Nickel-Zn-Pt-Pd-Re Hyper-Enriched Black Shale and Its Spatiotemporal Link With Clastic-Dominated Zn-Pb Mineralization at the Akie Property, Kechika Trough (British Columbia): Insights From Pyrite Chemistry and Geochronology

Abstract Nickel-Zn-Pt-Pd-Re–bearing hyper-enriched black shale (HEBS) occurs ≈3 to 30 m stratigraphically below the base of the Cardiac Creek Zn-Pb-Ag-Ba clastic-dominated (CD) deposit in the Late Devonian Gataga district, northeastern British Columbia. The HEBS horizon is ≈50 to 70 cm thick, stratiform and stratabound, and comprises disseminated sulfide mineralization within silty, calcareous shale. Bulk HEBS analyses have up to 0.9 wt % Ni, 2.7 wt % Zn, 110 ppb Pt, 126 ppb Pd, and >1 ppm Re. Sulfide mineral textures and crosscutting relationships suggest three stages of precipitation. Framboidal to microcrystalline pyrite (py1), very fine grained sphalerite (sp1), and very fine grained millerite (mlr1) are the earliest sulfides, all intergrown within the shale matrix and pyrobitumen (pyro1). Nodular pyrite (py2) forms anhedral masses on py1 or discrete nodules within the shale matrix. Subhedral to euhedral pyrite (py3), sphalerite (sp2), millerite (mlr2), gersdorffite, galena/clausthalite, chalcopyrite, and tennantite/tetrahedrite are the latest sulfides. Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) analyses of sulfides (especially pyrite) within HEBS reveal a complex distribution of Re, Mo, and Pt within early phases (py1, mlr1-py1, and sp1). Later py2 and py3 have lower abundances of these metals, and sp2, mlr2, and galena/clausthalite contain no detectable amounts. Whole-rock Re-Os geochronology of an unmineralized shale 2.8 m stratigraphically above HEBS gives an isochron age of 380.2 ± 3.6 Ma and initial Os ratio (Osi) of 0.25 ± 0.04 (2σ), which is identical to contemporaneous seawater, indicating an undisturbed depositional age. Two HEBS samples yield ages of 362.1 ± 25.6 and 358.5 ± 5.4 Ma, with highly radiogenic Osi of 1.54 ± 1.78 and 1.03 ± 0.13, respectively, that are considerably younger than the overlying shale. Rhenium, Mo, and Pt enrichment in py1 suggests syngenetic to early diagenetic sulfide precipitation, whereas the radiogenic Osi values are likely the result of later (≈360 Ma) fluid flow that reset the primary age (380–390 Ma) of the HEBS during the formation of synsedimentary to early diagenetic hydrothermal, CD Zn-Pb deposits in the area. This work demonstrates that there can be spatial and temporal juxtaposition and preservation of polymetallic HEBS and hydrothermal Zn-Pb-Ag-Ba CD mineral systems in a common depositional setting. It also highlights the value of pyrite petrographic and geochemical investigations in resolving complex geologic problems.