Mineralization and genesis of the Greens Creek volcanogenic massive sulfide (VMS) deposit, Alaska, USA

The Greens Creek Zn-Pb-Ag-Au volcanogenic massive sulfide (VMS) deposit is located on Admiralty Island, southeast Alaska, USA. The deposit has a global resource (combined resource, reserve and past-production) of 24.2 Mt at 13.9% Zn, 5.1% Pb, 658 g/t Ag, and 5.1 g/t Au, and is one of the most Au- and Agrich massive sulfide deposits in the world. The deposit is hosted by Late Triassic rocks along the eastern margin of the Alexander Terrane that formed as part of an oceanic back-arc or intra-arc rift-related sequence. These rocks are referred to as the Alexander Triassic metallogenic belt (ATMB) and host the 300 Mt Windy Craggy Cu-Co-Au deposit and the 10 Mt Palmer Cu-Zn VMS deposit. The ore occurs at the base of a thick section of graphitic argillite (SA) intercalated with dolomite (MA) beds (ca. 227 Ma) and is hosted by dolomitic rocks and rift-related sedimentary breccias. Stratigraphically equivalent but distal tholeiitic basalt and subordinate rhyolite (ca. 227 Ma) underlie the argillite. This Late Triassic sequence unconformably overlies a footwall package of Carboniferous (ca. 340–330 Ma) volcanic and volcaniclastic rocks intercalated with graphitic sedimentary rocks. These footwall rocks are variably altered to chlorite, sericite, quartz, and pyrite with proximity to massive sulfide. Serpentinized and carbonatealtered ultramafic rocks occur throughout the footwall. Late Triassic (219 ± 8 Ma) gabbroic dykes and sills cut footwall and hanging wall rocks. Basement rocks are quartz-muscovite schists with subordinate marble and felsic volcanic rocks. Six episodes of deformation affect rocks at Greens Creek: a pre-mineralization Permian (273–260 Ma) compression event (S1), three post-mineralization Cretaceous folding events (F2–F4) and a related shearing event (S2.5), and a mid- to late-Tertiary brittle faulting event. Eleven orebodies are distinguished, separated by faults, shear zones, or an attenuation of the ore thickness. Mineralization styles are divided into massive pyrite-rich (MFP), massive base metal-rich (MFB), barite-rich (WBA), carbonaterich (WCA), and siliceous (WSI) endmembers based on mineralogy. 3D modeling of the geology, mineralization styles, and metals identified a northern lens and a southern lens, with cores of high Fe, As/Sb, Tl, Cu, Cu ratio (100*(Cu/(Cu + Zn)), Zn ratio (100*(Zn/Zn+Pb)), and abundant MFP that define major vent centers. The lenses are laterally zoned from MFP outward to MFB, WBA, and WSI. WCA occurs throughout and grades into massive sulfide. The southern lens is enriched in Au, Ag, As, Sb, and Hg, high Zn and corresponding Zn ratios, with little to no Cu relative to the northern lens. The northern lens has large zones of high Cu and Cu ratios, indicating that it formed at higher temperatures. Fluid-assisted transfer products such as stylolites, pressure-solution cleavage, grain overgrowths, and dilational veins are common in brittle pyrite-rich and carbonate-rich rocks (MFP, WCA, MA). Solid-state transfer products, such as cm-scale folding, compositional banding, layer-parallel veins, durchbewegung texture, and boudinage, are common in ductile barite- and base metal-rich rocks (WBA, MFB). There is no evidence for significant remobilization of metals or mineralization styles by deformation at the deposit-, orebody-, or heading-scale. Metals are remobilized at the centimeter to meter scale during F2–F4 deformation. Mid- to late-Tertiary brittle faulting affects all rocks, forming cataclastic breccias, grain size reduction, and late brittle microfractures through all minerals. Greens Creek ore has a complex, fine-grained mineralogy and an Fe, Zn, Pb, Cu, Au, Ag, As, Sb, Tl, Hg, Ba, Cd, and Sr (± Mn, Mg) geochemical signature. Graphite is ubiquitous in all mineralization styles. Pyrite is the most refractory of the ubiquitous ore-related minerals and is used as a framework for paragenesis. Six pyrite types are defined (Py1–Py6). Stage 1 mineralization formed Py1a, Py1b, Py2, sphalerite, tetrahedrite, galena, graphite, barite, and various silicate and carbonate minerals during sedimentation and diagenesis. Stage 2 mineralization formed Py3, galena, sphalerite, tetrahedrite, chalcopyrite, arsenopyrite, and gold. Colusite and proustite are common in and around barite-rich ore. Py4 and Py6 formed during mid-Cretaceous metamorphism and deformation as a result of pressure-solution and reprecipitation of fine-grained hydrothermal pyrite. Py5 formed by in situ dynamic recrystallization of earlier pyrite. LA-ICPMS analyses show that Py1a, Py1b, Py2, and Py3a are enriched in Mn, Tl, As, Sb, Au, and Ag compared to Py4–Py6. Py4–Py6 contain higher Ni and Co but lack other trace elements. Pyrite chemistry and low Fe content of sphalerite indicate low temperature hydrothermal deposition (150–250°C). A Cu-Ag-S mineral assemblage of stromeyerite, covellite, chalcocite, and electrum (± bornite and enargite) is common in and around barite-rich ore and is newly defined at Greens Creek. This assemblage partially replaced stage 1 minerals. A distinct whole rock ‘sedimentary’ geochemical correlation of P, V, Cr, Ni, Co, U, Th, Al, Ti, Zr, K, Rb, Th, organic C, and ΣREE identifies lithic material throughout mineralized and ore samples. Fine-grained graphitic argillite beds (SA) plot as non-hydrothermal hemipelagic sediments and can be effectively used to determine ambient redox conditions during deposition. Massive dolomitic beds (MA) plot within the non-hydrothermal to hydrothermal fields, with 10–70% hydrothermal component. Framboids in ore and argillite are >10 μm and variably sized, indicating formation in pore water during diagenesis. Negative Ce/Ce* anomalies with correlative high Y/Ho, and Mo\(_{EF}\) and U\(_{EF}\) systematics, the presence of framboids, and very light S isotopic signature indicates that argillite was deposited in an open-ocean system within an oxic to suboxic water column, and a chemocline at or near the sediment-water interface, with anoxic, bacteriogenic H\(_2\)S-bearing pore fluid. A new U-Pb intercept age of 209 ± 9.4 Ma from hydrothermal monazite was determined, but is considered unreliable. The best age of mineralization is ca. 227 Ma, coeval with conodonts from hanging wall argillite and ca. 227 Ma U-Pb age dates from nearby stratigraphically equivalent Hyd volcanic rocks. A new emplacement age of 385 ± 4.4 Ma was determined for the Lil’ Sore dacite to the north of Greens Creek, but is considered unreliable. This age does indicate the presence of Middle to Late Devonian felsic volcanic rocks on Admiralty Island. Gold and Ag enrichment can be explained by: 1) a crustal basement source for Ag and a mafic-ultramafic footwall source for Au; 2) low temperature (150–250°C), weakly acidic (pH 4–5) fluid, rich in epithermal suite elements; and 3) a highly effective trap by subseafloor replacement of calcareous rocks below less-permeable hemipelagic muds. A dominant lateral zonation for metals and mineralization styles, clasts and partially replaced beds of sedimentary material within ore, trace element-rich framboids overprinted by ore-forming minerals, ubiquitous graphite, and a ‘sedimentary’ geochemical signature for ore supports a subseafloor replacement model. Ore formed initially as low temperature seafloor exhalative white smoker deposits during sedimentation (stage 1) and continued after burial to form higher temperature subseafloor replacement-style massive sulfide deposits (stage 2)