Abstract The Mascot-Jefferson City zinc district is the most important of the carbonate-hosted, Mississippi Valley-type (MVT) Zn ± Pb districts in eastern Tennessee, all of which are located in the Appalachian Valley and Ridge physiographic province (Fig. 3). Another producing zinc district in Tennessee, also of the Mississippi Valley-type and with many characteristics similar to those of the M-J district, is the Central Tennessee zinc district (Elmwood-Gordonsville mines) located along the crest of the Nashville Dome, a southwestward continuation of the Algonquin-Cincinnati Arch system. Plans are also under way to resume production in the Idol mine of the Copper Ridge district, which has been inactive since 1981.

Abstract The Río Blanco-Los Bronces copper-molybdenum porphyry district in the late Miocene to early Pliocene magmatic arc of central Chile is currently being mined by state mining company CODELCO (Río Blanco) and Anglo American Sur (Los Bronces). Combined annual production in 2011 was nearly 450,000 metric tons (t) of copper plus by-product molybdenum. With Anglo American's recent high-grade discoveries in the district (3.7 billion tons (Bt) grading 0.7% Cu at San Enrique-Monolito, and 4.5 Bt grading 0.9% Cu at Los Sulfatos) adding more than 65 Mt of fine copper to the mineral inventory, the district now ranks as the world's largest by contained metal, with more than 200 Mt of copper. Volcanic and volcaniclastic rocks of the Abanico and Farellones Formations represent premineralization host rocks ranging in age between 22.7 ± 0.4 and 16.8 ± 0.3 Ma (U-Pb dating of zircons). The bulk of the copper-molybdenum porphyry endowment is related to evolution of the San Francisco batholith, a large (200 km 2) granodiorite-dominated complex with U-Pb zircon ages between 16.4 ± 0.2 to 8.4 ± 0.2 Ma. Three geologic domains are defined in the district on the basis of rock types, structural breaks, and age determinations: the Los Piches-Ortiga block in the west, the San Manuel-El Plomo block in the center, and the Río Blanco-Los Bronces-Los Sulfatos block in the east. These geologic domains are younger progressively to the east, with most of the known copper endowment on the easternmost (Río Blanco-Los Bronces-Los Sulfatos) block. Intrusive and hydrothermal activity in the Los Piches-Ortiga block spanned ∼ 2.5 m.y., from 14.8 ± 0.1 to 12.3 ± 0.1 Ma. Although these events apparently did not produce high-grade copper deposits, silver-bearing veins associated with a high sulfidation hydrothermal system are present in the block. To the east, in the San Manuel-El Plomo block, a series of magmatic-hydrothermal systems developed during a ∼ 3-m.y. period between 10.8 ± 0.1 and 7.7 ± 0.1 Ma. These events also apparently failed to generate high-grade copper systems. Magmatic-hydrothermal activity in the eastern Río Blanco-Los Bronces-Los Sulfatos block, hosting virtually all of the copper endowment recognized in the district, spanned a ∼ 4-m.y. period from 8.2 ± 0.5 to 4.31 ± 0.05 Ma. Copper endowment in the district is associated with vertically continuous breccia bodies and quartz-veinstockworked porphyries. Hydrothermal assemblages follow a characteristic vertical and lateral zonation pattern. Remnants of high sulfidation and/or advanced argillic assemblages (quartz-enargite-tennantite-galenasphalerite-gypsum-anhydrite with dumortierite-pyrophyllite-alunite) and peripheral sericite-illite reflect preservation of shallow levels, whereas K-silicate alteration assemblages (biotite-K-feldspar-albite) are present in association with chalcopyrite-bornite and chalcopyrite-pyrite at depth. Between the mineralized bodies, a distal assemblage of hydrothermal chlorite-epidote-specularite-pyrite predominates. At Río Blanco-Los Bronces, igneous and/or hydrothermal- and hydrothermal-cemented breccias developed in intimate association with porphyry phases. At shallow depths, the hydrothermal breccia cement generally comprises quartz-sericitetourmaline, and contains pyrite > chalcopyrite/molybdenite. At deeper levels the breccia cement is predominantly biotite-K-feldspar containing bornite-chalcopyrite-molybdenite. These progressively grade outward into chalcopyrite-pyrite-dominated zones and ultimately to pyrite-dominated zones. Within the Río Blanco-Los Bronces-Los Sulfatos block, the upper presence of the K-silicate assemblage varies from ∼ 3,000 m above sea level (a.s.l.) in the poorly telescoped northern area (Río Blanco), to ∼ 4,000 m a.s.l in the highly telescoped southern area (Los Sulfatos). These differences may reflect varying rates of synmineral structural exhumation, or varying depth of porphyry emplacement along the Río Blanco-Los Bronces-Los Sulfatos structural corridor. Key factors contributing to the copper productivity in the district are considered to reflect both far-field tectonic conditions, and district-scale structural controls. Following the last significant phase of volcanism documented in the district (∼ 16.8 Ma), a temporally discrete period of peak compression and rapid exhumation, between ∼ 6 to 3 Ma, affected the central Chilean Andes. This period of uplift relates to flat-slab subduction of sea floor containing the Juan Fernandez Ridge into the Chile Trench and overlaps part of the emplacement history of the Río Blanco-Los Bronces-Los Sulfatos block (8.2–4.31 Ma). The lack of contemporaneous volcanism and concomitant tectonic uplift are interpreted to reflect a state of increased horizontal crustal compression due to shear coupling of the downgoing slab. By suppressing volcanism, these conditions are considered to promote the retention of magma in the deep crustal environment, where higher pressures promote greater solubility of magmatic volatiles and higher temperatures may promote longer lived magma chambers by slowing fractionation processes. Under such conditions, the potential is enhanced for increased amounts of metals and volatiles by addition of fresh batches of magma to the deep magma chamber. At the district scale, closely spaced (2 km) structures that control the position of the porphyry and breccia bodies in the Río Blanco-Los Bronces-Los Sulfatos block appear to have focused long-lived, multistage magmatic-hydrothermal activity within a narrow structural corridor, contributing to the development of large, high-grade porphyry/breccia systems.

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.

Abstract Massive sulfide deposits of the Vermont copper belt yielded -4 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 and minor mafic metavolcanic rocks of Silurian to Early Devonian age. At the largest deposits (Elizabeth, Ely, Pike Hill), massive sulfides are closely associated w5th metabasaltic hornblendeplagioclase 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 regional metamorphism took place under amphibolite-grade conditions, producing locally abundant kyanite and staurolite during peak prograde events. Geochemical studies of clastic metasedimentary host rocks of the district indicate a significant mafic component that suggests a continental island-arc provenance. The amphibolites, in contrast, have a geochemical signature similar to that of mid-ocean ridge basalt (MORB). Lithologically unusual wall rocks at the Elizabeth deposit, including coarse garnet-mica schist, plagioclase-rich granofels, quartz-mica-carbonate schist, actinolite-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, coticule, and magnetite iron formation in the district are believed to have formed as exhalative chemical precipitates. The overall geologic and geochemical features of the Vermont ores are similar to those of the Besshi deposits of 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.