CHROMIAN SPINEL FROM PGE-BEARING PLACER DEPOSITS, BRITISH COLUMBIA, CANADA: MINERALOGICAL ASSOCIATIONS AND PROVENANCE

摘要: We report results of electron-microprobe analyses for detrital grains chromian spinel ( n ≈ 800) and associated minerals recovered from modern fluvial buried paleochannel (Pt–)Au placer deposits = 30) mostly Holocene age distributed throughout British Columbia. The majority analyzed spinel-group are members the chromite–magnesiochromite solid-solution series, extend to Fe 3+ -rich compositions spinel. values mg# [100Mg/(Mg + 2+ )] cr# [100Cr/(Cr Al)] in all 30 samples occupy a relatively narrow range, 58–82 68–92, respectively, consistent with mafic ultramafic source. Four types core-to-rim patterns zoning have been identified: 1) weak decrease Cr coupled strong at typically constant fe #, as result limited – liquid fractionation subsolidus –Mg exchange ferromagnesian silicates; 2) sharp increase # [100Fe /(Al mg#, reflecting magmatic fractionation; 3) significant late-stage interaction Cr-rich magma followed by variable enrichment during cooling; 4) small likely due more oxidized melt re-equilibration an aluminous phase (Al-rich residual or amphibole). Values silicate (olivine, clinopyroxene, either Na–Al-poor Na–Al–Ti-rich, rare orthopyroxene, edenitic amphibole, among others) micro-inclusions attain 90–95. evaluated provenance placers using global database new one focusing on Alaskan-type intrusions. Chromian Atlin area Dease Lake exhibits pronounced Cr–Al trend indicative ophiolitic affinity (Slide Mountain Cache Creek oceanic terranes). In certain Tulameen area, defines Cr–Fe indicating derivation intrusion Quesnellia island-arc terrane. Placer central Columbia (Manson Cariboo region) display mixed heritage involving both sources. recognition source some Cr–PGE-bearing underscores potential bedrock mineralization where such intrusions presently poorly represented.