Geochemistry of porphyry-hosted Au-Ag deposits in the Little Rocky Mountains, Montana

Abstract

The Little Rocky Mountains, Montana, are a result of doming of Archean basement gneisses and Phanerozoic sedimentary rocks produced by the emplacement of a quartz monzonite to syenite intrusive complex of early Tertiary age. The intrusive rocks have whole-rock delta 18 O values near 8 per mil, variable initial epsilon Nd values from -11 to -19, and initial 87 Sr/ 86 Sr ratios from 0.7050 to 0.7061 all of which are compatible with derivation from lower crustal rocks having variable compositions. The Zortman and Landusky mining areas in the Little Rocky Mountains produce Au (and Ag) from fracture systems filled with clay minerals and low concentrations of Au-bearing pyrite within the intrusive rocks. K-Ar ages of illites associated with the Au mineralization are near 60 m.y. and are similar to K-Ar and fission-track ages of the intrusive rocks themselves. The delta D and delta 18 O values of these illites indicate that they, and the Au-bearing pyrite, formed at temperatures near 250 degrees C from meteoric fluids having delta 18 O values near 1 per mil and delta D values near -80 per mil. This Au event is characterized by illites having much higher initial 87 Sr/ 86 Sr ratios (as high as 0.7784) than the intrusive rocks which host the fracture system, suggesting that Sr, and by inference the Au, most likely was derived from Precambrian basement gneisses.After the gold was emplaced, later fluids released the gold by oxidizing the pyrite; they then formed kaolinite and hydrothermal fluorite and dolomite. The latter two are characterized by 87 Sr/ 86 Sr ratios near 0.708, similar to those of the intrusive rocks and Phanerozoic carbonates. Low-salinity fluid inclusions in fluorite homogenize at 100 degrees to 200 degrees C and have delta 18 O values near -10 per mil and delta D values near -110 per mil, consistent with those of Tertiary ground waters. The delta 34 S values of pyrite do not correlate with gold contents, and their wide variation from -11.4 to +6.3 per mil cannot be used to distinguish among igneous, sedimentary, and metamorphic sources.