Formation Of Ore Deposits Research Articles

Microstructural evolution and trace element mobility in Witwatersrand pyrite

Microstructural analysis of pyrite from a single sample of Witwatersrand conglomerate indicates a complex deformation history involving components of both plastic and brittle deformation. Internal deformation associated with dislocation creep is heterogeneously developed within grains, shows no systematic relationship to bulk rock strain or the location of grain boundaries and is interpreted to represent an episode of pyrite deformation that predates the incorporation of detrital pyrite grains into the Central Rand conglomerates. In contrast, brittle deformation, manifest by grain fragmentation that transects dislocation-related microstructures, is spatially related to grain contacts and is interpreted to represent post-depositional deformation of the Central Rand conglomerates. Analysis of the low-angle boundaries associated with the early dislocation creep phase of deformation indicates the operation of {100} slip systems. However, some orientation boundaries have geometrical characteristics that are not consistent with simple {100} deformation. These boundaries may represent the combination of multiple slip systems or the operation of the previously unrecognized {120} slip system. These boundaries are associated with order of magnitude enrichments in As, Ni and Co that indicate a deformation control on the remobilization of trace elements within pyrite and a potential slip system control on the effectiveness of fast-diffusion pathways. The results confirm the importance of grain-scale elemental remobilization within pyrite prior to their incorporation into the Witwatersrand gold-bearing conglomerates. Since the relationship between gold and pyrite is intimately related to the trace element geochemistry of pyrite, the results have implications for the application of minor element geochemistry to ore deposit formation, suggest a reason for heterogeneous conductivity and localized gold precipitation in natural pyrite and provide a framework for improving mineral processing.

Geological characteristics and metallogenesis of the shilu Fe-ore deposit in Hainan Province, South China

The Shilu Fe-ore deposit, a large-scale hematite-rich ore deposit in Hainan Province, South China, contains proven reserves of over 460Mt Fe-ore at 51% FeO, 4.07Mt Co-ore at 0.29% and 6.65Mt Cu-ore at 1.18%. The deposit is largely hosted within the Meso- to Neoproterozoic (ca. 0.85–1.30Ga) Shilu Group, a suite of metamorphosed neritic siliciclastic rocks and sedimentary carbonates, in which original sedimentary textures and structures are elegantly preserved. The Shilu Group, which hosts banded iron formations (BIFs), locally called amphibolitic itabirites, comprises six lithostratigraphic sequences. Fe-rich ores mainly reside in pyroxene–amphibole rocks of the middle sixth sequence, whereas the Co–Cu ores are located about 30–60m below the Fe-ore horizons and are primarily hosted by banded or impure dolostones of the lower sixth sequence. These ores are present mainly as lens-like or sheet-like bodies trending along troughs and/or transitional zones from limbs to troughs of the NW–SE trending synclinorium, and generally show banded or massive structures with lepidoblastic, cryptocrystalline or microcrystalline textures. Occasional oolitic and blastopsammitic textures are present, indicating an original sedimentary source for the Shilu Fe-polymetallic ores. At least two stages of deformation and associated metamorphism can be recognized. D1 deformation corresponds to a NW–SE trending synclinorium with associated regional greenschist facies metamorphism. D2 deformation records an early NW–SE orientated contraction (D2a), followed by doming with apparent skarnization (D2b) and finally transitional to a nearly E–W orientated extension (D2c), resulting in various amounts of folding, faulting and mineral recrystallization.Radiogenic isotopic analyses conducted on ores and host rocks record three metamorphic ages, including two Sm–Nd isochron ages of 830±16Ma and 212.9±6.6Ma, and a 40Ar–39Ar plateau age of 132.08±1.57Ma corresponding to D1, D2b, and D2c, respectively. Combined with previous U–Pb SHRIMP ages from ca. 960Ma to ca. 1300Ma and isotopic geochemical data (Xu et al., 2009), as well as other regional metamorphic events, an updated four-stage metallogenic model is proposed here: (1) deposition of the BIF-type ore source horizons between ca. 830 and 960Ma, (2) formation of a metamorphic sedimentary-type ore deposit from ca. 830–360Ma, (3) refinement of the deposit due to D2 deformation between ca. 250 and 210Ma and (4) superposed mineralizing stage formed by magma-related hydrothermal fluids between ca. 130 and 90Ma. The Shilu Fe-ore deposit can thus be better regarded as a structurally and hydrothermally reworked and re-enriched BIF-type ore deposit.

Iron and sulfur speciation in some sedimentary-transformation-type of lead–zinc deposits in West Kunlun lead–zinc ore deposit zone, Northwest China

Redox conditions are very important for almost all geological processes including ore deposit formation. However, the direct measurement of redox potential (Eh) values for solid materials is difficult. The chemical speciation of redox sensitive elements is one factor which determines the redox conditions. In this study, four groups of ore samples and their host rocks were collected from four sites along the west Kunlun lead-zinc ore deposit zone in northwest China and the iron- and sulfur- species analyzed using Mossbauer spectroscopy and X-ray absorption near edge fine structure (XANES). The results showed systematical change in redox conditions between the ore rocks and their associated host rocks. All ore samples contained more reducing sulfur species than their corresponding host-rocks indicating significantly enhanced reducing conditions in the ore bodies than their host rocks. Such reducing conditions should be more favorable for the sulfidization of metal during the ore formation. In addition very small concentrations of ferrous iron were found in only some of the small samples. This may indicate the removal of iron during metal sulfide ore deposit formation.

APPLYING STABLE ISOTOPES TO MINERAL EXPLORATION: TEACHING AN OLD DOG NEW TRICKS

The stable isotope ratios of various elements (e.g., H, C, O, S) have numerous uses to improve the understanding of the genesis and formation of hydrothermal and magmatic ore deposits, as well as having various applications to mineral exploration. However, stable isotope data has not been routinely collected during mineral exploration for various reasons related to cost per sample, the speed at which analytical data can be collected, and uncertainty regarding the benefits of stable isotope measurements to mineral exploration. Recent advances in analytical technologies which utilize infrared absorption spectroscopy (e.g., off-axis integrated cavity output spectroscopy [OA-ICOS]) mean that stable isotope data can now be collected in far greater quantities than has been previously possible. This advance in analytical technology, which allows for significantly more rapid and less expensive stable isotope analyses, has significant implications for the way in which stable isotope data can be collected and utilized during mineral exploration. Potential applications of stable isotope ratios to mineral exploration include delineating property- to district-scale stable isotope alteration halos and identifying “blind deposits” at depth, as well as vectoring toward new deposits within endowed districts. Stable carbon and oxygen isotope data collected using OA-ICOS from carbonate rocks surrounding the Screamer Carlin-type gold deposit in Nevada demonstrate that stable isotope alteration can be detected at distances of up to (and potentially more than) 3 km laterally around mineralization.

Banded iron formation to high-grade iron ore: a critical review of supergene enrichment models

All the major worldwide direct-shipping iron ore deposits associated with banded iron formations (BIF) are characteristically deeply weathered. They extend to considerable depths below the water table and show well-preserved primary structures and textures, but characteristically most deposits contain no evidence of chert bands being present prior to weathering. Recent studies have found evidence of hydrothermal and/ or metamorphic influences in the development of certain ore deposits and new genesis models such as the supergene-modified hypogene model have been postulated for major high-grade iron ore deposits. Nevertheless, there are many high-grade deposits that show no evidence of hypogene alteration and for which a hypogene or metamorphic genesis is unreasonable that are automatically ascribed to supergene enrichment, commonly erroneously attributed to lateritic weathering in tropical environments. Laterite (sensu lato) is a soil formation in which primary textures are destroyed and is underlain by a pallid zone showing the preservation of chert and the depletion, not enrichment, of iron oxides and thus is totally incompatible with the formation of the high-grade ore deposits. Various theories and models that purported to explain the conditions under which such a uniquely BIF-related dissolution of quartz and residual accumulation of hematite could occur by supergene processes typically conflict with current understanding of groundwater hydrology, chemistry, weathering processes and soil formation.Supergene enrichment of ore is universal in the leaching of gangue minerals such as iron silicates, carbonates and apatite and supergene enrichment of BIF to low-grade ore is common in near surface environments above the water table such as ferrugenised BIF outcrops, detrital ore deposits, and some shallow ore deposits that have been subjected to prolonged exposure to fresh meteoric water. In all cases of supergene enrichment traces of the chert bands are visible and the dissolution or replacement processes for the removal of quartz are clear, in direct contrast to the most important deep saprolite ore deposits that show no trace of chert bands.The widespread acceptance of an inappropriate and untenable supergene enrichment model inhibits search for the true origin of the ore and our ability to predict and find concealed high-grade ore deposits.

Timing of formation and geological setting of low-sulphidation epithermal gold deposits in the continental margin of NE China

The margin of NE China, a part of the West Pacific metallogenic belt, contains innumerable low-sulphidation mineral deposits. Gold deposits in this region can be classified into three distinct types based on geology and ore mineral paragenesis: (1) low-sulphidation epithermal silver–gold deposits, (2) low-sulphidation tellurium–gold deposits, and (3) low-sulphidation epithermal tellurium–gold deposits. Ores formed during the late Early Cretaceous and the early Late Cretaceous reflect three distinct metallogenic periods: the Fuxin Stage at 115.98 ± 0.89 Ma, the Quantou Stage at 107.2 ± 0.6 Ma or <103 Ma, and the Qingshankou or Yaojiajie Stage at < 97 Ma and 88.2 ± 1.4 Ma. The Fuxin Stage is dominated by trachyandesitic magmatism, with magmas emplaced at hypabyssal depths. In comparison, the Quantou Stage is characterized by high-K calc-alkaline, calc-alkaline, and sodic andesitic, dacitic, and rhyolitic magmatism of three different suites. The first of these is a high-K calc-alkaline andesitic magmatic suite that was accompanied by the emplacement of a calc-alkaline sodic dacite during the formation of the Ciweigou and Wufeng ore deposits. The second suite is dominated by calc-alkaline sodic rhyolite and high-K calc-alkaline sodic dacite magmatism associated with the formation of the Sipingshan ore deposit. The third suite is typified by high-K calc-alkaline andesitic magmatism associated with the emplacement of calc-alkaline hypabyssal granitoid complexes accompanying the formation of the Dong'an and Tuanjiegou ore deposits. The Qingshankou or Yaojia Stage is characterized by calc-alkaline sodic dacite magmatism associated with the formation of the Wuxing ore deposit. Metallogenesis during the Fuxin Stage characterized by trachytic magmatism is closely related to the formation of a deep-seated fault within a magmatic arc or the back-arc region of an immature continental margin and is associated with the Early Cretaceous subduction of the Pacific plate beneath Eurasia. Ore deposits that formed during the Fuxin Stage were generally related to magmato-hydrothermal fluids associated with mantle-derived magmas. In contrast, metallogenesis during the Quantou and Qingshankou or Yaojiajie stages was closely related to the formation of a mature high-K calc-alkaline magmatic arc within a continental margin setting again associated with the westward subduction of the Pacific plate. This metallogenic event was a product of magmato-hydrothermal systems derived from crust–mantle interaction and mixing of magmas derived from partial melting of different sections of the continental crust.

Diagenesis and metallogenetic mechanisms of the Tuanjiegou gold deposit from the Lesser Xing’an Range, NE China: Zircon U–Pb geochronology and Lu–Hf isotopic constraints

The Tuanjiegou gold deposit is a large-sized breccia-hosted epithermal gold deposit in Heilongjiang Province, eastern China, and was discovered in the late 1950s. The scale and unusual metallogenic setting of this gold deposit have made it the focus of many previous studies. We systematically analyzed ore samples, and volcanic and granitic rocks that are associated with the formation of the deposit by single zircon U–Pb dating, obtaining three distinct concordant groups of U–Pb ages from dacitic pyroclastic rocks and lavas: 119.3±3.3 (n=2), 113.2±1.1 (n=13), and 109.1±1.4Ma (n=8); four groups of concordant U–Pb ages from an ore-bearing granite porphyry: 112.5±2.2 (n=2), 107.0±1.2 (n=4), 103.0±0.70 (n=10), and 99.9±1.1Ma (n=4); two groups of concordant U–Pb ages from a pyrite-rich, quartz vein-cemented, brecciated and silicified granite porphyry: 104.23±0.51 (n=16) and 101.3±1.3Ma (n=2); and five groups of concordant U–Pb ages from pyrite-rich, quartz vein-cemented, brecciated or chunky ore samples: 139±4 to 144±3 (n=2), 127±4 (n=1), 113.8±2.1 (n=8), 101.3±3.4 (n=3), and 91±3Ma (n=1). Combining the whole-rock and Lu–Hf isotope geochemistry with the geology of the mining area and these new U–Pb dates, we have redefined the ore deposit-related temporal history of the area, with formation age of the magma associated with the Ningyuancun Stage volcanics being 119±1.4Ma and eruption of the volcanics at 113±1.1Ma. The granodiorite-porphyry and granite porphyry were emplaced at 107.0±1.2Ma and 103–102Ma, respectively, with ore deposit formation at 102–100Ma. The magmato–hydrothermal interaction that formed the deposit was closely related to Late Mesozoic crust–mantle interaction, with the early stage calc-alkaline and high-K calc-alkaline magmatism in this area potentially being derived from interaction between asthenospheric mantle material and the underplated lower crust. However, the magmas associated with late-stage intrusive calc-alkaline and high-K calc-alkaline magmatism may have been generated during interaction between adakite-like magmas derived from the subduction of an oceanic plate and lower crustal material. Here, we suggest that the fluids that formed this deposit formed by interaction with both early- and late-stage mantle-derived magmas and the lower crust, with subsequent aggregation after significant igneous differentiation. During ore formation, the area was in a back-arc initial (passive) rift and continental margin island arc tectonic environment associated with Late Mesozoic subduction of the Pacific Plate, with metallogenesis occurring during a change in the tectonic environment between the latest Early Cretaceous and the Late Cretaceous.

Halocarbons and other trace heteroatomic organic compounds in volcanic gases from Vulcano (Aeolian Islands, Italy)

Adsorbent-trapped volcanic gases, sublimates and condensates from active vents of the La Fossa crater on the island of Vulcano (Aeolian Islands, Italy) as well as ambient and industrial air were quantitatively analyzed by Short-Path Thermal Desorption–Solid Phase Microextraction–Cryotrapping–Gas Chromatography/Mass Spectrometry (SPTD–SPME–CF–GC–MS). Among the over 200 detected and quantified compounds are alkanes, alkenes, arenes, phenols, aldehydes, carboxylic acids, esters, ketones, nitriles, PAHs and their halogenated, methylated and sulfonated derivatives, as well as various heterocyclic compounds including thiophenes and furans. Most compounds are found at concentrations well above laboratory, ambient air, adsorbent and field blank levels. For some analytes (e.g., CFC-11, CH2Cl2, CH3Br), concentrations are up to several orders of magnitude greater than even mid-latitudinal industrial urban air maxima. Air or laboratory contamination is negligible or absent on the basis of noble gas measurements and their isotopic ratios.The organic compounds are interpreted as the product of abiogenic gas-phase radical reactions. On the basis of isomer abundances, n-alkane distributions and substitution patterns the compounds are thought to have formed by high-temperature (e.g., 900°C) alkyl free radical reactions and halide electrophilic substitution on arenes, alkanes and alkenes. The apparent abiogenic organic chemistry of volcanic gases may give insights into metal transport processes during the formation and alteration of hydrothermal ore deposits, into the natural volcanic source strength of ozone-depleting atmospheric trace gases (i.e., halocarbons), into possibly sensitive trace gas redox pairs as potential early indicators of subsurface changes on volcanoes in the state of imminent unrest, and into the possible hydrothermal origin of early life on Earth, as indicated by the presence of simple amino acids, nitriles, and alkanoic acids.

Rb-Sr age of metasomatism and ore formation in the low-temperature shear zones of the Fenno-Karelian Craton, Baltic Shield

Manifestations of the main types of metasomatites (beresite, propylite, listwaenite, aceite, and gumbeite) were identified in the shear zones of the Fenno-Karelian craton on the basis of the previously proposed systematics of metasomatic facies. These metasomatites were formed in shear stress environments, which determined their morphological features, in particular, finely banded texture. Comparatively low-temperature conditions of infiltration process and salting out effect (reduction of CO2 solubility with increasing salt content) lead to the heterogenization of fluid into two phases: aqueous salt solution and almost pure CO2. This results in more aggressive and mobile behavior of the fluid, and, correspondingly, more intense differentiation of the matter and contrast in metasomatic banding. Relations between metasomatic parageneses indicate an evolution trend of the processes from propylite, beresite and listwaenite to alkaline varieties and their repeated manifestation in the same shear zones. The results of Rb-Sr isochron dating of ore metasomatites from eight deposits and occurrences of the Fenno-Karelian craton (more than 100 samples of rocks and minerals) confirm previous assumptions. In general obtained data show that the shear zones controlling the distribution of the studied occurrences operated as fluid pathways during a long time period, up to 200 Ma, after the Svecofennian orogeny completion and did not show any correlation with Paleoproterozoic and Neoarchean magmatism. Rb-Sr isotope data on the metasomatites indicate three peaks of the post-Svecofennian metallogenic activity: 1700–1780, 1600–1650, and 1400–1500 Ma. Since the studied ore deposits were formed within tectonic structures, which evolved on the Archean crust and have a long prehistory, and fluid flows were subjected to intensive contamination by ancient crustal material, a relatively high initial Sr isotope ratios of formed ore-metasomatic systems were developed. High variablity of this value in the studied rocks ((87Sr/86Sr)0 from 0.706 to 0.750) is related to the heterogeneity of crustal protolith and to the relative storage and manifestation of the juvenile component of the fluid, which was responsible for the metasomatic transformation of the Archean and Paleoproterozoic rocks and ore-deposit formation.

Oxidation state of subarc mantle

Research Article| September 01, 2012 Oxidation state of subarc mantle K.A. Evans; K.A. Evans 1School of Applied Geology, Curtin University, GPO Box 1987, WA 6845, Australia Search for other works by this author on: GSW Google Scholar M.A. Elburg; M.A. Elburg 2Department of Geology and Soil Science, Ghent University, Krijgslaan 281 S8, 9000 Ghent, Belgium3School of Geological Sciences, University of KwaZulu-Natal, Private Bag x54001, 4000 Durban, South Africa Search for other works by this author on: GSW Google Scholar V.S. Kamenetsky V.S. Kamenetsky 4School of Earth Sciences, and CODES Centre of Excellence in Ore Deposits, University of Tasmania, Hobart, Tasmania 7001, Australia Search for other works by this author on: GSW Google Scholar Author and Article Information K.A. Evans 1School of Applied Geology, Curtin University, GPO Box 1987, WA 6845, Australia M.A. Elburg 2Department of Geology and Soil Science, Ghent University, Krijgslaan 281 S8, 9000 Ghent, Belgium3School of Geological Sciences, University of KwaZulu-Natal, Private Bag x54001, 4000 Durban, South Africa V.S. Kamenetsky 4School of Earth Sciences, and CODES Centre of Excellence in Ore Deposits, University of Tasmania, Hobart, Tasmania 7001, Australia *E-mail: k.evans@curtin.edu.au; elburg@ukzn.ac.za; dima.kamenetsky@utas.edu.au. Publisher: Geological Society of America Received: 18 Nov 2011 Revision Received: 12 Feb 2012 Accepted: 24 Mar 2012 First Online: 09 Mar 2017 Online Issn: 1943-2682 Print Issn: 0091-7613 © 2012 Geological Society of America Geology (2012) 40 (9): 783–786. https://doi.org/10.1130/G33037.1 Article history Received: 18 Nov 2011 Revision Received: 12 Feb 2012 Accepted: 24 Mar 2012 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 K.A. Evans, M.A. Elburg, V.S. Kamenetsky; Oxidation state of subarc mantle. Geology 2012;; 40 (9): 783–786. doi: https://doi.org/10.1130/G33037.1 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 SocietyGeology Search Advanced Search Abstract The subarc mantle is a primary control on the composition of arc magmas and the formation of arc-related ore deposits. Elevated ferric iron contents in arc lavas have been interpreted as a record of subarc mantle that is oxidized relative to mid-oceanic ridge basalt (MORB), but this conclusion is controversial. Measurements of spinel compositions in primitive arc lavas imply an arc magma source 1–4 log units more oxidized than MORB and ocean island basalts analyzed using the same technique. Samples from seven arcs show a significant correlation (P < 0.0005) between redox budget, subduction zone convergence rate, and subduction zone age. These results support the notion of oxidized arc lavas in the mantle source zone, but resolution of contradictory evidence regarding subarc mantle redox state requires further work. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Gold and copper in volatile saturated mafic to intermediate magmas: Solubilities, partitioning, and implications for ore deposit formation

The solubility of Au and Cu in hydrous andesite melts was determined at 1000°C and 200MPa. The data show that the solubility of Au is highly influenced by reduced S (S2−), and to a lesser degree, by the Cl concentration in the melt. Other melt compositional parameters such as K2O/Na2O ratio, or FeO concentration have no significant effect. The simultaneous presence of S2− and high concentrations of Cl (0.7–1.3wt.%) yield Au solubilities higher than expected from separate contributions of Au-sulfide and Au-chloride species. The strong effect of S and Cl on Au solubilities suggests dissolution mechanisms similar to those observed in magmatic volatiles. The most likely dominant dissolved species are AuSH or AuS(K/Na) in sulfur-bearing, and AuCl in Cl-bearing, S-free systems. At the S2−/S6+ transition (∼NNO+0.2), the solubility of Au sharply drops by a factor of 5. Copper shows markedly different behavior from that of Au; its solubility is influenced only slightly by the presence of S and Cl in the melt. This shows that the dominant Cu species is CuO0.5, with the exception of melts with unrealistically high Cl concentrations for natural systems, where Cu-chloride complexes become significant. Calculated volatile/melt partition coefficients of Au based on solubility data in melts and volatiles reach values up to ∼200 in a pyrrhotite saturated high-K calc-alkaline andesite melt below NNO+0.5, but sharply drop by an order of magnitude at the H2S to SO2 transition in the volatile phase. The predicted volatile/melt partition coefficients of Cu are much lower and vary in a narrow range (0.5–2). Consequently, volatiles released from mafic to intermediate magmas below NNO+1 may be important for the formation of magmatic-hydrothermal Au deposits and will favor high Au/Cu ratios in the overlying hydrothermal systems.

He-Ar-H-O isotopic signatures in Au[sbnd]Ag bearing ore fluids of the Sunshin epithermal gold-silver ore deposits, South Korea

Helium (3He/4He), argon (40Ar/36Ar) and oxygen (18O/16O) isotopic ratios in fluid inclusions trapped in quartz and pyrite from the Sunshin epithermal gold-silver ore deposits in South Korea were analyzed to investigate the magmatic contribution to ore forming fluids. Mantle helium with high 3He/4He ratios (up to 3He/4He~9RA, where RA=1.4×10−6 for air) were identified from fluid inclusions in auriferous quartz veins, which are characterized as low sulfidation (adularia-sericite) type. The ratios provide direct evidence of links between low sulfidation ore fluids in epithermal gold deposits and magma from a mantle source. Interestingly, the oxygen and hydrogen isotopic compositions of the ore fluids (δ18O=−1.9 to −6.6‰, δD=−82 to −100‰), calculated from ore quartz, indicate that meteoric waters were predominant during precipitation of the AuAg ore. Microthermometric measurements for fluid inclusions indicate relatively low temperatures (≤300°C) for the formation of the low sulfidation epithermal ore deposits.

The partitioning of sulfur and chlorine between andesite melts and magmatic volatiles and the exchange coefficients of major cations

Andesite melts were equilibrated with an H–O–S-bearing volatile phase to determine the partition coefficients for S and Cl as a function of melt composition and oxygen fugacity. The experiments were conducted in rapid-quench MHC vessel assemblies at 200MPa and 1000°C, and over a range of imposed fO2 between NNO−1.2 and NNO+1.8. High fluid/melt mass ratios (∼15) were employed, allowing precise and accurate partition coefficients to be obtained by mass balance calculations. Chlorine exhibits Henrian behavior at ClO−0.5 activities typical for arc magmas, with DClvolatile/melt=1.36±0.06 (1σ) below 0.2wt.% Cl in the melt; at higher ClO−0.5 activities, DClvolatile/melt increases linearly to 2.11±0.02 at 1wt.% Cl in the melt. In the volatile phase: FeCl2∼NaCl>KCl∼HCl. The determination of cation exchange coefficients for major cations yielded: KK,Navolatile/melt=1.23±0.10 (1σ) and ∗KFe,Navolatile/melt=DFevolatile/melt/DNavolatile/melt=1.08±0.16 (1σ). Under these conditions, the concentration of HCl in the vapor is negatively correlated with the (Na+K)/(Al+Fe3+) ratio in the melt. Reduced sulfur (S2−) appears to obey Henry’s law in andesite melt–volatile system at fH2S below pyrrhotite saturation. The partition coefficient for S at fO2=NNO−0.5 correlates negatively with the FeO concentration in the melt, changing from 254±25 at 4.0wt.% FeO to 88±6 at 7.5wt.% FeO. Pyrrhotite saturation is reached when approximately 3.2mol% S is present in the volatile phase at fO2=NNO−0.5. At the sulfide/sulfate transition, the partition coefficient of S drops from 171±23 to 21±1 at a constant FeO content of ∼6wt.% in the melt. At fO2=NNO+1.8, anhydrite saturation is reached at ∼3.3mol% S present in the volatile phase. Aqueous volatiles exsolving from intermediate to mafic magmas can efficiently extract S, and effect its transfer to sites of magmatic-hydrothermal ore deposit formation.

Uranium-ore giants

Toxic heavy metals can accumulate in Earth's near surface to form ore deposits. Experimental and direct measurements of ore fluids reveal the efficient mobilization and deposition of uranium, implying potentially rapid formation of economic-grade ore.

Thermal–hydraulic–mechanical–chemical coupling with damage mechanics using ESCRIPTRT and ABAQUS

We present a method to couple the thermal (T), hydraulic (H) and fluid–rock chemical (C) interaction capabilities of ESCRIPTRT with a rate-dependent mechanical (M) formulation of the finite element method including continuum damage mechanics for geomaterials implemented using Abaqus/Standard™. The resulting code builds on the modular architecture of ESCRIPTRT to solve sequentially these coupled THMC mechanisms and a number of feedback mechanisms including shear heating and damage. Damage in the host rock is linked to porosity evolution which in turn affects permeability, and this permeability-damage dependency is validated against published results. The motivation behind the development of this code is to improve our understanding of the formation of hydrothermal ore deposits, and we illustrate the use of this numerical tool with a simple geologic scenario involving shear zone development and albitisation. This application example focuses on the effects of damage on permeability enhancement and resultant fluid flow. Using damage mechanics allows for the localisation of narrower shear zones and reproduces the logarithmic pattern of rock pulverisation observed in nature on the sides of shear zones. The damage induced permeability evolution creates fluid pathways which may play a major role in forming hydrothermal ore-deposits.

New geochronological data from the Paleozoic and Mesozoic nappe structures, igneous rocks, and molybdenite in the North Wuyi area, Southeast China

The Wuyishan metallogenic belt, a part of the circum-Pacific tectonomagmatic belt, is considered one of the 19 key metallogenic belts in China because of its abundant mineral resources and vast prospecting potential. Specifically, the North Wuyi area is an important Cu–Pb–Zn polymetal ore-concentrated district in East China. We report new geochronological dates in this area, which suggest that the formation of mineralization-related magmatic rocks have mainly occurred during the Paleozoic and Mesozoic in four stages: the Caledonian (530Ma to 430Ma), Early–Middle Jurassic (183Ma to 160Ma), Early Cretaceous (140Ma to 110Ma), and Late Cretaceous (63Ma to 85Ma). The mineralization events are either coeval or slightly postdate the related magmatic activities. Nappe structures are widespread in the North Wuyi area and play an important role in the formation and preservation of ore deposits, as determined from field observations and structural analyses of mining tunnels. The regional distribution and structural styles of the thrust faults in the North Wuyi area and new 40Ar–39Ar dating results (415Ma to 390Ma) confirm that intensive tectonic activity during the Early Paleozoic, when the South China Block was about to separate from Gondwana, has wiped out evidence of Caledonian mineralization. All nappe structures with regional scales (several thousand meters) were formed during the Early Cretaceous period, with an 40Ar–39Ar age of 120Ma to 129Ma. The thrust direction of the nappe structures is mainly southeast during the Early Cretaceous in the main body and north piedmont of the Wuyi Mountain. These early thrust faults have not only broken or reformed the existing ore deposits, but have also provided hydrothermal ore-forming channels and spaces for metallogenism that are coeval or posterior to the thrust. The subsequent nappe structures thrust mainly from southeast to northwest and overprint the previous southeastward thrust faults. Using 40Ar–39Ar dating of the structural belt, U–Pb SHRIMP zircon dating of the wall rocks, and Re–Os dating of the ore formation, we analyze the genetic connection between the structures and ore bodies in different ore fields. Our results serve as useful information on prospecting for the latent ore body in the North Wuyi area, especially the ore bodies destroyed by faulting activities.

Recognizing recycled osmium

Understanding the behavior of noble metals like Os in Earth systems is important from both scientifi c and economic standpoints. The abundances of noble metals in the mantle have signifi cant implications for our understanding of the Earth’s evolution. The relative abundances of these strongly siderophile elements in the mantle potentially preserve a fi ngerprint of core formation, which likely scavenged much of the Earth’s budget of these elements, as well a record of the latest stages of accretion history, during which time the Earth’s mantle may have been re-enriched in these elements (e.g., Walter et al., 2000). Understanding processes that can modify these signatures in the Earth’s mantle is fundamental to such models. In addition, documenting environments under which noble metals are mobile, and those under which they become scavenged and concentrated, are critical for understanding the formation of precious metal ore deposits and developing strategies for prospecting. For example, many economic Au deposits are associated with magmatism at convergent margins, yet the source of the Au and the behaviors of other noble metals are generally not well constrained (McInnes et al., 1999). The paper by Suzuki et al. (2011, p. 999 in this issue of Geology) contributes important new results regarding the mobility of Os in subduction settings. The Re-Os isotope system, involving the decay of

Geological and geochemical characteristics and metallogenic model of the Wenquan molybdenum deposit

The Wenquan molybdenum deposit is a kind of large-sized porphyry molybdenum deposit found in recent years. In this paper, on the basis of deposit geology, geochemistry and isotope geochronology data, the metallogenic model of this deposit was established. The Wenquan granitic batholith belongs to the K-rich (alkali-rich) calc-alkaline rock series, which is the mineralization parent rock. The rock massif shows the characteristics of both crust-remelting granite and mantle-source granite. At the same time, the data of REE contents, hydrogen and oxygen isotopes and inclusion temperatures showed that the metallogenic hydrothermal solution is a mixed mesothermal solution of magmato-hydrothermal fluid and meteoric water. Mineralization was dated at 214±7.1 Ma, basically identical with the parent rock’s age (207–226 Ma). This reflects that molybdenum mineralization has a close relation to tectonic magmatism evoked by orogenic processes, and molybdenum mineralization occurred mainly at the petrogenesis stage at the late stage of magma emplacement. Mixing with meteoric water led to a decrease in the salinity of magmato-hydrothermal solution and changes in other physical and chemical properties. During the tectonic process, ore-bearing hydrothermal solution ascended along favorable fault structure channels. With physicochemical changes, it filled in the surrounding rock joints on both sides of faults, forming ore deposits.

THE ADVANCES IN THE STUDY OF THE REMOTE SENSING EXPLORATION MODEL: A REVIEW

The remote sensing exploration model is characterized by the systematic arrangement of the essential remote sensing features of a group of similar mineral deposits. These features are such attributes or properties of that group of deposits as geological setting, wall-rock alteration and ore-control tectonics. This paper points out that the study of remote sensing exploration model must include prospecting criteria of ore deposit formation, wall-rock alteration and ore deposit reformation.

Deciphering fluid sources of hydrothermal systems: A combined Sr- and S-isotope study on barite (Schwarzwald, SW Germany)

Primary and secondary barites from hydrothermal mineralizations in SW Germany were investigated, for the first time, by a combination of strontium (Sr) isotope systematics ( 87Sr/ 86Sr), Sr contents and δ 34S values to distinguish fluid sources and precipitation mechanisms responsible for their formation. Barite of Permian age derived its Sr solely from crystalline basement rocks, whereas all younger barite also incorporate Sr from formation waters of the overlying sediments. In fact, most of the Sr in younger barite is leached from Lower and Middle Triassic sediments. In contrast, most of the sulfur (S) of Permian, Jurassic and northern Schwarzwald Miocene barite originated from basement rocks. The S source of Upper Rhinegraben (URG)-related Paleogene barite differs depending on geographic position: for veins of the southern URG, it is the Oligocene evaporitic sequence, while central URG mineralizations derived its S from Middle Triassic evaporites. Using Sr isotopes of barite of known age combined with estimates on the Sr contents and Sr isotopic ratios of the fluids' source rocks, we were able to quantify mixing ratios of basement-derived fluids and sedimentary formation waters for the first time. These calculations show that Jurassic barite formed by mixing of 75–95% ascending basement-derived fluids with 5–25% sedimentary formation water, but that only 20–55% of the Sr was brought by the basement-derived fluid to the depositional site. Miocene barite formed by mixing of an ascending basement-derived brine (60–70%) with 30–40% sedimentary formation waters. In this case, only 8–15% of the Sr was derived from the deep brine. This fluid-mixing calculation is an example for deposits in which the fluid source is known. This method applied to a greater number of deposits formed at different times and in various geological settings may shed light on more general causes of fluid movement in the Earth's crust and on the formation of hydrothermal ore deposits.