Reinterpretation of the Ambient Paleoenvironmental Redox Conditions and Timing of Mineralization at SEDEX Zn-Pb Deposits in the Howard’s Pass District, Yukon
Gadd, Michael G.
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The Howard’s Pass district (HPD) of sedimentary exhalative (SEDEX) Zn-Pb deposits is located in the Selwyn basin, Yukon, and comprises 14 Zn-Pb deposits that contain an estimated 28 Mt of Zn+Pb metals. Semi-massive to massive sulfide mineralization is hosted in carbonaceous and calcareous to siliceous mudstones. Host rocks to these deposits lack visual and chemical evidence of hydrothermal alteration and mineralogical haloes and, accordingly, several aspects regarding mineral deposit genesis in the HPD are poorly constrained or unconstrained. Detailed petrographic and combined electron probe microanalysis (EPMA), laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) and secondary ion mass spectrometry (SIMS) studies were done on pyrite and apatite to better understand the mineralizing process(es), source(s) and pathways of sulfur and the ambient marine paleoenvironment at the time of mineralization. Pyrite is a minor, but ubiquitous component of the host rocks and mineralization. Individual crystals or crystal aggregates of pyrite are texturally complex and show multi-stage growth. Growth stages range from syngenetic to earliest diagenesis, to early to late diagenesis, through to metamorphism; LA-ICP-MS and SIMS analyses reveal that pyrite of different textures possesses different trace element and sulfur isotope compositional ranges. Moreover, these data support a predominantly diagenetic origin for base-metal sulfide mineralization, formed from thermochemically-reduced sulfate below, or at, the sediment-water interface. In places, the host rocks also contain abundant apatite that occurs as fine-grained layers stratigraphically above, within, and below the SEDEX deposits. EPMA and LA-ICP-MS reveal that apatite major, minor, trace and rare earth element-yttrium (REE-Y) compositions are remarkably similar throughout the stratigraphic succession. Several REE-Y proxies for depositional conditions and comparisons among apatites with diverse origins indicate that apatite from the HPD is hydrogenous rather than hydrothermal in origin. Previous workers concluded that sustained euxinic conditions were requisite in the formation and preservation of SEDEX deposits in the HPD, but the data presented in this thesis contradict this. These data include uniformly negative Ce anomalies in apatite (i.e., suboxic phosphogenic conditions) and extremely negative sulfur isotope values in framboidal pyrite (i.e., incremental depletion of seawater sulfate in the water column did not occur). Collectively, the current research presented here has redefined the ambient environmental and genetic models for the HPD.