Origin and evolution of skarn fluids, Empire zinc skarns, Central Mining District, New Mexico, U.S.A.

Abstract

Garnet-pyroxene-sphalerite skarns in the Empire Mine replace Paleozoic carbonates adjacent to the Tertiary Hanover-Fierro granodiorite. Skarn geometry suggests that fluids migrated up pre-ore dikes, faults and the igneous contact, and were deflected laterally into the permeable Tierra Blanca Limestone beneath the relatively impermeable Parting Shale. Silicates associated with propylitically altered pre-ore dikes are enriched in deuterium (D), and depleted in 18O relative to the Hanover-Fierro pluton and post-ore igneous rocks. Early skarn silicates are also depleted in 18O with respect to the pluton, while later skarn minerals are depleted in both D and 18O. Variations in isotope composition of alteration and skarn minerals indicate isotope heterogeneities in mineralizing fluids, even at the small scale of centimeters. Isotope thermometry indicates that there is some degree of subsolidus re-equilibration of igneous and alteration minerals. Several possible fluid flow regimes may have operated to produce the fluids calculated to be in exchange equilibrium with the various rocks and minerals of the Empire skarn system, and mixing of end-member meteoric, formation and magmatic fluids in different proportions can produce observed δD δ 18O trends. An end-member magmatic fluid could produce the D-enrichment observed for early skarn fluids, but this would require isolating magmatic fluids from external fluid sources during cooling of the system from magmatic temperatures of 700°C to skarn temperatures of the order of ≤ 400°C. The D-enrichment may also be explained by the mixing of magmatic and formation waters. Lower δD values, however, require that a large proportion of late-stage skarn fluids must be a D-depleted Tertiary meteoric water, and magmatic water is restricted to a relatively minor component. The end-member mixing approach indicates significant changes in fluid flow systematics over a relatively narrow range in temperature. Alternatively, observed trends in both δD and δ 18O for skarn fluids can also be reproduced by interacting a D-depleted meteoric water with the Hanover-Fierro pluton at low and variable system water-rock ratios, and temperatures between 250 and 400°C. During migration along the long fluid flow paths implied by the low system water-rock ratios (≤0.1), the salinity of dilute meteoric waters could increase through interaction with minerals or leaking fluid inclusions in the country rock. Correlation of isotope depletions of the carbonate wallrocks with inferred fluid flow conduits, suggests significant amounts of fluid-rock exchange at relatively high local water-rock ratios during focusing of flow by critical structures. Although different C sources might require smaller values, it is clear that large (>1) local water-rock ratios are required to produce depletions observed in both 18O and 13C in hydrothermal calcites. Stable isotope evidence does not require the presence of a significant magmatic fluid component, and suggests that the bulk of the skarn fluids could instead be derived predominantly from a D-depleted meteoric water.