THE GENESIS OF STRATIFORM LEAD-ZINC DEPOSITS, JASON PROPERTY, MACMILLAN PASS, YUKON (LATE DEVONIAN)

In modern geothermal systems, the factor limiting the formation of metal sulfides is the availability of reduced sulfur; in anoxic watermasses, the limiting factor is the availability of metal. The coincidence, therefore, of an exhalative metal-bearing geothermal system and a reduced watermass is very favorable for the formation of stratiform sulfide mineralization. The occurrence of stratiform Pb-Zn sulfide deposits throughout the Proterozoic and Lower Paleozoic and their absence following the oxygenation of the world ocean in the middle Paleozoic supports the contention that anoxic ocean waters were prerequisite for the formation and preservation of stratiform Zn-Pb deposits.;The presence of abundant resedimentation of stratiform mineralization, compositional and textural grading of stratiform laminae and beds, and lack of evidence of sequential deposition of minerals comprising the distal strata supports a sedimentary origin. The monominerallic tendency of laminae, the lateral zoning of lead:zinc within the stratiform bodies, and compositionally graded strata suggest effective segregation mechanisms during sedimentation. Formation of stratiform mineralization below a submarine brine pool is argued for by: (1) strontium isotopic data from stratiform carbonates and barite samples; (2) the textural similarity of stratiform mineralization with modern brine pool sediments; (3) stratigraphic evidence for the deposition of stratiform mineralization within a bathymetric depression and (4) the quartz-rich nature of the stratiform mineralization. (Abstract shortened with permission of author.).;Stratiform lead-zinc mineralization occurs within marine sediments of the Lower Earn Group of Middle to Late Devonian age that were deposited below a deep water, euxinic sea during rifting or wrench tectonism of the outer miogeocline of northwestern North America. Stratiform mineralization occurs adjacent to a synsedimentary fault and is interstratified with organic-rich siliceous shale, thin-bedded siltstone turbidite, and sedimentary breccias derived from the fault scarp. Mineralization includes veining and brecciation within and adjacent to the fault, thick bedded stratiform mineralization adjacent to the fault, and sheetlike bodies of finely laminated stratiform mineralization distal to the fault. The stratiform deposit is divided spatially into distinctive assemblages of laminae types. Near the Jason fault, stratiform mineralization is cut by abundant irregular veinlets and nodules, and replacement of carbonates by sulfide, and sulfide by sulfide is common.