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

Abstract

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