Clark et al-1990

Economic Geology Vol. 85, 1990, pp. 1651-1668

THE SAN JUDAS TADEO W (-Mo, Au) DEPOSIT: PERMIAN LITHOPHILE MINERALIZATION IN SOUTHEASTERN PERU

ALAN H. CLARK, DANIEL J. KONTAK,* AND EDWARD FARRAR Department of Geological Sciences, Queen's University, Kingston, Ontario, Canada K7L 3N6

Introduction

Examination of the 1:2,500,000 metallogenic map of Peru (Bellido et al., 1972) reveals an apparent anomaly in the vicinity of the Altiplano-Cordillera Occidental boundary in the southeastern part of the country. Whereas wolframite occurs locally (Fig. 1) in epithermal veins in southern Peru (e.g., Julcani; Petersen et al., 1977) and as a facies of the Santa Rosa (Cerro Verde) porphyry copper center (Valencia, 1975) and minor scheelite in several small skarn deposits (e.g., Tentadora; Vidal, 1985), tungsten is a subordinate constituent of mineralization in the central Andean Main Arc magmatic-tectonic domain (Clark et al., 1984). In contrast, wolframite (ferberite) is the major ore mineral exploited at the small San Judas Tadeo mine (lat 15o34'54" S; long 7022'18" W); its lithophile metal-rich quartz vein system would be more characteristic of the Triassic-Lower Jurassic and Oligo-Miocene subprovinces (Clark et al., 1990) of the Inner Arc domain of the Cordillera Oriental (Fig. 1). The mine, also known as Milagro de San Judas Tadeo and formerly as Rosa Carela or Porvenir, has been sporadically active over the past 40 years; Kiils- gaard and Bellido (1959) record that monthly production amounted to about 2 metric tons of 60 to 68 percent WO3 concentrates in the period 1953 to 1955. The most recent operator, from 1983, was Compafila Minerales del Sur; the mine has been closed since 1985.

We summarize here our preliminary observations on this deposit which, although probably of modest dimensions, assumes importance in the overall metallogenic evolution of this transect of the central Andes.

Local Geology Following the pioneering studies of Newell (1949),

Laubacher (1978) prepared a reconnaissance geologic map of the Cabanillas district which encompasses the mineralized area (Fig. 2). The mine is at an altitude of 4,000 to 4,200 m a.s.1. on the slopes ofCerro Som- breruni, 6 km west-northwest of the village of Ca- banillas; its workings straddle the valley of the Rio Chaquemayo (or Achafiamayo) a short distance above the point at which it debouches onto the Altiplano.

Present address: Nova Scotia Department of Mines, Mineral Resources Division, 1496 Lower Water Street, Box 1087, Halifax, Nova Scotia, Canada B3J 2X1.

The immediate country rocks comprise Lower De- vonian sandstones and subordinate quartzites and shales, in part graphite bearing, constituting the upper exposed section of the Silurian-Devonian Caba- nillas Group (Newell, 1949; Boucot and M6gard, 1972), represented in this area by a 2.5-km-thick succession dipping homoclin ally 15 o_20 o to the east- northeast (Fig. 2). The lower Paleozoic rocks are overlain with angular discordance by Jurassic and Cretaceous strata at Taya Taya, 12 km southwest of the mine, a relationship ascribed by Laubacher (1978) to an "early-Hercynian" orogenic pulse of Late De- vonian-Early Mississippian age. Intrusive rocks

A 40-km 2 pluton of intermediate granitoid rocks 5 km west of the mine (Fig. 2) is named here after Cerro Antarane, a 4,850-m mountain summit in its northwestern quadrant. The Antarane intrusion cuts the Devonian sediments, but is disconformably overlain on its northwest margin by volcanic rocks assigned to the middle Miocene (France et al., 1985) Palca Group by Klinck et al. (1986). The dominant rock type in the stock is medium-grained hypidiomorphic-granular hornblende-biotite granodiorite (Fig. 3a), but the southeastern area of the intrusion has a monzogranitic mode. Boulders of an equigranular granodiorite with an aplitic texture occur in the valley alluvium. The main phase granodiorite is dominated by normally zoned plagioclase (Ans0-70), greenish-brown hornblende, and alkali feldspar (Or79Ab21), with lesser amounts of reddish-brown biotite and late-stage quartz. Whole-rock and mineral chemical data are presented in Table 1. The optical properties (2V = 70; Z2C -- 24 ) and composition of the amphibole conform to those of common hornblende, albeit with a low A1TM content. The alkali feldspar lacks perthitic lamellae and its intermediate 2V (50-60 ) indicates only moderate ordering (i.e., tlo q- tim = 0.75--0.80; Suet al., 1984), suggesting that it cooled rapidly and that, despite its considerable size, the pluton was emplaced at shallow depths. The biotite is rich in MgO [Mg/(Mg + Fe) = 0.48] and TiO (4.58 wt %). The occurrence of abundant magnetite and the metaluminous composition (A/CNK -- 0.98) of the granodiorite are in conformity with an I-type, magnetite series classification (Ishihara, 1977). The rock is unusually rich in potassium (KaO: NaaO = 1.18), and in this respect is closely comparable to the high K granodiorites of the Yarabamba and Tiabaya Groups

1651

1652 SCIENTIFIC COMMUNICATIONS

JULCANI

% -..., TENTADORA ''0 (ca. 75 Ma) . ,

' SAmTA .. (193 Ma) M A I N A R C

oArOqul SANTA ROSA

(57 Ma)

CHILE , , , , ,

FIG. 1. Sketch map of southern Peru showing the location of the San Judas Tadeo W-Mo deposit and other tungsten-bearing mineralization in the region. Dashed line indicates the approximate boundary of the Mesozoic-Cenozoic Inner Arc tectono-magmatic domain (see Clark et al., 1984). Age data from Petersen et al. (1977), Vidal (1985), Clark et al. (1990), and Farrar et al. (1990).

of the Upper Cretaceous-Paleogene Coastal batholith in the Arequipa area (Le Bel et al., 1985). Shoshonitic affinities are apparent.

The chondrite-normalized rare earth element spectrum of the granodiorite (Fig. 4) reveals significant LREE enrichment, no negative europium anomaly, and little fractionation of the HREE. This pattern is quite different from those of the Triassic granodiorites and monzogranites of the Cordillera de Cara- baya segment of the Inner Arc (Fig. 4), which reveal clear evidence of crustal anatexis (Kontak et al., 1985). It also differs from the REE patterns for Permo-Trias- sic alkali basalts from the Cordillera Oriental, which display greater enrichment in LREE and marked fractionation of the HREE (Kontak et al., 1985, unpub. data). The pattern is, however, very similar to those characteristic of the Neogene, medium K, calc-alkaline intermediate volcanics of the Main Arc, both in southeastern Peru (Fig. 4; Dostal et al., 1977a) and northern Chile (Dostal et al., 1977b).

Quartz grains in the granodiorite contain numerous secondary fluid inclusions. Most comprise aqueous vapor and liquid with variable phase ratios, but halite daughter crystals indicate that some moderate salinity inclusions occur.

Whereas Kiilsgaard and Bellido (1959) record that the sedimentary rocks are intruded by granite to the north of the mine, no outcrops ofphaneritic granitoid rocks have been observed at surface or underground in the immediate mine area. However, a 6-m-wide

north-northeast-striking dike of a mesocratic porphyritic rock (Fig. 3b) is exposed in the mine. Strong argillic alteration locally affects the dike, but the composition of a weakly altered sample (Table 1), and the relict plagioclase (An4.5_52), hornblende, and biotite phenocrysts suggest a trachyandesitic (Le Bas et al., 1986) or high K andesitic (Peccerillo and Taylor, 1976; Ewart, 1982) composition. Tourmaline (schorl) is unusually abundant (ca. 0.3 modal %), occurring as a euhedral, probably late magmatic, accessory mineral. Small ("2 cm) hornblende-rich melanocratic inclusions are abundant and are tentatively interpreted as either cognate (? cumulus) bodies or relics of a magma more mafic than the host.

A striking feature of the dike is a marginal zone (Fig. 5a), 5 to 30 cm in width, which displays a texture ranging from vesicular through flow banded, commonly with disharmonic folding of laminae, to micro- brecciated. Numerous small angular fragments of quartzite occur in this zone, several crosscut by pyrite- molybdenite-quartz veinlets,

SCIENTIFIC COMMUNICATIONS 1653

FIG. 3. Petrology of intrusive rocks, San Judas Tadeo area. a. Hornblende-biotite granodiorite, the major facies of the Antarane pluton. b. P!agioclase and hornblende porphyritic trachyandesite from a dike exposed in the mine. Finer grained inclusion has a composition similar to that of the matrix.

sion reveals a high K20/Na20 ratio, again comparable to the intrusive rocks of the Coastal batholith in the Arequipa area. However, the plagioclase and alkali

feldspar of the rock are strongly sericitized, and the analysis should be treated with caution. The meta- morphic rocks are tentatively interpreted as compo-

TABLE 1. Chemical Data for Antarane Granodiorite and San Judas Tadeo Monzogranite and Trachyandesite Granodiorite Monzogranite Trachyandesite

Whole rock Biotite Hornblende P!agioclase Orthoclase Whole rock Whole rock SiO2 63.45 36.34 50.44 56.95 66.55 69.10 58.18 TiOa 0.63 4.58 0.54 0.00 0.00 0.55 0.91 AlaOa 16.03 14.36 4.75 28.89 19.40 14.78 15.63 FeaOa 5.12 2.33 5.98 FeO 20.50 14.87 0.00 0.00 MnO 0.11 0.61 0.95 0.00 0.00 0.12 0.16 MgO 1.79 10.62 13.62 0.00 0.00 1.01 3.72 CaO 4.13 0.22 12.34 9.86 0.07 1.95 6.04 Na20 3.02 0.00 0.45 5.70 1.52 3.09 3.40 KaO 3.57 9.39 0.35 0.18 14.95 5.09 3.43 PaO5 0.20 0.09 0.46 HaO a 1.47 3.94 1.96 1.82 1.63 Total 99.52 100.56 101.28 101.58 102.42 99.93 99.54

Number of ions

24(o) 24(o) 32(o) a2(o) Si 5.526 6.605 9.816 11.73 AI TM 2.472 {0.78 {6.258 {4.298 AI TM 0.102 Ti 0.524 0.057 Fe +a Fe + 2.604 1.736 Mn 0.078 0.122 Mg 2.408 2.835 Ca 0.036 1.847 1.942 0.014 Na 0.000 0.041 2.031 0.554 K 1.822 0.023 0.042 3.585

Rock analyses obtained by X-ray fluorescence techniques, and mineral data by energy-dispersive electron microprobe analysis Total Fe expressed arbitrarily as FeO or FeaOs a Water contents computed


lOO .

-;..

f 10-

i i i i i i

I C Pr Nd $m Eu Gd Tb Dy Er Yb

FIG. 4. Chondrite-normalized (normalized to 0.8 times the Leedey values of Masuda et al., 1973) REE patterns for the An- tarane granodiorite. For comparison, selected Upper Triassic granitoid rocks from the Cordillera de Carabaya (Kontak, 1985, and unpub. data) and Neogene, medium K, calc-alkaline volcanic rocks from the Cordillera Occidental of southeastern Peru (Dostal et al., 1977a) are given.

nents of the sub-Paleozoic basement (see Injoque et al., 1983) and the granite as being derived from an immediately subjacent pluton. The textural relationships of the dike are suggestive of the forceful emplacement of a volatile, boron-rich intermediate melt. The metapelitic rocks in the immediate mine area have a hornfelsic texture and are rich in biotite, al- mandine, and andalusite, probably representing the thermal aureole of the postulated underlying pluton.

The Vein System The San Judas Tadeo deposit comprises a series of

steeply dipping (avg 80 , SW or NE) quartz veins with a strike of N 110-160 (avg 140 ) and less important swarms of north-northeast-trending steep veins (Fig. 6a) and subhorizontal quartz veinlets (Fig. 6b), the latter subparallel to bedding. All veins have the form of sinuous, discontinuous lenses and appear to represent joint fillings. At least eight northwest- trending veins have been recognized, but almost all wolframite production has been derived from the 15- to 50-cm-wide Gloria, or Porvenir, vein, which strikes at ca. 130 and dips to the southwest at 65-85 , steepening with depth. The vein contains sporadic ore-grade shoots, up to 15 m in length, over a vertical

interval of 225 m and a strike length of 550 m. Ore grades of ca. 1.5 percent WOa and up to I percent MoS2 are reported (Kiilsgaard and Bellido, 1959); only wolframite was recovered. Scheelite- and pyrite-

FIG. 5. Exotic inclusions in the trachyandesite dike. a. Apha- nitic marginal zone of dike (upper), charged with angular fragments of hornfels (black) and quartzite (white), the latter containing molybdenite-quartz veinlets (not visible). Diameter of coin, 2.5 cm. b. Inclusion of coarse-grained biotite-chlorite schist in central part of the dike. c. Inclusion of coarse-grained biotite- tourmaline monzogranite, postulated in the text to be a progenitor of mineralization.


FIG. 6. a. Wolframite-rich quartz vein, San Judas Tadeo deposit. Wolframite (dark-gray) concentrated along hanging wall. Irregular patch (medium gray) of calcite, ankerite, and minor scheelite in vein core at right.Pale gray areas in vein are clay rich. Vein 570B, looking south, 000F level. b. Open stockwork of 1- to 3-era high-grade wolframite-bearing, quartz veinlets. Gently dipping veinlets are approximately parallel to bedding of host hornfelsic Cabanillas Group metapelites (dark) and quartzites (pale). Width of view, 120 cm; looking northwest. P-6 topographic point, 000F level.

bearing veinlets of the minor north-northeast-trending system are observed on the OOOF level (Porvenir adit; 4,000 m a.s.l.) to crosscut the porphyritic dike.

The Gloria vein is characteristic in texture and mineralogy of wolframite-rich quartz veins worldwide. A minor early assemblage, comprising coarse- grained molybdenite, finely granular quartz, and muscovite, occurs discontinuously at the vein margins. Much of the vein is made up of massive milky quartz

with irregular patches of ferberite, with the approximate composition HblsFbas, molybdenite, arseno- pyrite, pyrite, siderite, and multicolored fluorite, all of which form subhedral to euhedral crystals pro- jecting into local central vugs. Some ferberite blades in this main-stage assemblage are partially surrounded by clear, glassy, in part euhedral quartz, interpreted as earlier than the milky quartz and having crystallized in equilibrium with the wolframite (see fluid inclusion discussion). Sulfide veinlets crosscut ferberite blades, but some depositional overlap is suggested by the occurrence of rounded sulfide blebs within wolframite. Minor pyrrhotite (in part replaced by marcasite and siderite), chalcopyrite, acanthite, and stannite occur erratically within the quartz. A subordinate late-stage assemblage of green fluorite, ankerite, calcite, and sericite or muscovite occurs locally. Ferberite and the sulfides show an antipathetic distribution overall (mine personnel, pers. commun., 1983). Scheelite replaces ferberite in carbonate-rich vein sections, but some is subhedral and appears primary. A phyllosil- icate mineral which forms irregular patches in vuggy vein quartz was identified by Klinck et al. (1986) as pyrophyllite. Electrum, not previously recorded in the veins, has been observed as isolated rounded grains intergrown with pyrrhotite and chalcopyrite.

Wall-rock alteration zones, 4 to 8 cm in width, are dominated by quartz, muscovite, fluorite, and coarse- grained Fe-rich chlorite (Kontak, 1985). Tourmalin- ization is locally intense. Concentrations of aiman- dine-rich garnet in hornfels adjacent to the Gloria vein probably represent alteration of calcareous zones. Zones of bleaching extending to 1 m from the veins may reflect either a weak sericitization or destruction of graphite. Alteration is predictably stronger and more extensive in metapelitic than metapsammitic host rocks.

Gold mineralization

Local tradition records small-scale working of gold from placers in the Rio Chaquemayo below the San Judas Tadeo mine. Gold-bearing placers and, more recently, veins have been developed since 1986 by local cooperatives (Comunidad Minera Tancuafia) in the valleys of the Quebradas Torini and Fiaranco (locally "Barranca"), the contiguous northern tributaries of the Rio Chaquemayo in the mine area. A north- northwest-striking swarm of ca. 5-cm quartz veins (Surapata showing) on the upper south slope of the Torini valley and a 1.5-m quartz-pyrite-chalcopyrite vein (Jacahuata showing) on the northern slope of Quebrada Fiaranco contain visible gold. Wolframite is reported from alluvial sediments in the Torini and Fiaranco Valleys and as a trace constituent of the Ja- cahuata vein. These relationships strongly suggest that the gold veins are cogenetic with the San Judas Tadeo deposit and imply the existence of a lateral zonation


from W-Mo, with traces of Au, in the south to Au (-W) in the north. A similar zonation from lithophile metal (W-Sn)- to Au-dominant assemblages is evident in the veins of the Ananea district in the Cordillera de Carabaya, in the Inner Arc domain of this Andean transect (Clark et al., 1990). Fluid inclusion data

Some 650 thermometric measurements have been carried out on fluid inclusions occurring in both milky and transparent quartz and in purple fluorite from four representative specimens taken from the Gloria vein. Although only four hand specimens were studied, microthermometric data were determined for 25 separate areas in nine separate chips. The studied mineral grains have well-defined paragenetic relationships and are considered to delimit satisfactorily the economically important main stage of wolframite and molybdenite deposition. Details of the studied samples are given in Table 2. Most runs were made at the Nova Scotia Department of Mines using a mod- ified U.S.G.S. gas-flow stage manufactured by FLUID Inc. and calibrated using synthetic standards. The temperatures of phase changes are considered accu- rate to _0.5C. Difficulties were encountered in the homogenization of vapor-rich inclusions. Salinity es- timates are derived from Potter et al. (1978) data for the system H20-NaC1 for aqueous inclusions, the melting point of halite in saturated NaC1-H20 inclusions (Sourirajan and Kennedy, 1962), and the melting of clathrate in COz-bearing inclusions (Collins, 1979). The methane component of the carbon-bearing phase

was estimated using appropriate phase diagrams (e.g., Burruss, 1981).

The dominant milky quartz is host to a very large number of fluid inclusions, predominantly concentrated along a multiplicity of crosscutting planar frac- tures interpreted as evidence for repeated breakage and healing. Many such inclusions are definitely secondary. In the absence of clearly defined growth zones in the host quartz, a pseudosecondary rather than primary origin is inferred for other inclusions occurring as isolated clusters away from annealed fracture planes and yielding higher homogenization temperatures than the above. Most inclusions fall in the range 3 to 20 #m in maximum diameter. A small population of unusually large inclusions, with lengths of up to >300 #m and aspect ratios of 5/1 to 100/1, lies along probable growth surfaces in quartz. Their high homogenization temperatures (see below) and occurrence are consistent with a primary origin; as noted above, the glassy host quartz, unaffected by later depositional or fracturing events, may have been contemporaneous with the wolframite. Progressive an- nealing of smaller inclusions is reflected in a spectrum of forms from irregular and "amoeboid" to smaller, equant, bodies of very consistent size and shape (Roedder, 1968). The inclusions are classified on compositional grounds in Table 2 and illustrated in Figures 7 and 8.

Thermometric data obtained for the inclusions are summarized in Table 3 and in Figures 9 and 10. Final homogenization temperatures exhibit maxima at ca. 375 and 200C (Fig. 9); sample SJT-1C displays a high-temperature "tail." The higher temperatures (to

TABLE 2. Fluid Inclusion Characteristics and Sample Descriptions, San Judas Tadeo Deposit

Fluid inclusion classification

Type host Composition Abundance Association

1 Cloudy quartz; transparent quartz; fluorite

2 Cloudy quartz

3 Cloudy quartz 4 Cloudy quartz 5 Cloudy quartz

Aqueous, L + V (V -- 20-40%); 99% of all Th variable inclusions

H20 - CO2 (CO = 20-80%) Minor CO - dominant Uncommon Aqueous; halite bearing Rare (40.1%) Aqueous, varied solid phase Rare (40.1%)

_+ halite

Many episodes

With type I and, less commonly, type 4

Local Associated with type 3

Sample description

SJT-1C Cloudy quartz intergrown with sericite and fluorite SJT-1D Cloudy quartz and green fluorite intergrown with sericite, and overprinting molybdenite and wolframite mineralization SJT-1E Cloudy white quartz intergrown with sericite, and postdating clear quartz-wolframite-molybdenite association; fluid

inclusions in the late quartz associated with repeated fracturing and healing episodes SJT-2 Sample comprises early clear, euhedral quartz intergrown with coarse (2-5 cm) wolframite blades and radiating clusters

of molybdenite, and later, finer grained clear to cloudy quartz

cf. Table 3


396.5 ___ 0.8C in SJT-2; n = 6) derive entirely from type 1 inclusions, especially the large rodlike bodies inferred to be primary and other inclusions in clear quartz. Among the lower temperature inclusions, sta- tistically significant differences in Th were measured for populations restricted to specific fracture domains. The data for fluorite overlap those for the lower temperature inclusions in quartz.

The Th determinations for CO2-bearing inclusions are sparse, but they define a maximum around 310C; many decrepitated at 200 to 250C. In several cases, Th occurred, while others decrepitated prior to homogenization to the vapor phase. We therefore infer that effervescence of a CO2-HO parental fluid locally or periodically occurred during vein formation.

The T values for type 4 inclusions were determined for only one specimen; they correspond (191.8 _ 11.6C; n = 6) to the lower temperature population. In all such inclusions, fusion of halite occurred prior to homogenization. No T data were determined for type 5 inclusions.

The CO phase (L, V) homogenized over a wide temperature range, from 5 to 30C; all but one inclusion homogenized to the liquid phase. These data and the phase proportions at 40C yield Xo2 esti- mates of ca. 0.10 for the fluid (Burruss, 1981). The measurements indicate a fluid density in the 0.57- to 0.88-g/cc range, assuming other dissolved species to be negligible. Melting temperatures for CO were -56.4 ___ 0.5C (n -- 16; sample SJT-1B) and -56.9 +_ 0.4C (n = 8; sample SJT-1C), approximately the melting point of pure CO (-56.6C); methane contents are therefore inferred to be below at 1 mole percent.

The salinity of the fluids ranges from


From the above data, we conclude that the Antar- ane pluton and the San Judas Tadeo vein deposit were emplaced in the Early Permian, at ca. 277 Ma, cor- responding to the Asselian or Sakmarian and cooled through the blocking temperature of biotite (ca. 200-250C) at ca. 273 Ma. Hydrothermal activity that led to vein mineralization significantly postdated intrusion at ca. 260 Ma.

The apparent ages of the initial steps in the spectra for the biotite (117.3 _+ 11.2 Ma) and the muscovite (152.6 _+ 15.7 Ma) and for the second step in the hornblende spectrum (147.3 _+ 11.9 Ma), combined with the shapes of the spectra in their lower temperature portions, strongly suggest (Harrison and Mc- Dougall, 1980) that the region was thermally over- printed during an event at ca. 120 to 130 Ma. The high age recorded for the first step of the hornblende spectrum suggests (Harrison and McDougall, 1980) that fluids containing excess argon permeated the rocks of the intrusion after the reheating event. Such excess argon would be more readily incorporated in the biotite and could possibly explain why the biotite has a higher integrated age than the hornblende. The origin of the thermal overprint recorded by all three minerals is uncertain, but the data strongly suggest an event in the Neocomian. No clear evidence of igneous activity of this general age has been encountered in our research in the region (see Clark et al., 1990) or by Klinck et al. (1986). Yoshikawa et al. (1976), however, recorded Jurassic and Cretaceous K-Ar dates for granitoid rocks in the Coroccohuayco- Tintaya area, some 175 km northwest of the Cabanil- las district, and the possibility of such Mesozoic intrusive or volcanic activity in the area cannot be ruled out (see also Clark et al., 1990). This is more probable

than that resetting was caused by an episode of major uplift and cooling, given the relatively shallow environment of mineralization (see below).

Conclusions

Our observations permit a preliminary assessment of the P-T-X conditions ofW-Mo (-Au) mineralization at San Judas Tadeo and have broader implications for metallogenic relationships in southeastern Peru and contiguous Bolivia.

Conditions of ore formation

In most major aspects, the San Judas Tadeo quartz vein system conforms closely to the salient characteristics of this worldwide tungsten _+ molybdenum or tin ore deposit type, as exemplified by Panasqueira, Portugal (Kelly and Rye, 1979), Sannae, South Korea (Shelton et al., 1986), and in the central Andes, Chojlla, Bolivia (Ahlfeld and Schneider-Scherbina, 1964). Such deposits were typically eraplaced in joint or fault systems surrounding and within granitic cu- polas, and wolframite was generally deposited at temperatures of _

1 6{50 SCIENTIFIC COMMUNICATIONS

:.Q ..

FIG. 8. Photomicrographs of fluid inclusions in vein material from San Judas Tadeo and in Antarane granodiorite. Plane-polarized light. a. Plane containing abundant two-phase fluid inclusions in fluorite. T for these inclusions is 175-180C. (Scale bar = 50 lam; sample SJT-10.) b. Large isolated two- phase fluid inclusion in fluorite. This inclusion is isolated from planar elements and may be primary. (Scale bar --- 25 m; sample SJT-1D.) c. Group of equant and negative crystal-shaped, two-phase fluid inclusions in quartz. (Scale bar = 50 pm; sample SJT-1C.) d. Group of two-phase fluid inclusions in quartz with similar L/V ratios. (Scale bar = 50 pm; sample SJT-1C.) e. Group of secondary fluid inclusions with variable L/V ratios in quartz in the Antarane granodiorite. (Scale bar = 25 tm.)


TABLE 3. Homogenization Data for Aqueous Inclusions, San Judas Tadeo Deposit

Host

Sample mineral Area Th ----- la n

SJT-1C Chip I Quartz Chip 2 Quartz

Chip 3 Quartz SJT-1D

Chip I Fluorite

SJT-1E Chip 1

Chip 2

Quartz

1 191.8 _ 11.6 6 1 210.5 _ 20.9 24 2 226.4 ___ 11.2 9 3 211.5 _ 15.4 17 i 192.8 _ 12.0 17

1 166.8 _ 7.9 12 2 210.9 9.1 14 3 169.0 _ 3.9 7 4 169.0 12.0 7 5 201.6 ___ 2.0 3

Quartz 1 219.6 18.2 20 2 222.6 _ 15.4 12 3 209.0 5.7 24 4 219.1 11.8 6 5 220.2 _ 15.3 10

Quartz I 185.1 _ 12.5 9 2 203.5 20.1 15 3 193.6 _ 9.0 8

SJT-2 Chip 1 Quartz 1 372.7 ___ 12.4 17

2 396.5 0.8 6

Chip 2 Fluorite i 189.1 ___ 9.8 45 2 215.6_ 2.7 21

Quartz 3 357.1 _ 0.6 15 4 356.3 4.3 6

Chip 3 Fluorite i 193.6 _ 15.2 33

though we obtained no Th or salinity data [br the secondary inclusions in the quartz of the granodiorite, these are also predominantly of moderate to low salinity. Fluids of this composition would be consistent with the experimental data of Kilinc and Burnham

3O

20

io

1 Areas defined on basis of textural environment and composition of inclusions +oo

+ I00 + 200 + I0 +30

] quartz

[fl fe .... CO 2 I1Clu$101$ - o Th(L) I SdT-iE I Th(Vl Decrepifate +300 +400 -20 -I0 0 -58 -56

r-1 ,

+200 +300 +400 -;0 -

Our fluid inclusion observations place constraints on the conditions of subsequent W-rich vein formation. Because the CO2-bearing inclusions are interpreted as having been trapped at or near the CO2- H20 solvus, their homogenization temperatures (avg 310C) require no correction for confining pressure. The absence of vapor-rich H20 inclusions coexisting with the higher temperature type 1 aqueous (L + V) bodies (avg salinity, ca. 6 equiv wt % NaC1 and Th of 375C) indicates that confining pressures exceeded approximately 20.5 MPa, or 205 bars (Sourirajan and Kennedy, 1962). Moreover, the common decrepita- tion of the CO,-bearing inclusions in quartz suggests that the pressure of entrapment, interpreted as being predominantly lithostatic, was approximately 100 MPa (1 kbar), according to the data of Naumov and Malinin (1968), Leroy (1979), and Swanenberg (1980). The pressure correction for the high-temperature aqueous inclusions is therefore estimated to be approximately 80 to 100C (Potter, 1977).

The highest temperature fluids observed (inferred trapping temperatures in the approximate range, 450-500C) are of low salinity (

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20 z

o o

0 s''' quartz f uor te +

..

+ +

.... i , ,+ . , i , . . i .... i . , . 200 5OO 400

HOMOGENIZATION TEMPERATURE

FIG. ]0. Plot of equiv wt percent NaCI (determined from T.i, and fusion temperature of halite) versus Ti for aqueous inclusions hosted by quartz and fluorite. Although the low-temperature, high-salinity data trend toward the NaCI saturation surface as projected into this plane, they do not fall on it.

(1972) for chloride partitioning between an inter- 100 mediate granitoid melt and aqueous fluids, and we tentatively infer a juvenile origin for these inclusions. 00 Broadly similar fluids have been observed to occur in the early-stage parageneses in several Bolivian wolframite-rich vein systems by Kelly and Turneaure 260 (1970) and Thorn (1988). However, the earliest fluids represented in both the Miocene Chicote (Thorn, 220 1988) and Lower Jurassic Chojlla (Harwood, 1985) deposits are, in part, considerably more saline (to ca. 47% NaC1 at Chicote) than those at San Judas Tadeo, and temperatures higher than those we record have been confirmed. We infer that the deposits exhibiting 40 high-salinity fluids were emplaced at shallower levels than the others; the fluids intersected the dew-point curve and boiled, leading to segregation of a more saline fraction. The data presented by the above au- 300 thors and recorded here suggest that the early fluids in the central Andean wolframite vein systems range in chemistry from simple NaCl-dominated to more 260 complex Na-, Ca-, and Mg-bearing ones, but were all of low salinity. =- 220

We conclude that the ferberite, and probably the molybdenite, in the deposit under discussion crystallized from a magmatogenic aqueous fluid in the tern- 80 perature range 300 to 500C. The subsequent evolution of the hydrothermal system was more complex. 40 It is evident from Figure 10 that none of the sampled fluids fall on or close to the NaC1 saturation surface, rendering it unlikely either that the high-salinity fluids 00 originated by boiling as in a "porphyry-type" system (cf. Reynolds and Beane, 1985; fig. 5) or are lower temperature derivatives of such fluids. The small hia- tus in the T data for vein quartz may be evidence for a pressure drop during mineralization, but it should be emphasized that (1) salinity values also show a bi-

modal distribution, such as would be expected from isobaric cooling and intersection of a solvus at lower temperatures; and (2) the veins do not display evidence of intense repeated brecciation. The irregular distribution of fluid compositions in T salinity space also suggests that no simple fluid mixing occurred (Shelton et al., 1988), although several fluids may

3OO

26O

22O

180

140

0A-1026 biotite

, , ntegrated age = .2 +/- 1.4 Ma I I I a COCA-1026 hornblende

I

grated age = 258.3 +/- 4.9 Ma

b

$JT- 1 muscovite

i I

Integrated age = 255.5 +/- 1.9 Ma

i i i i i i i i i

0 0.i 0.2 0.3 0.4 0.5 0.6 0,7 0,8 0.9

FRACTION 39Ar RELEASED

FIG. 11. 4Ar/39Ar age spectra for magmatic biotite (a) and hornblende (b) from granodiorite from the southeastern extremity of the Antarane pluton, and for hydrothermal muscovite (c) from the Gloria vein, San Judas Tadeo deposit.


TABLE 4. Conventional K-Ar Age Determinations, San Judas Tadeo Mine Area, Southeastern Peru

40Ar Mineral K 2 4Ar(rad) cm a (atm) Age and

Sample no. Location Rock analyzed (%) X 10 -5 STP/g (%) error a (Ma) COCA1026 Eastern margin, main Antarane Bt 7.061 8.157 11.0 275.2 ___ 5.8

granodiorite granodiorite pluton

SJT- 1 Gloria vein Mc 8.514 9.391 1.1 Wolframite-bearing quartz vein

263.6 q- 5.2

Bt = biotite; Mc -- muscovite 2 Potassium results represent the mean of duplicate analyses using AAS technique; precision estimated at 0.7% (2a) a Ages were calculated using the constants recommended by Steiger and JSger (1977); error represents the analytical precision at

2a; rad = radiogenic

have been involved in the intermediate stage of mineralization. The close association of highly saline aqueous and low-salinity COs-bearing inclusions is instead interpreted as evidence for unmixing of an H20-COs (-NaC1) parental fluid at ca. 280 to 320C. Although the COs may have been a component in an earlier undetected juvenile fluid, the erratic distribution of COs-bearing inclusions suggests that this constituent was introduced from the country rocks, perhaps as a result of the oxidation of graphite.

Considerable attention has been paid to the role of COs and CH4 in the deposition ofwolframite (Lan- dis and Rye, 1974; Noronha, 1974; Kelly and Rye, 1979; Higgins, 1980; Ramboz' et al., 1985) and scheelite (So et al., 1983; Seal et al., 1987). Models range from the transport of tungsten as carbonate- bicarbonate complexes (Higgins, 1980) to the desta- bilization of monomeric or polymeric aqueous W species through fluid neutralization (Seal et al., 1987). Ramboz et al. (1985) provide a detailed analysis of the controls on ferberite deposition in the small Ser- recourte showing, France, demonstrating that ore formation took place when Xco2 in the fluid decreased and CH 4 replaced COs as the dominant carbonic species in solution. It should, however, be emphasized that the COs and CH4 contents of fluid inclusions in some major tungsten vein systems are low or negligible, as is particularly the case in the Triassic and Oligo-Miocene subprovinces of Bolivia (e.g., Kelly and Turneaure, 1970; Harwood, 1985; Thorn, 1988). Moreover, our textural observations at San Judas Tadeo strongly suggest that wolframite crystallized largely prior to the introduction of significant amounts of COs into the hydrothermal system. We therefore see no reason to ascribe wolframite deposition to phase changes involving carbonic species.

Recent experimentally derived data for tungsten speciation in aqueous fluids comparable to those involved in ore formation concur in assigning a minimal role to chloride complexing. Wesolowski et al. (1984) propose that monomeric tungstate species are pre-

dominant in alkali chloride solutions at temperatures in excess of 250C at high ionic strengths and at low Z W. Wood and Vlassopoulos (1989) infer that cation (Na + or K+)-tungstate ion pairing may contribute to WO3 solubility. In either case, wolframite precipita- tion would be favored by fluid neutralization (pH in- crease) such as would attend hydrogen metasomatism (greisening) of the vein wall rocks, as is observed at San Judas. On the basis of limited experimentation and thermodynamic calculation, Polya (1987) con- cluded that ferberite solubility in aqueous brines is retrograde at 250 to 300C but increases with in- creasing pressure to 200 MPa, particularly above 300C; wolframite deposition would be favored by a decrease in pressure. This effect may have contributed to tungsten mineralization in the present instance, but is not considered to have played a major role. Local and regional tectonic relationships

Our geochronologic data demonstrate that the magmatic hydrothermal activity of the Cabanillas district constituted a late event in the evolution of the Andean basement complex. Thus, the anomalous lithophile metal concentration represented by the deposit reflects its temporal isolation from the superimposed Mesozoic-Quaternary Andean orogenic and metallogenic epoch (see Clark et al., 1990).

Granitoid intrusion and W-Mo mineralization at San Judas Tadeo were contemporaneous with the devel- opment of an ensialic rift zone immediately to the northeast (Fig. 12). Stratigraphic studies by Newell (1949), Laubacher (1978), and Klinck et al. (1986) have delimited a northwest-trending belt of red, dominantly continental, fine to coarse clastic strata now exposed mainly along the southwest margin of the Altiplano and on the lower and intermediate slopes of the Cordillera Oriental, and inferred to con- stitute the fill of a longitudinal basin, the Mitu trough. These Mitu Group strata unconformably overlie marine units of the Copacabana Group, deposition of which extended, at least in east-central Peru, into the


TABLE 5. 4Ar/9Ar Gas-Release Data for Minerals from the Cabanillas District

Temp Vol. 9Ar K Apparent Error (C) 4Ar/a9Ar a6Ar/a9Ar a7Arca/9ArK (10 -6 cm a NTP) % a9Arc % 4Ar* age (Ma) +2a (Ma)

(a) Biotite in Antarane granodiorite (COCA-1026) 500 18.46 2.887E-2 1.334E- 1 3.151E-3 1.221 53.8 117.3 11.2 600 21.82 7.598E-3 5.905E-2 1.332E-2 5.161 89.7 224.3 3.4 675 24.02 1.355E-3 9.204E-3 4.271E-2 16.55 98.3 267.5 1.4 750 23.68 7.518E-4 8.662E-3 4.271E-2 16.55 99.0 265.7 0.8 825 23.78 9.485E-4 2.075E-2 2.413E-2 9.351 98.8 266.2 0.7 880 23.37 1.072E-3 4.841E-2 1.731E-2 6.709 98.6 261.5 1.2 940 23.54 1.068E-3 5.269E~2 3.226E-2 12.51 98.6 263.3 1.5

1,000 23.73 7.650E-4 3.170E-2 6.282E-2 24.35 99.0 266.2 0.8 1,200 24.55 1.265E-3 5.297E-2 1.959E-2 7.593 98.4 273.3 2.1

J value -- 0.006769 Integrated age -- 262.2 _+ 1.4 Ma

(b) Hornblende in Antarane granodiorite (COCA-1026) 500 98.49 2.376E- 1 2.886 4.220E-4 0.559 28.9 318.8 1.1 650 18.63 2.065E-2 0.566 2.734E-3 3.881 67.4 147.3 11.9 750 19.65 1.089E-2 0.530 2.711E-3 3.850 83.7 190.7 6.4 840 21.86 1.227E-2 4.289 2.519E-3 3.577 84.8 213.9 6.4 880 22.45 1.276E-2 9.705 2.275E-3 3.230 86.4 223.8 15.0 905 23.92 9.387E-3 11.13 2.979E-3 4.229 91.8 251.8 7.7 930 24.48 8.542E-3 10.20 5.514E-3 7.830 92.7 259.5 9.0 960 24.92 6.555E-3 8.588 1.368E-2 19.42 94.7 268.9 3.3

1,000 24.08 2.875E-3 7.873 1.955E-2 27.76 98.8 270.8 3.4 1,050 24.58 5.615E-3 8.977 7.324E-3 10.40 95.9 268.6 3.5 1,200 25.80 8.410E-3 13.15 1.071E-2 15.21 94.1 276.7 1.3

J value = 0.006771 Integrated age = 258.3 _+ 4.9 Ma

(c) Muscovite in Gloria vein, San Judas Tadeo (SJT-1) 500 27.15 4.774E-2 2.828E-2 3.335E-3 1.042 48.0 152.6 15.7 650 21.10 5.610E-3 1.761E-2 2.366E-2 7.395 92.1 223.1 1.6 750 23.14 2.712E-3 1.011E-2 4.789E-2 14.97 96.5 254.1 1.1 800 23.76 2.829E-3 4.394E-3 4.955E-2 15.49 96.4 260.3 2.5 850 23.42 1.963E-3 1.768E-3 9.000E-2 28.13 97.5 259.4 1.0 900 23.77 2.909E-3 2.952E-3 6.974E-2 21.80 96.3 260.2 1.4 950 25.10 6.858E-3 1.381E-2 3.012E-2 9.414 91.9 261.9 1.6

1,000 43.56 6.890E-2 1.321E-1 3.719E-3 1.163 53.3 263.4 21.9 1,200 118.7 3.211E-1 5.168E- 1 1.916E-3 0.599 20.1 270.2 18.9

J value -- 0.006775 Integrated age = 255.5 _+ 1.9 Ma

early Artinskian (i.e., early Leonardian), as is shown by the occurrence of a faunule including primitive Parafusilina (Newell et al., 1953). The Copacabana Group-Mitu Group unconformity is interpreted by Laubacher (1978) to reflect the late Hercynian orogeny of inferred mid-Permian age.

In the Cordillera de Carabaya (Fig. 12), the intermediate and upper parts of the sequence display in- tercalations of alkali basalt (Kontak, 1985; Kontak et al., 1990). However, in the Juliaca district (Fig. 12), Klinck et al. (1986) distinguish the upper largely volcanic strata, including (trachy-) andesitic and tuff- aceous horizons, as the Iscay Group. To the southeast,

the depocenter presumably persists beneath Lake Ti- ticaca, but the Mitu (-Iscay) Group is only locally exposed as thin subaqueous basaltic flows in the Serranla de Chilla, near Tiahuanacu, on the Bolivian shores of the lake. The Early Permian age (Asselian-Sakmarian) of the volcanism is tentatively established by K-Ar ages of 280 Ma (Chilla; McBride et al., 1983; Kontak et al., 1985) and 272 Ma (Hacienda Chafiocahua, NW ofJuliaca: Klinck et al., 1985). It is therefore apparent that, even allowing for diachronous onlap, the mid- Early Permian interval saw the termination of marine sedimentation (Copacabana Group), significant uplift, rifting, thick molassic sedimentation, and initial vol-


_.u,sA.OOUE / ' -

' ,, -. :

?' - g,.- iG. l g. Regional setting of the San Judas Tadeo deposit in

relation to the inferred boundaries of the Early Permian Mitu ensialie rift and the Upper Triassic granitoid intrusions of the Cordillera Oriental in southeastern Peru and contiguous Bolivia. After Newell (1949), Laubaeher (1978), McBride et al. (1983), Kontak et al. (1985), and Klinek et al. (1986).

canic activity in this region. Emplacement of the Antarane pluton was coeval with this volcanic activity, at least along the southwestern margin of the rift and at its southeastern extremity, and the subsequent hydrothermal activity may have been associated with continued fault activity along the rift margin. It is evident that the late Hercynian orogeny in this area occurred in the Early, rather than mid-Permian (cf. Laubacher, 1978).

The northeastern margin of the rift in the Cordillera de Carabaya (Kontak et al., 1985) also formed a nu- cleus for plutonism associated with W(-Mo, Sn) mineralization (Clark et al., 1984, 1990), but this larger scale event is predominantly of Late Triassic-Early Jurassic (ca. 205-230 Ma; Lancelot et al., 1978; Kon- tak et al., 1990), rather than Early Permian age. Sim- ilarly, the granitoid batholiths which lie along the northeastern margin of the Mitu rift zone in the Cor- dillera Real of northwestern Bolivia (Fig. 12) yield Triassic ages (McBride et al., 1983). The Carbonif- erous-Early Permian (284-300 Ma) Rb-Sr dates reported for the Cordillera Real granitoids by Miller and Harris (1989) are difficult to assess in the absence of analytical data. However, there is some evidence that intrusion began in this area in the Late Permian; McBride et al. (1987) report a 254 ___ 10-Ma hornblende date for a granodiorite-tonalite unit exposed in the upper Rio Cooco Valley on the eastern slopes of Nevado Illampu (Fig. 12). The coexisting biotite

yielded a considerably younger age (162 Ma), but the sample area lies within the Zongo-San Gabln tectono- thermal zone (Farrar et al., 1988), along which K-Ar mica ages were radically reset at ca. 38 Ma. Moreover, the granodiorite is crosscut by a swarm of aplitic dikes and, within 300 m, by a muscovitic leucogranite stock. The hornblende age is therefore regarded as a mini- mum for intrusion of the Illampu granodiorite. Horn- blende-bearing mesocratic rocks of this type, although not well documented, apparently form a discontinuous marginal facies around the northern part of the Sorata (Illampu) batholith, the central and southern areas of which are dominated by peraluminous biotite granodiorite and biotite-muscovite monzogranite (Tistl, 1985). It is interesting to note that the only significant molybdenum mineralization in the Bolivian Cordillera Oriental occurs in the Millipaya area (Ahl- feld and Schneider-Scherbina, 1964), where molybdenite-wolframite-chalcopyrite veins and stockworks are associated with leucogranitic and pegmatitic dikes adjacent to the hornblende-biotite granodiorite of the western margin of the Sorata batholith (Fig. 12). Al- though the areal extent of such mesocratic I-type granitoids in this region is incompletely defined, we suggest that plutonic activity occurred locally along the northeastern as well as the southwestern margin of the Mitu rift in the Permian. Klinck et al. (1986) report a 236 _ 6-Ma K-Ar age for a hornblende-biotite tonalitc pluton exposed near Serial Huisaroque (Fig. 12), close to the axis of the Mitu rift, demonstrating that magmatism of this broad composition persisted in the rift well into the Triassic.

In the broader context of the central Andean tectonic evolution at the close of the Paleozoic, it is clear that the Permian magmatic-hydrothermal activity under discussion took place at a considerable distance from the western margin of a recently assembled Gondwana continental mass (Ramos, 1988). A major calc-alkaline volcano-plutonic arc was generated in the Carboniferous along a considerable length of the margin in Chile, presumably recording eastward-di- rected subduction (McBride et al., 1976; Coira et al., 1982). This arc apparently does not persist to the Chile-Peru border and cannot be shown to have been active oceanward of the immediate study area; moreover, no clear record of Carboniferous magmatism has been documented in this transect. By the Early Permian, felsic metaluminous and peraluminous suites interpreted to result from incipient rifting of the western margin of Gondwana (Kay et al., 1989) were being emplaced throughout northern Chile. In broad terms, the rift-related Lower Permian-Upper Triassic granitoid and volcanic rocks in southeastern Peru and northwestern Bolivia represent the northerly exten- sion of this province. The K-rich, in part shoshonitic, metaluminous granitoids of the Cabanillas district and the Cordillera Real have compositions entirely in ac-


cord with generation as an are assemblage resulting from subduction. In southeastern Peru, these rocks are coeval and contiguous with alkaline basalts more characteristic of ensialic rift environments (Kontak et al., 1985, 1990). We therefore conclude that, in southeastern Peru, eastward subduction beneath the Gondwana continental margin was initiated in the Early Permian, during or immediately following the late Hercynian orogeny and overlapping in time with the extensional tectonism which generated the Mitu trough. The subduction zone magmatism is unusual in its great distance (ca. 350-450 kin) from a plate boundary and in its restriction to a major ensialic rift.

Emplacement of the Permian W-Mo deposit at San Judas Tadeo, and probably the Mo(-W) veins of the Millipaya area of northwestern Bolivia, adumbrated the far more extensive Triassic Sn-W mineralization of the Cordillera Oriental (Clark et al., 1984). How- ever, these small deposits differ in their minimal tin content and an affiliation with magmatism of, ulti- mately, mantle-lower crustal rather than upper crustal derivation. The broad tectonic setting of the mineralization, a rift zone close to the foreland boundary of a cordilleran orogen, is reminiscent of that of the Oligocene Mo(-W) porphyry deposits of the Colorado mineral belt and contiguous New Mexico, an environment of incipient rifting (Sillitoe, 1980), but the associated igneous rocks are of I rather than A type.

Acknowledgments Field work in Peru was supported by Natural Sci-

ences and Engineering Research Council of Canada (NSERC) grants to A.H.C. and laboratory studies by NSERC grants to E.F. and A.H.C. David F. Strong, Memorial University of Newfoundland, is thanked for providing the REE data, and Peter L. Roeder, Queen's University, for assistance in microprobe analysis. We are particularly indebted to Ronald C. R. Robertson for his initial sampling at the mine.

The generous logistical assistance of Minsur, S. A., is gratefully acknowledged, as is the encouragement and advice of Mario Arenas, F. Access to the San Judas Tadeo mine was granted by Compaia Minerales del Sur, Lima. Trent Hutchinson, R. Morrison, J. Camp- bell, and Ela Rusak prepared the figures, and Sheila McPherson and Diane Parr typed the manuscript, which was improved through the comments of John Guilbert and an anonymous Economic Geology re- viewer.

This paper is a contribution to the Queen's Uni- versity Central Andean Metallogenetic Project (CAMP).

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