Epithermal Precious Metal Deposits Research Articles

Metallogeny and Mineral Exploration – Some Personal Reminiscences

The following personalised narrative aims to document the highlights of my involvement in some of the ground breaking developments in Economic Geology and their direct application to mineral exploration and discovery over the past half century. The story begins with my introduction to geology at secondary school and university, followed by doctoral research based on fieldwork in the Andes of South America. Then, as an employee of the Chilean Geological Survey, I got my introduction to porphyry copper deposits before returning to the UK to take up a post-doctoral research fellowship. This formative period concluded with my starting out as an independent geological consultant to the global exploration and mining industry. These early years happened to coincide with the plate tectonics revolution and its radical implications for metallogeny. I realised that porphyry copper and related deposits are integral parts of volcano-plutonic arcs generated during subduction of oceanic lithosphere, and volcanogenic massive sulphide deposits in ophiolite complexes must have formed at oceanic spreading centres. At approximately the same time, application of K-Ar dating to copper deposits led to definition of metallogenic belts and corresponding epochs in the Andes, and then established the timing of their economically important supergene oxidation and enrichment. Subsequently, using more modern and precise U-Pb zircon and Re-Os molybdenite methods, collaborative attempts were made to determine porphyry copper deposit lifespans and ages of various copper belts, deposits and prospects around the world, including the Zambian Copperbelt. The focus on porphyry copper deposits led first to an appreciation of the linkage between them and subaerial volcanism and the importance of potassic alteration as a major host of hypogene copper mineralisation, and then to geological characterisation of the increasing number of gold-rich examples. Appreciation of the importance of hydrothermal breccias in porphyry copper deposits, including recognition of mineralised diatremes, resulted in a classification scheme for breccias that may be extended to related deposit types. Extensive fieldwork showed that zones of advanced argillic alteration, termed lithocaps, constitute the shallow parts of porphyry copper systems. The role of tectonic uplift in both porphyry copper formation and subsequent supergene modification was also charted. The end result of this body of work was a porphyry copper model that can be used as a basic exploration guide. In response to a marked increase in the world gold price in the late 1970s, more effort was devoted to gold concentrations in magmatic arc terranes, commencing with epithermal gold deposits in the shallow lithocaps of porphyry copper systems. This led to an input to classification schemes for epithermal precious metal deposits and, eventually, to assignment of the three main epithermal types to specific tectono-magmatic settings. After years of speculation, porphyry gold deposits were recognised for the first time in northern Chile, followed by definition of a new gold deposit class in association with relatively reduced granitic intrusions. A magmatic-hydrothermal origin for Carlin-type gold deposits was steadfastly supported over many years, notwithstanding its unpopularity until relatively recently, culminating in a proposal for modern analogues. For decades, metallogenic provinces and corresponding epochs have been widely appreciated, but debate concerning their origin(s) persists. The nature of accompanying magmatism could well provide an adequate explanation for at least some provinces (e.g., tin, molybdenum and possibly silver), but precursor metal enrichment in the lowermost crust and/or subcontinental lithospheric mantle may well be required in the case of gold and copper provinces. The story concludes with brief commentary on mineral exploration, which has been my lifelong (pre)occupation. Requirements for success in mineral exploration are discussed, based primarily on familiarity with the circum-Pacific region, followed by analysis of the types of companies and individuals involved and the burgeoning challenges to the exploration process, which, if not remedied, will have dire consequences for future metal production and global plans for the energy transition. My involvement as a lone practitioner in both mineral exploration and metallogenic research probably says something about my independent character traits, but nonetheless has been largely unstructured and certainly unplanned. I have no hesitation in recommending a similar career path for any recent graduate who relishes adventure and is prepared to endure significant work-life imbalance.

Natural growth of gold dendrites within silica gels

Abstract High-grade ores in low-sulfidation epithermal precious metal deposits include banded quartz veins that contain gold dendrites. The processes by which dendrite growth takes place have been subject to debate for decades, especially given that these deposits are known to form from dilute thermal liquids that contain only trace amounts of gold. It is shown here that growth of gold dendrites in epithermal veins at the McLaughlin deposit in California (western USA) originally took place within bands of gel-like noncrystalline silica. The gel provided a framework for the delicate dendrites to form. The high permeability of the gel allowed the diffusion and advection of gold from the thermal liquids flowing across the top of the silica layers to the sites of crystal growth within the gel. Over time, the gel hardened to form opal-AG. This silica phase is thermodynamically unstable and recrystallized to quartz that has a distinct mosaic texture.

Epithermal Precious Metal Deposits in South Korea—History and Pursuit

Abstract The gold and silver endowment of Korea has historically been well known, with records alluding to production as far back as 1122 BC. The main gold production period was from 1925 to 1943 during the Japanese occupation of Korea, with more than 1 Moz recorded in 1939. Muguk was the most productive gold mining operation, located within the central region of South Korea, with a recorded 590 koz of gold produced from 1934 to 1998 (first mined in AD 912). The majority of the historical mining operations were closed by government order in 1943 during the Second World War and never reopened. A number of small mines operated between 1971 and 1998, with limited production during a period of gold prices generally lower than at present (~25–50% of current inflation adjusted prices, apart from a four-year period 1979–83). It is likely that significant resources remain within these historical mining areas. Gold-silver deposit types historically recognized and exploited in Korea include placers and orogenic and intrusion-related vein systems. Only more recently have epithermal vein and breccia systems been recognized. This is not surprising, given that the geologic and tectonic setting of the Southern Korean peninsula is prospective for epithermal precious metal deposits, spatially associated with basin-scale brittle fault systems in Cretaceous volcanic terranes. South Korea is an underexplored jurisdiction, with limited modern exploration and drilling until the mid-1990s, when Ivanhoe Mines Ltd. discovered the Gasado, Eunsan, and Moisan epithermal gold-silver deposits, all of which became mines. Exploration was limited for another 20 years until Southern Gold Ltd., an Australian Securities Exchange (ASX)-listed company, commenced regional-scale exploration for epithermal deposits, using a strategy similar to that successfully employed by Ivanhoe.

Genesis and mining potential of kaolin deposits in Patagonia (Argentina)

Kaolin occurs in Patagonia as residual (weathering or hydrothermal) deposits at the surface of an extended Jurassic rhyolite province or in the upper sedimentary Cretaceous or Danian–Paleocene layers. On the same paleogeographic surface, numerous epithermal Au–Ag lodes occur, making kaolin genesis a crucial point in mining exploration. The weathering or sedimentary genesis of some deposits (Puma, Súper, FPS, Espingarda and Marta) was confirmed through clay isotope results. The origin of some corrective clays (Bajo Grande and White Bentonite) was analyzed and compared with that of one sample from Ukraine and one from a hydrothermal deposit in Furtei, Sardinia, Italy. In Patagonia, the residual and sedimentary kaolin deposits have resources of over 12 million tons. The identified hydrothermal deposits have more limited resources, due to their strong mineralogical zonation, which requires their selective “pocket” kaolin exploitation. The Patagonian region is the southernmost part of a continent where a Gondwana paleosurface of Late Mesozoic age developed on Jurassic rhyolite volcanic units. This surface is exposed along tens of thousands square kilometers in the cratonic units of northern and southern Patagonia, having a strong potential for finding new kaolin or epithermal precious metal deposits.

Petrologic, tectonic, and metallogenic evolution of the southern segment of the ancestral Cascades magmatic arc, California and Nevada

Ongoing arc magmatism along western North America was preceded by ancestral arc magmatism that began ca. 45 Ma and evolved into modern arc volcanism. The southern ancestral arc segment, active from ca. 30 to 3 Ma, adjoins the northern segment in northern California across a proposed subducted slab tear. The east edge of the Walker Lane approximates the east edge of the southern arc whose products, mostly erupted from stratovolcanoes and lava dome complexes arrayed along the crest of the ancestral arc, extend down the west flank of the Sierra Nevada. Southern arc segment rocks include potassic, calc-alkaline intermediate- to silicic-composition lava flows, lava dome complexes, and associated volcaniclastic deposits. Northern and southern segment rocks are similar to other convergent-margin magmatic arc rocks but are compositionally distinct from each other. Southern segment rocks have lower TiO 2 , FeO*, CaO, and Na 2 O contents and higher K 2 O contents, and exhibit less compositional-temporal variation. Compositional distinctions between the northern and southern segment rocks reflect the composition and thickness of the crust beneath which the associated magma systems were sourced. Northern segment rock compositions are consistent with generation beneath thin, primitive crust, whereas southern segment rocks represent magmas generated and fractionated beneath thicker, more evolved crust. Although rocks in the two arc segments have similar metal abundances, they are metallogenically distinct. Small porphyry copper deposits are characteristic of the northern segment whereas significant epithermal precious metal deposits are most commonly associated with the southern segment. These metallogenic differences are also fundamentally linked to the tectonic settings and crustal regimes within which these two arc segments evolved.

Re-Os AGE OF THE PUEBLO VIEJO EPITHERMAL DEPOSIT, DOMINICAN REPUBLIC

Research Article| March 01, 2014 Re-Os AGE OF THE PUEBLO VIEJO EPITHERMAL DEPOSIT, DOMINICAN REPUBLIC Jason D. Kirk; Jason D. Kirk † 1Department of Geosciences, University of Arizona, Tucson, Arizona 85721 †Corresponding author: e-mail, jdkirk@email.arizona.edu Search for other works by this author on: GSW Google Scholar Joaquin Ruiz; Joaquin Ruiz 1Department of Geosciences, University of Arizona, Tucson, Arizona 85721 Search for other works by this author on: GSW Google Scholar Stephen E. Kesler; Stephen E. Kesler 2Department of Earth and Environmental Sciences, University of Michigan, Ann Arbor, Michigan 48109 Search for other works by this author on: GSW Google Scholar Adam Simon; Adam Simon 2Department of Earth and Environmental Sciences, University of Michigan, Ann Arbor, Michigan 48109 Search for other works by this author on: GSW Google Scholar John L. Muntean John L. Muntean 3Nevada Bureau of Mines and Geology, Mail Stop 178, University of Nevada Reno, Reno, Nevada 89557-0178 Search for other works by this author on: GSW Google Scholar Author and Article Information Jason D. Kirk † 1Department of Geosciences, University of Arizona, Tucson, Arizona 85721 Joaquin Ruiz 1Department of Geosciences, University of Arizona, Tucson, Arizona 85721 Stephen E. Kesler 2Department of Earth and Environmental Sciences, University of Michigan, Ann Arbor, Michigan 48109 Adam Simon 2Department of Earth and Environmental Sciences, University of Michigan, Ann Arbor, Michigan 48109 John L. Muntean 3Nevada Bureau of Mines and Geology, Mail Stop 178, University of Nevada Reno, Reno, Nevada 89557-0178 †Corresponding author: e-mail, jdkirk@email.arizona.edu Publisher: Society of Economic Geologists Received: 22 Mar 2013 Accepted: 26 Aug 2013 First Online: 09 Mar 2017 Online ISSN: 1554-0774 Print ISSN: 0361-0128 © 2014 Society of Economic Geologists. Economic Geology (2014) 109 (2): 503–512. https://doi.org/10.2113/econgeo.109.2.503 Article history Received: 22 Mar 2013 Accepted: 26 Aug 2013 First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Jason D. Kirk, Joaquin Ruiz, Stephen E. Kesler, Adam Simon, John L. Muntean; Re-Os AGE OF THE PUEBLO VIEJO EPITHERMAL DEPOSIT, DOMINICAN REPUBLIC. Economic Geology 2014;; 109 (2): 503–512. doi: https://doi.org/10.2113/econgeo.109.2.503 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyEconomic Geology Search Advanced Search Abstract Re-Os analyses of pyrite from the Pueblo Viejo high-sulfidation epithermal deposit (both Monte Negro and Moore ore zones) yield an isochron age of 111.9 ± 3.7 Ma and initial 187Os/188Os ratio of 0.22 ± 0.18 (mean square of weighted deviates = 84, n = 21). Isochrons grouped by pyrite setting (layered versus vein) or locations yield ages within uncertainty of the overall isochron and with each other and, as such, do not yield temporally distinguishable ages. Sphalerite vein samples as well as shale source rocks show evidence of open system behavior and loss of Re and therefore are not included in the above regression. These results confirm that the Pueblo Viejo deposit is Early Cretaceous in age and coeval with the volcanic Los Ranchos Formation, which hosts the deposit. Recent paleontological age assignments for the Hatillo Formation, which overlies the Los Ranchos Formation, show that it is only slightly younger, opening the possibility that mineralization in the Hatillo Formation could also be related to late stages of Los Ranchos magmatism. Whether these revised Los Ranchos-Hatillo relations provide sufficient cover to account for the paragenetically late formation of pyrophyllite at Pueblo Viejo remains unclear. Recognition that Pueblo Viejo is coeval with the Los Ranchos Formation shows that large-scale, high-sulfidation epithermal mineralization can form from basal island-arc tholeiitic magmatism. A review of available experimental data confirms that these magmas can generate hydrothermal solutions of the necessary composition; favorability of the Los Ranchos Formation might have been enhanced by its bimodal composition. These results suggest that exploration for epithermal precious metal deposits should include basal island-arc tholeiite terranes. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

The metallogenic evolution of the Greater Antilles

The Greater Antilles host some of the world’s most important deposits of bauxite and lateritic nickel as well as significant resources of gold and silver, copper, zinc, manganese, cobalt and chromium. Beginning in Jurassic time, sedimentary exhalative base metal deposits accumulated in marine sedimentary rift basins as North and South America drifted apart. With the onset of intraoceanic subduction during the Early Cretaceous, a primitive (tholeiitic) island arc formed above a southwesterly-dipping subduction zone. Podiform chromite deposits formed in the mantle portion of the supra-subduction zone, directly above subducted Proto-Caribbean oceanic lithosphere. Within the nascent island arc, bimodal-mafic volcanogenic massive sulfide deposits formed in a fore-arc setting; mafic volcanogenic massive sulfide deposits formed later in mature back-arc basins. The Pueblo Viejo gold district, with five million ounces in production and twenty million ounces in mineable reserves, formed at 108-112Ma, in an apical rift or back-arc setting. By Late Cretaceous time, calc-alkaline volcanism was well established along the entire length of the Greater Antilles. Volcanogenic massive sulfide deposits including shallow submarine deposits characteristic of the primitive island arc gave way to porphyry copper and epithermal precious metal deposits typical of the mature island arc. Oblique collision of the Greater Antilles with North America began in the Late Cretaceous in Cuba and migrated eastward. Orogenic gold and tungsten deposits that formed during the collision event are preserved in ophiolites and in metamorphic core complexes. Since the Eocene, regional tectonism has been dominated by strike-slip motion as the North American continent moved westward relative to the Caribbean Plate. Large nickel-cobalt laterite deposits were formed when serpentinites were exposed to weathering and erosion during the mid-Tertiary. Bauxite deposits were derived from the weathering of volcanic ash within a carbonate platform of Eocene to Miocene age

Epithermal deposits in North Xinjiang, NW China

The North Xinjiang region (NW China) is an important part of the Central Asia Orogenic Belt, situated at the junction of Siberia, Tarim and Kazakhstan plates. It is an area characterized by multiple stages of Phanerozoic continental growth, during which several porphyry and epithermal systems were formed. The relationship of these mineral systems to the geodynamic evolution of the region has not yet been well understood. In this paper, we list the main geological characteristics of 21 significant epithermal precious and base metal deposits in North Xinjiang, and classify them into high-sulfidation and low-sulfidation styles, with the latter being predominant. We have selected seven epithermal deposits representing different styles formed under different tectonic regimes and discuss their geology and geochemistry in some detail. The deposit-scale geology and geochemistry of epithermal systems in North Xinjiang are essentially similar to those in other parts of the world. All epithermal deposits in North Xinjiang are hosted in volcanic rocks with ages ranging from Devonian to Triassic, with the Early Carboniferous volcanic sequences being the most important, followed by the Permian and Triassic. The Devonian–Early Carboniferous host rocks belong to the calc-alkaline series that developed in pre-collisional arc-back-arc basin systems; whereas the Permian–Triassic host volcanic rocks of shoshonite series formed in post-subduction regimes. Available isotopic ages of these epithermal systems cluster in two periods: Early Carboniferous (>320 Ma) and Late Carboniferous–Triassic (320–220 Ma), reflecting two metallogenic episodes that occurred during subduction-related accretion and post-subduction collision regimes, respectively. Accordingly, three groups of epithermal deposits in North Xinjiang can be recognized as (1) pre-collisional deposits without or with negligible collisional-related modification, (2) deposits formed in collision regime and (3) ore systems strongly overprinted by fluid flow in post-subduction collision regime.

The La Unión Au ± Cu prospect, Camagüey District, Cuba: fluid inclusion and stable isotope evidence for ore-forming processes

The Camaguey district, Cuba, is known for its epithermal precious metal deposits in a Cretaceous volcanic arc setting. Recently, the La Union prospect was discovered in the southern part of the district, containing gold and minor copper mineralization interpreted as porphyry type. Mineralization is hosted in a 73.0 ± 1.5 Ma calc–alkaline I-type oxidized porphyry quartz diorite intrusive within volcanic and volcaniclastic rocks of the early Cretaceous Guaimaro Formation. The porphyry is affected by propylitic alteration and crosscut by a network of quartz and carbonate veinlets and veins. Chlorite, epidote, sericite, quartz, and pyrite are the main minerals in the early veins which are cut by late carbonate and zeolite veins. Late barite pseudomorphously replaces pyrite. Gold is associated with pyrite as disseminations in the altered quartz diorite and in the veins, occurring as inclusions or filling fractures in pyrite with 4 g/t Au in bulk samples, and up to 900 ppm Au in in pyrite. Fluid inclusion and oxygen isotope data are consistent with a H2O–NaCl–(KCl) mineralizing fluid, derived from the quartz diorite magma, and trapped at least at 425°C and 1.2 kbar. This primary fluid unmixed into two fluid phases, a hypersaline aqueous fluid and a low-salinity vapor-rich fluid. Boiling during cooling may have played an important role in metal precipitation. Pyrite δ34S values for the La Union prospect range between 0.71‰ and 1.31‰, consistent with a homogeneous magmatic sulfur source. The fluids in equilibrium with the mineralized rocks have estimated δ18O values from 8‰ to 11.8‰, calculated for a temperature range of 480–505°C. The tectonic environment of the La Union prospect, its high gold and low copper contents, the physical–chemical characteristics of the mineralizing fluids and the isotopic signature of the alteration minerals and fluids indicate that the La Union gold mineralization is similar to the porphyry gold type, even though the ore-related epidote–chlorite alteration can be classified as propylitic and not the classic potassic and/or phyllic alteration. The low copper contents in the prospect could be due to a mineralizing fluid previously saturated in copper, which is indicated by trapped chalcopyrite crystals in high-temperature fluid inclusions. The low-temperature paragenesis, represented by carbonate, zeolite and barite, indicates epithermal overprint. The study shows the potential for other gold porphyry-type deposits in the Cretaceous volcanoplutonic arc of Cuba.

From Source to Sinks in Auriferous Magmatic-Hydrothermal Porphyry and Epithermal Deposits

Auriferous porphyry Cu deposits are restricted to convergent plate settings, whereas epithermal precious metal deposits form at extending convergent plate settings and in rifts. Both deposit types are linked to magma carrying metals and ligands that rises to form an upper-crustal magma chamber. Magma convection and fractionation lead to volatile exsolution and collection in the apical parts of the chamber, from which exsolved hydrothermal fluid ascends to form either a porphyry Cu-Au deposit associated with stocks and dykes generally at 2-5 km depth, or an epithermal deposit associated with coeval volcanic rocks at depths of <1 km.

Age of the Camaguey Gold-Silver District, Cuba: Tectonic Evolution and Preservation of Epithermal Mineralization in Volcanic Arcs

Epithermal precious metal deposits commonly form in middle to late Cenozoic volcanic arcs several kilometers above the level at which batholiths are emplaced. This relationship has led to the guideline that epithermal deposits should not be sought in volcanic arcs where batholiths are widely exposed. The Camaguey district in central Cuba violates this tenet and provides important insights into arc-forming processes that create and expose epithermal deposits. The Camaguey district hosts the Jacinto and Florencia low-sulfidation vein deposits and the Deseada and Golden Hill high-sulfidation prospects, all of which are within a few kilometers of the large Camaguey batholith. Some of the deposits, such as Jacinto, lie immediately below a local paleosurface that exposes the batholith and probably formed when rhyolite domes were emplaced onto this surface. A second group of deposits, such as the Deseada high-sulfidation system, is hosted by volcanic rocks that overlie the Camaguey batholith and stand above the paleosurface. New Ar-Ar age measurements indicate that Jacinto-type deposits and prospects cooled through Ar/Ar closure temperatures for adularia and muscovite at about 71 Ma and are slightly younger than the widespread granodiorite facies of the Camaguey batholith. In contrast, Deseada-type deposits and mineralized zones cooled through roughly similar closure temperatures for sericite, illite, and alunite at about 80 Ma, which is older than the age of the immediately underlying Camaguey batholith. These ages are best explained if the Camaguey batholith and surrounding volcanic rocks underwent rapid uplift, probably by tectonic unroofing, to form the paleosurface below which Jacinto-type deposits formed, and if the older Deseada-type deposits were thrust into place over this surface. This tectonic unroofing probably took place in an extensional regime generated during oblique convergence of Cuba with Yucatan as the Greater Antilles arc moved north into the Caribbean region during late Cretaceous time. Thrusting probably accompanied late stages of this convergence and/or later collision with the Bahamas Bank in Eocene time. All of the epithermal deposits were covered by Late Cretaceous-Eocene sedimentary rocks that have been removed only recently by erosion, thus accounting for the preservation of these unusually old epithermal deposits. Extensional events causing tectonic unroofing are probably common in arcs that undergo oblique convergence and could be an important agent for formation of epithermal deposits in other parts of the world. Preservation of these deposits might be aided by deposition and later erosion of sedimentary cover related to later collisions.

The Gandy and Abolhassani Epithermal Prospects in the Alborz Magmatic Arc, Semnan Province, Northern Iran

The Gandy and Abolhassani epithermal precious and base metal deposits occur in the Torud-Chah Shirin mountain range in the Alborz magmatic belt of northern Iran. The mountain range is considered to be part of the Paleogene Alborz volcanic arc. The exposed rocks in the study area consist of a volcaniclastic sequence of thin-bedded siltstones and sandstones, lapilli tuffs, volcanic breccias, and intermediate lava flows at Gandy, and mostly andesitic flows at Abolhassani. The flows are middle to upper middle Eocene age and they show a typical arc geochemical signature, with low concentrations of Nb, Ta, Zr, Hf, and Ti. Variable hydrothermal alteration occurs in scattered outcrops, covering about 4 km^2 at Gandy and 1 km^2 at Abolhassani. The Gandy and Abolhassani areas are about 3 km apart, and each contains a small abandoned Pb-Zn mine. Mineralization at Gandy occurs in quartz sulfide veins and breccias and is accompanied by alteration halos of quartz, illite, and calcite up to 2 m wide. The mineralization is divided into three main stages: brecciation (I), fracture filling (II), and crustiform banding (III). Stage I is economically important in terms of precious metal content. Stage II consists of four substages and contains the majority of base metal ore with quartz, calcite, and barite. Native gold is commonly found within partially oxidized pyrite and secondary iron (hydr)oxides such as goethite in stage I and coexists with galena and chalcopyrite in stage II. The final stage is dominated by quartz and calcite. Mineralization in the Abolhassani veins occurred in three main stages. The first two stages, which are economically important, contain similar mineral assemblages, including quartz, calcite, barite, galena, sphalerite, pyrite, and chalcopyrite, whereas the final stage is dominated by quartz and calcite. No gold grains were found in the Abolhassani samples. The average (max) assays from 14 channel samples of Gandy veins are 14.5 (68.3) g/t Au, 30.6 (161) g/t Ag, 3.1 (13) wt percent Pb, 0.84 (3.8) wt percent Zn, and 1.0 (6.3) wt percent Cu. For comparison, the values from 19 channel samples of Abolhassani veins are 0.85 (6.0) g/t Au, 29.5 (115) g/t Ag, 6.4 (16.5) wt percent Pb, 1.2 (5.2) wt percent Zn, and 0.83 (7.7) wt percent Cu. Fluid inclusion and sulfur isotope compositions were analyzed for the sulfide-sulfate assemblages of stage II at Gandy and stages I and II at Abolhassani. In both cases fluid inclusion assemblages were examined mostly in subhedral crystals of sphalerite. The average homogenization temperatures (T_h) and salinities of fluid inclusion assemblages from Gandy range from 234° to 285°C, with a peak at about 250°C and 4.2 to 5.4 wt percent NaCl equiv. These T_h values are in good agreement with isotopic temperatures from two sphalerite and galena pairs (236° and 245°C). The temperature and salinity values in fluid inclusion assemblages from the Abolhassani deposit range from 234° to 340°C and 6.7 to 18.7 wt percent NaCl equiv. Sulfide pairs of sphalerite-galena do not give reasonable isotopic equibrium temperatures at Abolhassani. Comparison of Th versus ice melting (T_(m(ice))) values for the two deposits indicates the presence of a moderate-salinity fluid (5–6 wt % NaCl equiv) of similar temperature (~250°C) in each deposit but with a higher temperature and salinity component also present at Abolhassani. The base metal-rich mineralization at Abolhassani may thus have been caused by the periodic injection of this higher salinity fluid. The Abolhassani deposit has a higher average Ag/Au ratio (~35) and Pb + Zn concentration (up to 7.6 wt %) than Gandy (Ag/Au ~ 2 and 3.9 wt %), consistent with this interpretation. The minimum depth of formation was at least 430 m below the paleowater table for Gandy and possibly as much as 600 m at Abolhassani. The lower grades of gold and the presence of higher salinity fluids at Abolhassani suggest that the occurrence of higher grade precious metal zones is unlikely at greater depth. By contrast, Gandy may have potential at depth for extensions of the high-grade gold veins. Exploration in the region should focus on areas with geologic evidence for relatively little posthydrothermal erosion, <200 to 300 m, thus increasing the preservation potential of epithermal veins with high gold grades, similar to those at Gandy. Results from Gandy and Abolhassani may aid exploration and assessment of the numerous, untested epithermal and related prospects along this 1,800-km-long volcanic belt in northern Iran.

New Mexico structural zone—an analogue of the Colorado mineral belt

Updated aeromagnetic maps of New Mexico together with current knowledge of the basement geology in the northern part of the state (Sangre de Cristo and Sandia–Manzano Mountains)—where basement rocks were exposed in Precambrian-cored uplifts—indicate that the northeast-trending Proterozoic shear zones that controlled localization of ore deposits in the Colorado mineral belt extend laterally into New Mexico. The shear zones in New Mexico coincide spatially with known epigenetic precious- and base-metal ore deposits; thus, the mineralized belts in the two states share a common inherited basement tectonic setting. Reactivation of the basement structures in Late Cretaceous–Eocene and Mid-Tertiary times provided zones of weakness for emplacement of magmas and conduits for ore-forming solutions. Ore deposits in the Colorado mineral belt are of both Late Cretaceous–Eocene and Mid-Tertiary age; those in New Mexico are predominantly Mid-Tertiary in age, but include Late Cretaceous porphyry-copper deposits in southwestern New Mexico. The mineralized belt in New Mexico, named the New Mexico structural zone, is 250-km wide. The northwest boundary is the Jemez subzone (or the approximately equivalent Globe belt), and the southeastern boundary was approximately marked by the Santa Rita belt. Three groups (subzones) of mineral deposits characterize the structural zone: (1) Mid-Tertiary porphyry molybdenite and alkaline-precious-metal deposits, in the northeast segment of the Jemez zone; (2) Mid-Tertiary epithermal precious-metal deposits in the Tijeras (intermediate) zone; and (3) Late Cretaceous porphyry-copper deposits in the Santa Rita zone. The structural zone was inferred to extend from New Mexico into adjacent Arizona. The structural zone provides favorable sites for exploration, particularly those parts of the Jemez subzone covered by Neogene volcanic and sedimentary rocks.

La Marciana Jurassic sinter—implications for exploration for epithermal precious-metal deposits in Deseado Massif, southern Patagonia, Argentina

A well-preserved Jurassic siliceous sinter has been identified at La Marciana farm in southern Patagonia, Argentina. Fossil sinters are a rare occurrence and this example bridges the gap between Devonian/Carboniferous examples and more abundant Tertiary sinters. Its presence is significant in the context of exploration for epithermal precious-metal deposits in the Deseado Massif geological province of southern Patagonia.Field and textural characteristics of the La Marciana siliceous deposit provide unequivocal evidence of its origin. It is located in an appropriate volcanic and epithermal environment and it displays a strongly developed non-planar depositional lamination, elongate cavities parallel to this lamination, small-scale stromatolitic columnar structures, moulds of stems and roots of plants and desiccation cracks.Geochemical and oxygen isotope studies provide additional information compatible with a sinter origin. The content of precious-metal elements—apart from in anomalous hydrothermal breccia—is low, and the average Au, Ag, As and Sb concentrations are similar to those of most known fossil sinters. The 18O SMOW values obtained on La Marciana sinter range from 7.3 to 16‰ and could indicate a lower temperature of formation when compared with quartz-vein isotope data from the Cerro Vanguardia area—the best-known example of an epithermal precious-metal deposit in the Deseado Massif.Hydrothermal alteration of the volcanic rocks and the presence of quartz veinlets and hydrothermal breccias with Au and Ag anomalies in the La Marciana area are indicative of gold- and silver-rich hydrothermal activity. Taken in conjunction with the presence of the La Marciana sinter, which is further evidence of Jurassic hot-spring deposits in the western border of the Deseado Massif, this suggests that exploration in the area should focus on locating epithermal quartz vein systems that may be preserved at depth.

Modern hot springs in the southern volcanic Cordillera of Peru and their relationship to Neogene epithermal precious-metal deposits

Modern hot springs associated with surface precipitates and acid alteration exist in several areas of the Southern Volcanic Cordillera of Peru. The hot springs are related to continental geothermal systems, which formed important epithermal precious-metal deposits during the Cenozoic. There are several high-temperature geothermal fields in the areas studied, with reservoir temperatures up to 190°C and siliceous sinter deposits that indicate favourable conditions for possible epithermal mineralisation at depth. To evaluate whether the modern geothermal systems are capable of forming epithermal deposits, as were ancient ones during the Cenozoic, the siliceous sinter deposits have been compared with those of geothermal systems in other areas, interpreted as modern examples of epithermal mineralisation. Also, the oxygen and hydrogen isotope composition of hot-spring waters has been compared with those of other geothermal systems and fluids of epithermal deposits in North and South America. The data suggest that the Cenozoic epithermal precious-metal deposits were formed by deeper and longer-term geothermal systems than those existing today. Also, the estimated reservoir temperatures of the geothermal systems in southern Peru are low compared to systems elsewhere which show evidence of subsurface epithermal mineralisation. It is therefore possible that the current formation of economic epithermal deposits, even beneath higher-temperature geothermal fields in the Southern Volcanic Cordillera of Peru, is rather restricted.

Combarbalá advanced argillic alteration zone, Chile: geology, geochemistry, mineralogy and mineralization potential

Central and northern Chile hosts several well-known epithermal precious-metal deposits associated with kaolinite-alunite-quartz alteration, but only Combarbalá district (Coquimbo region) is the altered rock extracted for ornamental purposes. This ornamental rock, called combarbalita is mined from pockets and veins hosted by altered, sub-horizontal, Cretaceous age that crop out around the town of Combarbalá. A study has been undertaken to define the conditions required for combarbalite formation and to evaluate, by analogy with Cu-Au and Cu-Pb deposits in adjacent districts, the significance of silicified bodies within the advanced argillic alteration as possible indicators of metalliferous mineral concentrations in the underlying propylitic structural analysis, microscopy, X-ray diffraction and chemical analysis were the principal methods used.The pyroclastic sediments that host combarbalite are or combarbalite, pyroclastic sedimentary strata of andesites. Field observation, crosscut by cylindrical, massive silica bodies interpreted as vents and considered to be the principal conduits for hydrothermal fluids. The silica bodies in the contain Cu and Pb minerals, have anomalous contents of As, Sb, Hg, Ag and Au and are cut by barite veins. Regional structural control for the silicic and advanced argillic alteration was exerted by faults orientated N 60° W, obliquely to the present north-south graben structure in which the district is located. Malachite and cerussite mineralization roughly follows these northwest-trending fractures, whereas fracture-bound chrysocolla, pyrite and chalcopyrite are found in the underlying propylitic alteration. Sulphide mineral concentrations are likely to be located close to the intersection of the vent structures with the propylitic alteration. As to the existing mining activity, the best, massive varieties of combarbalite are developed in conglomerates, where very fine-grained mineralogy gives the rock workability and preserves its attractive original textures.

Geology, geochemistry, and origin of volcanic rock-hosted uranium deposits in northwestern Nevada and southeastern Oregon, USA

Northwestern Nevada and southeastern Oregon have the largest uranium (U) deposits in Tertiary volcanic rocks in the US. Most deposits are in or adjacent to calderas or rhyolite lava-dome fields and are hydrothermal. Almost all are associated with rhyolites that have high primary U concentrations (9 to 20 ppm), but the rhyolites range from peralkaline to peraluminous. Caldera-related occurrences are hosted by peralkaline rocks in the McDermitt and Virgin Valley calderas. At McDermitt, major deposits are along or just inboard of the southwestern (Kings River) and northern ring fractures (Bretz–Opalite) of the caldera complex. The Kings River deposits are strongly enriched in trace elements associated with epithermal precious-metal deposits (e.g., As, Au, Ag, Mo, and Sb). Much of the U occurs as hydrothermal uraniferous zircon, and whole rock samples contain as much as 5% Zr. Gangue minerals include quartz, adularia, fluorite, and numerous sulfides. 40 Ar/ 39Ar dating of adjacent volcanic rocks and adularia indicates that mineralization was contemporaneous with igneous activity. The Bretz–Opalite deposits, which include the large Aurora resource (7.5×10 6 kg U 3O 8), are less enriched in epithermal trace elements and contain uraninite, coffinite, chalcedony, opal, and numerous sulfides. At Virgin Valley, deposits are mostly in tuffaceous sediments adjacent to ring-fracture rhyolites. Trace-element enrichment is minor, U is hexavalent in all identified U minerals, and associated phases include opal, organic material, and pyrite. Deposits in the Lakeview U district, Oregon are hosted by weakly peraluminous rhyolite domes and tuffs and are highly enriched in epithermal trace elements; the mineral assemblage includes coffinite, opal, and numerous sulfides. Minor U occurrences at Buff Peak, Nevada, the newly recognized, westernmost topaz (peraluminous) rhyolite in the US, are also trace element enriched; gangue minerals include quartz, adularia, and sulfides. On the basis of their geologic setting, geochemistry, age, and mineralogy, the McDermitt, Lakeview, and Buff Peak deposits are hydrothermal; sparse, published fluid inclusion data indicate temperatures between 200°C and 330°C. Virgin Valley deposits most likely are hydrothermal. Minor deposits in outflow ash-flow tuffs are distant from any intrusive heat source and probably formed from low-temperature groundwater.

Regional Crustal Structures and Their Relationship to theDistribution of Ore Deposits in the Western United States, Based on Magnetic andGravity Data

Upgraded gravity and magnetic databases and associated filtered-anomaly maps of western United States define regional crustal fractures or faults that may have guided the emplacement of plutonic rocks and large metallic ore deposits. Fractures, igneous intrusions, and hydrothermal circulation tend to be localized along boundaries of crustal blocks, with geophysical expressions that are enhanced here by wavelength filtering. In particular, we explore the utility of regional gravity and magnetic data to aid in understanding the distribution of large Mesozoic and Cenozoic ore deposits, primarily epithermal and porphyry precious and base metal deposits and sediment-hosted gold deposits in the western United States cordillera. On the broadest scale, most ore deposits lie within areas characterized by low magnetic properties. The Mesozoic Mother Lode gold belt displays characteristic geophysical signatures (regional gravity high, regional low-to-moderate background magnetic field anomaly, and long curvilinear magnetic highs) that might serve as an exploration guide. Geophysical lineaments characterize the Idaho-Montana porphyry belt and the La Caridad-Mineral Park belt (from northern Mexico to western Arizona) and thus indicate a deep-seated control for these mineral belts. Large metal accumulations represented by the giant Bingham porphyry copper and the Butte polymetallic vein and porphyry copper systems lie at intersections of several geophysical lineaments. At a more local scale, geophysical data define deep-rooted faults and magmatic zones that correspond to patterns of epithermal precious metal deposits in western and northern Nevada. Of particular interest is an interpreted dense crustal block with a shape that resembles the elliptical deposit pattern partly formed by the Carlin trend and the Battle Mountain-Eureka mineral belt. We support previous studies, which on a local scale, conclude that structural elements work together to localize mineral deposits within regional zones or belts. This study of mineral deposits of the western United States demonstrates the ability of magnetic and gravity data to elucidate the regional geologic framework or structural setting and to contribute in locating favorable environments for hydrothermal mineralization.

Gold dispersion in Andean desert environments (Atacama, Chile)

The mechanism of Au dispersion in sediments of mountainous desert environments has been studied in two different areas of the Chilean Andes. The San Pedro de Cachiyuyo placer (ca. 1800 m a.s.l.) consists of alluvial fans and ephemeral stream sediments deposited in a stable piedmont. The primary Au source is Cu–Au-bearing hydrothermal tourmaline breccia. The relief of this area is low (1850 m) and the average annual rainfall is ca. 20 mm. The La Coipa area (ca. 4000 m a.s.l.) is characterized by a rugged relief. The annual average rainfall is ca. 100 mm. The sedimentary deposits are less sorted than in San Pedro de Cachiyuyo and consist of ephemeral stream alluvium and slope deposits formed in a periglacial environment. The primary Au sources are two volcanic-hosted epithermal precious metal deposits. At San Pedro de Cachiyuyo, the halo is less than 1 km in length and the highest Au concentration are observed at the break in slope between the hillside and the piedmont. In the La Coipa area, Au was only detected by chemical analysis in the 125–63 μm and <63 μm fraction; however, the dispersion halo extends for over 10 km from the source. In both cases the geochemical signal of gold was strongest in the fraction <63 μm.

Epithermal precious metal deposits of Italy-an overview

Epithermal precious metal deposits have only quite recently been added to the metallogenic view of Italy. It was only in the late 1980s that two districts were recognized, in Sardinia and southern Tuscany, previously long known for a large variety of other commodities. The Sardinian epithermal precious metal district is associated with the Oligocene-Miocene Sardinian calc-alkaline magmatic cycle. The most relevant areas include the Au deposit at Furtei, already in production, and the Osilo prospect, where extensive exploration is under way. The deposit at Furtei contains at least six tonnes of gold metal. The mineralogy of the deposit and of the alteration assemblages is typical of the acid-sulfate (high-sulfidation) class of volcanic-hosted epithermal deposits. Fluids associated with alteration and mineralization have moderate temperatures (200–300 °C) and low salinities (less than 6% wt. NaCl equivalent); high-temperature, high-salinity fluids similar to porphyry-style systems also circulated at Furtei. At Osilo, a number of quartz veins containing up to several ppm Au have been identified. The alteration assemblage includes adularia and illite, i.e. is typical of low-sulfidation deposits. Fluid inclusion homogenization temperatures range from 198 to 270 °C, with salinities of less than 4% wt. NaCl equivalent. In southern Tuscany, a number of carbonate-hosted (“Carlin type”) gold showings occur at the edges of the geothermal fields of Larderello, Amiata and Latera, an area previously known for Sb mineralization. In fact, many showings coincide with former Sb mines, and stibnite, along with pyrite, is the most common sulfide mineral. Gold is typically invisible. Fluids hosted in a variety of minerals from these occurrences span a relatively large temperature range (132 to 245 °C), with constantly low salinities (less than 7% wt. NaCl equivalent). The onset of (presumably meteoric) hydrothermal fluid circulation can be ascribed to the emplacement of Neogene Tuscan magmatic rocks, but the ultimate source of gold remains speculative.