Rhodonite from the Bridge River Assemblage, Downton Creek (NTS 092J/09), Southwestern British Columbia

by Z.D. Hora¹, A. Langrova² and E. Pivec³

KEYWORDS: in dus trial min er als, rhodo -nite, sed i men tary man ga nese min er als,cher t , rhodochros i te , ku tnohor i te ,cobalt i te , gersdorffi te, ul lmannite ,tucekite, re place ment min er al iza tion

INTRODUCTION

A new rhodo nite oc cur rence was dis -cov ered in the Downton Creek area, about20 km west of Lillooet (Fig 1). Pros pec torG. Polischuk of Lillooet made the dis cov -ery while car ry ing out work be tween 1996and 1999, un der a 1995–1996 Pros pec tor’s Grant. Float in Downton Creek was tracedto an out crop in a small un named north erntrib u tary, and sub se quent stak ing of twomin eral claims, South ern Gem 1 and 2,cov ered the rhodonite outcrops.

Sam ples of Downton Creek rhodo nitedif fer from those stud ied from sim i lar show ings in Brit ishCo lum bia (Nel son et al., 1990; Simandl and Church, 1996;Hora et al., 2005). Hy dro ther mal re place ment of theDownton Creek rhodo nite has com pletely de stroyed itsorig i nal banded structure.

Based upon petrographic tex tures, it ap pears thatrhodo nite grains first recrystallized into automorphicshapes and, dur ing later hy dro ther mal al ter ation, were re -placed by Mn car bon ates and fi nally quartz. Veinlets rich inNi-Co-Sb-As sulphides, to gether with a Bi-Te min eral, in -di cate a fluid con tri bu tion from a source other than the Mnprotolith.

ANALYTICAL PROCEDURES

An a lyt i cal pro ce dures were sim i lar to those re ported in Hora et al. (2005). The only dif fer ence is that, in or der tostudy the base metal min er als, a beam di am e ter of ap prox i -mately 2 µm, an ac cel er at ing po ten tial of 20 KeV and abeam cur rent 10 nA was used. Nine new stan dards were

used: marcasite, GaAs, Ni, Co, Mn3O4, Ag, Bi2SO3, pyriteand stibnite.

GEOLOGY

The Downton Creek rhodo nite oc cur rence is lo catedalong the Downton Creek fault in the cen tral CayooshRange. It oc curs within the East ern as sem blage of theBridge River Com plex (Journeay and North cote, 1992),which con sists pri mar ily of sheared greenstone, ser pen tin -ite, chert and fine-grained volcaniclastic rocks (Journeay,1993).

Rhodo nite oc curs in slabs of up to 30 cm2 in as so ci a -tion with cherty quartz ite and greenstone (G. Polischuk,pers comm, 2006). Quartz vein ing with ar seno py rite, py rite and chal co py rite is com mon along faults and shear zonesthat are ex posed in log ging-road cuts (Polischuk, 1999).

Be tween the Duffy Lake road and An der son Lake, thenorth west-trending Downton Creek fault marks the bound -ary be tween two fa cies of the Bridge River as sem blage, aco her ent greenstone-chert argillite in the west and shearedgreenstone-chert mélange in the east. The fault cuts theover turned limb of a re gional south west-verg ing synclineand as so ci ated thrust faults in the Downton Creek head wa -ter re gion (Journeay et al., 1992).

MINERALOGY

The dom i nant min eral in the sam ples stud ied is typ i -cally pink rhodo nite. Com pared to sim i lar sam ples fromAr thur Point, near Bella Coola, it is coarser grained (up to1 mm) with fre quent automorphic-shaped pinacoidal crys -tals (Fig 2, 3).

Geo log i cal Field work 2006, Pa per 2007-1 39

¹Brit ish Co lum bia Geo log i cal Sur vey (re tired)

²Geological Institute, Academy of Sciences of the CzechRepublic, Prague, Czech Republic

³Geological Institute, Academy of Sciences of the CzechRepublic, Prague, Czech Republic (retired)

This publication is also available, free of charge, as colourdigital files in Adobe Acrobat® PDF format from the BCMinistry of Energy, Mines and Petroleum Resources website athttp://www.em.gov.bc.ca/Mining/Geolsurv/Publications/catalog/cat_fldwk.htm

Fig ure 1. Lo ca tion of rhodo nite oc cur rence on Downton Creek, near Lilooet.

The crys tals grow into the cav i ties, which were laterfilled with vein quartz (Fig 4, 5). Quartz re places rhodo nitecrys tals so that, in some sam ples, only frag ments of theorig i nal columnal rhodo nite crys tals re main (Fig 6, 7, 8).The chem i cal com po si tion of rhodo nite is al most pureMnSiO3 (Ta ble 1), ex cept for Ca, which may con sti tutefrom 1 to 7 wt. % of the min eral for mula (as CaO). Taken to -gether, the re main ing el e ments form less than 1 wt%. El e -vated Zn (0.11%) is com mon in rhodo nite as so ci ated withchert (Crerar et al., 1982) and cor re lates pos i tively withFeO.

The sec ond most com mon min eral in Downton Creekrhodo nite sam ples is quartz, which is the lat est hy dro ther -mal min eral, re plac ing all other min eral phases. Quartzveinlets cut along cleav age and pres sure frac tures in rhodo -nite crys tals (Fig 4, 5) and dis play un du la tory ex tinc tion(Fig 9). Whether the quartz is a prod uct of the trans for ma -tion of rhodo nite to rhodochrosite, or a prod uct of someother hy dro ther mal event, has not been established.

A rel a tively com mon com po nent of the sam ple iskutnohorite Ca(Mn, Mg, Fe)(CO2)2 (see x-ray dif frac tionre cord, Fig 10) which is pres ent as grey ish pink ag gre gatesof xenomorphic grains ap prox i mately 0.05 mm in di am e ter(Fig 11, 12). Com pared to the kutnohorite from a clas si caloc cur rence (Frondel and Bauer, 1955; Trdlièka and Ševcù,1968), the Downton Creek sam ples have lower con tents ofMgO and FeO (Ta ble 1). Their un usual com po si tion may be ex plained by for ma tion via replacement of rhodonite.

Veinlets and ag gre gates of kutnohorite (Fig 11, 12)pen e trate into rhodo nite along cleav age and bound aries ofad ja cent crys tals. In some sam ples, kutnohorite is re placedby quartz (Fig 5). Rhodochrosite oc curs in veinlets and ag -gre gates up to 100 µm in size (Fig 13, 14), lo cally withmanganocalcite. Both are later than rhodo nite, but ear lierthan kutnohorite and quartz. Rel ics of manganocalcite andrhodochrosite (Fig 3) within quartz and along rims ofrhodo nite sug gest that rhodo nite was orig i nally re placed by both, fol lowed by quartz replacement.

40 Brit ish Co lum bia Geo log i cal Sur vey

Fig ure 2. Automorphic rhodo nite crys tals in quartz-filled vugs, withquartz par tially re plac ing rhodo nite; cross-po lar ized light.

Fig ure 3. Same as Fig ure 2, show ing rhodo nite (Rh) be ing re -placed by rhodochrosite (Rc) and suc ces sively by quartz (Q); white grains are ore min er als (sulphides of the nic co lite-cobaltite-skut -teru dite group); back-scat tered elec tron im age with lo ca tion ofmicroprobe tests.

Fig ure 4. Automorphic crys tals of rhodo nite (1 mm in size) in quartzfilled vug; quartz also pen e trates rhodo nite along frac tures andcleav age planes; cross-po lar ized light.

Fig ure 5. Same as Fig ure 4, with rhodo nite (Rh) be ing re placed bykutnohorite (Ku); ore min er als (white grains) are ir reg u larly dis -persed, mainly in rhodo nite; back-scat tered elec tron im age with lo -ca tion of microprobe tests.

Cobaltite (CoAsS), gersdorffite (NiAsS), ullmannite(NiSbS) and tucekite (Ni9Sb2S8) all oc cur as tiny grains (up

to 2 µm) along frac tures in rhodo nite and less com monly inquartz (Ta ble 2; Fig 2, 5). Some grains ex hibit zon ing(Fig 15).

A grey me tal lic min eral con tain ing 48.69% Bi, 36.58% Te, 2.91% Ni and 0.21% Se could not be iden ti fied with cer -tainty due to its tiny grain size (less than 2 µm). Its chem i calcom po si tion is sug ges tive of tetradymite affinity.

Geo log i cal Field work 2006, Pa per 2007-1 41

Fig ure 6. Skel e tal ar range ment of rhodo nite crys tals par tially re -placed by quartz; cross-po lar ized light.

Fig ure 7. Same as Fig ure 6, in plane-po lar ized light.

Fig ure 8. Same as Fig ures 6 and 7, with quartz (Q) re plac ing rhodo -nite (Rh) and kutnohorite (Ku); back-scat tered elec tron im age withlo ca tions of microprobe tests.

Fig ure 9. Rhodo nite re placed by quartz with un du la tory ex tinc tion;cross-po lar ized light.

Fig ure 10. X-ray dif frac tion re cord of calcian kutnohorite (blue,kutnohorite; red, quartz; green, alu mina holder).

TABLE 1. SELECTED ANALYSES OF MINERALS FROMDOWNTON CREEK RHODONITE OCCURRENCE.

SUMMARY

The rhodo nite sam ple from Downton Creek is sig nif i -cantly dis tinct from ma te rial de scribed ear lier from Ar thurPoint. Ex ten sive hy dro ther mal re place ment of theDownton Creek rhodo nite has de stroyed all orig i nal band -ing within the protolith. Rhodo nite was recrystallized intoidiomorphic grains, and was sub se quently re placed by Mncar bon ate and finally by quartz.

The oc cur rence of Ni-Co-Sb-As min er als to getherwith Bi-Te min er als is un usual in rhodo nite de pos its. TheBi-Te min er als are com mon ac ces sory min er als inmesothermal lode gold de pos its and are known from theBridge River camp (Cairnes, 1937). Free gold has also been re cov ered from the ad ja cent Ra ven claims (G. Polischuk,pers comm, 2006). Sim i lar as so ci a tions of min er als of Niand Co are known as coproducts of serpentinization, oc cur -ring as veins within rodingite rims of ser pen tin ite bod ies inboth Mo rocco (Leblanc and Billaud, 1982) and the Ital ianAlps (Castelli and Rosetti, 1994). The prox im ity of thisrhodo nite show ing to a ma jor fault and its as so ci a tion withgreenstone-ser pen tine mélange sug gest that the Ni-Comin er als were mo bi lized from the oce anic crust and mantlefacies of the Bridge River assemblage.

42 Brit ish Co lum bia Geo log i cal Sur vey

Fig ure 11. Fine-grained rhodo nite (darker) is re placed byrhodochrosite (lighter); bound ary be tween the two is formed byquartz; plane-po lar ized light.

Fig ure 12. Same view as in Fig ure 11, show ing the man ga nese car -bon ates rhodochrosite (Rc) and kutnohorite (Ku) re plac ing rhodo -nite (Rh), which re sults in the sil ica com po nent be ing freed from the rhodo nite as quartz (Q); back-scat tered elec tron im age with lo ca -tion of microprobe tests.

Fig ure 13. Lad der ar range ment of par tially re placed rhodo nitecrys tals in a quartz vein; plane po lar ized light.

Fig ure 14. Rhodo nite ‘lad der’ ar range ment in de tail: rhodo nite(Rh), rhodochrosite (Rc), kutnohorite (Ku) and Mn cal cite (Mc) arere placed by quartz (Q); back-scat tered elec tron im age with lo ca -tion of microprobe tests.

Fig ure 15. Ex am ple of zon ing in oremin eral, with cobaltite sur roundedby ullmannite.

ACKNOWLEDGMENTS

The au thors wish to thank Gary Polischuk for bring ingthis dis cov ery to their at ten tion and for pro vid ing sam plesfor this study. J. Pávková and Z. Korbelová pro vided tech -ni cal as sis tance; J. Brožek, V. Šrein and B. Šreinová helpedwith photo-doc u men ta tion; and J. Dobrovoln& helped withX-ray re cords. Con struc tive com ments and ed i to rial im -prove ments were pro vided by Brian Grant, MitchMihalynuk and Paul Schiarizza.

REFERENCES

Cairnes, C.E. (1937): Ge ol ogy and min eral de pos its of BridgeRiver min ing camp, Brit ish Co lum bia; Geo log i cal Sur vey of Can ada, Mem oir 213, 140 pages.

Castelli, D. and Rossetti, P. (1994): Sul phide-ar se nide min er al iza -tion in rodingite from the Balangero mine (Ital ian west ernAlps): hydrothermalism re lated to meta mor phic flu ids;Giornata di stu dio in ricordo del Prof. Stefano Zucchetti,Politecnico di Torino, pages 103–110.

Crerar. D.A., Namson, J., Chyi, M.S., Wil liams, L. and Feigenson,M.D. (1982): Manganiferous cherts of the Fran cis can As -sem blage: I. Gen eral ge ol ogy, an cient and mod ern an a -logues, and im pli ca tions for hy dro ther mal con vec tion atoce anic spread ing cen ters; Eco nomic Ge ol ogy, vol ume 72,pages 519–540.

Frondel C. and Bauer L.H. (1955): Kutnahorite: a man ga nese do -lo mite; Amer i can Min er al o gist, vol ume 40, pages 748–759.

Hora, Z.D., Langrova, A. and Pivec, E. (2005): Con tri bu tion to themin er al ogy of the Ar thur Point rhodo nite de posit, south -west ern Brit ish Co lum bia; in Geo log i cal Field work 2004,BC Min is try of En ergy, Mines and Pe tro leum Re sources, Pa -per 2005-1, pages 177–185.

Journeay, J.M. (1993): Tec tonic as sem blages of the east ern CoastBelt, south west ern Brit ish Co lum bia: im pli ca tions for thehis tory and mech a nisms of terrane ac cre tion; in Cur rent Re -search, Part A, Geo log i cal Sur vey of Can ada, Pa per 93-1A,pages 221–233.

Journeay, J.M. and North cote, B.R.(1992): Tec tonic as sem blagesof the east ern Coast Belt, south west Brit ish Co lum bia; inCur rent Re search, Part A, Geo log i cal Sur vey of Can ada, Pa -per 92-1A, pages 215–224.

Journeay, J.M., Sanders, C., Van-Konijnenburg, J.H. and Jaasma,M. (1992): Fault sys tems of the east ern Coast Belt, south -west Brit ish Co lum bia; in Cur rent Re search, Part A, Geo -log i cal Sur vey of Can ada, Pa per 92-1A, pages 225–235.

Leblanc, M. and Billaud, P. (1982): Co balt ar se nide orebodies re -lated to an Up per Pro tero zoic ophiolite, Bou Azzer (Mo -rocco); Eco nomic Ge ol ogy, vol ume 77, pages 162–175.

Nel son, J.A., Hora, Z.D. and Harvey-Kelly, F. (1990): A newrhodo nite oc cur rence in the Cassiar area, north ern Brit ishCo lum bia (104P/5); in Geo log i cal Field work 1989, BC Min -is try of En ergy, Mines and Pe tro leum Re sources, Pa per1990-1, pages 347–350.

Polischuk, G. (1999): Pros pect ing as sess ment re port on the Sammin eral claim, Lillooet Min ing Di vi sion, Can ada; BC Min -is try of En ergy, Mines and Pe tro leum Re sources, As sess -ment Re port 26987, 18 pages.

Simandl, G.J. and Church, B.N. (1996): Clearcut pyrox -mangite/rhodo nite oc cur rence, Green wood area, south ernBrit ish Co lum bia (82E/E2); in Geo log i cal Field work 1995,BC Min is try of En ergy, Mines and Pe tro leum Re sources, Pa -per 1996-1, pages 219–222.

Trd l i èka , Z . and Ševcù , J . (1968) : Chemisches undroentgenographisches Studium des Kutnohorits von KutnaHora; Acta Universitas Carolinae, Geologica, vol ume 3,pages 175–189.

Geo log i cal Field work 2006, Pa per 2007-1 43

Sample 1 1 2 1 2 2 1 1 1 1Text figure 3 5 11 3 13 11 5 8 3 (ni) 13Analyses 2 7 24 4 22 25 8 14 5 18

Oxide (wt%):

SiO2 46.97 47.40 46.73 0.08 0.00 0.01 0.00 0.01 0.13 0.03

TiO2 0.00 0.02 0.02 0.00 0.00 0.00 0.02 0.00 0.00 0.00

Al2O3 0.02 0.00 0.03 0.00 0.03 0.02 0.00 0.04 0.00 0.01

Cr2O3 0.00 0.03 0.00 0.06 0.00 0.00 0.03 0.04 0.12 0.00

FeO 0.60 0.36 0.22 0.06 0.00 0.00 0.00 0.12 0.00 0.10MgO 0.67 0.22 0.37 0.55 0.49 0.38 0.22 0.24 0.22 0.43MnO 43.28 43.75 45.48 52.28 50.28 48.60 44.60 46.78 24.26 38.22CaO 7.17 7.43 7.17 6.24 8.22 9.80 12.69 12.86 36.00 18.99ZnO 0.11 0.00 0.01 0.00 0.00 0.00 0.00 0.09 0.00 0.00BaO 0.00 0.00 0.07 0.00 0.00 0.00 0.02 0.00 0.00 0.00

Na2O 0.00 0.00 0.03 0.00 0.01 0.01 0.01 0.00 0.00 0.00

K2O 0.00 0.02 0.00 0.00 0.03 0.00 0.01 0.00 0.00 0.01

Total 98.82 99.23 100.13 59.27 59.06 58.82 57.60 60.18 60.73 57.79Abbreviation: ni, not included

Rhodonite Rhodochrosite Kutnohorite Mn calcite

TABLE 2. SELECTED ANALYSES OF DISSEMINATED ORE MINERALS FROM DOWNTONCREEK RHODONITE OCCURRENCE.

44 Brit ish Co lum bia Geo log i cal Sur vey