Timing Of Ore Formation Research Articles

Genesis of Devonian volcanic-associated Lahn-Dill-type iron ores – part II: trace element fractionation evidences diffuse fluid venting

The iron (Fe)-oxide deposits of the Lahn-Dill-type are composed of haematite-quartz and rare siderite-haematite ores. These ores formed as marine chemical sediments on top of volcaniclastic rocks near the Middle to Late Devonian boundary (∼ 380 Ma). As such, their trace element fractionation patterns provide key information on venting style, ocean chemistry, particle-solution interaction, and depositional environment at the time of ore formation. This study combines WDXRF and ICP-MS/OES whole-rock geochemistry with complementary in-situ LA-ICP-MS analysis, and TEM element mapping of ore samples from the Fortuna Mine (Rhenish Massif, Germany). In-situ measurments were conducted on quartz-haematite, haematite, and siderite-haematite microdomains. Bulk major element contents of the ores indicate (volcani)clastic contamination and post-depositional hydrothermal alteration. Microdomain trace element distributions reveal four different trace element signatures, which are related to: (1) syngenetic apatite formation due to sorption of P and REY from seawater; (2) Fe-(oxyhydr)oxide-specific trace element scavenging and fractionation within the seawater column; (3) diagenetic Fe(III) reduction and trace element mobilisation in pore water; and (4) simultaneous deposition of (volcani)clastic material and Fe-(oxyhydr)oxides. These results show that Lahn-Dill-type iron ore formation resulted from mixing of a low-temperature vent fluid with ambient seawater at high seawater to vent fluid ratios. This likely was related to an environment in which diffuse venting dominated over focused venting, and in which quick Fe-particle precipitation led to formation of haematite-quartz ores. Local diagenetic Fe(III) reduction resulted in post-depositional siderite-haematite ore formation during which trace elements were partially remobilised in pore water.

Ultrapotassic plutons as a source of uranium of vein-type U-deposits (Moldanubian Zone, Bohemian Massif): insights from SIMS uraninite U–Pb dating and trace element geochemistry

The Bohemian Massif hosts significant hydrothermal U-deposits associated with shear zones in the high-grade metamorphic basement. But there is a lack of evidence of a genetic link between mineralization and U-fertile igneous rocks. This contribution provides constraints on the major U source of the vein-type U-deposits, the timing of ore formation and the metallogenetic model. The anomalous trace element signatures of the low-temperature hydrothermal deposits (high Zr, Y, Nb, Ti, ∑REE) and their close spatial relation with ultrapotassic rocks of the durbachite series point to a HFSE and REE enriched source rock. The durbachites have high U content (13.4–21.5 ppm) mainly stored in magmatic uraninite and other refractory minerals (e.g., thorite, zircon, allanite) that became metamict over a time interval sufficient to release U from their crystal structure, as suggested by the time gap between emplacement of the durbachites (EMP uraninite U–Pb age ~ 338 Ma) and hydrothermal activity (SIMS uranium ore U–Pb age ~ 270 Ma). Airborne radiometric data show highly variable Th/U ratios (1.5–6.0), likely reflecting a combination between (1) crystallization of magmatic uraninite, (2) hydrothermal alteration, and (3) leaching and mobilization of U along NW–SE-trending fault zones, manifested by elevated Th/U values in the radiometric map. The presence of rare magmatic uraninite in durbachites suggests almost complete uraninite dissolution; EMP imaging coupled with LA-ICP-MS analyses of refractory accessory phases revealed extensive mobilization of U together with HFSE and REE, providing direct evidence for metal leaching via fluid-driven alteration of radiation-damaged U-rich minerals. The large-scale HFSE and REE mobilization, demonstrated by the unusual trace element signatures of the U-deposits, was likely caused by low-temperature (270–300 °C), highly alkaline aqueous solutions containing F-, P-, and K-dominated complexing ligands. The first SIMS U–Pb age of 270.8 ± 7.5 Ma obtained so far for U-mineralization from the Bohemian Massif revealed a main Permian U mineralizing event, related to crustal extension, exhumation of the crystalline basement, and basin formation, as recorded by U–Pb apatite dates (280–290 Ma) and AFT thermal history models of the durbachites. The Permo-Carboniferous sedimentary cover probably represented a source of oxidized basinal brines infiltrating the basement-hosted durbachite plutons and triggering massive metal leaching. The interaction between basin-derived brines and durbachites resulted in significant modification of the chemical composition of the hydrothermal system (K and F release during biotite chloritization, P liberation through monazite alteration), leading to the formation of ore-bearing fluids responsible for the metallogenesis of the basement-hosted unconformity-related U-deposits in shear zones in the Bohemian Massif.

Intrusion-related orogenic gold deposit in the East Kunlun belt, NW China: A multiproxy investigation

The Wulonggou gold district is one of the most important gold producers of the East Kunlun metallogenic belt in China. Due to its complex geological background and limited investigations, the age of mineralization and ore-forming process remain unclear. Here, we focus on the Yanjingou gold deposit which is the largest and most typical deposit in the Wulonggou gold district, with a view to investigate the metallogenic mechanism and timing of ore formation with a view to evaluate the ore-forming processes and sources through a comprehensive study of the ore mineralogy, monazite and zircon U-Pb dating, in situ trace element analysis and mapping, and in situ S isotopes. Our results show that the magmatism and mineralization were coeval, during 239 Ma and 244 Ma in the Middle Triassic. Four stages of mineralization are identified with distinct textures, mineral assemblages, trace element characteristics and S isotopes. Stage 1 is represented by euhedral pyrite bearing quartz veins, which originated from deep metamorphic fluid, and was rarely affected by magmatic hydrothermal fluids. Stage 2 is characterized by the occurrence of abundant oriented arsenopyrite, which originated from the mixture of deep metamorphic fluid and magmatic hydrothermal fluid. Stage 3 is characterized by the occurrence of polymetallic sulfide of copper, with two generation of pyrite (stage 3 Py1 and Py2). Stage 3 Py1 formed from magmatic hydrothermal fluids whereas Py2 originated from deep metamorphic fluid and was also slightly influenced by magmatic hydrothermal fluids. Stage 4 is represented by late calcite and pyrite combination, which originated from another stage of deep metamorphic fluid. Stage2 and 3 are the main gold mineralization stages, and Au has a close relationship with As, and an obvious inverse correlation with Co and Ni. Combined with the trace element characteristics, we infer that the denudation degree of the Yanjingou gold deposit is very shallow with good prospects for exploration. Through multi-proxy investigation of the different stages and types of pyrite, we postulate that the Yanjingou gold deposit is an orogenic gold deposit related to intrusive rocks formed during Middle Triassic compressional transformation.

A Synthetic Haematite Reference Material for LA‐ICP‐MS U‐Pb Geochronology and Application to Iron Oxide‐Cu‐Au Systems

In both nature and synthetic experiments, the common iron oxide haematite (α‐Fe2O3) can incorporate significant amounts of U into its crystal structure and retain radiogenic Pb over geological time. Haematite is a ubiquitous component of many ore deposit types and, therefore, represents a valuable hydrothermal mineral geochronometer, allowing direct constraints to be placed on the timing of ore formation and upgrading. However, to date, no suitable natural haematite reference material has been identified. Here, a synthetic haematite U‐Pb reference material (MR‐HFO) is characterised using LA‐ICP‐MS and ID‐TIMS. Centimetre‐scale ‘chips’ of synthesised α‐Fe2O3 were randomly microsampled via laser ablation‐extraction and analysed using ID‐TIMS. Reproducible U/Pb and Pb/Pb measurements were obtained across four separate chips (n = 13). Subsequently, an evaluation of the suitability MR‐HFO in constraining U‐Pb data via LA‐ICP‐MS is presented using a selection of natural samples ranging from Cenozoic to Proterozoic in age. The MR‐HFO normalised U‐Pb ratios are more concordant and ages more accurate versus the same LA‐ICP‐MS spot analyses normalised to zircon reference material, when compared with independently acquired ID‐TIMS data from the same natural haematite grains. Results establish MR‐HFO as a suitable reference material for LA‐ICP‐MS haematite U‐Pb geochronology.

Using Models of the Deep Structure of the Earth’s Crust and Upper Mantle Based on GOCE Gravity Data in Metallogenic Analysis

Gravimetric data obtained by the GOCE spacecraft contributes to the development of global models of the deep structure of the Earth’s crust and upper mantle (referred to below as global models), improving the understanding of the metallogenic specialization of ore regions. Several examples of using these models in metallogenic analysis are considered here. It is shown that the concept of the global metallogenic homogeneity and zonality of the Pacific Ore Belt (POB) and its boundaries, is well supported by modern maps based on global models. Disputed POB boundaries also receive additional support in the thermal and density model of the upper mantle. Based on the analysis of global models, it is noted that the ore deposits of central and northern Chile clearly gravitate to a region with a denser and colder mantle at depths of 60 km, which corresponds to a flat segment of the subduction zone at the time of ore formation (Miocene), where the dehydration of the submerging oceanic plate and hydration of the upper mantle occurred. The distribution of polymetallic deposits in the Balkans toward the northwest corresponds to the spread of the mean values of density and temperature of the upper mantle. A similar direction is also observed in the structure of the Moho’s surface. Analysis of global models shows that tectonic–magmatic activation during the formation of magmatic arcs due to erosion unearths deposits associated with ophiolites and rifting conditions (Cu–Ni, SEDEX) and the forming of new porphyry Cu, skarn, and/or vein (Pb–Zn) deposits. These results are of great importance in predicting new deposits.

Spatial Regularities of Localization of Gold Ore Occurrences in the Yana–Kolyma Province

Abstract—To solve the problems of regional forecast within the Yana–Kolyma gold ore province, analysis of the spatial distribution of 2140 orogenic ore objects and their gold grades was carried out. Ore objects with >1 ppm Au are mapped on a scale of 1: 2,500,000, and areas of high mineralization density points are outlined in the sequence: general contour–contours around clusters of points with high Au grade–contours around proximal clusters of points–long axes of anisotropic contours. The curves obtained after the interpolation between the axes, with regard to the actual position of the ore objects, are interpreted as intersections of the recent topographic surface with the planes of faults that were active at the time of ore formation (ore-hosting faults). We propose to call the curves intersecting the known deposits “trends” (arched and linear). If no deposits have been revealed, the curves should be called “ore lineaments”, regardless of their curvature. The shape of the general contour around the gold ore occurrences and the distribution of ore objects within this contour permit outlining the Upper Indigirka (UID) and Central Kolyma (CKD) megadistricts.The geometry and spatial position of trends and ore lineaments are compared with the recent structural plan of the area, with geophysical fields, and with the existing ideas of the kinematic types of faults arising under certain geodynamic regimes of evolution of structures in the Yana–Kolyma province.It is suggested that the ore-hosting faults formed successively during the collisional interaction of the passive margin of the Siberian continent with the Kolyma–Omolon superterrane and during the formation of the volcanic arcs of the Okhotsk–Chukchi volcanic belt. Trends and ore lineaments can be correlated with the groups of structures of ore fields formed at the late collisional and subduction (postcollisional) stages of the area evolution. The trends and ore lineaments of NW orientation are attributed to the folded reverse faults and thrusts and to shears of different kinematics. At the postcollisional stage, the ore lineaments and trends of NE orientation formed in the zones of tectonomagmatic activity, subparallel to the strike of volcanic arcs of the Okhotsk–Chukchi volcanogenic belt. The distribution of clusters of gold ore occurrences and “empty” intervals between them along the strike of the recognized structures probably corresponds to the distribution of areas of extension and compression in the plane of the ore-hosting faults.The correlation among the trends, ore lineaments, and ore-hosting faults permits forecasting for the position and approximate size of new ore bodies within the Yana–Kolyma province. Prospective areas for a gold ores prospecting and exploration have been outlined on the extrapolated and interpolated extensions of the trends and at the sites of their intersection.

The role of hydrocarbons in ore formation at the Pillara Mississippi Valley-type Zn-Pb deposit, Canning Basin, Western Australia

Hydrocarbons have been suggested previously to play a role as reducing agents for in-situ sulfate reduction — one of several key components of the model proposed for ore formation in Mississippi-Valley-Type (MVT) deposits. Here, we provide direct evidence for hydrocarbon-brine interaction occurring synchronous to ore formation at the Pillara MVT deposit, through the analysis of brine and composite brine-hydrocarbon fluid inclusions by LA-ICP-MS combined with thermodynamic and reactive transport modeling. The occurrence of coeval primary hydrocarbon and brine inclusions in ore-stage calcite and sphalerite implicates the co-existence of hydrocarbons and brine at the time of ore formation. Elevated average concentrations (± 1σ) of 147 ± 141 ppm Zn, 53 ± 64 ppm Pb, 38 ± 36 ppm Cu and 11 ± 9 ppm Ag were measured in brine end member fluid inclusions. Hydrocarbon-bearing fluid inclusions are enriched several orders of magnitude in metal concentrations compared to the brine end member fluid inclusions. The analyzed fluid inclusion assemblages combined with numerical reactive transport simulations indicate that an oxidized acid-sulfate brine, which leached its metals from feldspathic sandstones in the deeper basin, represents a thermodynamically stable ore fluid. Interaction with feldspathic sandstones, granitic basement rocks and/or carbonates, and neutralization of pH, did not affect the Pb and Zn transport capabilities of the fluid. These findings indicate that Pb and Zn can be transported over long distances through carbonate-rich rocks, even though the original ore fluid may have been an acidic brine. The controlling factor for metal deposition at Pillara and similar MVT deposits can be related to the presence of hydrocarbons, permitting in-situ sulfate reduction and precipitation of sphalerite, galena, Ag and Cu sulfides.

3D reconstruction of volcanic and ore-forming environments of a giant VMS system: A case study from the Kidd Creek Mine, Canada

The Archean North Rhyolite is a bimodal suite of rocks located directly northwest of, but structurally separate from, the Kidd Creek volcanogenic massive sulfide deposit, which has produced 140.4 million tonnes of ore grading 2.29 wt% Cu, 6.15 wt% Zn, 0.22% Pb and 86.2 g/t Ag. The stratigraphy of the North Rhyolite is identical to that of the Kidd Creek mine consisting, from oldest to youngest, of ultramafic flows, aphyric rhyolite, quartz porphyritic rhyolite, and basalt flows, divisible into a “low TiO2” basalt unit (<1.55 wt%) and a “high TiO2” basalt unit (>1.55 wt%), with minor interflow graphitic argillites.The North Rhyolite is interpreted to be the folded continuation of the Kidd Creek mine stratigraphy. Similar alteration patterns in the North Rhyolite and the Kidd Creek mine stratigraphy, as well as significant copper and zinc values within the thickest portions of rhyolite domes and ridges of the North Rhyolite, suggests the potential for economically significant mineralization within the North Rhyolite stratigraphy. Correlation between alteration, mineralization and the rhyolite domes and ridges demonstrate the volcanic structures through which the rhyolite was extruded also acted as hydrothermal fluid pathways and were controlling structures in ore formation.A 3D volcanic reconstruction, showing the paleo-seafloor at the time of ore-formation, illustrates that the rhyolite flows of the North Rhyolite were emplaced into the same subsidence structure as the strata of the Kidd Creek mine area. This 3D model, generated by translating complex geological data acquired through core logging and underground mapping indicates the volcanic setting of, and ore-forming environment at, Kidd Creek is much larger than previously thought and provides new spatial data that can be used in exploration targeting.

Sintering with Kovdor magnetite concentrates in the batch

Electron-microscope data show that the magnetite crystals in iron ore have different microstructure, depending on the temperature and time of ore formation. Thus, in sedimentary–metamorphic iron quartzites and magmatic skarns, the structure of the magnetite crystals is homogeneous and the composition is close to stoichiometric. In Kovdor ore, the magnetite crystals are heterogeneous. The matrix contains isomorphic Al, Mg, Ti, and other impurities as individual spinel microphases. The reduction of magnetite crystals in conditions that resemble sintering indicates that heterogeneous crystals disintegrate on sintering, with the formation of two ore phases: solid solutions of magnetite and wustite that are not involved in liquid-phase strengthening of the sinter. In the final stage of fluxed-sinter production, calcium–silicon silicate binders of melilite composition are formed in the product in place of the melt; these binders are not strong. On the basis of the research findings, it is important, in assessing iron-ore fields, to pay attention not only to the content of iron and silicon oxides in the ore but also to the structure of the magnetite crystals, since the iron in the magnetite determines the direction of melt formation in processing.

Geology and Timing of Ore Formation in the Willow Creek Gold District, Talkeetna Mountains, Southern Alaska

The Willow Creek mining district was the third-largest lode gold district in Alaska in the 20 th century, having produced 19 metric tons (t) of gold from vein deposits and another 2 t of gold from associated placer deposits. The district is located in the southern Talkeetna Mountains, north of the Castle Mountain strike-slip fault system. Most gold occurs in widespread mineralized quartz veins hosted by granitic rocks of the Willow Creek pluton. A geochronologic study of granitic rocks, alteration minerals, and adjacent metamorphic rocks was conducted in order to better understand the timing of magmatism, thermochronologic evolution, and age of gold mineralization in the district. New U-Pb zircon dates from the Talkeetna batholith, including the Willow Creek pluton, range from 72.1 to 71.3 Ma. The U-Pb dates are similar to reported U-Late Cretaceous Pb and K-Ar dates from elsewhere in the study area, and tightly constrain the age of plutonic rocks that host the Willow Creek lode gold deposits. Granitic rocks that intrude the Willow Creek pluton along its northwestern margin yield U-Pb dates of 75.7 and 70.8 Ma. The 40 Ar/ 39 Ar thermochronologic data for the Willow Creek pluton and other granitoids reveal rapid postemplacement cooling through hornblende, muscovite, and biotite closure temperatures. Thermochronologic data from K-feldspars document slower cooling rates through lower temperatures, which may be related to slow uplift and exhumation during the Paleocene-Eocene. The data are compatible with a simple tectonothermal history following emplacement of the plutonic rocks, with essentially no evidence of significant thermal disturbance of the rocks after about 64 Ma. Uranium-thorium-lead dating of hydrothermal phosphate minerals (monazite and xenotime) and 40 Ar/ 39 Ar dates from hydrothermal sericite from wall rocks and veins indicate that the main stage of gold mineralization occurred at about 67 to 65 Ma. Combining U-Th-Pb data from monazites from the Independence, Talkeetna, Gold Bullion, and Fern deposits gave an average of 65.4 ± 2.3 Ma; hydrothermal xenotime from the Independence vein also yields a statistically indistinguishable age of 64.5 ± 2.3 Ma. These ages overlap 40 Ar/ 39 Ar dates of 67 to 66 Ma obtained from hydrothermal sericite associated with formation of the gold-bearing veins, and the age of a muscovite from a weakly mineralized pegamatite that cuts the Willow Creek pluton. In contrast to previous studies that suggested that at least some mineralization was Eocene in age, we found no compelling evidence for significant gold mineralization within the district at this time. The 40 Ar/ 39 Ar and K-Ar dates from graphitic white micas from schist south of the Hatcher Pass fault zone yield ages of 61 to 50 Ma that are probably related to uplift and cooling of these Late Cretaceous metasedimentary rocks. The absence of evidence for postemplacement thermal events affecting the plutonic rocks of the Willow Creek district to the north indicates that the schists and granitoids do not share a similar tectonothermal history, and that they have been juxtaposed by subsequent faulting along the Hatcher Pass fault zone.

The role of the thermal convection of fluids in the formation of unconformity-type uranium deposits: the Athabasca Basin, Canada

In the global production of uranium, ~18% belong to the unconformity-type Canadian deposits localized in the Athabasca Basin. These deposits, which are unique in terms of their ore quality, were primarily studied by Canadian and French scientists. They have elaborated the diagenetic–hydrothermal hypothesis of ore formation, which suggests that (1) the deposits were formed within a sedimentary basin near an unconformity surface dividing the folded Archean–Proterozoic metamorphic basement and a gently dipping sedimentary cover, which is not affected by metamorphism; (2) the spatial accommodation of the deposits is controlled by the rejuvenated faults in the basement at their exit into the overlying sedimentary sequence; the ore bodies are localized above and below the unconformity surface; (3) the occurrence of graphite-bearing rocks is an important factor in controlling the local structural mineralization; (4) the ore bodies are the products of uranium precipitation on a reducing barrier. The mechanism that drives the circulation of ore-forming hydrothermal solutions has remained one of the main unclear questions in the general genetic concept. The ore was deposited above the surface of the unconformity due to the upflow discharge of the solution from the fault zones into the overlying conglomerate and sandstone. The ore formation below this surface is a result of the downflow migration of the solutions along the fault zones from sandstone into the basement rocks. A thermal convective system with the conjugated convection cells in the basement and sedimentary fill of the basin may be a possible explanation of why the hydrotherms circulate in the opposite directions. The results of our computations in the model setting of the free thermal convection of fluids are consistent with the conceptual reasoning about the conditions of the formation of unique uranium deposits in the Athabasca Basin. The calculated rates of the focused solution circulation through the fault zones in the upflow and downflow branches of a convection cell allow us to evaluate the time of ore formation up to the first hundreds of thousands years.

The post-collisional Cihai iron skarn deposit, eastern Tianshan, Xinjiang, China

The Cihai iron skarn deposit is located in the southern part of the eastern Tianshan, Xinjiang, northwestern China. The major iron orebodies are banded and nearly parallel to each other. The iron ores are hosted in an early diabase dike and in skarn. Post-ore diabase dikes cut the iron ores and their hosting diabase. Hydrothermal activity can be divided into four stages based on geological and petrographic observations: initial K–Na alteration (stage I), skarn-minor magnetite event (II), retrograde skarn-magnetite main ore event (III), and quartz–calcite–sulfide veining (IV). Zircon U–Pb dating yields ages of 286.5±1.8Ma for early diabase and 275.8±2.2Ma for post-ore diabase dikes. Amphibole separated from massive magnetite ore gives a 40Ar–39Ar plateau age of 281.9±2.2Ma and is the time of ore formation. Formation of the Cihai iron deposit is closely related to post-collisional magmatism and associated Cu–Ni–Au polymetallic mineralization in the eastern Tianshan.

The Nature and Timing of Ore Formation in the Budunhua Copper Deposit, Southern Great Xing'an Range

The Nature and Timing of Ore Formation in the Budunhua Copper Deposit, Southern Great Xing'an Range

Tectonic, magmatic, and metallogenic evolution of the Tethyan orogen: From subduction to collision

This paper reviews the tectonic, magmatic, and metallogenic history of the Tethyan orogen from the Carpathians to Indochina. Focus is placed on the formation of porphyry Cu±Mo±Au deposits, as being the most characteristic mineral deposit type formed during both subduction and collisional processes in this region. Relatively little is known about the history of the Paleotethys ocean, which opened and closed between Gondwana and Eurasia in the Paleozoic, and few ore deposits are preserved from this period. The Neotethyan ocean opened in the Permian–Early Triassic as the Cimmerian continental fragments (the cores of Turkey, Iran, Tibet, and Indochina) rifted from the northern Gondwana margin and drifted northwards. These microcontinents docked with the Eurasian margin at various points in the Mesozoic and Cenozoic, and formed a complex archipelago involving several small back-arc basins and remnants of the Paleotethyan ocean. The main Neotethyan ocean and these smaller basins were largely eliminated by collision with India and Africa–Arabia in the early Eocene and early-mid Miocene, respectively, although Neotethyan subduction continues beneath the Hellenic arc and the Makran.The majority of porphyry-type deposits are found in association with Neotethyan subduction (mainly in the Mesozoic and Paleogene), and syn- to post-collisional events in the mid-Paleogene to Neogene. They are found throughout the orogen, but some sections are particularly well-endowed, including the Carpathians–Balkans–Rhodopes, eastern Turkey–Lesser Caucasus–NW Iran, SE Iran–SW Pakistan, southern Tibet, and SE Tibet–Indochina. Other sections that appear barren may reflect deeper levels of erosion, young sedimentary cover, or lack of exploration, although there may also be real reasons for low prospectivity in some areas, such as minimal subduction (e.g., the western Mediterranean region) or lithospheric underthrusting (as proposed in western Tibet).Over the last decade, improved geochronological constraints on the timing of ore formation and key tectonic events have revealed that many porphyry deposits that were previously assumed to be subduction-related are in fact broadly collision-related, some forming in back-arc settings in advance of collision, some during collision, and others during post-collisional processes such as orogenic collapse and/or delamination of subcontinental mantle lithosphere. While the formation of subduction-related porphyries is quite well understood, collisional metallogeny is more complex, and may involve a number of different processes or sources. These include melting of: orogenically thickened crust; previously subduction-modified lithosphere (including metasomatized mantle, underplated mafic rocks, or lower crustal arc plutons and cumulates); or upwelling asthenosphere (e.g., in response to delamination, slab breakoff, back-arc extension, or orogenic collapse).The most fertile sources for syn- and post-collisional porphyry deposits appear to be subduction-modified lithosphere, because these hydrated lithologies melt at relatively low temperatures during later tectonomagmatic events, and retain the oxidized and relatively metalliferous character of the original arc magmatism. Unusually metallically enriched lithospheric sources do not seem to be required, but the amount of residual sulfide phases in these rocks may control metal ratios (e.g., Cu:Au) in subsequent magmatic hydrothermal ore deposits. Relatively Au-rich deposits potentially form in these settings, as observed in the Carpathians (e.g., Roşia Montană), Turkey (Kisladag, Çöpler), and Iran (Sari Gunay, Dalli), although the majority of syn- and post-collisional porphyries are Cu–Mo-rich, and resemble normal subduction-related deposits (e.g., in the Gangdese belt of southern Tibet). This similarity extends to the associated igneous rocks, which, being derived from subduction-modified sources, largely retain the geochemical and isotopic character of those original arc magmas. While still retaining a broadly calc-alkaline character, these rocks may extend to mildly alkaline (shoshonitic) compositions, and may display adakite-like trace element signatures (high Sr/Y and La/Yb ratios) reflecting melting of deep crustal garnet amphibolitic sources. But they are otherwise hard to distinguish from normal subduction-related magmas.Small, post-collisional mafic, alkaline volcanic centers are common throughout the orogen, but for the most part appear to be barren. However, similar rocks in other post-subduction settings around the world are associated with important alkalic-type porphyry and epithermal Au±Cu deposits, and the potential for discovery of such deposits in the Tethyan orogen should not be overlooked.

The nature and timing of ore formation in the Budunhua copper deposit, southern Great Xing'an Range: Evidence from geology, fluid inclusions, and U–Pb and Re–Os geochronology

The Budunhua copper deposit is located in the southern Great Xing'an Range where the stockwork ore bodies are mainly hosted in the Lower Permian Dashizhai Formation. The mineralization occurs in two blocks – the Jinjiling and Kongqueshan – and both are characterized by four main stages: arsenopyrite–pyrite–(molybdenite)–quartz, chalcopyrite–pyrrhotite–quartz, galena–sphalerite–chalcopyrite–quartz and calcite–fluorite–quartz. Wall-rock alteration, which is related to mineralization, is dominated by silicification, sericitization, and chloritization. Fluid inclusion studies show that the ore-forming fluids from Cu stages in both blocks are broadly identical with trapping temperatures of 280 to 400°C for the Kongqueshan, and 300 to 420°C for the Jinjiling. Estimated trapping pressures for the Kongqueshan are 15.0 to 26.6MPa and for the Jinjiling are 14.9 to 29.4MPa, corresponding to entrapment depth of 1.5 to 2.7km and 1.5 to 2.9km below the paleowater table, respectively. Zircon grains from the tonalite porphyry in the deposit yielded a weighted 206Pb/238U mean age of 152±0.7Ma, which is consistent with a molybdenite Re–Os model age of 150±2.2Ma, indicating that both the porphyry intrusion and the copper deposit are of Late Jurassic age. Our data suggest that the Budunhua deposit is an atypical porphyry deposit related to a Late Jurassic magmatic-hydrothermal system.

Stable isotope geochemistry and Re–Os ages of the Yinan gold deposit, Shandong Province, northeastern China

The Yinan gold deposit in the Luxi area of Shandong Province in northeastern China is a skarn-type ore deposit. In this article, we present results from sulphur, lead, carbon–oxygen, and helium–argon isotope chemistry to characterize the ore genesis and source features. We also present rhenium–osmium ages from molybdenite to evaluate the timing of ore formation. The δ34S values of pyrite from the ore deposit range from 0.7‰ to 5.60‰ with a mean at 2.70‰, close to mantle and meteorite sulphur. Among Pb isotopes, 206Pb/204Pb values range from 18.375 to 18.436, 207Pb/204Pb values from 15.694 to 15.8, and 208Pb/204Pb values from 38.747 to 39.067. The δ13C values of calcite associated with the ores range from −0.2‰ to −0.5‰ and their δ18O values show variation from 9.4‰ to 12.6‰, suggesting a mixed fluid source. The 3He/4He and 40Ar/36Ar ratios of fluids trapped in pyrite are in the range of 0.27–1.11 Ra and 439.4–826, respectively, with calculated proportion of the mantle-derived He ranging from 3.25% to 14.03% and atmosphere argon ranging from 35.8% to 67.3%. The data suggest that the ore-forming fluids were derived from the crust and were mixed with a distinct contribution of mantle helium. The Re and Os values vary from 32 × 10−6 to 93.02 × 10−6 and from 0.01 × 10−9 to 0.34 × 10−9, respectively. The model ages of molybdenite range from 126.96 ± 1.82 Ma to 129.49 ± 2.04 Ma, with a weighted mean age of 128.08 ± 0.75 Ma and isochron age of 130.3 ± 3 Ma. These ages are close to the age of the associated quartz diorite porphyrite pluton, suggesting a close relationship between Cretaceous magmatism and metallogeny in NE China. A comparison of the Yinan gold deposit in the Luxi area with those of the Jiaodong area shows that the contrast in metallogenic features between the two are linked with the tectonic and geodynamic history.

Geology of the Skorpion Supergene Zinc Deposit, Southern Namibia

The Skorpion zinc deposit in southwestern Namibia comprises a significant nonsulfide orebody (24.6 Mt at 10.6% Zn) and subordinate amounts of primary base metal sulfide mineralization, which structurally underlies the nonsulfide ores at depth. The host rocks of the blind, nonsulfide orebody and relict sulfide mineralization consist of a mixed volcano-sedimentary sequence of Neoproterozoic age that has formed in an initial continental rift system between the Kalahari cratonic province and the Rio de la Plata cratonic province. The metavolcanic and subvolcanic rocks display a distinct bimodal geochemical composition of within-plate affinity and have been emplaced under an extensional crustal regime. Zircons from a rhyolitic vesicular flow, extruded approximately contemporaneously with the volcanic-hosted sulfide mineralization, have been dated by SHRIMP U-Pb analysis and yield an age of 751.9 ± 5.5 Ma. The basin that hosts the Skorpion deposit has been shaped by horst and (half-)graben tectonics, which also controlled the sedimentary deposition of siliciclastic and carbonate sediments within both shallow and deeper water environments. The rock sequence was affected by a major tectono-metamorphic event the Pan African-Brasiliano orogeny at approximately 550 to 545 Ma. Metamorphism reached uppermost greenschist to lowermost amphibolite facies, and the complex deformational style of the region is the result of intense faulting, folding, and thrusting. The Skorpion orebody is hosted by arkosic metaarenites and subordinately by volcaniclastic metasedimentary rocks. It is irregularly shaped, transgressive to sedimentary layering, and has a relatively flat top, which is covered by a blanket of unmineralized overburden consisting of calcrete, boulder beds and recent sand dunes. At depth, the orebody has a relatively sharp contact with a barren marble unit on the east and south and a more gradational boundary within the siliciclastic host rocks on the west and north. The nonsulfide ore minerals comprise predominantly sauconite (Zn smectite), substantial amounts of hemimorphite and smithsonite, and subordinate amounts of scholzite, tarbuttite, atacamite, hydrozincite, and hetearolite. The ore minerals are mainly euhedral and subhedral crystals and occur as open space fillings in intergranular voids, fractures, and breccias and as replacements of feldspar and mica. The metasedimentary host rocks of the nonsulfide ore contain abundant iron hydroxide pseudomorphs after disseminated sulfides and stringers and after rare semimassive stratiform layers of sulfides. Gossanous, manganese and iron hydroxide-rich, siliciclastic, metasedimentary rocks are common in the western and northwestern vicinity of the nonsulfide orebody. Alteration of the supergene mineralized host rocks includes removal of calcite cement and the breakdown of detrital albite-rich plagioclase and of minor orthoclase. Local alteration phenomena also include silicification and rare secondary barite, both a result of the alteration and breakdown of feldspar. Minor base metal sulfide mineral deposits occur in metarhyolitic flows and felsic hyaloclastic rocks, structurally below the nonsulfide orebody. The primary sulfide minerals comprise pyrite, pyrrhotite, sphalerite, minor chalcopyrite, and galena as well as secondary chalcocite and colloidal sphalerite (brunckite). The volcanic-hosted sulfide ore minerals are deformed and partly recrystallized and thus of premetamorphic and predeformation origin. In contrast, the Skorpion nonsulfide zinc deposit is of postmetamorphic origin, as shown by two hydrous, low-P, low-T ore assemblages. The secondary assemblages occur, at least partly, as delicate euhedral and subhedral crystals in secondary pore space, open fractures, veins, and breccias. The nonsulfide ore contains pseudomorphs from in situ oxidation and replacement of preexisting sulfides and ore and also from the remobilization or precipitation of base metals. The Skorpion nonsulfide zinc deposit is of supergene origin and formed from the oxidation of sulfide protore by circulating meteoric water, as suggested by textural evidence and by initial stable isotope data. The timing of ore formation is still undefined but lies within the large age bracket between 545 Ma (peak metamorphism) and subsequent partial exhumation and the deposition of the nonmineralized overburden of Namib Desert gravels (<21 Ma) and dune sands.

The Minas Azules Deposit—An Example of Orogenic Lode Gold Mineralization in the Sierra de Rinconada, Northern Argentina

The Minas Azules gold deposit, located in the northernmost part of the Sierra de Rinconada, Argentina, is hosted by low-grade metamorphosed turbidites of Middle Ordovician age. Quartz veinhosted, stockwork and disseminated gold mineralization is structurally controlled and intimately associated with a large cylindrical-type anticline that developed during the Upper Ordovician-Lower Silurian Ocloyic orogeny. Several styles of gold-bearing quartz veins were emplaced during folding and the later stages of episodic thrust faulting, but the main gold mineralization episode occurred at a particular stage in the deformation history and can be shown to be related to pressure solution reactions that occurred following cleavage development and prior to the onset of transpressional deformation. Due to post-ore faulting, mineralization is confined entirely to the eastern flank of the Minas Azules anticline. Petrological and geochemical studies reveal that, within the eastern flank, hydrothermal alteration of the wallrock is well-developed and, together with low-grade disseminated mineralization, extends for at least 70 meters from the vein system. Alteration in the Minas Azules deposit includes chloritization, sericitization, (de)silicification, sulfidation, and carbonatization. Lithogeochemical changes related to alteration are characterized by the addition of K2O, CO2, As, Au, S, and concomitant depletion of SiO2, MgO, and Na2O. Based on the style of mineralization, relative timing of ore formation, and characteristic alteration features, the Minas Azules deposit bears many similarities with well-documented orogenic gold deposits in Phanerozoic fold belts elsewhere (e.g., Tasmanides, eastern Australia; Meguma Terrane, Nova Scotia; Tien Shan, Central Asia; Mongol-Okhotsk fold belt, Russian Far East) and highlights the potential of the Sierra Rinconada as a significant producer of orogenic lode-gold and disseminated mineralization in the Andes.

The lode-porphyry model as dediced from the Camaru mesothermal granitoid-hosted gold deposit, southern Pará, Brazil

A new type of gold deposit is described for the Amazon Craton, in the Carajas greenstone belt province. Instead of mesothermal mylonite-hosted lodes or exhalative deposits already recognized in the Carajas Province, the Cumuru gold deposit is hosted by a calc-alkaline 2.82 Ga granodiorite pluton of volcanic arc affiliation. The Cumaru granodiorite is intruded in the horse-tail dilation sites of a shear zone that crosscuts the volcano-sedimentary mylonitic rocks of the Archean Gradaus greenstone belt. The Cumaru gold-bearing quartz veins lie along NE-SW trending fractures at the NW edge of the granodiorite pluton where disseminated ore in a stockwork array of veinlets is associated with the main lodes in a groundmass of hydrothermal phyllic alteration. Three kinds of fluids were identified in the Cumaru quartz veins. An aquo-carbonic fluid, immiscible at the time of ore formation, is interpreted as a metamorphic fluid related to shear zone. The second type of fluid comprises brines within the H2O-NaCl-KCl-CaCl2 system, interpreted as derived from the granodiorite residual fluids. The third fluid is a low-salinity meteoric water. The T-P conditions for the gold deposition constrained by the hydrothermal chlorite geothermometer and isochors calculated from the fluid inclusions microthermometric data. Values range from 300 to 350°C and 1.3 to 3.8kb. The low fO2 values of the ore fluids and the gold-sulfide paragenesis suggest that sulfur was in reduced state at the time of ore formation, conditions that favor gold transport as thiocomplexes. Two gold deposition events have been identified in the Cumaru deposit, both related with oxidation of the ore fluids. Initially oxidation was caused by immiscibility of the aquo-carbonic fluid and by wallrock sulfidation with decrease in fSa of the ore fluids. Subsequent mixing between aquo-carbonic fluid and aqueous brines brought about additional oxidation of the ore fluids with increase in fO2 and decrease in pH, favoring gold precipitation. Stable isotope data place important constraints on the source of the ore fluids: δ18O and δD values support the mixing between metamorphic and magmatic water and the δ 13 C data are consistent with mantle-derived carbonic fluids. The cumulative geological, structural, geochemical, fluid inclusions and isotopic data of the Cumaru gold deposit are similar both to Archean greenstone-hosted lode gold deposits that occur in shear zones and to porphyry-style mineralizations typical of Phanerozoic magmatic arcs. Such a situation suggests that both shear zone activation and granodiorite intrusion were involved in the genesis of the Cumaru mesothermal gold deposit, with is here assigned to a new classification of lode-porphyry gold deposit.

The significance of “diagenetic crystallization rhythmites” at the Nanisivik Pb-Zn-Ag deposit, Baffin Island, Canada

Rhythmically banded ore textures at the Nanisivik Pb-Zn-Ag deposit are identical to those commonly referred to as diagenetic crystallization rhythmites or DCRs. The presence of such textures in a demonstrably epigenetic orebody indicates that their formation is not necessarily indicative of early diagenetic sulfide mineralization. Although the development of such textures may occur in near-surface and/or early diagenetic environments, their presence alone is not a compelling constraint on the timing of ore formation.