Catudian Minerologtst Vol. 13, pp. 8-14 (1975) TEXTURAL A,ND COMPOSITIONAL VARIATIONS IN A Ni-Co-As ASSEMBIAGE K. C. MISRA Departmento! Geophysics M. B. FLEET Departmentof Geology University of Western Ontario, London, Ontario AISTRACT semblage from the Cobalt area, hoping to clarify tle mode of formation and chemical A Ni-Co-As assemblage from Cobalt, history of Ontario, phases, consistlng of plumose rosettes of rammelsbergite its constituent The material studied was enclosing large prismatic crystals of pararammels- obtained from Ward's Natural Science Estab- bergite and rimmed by cobaltite and safflorite has lishmenf Rochester, New York, and was used been examined by electron microprobe analysis originally as the source of pararammelsbergite for Fe, Ni, Co, S, As and Sb. Approximate aver- for the crystal structure investigation of Fleet age metal compositions for these phases are: para- (1,972). A preliminary discussion of the texture raormelsbergite, axial blades, Ni.su Co.g3 Fe.or, in- of the assemblage and of the chemistry of tle terlocking crystals at the cores of rosettes, Ni.", pararammelsbergite was included in his paper. Co.67 Fe.s1; rammelsbergite, Ni.67 Co.ss Fe.6s; co- baltite, Ni.uo Co.ae Fe.16 to Ni.24 Co.76 Fe.sa; and safflorite, inner zone, Ni.26 Co.6s Fe.2o, outer zone, Onr MrNsRALocv Ni32 Co.3s Fe.rs. The rammelsbergite is relatively homogeneous but there are significant composition- The ore mineral assemblage in the specimen al variations within the cobaltite zone. Ni de- consis8 of pararammelsbergite,rammelsbergite, creases and Co increases outward, and Fe and cobaltite, safflorite and skutterudite in a cherty S distributions across the cobaltite zone have fairly rnatrix. Petruk (l97La) classified the arsenide pronounced V-shaped profiles, both tending to be assemblagesof the Cobalt-Gowganda area into relatively enriched toward the inner and outer fivo gradational classes: (1) Ni-As, (2) Ni-Co- margins. As, (3) Co-As, (4) Co-Fe-As, and (5) Fe-As. Al- Although pararammelsbergite appears to have though the assemblageunder study does not formed by inversion of rammelsbergite, tle Co re- fit stdctly leased in this process has migrated away from into any of the above classes, it most pararammelsbergite and must have been redis- nearly correqponds to the Ni-Co-As assernblage tributed within tle remaining rammelsbergite and, of Petruk, which is transitional between the Ni- probably, within the cobaltite. Also, the V-shaped As and Co-As assemblages: the NiAs assem- Fe and S profiles across the cobaltite zone, idio- blage is characterized by nickeline as tle main morphic cobaltite grains cutting across the grain Ni-arsenide and the presence of rammelsbergite, bouodaries of rammelsbergite and of safflorite, and the Co-As assemblagesby pararammelsber- and cobaltite islands enclosed in rammelsbergite gite as the main Ni-arsenide rather than nickel- aod safflorite are inconsistent with sequential, ine. The phases notably absent in present primary deposition from the ore-forming fluid. the Thus, whilst the gross chemical 2ening may be assemblage are nickeline, breithauptite, "Ag-Sb primary, other features in these rosettes seem to minerals", gersdorffite and sulphides. be the result of later chemical readjustments through solid state diffusion. Dendrites and zoned rosettes INrnopucrroN The most striking textural features of the specimen are the over-all dendritic, almost fern- Tho mineralogy of the Ni-Co-Fe arsenide and like, habit of the arsenide assemblage(Fig. 1), sulpharsenide assemblages associated with the and the consistent zonal arrangement of the ar- silver-bearing veins in the Cobalt-Gowganda re- senide minerals in individual rosettes. A typical gion of Ontario has been described by Petruk dendrite is about 5 cm long and 5 mm across, (1,97La)and Potruk et al. (1971).In thi present the main and subsidiary branches of which are work we have investigatedin detail the textural characterized by narrow (less than L mm across) and compositionalvariations in a Ni-Co-As as- axial zones of bladed parargmmelsbergite(and Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/13/1/8/3434975/8.pdf by guest on 01 October 2021 Frc. 1. A portion of the specimen showing a typical dendrite, with axial pararammelsbergite (black) and plumose rosettes, occurring in a cherty matrix. Frc. 2. Photomicrograph of a portion of a typical zoned rosette showing roughly concentric zones of para- rammelsbergite (white, interlocking prismatic crystals in the core, prm), rammelsbergite (greyish white, rm), cobaltite Oight grey, cob) and safflorite (greyish white, conical prismatic crystals at ttre rim, saf). The safflorite zone consists of two subzones WA and IVB); the sharp boundary between them is marked with minute inclusions of gangue. Frc. 3. Photomicrograph showing nature of tle boundary between rammelsbergite (liebt grey) and cobal- tite (medium grey) zones. The idiomorphic grain boundaries of relatively large cobaltite crystals cut acres many smaUer grains of recrystallized, polygonal rammelsbergite. Frc. 4. Photomicrograph showing a typical g;radational boundary between the cobaltite (medium grey) and safflorite (greyish white) zones. Frc. 5. Photomicrograph showing the junction between outward growing safflorite (light gey) zones of two rosettes. Many of the islands of euhedral cobaltite (medium grey) in the safflorite zones cut across the grain boundaries of prismatic safflorite. Fro. 6. Photomicrograph showing idiomorphic skutterudite (ight grey, sk) transecting zone boundaries in a rosette, and containing islands of pararammelsbsrgite (white, prm), rammelsbergite (greyish white, rm) and cobaltite (medium grey, cob). Note the discontinuity of tle safflorite zone (greyish white, conical prismatic crystals, saf) as a result of the growth of the skutterudite. Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/13/1/8/3434975/8.pdf by guest on 01 October 2021 l0 TIIE CANADIAN MINERALOGIST clert) and longi.tudinal and lateral envelopes of no consistent compositional difference between plumose rosettes (Fig. 1). the two types, .and the turbid appearanc€ ap- - The rosettes, which apparently are lateral sec- pears to be due to a contrast in polishirig char. tions of dendrite branches, range from less than acteristics of the two minerals, or to minu& in- I mm to about 3 mm in diameter. and are clusions of gangue phases in rammelsbergite, or chglagtgrized by arsenide minerals arranged in to a combination of both. welldefined, roughly concentric zones. The se- quence of phase zoning outward from the core Cobaltite rq a typical rosette (Frg. 2) is: pararammelsber- qtte (zone I), rammelsbergite (zone II), cobaltiG Cobaltite is easily identified in tle present as- (zone III), and safflorite (zone fV). ihis zonal semblage because of its high relief and its pink- pattern is consistent in all the rosettes associated ish tinge, under reflected light, against the white cf,ith dendrites. The same zonal pattern may be of the other arsenide minerals. The wavy na- observed in isolated rosettes emfeAaea in- the ture of the boundary between rammelsbergite cheffy matrix, but many of them lack either the and cobaltite zones (see Fig. 2) is due to idio- ;nrarammelsbergite zone or the -Thepararammels- morphic outlines of relatively large cobaltite bergite and rammelsbergite zones. rosettes crystals that terminate against several smaller have preferred orientation within a dendrite. so grains of rammelsbergite (Fig. 3). On careful [hat their appearance varies with the orientation examination one can discern suggestions of out- of a section. No case of oscillatory or reverse lines of relict rammelsbergite grains within the zoning was observed among the hundreds of ro- cobaltite areas, and, along cobaltite-rammels- settes studied in polished sections of different bergite grain boundaries, these relict outlines orientation. Aa outer cobaltite zone following appear to pass into the grain boundaries of the the safflorite zone, as reported by petruk (L97La, rammelsbergite. The boundary between the co- !ig. 57) was not found in the present assem- baltite and safflorite zones may be sharp locally, hlage. but typically it is gradational, with the contact zone marked by innumerable islands of cobal- Patarammelsbergite tite, often with idiomorphic outlines, in safflo- rite and a few tiny islands of safflorite in cobal- TWo textural varieties of pararammelsbergite tite (Fig. 4). The resulting texture resembles the can be recognized in the specimen: (1) axial pseudo-eutectictexture of Lindgren (1930). Pods blades, some of which are up to 2 cm long, that of polygonal rammelsbergite in the cobaltite rnark the .axial zones of the main and subiidiary zone and islands of cobaltite in the rammlesber- branc,hes of dendrites, and (2) interlocking ag- gite zone are common, and in all such casesco- gregates of prismatic crystals, with minor baltite shows its characteristic idiomorphic out- amounts of interstitial rammelsbergite, in cores line against rammelsbergite. Islands of cobaltite of rosettes (Fig. 2) which represenithe terminal are also common in the safflorite zone and many and lateral limbs of the axial blades. Three com- of them are euhedral grains cutting across the positiolatly distinct textural varieties of para- boundaries of safflorite prisms (Fig. 5). They rammelsbergite were recognized by Fleet (1972) also ocsur in the pararammelsbergite zone at in a crushed concentrate of the present assem- the cote of rosettes, but invariably enclosed