Application of Pyrite Mineralogy and Chemistry to Determining Paleoredox, Hydrothermal History, and Target Vectoring in the MacMillan Pass Sedex District, Yukon, Canada
The MacMillan Pass district in Yukon, Canada, hosts the Tom and Jason sedimentary exhalative (SEDEX) Pb-Zn-Ag-(Ba) deposits. Pyrite-bearing drill core samples were collected from seven drill holes that intersected mineralization and time-stratigraphically equivalent rocks 2 – 3 km away. Detailed petrographic analyses reveal four pyrite textural varieties, each with distinct morphological characteristics: earliest diagenetic (Py1), early diagenetic (Py2a), late diagenetic (Py2b), and metamorphic (Py3). The timing for SEDEX Pb-Zn mineralization was syn- and/or post-late diagenesis (Py2b). A representative subset of pyrite grains was analysed for trace element contents and distributions by laser ablation-inductively coupled plasma mass spectrometry (LA-ICP-MS) using raster grids to create quantitative trace element composition maps. These were used to characterise the trace element composition of each pyrite generation and investigate how trace elements spatially vary. LA-ICP-MS analyses reveal each textural variety of pyrite has a distinct trace element composition that varies depending on the stratigraphic unit it is hosted in. A suite of SEDEX-mineralization-related elements (Zn, As, Pb, Tl, Bi) is incorporated into Py2 within mineralized units at greater abundances than in unmineralized units. Lead, Pb/Se and As/Mo contents in pyrite are the most robust potential vectors toward mineralization. Bulk geochemical paleo-redox proxies (Eu/Eu*, Ce/Ce*, Mo, Re/Mo, and Ni/Co) indicate that an anoxic – dysoxic water column and euxinic porewater conditions persisted during the deposition of the Lower Earn Group strata. A Ba-horizon was identified in rocks that are distal and time-equivalent to Pb-Zn mineralization. The Ba-horizon contains Py2 with anomalous metal (Tl, Co, Mn, Cd, Zn, Sb) contents and abundant barite, and is interpreted to represent the distal expression of hydrothermal activity. Two models are proposed to explain the formation of the Ba-horizon. In the first model, venting hydrothermal fluid debouched into the water column, ponded at the seafloor and infiltrated downward into the sediment. In the other model, hydrothermal fluid did not vent into the water column, it travelled up the fluid conduit and then migrated laterally within the unconsolidated sediment column to the distal locations. Both models invoke hydrothermal fluid metal contributions to Py2 within the Ba-horizon, and barite formation is facilitated by biological activity that is stimulated by warm, hydrothermal fluid.
URI for this recordhttp://hdl.handle.net/1974/26400
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