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Abstract

Clastic-dominated (CD-type) Zn-Pb ± Ag deposits account for significant global Zn and Pb resources. In this contribution, we describe a new Zn-Pb ± Ag deposit, Boundary Zone, recently discovered in the Macmillan Pass district, Yukon, Canada. Nine drill holes were sampled and studied using petrography, mineralogy (whole-rock and clay fraction X-ray diffractometry), and U-Pb geochronology on fluorapatite. These methods were used to develop a mineralogical paragenesis in order to constrain the timing of and controls on sulfide mineralization.

The mineralization at Boundary Zone is hosted by the Late Ordovician-Early Silurian Duo Lake and the Middle-Late Devonian Portrait Lake Formations. The discovery of mineralized Ordovician-Silurian rocks at Boundary Zone is the first time that significant stratabound mineralization has been identified in rocks of this age within the Macmillan Pass district. The premineralization stage is dominated by early diagenetic phases, including quartz, barite, pyrite, fluorapatite, and phyllosilicates. Two genetically distinct mineralization stages have been identified. Mineralization stage I comprises stratabound fine-grained sphalerite, pyrite, galena, sulfosalts, and barian mica. Mineralization stage I sulfides formed via barite replacement, nucleation on premineralization pyrite, and porosity exploitation during early biogenic silica transformation of opal-A to cryptocrystalline and microquartz in highly siliceous mudstones (up to 85 wt % quartz). The high-grade and volumetrically major mineralization stage II formed following significant hydrothermal fluid-induced brecciation and veining of the host rocks and is accompanied by silicification and siderite formation. The presence of kaolinite, pyrophyllite, quartz, and fluorapatite suggests that hydrothermal fluids were likely F rich with temperatures up to 240°C.

In one sample from the Niddery Lake Member, a discordant vein crosscuts mineralization stage I, but its paragenetic relationship with mineralization stage II could not be determined. Fluorapatite in this vein records a Middle Jurassic U-Pb age. We suggest the multiple mineralizing events at Boundary Zone formed during a prolonged period of fluid flow, spanning from diagenetic stages in the basin, possibly to periods of Cordilleran-related deformation in the Selwyn basin. These findings have significant implications for exploration strategies in the Macmillan Pass district and similar geologic settings, where biosiliceous mudstone deposition, diagenetic barite formation, and multiple hydrothermal fluid pulses are evident.