Geology and Geochemistry of Besshi-Type Massive Sulfide Deposits of the Vermont Copper Belt

Abstract

Abstract Massive sulfide deposits of the Vermont copper belt yielded approximately 3.6 Mt of ore during intermittent production from 1793 to 1958. The deposits consist of stratabound and generally stratiform pyrrhotite, chalcopyrite, and minor sphalerite and pyrite within metasedimentary rocks and minor mafic metavolcanic rocks of Silurian to Early Devonian age. At the largest deposits (Elizabeth, Ely, Pike Hill), massive sulfides are generally associated with metabasaltic amphibolite. The deposits are structurally complex, and have been deformed together with their host rocks during two stages of nappe-related, largely isoclinal folding, and during a later stage of dome-related folding; syntectonic shears and thrust faults commonly mark the contacts between massive sulfide bodies and silicate wall rocks. Postore re-gional metamorphism took place under amphibolite-grade conditions, producing locally abundant kyanite and staurolite in pelitic country rocks during peak prograde events. Geochemical studies of clastic metasedimentary host rocks in the district indicate a significant mafic component that suggests a continental island-arc provenance. The amphibolites, in contrast, have immobile trace element and rare earth element (REE) geochemical signatures similar to that of midocean ridge basalt (MORB). Lithologically unusual wall rocks at the Elizabeth deposit, including coarse garnet-mica schist, laminated plagioclase-rich granofels, quartz-mica-carbonate schist, tremolite-phlogopite schist, and quartz-albite tourmalinite, have high contents of Cr and MORB-type REE patterns that suggest protoliths of tholeiitic basalt. Massive sulfide, metachert, Mn-rich garnet-quartz rocks (coticule), and magnetite iron formation in the district are believed to have formed as exhalative chemical precipitates on the sea floor. Chemical analyses of unoxidized massive sulfide from the Elizabeth, Ely, and Pike Hill mines show that in addition to very high Cu (to 23.6 %) and in rare cases very high Zn (to 26.2 %), some ore samples contain minor Ag (to 100 ppm), Au (to 0.85 ppm), Cd (to 1500 ppm), Co (to 1469 ppm), Mn (to 5600 ppm), Mo (to 420 ppm), and Se (to 87 ppm). The ores have uniformly low As, Ba, Bi, Cr, Hg, Ni, Pb, REEs, Sb, Sn, Te, Th, Tl, U, and W. The overall geologic and geochemical features of the Vermont copper belt ores are similar to those of the Besshi deposits in Japan. Possible modern analogs include the actively forming massive sulfides of Guaymas Basin in the Gulf of California, Escanaba Trough on the Gorda Ridge, and Middle Valley on the Juan de Fuca Ridge.