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Miles L. Silberman

Publications and source records attributed to Miles L. Silberman.

13 recordsLinked to original sources
USGS1985

Geologic characteristics of sediment- and volcanic-hosted disseminated gold deposits - Search for an occurrence model

The current expansion of resource information, particularly on "disseminated" gold, and the improved technologies now available for resource investigations should place us in an enhanced position for developing a better predictive methodology for meeting one of the important responsibilities of the U.S. Geological Survey-to examine and assess the mineral resources of the geologic terranes composing the public (and privately owned) lands of the United States. The first step is systematic organization of these data. Geologic-occurrence models are an effective systematic method by which to organize large amounts of resource information into a logical sequence facilitating its use more effectively in meeting several industry and Survey objectives, which include the exploration for resources and the assessment of resource potential for land-use decisions. Such models also provide a scientific basis for metallogenesis research, which considers the observable features or attributes of ore occurrence and their "fit" into the Earth's resource puzzle. The use of models in making resource assessments/appraisals was addressed by Shawe (1981), who reported the results of a workshop on methods for resource appraisal of Wilderness and Conterminous United States Mineral Appraisal Program (CUSMAP; 1:250,000-scale quadrangles) areas. The Survey's main objective in the 1982 workshop was to evaluate the status of knowledge about disseminated or very fine grained gold deposits and, if possible, to develop an occurrence model(s). This report on the workshop proceedings has three main objectives: (1) Education through the publication of a summary review and presentation of new thinking and observations about the scientific bases for those geologic processes and environments that foster disseminated gold-ore formation; (2) systematic organization of available geologic, geochemical, and geophysical information for a range of typical disseminated gold deposits (including recognition of gaps in those data); and (3) assessment of current understanding (as presented in objective 2) toward formulating an empirical ore-occurrence model for this type of deposit. As such, this volume represents a preliminary first step at classification and provides a source of pertinent background information. Readers of this volume will soon discover, however, that full agreement has not yet been achieved in the interpretation of some of the geologic evidence. The resulting variations in tentative occurrence models for these controversial deposits ultimately will be resolved by filling the gaps in information that have already been identified. Thus, this volume does not report a U.S. Geological Survey consensus; the conclusions expressed in each chapter represent the particular interpretations of the various workshop participants.

Arizona, California, Idaho, Nevada, Utah↗
USGS1978

Age and composition of igneous rocks, Edna Mountain quadrangle, Humboldt County, Nevada

Six pulses of igneous activity ranging in age from Jurassic to Pliocene have been identified in the Edna Mountain quadrangle, Humboldt County, Nev. Porphyritic syenite am! quartz monzonite of Jurassic age (146-164 million years) at Buffalo Mountain are highly potassic through a wide range in SiO 2 content from olivine-bearing syenite to quartz-rich monzonite, and their composition contrasts sharply with plutons elsewhere in north-central Nevada. Granodiorite and quartz monzonite plutons of Cretaceous age (88- 106 m.y.) are chemically and mineralogically similar to other calc-alkaline plutons in north-central Nevada. Four episodes of Tertiary volcanism include rhyolite ashflow tuffs and slightly younger andesitic basalt flows and tuffs of Oligocene age, rhyolite vitrophyre of late Miocene age, and olivine basalt flows of Pliocene age. Their age and mineralogical and chemical compositions are similar to other Tertiary volcanic rocks in north-central Nevada.

Nevada↗
USGS1978

Comendite (peralkaline rhyolite) and basalt in the Mitu Group, Peru: Evidence for Permian-Triassic lithospheric extension in the central Andes

The Mitu Group consists of generally coarse clastic strata and volcanic rock of Permian and (or) Triassic age filling elongate basins that parallel the general structural trend of the Peruvian Andes. Volcanic rocks of the Mitu Group include peralkaline and nonperalkaline rhyolite and subalkaline basalt. To our knowledge, the peralkaline rhyolites are the first of this rock type reported from South America. The presence of appreciable volumes of peralkaline rhyolite and basalt, supports the interpretation that the Mitu Group was deposited in major graben structures that resulted from lithospheric extension produced by rifting or possible backarc extension.

central Andes Mountains↗
USGS1977

Cauldron subsidence of Oligocene age at Mount Lewis, Shoshone Range, Nevada: A reasonable interpretation

James Gilluly has rejected the interpretation of Wrucke and Silberman (U.S. Geol. Survey Prof. Paper 876, 1975) that a thrust fault and tear fault mapped by Gilluly and Gates (U.S. Geol. Survey Prof. Paper 465, 1965) as structures bounding the upper plate of the Roberts Mountains thrust at Mount Lewis are parts of a ring fracture around an area that underwent volcanic collapse. In his discussion (this volume) of our paper, Gilluly fails to consider important questions that we presented in support of the subsidence hypothesis. Instead of answering these critical questions, Gilluly merely recapitulates the interpretations that he and Gates gave in Professional Paper 465. We presented new information, including a map of one critical area along the cauldron boundary where, among other significant differences in geologic interpretation, we found the ring fault where previously no steep fault was shown. Gilluly believes that the paucity of dikes along the ring fracture at Mount Lewis is highly anomalous for cauldrons. However, the amount of dike rock is comparable to that in known cauldrons (some cauldrons have none) and is what might be expected at high levels in subsidence structures that have undergone relatively little resurgent igneous activity after collapse. Gilluly concludes that in formulating our interpretation of volcanic collapse, we have ignored much evidence that he and Gates have presented on thrust faulting; in the Shoshone Range. On the contrary, we have considered their ideas and have reinterpreted them using new evidence that strongly supports the concept of cauldron subsidence at Mount Lewis.

Nevada↗
USGS1976

Potassium-argon ages of basement rocks from St. George Island, Alaska

St. George Island is one of the Pribilof Islands which lie between 56°35' and 57°11' N. lat. in the Bering Sea, 350 km north of the Aleutian chain. The islands are situated near the margin of the continental platform that underlies most of the northern half of the Bering Sea (fig. 1). The islands are made up mostly of olivine basalt and basanite flows, pillow breccias, pyroclastic deposits, sills and dikes, most of which are nepheline normative (Barth, 1956). The volcanic rocks are late Pliocene to Holocene in age (Cox and others, 1966; D. M. Hopkins and M. L. Silberman, unpub. data) and are interbedded with marine sand and gravel, glacially derived sediments, frost breccia, and windblown sand and silt.

Alaska↗
USGS1975

The Oligocene volcanic center at Eureka, Nevada

A volcanic center covering an area of about 80 km 2 near Eureka, Nev., and active in the early Oligocene, is characterized by rhyolitic, rhyodacitic. and andesitic pyroclastic rocks, lava flows, and shallow intrusive bodies. These rocks were emplaced as intertonguing and interpenetrative units during a 5-m.y. interval; most of the volcanism was in the last 3 million years of this period (36 to 33 m.y. ago).

Nevada↗
USGS1974

K-Ar Age Relations of Granodiorite Emplacement and Tungsten and Gold Mineralization near the Getchell Mine, Humboldt County, Nevada

A granodiorite stock intrudes complexly folded and thrust-faulted Paleozoic sedimentary rocks in the Osgood Mountains of eastern Humboldt County, Nevada. Within the metamorphic aureole surrounding the pluton, the sedimentary rocks are converted to cordierite hornfels and marble; tungsten-bearing tactites developed along the contacts of the granodiorite. Cutting the granodiorite and sedimentary rocks is the Getchell fault, along which the disseminated gold ore bodies of the Getchell mine are localized. K-Ar ages of the granodiorite, andesite porphyry, tungsten-bearing tactites, and altered granodiorite from the Getchell mine indicate that emplacement, alteration, and mineralization are all part of a magmatic-thermal episode which took place approximately 90 m.y. ago.

Nevada↗
USGS1970

Geochronology of Tertiary igneous rocks in central Nevada

Potassium-argon dating of Tertiary igneous rocks in Lander County, central Nevada, indicates that igneous activity was episodic and can be separated into three periods. Igneous activity started abruptly about 37 m.y. ago with local extrusion of andesitic to quartz-latitic lava flows and intrusion of hypabyssal rocks of similar composition. This activity ceased about 33 m.y. ago and was followed by extrusion of rhyolite ash-flow sheets that blanketed large parts of the region. These ash-flow sheets range from about 34 to 22 m.y. in age. The final phase, represented by basalt and basaltic-andesite flows and intrusive rhyolite flow-dome complexes, took place about 16 to 10 m.y. ago. Andesitic to dacitic lava and hypabyssal rocks about 35 m.y. old are widespread east of Lander County and rhyolitic ash-flow tuffs 34 to 20 m.y. old are found south and east of Lander County. The younger (16 to 10 m.y.) basalt and basaltic-andesite flows are related to volcanism of the Snake River plain province to the north. The precision of the ages was evaluated by means of: (1) repeat analyses of the same mineral separate, (2) age determination of mineral pairs from the same hand specimen, and (3) age determinations on widely spaced samples from the same geologic body or formation. The last method seems most meaningful from a geologic point of view.

Nevada↗