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USGS2000

Water-quality characteristics for selected streams in Lawrence County, South Dakota, 1988-92

During the 1980’s, significant economic development and population growth began to occur in Lawrence County in the northern part of the Black Hills of western South Dakota. Rising gold prices and heap-leach extraction methods allowed the economic recovery of marginal gold ore deposits, resulting in development of several large-scale, open-pit gold mines in Lawrence County. There was increasing local concern regarding potential impacts on the hydrologic system, especially relating to the quantity and quality of water in the numerous streams and springs of Lawrence County. In order to characterize the water quality of selected streams within Lawrence County, samples were collected from 1988 through 1992 at different times of the year and under variable hydrologic conditions. During the time of this study, the Black Hills area was experiencing a drought; thus, most samples were collected during low-flow conditions. Streamflow and water-quality characteristics in Lawrence County are affected by both geologic conditions and precipitation patterns. Most streams that cross outcrops of the Madison Limestone and Minnelusa Formation lose all or large part of their streamflow to aquifer recharge. Streams that are predominantly spring fed have relatively stable streamflow, varying slightly with dry and wet precipitation cycles. Most streams in Lawrence County generally have calcium magnesium bicarbonate type waters. The sites from the mineralized area of central Lawrence County vary slightly from other streams in Lawrence County by having higher concentrations of sodium, less bicarbonate, and more sulfate. False Bottom Creek near Central City has more sulfate than bicarbonate. Nitrogen, phosphorous, and cyanide concentrations were at or near the laboratory reporting limits for most sites and did not exceed any of the water-quality standards. Nitrite plus nitrate concentrations at Annie Creek near Lead, Whitetail Creek at Lead, Squaw Creek near Spearfish, and Spearfish Creek below Robison Gulch were somewhat higher than at other sites. Mining activity, agricultural activity, and domestic development are possible sources of nitrogen to the streams. Increased mining activities were identified as the probable cause of increased nitrogen concentrations in Annie Creek. In the mineralized area of the northern Black Hills, detectable concentrations of trace elements are common in stream water, occasionally exceeding beneficial-use and aquatic-life criteria. In addition, many basins have been disturbed by both historical and recent mining operations and cleanup activities. The maximum dissolved arsenic concentration at Annie Creek near Lead (48 micrograms per liter) approached the current arsenic drinking-water standard. Concentrations at or greater than 5 micrograms per liter were found in samples from Annie Creek near Lead, Spearfish Creek above Spearfish, Whitetail Creek at Lead, and False Bottom Creek near Spearfish. Bear Butte Creek near Deadwood had one sample with a dissolved copper concentration that exceeded acute and chronic aquatic-life criteria. Bear Butte Creek near Deadwood had several manganese concentrations that exceeded the secondary maximum contaminant level of 50 micrograms per liter. Bed-sediment and water-quality data from selected sites in small drainage basins were used to determine if factors such as pH, arsenic concentrations in bed sediments, and calcite saturation control dissolved arsenic concentrations. Arsenic solubility is controlled by adsorption, mainly on ferrihydrite. In addition, adsorption/desorption of arsenic is controlled by the pH of the stream, with high arsenic concentrations appearing only at higher pH conditions (above 8). There are significant arsenic sources available to almost all the small streams of the northern Black Hills mining area, but arsenic is less mobile in streams that are not influenced to the higher pH values by calcite. Streams where arsenic is more mobile have lower iron concentrations in their bed sediments, and they have relatively high concentrations of calcite Additional water-quality data have been collected as part of other studies or monitoring programs by the South Dakota Department of Environment and Natural Resources, U.S. Environmental Protection Agency, U.S. Forest Service, and the U.S. Geological Survey. Summaries of selected data from these other sources are included as additional information.

South Dakota↗

USGS2017

Characteristics and 40Ar/39Ar geochronology of the Erdenet Cu-Mo deposit, Mongolia

The Early to Middle Triassic Erdenet porphyry Cu-Mo deposit, in northern Mongolia, developed in a continent-continent arc collision zone, within the Central Asian orogenic belt. The porphyry system is related to multiple intrusions of crystal-crowded biotite granodiorite porphyry, which formed a composite stock about 900 m in diameter, with multiple porphyritic microgranodiorite dikes. Wall rocks are Late Permian to Early Triassic, medium-grained granodiorite, with similar whole-rock geochemistry, mineralogy, and composition to the granodiorite porphyry. Whole-rock analysis of the granodiorite porphyry and wall rocks shows that these rocks cannot be discriminated, but both have depleted middle heavy rare earth elements and Y, typical of fertile porphyry magmatic suites. At the current pit level (1,250 m elev), early porphyry-style quartz veins (A and B type) are locally infilled by pyrite-chalcopyrite, with subordinate bornite, but most of the chalcopyrite occurs in D veins that constitute more than 50% of the Cu grade (~0.5 wt % Cu). The 0.3 wt % Cu shell resembles a molar tooth, enveloping the granodiorite porphyry, with deeper roots extending down the wal-rock contacts. Molybdenite occurs in monomineralic veins, and in finely laminated to massive quartz-molybdenite veins. The most important alteration is quartz-muscovite, which occurs as relatively coarse (100–500 μ m) alteration selvages (1–5 cm) that envelop D veins. The D veins cut illite ± kaolinite-smectite (or intermediate argillic) alteration. Intermediate argillic alteration, together with abundant pink anhydrite (commonly hydrated to gypsum), extends from at least 1,300- to 900-m elevation in the deepest drill holes, and has overprinted early potassic alteration, or relatively unaltered red granodiorite. Meter-wide zones of kaolinite cut the anhydrite-gypsum at all levels. There is an abrupt transition outward from the intermediate argillic alteration to chlorite-epidote (propylitic) alteration, at 50 to 200 m from the granodiorite porphyry contact, although D veins (and chalcopyrite) extend outward to the propylitic zone. The Erdenet porphyry system, was overprinted by advanced argillic alteration, which outcrops 2 km northwest of the pit, and forms a lithocap that extends over 10 × 2.5 km. It is characterized by residual quartz, andalusite, Na-Ca and K-alunite, diaspore, pyrophyllite, zunyite, topaz, dickite, and kaolinite. The upper part of the porphyry Cu-Mo deposit (removed by mining), comprised a bornite-chalcocite enriched zone up to 300 m thick with an average grade of 0.7 wt % Cu and up to 5 wt % Cu locally. Based on hypogene bornite-chalcocite mineral textures and high-sulfidation state mineralogy, the enriched zone is inferred to be of hypogene origin, but modified by supergene processes. Consequently, it may be related to formation of the lithocap. Previous Re-Os dates of 240.4 and 240.7 ± 0.8 Ma for molybdenite in quartz veins are comparable to new 40 Ar/ 39 Ar dates of 239.7 ± 1.6 and 240 ± 2 Ma for muscovite that envelops D veins. One 40 Ar/ 39 Ar date on K-alunite from the lithocap of 223.5 ± 1.9 Ma suggests that it may be about 16 m.y. younger than Erdenet, but this result needs to be verified by further dating.

Economic Geology↗

USGS1985

Geologic setting, petrology, and geochemistry of stratiform sphalerite-galena-barite deposits, Red Dog Creek and Drenchwater Creek areas, northwestern Brooks Range, Alaska

Similar stratiform sphalerite-galena-barite deposits occur in the Red Dog Creek area, De Long Mountains quadrangle, and in the Drenchwater Creek area, Howard Pass quadrangle, northwestern Brooks Range, Alaska. The deposits, located approximately 180 km apart, are hosted by Mississippian and Pennsylvanian strata of the Kagvik structural sequence of late Paleozoic and early Mesozoic age. This sequence is situated in the lowermost structural plate of a series of predominantly east-striking, shallow-dipping thrust plates that characterize the regional structure. The deposits occur in shale, tuff, tuffaceous sandstone, altered chert, and quartz exhalite. These units are in places interlayered with or adjacent to bedded or nodular barite or submarine keratophyre flows. Chert and shale commonly contain abundant highly altered, fine-grained feldspar, pumice lapilli, and mafic volcanic rock fragments.The main types of occurrences at Red Dog Creek are: (1) disseminated and stratiform sulfides thinly bedded in organic-rich Mississippian and Pennsylvanian shale and chert, (2) massive sulfide veins and breccia fillings in silicified Mississippian and Pennsylvanian shale, and (3) stratiform sulfide-bearing lenses subdivided into (3a) quartz-exhalite with up to several percent sulfides and barite, (3b) massive sulfide-quartz lenses with up to several percent barite, and (3c) barite-quartz lenses with up to several percent sulfides.At Drenchwater Creek sulfides and barite occur as: (1) disseminations in shale, chert, tuff, and tuffaceous sandstone; (2) disseminations to aggregates in quartz exhalite; and (3) occurrences in veins crosscutting cleavages in brecciated chert and shale.The stratiform sulfides occur in zones up to several thousand meters long and several tens of meters thick. Preliminary grades at Red Dog Creek are 17.1 percent zinc, 5.0 percent lead, and 2.4 oz silver per ton contained in at least 85 million tons.The range of sulfide sulfur isotope values at Red Dog Creek is -16.6 per mil to +3.6 per mil. Sphalerite averages 0.8 per mil. Pyrite is isotopically lighter than coexisting sphalerite in quartz exhalite and massive sulfide lenses. This isotopic relationship is compatible with paragenetic relationships which show pyrite crystallized both earlier and later than other sulfides. Galena is isotopically lighter than coexisting sphalerite. Three groups of geologically and isotopically distinct barites exist including isotopic values appropriate for derivation from late Paleozoic seawater sulfate.Early in the depositional sequence isotopically light pyrite formed from organically reduced seawater sulfate. Sphalerite, galena, late pyrite, and some barite precipitated from two sulfur-bearing solutions that mixed at the mineralization site. Isotopically light barite associated with sulfides precipitated from oxidized H 2 S without seawater sulfate contribution. Lead isotope data from Red Dog and Drenchwater Creeks are compatible with generation of lead in a Phanerozoic mature island-arc setting, a typical orogene, or possibly a back-arc type intracratonic basin.The stratiform sphalerite-galena-barite deposits probably formed in an incipient island-arc environment near a continental margin, containing magmas of keratophyric and andesitic composition, where metal-laden hydrothermal fluids discharged into a deep (?) water, low-energy environment. Later intense deformation, associated with plate docking and thrusting, disrupted and partly remobilized and stratiform deposits.

Alaska↗

USGS1983

Accumulation of fossil fuels and metallic minerals in active and ancient rift lakes

A study of active and ancient rift systems around the world suggests that accumulations of fossil fuels and metallic minerals are related to the interactions of processes that form rift valleys with those that take place in and around rift lakes. The deposition of the precursors of petroleum, gas, oil shale, coal, phosphate, barite, Cu-Pb-Zn sulfides, and uranium begins with erosion of uplifted areas, and the consequent input of abundant nutrients and solute loads into swamps and tectonic lakes. Hot springs and volcanism add other nutrients and solutes. The resulting high biological productivity creates oxidized/reduced interfaces, and anoxic and H 2 S-rich bottom waters which preserves metal-bearing organic tissues and horizons. In the depositional phases, the fine-grained lake deposits are in contact with coarse-grained beach, delta, river, talus, and alluvial fan deposits. Earthquake-induced turbidites also are common coarse-grained deposits of rift lakes. Postdepositional processes in rifts include high heat flow and a resulting concentration of the organic and metallic components that were dispersed throughout the lakebeds. Postdepositional faulting brings organic- and metal-rich sourcebeds in contact with coarse-grained host and reservoir rocks. A suite of potentially economic deposits is therefore a characteristic of rift valleys.

Tectonophysics↗

USGS2011

Book review: Hollowed ground—Copper mining and community building on Lake Superior, 1840s–1990s

In 1843, six years before the Forty-niners headed west for the goldfields of California, the United States’ first great mineral rush began to a land that was, as Patrick Henry told Congress, “beyond the most distant wilderness and remote as the moon.” He was referring to the Keweenaw Peninsula of northern Michigan. This rush was not for gold or silver, but for copper. And not just any copper, but native copper, so pure it required little refining before use. The early horde of fortune-seekers came with visions of finding mountains of solid copper, spurred on by stories of large masses of “float copper” that included the famous Ontonagon Boulder, a large mass of native copper originally found lying 32 km up the steep and rugged valley of the Ontonagon River (and now gathering dust in the Smithsonian Museum).

Economic Geology↗

USGS1966

Sulfide-mineral zoning in the basal nonesuch shale, Northern Michigan

A zone, 1 to 50 feet thick, at the base of the Nonesuch Shale , is relatively rich in copper, chiefly as chalcocite. Pyrite is the characteristic sulfide mineral of the overlying 400 to 600 feet of the formation. The boundary of the cupriferous zone with the pyrite zone regionally transgresses both stratigraphic layering and fades gradations indicative of ancient environmental boundaries. The top of the cupriferous zone is marked by the upward sequence, chalcocite-bornite-chalcopyrite-pyrite, and by the cadmium sulfide , greenockite. The sulfide zoning does not appear to be syngenetic. The regional configuration of the cupriferous zone suggests, instead, that the copper was introduced after deposition of the entire 50-foot sequence of strata. The area in which the top of the cupriferous zone is stratigraphically highest is related spatially, and probably genetically, to a zone of wedging in the underlying Copper Harbor Conglomerate.

Michigan↗

USGS1990

Fractionation of palladium and platinum in a Mesozoic diabase sheet, Gettysburg basin, Pennsyvania: Implications for mineral exploration

The York Haven diabase sheet displays clear-cut evidence of fractionation of Pd and Pt during differentiation of a high-Ti (about 1.1%) quartz-normative tholeiitic magma (York Haven type). At York Haven the sheet is about 750 m thick. It is characterized by abundant cumulus MgO-rich orthopyroxene (bronzite), and is markedly depleted in incompatible elements relative to the chilled margins. In contrast, at Reesers Summit, 16 km to the northwest, the sheet is about 500 m thick and consists of evolved rocks that have contents of incompatible elements two to three times greater than in the enclosing chilled margins. These evolved rocks represent complementary fractions to the cumulate rocks at York Haven. Mineralogic, petrologic and geochemical variations suggest considerable lateral migration and fractionation of the initial magma. Chilled margins of both sections have essentially the same Pd and Pt contents (10 ppb each) and similar Pd to Pt ratios (1.2). During differentiation, the cumulate rocks at York Haven were enriched in Pt and depleted in Pd, whereas at Reesers Summit, the low-MgO diabase and ferrogabbro zone were enriched in Pd relative to Pt. Anomalously high contents of Pd (to 165 ppb), Au (to 54 ppb), and Te (to 26 ppb) were found in an iron- (to 18%) and chlorine- (to 0.44%) rich ferrogabbro at Reesers Summit, suggesting possible late or post-magmatic enrichment of precious metals. Field relations, geochemical and petrographic data provide guides for further exploration for Pd and Pt in differentiated high-Ti quartz-normative diabase sheets. Based on present information, the most favorable sites for economic deposits are late-stage differentiates enriched in Fe and Cl. ?? 1990.

Pennsylvania↗

USGS2001

Miocene and early Pliocene epithermal gold-silver deposits in the northern Great Basin, western United States: Characteristics, distribution, and relationship to Magmatism

Numerous important Miocene and early Pliocene epithermal Au-Ag deposits are present in the northern Great Basin. Most deposits are spatially and temporally related to two magmatic assemblages: bimodal basalt-rhyolite and western andesite. These magmatic assemblages are petrogenetic suites that reflect variations in tectonic environment of magma generation. The bimodal assemblage is a K-rich tholeiitic series formed during continental rifting. Rocks in the bimodal assemblage consist mostly of basalt to andesite and rhyolite compositions that generally contain anhydrous and reduced mineral assemblages (e.g., quartz + fayalite rhyolites). Eruptive forms include mafic lava flows, dikes, cinder and/or spatter cones, shield volcanoes, silicic flows, domes, and ash-flow calderas. Fe-Ti oxide barometry indicates oxygen fugacities between the magnetite-wustite and fayalite-magnetite-quartz oxygen buffers for this magmatic assemblage. The western andesite assemblage is a high K calc-alkaline series that formed a continental-margin are related to subduction of oceanic crust beneath the western coast of North America. In the northern Great Basin, most of the western andesite assemblage was erupted in the Walker Lane belt, a zone of transtension and strike-slip faulting. The western andesite assemblage consists of stratovolcanoes, dome fields, and subvolcanic plutons, mostly of andesite and dacite composition. Biotite and hornblende phenocrysts are abundant in these rocks. Oxygen fugacities of the western andesite assemblage magmas were between the nickel-nickel oxide and hematite-magnetite buffers, about two to four orders of magnitude greater than magmas of the bimodal assemblage. Numerous low-sulfidation Au-Ag deposits in the bimodal assemblage include deposits in the Midas (Ken Snyder), Sleeper, DeLamar, Mule Canyon, Buckhorn, National, Hog Ranch, Ivanhoe, and Jarbidge districts; high-sulfidation gold and porphyry copper-gold deposits are absent. Both high- and low-sulfidation gold-silver and porphyry copper-gold deposits are affiliated with the western andesite assemblage and include the Comstock Lode, Tonopah, Goldfield, Aurora, Bodie, Paradise Peak, and Rawhide deposits. Low-sulfidation Au-Ag deposits in the bimodal assemblage formed under relatively low oxygen and sulfur fugacities and have generally low total base metal (Cu + Pb + Zn) contents, low Ag/Au ratios, and notably high selenide mineral contents compared to temporally equivalent low-sulfidation deposits in the western andesite assemblage. Petrologic studies suggest that these differences may reflect variations in the magmatic-tectonic settings of the associated magmatic assemblages-deposits in the western andesite assemblage formed from oxidized, water-rich, subduction-related calc-alkaline magmas, whereas deposits in the bimodal assemblage were associated with reduced, water-poor tholeiitic magmas derived from the lithospheric mantle during continental extension. The contrasting types and characteristics of epithermal deposits and their affinities with associated igneous rocks suggest that a genetic relationship is present between these Au-Ag deposits and their temporally associated magmatism, although available data do not prove this relationship for most low-sulfidation deposits.

Economic Geology↗

USGS1998

Comparative geology and geochemistry of sedimentary-rock-hosted (Carlin Type) gold deposits in the People's Republic of China and in Nevada, USA

Sedimentary-rock-hosted (Carlin-type) gold deposits have been considered economically significant and geologically distinct since the early 1960's. This report consists of a nine-part text and an interactive database. This small database is to help Western companies get more information about these gold deposits in China, and to help geologists who are interested in world Carlin-type deposits conduct research on them. Because of their economic significance and geological distinctiveness, these deposits have caught the interest of economic geologists all over the world since the early 1960's. Similar deposits have been discovered in China, Australia, Dominican Republic, Spain, and Russia besides Nevada. Perhaps most significant are the 165 Carlin-type gold deposits that were found in southwest China during the past 15 years. Of these, at least 19 deposits have proven to be of substantial tonnage, making China the second leading country to exploit such deposits. With the increasing interest in Chinese Carlin-type gold deposits, some western companies and geologists desire to get more information about these Chinese deposits. This seems to have been very difficult because the literature was in Chinese. It is estimated that several hundred scientific publications (including papers, books, and technical reports) have been published. This database of Chinese Carlin-type Gold deposits is built on the documentation published during the most recent 10 years and includes six subjects, which consist of 165 records and 30 fields. A new Proterozoic-age sedimentary-rock-hosted gold deposit in northeastern P.R. China also is described. Note that for the old version 1.1 on the CD-ROM, the latitude and longitude locations of the mineral occurrences have been estimated from sketch maps and journal articles and are not intended for digital analysis. One of the improvements in this version 1.2 is the accuracy of geographic data. Version 1.3 updates to the database and includes maps and photos of deposits, deposit information and a geochemical model. See the version history for details.

Open-File Report↗

USGS2012

Global exploration and production capacity for platinum-group metals from 1995 through 2015

Platinum-group metals (PGMs) are required in a variety of commercial, industrial, and military applications for many existing and emerging technologies, yet the United States is highly dependent on foreign sources of PGMs. Information on global exploration for PGMs since 1995 has been used in this study as a basis for identifying locations where the industry has determined that exploration has provided data sufficient to warrant development of a new mine or expansion of an existing operation or where a significant increase in capacity for PGMs is anticipated by 2015. Discussions include an overview of the industry and the selected sites, factors affecting mineral supply, and circumstances leading to the development of mineral properties with the potential to affect mineral supply. Of the 52 sites or regional operations that were considered in this analysis, 16 sites were producing before 1995, 28 sites commenced production from 1995 through 2010, and 8 sites were expected to begin production from 2011 through 2015 if development plans came to fruition. The United States imports PGMs primarily from Canada, Russia, South Africa, and Zimbabwe to meet increasing demand for these materials in a variety of specialized and high-tech applications. Feed sources of PGMs are changing in South Africa and Russia, which together accounted for about 89 percent of platinum production and 82 percent of palladium production in 2009. A greater amount of South African PGM capacity is likely to come from deeper, higher cost Upper Group Reef seam 2 deposits and deposits in the Eastern Bushveld area. Future Russian PGM capacity is likely to come from ore zones with generally lower PGM content and different platinum-to-palladium ratios than the nickel-rich ore that dominated PGM supply in the 1990s. Because PGM supply from Canada and Russia is derived as a byproduct of copper and nickel mining, the PGM supply from these countries is influenced by economic, environmental, political, and technological factors affecting exploration for and development of copper and nickel, as well as factors affecting the PGM industry. The recovery of PGMs from mill tailings since 2004 and the recycling of PGMs from catalytic converters, electrical components, and jewelry has increased since 1995 so that recycled PGMs recovered from these products accounted for about 30 percent of the supply of platinum worldwide and 29 percent of the supply of palladium worldwide in 2010. Economic and geopolitical conditions have influenced PGM supply and demand. The global recession of 2008 and 2009 temporarily decreased demand for PGMs and constrained PGM mine exploration and development, at least through 2010. Legislation regulating the structure of the mining sector has affected mining in Russia, South Africa, and Zimbabwe. Stricter vehicle emissions standards in established markets since the 1980s have led to mandatory use of pollution control devices, such as catalytic converters, that contain PGMs and are required on vehicles in expanding markets, such as China and India. It is expected that South Africa, Russia, Canada, and Zimbabwe will continue to be the principal sources of PGM at least for the next decade. Based on this review of the PGM industry, the world’s platinum capacity, expressed in terms of recoverable platinum metal, increased from 1995 through 2010 by 77,000 kilograms (kg) in South Africa, 9,000 kg in Zimbabwe, 6,000 kg in Russia, 2,000 kg in Botswana, and 2,000 kg in Canada. For the same period, palladium capacity worldwide increased by 44,000 kg in South Africa, 22,000 kg in Russia, 8,000 kg in Canada, 8,000 kg in the United States, 7,000 kg in Zimbabwe, and 3,000 kg in Botswana. Platinum capacity worldwide is expected to further increase by 24,000 kg in South Africa, 9,000 kg in Russia, 3,000 kg in Canada, and 2,000 kg in Zimbabwe from 2011 through 2015. Palladium capacity worldwide is likewise expected to increase an additional 16,000 kg in Russia, 14,000 kg in South Africa, 4,000 kg in Zimbabwe, and 1,000 kg in Canada if new or expanded mine and associated processing capacity comes into production as planned. It is likely that the magnitude of these changes in PGM capacity has been influenced by such factors as the global economy, electrical capacity shortages and mine safety concerns in South Africa, and geopolitical conditions in the major PGM producing countries.

Scientific Investigations Report↗

USGS2011

Structural controls and evolution of gold-, silver-, and REE-bearing copper-cobalt ore deposits, Blackbird district, east-central Idaho: Epigenetic origins

The Cu-Co ± Au (± Ag ± Ni ± REE) ore deposits of the Blackbird district, east-central Idaho, have previously been classified as Besshi-type VMS, sedex, and IOCG deposits within an intact stratigraphic section. New studies indicate that, across the district, mineralization was introduced into the country rocks as a series of structurally controlled vein and alteration systems. Quartz-rich and biotite-rich veins (and alteration zones) and minor albite and siderite veinlets maintain consistent order and sulfide mineral associations across the district. Both early and late quartz veins contain chalcopyrite and pyrite, whereas intermediate-stage tourmaline-biotite veins host the cobaltite. Barren early and late albite and late carbonate (generally siderite) form veins or are included in the quartz veins. REE minerals, principally monazite, allanite, and xenotime, are associated with both tourmaline-biotite and late quartz veins. The veins are in mineralized intervals along axial planar cleavage, intrafolial foliation, and shears. Mineralized intervals are hosted by a variety of metasedimentary rocks, including three phyllitic units of Mesoproterozoic age and two schistose units. All of these units are S-tectonites in the footwall of a regional thrust fault. Specifically, the district lies within an oblique thrust ramp containing a series of structural horses (three domains) in a duplex system. The deposits span the three domains and are hosted by metamorphic rocks that range from lower amphibolite facies in the structurally upper domain to lower-middle greenschist facies in the lower domain (an inverted metamorphic sequence). Early quartz and biotite veins were introduced during progressive folding and prolonged peak metamorphic conditions and they underwent late-tectonic retrograde recrystallization and metamorphic mineral growth, to the same extent as the country rocks in each domain. Where little subsequent deformation occurred, early veins are discordant to bedding but, where folding was polyphase and fabrics are penetrative, mineralized zones are concordant with metamorphic compositional layering. Late quartz veins in the zones are associated with retrograde minerals and textures and are only locally deformed. 40 Ar/ 39 Ar dating of unoriented muscovite from the selvage of a late quartz vein yields a Late Cretaceous age of about 83 Ma, the time of retrograde metamorphism associated with introduction of late quartz veins. Textural data at all scales indicate that the host sites for veins and the tectonic evolution of both host rocks and mineral deposits were kinematically linked to Late Cretaceous regional thrust faulting. Heat, fluids, and conduits for generation and circulation of fluids were part of the regional crustal thickening. The faulting also juxtaposed metaevaporite layers in the Mesoproterozoic Yellowjacket Formation over Blackbird district host rocks. We conclude that this facilitated chemical exchange between juxtaposed units resulting in leaching of critical elements (Cl, K, B, Na) from metaevaporites to produce brines, scavenging of metals (Co, Cu, etc) from rocks in the region, and, finally, concentrating metals in the lower-plate ramp structures. Although the ultimate source of the metals remains undetermined, the present Cu-Co ± Au (± Ag ± Ni ± REE) Blackbird ore deposits formed during Late Cretaceous compressional deformation .

Idaho↗

USGS2013

Silicate melt inclusion evidence for extreme pre-eruptive enrichment and post-eruptive depletion of lithium in silicic volcanic rocks of the western United States: implications for the origin of lithium-rich brines

To evaluate whether anatectic and/or highly fractionated lithophile element-enriched rhyolite tuffs deposited in arid lacustrine basins lose enough lithium during eruption, lithification, and weathering to generate significant Li brine resources, pre-eruptive melt compositions, preserved in inclusions, and the magnitude of post-eruptive Li depletions, evident in host rhyolites, were documented at six sites in the western United States. Each rhyolite is a member of the bimodal basalt-rhyolite assemblage associated with extensional tectonics that produced the Basin and Range province and Rio Grande rift, an evolving pattern of closed drainage basins, and geothermal energy or mineral resources. Results from the 0.8 Ma Bishop tuff (geothermal) in California, 1.3 to 1.6 Ma Cerro Toledo and Upper Bandelier tephra (geothermal) and 27.9 Ma Taylor Creek rhyolite (Sn) in New Mexico, 21.7 Ma Spor Mountain tuff (Be, U, F) and 24.6 Ma Pine Grove tuff (Mo) in Utah, and 27.6 Ma Hideaway Park tuff (Mo) in Colorado support the following conclusions. Melt inclusions in quartz phenocrysts from rhyolite tuffs associated with hydrothermal deposits of Sn, Mo, and Be are extremely enriched in Li (1,000s of ppm); those from Spor Mountain have the highest Li abundance yet recorded (max 5,200 ppm, median 3,750 ppm). Forty-five to 98% of the Li present in pre-eruptive magma was lost to the environment from these rhyolite tuffs. The amount of Li lost from the small volumes (1–10 km3) of Li-enriched rhyolite deposited in closed basins is sufficient to produce world-class Li brine resources. After each eruption, meteoric water leaches Li from tuff, which drains into playas, where it is concentrated by evaporation. The localized occurrence of Li-enriched rhyolites may explain why brines in arid lacustrine basins seldom have economic concentrations of Li. Considering that hydrothermal deposits of Sn, Mo, Be, U, and F may indicate potential for Li brines in nearby basins, we surmise that the world’s largest Li brine resource in the Salar de Uyuni (10 Mt) received Li from nearby rhyolite tuffs in the Bolivian tin belt.

California;Colorado;New Mexico;Utah↗

USGS2005

U-Pb zircon ages and Pb isotope geochemistry of gold deposits in the Carolina slate belt of South Carolina

Volcanic rocks of the Persimmon Fork Formation host the largest known gold mines of the Carolina slate belt. U-Pb (SHRIMP) zircon ages have been obtained from rocks closely associated with pyrite-enargite-gold deposits at Brewer (quartz-topaz rhyolite breccia from the argillic alteration zone in the Brewer pit and felsic ash-flow tuff from the quartz sericite alteration zone), from the disseminated and semimassive pyrite-gold deposits at Haile (crystal lithic rhyolitic ash-flow tuffs from the Champion pit), and from the Ridgeway deposit (felsic ash-flow tuff from the stratigraphic host of the North pit gold deposit). Generally, the zircons are fine grained, fractured, and contain crystal imperfections (corrosion, inclusions, and pits). 206 Pb/ 238 U zircon spot ages for all deposits span a wide range, mostly from 400 to 760 Ma. Inclusions and cores indicative of inherited domains in the zircons were not found, and only a few analyses range from 1.1 to 1.8 Ga. A distinct xenocrystic zircon population was not identified. The 206 Pb/ 238 U weighted age averages of zircon indicate the following crystallization dates for the volcanic and volcaniclastic rocks closely associated with the gold deposits: 550 ± 3 Ma for Brewer, 553 ± 2 Ma for Haile, and 556 ± 2 Ma for the Ridgeway deposit. These zircon crystallization ages represent close estimates of the age of the original gold mineralizing events. Younger zircon spot ages can be attributed to the effects of Paleozoic regional metamorphism. Pb isotope compositions of sulfide minerals (galena, pyrite, enargite, sphalerite, chalcopyrite, and molybdenite) and silicate minerals (K-feldspar, and sericite) in the gold deposits help to constrain the sources of fluids and metals during the mineralizing events. The deposits are pyrite rich, containing multiple generations of pyrite, including early-crystallized pyrite that is closely associated with the original gold mineralizing event, as well as recrystallized pyrite formed in response to Paleozoic metamorphism. Pb isotope compositions of pyrite span a wide range, including the most radiogenic values for the sulfides. Galena and K-feldspar are not abundant but where present they are typically the least radiogenic minerals. Galena has a limited range of Pb isotope compositions that are representative of the gold deposits as a group ( 206 Pb/ 204 Pb = 18.020–18.326, 207 Pb/ 204 Pb = 15.550–15.639, 208 Pb/ 204 Pb = 37.605–38.286). Values of 207 Pb/ 204 Pb straddle the average crustal Pb growth curve, consistent with contributions involving the mantle and continental crust. Whole-rock Pb isotope compositions of volcanic and volcaniclastic rocks of the Persimmon Fork Formation nearly match the range for sulfides in the gold deposits. Subtle regional contrasts in Pb isotope compositions exist among the deposits. Sulfide minerals from Barite Hill (e.g., galena 206 Pb/ 204 Pb < 18.077) in southern South Carolina are generally less radiogenic than sulfides from Ridgeway ( 206 Pb/ 204 Pb > 18.169), Haile ( 206 Pb/ 204 Pb > 18.233), and Brewer ( 206 Pb/ 204 Pb > 18.311) in northern South Carolina. Because Pb isotope compositions of basement rocks from Grenville massifs in the southern Appalachians and sulfide minerals from the gold deposits do not match, a direct genetic connection cannot be established. Diversity in values of 206 Pb/ 204 Pb and the relatively high values of 207 Pb/ 204 Pb suggest that the deposits evolved adjacent to or closely related to continental blocks, perhaps linked to a back-arc tectonic setting. Among potential younger analogues of the slate belt gold deposits are the sulfide deposits of the Okinawa trough in the western Pacific. Mantle-derived isotopic contributions were more important at Barite Hill in southern South Carolina, the least radiogenic among the deposits where oceanic crust had developed, than at Brewer, Haile, and Ridgeway in northern South Carolina where rifting thinned the continental crust.

Georgia, North Carolina, South Carolina, Virginia↗

USGS1970

Distribution of silver and copper in placer gold derived from the northeastern part of the Colorado Mineral Belt

Placer gold grains from the modern streams originating in the Colorado Mineral Belt were examined for silver and copper content on a quantitative basis utilizing the electron microprobe. The variation among grains from a particular locality is large, but the mean silver content of the interior of the placer gold grains from each locality and (or) the variation in copper content may be of value in distinguishing lode sources and gold mining districts. Microprobe analysis of the interior of gold grains is independent of chemical actions that affect the border of placer gold grains during their transport history, and it is shown that distinct compositional groups of different lode sources may be identified even in a single sample--information that may aid in recognizing the existence of concealed lodes that once contributed to a placer environment.

Colorado↗

USGS2000

Cruise report RV Ocean Surveyor cruise O-1-00-GM the bathymetry and acoustic backscatter of the Pinnacles area, northern Gulf of Mexico May 23, through June 10, 2000 Venice, LA to Venice, LA

An extensive deep (~100 m) reef tract occurs on the Mississippi-Alabama outer continental shelf (OCS). The tract, known as "The Pinnacles", is apparently part of a sequence of drowned reef complexes along the "40-fathom" shelf edge of the northern Gulf of Mexico (Ludwick and Walton, 1957). It is critical to determine the accurate geomorphology of deep-reefs because of their importance as benthic habitats for fisheries. The Pinnacles were mapped by Ludwick and Walton (1957) using a single-beam echo sounder but the spatial extent and morphology were interpreted from a series of widely separated, poorly navigated bathymetric profiles. Other recent studies, supported by Minerals Management Service (MMS), used towed sidescan sonars and single-channel seismic-reflection profiling. None of the existing studies provide the quality of geomorphic data necessary for reasonable habitat studies. The fish faunas of shallow hermatypic reefs have been well studied, but those of deep ahermatypic reefs have relatively ignored. The ecology of deep ahermatypic reefs is fundamentally different from hermatipic reefs because autochthonous intracellular symbiotic zooxanthellae (the carbon source for hermatypic corals) do not form the base of the trophic web. Instead, exogenous plankton, transported to the reef by currents, serves as the primary carbon source. Deep OCS reefs also lie below the practical working depths for SCUBA; consequently, remote investigations from a ship or in situ investigations using submersibles or ROVs are required. Community structure and trophodynamics of demersal fishes of the Pinnacles are presently the focus of USGS reseach. A goal of the research is to answer questions concerning the relataive roles played by geomorphology and surficial geology in the interaction with and control of biological differentiation. OCS reefs are important because we now know that such areas are important coral reef fish havens, key spawning 2 sites, and a critical early larval and juvenile habitats for economically important sport/food fishes. Also, deep-reef ecosystems as well as the fish populations they sustain are impacted by intensive oil-field development. It is now known that deep OCS reefs function as a key source of re-population (via seasonal and ontogenetic migration) of already heavily impacted inshore reefs. A reflection of this realization is the recent closure by the Gulf States Fisheries Management Council of a 600 mi 2 area of the Florida Middle Grounds (another unmapped major "40-fathom" OCS reef complex) to commercial fishing to preserve grouper spawning aggregations. It is known that the Pinnacles reefs support a lush fauna of ahermatypic hard corals, soft corals, black corals, sessile crinoids and sponges—together forming a living habitat for a well-developed fish fauna. The fish fauna comprises typical Caribbean reef fishes and Carolinian shelf fishes, plus epipelagic fishes, and a few deep-sea fishes. The base of the megafaunal invertebrate food web is plankton, borne by essentially continuous semi-laminar currents flowing predominantly out of the SW, up, along and across the shelf edge. These currents are intercepted by pinnacles reefs, which lie roughly in two linear tracts, parallel to the coastline (see fig. 1 in report). USGS research initiated in 1997 (Sulak et al., in progress) has demonstrated that the Pinnacles reef fish fauna is dominated by planktivorous fishes. Ongoing food habits, trophic web and stable isotope analyses by the USGS are reinforcing a basic picture of deep OCS reefs as ecosystems based on exogenous current-borne plankton. Long-term current meter deployments have demonstrated that the >3 m, <16m relief of the Pinnacles reefs disrupts the prevailing currents, inducing local complexity (F. Kelly, Texas A&M Univ., pers. comm.) favorable to plankton and planktivores. Geodetically accurate bathymetry maps, coregistered with calibrated acoustic backscatter maps, are critical to delineate benthic biotopes, and are essential to correlate biological community differentiation with the physical environment. Previous mapping of the Pinnacles area using the TAMU towed single-beam sidescan system (Anonymous., 1999) employed technology with inherent deficiencies in backscatter calibration, bathymetric precision, and geo-referencing. The resulting bathymetric interpretations of TAMU data are further degraded by ship-track-parallel artifacts that obliterate geomorphology rendering them inadequate to provide high-resolution mapping of individual target reefs (typical maximum relief 15 m, diameter 200-500 m). The sidescan-sonar surveys of the area (Laswell et al., 1990) suffer many of the same deficiencies as the TAMU data so they add no quantitative and little qualitative information about the geomorphology and surficial geology.

Gulf of Mexico, the Pinnacles area↗

USGS2012

Origins of mineral deposits, Belt-Purcell Basin, United States and Canada: An introduction

The fill of the Mesoproterozoic Belt-Purcell Basin, which straddles the United States-Canada border within the Rocky Mountains of western North America ( Fig. 1 ), consists of marine and nonmarine clastic and carbonate strata 15 to 20 km thick. Three giant metal-producing ore deposits or districts account for the bulk of the known metal endowment within the bounds of the Belt-Purcell Basin: (1) the syndepositional Sullivan Pb-Zn-Ag deposit in southern British Columbia (total production: Pb, 8.4 million tonnes [Mt]; Zn, 7.9 Mt; Ag, 0.0093 Mt; Lydon, 2000 ), (2) the mesothermal Pb-Zn-Ag veins of the Coeur d’Alene district in northern Idaho (total production: Pb, 7.5 Mt; Zn, 3.0 Mt; Ag, 0.052 Mt; Long, 1998 ; post-1997 data from USGS Annual Minerals Yearbooks), and (3) the Cretaceous porphyry copper deposit and associated polymetallic veins in the Butte district in Montana (total resource: Cu, 35 Mt; Zn, 4.6 Mt; Ag, 0.044 Mt; Long et al., 1998 ). The Sullivan Mine closed in 2001 after more than 92 years of production. Mining of 26 major vein deposits in the Coeur d’Alene district began in the 1880s and peaked about 1950. Production in the Coeur d’Alene district continues today from the Galena and Lucky Friday Mines (the latter closed for 2012 to refurbish the mile-deep vertical access shaft). Mining at Butte began in 1875, with copper production peaking in 1917. Mining continues today in the eastern upfaulted portion of the Butte porphyry copper deposit at the Continental Mine.

Alberta, British Columbia, Idaho, Montana↗

USGS1972

Ground-water in the Teresina-Campo Maior area, Piaui, Brazil

The Teresina-Campo Maior area lies in a presently developing farming and grazing region near the margin of drought-prone northeast Brazil where irrigated farming offers the best potential for economic development. The area comprises 9,700 square kilometers largely of catinga-covered tabular uplands which are drained by the perennial Rio Parnatba. The climate is hot and humid most of the year but with distinct wet and dry seasons. Temperature extremes range from 20?C to 39?C and the annum rainfall averages 1,200 millimeters. The area's ground-water reservoir is contained chiefly in sandstone aquifers of six westward-dipping sedimentary rock formations, all part of the Maranhao sedimentary basin. The youngest of these formations, namely the Piaut (Pennsylvarian), Poti (Mississippian), Longa (Upper Devonian), and Cabecas (Middle Devoniar), contain the principal aquifers. Precipitation is the primary source of recharge to these aquifers and is more than sufficient to replenish current withdrawals from wells. Underlying the principal aquifers are the untapped Pimenteiras and Serra Grande Formations (both Lower Devonian) which in areas adjacent to the report area are moderately good to excellent water producers. These aquifers are recharged principally by lateral inflow from the east. Water also occurs in the alluvial deposits (Quaternary) underlying the flood plain of the Rio Parnatba but recurrent and uncontrolled flooding at present (1966) precludes their development. Of little economic importance, because they lie above the zone of saturation, are the thin erosional remnants of the Pastos Bons (Upper Triassic), Matuca, and Pedra de Fogo (both Permian) Formations. There are in the report area about 200 drilled wells most of which are pumped with power-driven engines. The wells range from 40 to 500 meters deep but most do not exceed 150 meters, and practically all are completed open hole. Yields range from 500 liters per day for 6-inch-diameter domestic wells to 240,000 liters per hour for 10-inch high-capacity municipal wells. Although there are many more dug wells than drilled wells, dug wells account for less than 1 percent of the current (1966) draft. The current annual withdrawal from the principal aquifers is approximately 5 million cubic meters of which almost half is used for municipal supply and the rest for rural household and irrigation uses. Additional water for public supply is available from aquifers now being pumped, and larger yields probably could be obtained from rural wells designed to take full advantage of the aquifer. Analyses of 28 samples show that the chemical quality of the water is well below the accepted limits of mineral concentration for most uses. Water from the Longa Formation averages 842 milligrams per liter in total dissolved solids and is more mineralized than that in the Piaul and Port Formations which contain water averaging less than 300 milligrams per liter. The water in the Piaui and Poti aquifers is the most suitable in the area for irrigation and has SAR values of C1-S1 and C2-S1. The quantities of water currently being used for irrigation are relatively small (600,000 cubic meters annually) but will increase substantially when intensive irrigation becomes a reality. Divisio de Hydrogeologia da Superintendancia do Desenvolvimento do Nordeste estimates that about 2,500 million cubic meters of water per year would be needed to irrigate about 250,000 hectares in the Teresina-Campo Maior area (about 25 percent of the total area). This goal, however, is not likely to be realized as the water requirement is five times the estimated natural recharge to the aquifers of the area. Most of the water-bearing formations in the report area have barely been tapped and can be developed a great deal more. In fact, the current annual withdrawal from the principal aquifers is less than 0.0025 percent of a conservative estimate of annual replenishment from rainfall. Additionally, only the

Water Supply Paper↗

USGS2000

Late-kinematic timing of orogenic gold deposits and significance for computer-based exploration techniques with emphasis on the Yilgarn Block, Western Australia

Orogenic gold deposits are a widespread coherent group of epigenetic ore deposits that are sited in accretionary or collisional orogens. They formed over a large crustal-depth range from deep-seated low-salinity H 2 O–CO 2 ±CH 4 ±N 2 ore fluids and with Au transported as thio-complexes. Regional structures provide the main control on deposit distribution. In many terranes, first-order faults or shear zones appear to have controlled regional fluid flow, with greatest ore-fluid fluxes in, and adjacent to, lower-order faults, shear zones and/or large folds. Highly competent and/or chemically reactive rocks are the most common hosts to the larger deposits. Focusing of supralithostatic ore fluids into dilatant zones appears to occur late during the evolutionary history of the host terranes, normally within D 3 or D 4 in a D 1 –D 4 deformation sequence. Reactivation of suitably oriented pre-existing structures during a change in far-field stress orientation is a factor common to many deposits, and repeated reactivation may account for multiple mineralization episodes in some larger deposits. Absolute robust ages of mineralization support their late-kinematic timing, and, in general, suggest that deposits formed diachronously towards the end of the 100 to 200 m.y. long evolutionary history of hosting orogens. For example, in the Yilgarn Block, a region specifically emphasised in this study, orogenic gold deposits formed in the time interval between 40 and 90 m.y., with most about 60 to 70 m.y., after the youngest widespread basic-ultrabasic volcanism and towards the end of felsic magmatism. The late timing of orogenic gold deposits is pivotal to geologically-based exploration methodologies. This is because the present structural geometries of: (i) the deposits, (ii) the hosting goldfields, and (iii) the enclosing terranes are all essentially similar to those during gold mineralization, at least in their relative position to each other. Thus, interpretation of geological maps and cross-sections and three-dimensional models can be used to accurately simulate the physical conditions that existed at the time of ore deposition. It is particularly significant that the deposits are commonly related to repetitive and predictable geometries, such as structural heterogeneities within or adjacent to first-order structures, around rigid granitoid bodies, or in specific “locked-up” fold-thrust structures. Importantly, the two giant greenstone-hosted goldfields, Kalgoorlie and Timmins, show a remarkably similar geometry at the regional scale. Computer-based stress mapping and GIS-based prospectivity mapping are two computer-based quantitative methodologies that can utilize and take advantage of the late timing aspect of this deposit type to provide important geological aids in exploration, both in broad regions and more localized goldfields. Both require an accurate and consistent solid geology map, stress mapping requires knowledge of the far-field stresses during mineralization, and the empirical prospectivity mapping requires data from a significant number of known deposits in the terrane. The Kalgoorlie Terrane, in the Yilgarn Block, meets these criteria, and illustrates the potential of these methodologies in the exploration for orogenic gold deposits. Low minimum stress anomalies, interpreted to represent dilational zones during gold-related deformation, coincide well with the positions of known goldfields rather than individual gold deposits in the terrane, and there are additional as-yet unexplained anomalies. The prospectivity analysis confirms that predictable and repetitive factors controlling the siting of deposits are: (i) proximity to, and orientation and curvature of, granitoid-greenstone contacts, (ii) proximity to segments of crustal faults which strike in a preferred direction, (iii) proximity to specific lithological contacts which have similar preferred strike, (iv) proximity to anticlinal structures, and (v) the presence of preferred reactive host rocks (e.g., dolerite). The prospectivity map defines a series of anomalous areas, which broadly conform to those of the stress map (>78% correspondence). The most prospective category on this map covers less than 0.3% of the greenstone belts and yet hosts 16% of the known deposits, which have produced>80% of known gold. Thus, it discriminates in favour of the larger economically more-attractive deposits in the terrane. The successful application of stress mapping and prospectivity mapping to geology-based exploration for orogenic gold deposits indicates that more quantitative analysis of geological map data is a profitable line of research. The computer-based nature of these methodologies is ideal for the production of an ultimate, integrated, deposit target map, which can be compared to other, more conventional, targeting parameters such as geophysical and geochemical anomalies. Such an integrated strategy appears the way forward in the increasingly difficult task of cost-effective global exploration for orogenic gold deposits in poorly exposed terranes.

Western Australia↗