Platinum, Pd, Mo, Au and Re deportment in hyper-enriched black shale Ni-Zn-Mo-PGE mineralization, Peel River, Yukon, Canada

Abstract

Middle Devonian Ni-Mo-Zn-Pt-Pd-Au-Re hyper-enriched black shales (HEBS) from Peel River in north Yukon have high abundances of platinum group elements (PGE; ca. 500 ppb), Au (40–130 ppb) and Re (8–55 ppm). Mineralization occurs in up to three thin (0.5–10 cm thick) layers that include the regionally extensive HEBS at the Road River Group-Canol Formation stratigraphic contact (e.g., correlative with the Nick Ni-Mo-Zn-PGE-Au-Re prospect, northwestern Yukon); two other previously undocumented HEBS layers stratigraphic layers underlie the upper layer. Although the three different layers have minor textural differences, they display marked mineralogical and geochemical similarities to each other. Millerite is the predominant Ni sulfide, and it postdates several textural varieties of pyrite.A longstanding, unresolved question regarding this mineralization in Yukon is the mineralogical host of PGE, Mo, Au and Re. We analyzed the sulfide mineral assemblages using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). LA-ICP-MS analyses reveal that pyrite within all of the layers is the principal host of PGE, Re, Au and Mo, and that paragenetically late pyrite (spatially associated with millerite) contains the highest abundances, with up to 4 ppm Pd, 8 ppm Pt, 500 ppm Re, 1.5 ppm Au and 7000 ppm Mo. Rare, late pyrite-marcasite veins in the upper HEBS layer cross-cut all previous generations of sulfides and also postdate quartz cement. The late sulfide veins are devoid of Mo-Pd-Re-Pt-Au, indicating that mineralization coincided with early to late diagenesis and predates lithification. The high abundances of these elements in diagenetic varieties of pyrite suggests that hyper-enrichment occurred near the seafloor, and that this suite of elements was likely derived from ambient seawater.