August 2009 Geology and GSA Today media ...( Boulder CO USA - GEOLOGY studies inc...)

Boulder, CO, USA - GEOLOGY studies include some curious associations: air hockey and plate tectonics; calcite and Earth's orbit; Columbia River Flood Basalts and the Congo Fan; and rock hyrax middens and global climate change. Also in Geology: briny eruptions on Mars; the only basin known to have both a marine and terrestrial P-T boundary; a major Silurian fish die-off; and mapping naturally occurring asbestos in the Sierra Nevada. GSA TODAY illustrates the Klondike goldfields and megafauna of Beringia.

High Cu grades in porphyry Cu deposits and their relationship to emplacement depth of magmatic sources
John M. Proffett, P.O. Box 772066, Eagle River, Alaska 99577, USA. Pages 675-678.

In porphyry copper (Cu) deposits, which produce most of the world's copper, Cu-sulfides are disseminated along small veins and fractures associated with small bodies of porphyritic-textured granitic rock. These are emplaced above larger bodies of granitic magma, which are the sources of both mineralizing fluids and the porphyritic rock. Average copper grades of these deposits are relatively low, but zones of higher grades contained within them are important economically, and for an understanding of their origin. The paper by Proffett identifies two types of higher-grade copper mineralization, one or the other of which may be dominant in different deposits. He shows that the type that is dominant correlates with the depth of emplacement of the underlying magmatic source. Deposits in which most high Cu grades are associated with small, early, granular quartz veins are found above magmatic sources emplaced to depths of about 4 km or less. Those in which most high Cu grades occur in narrow halos of altered rock along early fractures are located above magmatic sources emplaced to depths of about 5 km or more. Fluids with water, salt, and sulfur concentrations typical for magmatic sources would exist as separate brine and vapor phases at pressures corre
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