Skarn zonation and fluid evolution in the Groundhog Mine, Central mining district, New Mexico

Abstract

The Groundhog mine contains one of the largest and best exposed Zn-Pb-Cu-Ag skarn deposits in the United States. Skarn is formed in Carboniferous limestones spatially associated with a nearly vertical swarm of Early Tertiary granodiorite porphyry dikes. Metal ratios, mineralogy, composition, and fluid inclusion characteristics of the skarn are systematically zoned from northeast to southwest along the >3-km strike length of the system and relative to the original dike-limestone contact. Zn/Cu, Zn/Ag, Pb/Cu, Pb/Zn, and Pb/Ag ratios all increase toward the distal southwest end of the mine where highest Zn, Pb, and total metal grades also occur. Endoskarn is zoned in terms of the relative proportions and manganese content of epidote and chlorite. In the Northeast zone, skarn is zoned away from dikes in the sequence: garnet to pyroxene to pyroxenoid to marble. In the Central skarn zone, garnet is absent and pyroxene begins the zonation sequence at the dike contact. In the Southwest zone, pyroxene is less abundant and pyroxenoid locally begins the zonation sequence. Sphalerite-galena-pyrite-quartzcarbonate are present in all skarn zones and in carbonate replacement mantos and chimneys beyond skarn. Distal mineralogical zones (e.g., pyroxenoid and manto) are more extensive to the southwest. In general, the skarn zonation sequence is wider and more fully developed in pure, coarse-grained limestone than in silty, carbonaceous, or fine-grained limestone. High permeability and water/rock ratio are considered the dominant controls on skarn size. Most skarn minerals, including garnet, pyroxene, pyroxenoid, ilvaite, amphibole, chlorite, carbonate, and sphalerite, are enriched in manganese. Pyroxene becomes more manganese rich (up to 85 mole % johannsenite):(1) marble, (2) along strike to the southwest, (3) at higher elevations, and (4) in coarser grained, cleaner limestones. The more manganese-rich pyroxenes formed from fluids which are distal in the overall zonation sequence. In contrast, pyroxene formed in proximal locations (dike contact, Northeast skarn zone, low elevation, fine-grained, impure limestone) is enriched in magnesium. Pyroxene-forming fluids are depleted first in Mg, then in Fe, and finally in Mn, as proposed by Burt (1977). Fluid inclusion homogenization temperatures are lower than for most zinc skarns. In the Northeast skarn zone the average homogenization temperature is 337oC and salt daughter minerals are present in garnet and pyroxene (inclusions without daughter minerals average 8.5 wt % NaCI), whereas salt daughter minerals are absent in the Central and Southwest skarn zones and average homogenization temperatures and salinities are 320oC, 7.3 wt percent NaC1, and 293oC, 6.2 wt percent NaC1, respectively. Pressure estimates from rare clusters of fluid inclusions with evidence for boiling are between 155 and 175 bars. Skarn thermal gradients calculated from fluid inclusion homogenization temperatures are 35oC from dike to marble front and 50oC from dike to the limit of alteration (carbonate replacement mantos and chimneys). The calculated average thermal gradient along strike ranges from 8oC/km along the dike contact to 23oC/km along the marble front. The calculated thermal gradients and measured pyroxene compositions are in general agreement with published experimental studies of clinopyroxene stability relations. The variations in skarn metal ratios, mineralogy, composition, and fluid inclusion characteristics have been incorporated into a general model for zinc skarn formation which can be applied to the hierarchy of zinc skarns summarized by Einaudi et al. (1981) and as a guide to exploration. The distal end of this zinc skarn hierarchy is transitional to manto and chimney carbonate replacement deposits which contain few, if any, calc-silicate minerals.