Seeing Through Metamorphism: Geochemical and Isotopic Studies of Amphibolite Facies Mafic-Siliciclastic (Besshi-Type) Massive Sulfide Deposits at the Elizabeth Copper Mine, Vermont, USA
Authors: Slack John F. , Shanks Wayne C. , Woodruff Laurel G. 
Published: November 2024

The Elizabeth volcanogenic massive sulfide (VMS) deposit in Vermont is atypical among such deposits in containing both stratiform Fe-Cu-Zn-Mn mineralization and stratabound Al-Na-K-B enrichments and Ca-Mg depletions in wall rocks. This mafic-siliciclastic (Besshi-type) deposit is contained in a thick sequence of Lower Devonian pelitic schist, minor quartzite, and sparse amphibolite. Wall rocks to the sulfide ores are predominantly amphibole-bearing rocks lacking quartz. The deposit has been metamorphosed to middle amphibolite facies and is complexly deformed by two folding events and syndeformational thrust faults.

Whole-rock analyses of altered metabasaltic wall rocks reveal locally high concentrations of SiO2 (up to 85.0 wt %), Al2O3 (up to 32.5 wt %), K2O (up to 4.02 wt %), or Na2O (up to 6.02 wt %). Data for relatively immobile trace elements (Sc, Cr, Ti, Zr, Th) and rare earth elements (REEs) indicate protoliths of low-Ti tholeiitic basalt broadly of normal mid-ocean ridge basalt (N-MORB) affinity. The altered wall rocks are mineralogically distinctive in containing, in many samples, abundant muscovite, phlogopite, albite, dolomite, tourmaline, and/or tremolite-actinolite. One very aluminous unit of coarse garnet-mica schist, also with a tholeiitic basalt protolith, has local domains composed of abundant staurolite, minor margarite and sillimanite, and rare corundum. All of these altered wall rocks are stratabound but generally discontinuous along strike, in contrast to Mn-rich stratiform lenses (coticules), for which mineralogical and geochemical data suggest deposition as VMS-related chemical sediments.

Al-normalized calculations of whole-rock geochemical data for mineralogically different types of altered basalts, relative to an inferred least altered precursor in the mine sequence, show average major additions (&gt;50%) of Mn, Na, and K, and major losses (&gt;50%) of Mg and Ca for most types. Changes in Si, Ti, and Fe are generally negligible (±5%), except for siliceous, tourmaline-rich rocks that display on average a major addition of 254% Si. Whole-rock δ18O values of silica-poor samples with metabasaltic precursors range from 10.7 to 13.4‰. These values are uncorrelated with SiO2 contents and are mostly higher than that of the least altered metabasalt in the Elizabeth mine area (δ18O = 11.1‰), suggesting premetamorphic subseafloor alteration by relatively low-temperature (ca. 150°–250°C) VMS-related hydrothermal fluids. A lack of depletion of light REEs for most of the altered metabasalts, compared to the least altered precursor, is also consistent with a low-temperature alteration process. Sulfur isotope values for pyrrhotite, chalcopyrite, sphalerite, and pyrite in massive sulfide and disseminations in wall rocks range from 4.3 to 9.3‰, a typical range for sediment-hosted sea-floor hydrothermal systems, reflecting mixed sulfur sources derived mainly from footwall basalts and coeval seawater sulfate.

Shale-normalized REE data for coticules show small negative Ce and Eu anomalies that suggest deposition of precursor Mn-rich sediments in mildly oxic waters. In contrast, most samples of massive sulfides lack Ce anomalies but have small to moderate positive Eu anomalies, reflecting mineralization under anoxic and reducing conditions at or above 250°C. The presence of negative Ce anomalies in the coticules and two samples of massive sulfide, but the lack of such anomalies in other massive sulfide samples, suggest deposition within a stratified water column in which the redoxcline fluctuated due to hydrothermal venting of reductants such as Fe2+, Mn2+, H2S, H2, and CH4, which episodically produced anoxic bottom waters during VMS mineralization.

The length (≥3.4 km) and relatively narrow width (~500 m) of the Elizabeth sulfide deposit is attributed to formation in an elongate sea-floor graben that served as a locus of tholeiitic basaltic volcanism and hydrothermal mineralization. Morphological and geochemical data for highly altered metabasaltic wall rocks of the deposit provide evidence for pervasive subseafloor alteration, during and after exhalative chemical precipitation of sulfides and Mn-Fe sediments. Possible modern analogs are the predominantly sediment-hosted VMS deposits in Middle Valley and Escanaba trough in the northeast Pacific Ocean and Guaymas basin in the Gulf of California. The metalliferous sulfide deposits of the Atlantis II Deep in the Red Sea may be the best modern analog, based on their sheetlike morphology, sulfur isotope values, and associated Fe and Mn oxyhydroxide sediments that are potential protoliths of the coticule rocks in the Elizabeth mine sequence.