Geology, lithogeochemistry, age, and genesis of the Zn-Pb-Cu-Ag-(Au)-barite AG volcanogenic massive sulfide (VMS) deposit, Haines, Alaska

Quinn, Kei (2024) Geology, lithogeochemistry, age, and genesis of the Zn-Pb-Cu-Ag-(Au)-barite AG volcanogenic massive sulfide (VMS) deposit, Haines, Alaska. Masters thesis, Memorial University of Newfoundland.

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Abstract

The AG volcanogenic massive sulfide (VMS) deposit is hosted within the Late Triassic (Hyd and Tats Group) bimodal volcanic rocks of the Alexander Triassic metallogenic belt (ATMB). The AG deposit is composed of both exhalative- and replacement-style, barite-rich VMS mineralization with an inferred resource of 4.3 Mt grading 4.64% Zn, 0.12% Cu, 0.96% Pb, 119.5 g/t Ag, 0.53 g/t Au and 34.8% BaSO₄. The volcanic sequence that hosts the AG deposit includes: (1) enriched mid-ocean ridge basalt (EMORB)-like pillowed flows of tholeiitic basalts, (2) effusive flows of ferroandesites (FeA) and FIIIa ferrodacites (FeD) that are capped by FIIIa ferrorhyolitic lapilli tuffs (FeR), all of which have weak “arc-like” geochemical signatures and mixed tholeiitic/calc-alkaline affinities (3) EMORB-like pillowed flows of FeTi-rich, variolitic basalts (Z-FeTiB) that are intercalated with barite clast-bearing heterolithic fragmental rocks, (4) pyroclastic deposits of EMORB-like, FeTi-rich, hanging wall (HW) basalts (HW-FeTiB), and (5) syn-volcanic sills of FIIIb high-silica rhyolites (HSR). The AG deposit is a bimodal-mafic deposit with a flow-dominated stratigraphic footwall and a volcaniclastic-dominated hangingwall. Most of the AG deposit formed at the contact of the Fe-rich, intermediate to felsic rocks with weak “arc-like” geochemical signatures and the EMORB-like FeTi-rich basalts. Mineralization is intercalated with FeTi-rich, variolitic pillowed basalts and heterolithic fragmental rocks. The heterolithic fragmental rocks are considered debris flow deposits that were emplaced along an interpreted syn-volcanic fault, the Finch fault. The Finch fault also controlled the distribution of HSR sills, sharp lateral changes in hydrothermal alteration intensity, the thicknesses and facies of units, and VMS mineralization. Litho- and chemo-stratigraphic reconstruction of the volcanic environment suggests the rocks hosting the AG VMS deposit formed in a propagating intra-arc rift associated with basin development where high temperature (> 900 oC), shallow (< 10 km) magmatic processes included basaltic underplating, crystal fractionation, assimilation of arc crust, and periodic magmatic replenishment. The tectono-magmatic conditions that were essential for initiating and sustaining the hydrothermal convection required to form the AG VMS deposit are both physically and chemically reflected in the AG volcanic sequence. Zircons from the FeR and HSR yield new chemical abrasion isotope dilution thermal ionization mass spectrometry (CA-ID-TIMS) U-Pb crystallization dates of 210.35 ± 0.27, and 210.52 ± 0.08 Ma, respectively (Fig. 2.17; Table 2.4, Table 2.5) that constrain the timing (210.60 Ma to 210.08 Ma) of the AG mineralization and corroborate the Norian (ca. 227–208.5 Ma) conodonts in the Tats Group rocks that host the Windy Craggy VMS deposit and the timing of VMS mineralization at the Palmer deposit (213 ± 5 Ma; Hyd Group). The AG volcanic rocks have geochemical and geologic features that are like assemblages documented in Neoarchean VMS-hosting rocks in the Abitibi greenstone belt, suggesting that the tectono-magmatic processes responsible for forming the AG volcanic rocks in the Late Triassic may have been operative in the Neoarchean.

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