Petrology and geochemistry of propylitic alteration at Southwest Tintic, Utah

Abstract

Systematic variations in mineral assemblages, fluid inclusion characteristics, mineral chemistry, and isotopic ratios document the chemical and thermal evolution of hydrothermal fluids responsible for late alteration in a small porphyry copper system near Tintic, Utah. Propylitic alteration of volcanic rock can be subdivided into actinolite, epidote, and chlorite subzones. An actinolite subzone containing actinolite, epidote, chlorite, and calcite extends 250 m away from the secondary biotite zone. An epidote subzone containing epidote, chlorite, and calcite is present up to 500 m away from the secondary biotite zone. A chlorite subzone of chlorite and calcite is present up to 2 km from the secondary biotite zone. Chlorite which occurs in each subzone varies systematically in Fe, Mg, and Mn content. The ratio Fe/(Fe + Mg) increases from 0.27 at the secondary biotite zone to 0.48 in the outer propylitic zone accompanied by a decrease in activity of clinochlore from 0.15 to 0.02. MnO increases from 0.23 to 0.44 wt percent. The Fe (super +3) content of epidote also systematically decreases from the actinolite subzone outward. The activity of clinozoisite in the epidote ranges from 0.038 at the secondary biotite zone to 0.21 700 m away from the secondary biotite zone in the outer epidote subzone. Systematic changes in chlorite and epidote compositions reflect increasing log a (sub Mg (super +2) ) /a 2 (sub H (super +) ) and log a (sub Fe (super +2) ) /a 2 (sub H (super +) ) with increasing distance from the hydrothermal biotite zone and decreasing temperature and salinity.Fluid inclusion homogenization temperatures decrease away from the secondary biotite zone with a horizontal thermal gradient of 78 degrees C/km. Temperatures range from 351 degrees to 160 degrees C in propylitic alteration. Salinities also decrease outward from 7.3 to 0.0 equiv wt percent NaCl.Hydrothermally altered rocks are systematically depleted in 18 O outward from the phyllic alteration zone into the chlorite subzone, as recorded by whole-rock samples (10.9 to -1.7ppm), calcite (10.4-1.6ppm), and quartz veins (2.8 to -0.8ppm). Calculated delta 18 O and delta D values for the actinolite subzone are +5 and -55 per mil, respectively; values as low as -15.1 and -120 per mil are recorded at the outer limits of sampling in the chlorite subzone.These latter values indicate that outer propylitic alteration is dominated by isotopically unevolved local meteoric water. However, the large variations in hydrogen and oxygen isotope compositions recorded across the zoned propylitic alteration are too large to have been produced from an isotopically homogeneous fluid, given the measured range of alteration temperatures (350 degrees -200 degrees C). Rather, the variations in isotopic compositions and salinity of the propylitic alteration fluids are consistent with the progressive mixture of unevolved ( 18 O- and D-depleted), low-salinity local meteoric water with a much more 18 O- and D-enriched, higher salinity fluid. The later phyllic alteration also formed from a similar, 18 O- and D-enriched fluid. We suggest that these isotopically enriched fluids more likely evolved from local meteoric water through extensive exchange at very low water/rock ratios with the igneous rocks at depth in the hydrothermal system at Tintic. An alternative explanation is that 18 O- and D-enriched fluids represent a late incursion of magmatic water unlike many other porphyry copper systems.Participation of such 18 O- and D-enriched fluids in the waning stages of hydrothermal alteration at Tintic contrasts with the usual domination of isotopically unevolved meteoric water at these stages (e.g., propylitic, phyllic) in many porphyry copper systems (Sheppard et al., 1971; Taylor, 1974; Gustafson, 1978). At Tintic, isotopically enriched fluids coexist with and even displace meteoric water during propylitic and phyllic alteration, respectively.