Norman J Page

Serpentinization at Burro Mountain, California

AbstractThe Burro Mountain ultramafic complex, Monterey County, California, consists of dunites and peridotites which are partially or wholly serpentinized. Primary minerals in both rock types are olivine, enstatite, diopside, and picotite which upon alteration yield chrysotile, lizardite, brucite, magnetite, talc, tremolite, and carbonate. Electron microprobe analyses show that enstatite, En85.8 to En90.8, alters to “bastite” composed only of lizardite (5.0–12.0 weight percent FeO), whereas olivine, Fo90.8 to Fo91.6, forms lizardite+chrysotile+brucite with or without magnetite. The chrysotile ranges from 3.0 to 5.0 weight percent FeO, the brucite from 16.0 to 43.0 weight percent FeO. As Serpentinization proceeds, the alteration products are enriched in FeO relative to MgO.Serpentinization probably originates in a changing

Serpentinization and alteration in an olivine cumulate from the Stillwater Complex, Southwestern Montana

P_{O_2 }

Modeling surficial sand and gravel deposits

-T environment by two different reactions:(a)Olivine+enstatite+H2O+O2⇄Mg, Fe+2 chrysotile+Mg, Fe+3, Fe+2 lizardite with or without magnetite.(b)Olivine+H2O+O2⇄Fe+2, Mg brucite+Mg, Fe+2 chrysotile+Mg, Fe+2, Fe+3 lizardite with or without magnetite. The presence of Fe (OH)2 in brucite indicates that temperatures of Serpentinization may be lower than temperature heretofore inferred. This is suggested by thermodynamic calculations, assuming ideal solid solution, and relating the substitution of Fe+2 for Mg+2 in brucite to the stability field of brucite.

Platinum-Group Element Mineralogy of the Pole Corral Podiform Chromite Deposit, Rattlesnake Creek Terrane, Northern California

Abstract Samples of 20 chromitite, 14 ultramafic and mafic rock, and 9 laterite and soil samples from the Precambrian Sukinda and Nausahi ultramafic complexes, Orissa, India were analyzed for platinum-group elements (PGE). The maximum concentrations are: palladium, 13 parts per billion (ppb); platinum, 120 ppb; rhodium, 21 ppb; iridium, 210 ppb; and ruthenium, 630 ppb. Comparison of chondrite-normalized ratios of PGE for the chromitite samples of lower Proterozoic to Archean age with similar data from Paleozoic and Mesozoic ophiolite complexes strongly implies that these complexes represent Precambrian analogs of ophiolite complexes. This finding is consistent with the geology and petrology of the Indian complexes and suggests that plate-tectonic and ocean basin developement models probably apply to some parts of Precambrian shield areas.

Platinum-Group Element Analyses of Serpentinites in the Eastern Central Alps, from Davos (Switzerland) to the Valmalenco (Italy)

Some of the olivine cumulates of the Ultramafic zone of the Stillwater Complex, Montana, are progressively altered to serpentine minerals and thompsonite. Lizardite and chrysotile developed in the cumulus olivine and postcumulus pyroxenes; thompsonite developed in postcumulus plagioclase. The detailed mineralogy, petrology, and chemistry indicate that olivine and plagioclase react to form the alteration products, except for H2O, without changes in the bulk composition of the rocks.

PALLADIUM, PLATINUM, RHODIUM, RUTHENIUM AND IRIDIUM IN PERIDOTITES AND CHROMITITES FROM OPHIOLITE COMPLEXES IN NEWFOUNDLAND.

Analyses of platinum-group elements (PGE) in rocks collected from the Voikar-Syninsky ophiolite in the Polar Urals suggest that the distribution and geochemistry of PGE in this Paleozoic ophiolite are similar to those in Mesozoic ophiolites from elsewhere. Chondrite-normalized PGE patterns for chromitite, the tectonite unit, and ultramafic and mafic cumulate unit have negative slopes. These results are similar to those found for chromitites from other ophiolites; stratiform chromities show positive slopes. If the magmas that form both types of chromitite originate from similar mantle source material with respect to PGE content, the processes involved must be quite different. However, the distinct chondrite-normalized PGE patterns may reflect differing source materials.

Palladium, platinum, rhodium, ruthenium, and iridium in chromitites from the Massif du Sud and Tiebaghi Massif, New Caledonia

Mineral-deposit models are an integral part of quantitative mineral-resource assessment. As the focus of mineral-deposit modeling has moved from metals to industrial minerals, procedure has been modified and may be sufficient to model surficial sand and gravel deposits. Sand and gravel models are needed to assess resource-supply analyses for planning future development and renewal of infrastructure. Successful modeling of sand and gravel deposits must address (1) deposit volumes and geometries, (2) sizes of fragments within the deposits, (3) physical characteristics of the material, and (4) chemical composition and chemical reactivity of the material. Several models of sand and gravel volumes and geometries have been prepared and suggest the following: Sand and gravel deposits in alluvial fans have a median volume of 35 million m3. Deposits in all other geologic settings have a median volume of 5.4 million m3, a median area of 120 ha, and a median thickness of 4 m. The area of a sand and gravel deposit can be predicted from volume using a regression model (log [area (ha)] =1.47+0.79 log [volume (million m3)]). In similar fashion, the volume of a sand and gravel deposit can be predicted from area using the regression (log [volume (million m3)]=−1.45+1.07 log [area (ha)]). Classifying deposits by fragment size can be done using models of the percentage of sand, gravel, and silt within deposits. A classification scheme based on fragment size is sufficiently general to be applied anywhere.

Distribution of platinum-group elements in the Bati Kef chromite deposit, Guleman-Elazig area, eastern Turkey

The Pole Corral deposit (Red Mountain group) was selected for mineralogic studies because it contained some of the highest platinum-group element (PGE) contents found in a geochemical survey of 280 podiform chromite deposits in California and Oregon. The deposit is located about 11 airline km southwest from the town of Beegum, CA. It occurs in serpentinized dunite hosted by serpentinized harzburgite, part of a disrupted ophiolite, in the fault bounded Rattlesnake Creek terrane, which is a subdivision of the western Palaeozoic and Triassic belt of the Klamath Mountains province. Chromitite crops out on the walls of a prospect pit and consists of 25–50 mm, thick layers in dunite, disrupted by faulting and shearing, with an aggregate length of about 12 m. In 11 samples of chromitite, Pd, Pt, Rh, Ir and Ru, contents range from 1 to 15, 35 to 2530, 3 to 74, 70 to 2930, and 70 to 4930 ppb, respectively, and average 4·3, 271, 23, 999, and 1909 ppb, respectively. The mineralogy also indicates Os is present. Increased PGE contents are not associated with increased Cu (maximum 7ppm) or Ni (maximum 1500 ppm) contents, which suggests that the chromitites are poor in base-metal sulphides.