Oxygen, hydrogen, and carbon isotope studies of the Stony Mountain Complex, western San Juan Mountains, Colorado

Abstract

The mid-Tertiary Stony Mountain ring-dike complex, approximately 1.3 km in diameter, is composed of an outer diorite, a main mass of biotite gabbro, and an inner diorite. This composite stock and most of the surrounding country rocks of the San Juan volcanic field in the vicinity of the Silverton caldera have experienced various degrees of 18 O depletion (up to 10 per mil) due to interaction with heated meteoric ground waters. Most of the isotopic effects are a result of exchange between H 2 O and solidified igneous rocks, but the inner diorite may have been intruded as an inhomogeneous, low 18 O magma. The delta 18 O values of the rocks decrease with decreasing grain size, compatible with subsolidus exchange. Quartz typically has delta 18 O = 6 to 8 and is more resistant to exchange than any other mineral studied. The order of increasing resistance to hydrothermal 18 O exchange is feldspar-pyroxene-biotite-magnetite-quartz. Hydrogen isotope analyses of sericites, chlorites, biotites, and amphiboles range from --117 to --150 (SMOW) and exhibit the same order of D/H enrichment as do normal igneous hydrous phases, except that each phase is drastically depleted in deuterium. The delta D in biotites varies inversely with Fe/Fe+Mg and positively with elevation in the intrusion over a range of 600 m. The calculated delta D of the Tertiary meteoric waters is --100. Carbonate delta 13 C values average --5.5 (PDB), within the generally accepted range for deep-seated carbon. Based on numerical simulation of thermally driven convective fluid flow of plutons in similar geologic environments, the average integrated fluid flux that persisted for most of the cooling history of the Stony Mountain complex is estimated to be 10 (super -7) g cm (super -2) s (super -1) , equivalent to an overall water/rock ratio of 0.2 (weight units). It is likely that the time-temperature circulation history of the meteoric-hydrothermal fluids associated with the Stony Mountain complex, and with gabbroic complexes in general, was such that any ore metals in solution were dispersed into the surrounding country rocks.

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