Ted G. Theodore

Petrogenesis of Mylonites of High Metamorphic Grade in the Peninsular Ranges of Southern California

A fairly continuous, narrow belt of mylonite gneisses extends approximately 60 miles across southern California and crops out prominently at Coyote Mountain, near Borrego Springs, San Diego County. At Coyote Mountain, both prebatholithic rocks and igneous rocks lithologically similar to rocks from the nearby southern California batholith have been deformed in the mylonite zone—a deformation that is the last plutonic event recorded in the rocks. Petrographic evidence within these mylonites at Coyote Mountain indicates that sillimamte-K feldspar-muscovite-quartz assemblages remained stable or recrystallized (or both) during mylonitization. In addition, a maximum “set” temperature of 580° C, inferred from the MgCO 3 content of calcite in deformed dolomite marbles, was determined. The physical conditions probable at the metamorphic peak accompanying mylonitization are: P-T conditions inferred from experimentally studied systems, T = 580° to 660° C and Pxotai = PH, O( ? ) = 3.4 to 7.0 kb; high H 2 O activity, as indicated by the continued stability of muscovite at such high temperatures. Mylonitization within this belt may be related to a rise of magma to higher crustal levels from within the southern California batholith.

Particles in fluid inclusions from Yellowstone National Park—Bacteria?

Micrometre-sized particles resembling microorganisms have been found in liquid-rich fluid inclusions in hydrothermal quartz crystals from a research drill hole in Yellowstone National Park, Wyoming. The particles are moving, apparently in Brownian motion. The 1-2-mm-long quartz crystals were obtained from fractures in late Pleistocene rhyolite at depths of 59.5,102.1, and 102.5 m. The measured temperature and pressure at 102.1 m, where about 99% of the moving particles were found, were about 190 °C and 1.24 MPa, respectively. Homogenization temperatures (190–280 °C) of fluid inclusions at this depth mostly exceed measured temperatures, which suggests that the fluid inclusions probably did not form under present-day conditions. Instead, the fluid inclusions probably formed ⩾45 000–14 000 B.P. at a much higher pressure owing to the weight of at least 490 m of glacial ice. Only 3 of the more than 200 quartz crystals examined contain up to several hundred rodlike, threadlike, or irregular-shaped moving particles. Fluid inclusions in 17 other quartz crystals each contain one or two moving particles that were observed only because of their continuous Brownian motion.

Geochemical and fluid zonation in the skarn environment at the tomboy—minnie gold deposits, Lander County, Nevada

The Tomboy—Minnie gold deposits are related to the middle Tertiary porphyry copper system centered at Copper Canyon. Gold-silver ores in the deposits occur mostly in a pyrrhotite- and pyrite-rich basal 30-m-thick sequence of altered calcareous conglomerate belonging to the Middle Pennsylvanian Battle Formation. The entire mineralized system contained at least 3.3 million troy oz gold before large-scale mining operations began. Alteration in the Tomboy—Minnie deposits includes actinolite- and chlorite-dominant assemblages, in marked contrast to the skarn, potassic, and phyllic assemblages characterizing the copper-gold-silver deposits of the system. Introduction of gold occurred penecontemporaneously with replacement of early diopside-alteration assemblages by actinolite and chlorite. Metals are zoned strongly in the Copper Canyon system: the West and East ore bodies occur in a copper-gold-silver zone that is followed outward by a gold-silver zone which includes the Tomboy deposit and in turn, is succeeded by a lead-zinc-silver zone. Locations of drill holes that have Au/Ag assay ratios of ⩾ 1 clearly outline the Tomboy—Minnie deposits within an area of rocks with Au/Ag ratios of ≈ 0.5. Fluid-inclusion studies suggest wide variations in temperature and chemistry prevailed in the fluids associated with mineralization at the Tomboy. Early fluids associated with diopsidequartz assemblages probably were dominantly CaCl2-rich brines and were boiling at temperatures higher than 500°C. These fluids were progressively enriched in sodium and potassium over time, and during the hydrosilicate stages, temperatures probably ranged from 320 to 500°C at the time actinolite formed, and from 220 to 320°C at the time chlorite was dominant. Sulfur isotopic data suggest that sulfur, mostly from a magmatic or deep-seated crustal source, was transported by hydrothermal fluids as aqueous H2S with a δ 34S of about 4 ± 1‰ to the West, East, and Tomboy deposits.

Sedimentology of the Pennsylvanian and Permian Strathearn Formation, Northern Carlin Trend, Nevada; with a section on microfossil controls on the age of the Strathearn Formation

2001 This repon is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards or with the North American Stratigraphic Code. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S.