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Richard J. Goldfarb

Publications and source records attributed to Richard J. Goldfarb.

29 records · Page 2Linked to original sources
USGS2012

Mineral potential tracts for orogenic, Carlin-like, and epithermal gold deposits in the Islamic Republic of Mauritania, (phase V, deliverable 68): Chapter H1 in Second projet de renforcement institutionnel du secteur minier de la République Islamique de Mauritanie (PRISM-II)

In 1996, at the request of the Government of the Islamic Republic of Mauritania, a team of U.S. Geological Survey (USGS) scientists produced a strategic plan for the acquisition, improvement and modernization of multidisciplinary sets of data to support the growth of the Mauritanian minerals sector and to highlight the geological and mineral exploration potential of the country. In 1999, the Ministry of Petroleum, Energy, and Mines of the Islamic Republic of Mauritania implemented a program for the acquisition of the recommended basic geoscientific information, termed the first Projet de Renforcement Institutionnel du Secteur Minier (Project for Institutional Capacity Building in the Mining Sector, PRISM-I). As a result of the PRISM-I efforts, a great deal of new geological, geophysical, geochemical, remote sensing, and hydrological data became available for evaluation and synthesis. However, the Ministry of Petroleum, Energy, and Mines recognized that additional work was required to extract the full benefit of the data before it could be of greatest use to the international community and of benefit to the Mauritanian minerals and development sector. To achieve this benefit, the Ministry of Petroleum, Energy, and Mines implemented a second Projet de Renforcement Institutionnel du Secteur Minier (PRISM-II) in 2006 to consolidate, synthesize, and interpret all of the existing data, create a new 1:1,000,000 scale geologic map, and define the mineral resource potential of the country. A consortium in which the USGS was the lead scientific agency carried out the majority of the PRISM-II work. In 2008, the USGS Mauritania Minerals Project was interrupted due to political changes in Mauritania. PRISM-II work resumed in 2011, and was completed in 2013 with the delivery of over 40 separate written reports and plates, an access file containing the Mauritanian National Mineral Deposits Database, and an interactive GIS containing all of the multi-disciplinary data and interpretive areas of mineral resource potential in Mauritania. This report contains the USGS results of the PRISM-II Mauritania Minerals Project and is presented in cooperation with the Ministry of Petroleum, Energy, and Mines of the Islamic Republic of Mauritania. The Report is composed of separate chapters consisting of multidisciplinary interpretive reports with accompanying plates on the geology, structure, geochronology, geophysics, hydrogeology, geochemistry, remote sensing (Landsat TM and ASTER), and SRTM and ASTER digital elevation models of Mauritania. The syntheses of these multidisciplinary data formed the basis for additional chapters containing interpretive reports on 12 different commodities and deposit types known to occur in Mauritania, accompanied by countrywide mineral resource potential maps of each commodity/deposit type. The commodities and deposit types represented include: (1) Ni, Cu, PGE, and Cr deposits hosted in ultramafic rocks; (2) orogenic, Carlin-like, and epithermal gold deposits; (3) polymetallic Pb-Zn-Cu vein deposits; (4) sediment-hosted Pb-Zn-Ag deposits of the SEDEX and Mississippi Valley-type; (5) sediment-hosted copper deposits; ( 6) volcanogenic massive sulfide deposits; (7) iron oxide copper-gold deposits; (8) uranium deposits; (9) Algoma-, Superior-, and oolitic-type iron deposits; (10) shoreline Ti-Zr placer deposits; (11) incompatible element deposits hosted in pegmatites, alkaline rocks, and carbonatites, and; (12) industrial mineral deposits. Additional chapters include the Mauritanian National Mineral Deposits Database are accompanied by an explanatory text and the Mauritania Minerals Project GIS that contains all of the interpretive layers created by USGS scientists. Raw data not in the public domain may be obtained from the Ministry of Petroleum, Energy, and Mines in Nouakchott, Mauritania.

Open-File Report↗
USGS2011

Petrogenesis of postcollisional magmatism at Scheelite Dome, Yukon, Canada: Evidence for a lithospheric mantle source for magmas associated with intrusion-related gold systems

The type examples for the class of deposits termed intrusion-related gold systems occur in the Tombstone-Tungsten belt of Alaska and Yukon, on the eastern side of the Tintina gold province. In this part of the northern Cordillera, extensive mid-Cretaceous postcollisional plutonism took place following the accretion of exotic terranes to the continental margin. The most cratonward of the resulting plutonic belts comprises small isolated intrusive centers, with compositionally diverse, dominantly potassic rocks, as exemplified at Scheelite Dome, located in central Yukon. Similar to other spatially and temporally related intrusive centers, the Scheelite Dome intrusions are genetically associated with intrusion-related gold deposits. Intrusions have exceptional variability, ranging from volumetrically dominant clinopyroxene-bearing monzogranites, to calc-alkaline minettes and spessartites, with an intervening range of intermediate to felsic stocks and dikes, including leucominettes, quartz monzonites, quartz monzodiorites, and granodiorites. All rock types are potassic, are strongly enriched in LILEs and LREEs, and feature high LILE/HFSE ratios. Clinopyroxene is common to all rock types and ranges from salite in felsic rocks to high Mg augite and Cr-rich diopside in lamprophyres. Less common, calcic amphibole ranges from actinolitic hornblende to pargasite. The rocks have strongly radiogenic Sr (initial 87 Sr/ 86 Sr from 0.711-0.714) and Pb isotope ratios ( 206 Pb/ 204 Pb from 19.2-19.7), and negative initial ɛ Nd values (−8.06 to −11.26). Whole-rock major and trace element, radiogenic isotope, and mineralogical data suggest that the felsic to intermediate rocks were derived from mafic potassic magmas sourced from the lithospheric mantle via fractional crystallization and minor assimilation of metasedimentary crust. Mainly unmodified minettes and spessartites represent the most primitive and final phases emplaced. Metasomatic enrichments in the underlying lithospheric mantle are attributes of the ancient North American cratonic margin that appear to be essential prerequisites to this style of postcollisional magmatism and associated gold-rich fluid exsolution. This type of magmatic hydrothermal activity occurs in a very specific tectonic setting that typically sets intrusion-related gold deposits apart from orogenic gold deposits, which are synorogenic in timing and have no consistent direct relationship to such diverse and contemporaneous lithospheric mantle-derived magmas, although they too are commonly sited adjacent to lithospheric boundaries.

Yukon↗
USGS2011

The Kharapeh orogenic gold deposit: Geological, structural, and geochemical controls on epizonal ore formation in West Azerbaijan Province, Northwestern Iran

The Kharapeh gold deposit is located along the northwestern margin of the Sanandaj–Sirjan Zone (SSZ) in the West Azerbaijan province, Iran. It is an epizonal orogenic gold deposit formed within the deformed zone between central Iran and the Arabian plate during the Cretaceous–Tertiary Zagros orogeny. The deposit area is underlain by Cretaceous schist and marble, as well as altered andesite and dacite dikes. Structural analysis indicates that the rocks underwent tight to isoclinal recumbent folding and were subsequently co-axially refolded to upright open folds during a second deformation. Late- to post-tectonic Cenozoic granites and granodiorites occur northeast of the deposit area. Mineralization mainly is recognized within NW-trending extensional structures as veins and breccia zones. Normal faults, intermediate dikes, and quartz veins, oriented subparallel to the axial surface of the Kharapeh antiform, indicate synchronous extension perpendicular to the fold axis during the second folding event. The gold-bearing quartz veins are >1 km in length and average about 6 m in width; breccia zones are 10–50 m in length and ≤1 m in width. Hydrothermal alteration mainly consists of silicification, sulfidation, chloritization, sericitization, and carbonatization. Paragenetic relationships indicate three distinct stages—replacement and silicification, brecciation and fracture filling, and cataclastic brecciation—with the latter two being gold-rich. Fluid inclusion data suggest mineral deposition at temperatures of at least 220–255°C and depths of at least 1.4–1.8 km, from a H 2 O–CO 2 ±CH 4 fluid of relatively high salinity (12–14 wt.% NaCl equiv.), which may reflect metamorphism of passive margin carbonate sequences. Ore fluid δ 18 O values between about 7‰ and 9‰ suggest no significant meteoric water input, despite gold deposition in a relatively shallow epizonal environment. Similarities to other deposits in the SSZ suggest that the deposit formed as part of a diachronous gold event during the middle to late Tertiary throughout the SSZ and during the final stages of the Zagros orogeny. The proximity of Kharapeh to the main tectonic suture of the orogen, well-developed regional fold systems with superimposed complex fracture geometries, and recognition of nearby volcanogenic massive sulfide systems that suggest a region characterized by sulfur- and metal-rich crustal rocks, collectively indicate an area of the SSZ with high favorability for undiscovered gold resources.

West Azerbaijan Province↗
USGS2010

Secular variation in economic geology

The temporal pattern of ore deposits on a constantly evolving Earth reflects the complex interplay between the evolving global tectonic regime, episodic mantle plume events, overall changes in global heat flow, atmospheric and oceanic redox states, and even singular impact and glaciation events. Within this framework, a particular ore deposit type will tend to have a time-bound nature. In other words, there are times in Earth history when particular deposit types are absent, times when these deposits are present but scarce, times when they are abundant, and still other times for which we lack sufficient data. Understanding of such secular variation provides a critical first-order tool for exploration targeting, because rocks that have formed or were deformed during a certain time slice may be very permissive for a given deposit type, whereas identification of rocks of less favorable ages would help eliminate large areas during exploration programs. Secular analysis, therefore, is potentially a powerful tool for mineral resource assessment in poorly known terranes, providing a quick filter for favorability of a given deposit type using age of host rocks. Factors bearing on the known age distribution of a particular type of deposit include the following: (1) uneven preservation, (2) data gaps, (3) contingencies of plate motions, and (4) long-term secular changes in the Earth System. The present special issue of Economic Geology is focused on the latter factor, although all of these are interrelated. The selective preservation of certain mineral deposit types and the greater susceptibility for shallowly formed ores in tectonically active environments to be lost to erosion define a pattern that is superimposed on the secular formational trends (e.g., Groves et al., 2005a , b ; Kerrich et al., 2005 ). With improved geochronological methods and the availability of information on important mineral deposits from most parts of the world, data gaps for defining broad temporal distributions of ore types are becoming smaller. It has been increasingly recognized that ore deposit formation is also correlated with plate tectonic setting. Nevertheless, a complex Earth history of supercontinent assembly and breakup has led to the fragmentation of many Paleozoic and Precambrian mineral provinces. The use of plate reconstructions in economic geology, although extremely controversial and conjectural before the late Paleozoic, is critical for defining these provinces prior to the added complications resulting from post-ore plate motions. It is now well established that the temporal patterns of many types of mineral deposits (Fig. 1 1 ) reflect the formation or break-up of supercontinents and the preservation potential of deposits formed during these periods ( Barley and Groves, 1992 ; Titley, 1993 ; Kerrich et al., 2005 ; Goldfarb et al., 2009 ). Approximate time periods for such formation and break-up, respectively, include 2800–2500 and 2450–2100 Ma for Kenorland, 2100–1800 and 1600–1300 Ma for Nuna/Columbia, 1300–1100 and 850–600 Ma for Rodinia, and 600–300 Ma and 200–60 Ma for Gondwanaland-Pangea. A new supercontinent, Amasia, has begun to form during the past 250 m.y., thus overlapping Pangea break-up. Many of the formation-preservation patterns are themselves controlled by progressive cooling of Earth, the change from a mantle-plume buoyancy style to subduction-dominated tectonics, a decreasing buoyancy of the subcontinental lithospheric mantle, and depth of ore formation. In general, orogenic Au, volcanogenic massive sulfide (VMS), epithermal Au-Ag, and porphyry Cu±Au and Mo porphyry deposits form in active margins during periods of supercontinent assembly. Numerous other ore deposit types show an association with supercontinent formation, but develop inland of the active margin. These include many of the MVT Pb-Zn deposits and unconformity-type U deposits. The Tertiary Carlin-type deposits within the deformed shelf sequences along the North American craton margin also appear to have formed during the ongoing growth of Amasia. Those ores associated with periods of supercontinent breakup or attempted breakup are more difficult to define. They probably include diamond, Bushveld-type Ni-Cu-PGE, IOCG, and clastic-dominated Pb-Zn (or SEDEX) deposits in intracontinental areas of failed rifting, and other clastic-dominated Pb-Zn deposits in areas of actual breakup. In all cases, however, these temporal/spatial distributions are ultimately controlled by the secular character of Earth history.

Economic Geology↗
USGS2007

Geology and origin of epigenetic lode gold deposits, Tintina Gold Province, Alaska and Yukon

More than 50 million ounces of lode gold resources have been defined in the previous 15 years throughout accreted terranes of interior Alaska and in adjacent continental margin rocks of Yukon. The major deposits in this so-called Tintina Gold Province formed around 105 to 90 million years ago in east-central Alaska and Yukon, and around 70 million years ago in southwestern Alaska, late in the deformational history of their host rocks. All gold deposits studied to date formed from CO 2 -rich and 18 O-rich crustal fluids, most commonly of low salinity. The older group of ores includes the low-grade intrusion-related gold systems at Fort Knox near Fairbanks and those in Yukon, with fluids exsolved from fractionating melts at depths of 3 to 9 kilometers and forming a zoned sequence of auriferous mineralization styles extending outward to the surrounding metasedimentary country rocks. The causative plutons are products of potassic mafic magmas generated in the subcontinental lithospheric mantle that interacted with overlying lower to middle crust to generate the more felsic ore-related intrusions. In addition, the older ores include spatially associated, high-grade, shear-zonerelated orogenic gold deposits formed at the same depths from upward-migrating metamorphic fluids; the Pogo deposit is a relatively deep-seated example of such. The younger gold ores, restricted to southwestern Alaska, formed in unmetamorphosed sedimentary rocks of the Kuskokwim basin within 1 to 2 kilometers of the surface. Most of these deposits formed via fluid exsolution from shallowly emplaced, highly evolved igneous complexes generated mainly as mantle melts. However, the giant Donlin Creek orogenic gold deposit is a product of either metamorphic devolatilization deep in the basin or of a gold-bearing fluid released from a flysch-melt igneous body.

Alaska, Yukon↗
USGS2007

Surface-water, ground-water, and sediment geochemistry of epizonal and shear-hosted mineral deposits in the Tintina Gold Province--arsenic and antimony distribution and mobility: Chapter G in Recent U.S. Geological Survey studies in the Tintina Gold Province, Alaska, United States, and Yukon, Canada--results of a 5-year project

Epigenetic mineral deposits in the Tintina Gold Province are generally characterized by high concentrations of arsenic and antimony in their mineral assemblage. A total of 347 samples (ground water, surface water, and stream sediment) were collected to investigate the distribution and mobility of arsenic and antimony in the environment near known mineral deposits. Samples were collected from east to west at Keno Hill and Brewery Creek, Yukon, Canada; and Cleary Hill, True North, Scrafford Mine, Fairbanks, Ryan Lode, Stampede Creek, Slate Creek, and Donlin Creek, all in Alaska. Surface- and ground-water samples are all slightly acidic to near-neutral in pH (5-8), have a wide range in specific conductance (surface water 17-2,980 microsiemens per centimeter and ground water 170-2,940 microsiemens per centimeter), and show elevated dissolved arsenic and antimony concentrations (arsenic in surface water is less than 1 to 380 micrograms per liter and in ground water is less than 1 micrograms per liter to 1.5 milligrams per liter; antimony in surface water is less than 2 to 660 micrograms per liter and in ground water is less than 2 to 60 micrograms per liter). Stream sediments downstream from these deposits have high concentrations of arsenic and antimony (arsenic median is 1,670 parts per million, maximum is 10,000 parts per million; antimony median is 192 parts per million, maximum is 7,200 parts per million). The mobility of arsenic and antimony is controlled by the local redox environment, with arsenic being less mobile in oxidized surface waters relative to antimony, and arsenic more mobile in reduced ground water. These factors suggest that both antimony and arsenic may be useful pathfinder elements in water and sediment for targeting similar style deposits elsewhere in the Tintina Gold Province.

Alaska;Yukon↗
USGS2002

Tectonics and distribution of gold deposits in China - An overview

Gold exploration in China has expanded rapidly during the last two decades since a modern approach to economic development has become a national priority. China currently produces 180 tonnes (t) of gold annually, which is still significantly less than South Africa, USA, and Australia. However, China is now recognized as possessing significant gold resources in a wide range of mineral deposit types. Present estimates of gold resources in China exceed 4,500 t, which comprise 60% in gold-only deposits, more than 25% in base metal-rich skarn, porphyry, and vein deposits, and more than 10% in placer accumulations. The major gold provinces in China formed during the main episodes of Phanerozoic tectonism. Such tectonism involved interaction of China's three major Precambrian cratons, North China, Tarim, and Yangtze (or South China when combined with Cathaysia block), with the Angara (or Siberian), Kazakhstan–Kyrgyzstan, and Indian cratons. Resulting collisions included deformation of accreted oceanic sequences between the cratonic blocks. The most important ore-forming orogenies were (1) the late Paleozoic Variscan (405–270 Ma), which led to amalgamation of the Angara, North China and Yangtze cratons, (2) the Indosinian (270–208 Ma), which led to the collision of North China and South China cratons, (3) the Yanshanian (208–90 Ma), which was largely influenced by the subduction of the Izanagi–Pacific plates beneath eastern China, and (4) the Himalayan (<90 Ma) indentation of the Indian continent into Eurasia. No important Precambrian gold systems are recognized in China, mainly because of reworking of exposed Precambrian rocks by these younger orogenies, but there are a few Caledonian (600–405 Ma) gold-bearing systems in northern Xinjiang. Most of China's orogenic, epithermal, and Carlin-like gold deposits are in the reworked margins of major cratonic blocks and in metasedimentary rock-dominated fold belts adjacent to these margins. Accordingly, the major gold provinces are present along the northern, southeastern and southern margins of the North China craton, along the southwestern and northwestern margins of the Yangtze craton, in the Tianshan and Altayshan orogenic belts in northern Xinjiang, and throughout the southeastern China fold belt. Gold-placer deposits derived from these primary deposits are concentrated in the northernmost part of northeastern China and along the northwestern margin of the Yangtze craton. The major provinces with significant gold in porphyry-related copper systems and base metal skarns are present in the Yangtze River area along the northeastern and southeastern margin of the Yangtze craton, in the fold belt in southwestern China, and scattered through northern China. Three-quarters of the Chinese gold-only deposits occur within the North China craton margins. Half are located in the uplifted Precambrian metamorphic rocks and most of the remainder are hosted in the Phanerozoic granitoids that intruded the reworked Precambrian terranes. The abundance of granite-hosted gold contrasts the North China craton with other Precambrian cratons, such as those in Western Australia, central Canada, and Zimbabwe, where gold is mainly hosted in the Archean greenstone belts. This difference may be explained by the multiple episodes of Phanerozoic tectonism along the North China craton margins resulting from the collision of the Angara, North China, and South China cratons, and from subduction of the Izanagi–Pacific oceanic plates underneath the eastern China continent.

Mineralium Deposita↗
USGS2000

The Tintina Gold Belt - A global perspective

The so-called Tintina Gold Belt extends for more than 1000 km along the length of the northern North American Cordillera. Middle to Late Cretaceous Au deposits within the belt have various similar characteristics, among which are a spatial and temporal association with magmatism; Bi-W-Te signatures in deposits hosted by granitod stocks and As-Sb signatures where hosted by sedimentary rocks and dyke systems; and δ180 values consistently > 12 per mil for Au-bearing quartz. Nevertheless significant differences in structural styles, levels of deposit emplacement, ore-fluid chemistry, and Au grades suggest that the characteristics represent a broad range of deposit types. Many of these are best classified as orogenic Au deposits in the Yukon-Tanana terrane, as epithermal and porphyry-style Au deposits in the Kuskokwim region, and as Au-bearing, granite-related veins and stockworks, replacements, and skarns, as well as associated polymetallic lodes, in central Yukon. The diverse types of Au deposits and associated plutons of the Tintina Gold Belt collectively define a 45-m.y.-long period of arc magmatism that migrated northwesterly, for about 1000 km, across the active collisional margin of Cretaceous northwestern North America. The initiation of fluid flow and plutonism in Albian time seems to correlate with the onset of oblique subduction and dextral strike-slip on the Denali-Farewell, Tintina-Kaltag, and related fault systems. Initial Au-vein formation and subduction-related magmatism at about 115-110 Ma (e.g., including the Goodpaster and Fortymile districts), within the seaward side of the Yukon-Tanana terrane, correlate with the arrival of the Wrangellia superterrane off the continental margin. Dextral translation of the allochthonous Wrangellia block was associated with the migration of the thermal pulse to the northwest at about 95-90 Ma. Orogenic (or so­ called mesotherrnal) and granitoid-related Au deposits formed across the width of the Yukon-Tanana terrane (e.g., Fort Knox, True North, Ryan Lode, Kantishna district) and inland into the passive-margin rocks of the Selwyn basin ( e.g., Scheelite Dome, Brewery Creek, Dublin Gulch), respectively. By 70 Ma, the arc had migrated to the vicinity of present-day southwestern Alaska, where it was associated with the formation of additional orogenic Au deposits (e.g., Willow Creek district) and, within still-preserved shallow crustal levels, epithermal Au systems (e.g., Donlin Creek). The Au-bearing deposits of the Tintina Gold Belt are typical of those found in most well-preserved, moderate- to high-temperature Phanerozoic collisional orogens. Around the circum-Pacific region, these would include large areas of Mesozoic tectonism along the Cordilleran orogen, throughout the Russian Far East, and along the margins of the North China craton. Favorable terrain for such Au belts of Paleozoic age worldwide include the active Gondwana margins (e.g., Tasman orogenic system, northern Africa, Telfer district), and the northern margins ( e.g., Caledonian Kazakhstania, Uralian orogen, Baikal orogen, Tian Shan orogenic system) and western margins ( e.g., southern European massifs) to the Paleo-Tethys Ocean. Gold lodes in all of the Phanerozoic belts are dominated by orogenic Au-deposit types; other deposit types are concentrated where relatively shallow levels to the orogens are locally preserved. A significant percentage of the lode-gold resource in many areas was lost to placer accumulation that began forming approximately 100 m.y. after hypogene ore formation, except where continent-continent collision "cratonized" highly mineralized terranes in central Asia.

Book chapter↗