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R. I. Grauch

Publications and source records attributed to R. I. Grauch.

29 records · Page 2Linked to original sources
USGS1994

Cyclic variations of sulfur isotopes in Cambrian stratabound Ni-Mo-(PGE-Au) ores of southern China

Cyclic variations of δ 34 S values over a range of at least 48‰ in pyrite nodules from stratabound Ni-Mo-PGE(Au) ores of southern China are attributed to biogenic reduction of seawater sulfate in an anoxic, phosphogenic, and metallogenic basin. Cyclic introduction and mixing of normal seawater into typically stagnant basin waters led to extreme variations in δ 34 S values of aqueous sulfide species present at different times. Intermittent venting of metal-laden hydrothermal fluids into such a bacteriogenic sulfide-rich environment resulted in precipitation of metal sulfides as pseudomorphous replacements of organic debris and as sulfide sediments that record large δ 34 S CDT variations from −26 to +22‰. Apatite and silica dominated the replacement of the organic debris when metals were not being introduced into the basin. The combination of abundant organic debris, localized topographic basins for accumulation of the debris, bacterial production of sulfide species, and introduction of metal-bearing hydrothermal fluids provided the environment necessary to form these unusually rich Ni-Mo ores.

Geochimica et Cosmochimica Acta↗
USGS1994

Hogtuvaite, a new beryllian member of the aenigmatite group from Norway, with new X-ray data on aenigmatite

Hogtuvaite is a new beryllian member of the aenigmatite group that was discovered in Nordland County, Norway. It is a metamorphic mineral, hosted by Proterozoic granitic gneisses and mafic pegmatites of metamorphic origin. Compositional variations within and between gneiss-hosted samples of hogtuvaite are minimal; however, pegmatite-hosted samples of hogtuvaite are significantly different, containing less Al and Sn, and more Ti and Mn, than those from the gneisses. The mineralogical, optical and crystallographic properties of hogtuvaite are described. A new and uniquely indexed set of X-ray powder diffraction data for aenigmatite is presented. -from Authors

Canadian Mineralogist↗
USGS1992

Gold and platinum in shales with evidence against extraterrestrial sources of metals

Few black shales contain concentrations of precious metals higher than average continental crust (i.e. ???5 ppb Au). Yet Au and Pt alloys have been reported from the Kupferschiefer in Poland. Moreover, thin sulfide beds in certain Chinese and Canadian shales contain several hundred ppb Au, Pd and Pt and average ???4% Mo and ???2.5% Ni in an association that is difficult to explain. Volcanic and non-volcanic exhalations, hydrothermal epigenesis involving either igneous or sedex fluids, biogenic processes and low-temperature secondary enrichment are among the possible factors involved in deriving Ni, PGE and Au for black shales and sulfide beds in black shales. Extraterrestrial sources have been invoked in some cases (e.g., the Cambrian of China). However, available data on abundances of PGE indicate relatively low values for Ir (<0.02-2 ppb) in comparison with amounts for other PGE (up to 700 ppb Pt and 1255 ppb Pd). These data and high contents for Mo are not consistent with extraterrestrial sources of metals for Chinese shales and Ni-Mo-sulfide beds. Data are less complete for the U.S. shales, but nevertheless are suggestive of earthly origins for PGE. ?? 1992.

Chemical Geology↗
USGS1987

Age of uranium mineralization at the Jabiluka and Ranger deposits, Northern Territory, Australia: New U-Pb isotope evidence

The Ranger and Jabiluka uranium deposits are the largest in the Alligator Rivers uranium field, which contains at least 20 percent of the world's low-cost uranium reserves. Ore occurs in early Proterozoic metasediments, below an unconformity with sandstones of the 1.65-b.y.-old Kombolgie Formation. This study has used U-Pb isotope data from a large number of whole-rock drill core samples with a variety of mineral assemblages and textures. Ranger samples indicate a well-defined age of 1,737 + or - 20 m.y., consistent with earlier suggestions of the antiquity of this deposit. This age is distinctly pre-Kombolgie, so the Ranger deposit cannot have been formed by processes requiring its presence. The age is consistent, however, with mineralization related to heating associated with either the emplacement of early postmetamorphic granites, or possibly, with intrusion of the nearby Oenpelli Dolerite. In contrast, data for the least-altered Jabiluka ores yield a concordia intercept age of 1,437 + or - 40 m.y.--significantly younger than the Ranger age and also younger than the Kombolgie Formation. This age may correspond to a regional thermal event, as indicated both by mafic dikes of roughly this age and a zircon lower intercept age from a nearby granite-gneiss. Thus our data indicate that there must be at least three distinct periods of major U mineralization in the Alligator Rivers uranium field (including the previously determined approximately 900-m.y. age of the Nabarlek deposit), despite the fact that all of the deposits share strong similarities in their host rock, mineralogy, alteration, and geologic setting.Both Ranger and Jabiluka reflect a common, profound isotopic disturbance at about 400 to 600 m.y. This disturbance, which was especially pronounced at Jabiluka, may correspond to the development of basins and associated basalt flows to the west and southwest, as suggested by Crick et al. (1980).

Economic Geology↗
USGS1985

U-Pb isochron age and Pb isotope systematics of the Golden Fleece vein; implications for the relationship of mineralization to the Lake City caldera, western San Juan Mountains, Colorado

A U-Pb isochron age of 27.5 + or - 0.5 m.y. has been determined for the Golden Fleece vein, southwestern Colorado, which contains an assemblage of pitchblende and gold-silver tellurides unique within the Lake City area. Initial Pb isotope ratios of ore samples from the vein ( 206 Pb/ 204 Pb = 19.08-19.58; 207 Pb/ 204 Pb = 15.643-15.682; 208 Pb/ 204 Pb = 38.533-38.84) are among the most radiogenic values reported from the San Juan Mountains and suggest that a significant component of common Pb, and possibly other metals, was derived from upper crustal Precambrian rocks by deeply circulating hydrothermal fluids. The 27.5-m.y. age of the Golden Fleece vein is identical, within analytical uncertainty, to the age of the quartz latite lavas that it cuts. This clearly documents the relationship of the vein to the waning stages of activity associated with the Uncompahgre caldera (27-29 m.y.) rather than to early activity related to the 23.1-m.y.-old Lake City caldera. Other veins in the Lake City area have Pb isotope ratios and mineral assemblages that are distinctly different from the Golden Fleece, and most of these veins are thought to be genetically related to the younger Lake City caldera.

Economic Geology↗