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, chert, rhodochrosite, kutnohorite, cobaltite, gersdorffite, ullmannite, 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, about 20 km west of Lillooet (Fig 1). Pros pec tor G. Polischuk of Lillooet made the dis cov - ery while car ry ing out work be tween 1996 and 1999, un der a 1995 1996 Pros pec tor s Grant. Float in Downton Creek was traced to an out crop in a small un named north ern trib u tary, and sub se quent stak ing of two min eral claims, South ern Gem 1 and 2, cov ered the rhodonite outcrops. Sam ples of Downton Creek rhodo nite dif fer from those stud ied from sim i lar show ings in Brit ish Co lum bia (Nel son et al., 1990; Simandl and Church, 1996; Hora et al., 2005). Hy dro ther mal re place ment of the Downton Creek rhodo nite has com pletely de stroyed its orig i nal banded structure. Based upon petrographic tex tures, it ap pears that rhodo nite grains first recrystallized into automorphic shapes 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 in Ni-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 Mn protolith. 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 to study 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 a beam cur rent 10 na was used. Nine new stan dards were ¹Brit ish Co lum bia Geo log i cal Sur vey (re tired) ²Geological Institute, Academy of Sciences of the Czech Republic, Prague, Czech Republic ³Geological Institute, Academy of Sciences of the Czech Republic, Prague, Czech Republic (retired) This publication is also available, free of charge, as colour digital files in Adobe Acrobat PDF format from the BC Ministry of Energy, Mines and Petroleum Resources website at http://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. used: marcasite, GaAs, Ni, Co, Mn 3 O 4, Ag, Bi 2 SO 3, pyrite and stibnite. GEOLOGY The Downton Creek rhodo nite oc cur rence is lo cated along the Downton Creek fault in the cen tral Cayoosh Range. It oc curs within the East ern as sem blage of the Bridge 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 cm 2 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 zones that are ex posed in log ging-road cuts (Polischuk, 1999). Be tween the Duffy Lake road and An der son Lake, the north west-trending Downton Creek fault marks the bound - ary be tween two fa cies of the Bridge River as sem blage, a co her ent greenstone-chert argillite in the west and sheared greenstone-chert mélange in the east. The fault cuts the over turned limb of a re gional south west-verg ing syncline and 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 from Ar thur Point, near Bella Coola, it is coarser grained (up to 1 mm) with fre quent automorphic-shaped pinacoidal crys - tals (Fig 2, 3). Geo log i cal Field work 2006, Pa per 2007-1 39
The crys tals grow into the cav i ties, which were later filled with vein quartz (Fig 4, 5). Quartz re places rhodo nite crys tals so that, in some sam ples, only frag ments of the orig 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 pure MnSiO 3 (Ta ble 1), ex cept for Ca, which may con sti tute from 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 with chert (Crerar et al., 1982) and cor re lates pos i tively with FeO. The sec ond most com mon min eral in Downton Creek rhodo nite sam ples is quartz, which is the lat est hy dro ther - mal min eral, re plac ing all other min eral phases. Quartz veinlets 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 some other hy dro ther mal event, has not been established. A rel a tively com mon com po nent of the sam ple is kutnohorite Ca(Mn, Mg, Fe)(CO 2 ) 2 (see x-ray dif frac tion re cord, Fig 10) which is pres ent as grey ish pink ag gre gates of 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 cal oc cur rence (Frondel and Bauer, 1955; Trdlièka and Ševcù, 1968), the Downton Creek sam ples have lower con tents of MgO 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 of ad ja cent crys tals. In some sam ples, kutnohorite is re placed by quartz (Fig 5). Rhodochrosite oc curs in veinlets and ag - gre gates up to 100 µm in size (Fig 13, 14), lo cally with manganocalcite. Both are later than rhodo nite, but ear lier than kutnohorite and quartz. Rel ics of manganocalcite and rhodochrosite (Fig 3) within quartz and along rims of rhodo nite sug gest that rhodo nite was orig i nally re placed by both, fol lowed by quartz replacement. Fig ure 2. Automorphic rhodo nite crys tals in quartz-filled vugs, with quartz par tially re plac ing rhodo nite; cross-po lar ized light. Fig ure 4. Automorphic crys tals of rhodo nite (1 mm in size) in quartz filled vug; quartz also pen e trates rhodo nite along frac tures and cleav age planes; 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 of microprobe tests. Fig ure 5. Same as Fig ure 4, with rhodo nite (Rh) be ing re placed by kutnohorite (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. 40 British Columbia Geological Survey
Cobaltite (CoAsS), gersdorffite (NiAsS), ullmannite (NiSbS) and tucekite (Ni 9 Sb 2 S 8 ) all oc cur as tiny grains (up to 2 µm) along frac tures in rhodo nite and less com monly in quartz (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 cal com po si tion is sug ges tive of tetradymite affinity. TABLE 1. SELECTED ANALYSES OF MINERALS FROM DOWNTON CREEK RHODONITE OCCURRENCE. 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 9. Rhodo nite re placed by quartz with un du la tory ex tinc tion; cross-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 with lo ca tions of microprobe tests. Fig ure 10. X-ray dif frac tion re cord of calcian kutnohorite (blue, kutnohorite; red, quartz; green, alu mina holder). Geo log i cal Field work 2006, Pa per 2007-1 41
Fig ure 11. Fine-grained rhodo nite (darker) is re placed by rhodochrosite (lighter); bound ary be tween the two is formed by quartz; 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 14. Rhodo nite lad der ar range ment in de tail: rhodo nite (Rh), rhodochrosite (Rc), kutnohorite (Ku) and Mn cal cite (Mc) are re placed by quartz (Q); back-scat tered elec tron im age with lo ca - tion of microprobe tests. 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 thur Point. Ex ten sive hy dro ther mal re place ment of the Downton Creek rhodo nite has de stroyed all orig i nal band - ing within the protolith. Rhodo nite was recrystallized into idiomorphic grains, and was sub se quently re placed by Mn car bon ate and finally by quartz. The oc cur rence of Ni-Co-Sb-As min er als to gether with Bi-Te min er als is un usual in rhodo nite de pos its. The Bi-Te min er als are com mon ac ces sory min er als in mesothermal lode gold de pos its and are known from the Bridge 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 Ni and Co are known as coproducts of serpentinization, oc cur - ring as veins within rodingite rims of ser pen tin ite bod ies in both Mo rocco (Leblanc and Billaud, 1982) and the Ital ian Alps (Castelli and Rosetti, 1994). The prox im ity of this rhodo nite show ing to a ma jor fault and its as so ci a tion with greenstone-ser pen tine mélange sug gest that the Ni-Co min er als were mo bi lized from the oce anic crust and mantle facies of the Bridge River assemblage. Fig ure 13. Lad der ar range ment of par tially re placed rhodo nite crys tals in a quartz vein; plane po lar ized light. Fig ure 15. Ex am ple of zon ing in ore min eral, with cobaltite sur rounded by ullmannite. 42 British Columbia Geological Survey
TABLE 2. SELECTED ANALYSES OF DISSEMINATED ORE MINERALS FROM DOWNTON CREEK RHODONITE OCCURRENCE. Rhodonite Rhodochrosite Kutnohorite Mn calcite Sample 1 1 2 1 2 2 1 1 1 1 Text figure 3 5 11 3 13 11 5 8 3 (ni) 13 Analyses 2 7 24 4 22 25 8 14 5 18 Oxide (wt%): SiO 2 46.97 47.40 46.73 0.08 0.00 0.01 0.00 0.01 0.13 0.03 TiO 2 0.00 0.02 0.02 0.00 0.00 0.00 0.02 0.00 0.00 0.00 Al 2 O 3 0.02 0.00 0.03 0.00 0.03 0.02 0.00 0.04 0.00 0.01 Cr 2 O 3 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 0.10 MgO 0.67 0.22 0.37 0.55 0.49 0.38 0.22 0.24 0.22 0.43 MnO 43.28 43.75 45.48 52.28 50.28 48.60 44.60 46.78 24.26 38.22 CaO 7.17 7.43 7.17 6.24 8.22 9.80 12.69 12.86 36.00 18.99 ZnO 0.11 0.00 0.01 0.00 0.00 0.00 0.00 0.09 0.00 0.00 BaO 0.00 0.00 0.07 0.00 0.00 0.00 0.02 0.00 0.00 0.00 Na 2 O 0.00 0.00 0.03 0.00 0.01 0.01 0.01 0.00 0.00 0.00 K 2 O 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 57.79 Abbreviation: ni, not included ACKNOWLEDGMENTS The au thors wish to thank Gary Polischuk for bring ing this dis cov ery to their at ten tion and for pro vid ing sam ples for this study. J. Pávková and Z. Korbelová pro vided tech - ni cal as sis tance; J. Brožek, V. Šrein and B. Šreinová helped with photo-doc u men ta tion; and J. Dobrovoln helped with X-ray re cords. Con struc tive com ments and ed i to rial im - prove ments were pro vided by Brian Grant, Mitch Mihalynuk and Paul Schiarizza. REFERENCES Cairnes, C.E. (1937): Ge ol ogy and min eral de pos its of Bridge River min ing camp, Brit ish Co lum bia; Geo logical Survey 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 ern Alps): 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 at oce 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 the min 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 Fieldwork 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 Coast Belt, south west ern Brit ish Co lum bia: im pli ca tions for the his tory and mech a nisms of terrane ac cre tion; in Cur rent Re - search, Part A, Geological Survey of Canada, Pa per 93-1A, pages 221 233. Journeay, J.M. and North cote, B.R.(1992): Tec tonic as sem blages of the east ern Coast Belt, south west Brit ish Co lum bia; in Cur rent Re search, Part A, Geological Survey of Canada, 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 - logical Survey of Canada, 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); Economic Geology, vol ume 77, pages 162 175. Nel son, J.A., Hora, Z.D. and Harvey-Kelly, F. (1990): A new rhodo nite oc cur rence in the Cassiar area, north ern Brit ish Columbia (104P/5); in Geological Fieldwork 1989, BC Min - is try of En ergy, Mines and Pe tro leum Re sources, Pa per 1990-1, pages 347 350. Polischuk, G. (1999): Pros pect ing as sess ment re port on the Sam min 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 ern British Columbia (82E/E2); in Geological Fieldwork 1995, BC Min is try of En ergy, Mines and Pe tro leum Re sources, Pa - per 1996-1, pages 219 222. Trdlièka, Z. and Ševcù, J. (1968): Chemisches und roentgenographisches Studium des Kutnohorits von Kutna Hora; Acta Universitas Carolinae, Geologica, vol ume 3, pages 175 189. Geo log i cal Field work 2006, Pa per 2007-1 43
44 British Columbia Geological Survey