TI – ANDRADITE IN GARNET FROM THE SAN PEDRO SKARN, NEW MEXICO: IMPLICATIONS FOR DYNAMIC FLUID INTERACTIONS AND GARNET GROWTH

Previous studies of major and minor element abundances in zoned and unzoned skarn garnets have provided valuable insights into the complex fluid-rock interactions that occur during skarn formation. Despite these contributions, there are still aspects of garnet growth in these complex systems that are not well constrained. We examined garnet-bearing skarn samples from the San Pedro Mine (SPM), NM as part of an ongoing investigation to explore growth mechanisms of skarn garnets. The SPM is a Cu skarn formed from the intrusion of a monzonitic porphyry into the Madera formation (alternating impure carbonate and shales). Scanning Electron Microscope imaging and Electron Microprobe analyses of these samples show that garnet occurs with distinct textural and compositional differences.

A representative skarn sample has a type of garnet with large (>500 µm), euhedral crystals with extensive oscillatory zoning. The growth zones of these garnets are dominated by variations in Adr and Grs components, but all have Adr-rich compositions (Adr_67.1-98.0 – Grs_0.2-30.5). This sample likely represents a later stage skarn development with an influx of oxidizing fluid, as suggested in previous work. In a different sample, there are two texturally distinct garnet types (A & B) with different compositions. Type A occurs as semi porous masses with very subtle, irregularly zoned regions. Type B occurs as small ( ̴ 20-50 µm), subhedral - anhedral grains with highly perturbated zoned cores and unzoned rims. Type B garnets occur within the type A garnet mass, but also as isolated grains within the matrix. Both textural types of garnet contain a component of Ti-Adr in solid solution that is present throughout individual garnet grains. Type A garnet has the compositional range: Adr_60.9-79.7; Grs_16.5-32.4; Prp_0.7-1.7 (0.18-0.3 a.p.f.u. Ti), while type B is more Adr-poor: Adr_21.2-29.4; Grs_58.9-68.2; Prp_9.8-11.5 (0.2-0.25 a.p.f.u. Ti). The higher concentration of Mg in the type B garnets is unusual for skarns but could reflect an interaction between meteoric waters and the impure limestone or a higher Mg content in the magmatic fluids. The consistency of Ti throughout the growth zones in the garnets suggests that the skarn-forming fluids were constantly replenished in Ti, even though the concentrations of Al, Fe, and Mg were changing significantly.