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M.L. Silberman

Publications and source records attributed to M.L. Silberman.

At least 19 recordsLinked to original sources
USGS1995

Geology of epithermal silver-gold bulk-mining targets, bodie district, Mono County, California

The Bodie mining district in Mono County, California, is zoned with a core polymetallic-quartz vein system and silver- and gold-bearing quartz-adularia veins north and south of the core. The veins formed as a result of repeated normal faulting during doming shortly after extrusion of felsic flows and tuffs, and the magmatic-hydrothermal event seems to span at least 2 Ma. Epithermal mineralization accompanied repeated movement of the normal faults, resulting in vein development in the planes of the faults. The veins occur in a very large area of argillic alteration. Individual mineralized structures commonly formed new fracture planes during separate fault movements, with resulting broad zones of veinlets growing in the walls of the major vein-faults. The veinlet swarms have been found to constitute a target estimated at 75,000,000 tons, averaging 0.037 ounce gold per ton. The target is amenable to bulkmining exploitation. The epithermal mineralogy is simple, with electrum being the most important precious metal mineral. The host veins are typical low-sulfide banded epithermal quartz and adularia structures that filled voids created by the faulting. Historical data show that beneficiation of the simple vein mineralogy is very efficient. ?? 1995 Oxford University Press.

Nonrenewable Resources↗
USGS1993

Alteration and geochemical zoning in Bodie Bluff, Bodie mining district, eastern California

Banded, epithermal quartz-adularia veins have produced about 1.5 million ounces of gold and 7 million ounces of silver from the Bodie mining district, eastern California. The veins cut dacitic lava flows, pyroclastic rocks and intrusions. Sinter boulders occur in a graben structure at the top of Bodie Bluff and fragments of sinter and mineralized quartz veins occur in hydrothermal breccias nearby. Explosive venting evidently was part of the evolution of the ore-forming geothermal systems which, at one time, must had reached the paleosurface. Previous reconnaissance studies at Bodie Bluff suggested that the geometry of alteration mineral assemblages and distribution of some of the major and trace elements throughout the system correspond to those predicted by models of hot-spring, volcanic rock hosted precious metal deposits (Silberman, 1982; Silberman and Berger, 1985). The current study was undertaken to evaluate these sugestions further.

Journal of Geochemical Exploration↗
USGS1983

Isotopic studies of mariposite-bearing rocks from the south- central Mother Lode, California.

Gold-bearing vein formation in the Mother Lode belt of the study area apparently occurred during the Early Cretaceous between 127 and 108 m.y. B.P. The hydrothermal fluids that carried the gold precipitated quartz and mariposite at approx 320oC, similar to the T of precipitation of gold-bearing quartz veins in the Allegheny district. The O- and H-isotopic composition calculated for the fluid indicate that it was similar to formation water or was metamorphic in origin. If the carbonate in the veins was in isotopic equilibrium with this same fluid, it apparently precipitated at a higher T of approx 400oC. The Sr in the carbonate is much less radiogenic than that in any known marine carbonate, but is similar in isotopic composition to that in metamorphosed mafic volcanic rocks of the general region. These mafic rocks could have been the source for the Sr in the hydrothermal veins. This observation supports the contention that the gold-mariposite-quartz-carbonate rocks were formed as an alteration product of serpentinite and other mafic igneous rocks.-A.P.

California Geology↗
USGS1982

K-Ar geochronology of basement rocks on the northern flank of the Huancabama deflection, Ecuador

The Huancabamba deflection, a major Andean orocline located at the Ecuador-Peru border, constitutes an important geologic boundary on the Pacific coast of South America. Crust to the north of the deflection is oceanic and the basement is composed of basic igneous rocks of Cretaceous age, whereas crust to the south is continental and felsic rocks of Precambrian to Cretaceous age make up the basement. The northern flank of the Huancabamba Deflection in El Oro Province, Ecuador, is underlain by Precambrian polymetamorphic basic rocks of the Piedras Group; shale, siltstone, sandstone, and their metamorphosed equivalents in the Tahuin Group (in part of Devonian age); concordant syntectonic granitic rocks; quartz diorite and alaskite of the Maroabeli pluton; a protrusion of serpentinized harzburgite that contains a large inclusion of blueschist-facies metamorphic rocks, the Raspas Formation, and metamorphic rocks north of the La Palma fault. Biotite from gneiss of the Tahuin Group yields a Late Triassic K-Ar age (210 ? 8 m.y.). This is interpreted as an uplift age and is consistent with a regional metamorphism of Paleozoic age. A nearby sample from the Piedras Group that yielded a hornblende K-Ar age of 196 ? 8 m.y. was affected by the same metamorphic event. Biotite from quartz diorite of the mesozonal Maroabeli pluton yields a Late Triassic age (214 ? 6 m.y.) which is interpreted as an uplift age which may be only slightly younger than the age of magmatic crystallization. Emplacement of the pluton may postdate regional metamorphism of the Tahuin Group. Phengite from politic schist of the Raspas Formation yields an Early Cretaceous K-Ar age (132 ? 5 m.y.). This age is believed to date the isostatic rise of the encasing serpentinized harzburgite as movement along a subjacent subduction zone ceased, and it is synchronous with the age of the youngest lavas of a coeval volcanic arc in eastern Ecuador. A Late Cretaceous K-Ar age (74.4 ? 1.1 m.y.) from hornblende in amphibolite north of the La Palma fault shows that rocks there are distinct from the superficially similar rocks of the Tahuin Group to the south. Biotite from schist in the Eastern Andean Cordillera yields an Early Eocene age (56.6 ? 1.6 m.y.). Metamorphic rocks in the northern part of the Eastern Andean Cordillera are Cretaceous in age and were metamorphosed in part in early Tertiary time. They are unrelated to and were metamorphosed later than any of the diverse rocks exposed on the northern flank of the Huancabamba Deflection.

Open-File Report↗
USGS1980

Metallogenic and tectonic significance of oxygen isotope data and whole-rock potassium-argon ages of the Nikolai Greenstone, McCarthy Quadrangle, Alaska

The Middle and (or) Late Triassic Nikolai Greenstone, part of the allochthonous terrane of Wrangellia, is typically altered and locally metamorphosed to prehnite-pumpellyite facies with chlorite and epidote as the most common secondary minerals. Intrinsic copper content averages 155 ppm, and two types of concentrations of copper in the Nikolai are common: (1) native copper fillings of amygdules and rubble zones typically near flow tops, and (2) veins and thin replacement zones that contain native copper and copper-iron sulfides in quartz-epidote or calcite gangue in faults and fractures. Oxygen isotope data from quartz and epidote from three copperbearing veins yield calculated ore fluid temperatures of approximately 200°C and 6180 of approximately +1 per mil in agreement with a metamorphicsegregation origin of these deposits, as suggested by Sinclair (1977). Seven K-Ar ages of chloritized greenstone, including those adjacent to veins fall on an initial argon diagram with a zero intercept and a slope which yields an isochron age of 112 + 11 m.y. The ages define a Cretaceous thermalmetamorphic episode which is responsible for alteration and mineralization. The episode is younger than a major Jurassic progeny, accompanied by granitic intrusion, in the area, and appears to be unaffected by minor granitic intrusion in the middle to late Tertiary. We believe the Cretaceous event is related to accretion of Wrangellia to its present relative position in North America. This age of accretion agrees with stratigraphic and structural evidence cited by other workers.

Alaska↗