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摘要
The Goodsprings district, Clark County, Nevada, includes zinc-dominant carbonate replacement deposits of probable late Paleozoic age, and lead-dominant carbonate replacement deposits, copper +/- precious metal-platinum group element (PGE) deposits, and gold +/- silver deposits that are spatially associated with Late Triassic porphyritic intrusions. The district encompasses similar to 500 km(2) although the distribution of all deposits has been laterally condensed by late Mesozoic crustal contraction. Zinc, Pb, and Cu production from about 90 deposits was similar to 160,000 metric tons (t) (Zn > Pb >> Cu), 2.1 million ounces (Moz) Ag, 0.09 Moz Au, and small amounts of PGEs-Co, V, Hg, Sb, Ni, Mo, Mn, Ir, and U-were also recovered.
Zinc-dominant carbonate replacement deposits (Zn > Pb; Ag +/- Cu) resemble Mississippi Valley Type (MVT) Zn-Pb deposits in that they occur in karst and fault breccias in Mississippian limestone where the southern margin of the regional late Paleozoic foreland basin adjoins Proterozoic crystalline rocks of the craton. They consist of calcite, dolomite, sphalerite, and galena with variably positive S isotope compositions (delta(34)S values range from 2.5-13 parts per thousand), and highly radiogenic Pb isotope compositions ((206)Pb/(204)Pb >19), typical of MVT deposits above crystalline Precambrian basement. These deposits may have formed when southward flow of saline fluids, derived from basinal and older sedimental), rocks, encountered thinner strata and pinch-outs against the craton, forcing fluid mixing and mineral precipitation in karst and fault breccias. Lead-dominant carbonate replacement deposits (Pb > Zn, Ag +/- Cu +/- Au) occur among other deposit types, often near porphyritic intrusions. They generally contain higher concentrations of precious metals than zinc-dominant deposits and relatively abundant iron oxides after pyrite. They share characteristics with copper +/- precious metal-PGE and gold +/- silver deposits including fine-grained quartz replacement of carbonate minerals in ore breccias and relatively low S and Pb isotope values (delta(34)S values vary from 0-similar to 4 parts per thousand; (206)Pb/(204)Pb <18.5). Copper +/- precious metal-PGE deposits (Cu, Co, Ag, Au, Pd, and Pt) consist of Cu carbonate minerals (after chalcocite and chalcopyrite) and fine-grained quartz that have replaced breccia clasts and margins of fissures in Paleozoic limestones and dolomites near porphyritic intrusions. Gold +/- silver deposits occur along contacts and within small-volume stocks and dikes of feldspar porphyry, one textural variety of porphyritic intrusions. Lead isotope compositions of copper +/- precious metal-PGE, gold +/- silver, and lead-dominant carbonate replacement deposits are similar to those of Mojave crust plutons, indicating derivation of Pb from 1.7 Ga crystalline basement or from Late Proterozoic siliciclastic sedimentary rocks derived from 1.7 Ga crystalline basement.
Four texturally and modally distinctive porphyritic intrusions are exposed largely in the central part of the district: feldspar quartz porphyry, plagioclase quartz porphyry, feldspar biotite quartz porphyry, and feldspar porphyry. Intrusions consist of 64 to 70 percent SiO(2) and variable K(2)O/Na(2)O (0.14-5.33) that reflect proportions of K-feldspar and albite phenocrysts and megacrysts as well as partial alteration to K-mica; quartz and biotite phenocrysts are present in several subtypes. Albite may have formed during emplacement of magma in brine-saturated basinal strata, whereas hydrothermal alteration of matrix, phenocrystic, and megacrystic feldspar and biotite to K-mica, pyrite, and other hydrothermal minerals occurred during and after intrusion emplacement. Small volumes of garnet-diopside-quartz and retrograde epidote-mica-amphibole skarn have replaced carbonate rocks adjacent to one intrusion subtype (feldspar-quartz porphyry), but alteration of carbonate rocks at intrusion contacts elsewhere is inconspicuous.
Uranium-lead ages of igneous zircons vary inconsistently from similar to 180 to 230 Ma and are too imprecise to distinguish age differences among intrusion subtypes; most ages are 210 to 225 Ma, yielding a mean of 217 +/- 1 Ma. K-Ar and (40)Ar/(39)Ar ages of magmatic (plagioclase, biotite) and hydrothermal (K-mica) minerals span a similar range (183-227 Ma), demonstrating broadly contemporaneous intrusion emplacement and hydrothermal alteration but allowing for multiple Late Triassic magmatic-hydrothermal events. Imprecision and range of isotopic ages may have resulted from burial beneath Mesozoic and Tertiary strata and multiple intrusion of magmas, causing thermal disturbance to Ar systems and Pb loss from zircons in intrusions.
Separate late Paleozoic (zinc-dominant carbonate replacement deposits) and Late Triassic (all other deposits) mineralizing events are supported by form, distribution, and host rocks of metal deposits, by hydrothermal mineral assemblages, isotope compositions, metal abundances, and metal diversity, and by small intrusion volumes. These characteristics collectively distinguish the Goodsprings district from larger intrusion related carbonate replacement districts in the western United States. They can be used to evaluate proximity to unexposed porphyritic intrusions associated with PGE and gold +/- silver mineralization.
- 出版日期2011-5
