The Experts below are selected from a list of 5946 Experts worldwide ranked by ideXlab platform
Jiahao Zheng - One of the best experts on this subject based on the ideXlab platform.
-
a synthesis of iron deposits in the eastern tianshan nw china
Geoscience frontiers, 2019Co-Authors: Jiahao ZhengAbstract:Abstract The northern Xinjiang region is one of the most significant iron metallogenic provinces in China. Iron deposits are found mainly within three regions: the Altay, western Tianshan, and eastern Tianshan orogenic belts. Previous studies have elaborated on the genesis of Fe deposits in the Altay orogenic belt and western Tianshan. However, the geological characteristics and mineralization history of iron deposits in the eastern Tianshan are still poorly understood. In this paper we describe the geological characteristics of iron deposits in the eastern Tianshan, and discuss their genetic types as well as metallogenic-tectonic settings. Iron deposits are preferentially distributed in central and southern parts of the eastern Tianshan. The known iron deposits in the eastern Tianshan show characteristics of magmatic Fe-Ti-V (e.g., Weiya and Niumaoquan), sedimentary-metamorphic type (e.g., Tianhu), and iron Skarn (e.g., Hongyuntan). In addition to the abovementioned iron deposits, many iron deposits in the eastern Tianshan are hosted in submarine volcanic rocks with well-developed Skarn mineral assemblages. Their geological characteristics and magnetite compositions suggest that they may belong to distal Skarns. SIMS zircon U-Pb analyses suggest that the Fe-Ti oxide ores from Niumaoquan and Weiya deposits were formed at 307.7 ± 1.3 Ma and 242.7 ± 1.9 Ma, respectively. Combined with available isotopic age data, the timing of Fe mineralization in the eastern Tianshan can be divided into four broad intervals: Early Ordovician–Early Silurian (476–438 Ma), Carboniferous (335–303 Ma), Early Permian (295–282 Ma), and Early Triassic (ca. 243 Ma). Each of these episodes corresponds to a period of subduction, post-collision, and intraplate tectonics during the Paleozoic and Mesozoic time.
-
mineralogy fluid inclusions and isotopes of the cihai iron deposit eastern tianshan nw china implication for hydrothermal evolution and genesis of subvolcanic rocks hosted Skarn type deposits
Ore Geology Reviews, 2017Co-Authors: Jiahao Zheng, Jingwen Mao, Fuquan Yang, Fengmei Chai, Yongfeng ZhuAbstract:Abstract Most Skarn deposits are closely related to granitoids that intruded into carbonate rocks. The Cihai (>100 Mt at 45% Fe) is a deposit with mineral assemblages and hydrothermal features similar to many other typical Skarn deposits of the world. However, the iron orebodies of Cihai are mainly hosted within the diabase and not in contact with carbonate rocks. In addition, some magnetite grains exhibit unusual relatively high TiO 2 content. These features are not consistent with the typical Skarn iron deposit. Different hydrothermal and/or magmatic processes are being actively investigated for its origin. Because of a lack of systematic studies of geology, mineral compositions, fluid inclusions, and isotopes, the genetic type, ore genesis, and hydrothermal evolution of this deposit are still poorly understood and remain controversial. The Skarn mineral assemblages are the alteration products of diabase. Three main paragenetic stages of Skarn formation and ore deposition have been recognized based on petrographic observations, which show a prograde Skarn stage (garnet-clinopyroxene-disseminated magnetite), a retrograde Skarn stage (main iron ore stage, massive magnetite-amphibole-epidote ± ilvaite), and a quartz-sulfide stage (quartz-calcite-pyrite-pyrrhotite-cobaltite). Overall, the compositions of garnet, clinpyroxene, and amphibole are consistent with those of typical Skarn Fe deposits worldwide. In the disseminated ores, some magnetite grains exhibit relatively high TiO 2 content (>1 wt.%), which may be inherited from the diabase protoliths. Some distinct chemical zoning in magnetite grains were observed in this study, wherein cores are enriched in Ti, and magnetite rims show a pronounced depletion in Ti. The textural and compositional data of magnetite confirm that the Cihai Fe deposit is of hydrothermal origin, rather than associated with iron rich melts as previously suggested. Fluid inclusions study reveal that, the prograde Skarn (garnet and pyroxene) formed from high temperature (520–600 °C), moderate- to high-salinity (8.1–23.1 wt.% NaCl equiv, and >46 wt.% NaCl equiv) fluids. Massive iron ore and retrograde Skarn assemblages (amphibole-epidote ± ilvaite) formed under hydrostatic condition after the fracturing of early Skarn. Fluids in this stage had lower temperature (220°–456 °C) and salinity (8.4–16.3 wt.% NaCl equiv). Fluid inclusions in quartz-sulfide stage quartz and calcite also record similar conditions, with temperature range from 128° to 367 °C and salinity range from 0.2 to 22.9 wt.% NaCl equiv. Oxygen and hydrogen isotopic data of garnet and quartz suggest that mixing and dilution of early magmatic fluids with external fluids (e.g., meteoric waters) caused a decrease in fluid temperature and salinity in the later stages of the Skarn formation and massive iron precipitation. The δ 18 O values of magnetite from iron ores vary between 4.1 and 8.5‰, which are similar to values reported in other Skarn Fe deposits. Such values are distinct from those of other iron ore deposits such as Kiruna-type and magmatic Fe-Ti-V deposits worldwide. Taken together, these geologic, geochemical, and isotopic data confirm that Cihai is a diabase-hosted Skarn deposit related to the granitoids at depth.
Andreas G Mueller - One of the best experts on this subject based on the ideXlab platform.
-
mineral equilibria and zircon garnet and titanite u pb ages constraining the ptt path of granite related hydrothermal systems at the big bell gold deposit western australia
Mineralium Deposita, 2018Co-Authors: Andreas G Mueller, Neal J McnaughtonAbstract:The Big Bell deposit (75 t gold) is located in a narrow spur of the Meekatharra greenstone belt, Yilgarn Craton, Western Australia. Two ore bodies are located in a calcic-potassic contact alteration zone overprinting lineated granodiorite dykes and amphibolite: almandine-cummingtonite-hornblende Skarn (1–3 g/t Au, 1700 g/t As, 330 g/t W) and the muscovite-microcline gneiss (3–5 g/t Au, 580 g/t Sb, 620 g/t W) of the Main Lode. Genetic models vary from pre- to post-metamorphic replacement. Hornblende-plagioclase pairs in amphibolite constrain peak metamorphic temperature to 670 ± 50 °C. In contrast, garnet-biotite thermometry provides estimates of 578 ± 50 and 608 ± 50 °C for garnet-cordierite-biotite schist bordering the Skarn and enveloping the Main Lode. Garnet-cordierite and garnet-hornblende pairs extend the range of fluid temperature to 540 ± 65 °C, well below peak metamorphic temperature. At 540–600 °C, the alteration assemblage andalusite + sillimanite constrains pressure to 300–400 MPa corresponding to 11–14 km crustal depth. Published U-Pb ages indicate that metamorphism took place in the aureole of the southeast granodiorite-tonalite batholith (2740–2700 Ma), followed by gold mineralization at 2662 ± 5 Ma and by the emplacement of biotite granite and Sn–Ta–Nb granite-pegmatite dykes at 2625–2610 Ma. Amphibolite xenoliths in granite northwest of the deposit record the lowest temperature (628 ± 50 °C), suggesting it lacks a metamorphic aureole. The rare metal dykes are spatially associated with epidote-albite and andradite-diopside Skarns (≤1.5 g/t Au), mined where enriched in the weathered zone. We analysed hydrothermal zircon intergrown with andradite. Concordant U-Pb ages of 2612 ± 7 and 2609 ± 10 Ma confirm the presence of a second granite-related system. The zircons display oscillatory zoning and have low Th/U ratios (0.05–0.08). Low-Th titanite from an albite granite dyke has a concordant but reset U-Pb age of 2577 ± 7 Ma.
-
copper gold endoSkarns and high mg monzodiorite tonalite intrusions at mt shea kalgoorlie australia implications for the origin of gold pyrite tennantite mineralization in the golden mile
Mineralium Deposita, 2007Co-Authors: Andreas G MuellerAbstract:Five Cu–Au epidote Skarns are associated with the Mt. Shea intrusive complex, located in the 2.7–2.6 Ga Eastern Goldfields Province of the Archean Yilgarn craton, in greenstones bounded by the Boulder Lefroy and Golden Mile strike-slip faults, which control the Golden Mile (1,435 t Au) at Kalgoorlie and smaller “orogenic” gold deposits at Kambalda. The Cu–Au deposits studied are oxidized endoSkarns replacing faulted and fractured quartz monzodiorite–granodiorite. The orebodies are up to 140 m long and 40 m thick. Typical grades are 0.5% Cu and 0.3 g/t Au although parts are richer in gold (1.5–4.5 g/t). At the Hannan South mine, the Skarns consist of epidote, calcite, chlorite, magnetite (5–15%), and minor quartz, muscovite, and microcline. Gangue and magnetite are in equilibrium contact with pyrite and chalcopyrite. The As–Co–Ni-bearing pyrite contains inclusions of hematite, gold, and electrum and is intergrown with cobaltite and Cu–Pb–Bi sulfides. At the Shea prospect, massive, net-textured, and breccia Skarns are composed of multistage epidote, actinolite, albite, magnetite (5%), and minor biotite, calcite, and quartz. Gangue and magnetite are in equilibrium with Co–Ni pyrite and chalcopyrite. Mineral-pair thermometry, mass-balance calculations, and stable-isotope data (pyrite δ34SCDT = 2.5‰, calcite δ13CPDB = −5.3‰, and δ18OSMOW = 12.9‰) indicate that the Cu–Au Skarns formed at 500 ± 50°C by intense Ca–Fe–CO2–S metasomatism from fluids marked by an igneous isotope signature. The Mt. Shea stock–dike–sill complex postdates the regional D1 folding and metamorphism and the main phase of D2 strike-slip faulting. The suite is calc-akaline and comprises hornblende–plagioclase monzodiorite, quartz monzodiorite, granodiorite, and quartz–plagioclase tonalite porphyry. The intrusions display a wide range in silica content (53–73 wt% SiO2), in \( {{\text{Mg}}} \mathord{\left/ {\vphantom {{{\text{Mg}}} {{\left( {{\text{Mg}} + {\text{Fe}}_{{{\text{TOTAL}}}} } \right)}}}} \right. \kern-\nulldelimiterspace} {{\left( {{\text{Mg}} + {\text{Fe}}_{{{\text{TOTAL}}}} } \right)}} \) ratio (0.37–0.89), and in \( {\text{K}} \mathord{\left/ {\vphantom {{\text{K}} {{\left( {{\text{K}} + {\text{Na}}} \right)}}}} \right. \kern-\nulldelimiterspace} {{\left( {{\text{K}} + {\text{Na}}} \right)}} \) ratio (0.02–0.31). Chromium (62–345 ppm), Ni (23–158), Sr (311–1361 ppm), and Ba (250–2,581 ppm) contents are high, Sr/Y ratios are high (24–278, mostly >50), and the rare earth element patterns are fractionated \( {\left( {{{\text{Ce}}_{N} } \mathord{\left/ {\vphantom {{{\text{Ce}}_{N} } {{\text{Yb}}_{N} = 17 - 41}}} \right. \kern-\nulldelimiterspace} {{\text{Yb}}_{N} = 17 - 41}} \right)} \). These features and a negative niobium anomaly relative to the normal mid-ocean ridge basalt indicate that the suite formed by hornblende fractionation from a subduction-related monzodiorite magma sourced from metasomatized peridotite in the upper mantle. The magnesian composition of many intrusions was enhanced due to hornblende crystallization under oxidizing hydrous conditions and during the subsequent destruction of igneous magnetite by subsolidus actinolite–albite alteration. At the Shea prospect, main-stage Cu–Au epidote Skarn is cut by biotite–albite–dolomite schist and by red biotite–albite replacement bands. Post-Skarn alteration includes 20-m-thick zones of sericite–chlorite–ankerite schist confined to two D3 reverse faults. The schists are mineralized with magnetite + pyrite + chalcopyrite (up to 0.62% Cu, 1.6 g/t Au) and are linked to Skarn formation by shared Ca–Fe–CO2 metasomatism. Red sericitic alteration, marked by magnetite + hematite + pyrite, occurs in fractured porphyry. The biotite/sericite alteration and oxidized ore assemblages at the Shea prospect are mineralogically identical to magnetite–hematite-bearing gold lodes at Kambalda and in the Golden Mile. Published fluid inclusion data suggest that a “high-pressure”, oxidized magmatic fluid (2–9 wt% NaCl equivalent, \( X_{{{\text{CO}}_{2} }} = 0.1 - 0.2 \), 200–400 MPa) was responsible for gold mineralization in structural sites of the Boulder Lefroy and Golden Mile faults. The sericite–alkerite lodes in the Golden Mile share the assemblages pyrite + tennantite + chalcopyrite and bornite + pyrite, and accessory high-sulfidation enargite with late-stage sericitic alteration zones developed above porphyry copper deposits.
-
u pb ages constraining batholith emplacement contact metamorphism and the formation of gold and w mo Skarns in the southern cross area yilgarn craton western australia
Economic Geology, 2000Co-Authors: Andreas G Mueller, Neal J McnaughtonAbstract:Gold mines in the Archean Southern Cross greenstone belt, central Yilgarn craton, have produced more than 220 metric tons (t) of gold. The deposits are characterized by a pyroxene-rich gangue and have been interpreted as synmetamorphic amphibolite facies replacement deposits and as postmetamorphic intrusion-related Skarns. We present ion microprobe U-Pb zircon data constraining the age of amphibolite facies contact metamorphism in the aureole of the Ghooli Dome, the largest granitoid batholith intruding the Southern Cross belt. Drill cores from the Copperhead gold mine provided zircons from an altered quartz porphyry sill, located 300 m southwest of the Ghooli Dome, and zircons from the monzogranite gneiss at the border of the batholith. The porphyry sill contains embayed igneous zircons dated by a concordant U-Pb age of 2912 ± 5 Ma and euhedral zircons of metamorphic crystal habitus dated by a second concordant age of 2772 ± 5 Ma. The igneous zircons are slightly younger than those recovered from another sill of quartz porphyry, sampled at the Southern Star mine, which define a single concordant U-Pb age of 2934 ± 7 Ma. The metamorphic zircons in the porphyry at Copperhead record the same age as the igneous zircons (2775 ± 10 Ma) in the monzogranite gneiss of the Ghooli Dome. This pluton extends more than 40 km to the southeast of Copperhead and also forms the batholith border at the Corinthian and Fraser’s mines. These gold deposits, too, are hosted by amphibolite facies greenstones contact metamorphosed at 2772 ± 5 Ma. The Corinthian Skarn (2620 ± 6 Ma) formed 150 m.y. after high-grade metamorphism and, consequently, cannot be classified as synmetamorphic or metamorphogenic. A review of field relationships and of published geochronometric data suggests that the gold Skarns in the Southern Cross area are related in space and time to a suite of magnetite series granite-pegmatite complexes. These granites, dated regionally at 2.66 to 2.60 Ga, are also associated with gold-bearing W-Mo Skarn-greisen or Skarn systems of up to 6-km strike length. The numerous Skarns exposed in the deeply eroded (14 km paleodepth), central Yilgarn craton are members of a recently recognized group of intrusion-related gold deposits which are part of continental-margin tungsten-tin provinces.
Jingwen Mao - One of the best experts on this subject based on the ideXlab platform.
-
comparative geochemical study of scheelite from the shizhuyuan and xianglushan tungsten Skarn deposits south china implications for scheelite mineralization
Ore Geology Reviews, 2019Co-Authors: Jingwen Mao, Zheng Zhao, Trevor Ireland, Fojun Yao, Yuping Yang, Weidong SunAbstract:Abstract Scheelite has been analyzed from the Shizhuyuan and the Xianglushan world-class W deposits from the Nanling W–Sn region and Jiangnan W belt, respectively. The Shizhuyuan deposit consists of proximal Skarn and greisen W–Sn–Mo–Bi and distal Pb–Zn–Ag veins. The Xianglushan deposit, contains layer-like sulfide–scheelite and Skarn W orebodies on granite cupolas overprinted by W greisen veins. Scheelite in Skarn ores from the Shizhuyuan contains higher concentrations of Mo than those in the sulfide–scheelite and Skarn ores from the Xianglushan deposit, reflecting differences between oxidizing and reducing magmatic-hydrothermal fluids. Under oxidizing conditions, W is accompanied by Mo partitions into exsolved fluids to form W–Mo garnet Skarns, whereas under reducing conditions, little Mo is carried by exsolved fluids to form W pyroxene Skarns. Trace element patterns of scheelite from both deposits show negative Ba, Sr, Zr, and Ti, and positive Ta anomalies. Rare earth element (REE) patterns of scheelite within Skarns from the Shizhuyuan deposit have negatively inclined and flat M-type tetrad patterns, and scheelite from the greisens displays flat and positively inclined M-type tetrad patterns. We infer that the fluids formed scheelite within the W Skarns and greisens inherited parental magma trace element and REE characteristics (depleted Ba, Sr, Zr, and Ti, enriched Ta, negative Eu anomalies, and tetrad effects). Whereas, scheelite from sulfide–scheelite veins and Skarns of the Xianglushan deposit also has W- and MW-type tetrad REE patterns. The W-type tetrad REE patterns are complementary to REE patterns from the Renjiashan granite, and the MW-type tetrad REE patterns occur during a single evolutionary stage within a complex hydrothermal environment. Sulfide mineralization can form after or before W Skarns (the former like Shizhuyuan deposit and the latter like Xianglushan deposit). The formation conditions of the latter included reducing conditions and sulfide firstly supersaturated in the melt, resulting in sulfide drops which carried W aggregated on the cupolas. W Skarns and greisens in both deposits underwent generally successive processes related to water supersaturation in the melt. Following a temperature decrease and crystallization, bubbles carried material changing from Si and metal to Si oxide complexes.
-
mineralogy and titanite geochronology of the caojiaba w deposit xiangzhong metallogenic province southern china implications for a distal reduced Skarn w formation
Mineralium Deposita, 2019Co-Authors: Guiqing Xie, Jingwen Mao, Leon Bagas, Zhiyuan ZhangAbstract:The Caojiaba tungsten deposit (19.03 Mt@ 0.37 wt% WO3) is hosted by Skarn along the contact between clastic and carbonate rocks in the Xiangzhong Metallogenic Province of southern China. The deposit is characterized by an early prograde Skarn containing low andraditic garnet (Ad0.7–21.9) and hedenbergitic pyroxene (Hd52.9–77.3) overprinted by a retrograde biotite–chlorite assemblage and then by quartz–scheelite veins, similar to well-studied reduced tungsten Skarns worldwide. Scheelite has low MoO3 (0.01–0.16 wt%), and ore commonly contains up to 1.5 ppm Au and up to 0.33 wt% Sb. Sensitive high-resolution ion microprobe (SHRIMP) U–Pb analyses of hydrothermal titanite coexisting with scheelite in three Skarn ore samples provide ages between 206 ± 5 Ma and 196 ± 3 Ma (2σ). Our new ages demonstrate that the tungsten mineralization took place at Caojiaba between 206 and 196 Ma, overlapping the 228–201 Ma emplacement age of granitic rocks in the Xiangzhong Metallogenic Province. Mineralogical and geochronological evidence collectively indicates that Caojiaba is a distal reduced W Skarn deposit. The 226–196 Ma granite-related W mineralization recognized throughout the province has a possible link with the widespread Sb–Au mineralization in the region.
-
mineralogy fluid inclusions and isotopes of the cihai iron deposit eastern tianshan nw china implication for hydrothermal evolution and genesis of subvolcanic rocks hosted Skarn type deposits
Ore Geology Reviews, 2017Co-Authors: Jiahao Zheng, Jingwen Mao, Fuquan Yang, Fengmei Chai, Yongfeng ZhuAbstract:Abstract Most Skarn deposits are closely related to granitoids that intruded into carbonate rocks. The Cihai (>100 Mt at 45% Fe) is a deposit with mineral assemblages and hydrothermal features similar to many other typical Skarn deposits of the world. However, the iron orebodies of Cihai are mainly hosted within the diabase and not in contact with carbonate rocks. In addition, some magnetite grains exhibit unusual relatively high TiO 2 content. These features are not consistent with the typical Skarn iron deposit. Different hydrothermal and/or magmatic processes are being actively investigated for its origin. Because of a lack of systematic studies of geology, mineral compositions, fluid inclusions, and isotopes, the genetic type, ore genesis, and hydrothermal evolution of this deposit are still poorly understood and remain controversial. The Skarn mineral assemblages are the alteration products of diabase. Three main paragenetic stages of Skarn formation and ore deposition have been recognized based on petrographic observations, which show a prograde Skarn stage (garnet-clinopyroxene-disseminated magnetite), a retrograde Skarn stage (main iron ore stage, massive magnetite-amphibole-epidote ± ilvaite), and a quartz-sulfide stage (quartz-calcite-pyrite-pyrrhotite-cobaltite). Overall, the compositions of garnet, clinpyroxene, and amphibole are consistent with those of typical Skarn Fe deposits worldwide. In the disseminated ores, some magnetite grains exhibit relatively high TiO 2 content (>1 wt.%), which may be inherited from the diabase protoliths. Some distinct chemical zoning in magnetite grains were observed in this study, wherein cores are enriched in Ti, and magnetite rims show a pronounced depletion in Ti. The textural and compositional data of magnetite confirm that the Cihai Fe deposit is of hydrothermal origin, rather than associated with iron rich melts as previously suggested. Fluid inclusions study reveal that, the prograde Skarn (garnet and pyroxene) formed from high temperature (520–600 °C), moderate- to high-salinity (8.1–23.1 wt.% NaCl equiv, and >46 wt.% NaCl equiv) fluids. Massive iron ore and retrograde Skarn assemblages (amphibole-epidote ± ilvaite) formed under hydrostatic condition after the fracturing of early Skarn. Fluids in this stage had lower temperature (220°–456 °C) and salinity (8.4–16.3 wt.% NaCl equiv). Fluid inclusions in quartz-sulfide stage quartz and calcite also record similar conditions, with temperature range from 128° to 367 °C and salinity range from 0.2 to 22.9 wt.% NaCl equiv. Oxygen and hydrogen isotopic data of garnet and quartz suggest that mixing and dilution of early magmatic fluids with external fluids (e.g., meteoric waters) caused a decrease in fluid temperature and salinity in the later stages of the Skarn formation and massive iron precipitation. The δ 18 O values of magnetite from iron ores vary between 4.1 and 8.5‰, which are similar to values reported in other Skarn Fe deposits. Such values are distinct from those of other iron ore deposits such as Kiruna-type and magmatic Fe-Ti-V deposits worldwide. Taken together, these geologic, geochemical, and isotopic data confirm that Cihai is a diabase-hosted Skarn deposit related to the granitoids at depth.
-
zircon u pb and phlogopite 40 ar 39 ar age of the chengchao and jinshandian Skarn fe deposits southeast hubei province middle lower yangtze river valley metallogenic belt china
Mineralium Deposita, 2012Co-Authors: Guiqing Xie, Jingwen Mao, Haijie Zhao, Chao Duan, Lei YaoAbstract:The Chengchao and Jinshandian deposits in the southeast Hubei Province are the two largest Skarn Fe deposits in the Middle–Lower Yangtze River Valley metallogenic belt (MLYRVMB), China. They are characterized by NW-striking orebodies that are developed along the contacts between the Late Mesozoic granitoid and Triassic carbonate and clastic rocks. New sensitive high-resolution ion microprobe and laser ablation inductively coupled plasma mass spectrometry zircon U–Pb dating of the mineralization-related quartz diorite and granite at Chengchao yield ages of 129 ± 2 and 127 ± 2 Ma, respectively, and those at Jinshandian of 127 ± 2 and 133 ± 1 Ma, respectively. These results are interpreted as the crystallization age of these intrusions. Hydrothermal phlogopite samples from the Skarn ores at Chengchao and Jinshandian have the plateau 40Ar–39Ar ages of 132.6 ± 1.4 and 131.6 ± 1.2 Ma, respectively. These results confirm that both intrusions and associated Skarn Fe mineralization were formed contemporaneously in the middle Early Cretaceous time. New zircon U–Pb and phlogopite 40Ar–39Ar ages in this study, when combined with available precise geochronological data, demonstrate that there were two discontinuous igneous events, corresponding to two episodes of Skarn Fe-bearing mineralization in the southeast Hubei Province: (1) 140–136 Ma diorites and quartz diorites and 141–137 Ma Skarn Cu–Fe or Fe–Cu deposits and (2) 133–127 Ma quartz diorites and granites and 133–132 Ma Skarn Fe deposits. This scenario is similar to that proposed for the entire MLYRVMB. The intrusions related to Skarn Fe deposits show obviously petrological and geochemical differences from those related to Skarn Cu–Fe or Fe–Cu deposits. The former are quartz diorite and diorite in petrology and have similar adakitic geochemical signatures and in equilibrium with a garnet-rich residue, whereas the latter are petrologically granite and quartz diorite that are distinguishable from adakitic rocks and in equilibrium with a plagioclase residue. These features indicated that two episodes of magmatism and the formation of Skarn Fe-bearing deposits in the southeast Hubei Province, MLYRVMB, might be associated lithosphere thinning induced by asthenosphere upwelling during the Late Mesozoic.
Anthony E Williamsjones - One of the best experts on this subject based on the ideXlab platform.
-
constraints on the uptake of ree by scheelite in the baoshan tungsten Skarn deposit south china
Chemical Geology, 2018Co-Authors: Wen Winston Zhao, Meifu Zhou, Anthony E Williamsjones, Zheng ZhaoAbstract:Abstract Scheelite is the main ore mineral in Skarn-type tungsten deposits, and a common accessory mineral in a variety of rock-types. The Baoshan deposit in South China is one of the most important polymetallic scheelite Skarn deposits in China, hosting 40,000 t of WO3 with economic concentrations of Zn, Cu, and Ag. It is hosted by a calcic Skarn that is zoned outwards mineralogically from garnet-clinopyroxene, through clinopyroxene-garnet, to wollastonite, and overprinted by retrograde minerals. Scheelite occurs in both the prograde and retrograde Skarns, and is complexly zoned. On the basis of its textures, the scheelite was classified into three types. Scheelite I and II belong to the early and late prograde stages, respectively, and Scheelite III precipitated during the retrograde stage. The molybdenum (Mo) content of these scheelite types ranges from 54 ppm to 24 wt%, and the total rare earth element content ranges from 12 to 321 ppm. Rare earth element (REE) concentrations and chondrite-normalized REE profiles vary with the distribution of major elements. The profiles indicate variable degrees of REE enrichment, which correlates negatively with the Mo content. Molybdenum-rich scheelite displays a negative Eu anomaly, and Mo-poor scheelite a positive Eu anomaly. Crystal structure provided the first-order control on the minor and trace element composition of the scheelite. Incorporation of REE3 + into scheelite was controlled partly by a coupled substitution involving Mo. The lattice strain model was used to estimate scheelite-fluid partition coefficients for the REE from the contents of these elements in the scheelite and to predict the relative distributions of the REE in the ore-forming fluids. It is proposed that conditions were initially oxidizing, leading to strong incorporation of Mo in Scheelite I, that they became more reducing with the crystallization of Scheelite II containing lesser Mo, and that during retrograde Skarn formation there was a return to oxidizing conditions due to an influx of meteoric waters, which altered Scheelite II giving rise to the formation of Scheelite III. The study shows that the composition of scheelite recorded the history of the Baoshan hydrothermal system, and that the behaviour of the REE could be used to quantitatively reconstruct the changing physicochemical conditions during ore formation.
Neal J Mcnaughton - One of the best experts on this subject based on the ideXlab platform.
-
mineral equilibria and zircon garnet and titanite u pb ages constraining the ptt path of granite related hydrothermal systems at the big bell gold deposit western australia
Mineralium Deposita, 2018Co-Authors: Andreas G Mueller, Neal J McnaughtonAbstract:The Big Bell deposit (75 t gold) is located in a narrow spur of the Meekatharra greenstone belt, Yilgarn Craton, Western Australia. Two ore bodies are located in a calcic-potassic contact alteration zone overprinting lineated granodiorite dykes and amphibolite: almandine-cummingtonite-hornblende Skarn (1–3 g/t Au, 1700 g/t As, 330 g/t W) and the muscovite-microcline gneiss (3–5 g/t Au, 580 g/t Sb, 620 g/t W) of the Main Lode. Genetic models vary from pre- to post-metamorphic replacement. Hornblende-plagioclase pairs in amphibolite constrain peak metamorphic temperature to 670 ± 50 °C. In contrast, garnet-biotite thermometry provides estimates of 578 ± 50 and 608 ± 50 °C for garnet-cordierite-biotite schist bordering the Skarn and enveloping the Main Lode. Garnet-cordierite and garnet-hornblende pairs extend the range of fluid temperature to 540 ± 65 °C, well below peak metamorphic temperature. At 540–600 °C, the alteration assemblage andalusite + sillimanite constrains pressure to 300–400 MPa corresponding to 11–14 km crustal depth. Published U-Pb ages indicate that metamorphism took place in the aureole of the southeast granodiorite-tonalite batholith (2740–2700 Ma), followed by gold mineralization at 2662 ± 5 Ma and by the emplacement of biotite granite and Sn–Ta–Nb granite-pegmatite dykes at 2625–2610 Ma. Amphibolite xenoliths in granite northwest of the deposit record the lowest temperature (628 ± 50 °C), suggesting it lacks a metamorphic aureole. The rare metal dykes are spatially associated with epidote-albite and andradite-diopside Skarns (≤1.5 g/t Au), mined where enriched in the weathered zone. We analysed hydrothermal zircon intergrown with andradite. Concordant U-Pb ages of 2612 ± 7 and 2609 ± 10 Ma confirm the presence of a second granite-related system. The zircons display oscillatory zoning and have low Th/U ratios (0.05–0.08). Low-Th titanite from an albite granite dyke has a concordant but reset U-Pb age of 2577 ± 7 Ma.
-
u pb ages constraining batholith emplacement contact metamorphism and the formation of gold and w mo Skarns in the southern cross area yilgarn craton western australia
Economic Geology, 2000Co-Authors: Andreas G Mueller, Neal J McnaughtonAbstract:Gold mines in the Archean Southern Cross greenstone belt, central Yilgarn craton, have produced more than 220 metric tons (t) of gold. The deposits are characterized by a pyroxene-rich gangue and have been interpreted as synmetamorphic amphibolite facies replacement deposits and as postmetamorphic intrusion-related Skarns. We present ion microprobe U-Pb zircon data constraining the age of amphibolite facies contact metamorphism in the aureole of the Ghooli Dome, the largest granitoid batholith intruding the Southern Cross belt. Drill cores from the Copperhead gold mine provided zircons from an altered quartz porphyry sill, located 300 m southwest of the Ghooli Dome, and zircons from the monzogranite gneiss at the border of the batholith. The porphyry sill contains embayed igneous zircons dated by a concordant U-Pb age of 2912 ± 5 Ma and euhedral zircons of metamorphic crystal habitus dated by a second concordant age of 2772 ± 5 Ma. The igneous zircons are slightly younger than those recovered from another sill of quartz porphyry, sampled at the Southern Star mine, which define a single concordant U-Pb age of 2934 ± 7 Ma. The metamorphic zircons in the porphyry at Copperhead record the same age as the igneous zircons (2775 ± 10 Ma) in the monzogranite gneiss of the Ghooli Dome. This pluton extends more than 40 km to the southeast of Copperhead and also forms the batholith border at the Corinthian and Fraser’s mines. These gold deposits, too, are hosted by amphibolite facies greenstones contact metamorphosed at 2772 ± 5 Ma. The Corinthian Skarn (2620 ± 6 Ma) formed 150 m.y. after high-grade metamorphism and, consequently, cannot be classified as synmetamorphic or metamorphogenic. A review of field relationships and of published geochronometric data suggests that the gold Skarns in the Southern Cross area are related in space and time to a suite of magnetite series granite-pegmatite complexes. These granites, dated regionally at 2.66 to 2.60 Ga, are also associated with gold-bearing W-Mo Skarn-greisen or Skarn systems of up to 6-km strike length. The numerous Skarns exposed in the deeply eroded (14 km paleodepth), central Yilgarn craton are members of a recently recognized group of intrusion-related gold deposits which are part of continental-margin tungsten-tin provinces.