Geology and geochemistry of the Kokomo mining district, Colorado

Abstract

The ores of the Kokomo mining district, Colorado, are Ag-Pb-Zn massive sulfide replacement deposits (mantos) controlled by northeast-striking, high-angle faults and hosted by limestone beds in the Pennsylvanian Minturn Formation. The Minturn Formation is as much as 1,830 m thick and consists of interbedded nonmarine, coarse clastic sedimentary rocks and thin marine limestones. The sedimentary section is greatly expanded by Cretaceous-Tertiary porphyritic granodiorite and quartz monzonite sills, dikes, and stocks.Hydrothermal alteration is dominated by skarns in calcareous sedimentary rocks centered on the Tucker Mountain area. Manto orebodies occur to the south in an updip direction distal to the skarns. Intrusive rocks throughout the Kokomo district have been affected by propylitic alteration. Argillic and sericitic alteration of sedimentary and intrusive rocks is restricted to areas immediately around the mantos and along faults that served as conduits for hydrothermal solutions. Mantos are surrounded by halos of siderite-manganosiderite, hydrothermal dolomite, and/or jasperoid in the limestone.Massive sulfide replacement ore exhibits banded and tabular textures very similar to ores at Leadville and Gilman, Colorado. Ore deposition occurred in the general sequence: jasperoid,; iron sulfides (pyrrhotite, marcasite, and pyrite) + siderite, quartz, high iron sphalerite, ferroan dolomite, galena, chalcopyrite and sulfosalts, barite, ferroan calcite, and calcite. Fluid inclusion analyses of quartz and carbonate gangue minerals indicate that temperatures averaged 350 degrees C during the early stages of ore deposition and decreased to about 160 degrees C during the late stages. Ore fluid salinities also decreased through the ore deposition event from 4.6 to 18 O and delta 18 C values compared to diagenetic dolomite due to isotopic exchange between limestones and hydrothermal fluids. Carbonate gangue minerals within the ore zone exhibit lower delta 18 O and delta 13 C values, and these values yield a calculated fluid delta 18 O (sub H 2 O) = 5.0 to 6.9 per mil, within the range for magmatic water. Sulfur isotope values of sulfide minerals range from -0.6 to + 0.8 per mil. The narrow range of delta 34 S values around 0 per mil suggests a magmatic sulfur source.The fission track age of the Kokomo ore system is about 40 Ma. Fission track dates are younger to the north and indicate a thermal center around Tucker Mountain. A porphyritic rhyolite plug on Tucker Mountain, described as a Pennsylvanian volcanic feature in previous literature, yields a fission track age of 39.5 Ma.The Kokomo stable isotope values, fluid inclusion homogenization temperatures, and salinities are comparable to data from other Leadville-type (magmatic hydrothermal) carbonate-hosted sulfide replacement deposits in the Colorado mineral belt, including Leadville and Gilman.