Organic geochemistry and petrology of sedimentary exhalative Pb-Zn and polymetallic hyper-enriched black shale deposits in the Selwyn Basin, Yukon

Abstract

Paleozoic strata of the Selwyn Basin host sedimentary exhalative (SEDEX) Pb-Zn deposits, and age-correlative strata of the Richardson trough host polymetallic hyper-enriched black shale (HEBS) deposits. In both deposit types, organic matter is spatially and temporally associated with mineralization. We investigated the characteristics of organic matter in mineralization and unmineralized host rocks in the XY Central SEDEX deposit in the Howard's Pass district, and the Nick and Peel River HEBS deposits in the Richardson trough using Rock-Eval pyrolysis, organic petrography, and solvent extraction and gas chromatography mass spectrometry (GCMS) analysis of the soluble organic matter (SOM). All samples experienced extremely high thermal maturity (Tmax up to 599°C), indicating they contain low SOM. Rock-Eval parameters S1, S2, HI, and OI values are low. Total organic carbon (TOC) values are low for Nick and Peel River and are generally higher for XY Central. Residual carbon values are universally high. Mineral carbon values are low for deposits studied (one outlier). Pyrobitumen reflectance is mostly below 5.80%. Full-scan GCMS analyses of SOM reveal that most, if not all, high molecular weight hydrocarbons, including biomarkers, have been lost due to thermal cracking and many detected peaks are likely due to contaminants introduced during sampling.