DEPOSITIONAL ENVIRONMENT OF MUDSTONE HOST ROCKS AT THE HOWARDS PASS ZN-PB DEPOSITS, YUKON TERRITORY, CANADA: INSIGHTS FROM IRON SPECIATION, SULFUR ISOTOPES, AND BULK FE/AL AND MO/TOC RATIOS

Sedimentary environments have been examined for mudstones of the Ordovician-Silurian Duo Lake Fm (DLF) and Ordovician Steel Fm (SF) in the Howards Pass sedex district using the proxies degree of pyritization of total Fe (DOPt), δ³⁴S of pyrite, bulk Fe/Al, and bulk Mo/total organic carbon (TOC). The analyzed samples, which are from drill core, represent mudstone layers that underlie, are interbedded with, or overlie massive sulfide layers of the XYC deposit. Some mudstone layers are high in Zn (up to 2%) and Pb (up to 0.3%), but most are not (median values are 75 and 44 ppm, respectively; n=41). The data were obtained to test whether paleoredox proxies developed for common marine sediments are reliable indicators of sedimentary environments proximal to sedex deposits, and to develop a finer resolution sulfur isotope record than reported in previous studies.

DOPt rises from 0.2 in the lower DLF, exceeds 0.7 through the massive sulfide interval (active member of DLF, or AM) and overlying phosphatic strata (upper siliceous mudstone member of DLF, or USM), and falls to 0.4 in the SF. Values of δ³⁴S increase from -15‰ in the lower DLF to 35‰ in the USM, then decrease to -2‰ in the SF. Fe/Al increases through the lower DLF then varies erratically between 0.14 and 1.5 in the AM and USM. Mo/TOC increases to 20 through the lower DFL, then decreases to 4 in the AM and to 1 in the USM before falling to <0.5 in the SF. The concentration of Mo reaches 110 ppm in the lower DLF, falls to 17 ppm in the AM and to 4 ppm in the USM, then falls to <2 ppm in the SF.

In the AM and USM, erratic changes in Fe/Al may indicate addition or removal of Fe associated with ore formation. Such redistribution of Fe would render DOPt an unreliable proxy of water column redox for these units. On the other hand, Mo/TOC rises and falls through the DLF as δ³⁴S rises, as expected of a basin that became restricted and reducing. The highest Mo/TOC ratios probably reflect euxinia, whereas the decline reflects depletion of Mo from stagnant waters prior to basin refreshment in the SF, which is bioturbated. Overall, our data corroborate earlier findings that the DLF accumulated under anoxic-euxinic conditions. They also reveal that Fe-Al-S relationships in the AM and USM are inconsistent with simple models involving contributions from sedimentary detritus, water column pyrite, and diagenetic pyrite.