Abstract Arsenide-silver-uranium vein deposits are epigenetic concentrations of silver minerals, arsenides, sulpharsenides, sulphides, and accompanying chalcophile and siderophile elements, and of uranium oxides and silicates and accompanying lithophile (oxyphile) elements. Proportions of the ore constituents in these deposits differ substantially in different areas. For instance, the deposits in the Great Bear Lake area, Northwest Territories, contain a whole spectrum of uranium, silver, cobalt, copper, and associated minerals, whereas in the deposits in the Cobalt area, Ontario, uranium minerals are absent. As demonstrated by analyses of paragenetic mineral sequences in the Jachymov deposit, Czech Republic, and the Eldorado deposit at Great Bear Lake, Canada, the introduction of uranium into these deposits took place at distinct stages in the mineralization process. Although the uranium and arsenide-silver assemblages may locally occur together, their deposition took place in separate mineralization stages (Ruzicka, 1971). Because of the above-mentioned features, deposits of the arsenide silver-uranium vein type have been subdivided. The subtypes are arsenide vein silver-cobalt and arsenide vein uranium-silver.

Abstract The Miocene Kışladağ deposit (~17 Moz), located in western Anatolia, Turkey, is one of the few global examples of Au-only porphyry deposits. It occurs within the West Tethyan magmatic belt that can be divided into Cretaceous, Cu-dominant, subduction-related magmatic arc systems and the more widespread Au-rich Cenozoic magmatic belts. In western Anatolia, Miocene magmatism was postcollisional and was focused in extension-related volcanosedimentary basins that formed in response to slab roll back and a major north-south slab tear. Kışladağ formed within multiple monzonite porphyry stocks and dikes at the contact between Menderes massif metamorphic basement and volcanic rocks of the Beydağı stratovolcano in the Uşak-Güre basin. The mineralized magmatic-hydrothermal system formed rapidly (<400 kyr) between ~14.75 and 14.36 Ma in a shallow (<1 km) volcanic environment. Volcanism continued to at least 14.26 ± 0.09 Ma based on new age data from a latite lava flow at nearby Emiril Tepe. Intrusions 1 and 2 were the earliest (14.73 ± 0.05 and 14.76 ± 0.01 Ma, respectively) and best mineralized phases (average median grades of 0.64 and 0.51 g/t Au, respectively), whereas younger intrusions host progressively less Au (Intrusion 2A: 14.60 ± 0.06 Ma and 0.41 g/t Au; Intrusion 2 NW: 14.45 ± 0.08 Ma and 0.41 g/t Au; Intrusion 3: 14.39 ± 0.06 and 14.36 ± 0.13 Ma and 0.19 g/t Au). A new molybdenite age of 14.60 ± 0.07 Ma is within uncertainty of the previously published molybdenite age (14.49 ± 0.06 Ma), and supports field observations that the bulk of the mineralization formed prior to the emplacement of Intrusion 3. Intrusions 1 and 2 are altered to potassic (biotite-K-feldspar-quartz ± magnetite) and younger but deeper sodic-calcic (feldspar-amphibole-magnetite ± quartz ± carbonate) assemblages, both typically pervasive with disseminated to veinlet-hosted pyrite ± chalcopyrite ± molybdenite and localized quartz-feldspar stockwork veinlets and sodic-calcic breccias. Tourmaline-white mica-quartz-pyrite alteration surrounds the potassic core both within the intrusions and outboard in the volcanic rocks. Tourmaline was most strongly developed on the inner margins of the tourmaline-white mica zone, particularly along the Intrusion 1 volcanic contact where it formed breccias and veins, including Maricunga-style veinlets. Field relationships show that the early magmatic-hydrothermal events were cut by Intrusion 2A, which was then overprinted by Au-bearing argillic (kaolinite-pyrite ± quartz) alteration, followed by Intrusion 3 and late-stage, low-grade to barren argillic and advanced argillic alteration (quartz-pyrite ± alunite ± dickite ± pyrophyllite). Gold deportment changes with each successive hydrothermal event. The early potassic and sodic-calcic alteration controls much of the original Au distribution, with the Au dominantly deposited with feldspar and lesser quartz and pyrite. Tourmaline-white mica and argillic alteration events overprinted and altered the early Au-bearing feldspathic alteration and introduced additional Au that was dominantly associated with pyrite. Analogous Au-only deposits such as Maricunga, Chile, La Colosa, Colombia, and Biely Vrch, Slovakia, are characterized by similar alteration styles and Au deportment. The deportment of Au in these Au-only porphyry deposits differs markedly from that in Au-rich porphyry Cu deposits where Au is typically associated with Cu sulfides.

Abstract The Eldorado Formation is a wedgeshaped sequence of fanglomerates and arenites which thickens from 200 m in the west to 600 m in the east. The formation rests unconformably on the Aandenk Formation and is overlain by the Ventersdorp lava. The rapid change in thickness is due to intraformational unconformities. Four distinct alluvial fan cycles with radii of between 5 km and 25 km prograded from west to east; these cycles define four members, from the bottom upward: the Rosedale Member, the Van den Heeversrust Member, the EA zone and Uitkyk Member. The lower two members are crude finingupward megasequences. The topmost two members of the formation in the west consist almost entirely of rudities which, 15 km to the east, grade into arenites. In the southern part of the area the entire Eldorado Formation is composed of clastsupported, poorly sorted polymictic conglomerates, which show crude stratification, and trough crossbedded litharenites and subgreywackes. This is called the Welkom facies. In the northern part of the area oligomictic conglomerates and quartzarenites are commonly developed in the lower three members, especially in the EA zone; this is called the Loraine facies. These two facies interfinger. Sedimentation units commonly between 60 cm and 120 cm thick fine upward into matrixsupported conglomerates capped in places by trough crossbedded arenites. Ephemeral sheet floods produced the gravels, whereas sands were deposited during waning flow conditions. Detrital pyrite, gold and uraninite occur sporadically and in low concentrations in the polymictic Rosedale Placer at the base of the formation. These heavy minerals are also concentrated at various horizons in oligomictic conglomerates of the Loraine facies. Pay zones as much as 400 m long are oriented from west to east perpendicular to the depoaxis of the basin. The amount of gold shows a sympathetic relation with the size and amount of pyrite grains which decreases sharply east of this axis. Each of the lower two members is interpreted as a geomorphological cycle which was deposited during uplift of the source area. In each case tectonism was followed by recession of the fan and retreat of the uplifted mountains. The upper two members were deposited during more frequent uplifts of the source area. Marginal unconformities and the wedging of units near the basin edge in the west suggest syndepositional folding and thrusting in the source area and accumulation of fanglomerates in a foreland molasse basin. Two entirely different sources, one oligomictic and the other polymictic, are postulated. Gravels and sands were distributed from west to east as alluvial fans from both sources. The sediments were redistributed longitudinally as gravel and sand bars on a broad braid plain down a southeastward paleoslope.

Abstract The El Creston gold deposit is located 47 km southeast of Hermosillo, Sonora. Gold and silver mining activity dates back to 1740, with the period of greatest activity from 1865 to 1916. The mine is currently operated by Exploraciones Eldorado S.A. de C.V., which commenced production in December 1993. The geological resource as of January 1, 1999, was 8,130,000 tonnes (t) with an average Au grade of 0.94 g/t, containing 245,000 oz of gold. A mineable reserve of 1,792,000 t grading 1.06 g/t Au is currently being exploited from the El Creston open pit at a rate of 9,000 t of ore per day. Rocks in the area are composed mainly of Paleozoic miogeosynclinal and eugeosynclinal sedimentary sequences that were intruded by a series of Late Cretaceous intrusions and overlain by Eocene-Oligocene calc-alkaline volcanic rocks. Laramide compressional deformation and Tertiary Basin and Range extension led to the development of at least four orientations of regional faults, and imposed structural tilting on large fault blocks. Gold mineralization is localized along faults within both the Paleozoic sedimentary rocks and Cretaceous intrusions. Gold typically occurs as fine-grained particles with quartz veins, stockworks, and base metal sul-fides, associated with silicification, potassic and propylitic alteration, and secondary iron and manganese oxides. The associated alteration includes quartz veins, stockwork and silicification, potassic and propylitic assemblages, and abundant secondary iron and manganese oxides. Fluid inclusions indicate ore deposition was associated with low-temperature, moderately high salinity fluids in a deep epithermal environment.