The world-class Howard’s Pass SEDEX Zn-Pb district, Selwyn Basin, Yukon. Part II: the roles of thermochemical and bacterial sulfate reduction in metal fixation

Abstract

The Howard's Pass district of sedimentary exhalative (SEDEX) Zn-Pb deposits is located in Yukon Territory and comprises 14 Zn-Pb deposits that contain an estimated 400.7 Mt of sulfide mineralization grading 4.5 % Zn and 1.5 % Pb. Mineralization is hosted in carbonaceous and calcareous and, to a lesser extent, siliceous mudstones. Pyrite is a minor but ubiquitous mineral in the host rocks stratigraphically above, within, and below mineralization. Petrographic analyses reveal that pyrite has a complex and protracted growth history, preserving multiple generations of pyrite within single grains. Sulfur isotope analysis of paragenetically complex pyrite by secondary ion mass spectrometry (SIMS) reveals that sulfur isotope compositions vary with textural zonation. Within the Zn-Pb deposits, framboidal pyrite is the earliest pyrite generation recognized, and this exclusively has negative δ 34 S values (mean = −16.6 ± 4.1 ‰; n = 55), whereas paragenetically later pyrite and galena possess positive δ 34 S values (mean = 29.1 ± 7.5 and 22.4 ± 3.0 ‰, n = 13 and 13, respectively). Previous studies found that sphalerite and galena mineral separates have exclusively positive δ 34S values (mean = 16.8 ± 3.3 and 12.7 ± 2.8 ‰, respectively; Goodfellow and Jonasson 1986). These distinct sulfur isotope values are interpreted to reflect varying contributions of bacterially reduced seawater sulfate (negative; framboidal pyrite) and thermochemically reduced seawater sulfate and/or hydrothermal sulfate (positive; galena, sphalerite, later forms of pyrite). Textural evidence indicates that framboidal pyrite predates galena and sphalerite deposition. Collectively, the in situ and bulk sulfur isotope data are much more complex than δ 34 S values permitted by prevailing genetic models that invoke only biogenically reduced sulfur and coeval deposition of galena, sphalerite, and framboidal pyrite within a euxinic water column, and we present several lines of evidence that argue against this model. Indeed, the new data indicate that much of the base metal sulfide mineralization was emplaced below the sedimentwater interface within sulfidic muds under reducing conditions during early diagenesis. Furthermore, thermochemical sulfate reduction provided most of the reduced sulfur within the Zn-Pb deposits.

Questioning the biogenicity of Neoproterozoic superheavy pyrite by SIMS

“…TSR has also been invoked as an important process in ore deposits (Rye and Ohmoto 1974;Powell and Macqueen 1984;Ghazban et al 1990;Tompkins et al 1994;Randell and Anderson 1996;Alonso et al 1999;Cooke et al 2000;Peevler et al 2003;Kelley et al 2004aKelley et al , 2004bBasuki et al 2008;Gadd et al 2017;Sooenicka and Lüders 2018). Hydrothermal sulfate-bearing fluids that percolated through preexisting evaporites could react with organic matter and form pyrite deposits.…”

The world-class Koushk Zn-Pb deposit, Central Iran: A genetic model for vent-proximal shale-hosted massive sulfide (SHMS) deposits – Based on paragenesis and stable isotope geochemistry

“…Nonetheless, the processes of sulfate reduction to sulfide in different orebodies and the subsequent deposition of sulfide minerals remain controversial (Gadd et al, 2017;Magnall et al, 2016b). Thermochemical and bacterial sulfate reduction (TSR and BSR, respectively), and anaerobic oxidation of methane coupled with sulfate reduction (AOM-SR, Magnall et al, 2016b) are the most relevant processes for supplying sulfur from the seawater sulfate and deposition of sulfide minerals in SHMS deposits (e.g., Kelley et al, 2004;Shanks III, 2001, 2014Magnall et al, 2016b;Gadd et al, 2017), discussed below in stratiform and stratabound sulfide ores.…”

He-Ar Isotopes and Trace Gas Compositions of Fluid Inclusions in Massive Sulphides from the Yushui Copper-Polymetallic Deposit, South China: Metallogenic Implications

“…In particular, one of the most important reactions controlling the sulphur behavior in deep sedimentary environments is the abiogenic reduction of sulphate to sulphide (occurring at temperature above 100-140 • C) coupled with the oxidation of hydrocarbons, which is termed thermochemical sulphatet reduction (TSR) [94][95][96]. Thus, the TSR process involving hydrocarbons may contribute significantly to the genesis of both MVT and SEDEX ore deposits [97][98][99]. Moreover, it has been suggested that siliceous limestones commonly experience insignificant decarbonation/decomposition of carbonates under contact metamorphic conditions; whereas for most sedimentary rocks enriched in organic matter, CH 4 rather than CO 2 will be released [100].…”