Geology topics
Miles L. Silberman
Publications and source records attributed to Miles L. Silberman.
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.
Genesis of gold vein mineralization in an Upper Cretaceous turbidite sequence, Hope-Sunrise District, southern Alaska
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Proceedings of the Symposium on Mineral Deposits of the Pacific Northwest: Geological Society of America, Cordilleran Section meeting at Corvallis, Oregon, March 20-21, 1980
Stratiform and volcanogenic zinc-lead-barium deposits occur within the pelagic Kagvik sequence in the Red Dog Creek and Drenchwater Creek areas in the DeLong Mountains and Howard Pass quadrangles. The sulfide deposits occur together with barite, tuff, sandstone, shale, chert, and keratophyre flows and sills. The three main occurrences of the stratiform zinc-lead-barium deposits in the Red Dog Creek area are: (1) thinly-bedded stratiform sulfide minerals in organic-rich Mississippian shale and chert; (2) massive sulfide vein and breccia fillings in Mississippian shale; and (3) massive stratiform barite lenses and nodules in chert and shale of the Siksikpuk Formation. The Drenchwater Creek area has only the first type. Sulfide-bearing zones of up to several thousand meters long occur in both areas with up to 19.5 percent Zn, 9.5 percent Pb, and locally more than 150 ppm Ag. Radiolaria from sulfide-bearing chert indicate in the Red Dog Creek area a Mississippian age of sulfide deposition. K/Ar ages of 319 + 10 m.y. and 330 + 17 m.y. obtained from biotite in keratophyre associated with sulfide-bearing tuff and chert in the Drenchwater Creek area indicate a Mississippian age of sulfide deposition. Sulfur isotope analyses of sulfur-bearing minerals from the Red Dog Creek area indicate that a combination of sea water sulfate and deep-seated sulfur was probably the source of the barite sulfate whereas sulfur in the sulfide minerals may have had a deep-seated source. Sulfur isotope values for sphalerite and galena are similar in vein and stratiform occurrences. Using sulfur isotope geothermetry, spalerite-galena pairs yield paleotemperatures of between 115° and 305° C. Lead isotope analyses of galena from both the Red Dog Creek and Drenchwater Creek areas yield Triassic model ages of approximately 200 m.y. Lead isotope data from both areas approximate typical orogene, i.e., Andean-type arc, or mature island-arc values and exclude ocean floor rifting for the generation of lead. The isotopic, petrologic, and field data indicate the stratiform zinc-lead-barium deposits in the Red Dog Creek and Drenchwater Creek areas formed during a short-lived period of Mississippian submarine volcanism in either an incipient Andean-type arc, or mature island arc environment.
New ages on intrusive rocks and altered zones in the Alaska Peninsula: A section in The United States Geological Survey in Alaska: Accomplishments during 1977
Preliminary potassium-argon dating of intrusive rocks and altered zones in the Chignik and Sutwik Island quadrangles of the Alaska Peninsula seems to indicate at least three and possibly four Tertiary ages of alteration and mineralization.
New potassium-argon data on the age of mineralization and metamorphism in the Willow Creek mining district, southern Talkeetna Mountains, Alaska
The now largely abandoned Willow Creek mining district, southern Talkeetna Mountains, Alaska, produced nearly $18,000,000 in gold and minor silver between 1909 and the early 1950's. Mineralized quartz veins, which contain gold and silver along with minor quantities of base metals (in pyrite, galena, chalcopyrite, sphalerite, molybdenite, and arsenopyrite), cut Late Cretaceous and early Tertiary tonalite and quartzmica schist of probable Jurassic age (Ray, 1954; Silberman and others, 1976; Bela Csejtey, Jr., unpub. data, 1978).
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.
Radiometric ages of some Cretaceous and Tertiary volcanic and intrusive rocks in South-central Arizona
Reconnaissance geologic mapping and radiometric ages provide the framework for correlation of Cretaceous and middle Tertiary volcanic and clastic rocks in the Vaca Hills and Eloy quadrangles, Arizona. The radiometric ages also record a middle Tertiary thermal event of much greater extent than .suggested by the volume of middle Tertiary igneous rocks present in outcrop.
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.
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.
Geochemical anomalies in the Willow Creek mining district, southern Talkeetna Mountains, Alaska
No abstract available.
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.
Cauldron subsidence of Oligocene age at Mount Lewis, northern Shoshone Range, Nevada
No abstract available.
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).
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.
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.