High Temperature Hydrothermal Metamorphism at The Geysers geothermal field, California: An Investigation of Mineral Chemistry and Vein Paragenesis from an Active Contact Metamorphic Environment

摘要

Core samples from eight wells in The Geysers geothermal field that define a transect from the intrusive core of the system through the contact metamorphosed Franciscan metagraywacke and argillite that hosts the high temperature vapor dominated geothermal reservoir have been examined by electron microprobe, scanning electron and petrographic microscopic techniques to document mineral chemistries and vein paragenesis. Vein paragenesis in the suite of core samples shows early veins of: biotite + apatite + quartz + pyrite ± plagioclase ± pyrite ± ilmenite; tourmaline + quartz + chlorite ± K-feldspar ± plagioclase ± epidote ± titanite ± pyrite ± ilmenite ± chalcopyrite; and actinolite + apatite + quartz + pyrite. With later: epidote + chlorite + quartz ± actinolite ± K-feldspar ± titanite; and K-feldspar ± pyrite ± titanite. Three hundred and sixty-six electron microprobe analyses of vein and wall rock mineral chemistry have been collected from: biotite (seventy-seven); tourmaline (one hundred and sixteen); calcic amphibole (twenty); apatite (thirty-three); plagioclase (fourteen); epidote (thirty-one); chlorite (forty-three); titanite (eight); and K-feldspar (twenty-five). Biotite chemistry data from veins and contact metamorphic wall rock were collected from two cores. The biotite is annite to phologopite in composition with Mg/(Mg+Fe) values ranging from 0.37 to 0.56 and Al contents ranging from 1.12 to 1.62 apfu with overlapping compositional ranges. Variations in Mg, Fe, Al and Cl content were observed between vein and wall rock biotite and disparities in F content were observed between the two cores with average F contents of 0.20 and 0.02 apfu. Vein and wall rock tourmaline chemistry data was collected from four cores. Tourmaline is dominantly foititc to scholitic in composition with less abundant dravitic species. All tourmaline measurements fall into the alkali and X-site vacant groups. No systematic zoning was observed in relation to distance from the pluton, but zoning within crystals was observed from base to tip in elongated grains and within adjacent domains of sector zoned tourmalines. Comparison of tourmaline in the contact metamorphic wall rock to vein tourmaline from the same core showed that vein tourmaline is enriched in Al and F compared to the wall rock tourmaline. Vein calcic amphibole chemistry data was collected from one core and was found to be actinolite to magnesiohornblende in composition. Apatite chemistry data was collected from two cores. The monovalent anion site showed variable composition between cores and at the grain scale, ranging between nearly end member F and OH apatite species in one core to OH-rich apatite with minor Cl (< 0.26 apfu) and negligible F in another. Plagioclase was observed in one core as Ca-rich (average An_(75)) vein selvages. Epidote from veins within the intrusive contained low abundances of rare earth elements (Ce < 0.17, Nd < 0.06, La < 0.11 apfu), but no relationship between Al and Fe~(+3) content and distance from the intrusive was observed. Chlorite chemistry data was collected from five cores and is dominantly clinochlore in composition with less abundant chamosite. Chlorite is found in veins across the entire core transect, from within the intrusive out into the geothermal reservoir and shows a trend of increasing Mg content with distance from the intrusive with Mg/(Mg+Fe) values ranging from 0.11 to 0.60. Vein titanite chemistry was collected from two cores and was found to be Al- and F-rich with average Al and F contents of 0.39 and 0.27 apfu respectively.