Zoned alteration in limestone at porphyry copper deposits, Ely, Nevada

Abstract

The porphyry copper deposits near Ely, Nevada, occur within at least six bodies of quartz monzonite porphyry, associated spatially with barren sills, plugs, and a large stock of porphyritic quartz monzonite. The mineralized intrusive rocks were largely localized by the thrust-faulted contact between the Pennsylvanian Ely Limestone and the Mississippian Chainman Shale. Near mineralized intrusions the limestone contains extensive bodies of calc-silicate iron oxide sulfide copper ores. Adjacent shale underwent extensive potash metasomatism and hornblende-hornfels facies metamorphism but is less well mineralized. Two or more generations of postore faulting were important in developing the present geometry of the deposits. Several plutons are now rootless; at depth they terminate on fault contacts against unmineralized rocks.Silicate alteration assemblages in fractured quartz monzonite porphyry correlate with those in adjacent Ely Limestone. In deeper parts of the mines, secondary biotite and orthoclase plus disseminated chalcopyrite permeate porphyry. Massive andradite and fine-grained diopsidic pyroxene replace adjacent quartz-bearing limestone and dolomite. Later pyrite-chalcopyrite veinlets cut the early calc-silicates and are rimmed by quartz, calcite, magnetite, and either actinolite or a nontronite clay of varying composition Such multicolored silicate rock extends several hundred feet from altered porphyry and is called the tactite alteration zone. A band of massive wollastonite plus quartz typically occurs at the outermost edge of tactite.The irregular peripheral alteration zone, characterized by bleaching and local development of saponite, diopside, and tremolite, extends outward to unaltered limestone. Minor vein and replacement ore deposits within this zone show a crude metal zoning pattern.Biotite-orthoclase alteration in porphyry grades upward into pervasive or veinlet-dominated quartz-sericite alteration. Adjacent Ely Limestone contains hydrothermal calcite, pyrite, nontronite, and fine-grained quartz, and is referred to as the silica-pyrite zone. This zone, which has a low copper content, was apparently once extensive above ore-grade tactite zone rocks. Numerous features present in the various alteration zones can best be explained by the presence of solute-rich high temperature hydrothermal fluids during the formation of the deposits.The relationships between the different alteration types described above can be used as a model to aid in determining the prefaulting shape of the Ely porphyry copper system. Several bodies of mineralized porphyry and limestone apparently were once cupolalike features above the large unmineralized pluton. Chemical data suggest that these porphyry bodies could have provided sufficient iron to form the adjacent tactite assemblages, but that copper, sulfur, and some other elements were derived from elsewhere.