The Origin of Hyper-Enriched Black Shales and Their Relationship to Hydrocarbon Generation

Abstract The Richardson Trough in northern Yukon hosts several occurrences of polymetallic hyper-enriched black shales (HEBS), comprising semimassive sulfide layers with metal concentrations several orders of magnitude above those of average black shales. Models seeking to explain the origin of such spectacular metal concentrations have focused on syndepositional, early diagenetic processes, proposing that the mineralization is entirely prelithification. These models do not provide satisfactory explanations for the mineral textures, paragenesis, and mineral chemistry and thus fail to capture the full story of HEBS formation. We present a new model for HEBS formation that explains mineral textures unaccounted for in previous genetic models. The sulfide fraction in HEBS is dominated by three types of pyrite: Ni-enriched framboidal pyrite (Py1a), euhedral pyrite (Py1b), and an As-enriched anhedral overgrowth (Py2). Two generations of millerite (NiS) have been identified. The first is blebby, disseminated millerite (Mlr1a) and interstitial millerite (Mlr1b), which replaced preexisting features in pyrite. The second millerite generation encases preexisting pyrite and locally replaced sphalerite (Mlr2a). It also forms laths in veinlets with cryptocrystalline quartz and bitumen and in fractures that crosscut bedding-parallel pyritic layers (Mlr2b). Some secondary millerite occurs in sulfide nodules (Mlr2c) containing sphalerite and gersdorffite. Much of the HEBS consists of biogenic quartz, detrital and diagenetic feldspar, and minor illite. The feldspars comprise K-, Ba-, and NH4-rich varieties. Detrital K-feldspar was altered to buddingtonite (Bud) during early diagenesis and to hyalophane (Hya-B) during late diagenesis. Authigenic hyalophane (Hya-A) precipitated concurrently with Hya-B, from pore-fluids in the HEBS matrix, or formed nodules (Hya-C) and veneers (Hya-D) on preexisting sulfides. We propose that the HEBS formed in three stages. Stage 1 involved extensive pyrite precipitation and significant accumulation of metal-rich organic material. Stage 2 coincided with the cessation of pyrite precipitation and the release of nickel and zinc from organic material to precipitate millerite and sphalerite. Stage 3 proceeded via reactions within the oil window that converted clay minerals to authigenic feldspar and released acid, partially dissolving sphalerite. Organic-hosted nickel reacted with sulfur released by sphalerite dissolution to precipitate the second generation of millerite. Our model provides the first explanation for the millerite-sphalerite textures, accounts for the multiple generations of millerite, and explains the various metal enrichments that characterize HEBS. It also demonstrates how diagenetic mineral reactions can strongly influence metal concentrations in black shale.