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 Fe3+-rich compositions of chromian spinel. The values of mg# [100Mg/(Mg + Fe2+)] 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 fe3+#, as a result of limited chromian spinel – liquid fractionation and subsolidus Fe2+–Mg exchange with ferromagnesian silicates; 2) a sharp decrease in Cr and cr# and increase in fe3+# [100Fe3+/(Al + Cr + Fe3+)] 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 Fe2+ during cooling; and 4) a strong decrease in Cr and cr# with a small increase in mg# and fe3+#, 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–Fe3+ 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.

You do not currently have access to this article.