USGS Science⌕ Search

Geology topics

Ted G. Theodore

Publications and source records attributed to Ted G. Theodore.

23 records · Page 2Linked to original sources
USGS1971

Ore fluids in the porphyry copper deposit at Copper Canyon, Nevada

The large, low-grade copper and gold deposit at Copper Canyon, Lander County, Nevada, formed in the contact metasomatic environment adjacent to a shallow Tertiary intrusion. Vein and disseminated chalcopyrite-pyrite-pyrrhotite-arsenopyrite mineralization, with lesser amounts of gold, galena, sphalerite, marcasite, and siderite occur in the Upper Cambrian Harmony and Middle Pennsylvanian Battle Formations. Studies of fluid inclusions in stages of vein quartz and in healed fractures through quartz phenocrysts and pebbles reveal the presence of very saline, sometimes CO 2 -rich fluids in the early and middle stages of mineralization. Ore fluid salinities during the base metal mineralization were approximately 40 wt percent, and temperatures were near 375 degrees C. Circulation of this hot brine was apparently restricted to an elongate highly fractured zone within 3,000 feet laterally of the intrusive. Later fluids in this central zone were somewhat cooler, near 300 degrees C, and had significantly lower salinities in the range 12 to 1.2 wt percent. Smaller lode deposits approximately 1 to 5 miles from the intrusion appear to have formed from low to moderate salinity fluids at temperatures generally in the range 250 to 335 degrees C. Geologic and fluid considerations suggest formation at approximately 6,000 feet in depth.

Nevada↗
USGS1970

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.

California↗