The Goldstrike property, located in the Carlin Trend in Nevada, contains a diverse group of Carlin deposits, including some of the largest and highest grade examples known. The largest deposit, Betze-Post, has a gold endowment of approximately 1,250 metric tons (t) Au, and the Meikle deposit, which contains 220 t Au, has a grade of 24.7 g/t Au. Goldstrike is part of the larger Blue Star-Goldstrike subdistrict, which has an areal extent of 58.5 by 2 km and a total gold endowment of 1,970 t. The first discovery of gold at Goldstrike was in 1962. Subsequent exploration culminated in the discovery in 1986 of large high-grade orebodies beneath smaller, lower grade orebodies. Exploration over a 40-yr period has relied on the evolution in understanding of geology and ore controls, supported by the application of geochemical and geophysical exploration techniques.

Pregold mineralization at the Getchell Carlin-type gold deposit includes quartz and base metal vein mineralization associated with intrusion of a Cretaceous granodiorite stock. The veins contain minor pyrite and trace chalcopyrite, arsenopyrite, galena, and sphalerite. The pyrite is moderately coarse and, in thin section, has high relief, is well polished, and is fractured and locally cemented by the gold ore assemblage. White micas are associated with veins near the granodiorite intrusion. Gold was not observed or detected by fire assay analyses of samples or electron microprobe analyses of pyrites. Microprobe analyses show that pregold pyrites have near-stoichiometric compositions. Variable, low arsenic is present in pyrite in samples overprinted by gold mineralization. Secondary ion mass spectrometry (SIMS) analyses detected trace gold in the coarse, near-stoichiomet-ric pyrite in overprinted samples. The pregold vein assemblage was fractured and cemented by gold ore-stage mineralization

The Rain mine includes two mined-out open pits that contained 36.4 t (1.17 Moz) gold, averaging 1.8 g/t gold, and underground reserves, including underground production, estimated at 4.9 t (157,000 oz) gold averaging 7.7 g/t. Rain orebodies are localized in a breccia complex within the hanging wall of the Rain fault and hosted within the Mississippian Webb Formation immediately overlying the contact with the Devonian Devils Gate Limestone.

The ore host includes four texturally and genetically distinct breccia types: (1) crackle breccia; (2) hy-drothermal breccia; (3) tuffisite with accretionary lapilli; and (4) collapse breccia. Crackle breccia forms a capping over multistage hydrothermal breccias that are cut by tabular- to pipe-shaped tuffisite dikes, with some containing accretionary lapilli. Pre- and synore hydrothermal breccias formed during at least three episodes of convective fluidization, followed by quartz-sulfide-barite cementation. High-grade gold was deposited as a late phase along the upper portion of the hydrothermal breccia mass and extended into the crackle breccia zone. Collapse breccias occur along the floor of the composite breccia mass and have irregular upper and lower contacts. The lower contact occurs on a dissolution boundary with the Devils Gate Limestone.

Newly recognized gold-rich sedimentary-exhalative (sedex) mineralization in Nevada, with an average gold grade of 14 g/tonne (t), and the occurrence of significant amounts of gold in classic sedex deposits like Rammelsberg, Germany (30 Mt at 1 g/t), Anvil, Canada (120 Mt at 0.7 g/t), and Triumph, Idaho (? at 2.2 g/t) demonstrate that basin brines can form gold ore. The sedex Au mineralization in Nevada represents a previously unrecognized end member in a spectrum of sedex deposits that also includes large Zn-Pb, intermediate Zn-Pb-Ba ± Au, and barite deposits. Study of ore deposits, modern brines, and chemical modeling indicates that variation in metal ratios and their abundance in sedex deposits are dominantly controlled by the concentration and redox state of sulfur in brines. For example, Au and Ba solubilities are highest in H2S-rich, SO4-poor fluids, whereas base metal solubilities are highest when H2S is not present. Chemical modeling indicates a typical reduced brine (15 wt % NaCl equiv, pH = 5.5, H2S = 0.01 m) at 200°C is capable of transporting as much as 1 ppm Au in solution.

The H2S content in brines is controlled by the rate of its production through thermochemical reduction of sulfate by organic matter and the rate of its removal from the fluid through the sulfidation of reactive Fe in the sediments. Thus, sedimentary basins with high organic carbon and sulfate in rocks low in reactive Fe, such as carbonates and shales, are most likely to produce H2S-rich brines that may form gold-rich sedex deposits. Because of the tremendous scale of sedex hydrothermal systems, evidence that basin fluids can transport gold identifies a new mechanism for concentrating gold in sedimentary basins and opens extensive areas to further gold exploration.

Carlin-type deposits are major sources of gold, yet their origins are enigmatic. Suggested genetic models make connections to magmatism, regional metamorphism, or regional extension. Depositional mechanisms are uncertain as well. We propose on the basis of geologic, physical, and chemical reasoning, a genetic model in which meteoric fluids were circulated by heat released during crustal extension. These fluids interacted at depth with the sedimentary rock pile and scavenged gold. Upon upwelling, these fluids interacted with various lithologies and/or other fluids and produced the characteristic alteration and metal suites of these deposits. To test the viability of this amagmatic model, we have investigated certain physical and chemical constraints implicit to the model.

Gold mineralization at Carlin is clearly younger than hydrocarbon maturation (pre-Cretaceous) and felsic dike intrusion (Cretaceous), and older than deep oxidation (late Tertiary). Within the episode of gold mineralization, the main gold ore (MGO) stage and late gold ore (LGO) stage are distinguished paragenetically, wi-th a variety of vein and mineralization types in each. MGO stage fluids contained 5 to 10 mole percent COa, appreciable H2S, and 3 ± 1 wt percent NaCl equiv. At least portions of MGO stage mineralization were characterized by two-phase boiling (COa exsolution) at 215° ± 30°C and 800 ± 400 bars. In contrast, LGO stage fluids were gas poor with salinities <1.5 wt percent NaCl equiv and record only nonboiling conditions. MGO stage fluids had 518Oh2o values of 5 to 9 per mil, whereas LGO stage fluids resembled unevolved meteoric water with 518Oh2o values < —3 per mil.

From the MGO stage to the LGO stage, calcite 5180 values shifted from near whole-rock values of 12 ± 3 per mil to around 0 ± 1 per mil as LGO stage fluids flooded the system. Jasperoids also record a large range (9-22%o) in 518Oh2o values. These data indicate the involvement of two very different fluids in ore deposition. Because MGO and LGO stage features are closely associated spatially with each other and with Au, As, Sb, Hg, and other ore elements, both fluids are believed to have both been present during most stages of ore deposition.

At pressures of 80 to 85 percent lithostatic, depths of 3.8 ± 1.9 km are required to accommodate the 800 ± 400 bars of pressure recorded in MGO stage fluid inclusions. Carlin, therefore, is not an epi-thermal or hot spring deposit. Carbon dioxide in gas-rich MGO stage fluids may have originated either directly from buried intrusions or their contact aureoles, or from low-grade regional metamorphism at depth. The water may have been originally meteoric, and Au may be magmatic or derived from leaching of deep metamorphic or sedimentary rocks. Ore deposition appears to have occurred in zones of throttling at a pressure seal between normally pressured and overpressured regimes, where fluids experienced a change from near-lithostatic to hydrostatic conditions. Such pressure seals are common in deep sedimentary basins and may be a key to highly localized gold deposition. Mixing of two fluids and interaction with host rocks along thin permeable bioclastic horizons are believed to have been the major factors in depositing ore.

В настоящее время все большее значение приобретают работы по выделению площадей, перспективных для поисков различных минеральных месторождений. Они проводятся геологами производственных организаций и научных учреждений и охватывают самые различные районы нашей страны. Для многих минеральных месторождений в последнее время предложены определенные поисковые критерии, составлены карты прогнозов и даны прогнозные предположения, нередко подтверждающиеся удачными поисковыми и разведочными работами. Тем не менее это только начало, и перед (Геологами стоит задача по выявлению все новых и новых перспективных площадей для успешных поисков рудных месторождений магматогенного и осадочного происхождения. 

Книга является первой обобщающей монографией по минералогия самородной серы и парагенных ей минералов (кальцита, арагонита, целестина, барита, гипса, киарца и его разновидностей, пирита и др.) Основное внимание уделено самородной сере. Детально рассмотрена ее структура, химический состав, физические свойства, морфология, рост и изменение реальных кристаллов, морфология и онтогения агрегатов, типоморфные особенности. На основании анализа состава к строения серы и парагенных ей минералов, изучения пар а генетических соотношений минералов и зональности серных тел рассмотрена история минералогенезиса в месторождениях самородной серы, выяснены физико-химические особенности сероотлагающих растворов, высказаны соображения о генезисе месторождений. В заключение рассмотрены поисково-оценочные минералогические критерии на самородную серу.

Вопрос о возрасте, самостоятельности и месте малых интрузий в истории развития юго-западных отрогов Гиссара до настоящего времени не рассматривался исследователями. В существующих схемах магматизма указанного района они относились к дайковым образованиям, венчающим герцинский цикл  (2).

В результате проведенных исследований нами в пределах хр. Сурхантау, Северного Байсунтау, Чакчара откартирована и изучена серия пород (диорит-порфириты, гранодиорит-порфиры, гранит-порфиры и др.), относящихся к разряду малых интрузий. При этом под малыми интрузиями мы подразумеваем «комплекс интрузивных пород, состоящий из родственных, но самостоятельных тел, которые обычно образуются в поздние этапы развития магматизма определенного цикла» (3).

The Carlin disseminated gold deposit occurs in an autochthonous sequence of Paleozoic sedimentary rocks exposed in a structural window in the Roberts Mountains thrust in north-central Nevada. The upper 175 m of the Silurian Roberts Mountains Formation hosts the majority of ore at Carlin and is characterized by laminated, fine-grained, calcareous and/or dolomitic argillaceous siltstone with local coarser grained siltstones and <0.25- to >50-cm-thick lenticular interbeds of sand- and granule-sized calcareous bioclastic debris or fossil hash. Detailed studies of drill core and exposures in the East pit of the Carlin mine show that alteration and mineralization are zoned away from crosscutting fault conduits and these more permeable bioclastic beds, indicating that these two features were major inflow zones for hydrothermal fluid.

In unoxidized rocks, unaltered calcareous siltstone (1) containing quartz, dolomite, calcite, illite, K feldspar, and pyrite is progressively converted to assemblages of (2) quartz + dolomite + calcite + illite + pyrite, (3) quartz + dolomite + illite-K mica + pyrite, (4) quartz + illite-K mica + pyrite, and (5) quartz + kaolinite-dickite + pyrite adjacent to inflow zones where jasperoids are developed. Gold most consistently enriches the zone of calcite and dolomite removal (3 and 4 above), though it occurs in all zones, locally in high concentrations. This zoned alteration was accomplished by a C02-rich acidic fluid. This acidic alteration enhanced the passage of fluids by extensive carbonate removal to form zones of higher permeability.

Oxidation is wholly a supergene effect related to deep weathering, because the oxidation is superimposed on both mineralized and altered rocks with only minor effect on the major element chemistry; it has produced low-temperature goethitic Fe oxides rather than higher temperature hematite and is not spatially related to Au distribution at the mine or on a district scale.

Because of extensive carbonate removal leading to local volume reduction through collapse and/or compaction, geochemical effects are examined using ratios to relatively immobile elements such as Al and Ti. Extensive depletion of Ca, Mg, and C02 and introduction of Si, Au, and S have occurred. Potassium is depleted in the conversion of illite to dickite-kaolin-ite in proximal silicified inflow zones, and Fe enriches some pyritized rock. Carbonate removal and silicification are two separate processes, both of which are spatially associated with mineralization. Mineralized decarbonated rocks and barren footwall rocks commonly are not silicified, and intensely silicified proximal alteration zones are generally low grade.