SKARN FLUID HISTORY RECORDED IN OH AND TRACE ELEMENT ZONING IN A RASPBERRY GARNET FROM SIERRA DE CRUCES, MEXICO

Red raspberry grossular garnets in skarn deposits in Sierra de Cruces, Mexico, are unique because of their pleasing colors, ranging from dark pink at the rim to pale pink and brown in the core. The pink outer zoning on these crystals is identified as the grossular variety of garnet, while the dark inner core is identified as the melanite variety of andradite due to the elevated levels of titanium present (Lueth, 2003). Because these garnets grew through the intrusion of hydrothermal fluids, we hypothesize that the variable fluid of the hydrothermal deposit is recorded in the OH concentration and trace chemistry of these garnet crystals. Infrared spectra of the OH bands in the garnet were collected using a microscope attachment on the Nicolet Magna-IR 750 FTIR spectrometer at JMU. Hydroxyl concentrations were calculated using the calibration of Rossman (2006) for grossular garnets. Semi-quantitative chemical analyses were obtained on the LEO 1430VP SEM/EDS at JMU. The EDS data show that the pink outer zoning is manganese rich, while the dark inner core is titanium and iron rich. The OH concentrations range from 229 - 2322 ppm H₂O. The results show that the water is most concentrated in the core of the crystal, and oscillates but is generally lower further from the core. Hydroxyl band energies change from core to rim. Spectra from the iron and titanium rich class 7 core have band peaks at 3640cm^-1, 3600cm^-1, and 3555cm^-1. Spectra from the class 3 pink rim have band peaks at 3631cm^-1, 3663cm^-1, and 3687cm^-1. Data from the water concentrations compared with semi-quantitative chemical analysis suggest that the raspberry garnet crystallized in pulses relating to changes in the mineralizing environment.