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Effects of Mother Lode-Type Gold Mineralization on ¹⁸⁷Os/¹⁸⁸Os and Platinum Group Element Concentrations in Peridotite: Alleghany District, California

Richard J. Walker

Department of Geology, University of Maryland, College Park, Maryland, 20742

John Karl Böhlke

U.S. Geological Survey, 431 National Center, Reston, Virginia 20192

William F. McDonough and Ji Li

Department of Geology, University of Maryland, College Park, Maryland, 20742

Corresponding author: e-mail, rjwalker{at}geol.umd.edu

Osmium isotope compositions and concentrations of Re, platinum group elements (PGE), and Au were determined for host peridotites (serpentinites and harzburgites) and hydrothermally altered ultramafic wall rocks associated with Mother Lode-type hydrothermal gold-quartz vein mineralization in the Alleghany district, California. The host peridotites have Os isotope compositions and Re, PGE, and Au abundances typical of the upper mantle at their presumed formation age during the late Proterozoic or early Paleozoic. The hydrothermally altered rocks have highly variable initial Os isotope compositions with _Os values (% deviation of ¹⁸⁷Os/¹⁸⁸Os from the chondritic average calculated for the approx. 120 Ma time of mineralization) ranging from –1.4 to –8.3. The lowest Os isotope compositions are consistent with Re depletion of a chondritic source (e.g., the upper mantle) at ca. 1.6 Ga. Most of the altered samples are enriched in Au and have depleted and fractionated abundances of Re and PGE relative to their precursor peridotites.

Geochemical characteristics of the altered samples suggest that Re and some PGE were variably removed from the ultramafic rocks during the mineralization event. In addition to Re, the Pt and Pd abundances of the most intensely altered rocks appear to have been most affected by mineralization. The ¹⁸⁷Os-depleted isotopic compositions of some altered rocks are interpreted to be a result of preferential ¹⁸⁷Os loss via destruction of Re-rich phases during the event. For these rocks, Os evidently is not a useful tracer of the mineralizing fluids. The results do, however, provide evidence for differential mobility of these elements, and mobility of ¹⁸⁷Os relative to the initial bulk Os isotope composition during hydrothermal metasomatic alteration of ultramafic rocks.