and 11 dis-play the geochemistry of our samples compared to previousstudies. Figure 12 shows the variations in thickness and faciesin the upper rhyolite using drill hole information, since thisunit does not crop out well.Interpretation

The main rhyolite can be divided in two subunits based onquartz crystals abundance, Zr/Y ratios, and Ti contents. This isimportant because without the separation into two subunits,the facies variations would have been interpreted differently.Facies variations in the low Ti (quartz-phyric) subunit sug-gest that it was emplaced as a dome with a massive core, thickank breccias, and a thin carapace (e.g., Yamagishi and Dim-roth, 1985; McPhie et al., 1993). The low Ti lava dome waspresumably emplaced on a paleohorizontal surface formed bymac lavas. The location of the massive core of the dome gives

the general location of the effusive center (volcanic vent)east of the Chemin de la Mine (Fig. 8B).

The high Ti (aphyric) subunit of the main rhyolite is interpreted as submarine lava ows or domes. It has mcomplex facies variations than the low Ti subunit. Mafacies are not located near the proposed volcanic vent arethe underlying low Ti rhyolite, suggesting that the two units do not share the same vent. Given the location ofmassive facies and the thickness variations within the higsubunit, we propose it erupted from two separate vents, on either side of the Chemin de la Mine (Fig. 8C).

The upper rhyolite, although thinner than the subunitsthe main rhyolite, is more complex; it is interpreted to hbeen produced by two distinct effusive episodes. The olepisode produced the thickest products and these thin w ward, from 70 to 75 m in DDHs HEB-01 and HEB-03 to 4in DDHs HEB-02 and HEB-08 (Fig. 12). The upper rhyoin DDH HEB-01 consists entirely of the massive facies

HEB-01

HEB-02 HEB-08

N HEB-03 Younging

500 m

C h e m

i n

d e l a m i n e

Hyaloclastite (F)

Pillow breccia (F)

Pillows (P)

Massive (M)

Other units(mostly intrusive)

Field station

Stratigraphic columns from drill core observations

Map projection

Other

Basaltic andesite facies

P

M

P

MF

P

MF

P

T r u et h i ck n

e s s i n

cl u d i n

gi nt r u

s i on

s

( m )

Top of basaltic andesite

Base of basaltic andesite0

50

100

150

200

250

M

P

P

P

P

P

MM

0

50

100

150

P

P

P

P

P

P

P

P

MF

MF0

50

100

150

200

P

P

F

F

P

0

50

P

P

F

P

FIG. 5. Volcanic facies variations in the uppermost occurrence of the Hébécourt basaltic andesite. Top: Graphic logs ofthe four diamond drill holes (DDHs) examined for this unit. All logs are at the same scale, which represents the true thick-ness of the units including intrusions. Thicknesses with intrusions removed are: HEB-03, 127 m; HEB-01, 132 m; HEB-02,85 m; and HEB-08, 56 m. Pie charts summarize the proportion of each volcanic facies within each DDH (see legend). Bot-tom: A map displaying the facies variations, integrating surface observations and drill core observations projected along theinferred 72°S bedding plane. The map has been rotated to show the correct way-up and does not have the same orientationas Figure 2.



70 ROGERS ET AL.

Hébécourt basaltic andesite

Hébécourt basalt Basalt previous data

Calc-alkaline andesite

Basaltic andesiteprevious data


previous data

Group 1 Group 2

Reneault-Dufresnoy formation

0.03 0.1 1 2Nb / Y

Z r

/ T i O

2

0.01

0.1

1

0.001

rhyolite

rhyodacite

daciteandesite

a l k a

l i - b a s a

l t

comendite/ pantellerite

sub-alkaline basalt

Tr/An

B

andesite/ basalt

0.001 0.01 0.1

40

50

80

70

60

S i O

( w t % )

2

rhyolite

rhyodacite

dacite

andesitebasaltic andesite

basalt

ABbasanitetrachybasanitenephelinite

phonolite

Tr/An

c/p

trachyte

Zr / TiO 2

0.3

A C

0

Calc-alkaline

T r a n s i

t i o n a l

Tholeiitic

Z r / Y =

4 . 5

Z r / Y = 2 . 8

Z r p p m

50

100

150

200

250

10 20 30 40 50 60 70 80 90

Y ppm

Yb ppm

L a p p m L a

/ Y b =

2 . 6

L a / Y b

= 5 . 3

Calc-alkaline Transitional

Tholeiitic

D

00

24

68

101214161820

2 4 6 8 10 12

Log Zr/Y10

L o g

T h / Y b

1 0

Calc-alkaline

Tholeiitic Transitional

1 1.250.750.50.25-1

-0.75

-0.5

-0.25

0

0.25

0.5E

FIG. 6. Geochemistry of mac to intermediate volcanic rocks in the study area. A. and B. Classication diagrams from Winchester and Floyd (1977). C. to E. Magmatic afnity diagrams from Ross and Bédard (2009). Analyses illustrated bysymbols on all diagrams were obtained from Activation Laboratories Ltd. in Ancaster, Ontario, using fusion ICP-AES formajor elements, and fusion ICP-MS for the trace element shown. The compiled data plotted as elds here and in Figure 10 were provided by Cogitore Resources Inc., with additional unpublished data from the Geological Survey of Canada (courtesyof E. Grunsky). In addition the Reneault-Dufresnoy eld in (E) includes lavas from locations S of the study area (from Rosset al., 2008a).




10

100

Th Nb Ta La Ce Pr NdSm Zr Hf Eu Ti Gd Tb Dy Y Er Yb Lu1

Hébécourt basalt

A

R o c k / p

a n

t l e

r i m i t i v e m

10

100

Th Nb Ta La Ce Pr Nd Sm Zr Hf Eu Ti Gd Tb Dy Y Er Yb Lu1

Hébécourt basaltic andesite

B

R o c k / p

a n

t l e

r i m i t i v e m

100

1

10

Th Nb Ta La Ce Pr NdSm Zr Hf Eu Ti Gd Tb Dy Y Er Yb Lu

Reneault-Dufresnoy formation(Group 1)

C

R o c k / p

a n

t l e

r i m i t i v e m

1

10

100


Reneault Group 2)and calc-alkaline andesite in Hébécourt Fm

-Dufresnoy fm ( D

R o c k / p

a n

t l e

r i m i t i v e m

100

1

10


E

R o c k / p

a n

t l e

r i m i t i v e m

1

10

100


R o c k / p

a n

t l e

r i m i t i v e m

OIB

N-MORB

E-MORB

Non-arc basalts Arc basalts

IAT

Med-K CAB

BABB

G

1 10 100.1

1

.1

.01

10

Magma-crustinteraction

Deepcrustalrecycling

V o l c a

n i c a r c a

r r a y

M O R B

- O I B a

r r a yOIB

N-MORB

E-MORB

Archean crust

Nb/Yb

T h / Y b

BRG

-8

-4

0

4

8

-8 -4 0 4 8

DF1

D F 2

F

H

CRB + OIB

MORB

IAB


Group 2Group 1

Basaltic andesite

BasaltBasalt, previous data

Reneault-Dufresnoy formation

Calc-alkaline andesite

FIG. 7. Geochemistry of mac to intermediate volcanic rocks in the study area and comparison with archetypical basaltsfrom known tectonic environments. A.-D. Extended multielement plots based on our data. Two unpublished analyses of theHébécourt basalt (Ta not determined) were supplied by E. Grunsky. Archetypal modern basalts: (E) from nonarc environ-ments from Sun and McDonough (1989): N-MORB = normal mid-ocean ridge basalt, E-MORB = enriched MORB; OIB= ocean island basalt; (F) from arc environments: BABB = back-arc basin basalt (avg of sites 834-839 in the Lau basin fromEwart et al., 1994); IAT = island-arc tholeiite (avg of samples 482-8-1, 482-8-8, 482-8-11, and 482-8-12 from Ata island,Tonga arc, after Turner et al., 1997); Med K CAB = medium K calc-alkaline basalt (avg of samples 67150 and 67154, island ofFlores, Sunda arc, after Stolz et al., 1990). In (A) to (F), primitive mantle normalization values are from Sun and McDonough(1989). G. Tectonic discrimination diagram from Agrawal et al. (2008). IAB = island-arc basalt; CRB = continental rift basalt.Discriminant factors (DF1, DF2) are functions combining Th, La, Sm, Yb, and Nb in log ratio format. H. Th/Yb vs. Nb/Ybdiagram from Pearce (2008), useful for identifying crustal inputs in oceanic basalts. BRG = other Blake River Group lavas with SiO2 <60%, from Ross et al. (2008a, b, 2009).



72 ROGERS ET AL.

increasing amounts of volcaniclastic facies in other drill holes.Bedded fragmental rocks with rounded rhyolite clasts areobserved in DDH HEB-03, suggesting transport of rhyoliticdebris on the ank of a dome or lava. Facies variations suggestthat the vent was located near DDH HEB-01 for this episode.Products from the younger episode are only evident in DDHHEB-08 and little can be said about its origin or vent area.

The tholeiitic rhyolites in the Hébécourt Formation havetrace element proles that are similar in form to those of themac tholeiitic rocks in the same formation, except for nega-tive Eu and Ti anomalies (relative to the rest of the elements).Rare earth element (REE) modeling by Robidoux (2008) sug-gested that the magma for the main rhyolite can be derivedfrom a magma that produced the Hébécourt basalt lava ows

following 70 to 80% plagioclase-dominated fractionation. Ifthis model is correct, then all of the tholeiitic rocks of theHébécourt Formation may ultimately have been derivedfrom the same magma. However, this interpretation wouldrequire that the inferred intermediate differentiation prod-ucts between the Hébécourt basaltic andesite and the Hébé-court main rhyolite were not erupted on the sea oor, becausethe Hébécourt Formation is strongly bimodal. The alternativeis that the tholeiitic rhyolites were formed by crustal meltingof hydrated basalt at shallow crustal depths (e.g., Hart et al.,2004; Leclerc et al., 2011).

The tholeiitic rhyolites in the Hébécourt Formation havean FIIIb-like signature in the diagrams of Lesher et al. (1986)

and Hart et al. (2004), although they do not plot entire within the predened elds (Fig. 11E, F). FIIIb rhyolites aassociated with VMS deposits at Matagami and Kidd Crefor example (Hart et al., 2004). This type of rhyolite is format shallow crustal depths (<10 km) in the absence of residgarnet or amphibole and the magmas were possibly emplacin an oceanic rift setting (Hart et al., 2004).

Calc-Alkaline Intercalations in the HébécourtFormation

DescriptionAlthough the Hébécourt Formation is dominantly thole

itic, minor intercalations of calc-alkaline rocks occur in

easternmost part of the study area (DDH HEB-03 and HEB01). These mostly felsic intercalations are summarizedTables 1 and 4, and their geochemistry is shown in Figuresand 11. Rogers (2010) provided a more detailed descriptiof the units.Interpretation

Intervals of calc-alkaline rocks within the Hébécourt Fmation were not anticipated. Rocks with transitional to caalkaline afnities were previously interpreted to have berestricted to the stratigraphically overlying Reneault-Dufrnoy formation (Goutier, 1997). The possibility that the rhdacitic calc-alkaline rocks in the Hébécourt Formation a

TABLE 3. Summary Facies Descriptions of the Felsic Tholeiitic Rocks in the Hébécourt Formation


Main rhyolite Massive Figure 8 Coherent volcanic rock, medium to ne grained, locally ow Interior of subm(both low and banded and medium gray in fresh surface (Fig. 9a);≤ 3% rhyolite domes and owshigh Ti subunits) amygdales 1−2 mm (quartz, calcite, or chlorite); 1−90% white,

gray, or beige, circular to irregular spherules,≤ 1 cm across;sericite, chlorite, and epidote alteration locally, from moderateto intense;

In low Ti subunit: 1−2% quartz phenocrysts 1−2 mm and raresmaller feldspar phenocrysts; 1−5% quartz microphenocrysts

Volcaniclastic Figure 8 Monomictic tuff breccias, lapilli tuffs and tuffs, typically not Flank and summ bedded; wide range in rhyolite clast sizes, from 1 mm to 35 cm breccias on submar (Fig. 9b-c); normal grading seen locally in rocks with clasts less rhyolite domes and than 2 cm in size; typically matrix supported, with a quartz ows; hyaloclastic cement; angular to subangular clasts, generally equant in shape fragmentation,1 in situ and randomly oriented or resedimented slightly

Upper rhyolite Massive Figure 12 Coherent volcanic rock, ne grained and medium gray in fresh Interior of subm surface (Fig. 9d); 2−3% quartz or chlorite amygdales, 1−2 mm rhyolite domes and and circular to irregular in shape; two spherulitic textures:

(1) discrete white spherules,≤ 2 mm across, circular to slightlyirregular in shape; (2) impinging spherules 1−2 cm across with

intervals of completely merged spherules; groundmasschloritized, epidote altered or siliciedAngular Figure 12 Monomictic lapilli tuff (by composition), nonbedded; typically Related to felsic

fragment matrix supported, with a ne-grained matrix (Fig. 9e); locally domes (ank brec lapilli tuff clast supported; 1−2 cm angular fragments, max 5 cm;

spherulitic texture visible within larger clasts, white spherulesup to 1 cm in size

Rounded Figure 12 Can be stratied (medium thickness); rounded to subrounded Resedimented mat fragment clasts, 2 mm across, max 1−2 cm; normal grading locally; (density currents) lapilli tuffs green or white color in fresh surface (epidote-altered rhyolite; lava or dome and tuffs siliceous rhyolite)

1 See White and Houghton (2006) for a complete denition of “hyaloclastite”





74 ROGERS ET AL.

intrusive (as thought by some previous workers) is not con-sistent with the large proportion of volcaniclastic facies thatdisplay multiple graded beds and rounded fragments requir-ing lateral transport by density currents, and a calc-alkalineandesite facies that consists of pillow breccias (formed on thesea oor). Therefore, near simultaneous extrusion of abun-dant tholeiitic and minor calc-alkaline magmas is required

during Hébécourt Formation volcanism. It is likely that t volcanic vents for each magma type were separate. The vfor calc-alkaline volcanism may have been located to the of where these rocks were encountered as there is a highproportion of calc-alkaline rhyodacite toward the east in observed drill holes. Also, clasts size in the volcaniclastic sis larger to the east.

10 cm

A

10 cm

5 cm

C

E

F

B

D

FIG. 9. Felsic rocks in the Hébécourt Formation. A. Massive facies in the low Ti subunit in the main rhyolite, with hairlinesericite veins and 1 to 2 mm “quartz eye” phenocrysts. B. Volcaniclastic facies of the low Ti rhyolite, clearly matrix supported with large clasts. C. Volcaniclastic facies of the high Ti rhyolite. D. Massive facies of the upper rhyolite, with impinging,irregularly shaped spherules. E. Lapilli-tuff facies of the upper rhyolite, with angular fragments. The smaller clasts are epi-dote altered and the larger clasts display a microspherulitic texture. F. Interbedded nely laminated tuff and argillite from theboundary between the rst and second volcanic episodes of the upper rhyolite.




A

B

C

D

E

F

0.001 0.01 0.1 1 240

50

80

70

60

( w t % )

S i O

2

rhyolite

rhyodacite/dacite

andesite

sub-alkalinebasalt

ABbasalt/

trach./neph.

phonolite

Tr/An

comp/par

trachite

Zr / TiO2

0.03 0.1 1 2Nb / Y

Z r

/ T i O

2

0.01

0.1

1

0.001

rhyolite

rhyodacitedacite

andesite

andesite / basalt

a l k a

l i - b a s a

l t

comendite/ pantellerite

sub-alkaline basalt

Tr/An

0100200

300400500600700800900

0 50 100 150 200 250 300 350

Calc-alkaline T r a

n s i t i o n

a l

Tholeiitic

Z r / Y

= 4 . 5

Z r / Y = 2 . 8

Y ppm

Z r p p m

0

0.10.20.30.40.50.60.70.8

0 200 400 600 800 1000 1200

T i O

w t %

2

Zr ppm

0100

200

300

400

500

600

700

800900

0 50 100 150 200 250 300 350

Calc-alkaline T r a

n s i t i o n

a l

Tholeiitic

Z r / Y

= 4 . 5

Z r / Y = 2 . 8

Y ppm

Z r p p m

Log Zr/Y10

L o g

T h / Y b

1 0

Calc-alkaline

Tholeiitic Transitional

1 1.250.750.50.25-1

-0.75

-0.5

-0.25

0

0.25

0.5

Upper rhyolite

Calc-alkaline rhyodacite

Low-Ti subunit

High-Ti subunit

Previous Data

Previous Data

Previous DataPrevious Data

Legend

Main rhyolite

FIG. 10. Geochemistry of felsic volcanic rocks from the Hébécourt Formation in the study area. A. and B. Classicationdiagrams from Winchester and Floyd (1977) for all units. C. D. and F. Magmatic afnity diagrams from Ross and Bédard(2009): (C) for the two subunits of the main rhyolite; (D) for the upper rhyolite and the calc-alkaline rhyodacite; (F) for allunits. (E) Plot of TiO2 vs. Zr for all units.



76 ROGERS ET AL.

Reneault-Dufresnoy FormationThe Reneault-Dufresnoy formation is the youngest strati-

graphic unit in the study area (Rogers, 2010). It consistsmostly of massive to pillowed submarine lavas in the rstfew hundred meters (Tables 1, 4). The Reneault-Dufresnoyrocks are geochemically variable (Figs. 6, 7), as also noted byRoss et al. (2011a, b) and Goutier et al. (2012). The whole ofthis formation west of Lake Duparquet (including the studyarea and units farther south) was interpreted as a now-tilted

submarine shield volcano built on the Hébécourt lava pla(Dimroth et al., 1985; Ross et al., 2011a).

Trace element proles of Group 2 lavas (Fig. 7D) resembthose of modern oceanic arc magmas (e.g., Fig 7F: mediuK calc-alkaline basalts). On a tectonic discrimination dgram, the Reneault-Dufresnoy lavas straddle the MORB-IAboundary (Fig. 7G), with most Group 2 samples plottingthe IAB region. Finally, on the Th/Yb versus Nb/Yb diag(Fig. 7H), Reneault-Dufresnoy samples dene a trend aw

100

0.1

1

10

1000

R o c k / p

a n

t l e

r i m i t i v e m


D

100

0.1

1

10

1000

R o c k / p

a n

t l e

r i m i t i v e m

C


100

0.1

1

10

1000

R o c k

/ p

a n

t l e

r i m i t i v e m

A


100

0.1

1

10

1000

R o c k / p

a n

t l e

r i m i t i v e m

B


Low-Ti subunit High-Ti subunit

Hébécourt upper rhyolite Calc-alkaline rhyodacite

150 200100500Y

Z r / Y

FI

FII

FIIIb

FIIIa

15

10

5

20

25

0

E

150 20100500Yb cn

1

10

100

.1

( L a

/ Y b ) c

n

FI

FII

FIIIb

FIIIaFIV

F


Low-Ti subunitHigh-Ti subunit

Hébécourt upper rhyoliteCalc-alkaline rhyodacite

Main rhyolite

FIG. 11. Geochemistry of felsic volcanic rocks from the Hébécourt Formation in the study area. A. to D. Extended multi-element plots. Primitive mantle normalization values are from Sun and McDonough (1989). E. and F. Rhyolite classicationdiagrams after Lesher et al. (1986) and Hart et al. (2004). FI to FIII elds in (E) after Piercey (2010). Normalization valuesin (F) from Nakamura (1974): average of 10 chondrites.




H E B

- 0 1

0 5 0

9 0

A s h C o a r s e A s h < 1 c m > 1 c m

H E B

- 0 8

True thickness including intrusions (m)

0 5 0

1 0 0

1 2 0


H E B

- 0 2

0 5 0


H E B - 0 3

0 5 0 8 0


B o u n d a r y

b e t w e e n v o l c a n i c e p i s o d e s

R e n e a u l t - D u f r e s n o y

f o r m a t i o n

H é b é c o u r t b a s a l t i c a n d e s i t e



T r u e t h i c k n e s s

( e x c l u d i n g i n t r u s i o n s ) : 7 0 m







4 3 % F r a g m

e n t a l

6 7 % F r a g m

e n t a l

0 % F r a g m

e n t a l

4 9 % F r a g

m e n t a l

6 1 5 m

6 1 5 m

8 0 0 m

M a s s i v e

M a f i c

t u f f

L a m i n a t e d

t u f f a n d a r g i l l i t e

F r a g m e n t a l

I n t r u s i o n


O t h e r r o c k

t y p e s

F I G .

1 2 . T h i c k n e s s a n d v o l c a n i c

f a c i e s v a r i a t i o n s

i n t h e H é b é c o u r t u p p e r r h y o l i t e , s h o w n

b y g r a p h i c

l o g s f r o m D D H H E B

- 0 3 , H E B

- 0 1 , H E B

- 0 2 , a n d

H E B

- 0 8 , a l l

p l o t t e d a t

t h e s a m e v e r t

i c a l s c a l e . T h e h o r i z o n t a l a x i s i s a g r a i n - s i z e s c a l e .

T h e r e d

l i n e i s i n t e r p r e t e d t o r e p r e s e n

t t h e b o u n d a r y b e t w e e n

t w o v o l c a n i c e p i s o d e s

i n t h e

H é b é c o u r t u p p e r r h y o l i t e , m a r k e d

b y n e l y l a m i n a t e d t u f f a n d a r g i l l i t e , a n d

i n H E B

- 0 8 b y a

t h i c k e r u n i t o f m a c t u f f .



78 ROGERS ET AL.

from the MORB-OIB array, suggesting signicant involve-ment of a subduction component or crustal contamination.

The calc-alkaline andesite intercalated in the HébécourtFormation plots with Group 2 samples from the Reneault-Dufresnoy formation in all geochemical diagrams shown.Figure 3 illustrates that the thin calc-alkaline intercalations(andesite and rhyodacite) occur at several stratigraphic levels.The Hébécourt Formation is a product of near contempora-neous tholeiitic and minor calc-alkaline volcanism with mag-mas erupted from different vents located in the same generalarea. This spatial and temporal coincidence favors crustalcontamination of the tholeiitic magma—not variable degreesof subduction involvement—as the easiest mechanism toproduce arc-like calc-alkaline trace element signatures inthe Reneault-Dufrenoy formation (and in the calc-alkalineintercalations in the Hébécourt Formation). This conclusionmay extend to most of the Blake River Group, for which manysamples plot between the MORB-OIB array and the pole forArchean crust in Figure 7H.

Mineralization and AlterationThe top of the Hébécourt Formation in the area between

the Ontario-Quebec border and Lake Hébécourt, includingthe two tholeiitic rhyolites described above, has historicallybeen recognized as a prospective area for VMS-type miner-alization since the discovery of Zn in this area in the 1970s(Cloutier, 1975; Cashin and Fraser, 1992). Since then, explo-ration has focused mainly on the main rhyolite east of theChemin de la Mine (Fig. 2). Alteration

The Ishikawa alteration index (AI; Ishikawa et al., 1976) isa good indicator of chlorite and sericite alteration commonly

associated with metamorphosed VMS deposits (e.g., Pauet al., 2001; van Ruitenbeek et al., 2011). Values beloware indicative of diagenetic trends, fresh rocks have AI valbetween 20 and 60 to 65, and rocks with higher values more altered (e.g., Large et al., 2001; Gifkins et al., 2005) values were calculated for compiled datasets of historic new surface samples (Fig. 13A) and drill core samples (F13B). Figure 13A shows that the low Ti rhyolite is much maltered than the overlying high Ti rhyolite near the Cheminla Mine. Differing degrees of alteration of the main rhyolsubunits was also observed in drill cores farther east (Fras1991; Martin, 1994; Bambic, 1998), as shown in Figure 1The low AI values for the high Ti rhyolite are indicativediagenetic trends, as shown by the alteration box plot for fe volcanic rocks (Fig. 13D). However, many samples of fe volcanic rocks show evidence of VMS-related hydrotheralteration as dened by chlorite ± pyrite ± sericite and seric+ chlorite ± pyrite trends in Figure 13D. The alteration bplot indicates that the mac volcanic rocks are predominanunaltered (Fig. 13C), perhaps with some carbonate alterati

and minor chlorite alteration.Mineralization in the main rhyolite

Assay data (compiled by Cogitore Ressources Inc.) indicthat sulde mineralization and anomalously high metal ccentrations dene two zones, informally called zone A (F14A) and zone B (Fig. 14B). Zone A is located in the cenregion of the main rhyolite (mostly within the high Ti subuand is characterized by anomalously high Zn, with almostCu. The highest assays for Zn (e.g., 6.17 wt % over 0.1 m)in DDH SC-13 and are represented by semimassive banor stringers of pyrite and sphalerite (Cloutier, 1975; Mart1994; Bambic 1998). Lesser Zn values occur to the east

TABLE 4. Summary Facies Descriptions for the Reneault-Dufresnoy Formation and the Calc-Alkaline Rocks of the Hébécourt Formati


Hébécourt FormationCalc-alkaline Massive ≤ 20 m thick Coherent volcanic rock, ne grained to aphanitic, medium to dark gray Submarinrhyodacite in fresh surface; 1−2% white spherules,≤ 2 cm across; 1−2% quartz ow or dome amygdales 2−3 mm across; locally 1−2% feldspar phenocrysts,

≤ 2 mm across Volcaniclastic ? Variable (see Rogers, 2010). Lapilli-tuff to tuff; includes graded beds Variable 2−3 m thick; angular to rounded clasts

Calc-alkaline Pillow breccia 12 m 0.5−1 cm hyaloclastite shards, angular, chloritized and jigsaw-t; Part oandesite larger aphyric, nonvesicular, pillow fragments with uidal shapes, submarine ≤ 15 cm across, with chilled margins; beige-whitish varioles lava ow,

(in fresh surface) in some pillow fragments,≤ 1 cm in size hyaloclasticfragmentation

Reneault-Dufresnoy formation (Groups 1 and 2), rst 300 m Massive Not mapped Coherent volcanic rock, medium to very ne grained, green to medium Submari gray in fresh surface;≤ 2% carbonate or chlorite lled vesicles≤ 2 mm

Pillowed Not mapped Pillows are aphyric, ne grained, 70−80 cm across, max≤ 2 m; they contain Submarine lavas 2−3% (although locally≤ 20%) quartz and chlorite amygdales which are 1−3 mm in diameter; varioles at margins, <0.5 cm across; interstitial

hyaloclastite, chlorite- and epidote-altered, angular fragments, 3−4 mm with local jigsaw-t textures

Volcaniclastic Not mapped Pillow breccia and tuff; hyaloclastite with angular, chloritized, jigsaw-t Subma fragments, 2−4 mm across;≤ 10% pillow fragments, >15 cm, uidal shapes and chilled margins




S C - 1

5

H B 9 4 - 0

2

7 5 3 - 0

3

7 5 3 - 0

5

H B 9 7 - 0

2

H E B - 0 5

H E B - 0

3

H E B - 0 1

H E B - 0

2

S C - 1

3

H E B - 0

8

H E B - 0

4

H E B - 0

9

6 1 7 0 0 0 m

E

5 375 000 mN 5 374 000 5 373 000 5 372 000

6 1 6 0 0 0

6 1 5 0 0 0

6 1 4 0 0 0

6 1

3 0 0 0

6 1 2 0 0 0

6 1 1 0 0 0

I s h i k a w a

D D H s a m p

l e s

( v e r t i c a

l p r o

j e c

t i o n

)

8 0

1 0 0

6 0

4 0

2 0

0

F r e s h

A l t e r e

d

D i a g e n e s i s

I s h i k a w a

i n d e x ( f o r a a n

d b )

D

H E B - 0

6

H E B - 0

7

7 7 7 4 6

S C - 1

2

H B - 9

4 - 0

1

S C - 1

5

H B 9 4 - 0

2

7 5 3 - 0

3

7 5 3 - 0

5

H B 9 7 - 0

2

H E B - 0 5

H E B - 0

3

H E B - 0 1

H E B - 0

2

S C - 1

3

H E B - 0

8

H E B - 0

4

H E B - 0

9

6 1 7 0 0 0 m

E

5 375 000 mN 5 374 000 5 373 000 5 372 000

6 1 6 0 0 0

6 1 5 0 0 0

6 1 4 0 0 0

6 1 3 0 0 0

6 1 2 0 0 0

6 1 1 0 0 0

I s h i k a w a s u r f a c e s a m p

l e s

( n o p r o

j e c

t i o n

)

I n t r u s i v e u n

i t

V o

l c a n

i c u n

i t

D i a m o n

d d r i l l h o

l e

C h e m

i n d e

l a M i n e

L e g e n

d ( f o r a a n

d b )

A

H E B - 0

6

H E B - 0

7

7 7 7 4 6

S C - 1

2

H B - 9

4 - 0

1

B

0

1 0

2 0

3 0

4 0

5 0

6 0

8 0

9 0

1 0 0

7 0

0 1 0 2 0 3 0 4 0 5 0 6 0 8 0 9 0 1 0 0 7 0

C h l o r i t e - C a r b o n a t e - P y t i t e i n d e x ( C C P I )

I s h i k a w a a

l t e r a

t i o n

i n d e x

( A I )

L e a s t a

l t e r e d

b o x e s

M a

f i c r o c

k s a m p

l e s

( t h i s s t u d y )

B a s a

l t ( a r c s )

A n d e s i t e

( a r c s )

C h l o r i t e S

e r i c

i t e

( p h e n g

i t e )

M u s c o v i t e

K - f e

l d s p a r

A l b i t e

E p

i d o

t e

C a

l c i t e

D o

l o m

i t e

A n

k e r i t e

A c

t i n o

l i t e

T r e m o

l i t e

B a s a

l t ( r i d g e s )

0

1 0

2 0

3 0

4 0

5 0

6 0

8 0

9 0

1 0 0

7 0

0 1 0 2 0 3 0 4 0 5 0 6 0 8 0 9 0 1 0 0 7 0

C h l o r i t e - C a r b o n a t e - P y t i t e i n d e x ( C C P I )

I s h i k a w a a

l t e r a

t i o n

i n d e x

( A I )

R h y o

l i t e ( a r c s )

D a c

i t e ( a r c s )

L e a s t a

l t e r e d

b o x e s

F e

l s i c r o c

k s a m p

l e s

( t h i s s t u d y )

C h l o r i t e S

e r i c

i t e

( p h e n g

i t e )

M u s c o v i t e

K - f e

l d s p a r

A l b i t e

E p

i d o

t e

C a

l c i t e

D o

l o m

i t e

A n

k e r i t e

A c

t i n o

l i t e

T r e m o

l i t e

e p + c c

± a

b

c h l ±

p y

± ( s

e r )

a b + c

h l

s e r i c

i t e

s e r +

c h l + p y

C

5 0 0 m N

5 0 0 m N

F i g

. 1 4 a

F i g

. 1 4 b

F I G .

1 3 . H y d r o t h e r m a l a l t e r a t i o n

i n t h e s t u d y a r e a .

A . a n d

B . M a

p s o f

t h e I s h i k a w a a l t e r a t i o n

i n d e x ( A I ; I s h i k a w a e t a l . ,

1 9 7 6 ) f o r s u r f a c e a n d

d r i l l c o r e s a m p l e s ,

r e s p e c t i v e l y . G e o l o g i c b o u n d a r i e s a r e p r o v i d e d

f o r r e f e r e n c e .

N o t e

t h a t f o r s i m p l i c i t y , c o r e s a m p l e s a r e p r o j e c t e d v e r t i c a l l y

f r o m v a r i a b l e

d e p t h s a n d

l i k e l y d o n o

t p r o j e c t

t o t h e c o r r e c t g e o l o g i c u n

i t a t s u r f a c e ( r e c a l l t h e 7 2 ° S d i p o f s t r a t a ) .

T h e

1 0 m

A I g r i d s w e r e p r o d u c e d

b y i n t e r p o l a t i o n

b e t w e e n s a m p l e s w

i t h a s e a r c h r a d i u s o f 1 0

0 m a n d

a m

i n i m u m o f t w o s a m p

l e s p e r g r i d p o i n t .

C . a n d

D . A

l t e r a t i o n b o x p l o t s o f A

I v s . c h l o r i t e - c a r b o n a t e - p y r i t e

i n d e x ( C

C P I ; L a r g e e t a l . ,

2 0 0 1 ) f o r m a c a n d

f e l s i c s a m p l e s ,

r e s p e c t i v e l y . T h e g e o c h e m

i c a l a n a l y s e s u s e d a r e

f r o m t h i s s t u d y a n d a c o m p i l e d

d a t a s e t s u p p l i e d

b y C o g i t o r e

R e s o u r c e s

I n c . T h e b o x o f u n a l t e r e d

b a s a l t s f r o m

y o u n g

s u b m a r i n e r i d g e s

h a s b e e n c o m p i l e d

f r o m t h e G E O R O C d a t a b a s e ( h t t p : / / g e o r o c . m p c h - m a i n z . g w

d g . d e / g e o r o c / , 2 7 4 s a m p l e s w

i t h o u t l i e r s e x c l u d e d ) , w

h e r e a s t h e f o u r

o t h e r b o x e s f o r u n a l t e r e d

y o u n g a r c r o c k s a r e

t a k e n f r o m G i f k i n s e t a l . ( 2 0 0 5 ) .



80 ROGERS ET AL.

5 372 0005 373 0005 374 000 mN

6 1 7 0 0 0 m

E

6 1 6 0 0 0

6 1 5 0 0 0

6 1 4 0 0 0

6 1 3 0 0 0

6 1 2 0 0 0

6 1 1 0 0 0

H E B - 0 5

H E B - 0 3

H E B - 0 1

H E B - 0 2

H E B - 0 8

H E B - 0 4

H E B - 0 9

H E B - 0 6

Z n

i n p y r i t e

f r o m s u l f i d e - b e a r i n g s t r a t i f i e d

h o r i z o n s

0 -

0 . 0 1 %

0 . 0 1 -

0 . 1 %

0 . 1 -

1 %

1 %

- 6 . 1 %

6 1 7 0 0 0 m

E

6 1 6 0 0 0

6 1 5 0 0 0

6 1 4 0 0 0

6 1 3 0 0 0

6 1 2 0 0 0

6 1 1 0 0 0

5 372 0005 373 0005 374 000 mN

H E B - 0 5

H E B - 0 3

H E B - 0 1

H E B - 0 2

H E B - 0 8

H E B - 0 4

H E B - 0 9

H E B - 0 6

C u

i n p y r i t e

f r o m s u l f i d e - b e a r i n g s t r a t i f i e d

h o r i z o n s

0 -

0 . 0 1 %

0 . 0 1 -

0 . 1 %

0 . 1 -

1 %

1 %

- 6 . 1 %

H E B - 0 1

H E B - 0 2

S C - 1 3

S C - 1 4

H B 9 4 - 0 1

7 5 3 - 0 8

H - 4 - 7 1

7 7 7 3 8

7 7 7 4 5

7 7 7 3 9

5 373 0005 373 500 mE

6 1 4 0 0 0

6 1 4 0 0 0 m

E

6 1 4 5 0 0

S C - 1 5

H B 9 4 - 0 2

7 5 3 - 0 3

7 5 3 - 0 5

H B 9 7 - 0 2

H E B - 0 5

S C - 1 6

7 5 3 - 1 0

7 5 3 - 0 1

7 5 3 - 0 4

7 5 3 - 0 2

H B 9 7 - 0 3

7 5 3 - 0 7

H B 9 7 - 0 1

7 5 3 - 0 6

6 1 6 5 0 0 m

E

6 1 5 5 0 0

6 1 6 0 0 0

5 374 0005 374 500 mN

A

B

b

a

C

D

0 . 1 %

Z n c o n t o u r

0 . 5 %

Z n c o n t o u r

0 . 1 %

C u c o n t o u r

H i s t o r i c a l a s s a y d a t a ( f o r a a n d

b )

C h e m i n d e l a m

i n e

D i a m o n d

d r i l l h o l e

V o l c a n i c u n i

t

I n t r u s i v e u n i

t

L e g e n d

8 0

1 0 0

6 0

4 0

2 0

0

F r e s h

A l t e r e d

D i a g e n e s i s

I s h i k a w a

i n d e x ( f o r a a n d

b )

2 5 0 m

2 5 0 m

5 0 0 m

N

5 0 0 m

F I G . 1 4 .

B a s e m e t a l m

i n e r a l i z a t i o n a n d p y r i t e g e o c h e m

i s t r y f o r s a m

p l e s i n t h e s t u d y a r e a .

A . a n d

B . C

o p p e r a n d

z i n c c o n t o u r s r e p r e s e n t i n g m

i n e r a l i z e d z o n e s

A a n d

B , r e s p e c t i v e l y

( s e e t e x t f o r d e s c r i p t i o n s ) , s u p e r i m p o s e d o n a g r i d o f t

h e I s h i k a w a a l t e r a t i o n

i n d e x ( A I ; I s h i k a w a e t a

l . , 1 9 7 6 ) f o r d r i l l c o r e s a m p l e s , p r e p a r e d a s i n

F i g u r e

1 3 . T h e C u a n d

Z n c o n t o u r s a r e

b a s e d o n

5 m g r i d s o f d r i l l c o r e a s s a y s u s i n g

i n t e r p o l a t i o n b e t w e e n s a m p l e s w

i t h a s e a r c h r a d i u s o f 5 0 m a n d a m

i n i m u m o f t w o s a m p l e s

p e r g r i d p o i n t .

S a m p l e s w

e r e p r o j e c t e d v e r t i c a l l y

f r o m v a r i o u s

d e p t h s t o t h e s u r f a c e

f o r b o t h a l t e r a t i o n a n d a s s a y s , s o

t h e g e o l o g i c

b o u n d a r i e s a r e p r o v i d e d

f o r r e f e r e n c e

o n l y . C

. a n d D

. T h e m a t i c

m a p s

f o r C u a n d

Z n v a l u e s

i n p y r i t e

f r o m s u l

d e - b e a r i n g s t r a t i e d

i n t e r v a l s i n d r i l l c o r e , b a s e d o n

L A

- I C P

- M S m e a s u r e m e n t s .

T h e s a m p l e s a r e

p r o j e c t e d t o t h e s u r f a c e u s i n g

t h e r e g i o n a l d i p o f 7 2 ° S , s o

t h e y p l o t a t

t h e c o r r e c t s t r a t i g r a p h i c p o s i t i o n .




west. To the west, in DDH 77738, Zn occurs in pyrite-richsulde disseminations and thin, fracture-controlled string-ers. In DDH SC-14 to the east, the mineralization consists ofpyrite and sphalerite stringers associated with quartz.

Zone B, with higher Cu and Zn values than zone A, is locatedin the northeast corner of the study area (Fig. 14B). It consistsof up to 40 m long mineralized DDH intersections within azone of chlorite and sericite alteration (Cloutier, 1975; Bam-bic, 1998), with both disseminated and stringer sulde min-eralization (Cloutier, 1975). Disseminated pyrite, sphalerite,and chalcopyrite occur mostly in the westernmost part of zoneB, whereas pyrite-dominated stringers occur in the easternpart (Cloutier, 1975). The highest Cu grades (0.19 wt % Cuover 1.9 m) and the most altered rocks (AI = 80−95) occur inDDH 753-07, where mineralization occupies the matrix of alow Ti rhyolite breccia containing angular fragments with anaverage size of 3 to 4 cm (Cloutier, 1975). This breccia is partof the east ank of the low Ti rhyolite dome. Much lower Cuand Zn values occur higher up in the high Ti rhyolite. Thehighest Zn intersection in zone B is 11.6 wt % Zn over 1.1 min DDH 753-01 (Cloutier, 1975).Interpretation

Known alteration and mineralization in the study area is typ-ical of Cu-Zn VMS systems, but an economic deposit is yet tobe discovered. Mineralization occurs as sulde stringers anddisseminations within chlorite and/or sericite altered rocks,consistent with formation below the sea oor by replacementprocesses (Doyle and Allen, 2003), for example, in permeablebreccias along the anks of rhyolites domes. Large portionsof the study area remain underexplored. In the following sec-tion, we discuss the use of sulde-bearing stratied intervals,in particular the mineral chemistry of pyrite from these intervals, as an aid for VMS exploration.

Sulde-Bearing Stratied Intervals, andTheir Use in Targeting Hydrothermal Upow ZonesTwo sulde-bearing stratied intervals, historically termed

“exhalites,” were previously identied at the contact betweenthe main rhyolite and the youngest basaltic andesite in theHébécourt Formation, and at the contact between the Hébé-court upper rhyolite and the Reneault-Dufresnoy forma-tion (Fraser, 1991; Carignan and Lafrance, 2008). CogitoreResources Inc. intersected Zn values up to 1.2 wt % over corelengths of several decimeters within these intervals (Carignanand Lafrance, 2008). During the examination and samplingof these intervals it was noted that they did not always occur

at the contacts mentioned above (see Fig. 3B), and that thenumber of these intervals in a DDH could range from one tothree. In addition, a few sulde stratied intervals were foundin the Reneault-Dufresnoy formation, especially in the west-ern portion of the study area (Rogers, 2010).General description of sulde-bearing stratied intervals

Sulde-bearing stratied intervals include: (1) thinly lami-nated to thinly bedded intervals, 20 cm thick on average(locally up to 50−60 cm thick) consisting of alternating ne-grained tuff, chlorite, argillite, and very ne grained pyrite(Fig. 15A, B); and (2) generally thicker tuffaceous intervals,60 to 70 cm thick on average, which typically lack internal

stratication. In the thinly laminated to thinly bedded ptions, individual layers range from 1 to 2 mm to 3 to 4 cthickness. Laminae containing ne-grained (≤ 1 mm) pyriteare commonly the thinnest, <1 to 2 mm in thickness, anmany cases the pyrite is hosted by argillite. Chlorite vei were also observed. The thicker tuffaceous portions coof tuff and lapilli tuff. The larger clasts in these units are cally 2 to 3 cm in size and are mostly angular. Grading icommonly observed. Microscopic grains of chalcopyritesphalerite are minor to trace phases in the sulde-bearstratied intervals, but the dominant sulde mineral toccurs in all samples is pyrite.Pyrite occurrences and textures in the thinly laminated to thinly bedded portions

Pyrite is most common in the thinly laminated to thbedded portions. Pyrite is typically ne grained (<2 mm)is associated with specic laminations (Fig. 15C). In sexamples the abundance of pyrite is high enough to pduce apparent massive sulde laminations. Disseminatiopyrite dispersed evenly through several laminations are rLocally pyrite forms coarse blebs within a lamination or acontact between laminations.

Discordant pyrite veinlets were observed in most of samples (e.g., Fig. 15A). In these veinlets, pyrite is typiassociated with quartz, although in some cases the veinare entirely pyrite. The pyrite grains are typically coarser 15D) than those within laminations, and better formed. Vlets cut the laminations or connect with them, suggestingat least some of the concordant pyrite may have been induced after sediment deposition.

Small inclusions of other sulde and silicate minerals wthe pyrite are very common. However, euhedral coargrained pyrite (>2 mm) contain far fewer inclusions, probas a result of metamorphic recrystallization (Tomkins e2007). Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) analyses of pyrite

Samples: Thick sections (~100 µm thickness) were preparedfor 19 representative samples of the thinly laminated to thbedded intervals within the sulde-bearing stratied interEach sulde-bearing stratied interval is represented by or two thick sections. Sections were made in areas of adant pyrite, irrespective of the type of occurrence (e.g., py veinlets vs. pyrite-rich laminations, ne or coarse grains,

Methods: LA-ICP-MS spot analyses were performed

pyrite grains at Laurentian University in Sudbury, Ontausing an ultraviolet laser beam from a NewWave Rese213 nm probe coupled with a Thermo-Fisher XSeries 2 IMS. Spots to be analyzed were rst cleaned of possibleface impurities using a 2 s preablation with a 55 µm widebeam, and the actual analysis used a stationary 40 µm widebeam for 25 s. In ve of the thick sections, the pyrite gr were too small and a 25 µm wide beam had to be used (samablation time, following cleaning with a 2 s preablation ua 30 µm wide beam).

Between 10 and 20 spots were analyzed per thick sectMost pyrite grains are represented by one spot, althouin larger grains there may be two or more points to de



82 ROGERS ET AL.

possible metal zonation. Analyzed spots were chosen to obtaininformation on all pyrite types within the samples (concordant vs. discordant pyrite, with or without inclusions, etc.).

When selecting the integration interval for trace elementcalculations from the raw spectra, inclusions of other miner-als in the pyrite grains, visible by spikes of elements such asCu, Au, and Zn in the spectra, could not be avoided due totheir small size, their relative abundance, and their presenceat depth within the analyzed grains. Due to the metamorphicrecrystallization, inclusions in the pyrite contain the bulk ofthe trace metals of interest. As these metals do not reside inthe pyrite lattice they are interpreted to have been derivedfrom primary, trace metal-enriched pyrite during regionalmetamorphism as this is common for metamorphosed sul-des (Huston et al., 1995; Abraitis et al., 2004). Thus, thetrace elements values reported were obtained from mixturesof pyrite and tiny inclusions of other minerals within thepyrite grains. The external standards used were the MASS1synthetic polymetallic sulde standard from the U.S. Geo-logical Survey for most trace elements (Ag, As, Au, Bi, Cd,Co, Cr, Cu, Hg, Mn, Mo, Sb, Sn, V, Zn) and Standard Refer-ence Material 612, a glass from the U.S. National Instituteof Standards and Technology (NIST), for Ni, Pb, Sc, Ti, Tl,Th, and U.

Results: The determined trace element content of pyrite iextremely variable between samples and within samplesfound in similar studies (e.g., Peter et al., 2003a, b, using etron microprobe; Chenery et al., 1995; Chapman et al., 20Maslennikov et al., 2009, using LA-ICP-MS). The spatialtribution of Cu and Zn values are shown as thematic mapsFigure 14C, D.

The distribution of values for four economically signicelements within pyrite grains is portrayed on a graph of mecontent versus DDH lateral position (Fig. 16). Based on vartions in the median and 90th percentile curves for Cu, twopeaks are observed: a western one centered on HEB-04, aan eastern one in the region of HEB-02 to HEB-03, with intervening gap clearly illustrated by a sharp decrease in median and 90th percentile curves (Fig. 16A). A similar patern was observed for Zn, although the western peak is odistinguished on the median curve and the eastern Zn pecontains a sharp decrease at HEB-01 (Fig. 16B). Gold aAg values also display eastern and western peaks that coin with those for Cu and Zn, and a gap illustrated by a shadecrease in the median and 90th percentile curves for Au andAg relative to neighboring drill holes at HEB-08 (Fig. 16D). Similar patterns exist for As, Bi, Cd, Hg, Mn, Pb, Sb, Tl (not shown).

D 0.1 mmC 0.1 mm

A B

FIG. 15. Photographs (A, B) and reected light photomicrographs (C, D) of the sulde-bearing stratied intervals. A. Thinly laminated to vded interval containing tuff and argillite. Pyrite is present in veinlets and in blebs between and within layers. B. Thinly to thickly laminated tuffshowing synsedimentary deformation. Fine-grained concordant pyrite is associated with some laminations. C. Fine-grained concordant pyrit veinlet-associated pyrite. Pits in the surface resulting from laser ablation are circled in red.




InterpretationSulde-bearing stratied intervals consist of tuff, argillite,

chlorite, and suldes. The tuff and argillite component inthese intervals represent a hiatus in effusive volcanic activityand these hiatuses may indicate favorable periods for VMSformation. The presence of chlorite veinlets suggests thatthe chlorite laminations were formed by replacement of tufflaminations; in the same way, discordant sulde veinlets con-nected to sulde laminations indicate that at least a portion ofthe suldes were introduced postdeposition. Therefore, thesulde-bearing stratied intervals are not true exhalites, asthe suldes were not exclusively deposited by precipitationand sedimentation from a hydrothermal plume in the watercolumn. However, these intervals can still be used to identifypossible hydrothermal upow zones. High metal values in thesulde-bearing stratied intervals may indicate mineralizedzones at deeper (or even possibly higher) stratigraphic levels,as the metal-bearing uids which introduced the suldes were

likely moving in a mostly vertical direction as is generalcase for ow-dominated VMS systems (Franklin et al., 2

Lateral, spatial variations of precious and base metalpyrite from sulde-bearing stratied intervals suggest existence of two distinct broad hydrothermal upow zonthe upper part of the Hébécourt Formation: a western oin the HEB-09 to HEB-04 region, and an eastern one in HEB-02 to HEB-03 region. These ndings have implicatfor exploration as discussed in the next section. Hydrotheuids also likely traversed the lower levels of the ReneDufresnoy formation, as shown by high Cu and Zn valuthis formation within DDH HEB-09 (Fig. 14C, D).

Discussion and ConclusionsChemical stratigraphy and identication of volcanic vents

The Hébécourt Formation has been divided into four tleiitic volcanic units: basalt; basaltic andesite; main rhy(comprising both a low and a high Ti subunit); and the up

Medianth90

percentile

Detection limit (D.L.)

n number of samples above D.L.

A

B

C

D

p p m

Cu

1

10

100

1000

10000

100000

D.L.

MD 03 HEB: 04 08 02 0103& 05

0609 Ho l e #

n4 49 29 20 47 67 95 9

0.001

0.01

0.10

1

10 n4 30 17 3 42 37 50 2

D.L.

MD 03 HE B: 04 08 02 0103& 05

0609 Hole #

p p m

Au

0.001

0.01

0.10

1

10

100

1000

D.L.

p p m

n4 48 28 17 47 67 90 8

MD 03 HEB: 04 08 02 0103& 05

0609 Hole #

Ag

p p m

1

10

100

1000

10000

100000 n2 48 29 22 47 75 101 9

D.L.

MD 03 HEB: 04 08 02 0103& 05

0609 Hole #

Zn

FIG. 16. Geochemical variations in pyrite from sulde-bearing straintervals based on LA-ICM-MS analyses. Metal contents for individualses are plotted against the DDH in which the samples were taken, as a rsentation of lateral variations: A. Copper. B. Zinc. C. Gold. D. Silver.



84 ROGERS ET AL.

rhyolite. As shown in Figure 2, the bimodal Hébécourt For-mation consists of two tholeiitic mac-felsic cycles, the sec-ond being signicantly thinner than the rst. The low Ti mainrhyolite subunit was initially erupted from a vent located justeast of the Chemin de la Mine (Fig. 17A). For the high Tisubunit two distinct felsic volcanic vents were probably active,one on each side of the low Ti dome (Fig. 17B). The overly-ing basaltic andesite was extruded from the east and owed westward (Fig. 17C). Utilization of the eastern effusive centerby both felsic and mac magmas suggests a control by synvolcanic structure (e.g., Settereld et al., 1995; DeWolfe etal., 2009). The effusive center probably shifted slightly westward (toward HEB-01) to produce the main part of the upperrhyolite (Fig. 17D). Facies variations allow reconstructionof the volcanic vent systems for the various units, but this isonly a two-dimensional image of a three-dimensional system.Feeder dikes were not observed in this study, thus the exactpositions of the vents in the third dimension are unknown.Possible tectonic setting and rhyolite fertility

Regionally, the Hébécourt Formation consists largely ofmonotonous tholeiitic basaltic lavas that were interpretedas a lava plain by Dimroth et al. (1982). Trace element pat-terns determined in this study are compatible with a back-arcbasin tectonic setting for these basalts (and basaltic andes-ites). Felsic complexes within the Hébécourt Formation areuncommon regionally, and calc-alkaline intercalations havenot been reported outside of the study area. There is no obvi-ous explanation in our data to account for the greater volumeof rhyolite in the study area; perhaps it may be related tothe development of a local, high-level magma chamber, and/ or to local crustal extension generating a thermal anomaly(Franklin et al., 2005; Galley et al., 2007). Regardless of themechanism of formation (e.g., differentiation from a tholeiiticbasaltic magma or partial melting of a mac source), the tho-leiitic rhyolites must have been generated at relatively shal-low crustal depth based on their at REE patterns (Hart etal., 2004), which would have created enhanced heat ow andfavored hydrothermal activity. FIII rhyolites, like those of theHébécourt Formation, are commonly associated with VMSdeposits in the Archean (Lesher et al., 1986; Hart et al., 2004).

Following the emplacement of the Hébécourt Formation,magmas were repeatedly extruded from the same generalareas and this led to the construction of a large shield vol-cano now exposed farther south in the Reneault-Dufresnoyformation (Ross et al., 2011a). The tholeiitic to calc-alkalinemagmas that produced the Reneault-Dufresnoy formation

had a greater inuence from a subduction component relativeto those that created the Hébécourt Formation, or else they were more contaminated by existing crust.Mineralization and hydrothermal upow

In the study area, there is a spatial coincidence betweenthe identied effusive centers, hydrothermal alteration zones,base metal mineralization, and metal concentrations in pyritefrom the sulde-bearing stratied intervals (Fig. 17). Speci-cally, the proposed effusive center for the Hébécourt basalticandesite is located immediately south of the zone B stringer-type mineralization, an area of most intense sericite-chloritealteration within breccias of the main rhyolite that is located

lower in the stratigraphic succession. This suggests that basaltic andesite magmas used the same or nearby synvolcastructures as the hydrothermal uids which altered and meralized the main rhyolite. This implies the continued extence of synvolcanic extension, which is critically importaprovide access to the near-surface environment for minerizing uids. Synvolcanic faults are likely to have been acelsewhere in the area and to have controlled pathways fboth magmas and hydrothermal uids, although such fauare difcult to locate precisely. Other studies in VMS-host volcanic successions, including in the Noranda mining cahave showed the importance of long-lived and reactivasynvolcanic structures, which are responsible for the obsercoincidence between volcanic and hydrothermal vents (Gson et al., 1999, and references therein; Galley et al., 2007

Pyrites in sulde-bearing stratied intervals from multipstratigraphic levels in the study area contain appreciable AAu, Cu, and Zn, among other elements. The interest here not necessarily the high trace element values themselves (tis typical of VMS settings; Huston et al., 1995) but rather thspatial distribution. The high metal values dene a broad zoeast of the Chemin de la Mine (Figs. 14, 17) and therefocorrespond to an area also containing the effusive center the low Ti dome, the main volcanic vent for the upper rhylite, and one of the vents for the high Ti rhyolite, as well as weak Zn mineralization in zone A.

The high metal contents of pyrite from sulde-bearinstratied intervals located west of the Chemin de la Minemetal values comparable to those in east—are not explainby known base metal mineralization and hydrothermal altetion within the underlying strata. This may be the result the limited exploration in this area as compared to the arfarther to the east, especially in the main rhyolite, raththan to a lack of alteration and mineralization. The westemetal anomaly indicated by elevated pyrite metal contencould potentially be associated with a proposed high Ti rhlite western effusive center (centered on DDH HEB-04, seFigs. 8, 14, 15, 17), or with other undened volcanic veand associated hydrothermal upow zones. This proposithas obvious implications for exploration and demonstrathe usefulness of eld mapping of volcanic facies and checal stratigraphy in parallel with the trace element analysispyrite within stratied intervals to target new mineralizareas within volcanic terranes.Implications for VMS exploration elsewhere

The combination of techniques used in this study repr

sents a new approach outside VMS mining camps and cobe applied elsewhere. Previous published investigationssulde-bearing stratied intervals, iron formations, etc., wdone in established mining camps, with a known stratigrapand structure, and the VMS deposits had already been founIf exhalite studies are to be applied outside of VMS camhowever, it is of prime importance to combine them with understanding of the volcanic architecture of the investigaarea. This requires establishing a litho- and chemostratigphy to constrain volcanic facies mapping and facies analin order to locate coincident effusive and hydrothermal ceters (Gibson et al., 1999). For example, our interpretationthe facies variations in the main rhyolite would have be




Low-Ti rhyolite

Low-Ti rhyoliteHigh-Ti rhyolite

Hébécourt basalt

Zone A Zone B

Basalticandesite

Hébécourt basalt


Basalticandesite

Upperrhyolite

Hébécourt basalt

other rhyolite


HEB-04 HEB-08 HEB-02 HEB-01 HEB-03 HEB-06HEB-09

Hydrothermal up-flow Hydrothermal up-flow

2702.0 ±1.0 Ma

2703.0 ±0.9 Ma A

B

D

C

C h

e mi n

d e l a mi n

e

Hébécourt basalt

FIG. 17. Geologic history of the study area, focusing on the top part of the Hébécourt Formation, illustrated by pretiltingschematic sections (not to scale). Hydrothermal upow within volcanic units probably occurred more or less continuouslyduring the time period shown. A. Deposition of the low Ti subunit of the main rhyolite. Triangles represent the volcaniclasticfacies and randomly orientated dashes represent the massive facies. B. Eruption of the high Ti subunit of the main rhyolite,from two separate vents. Also shown are the zones A and B alteration and mineralization. Thick red lines represent the loca-tion of known sulde-bearing laminated intervals (some may be more continuous than shown). C. Eruption of the youngestinterval of the Hébécourt basaltic andesite. Filled triangles represent hyaloclastite and the pillows decrease in size to the west.D. Eruption of the upper rhyolite from the easternmost vent, as the western vent is unknown. Calc-alkaline intercalations inthe Hébécourt Formation and eruption of the Reneault-Dufresnoy formation not shown.



86 ROGERS ET AL.

very different without its division into two subunits. Further,combining these data with trace element variations in suldesfrom laminated intervals may help to identify hydrothermalupow zones, which can correspond to volcanic effusive cen-ters. In many ancient volcanic sequences, deformation andmetamorphism may strongly hinder recognition of primaryfeatures, and the use of combined approaches in dening a VMS-prospective environment becomes critical.Exhalites

The term “exhalite” is sometimes overused in mineral explo-ration, with laminated sulde-bearing intervals automaticallyassumed to represent laterally extensive markers formed bysulde precipitation and sedimentation in seawater, following venting of a hydrothermal uid into the water column (e.g.,Kalogeropoulos and Scott, 1989; Liaghat and MacLean, 1992;Peter et al., 2003a, b; Chapman et al., 2008). While true exha-lites undoubtedly exist in many VMS camps, explorationistshave to be careful about using the term exhalite too quickly.

This study has shown that careful examination of laminatedsulde-bearing intervals indicates that they do not alwaysform one or several laterally extensive units but can insteadform several discontinuous and unconnected units (Fig. 3B).In such units, the suldes can be truly exhalative and depos-ited with the enclosing sediment, or they can also be intro-duced later by replacement, from hydrothermal uids movingmostly upward in the crust (e.g., Knuckey and Watkins, 1982,show chalcopyrite veinlets crosscutting laminated sulde-bearing intervals at the Corbet mine). Nevertheless, even ifthey are not classic exhalites, the sulde minerals within theseintervals can still contain useful information on the location ofhydrothermal upow zones that can aid in VMS exploration.Conclusions

Detailed physical volcanology, mapping, and chemicalstratigraphy in the Hébécourt Formation and the overlyingReneault-Dufresnoy formation in the northern part of theArchean Blake River Group were instrumental in locatingeffusive centers (volcanic vent areas). In addition, LA-ICP-MS analyses of pyrite from sulde-bearing stratied intervals intercalated with the volcanic units, combined with themapping of hydrothermal alteration zones using lithogeo-chemistry, allowed us to locate hydrothermal upow zonesassociated in some cases with areas of known mineralization.This approach helped to dene areas with potential to host VMS mineralization.

AcknowledgmentsBenoit Lafrance and Tony Brisson, formerly at Cogitore

Resources Inc., shared their knowledge of the Hébécourtproperty and supplied unpublished geologic compilations anddatabases. We also thank Gérald Riverin of Cogitore ResourcesInc. for his interest in the project. This study was funded inpart by the Geological Survey of Canada’s Targeted Geosci-ence Initiative program, phase 3, Abitibi project, led by Ben-oît Dubé, whom we thank for support. We also acknowledgenancial and logistical support from the Ministère des Res-sources naturelles et de la Faune (Quebec); in particular wethank Sylvain Lacroix. INRS-ETE provided an internationalfee-exemption scholarship to the rst author. Balz Kamber

organized the use of the Laurentian University LA-ICP-Mlab, and Thomas Ulrich was the analyst. Jan Peter and JeBédard are thanked for discussions on the sulde-bearing lered intervals and Archean petrogenesis. P. Robidoux andGiroux served as eld assistants. V. Bécu, P. Thurston, gueditor H. Gibson and an anonymous journal reviewer pr vided useful comments which helped improve the paper.

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