Fluid origin of fluorite-rich carbonate-hosted Pb–Zn mineralization of the Himalayan–Zagros collisional orogenic system: A case study of the Mohailaheng deposit,Tibetan Plateau,China

A significant belt of carbonate-hosted Pb–Zn mineralization occurs in the Himalayan–Zagros collisional orogenic system. Three differing types of these Pb–Zn deposits within this belt have been identified based on variations in gangue mineral assemblages, leading to the classification of carbonate-, quartz- and fluorite-rich classes of Pb–Zn deposits. The third Pb–Zn mineralization (fluorite-rich) type is common in this orogenic system, but little research has been undertaken on it. Here, we focus on the Mohailaheng deposit, a large-sized fluorite-rich carbonate-hosted Pb–Zn deposit (> 100 Mt Pb + Zn ores with average grade of 2.18%–4.23%); the deposit is located in the Sanjiang Cenozoic thrust-fold belt, an important part of the Himalayan–Zagros collisional orogenic system and an area that formed during the early Tertiary India–Eurasia collision. The main orebodies in this deposit are stratabound and are hosted by Carboniferous limestones that are located along secondary faults associated with a regional thrust fault. The main assemblage is a sphalerite + galena + pyrite sulfide assemblage associated with a calcite + fluorite + barite + quartz + dolomite gangue assemblage. Detailed field and experimental work indicates that the deposit formed during three distinct phases of hydrothermal activity. Studies on fluid inclusion and stable isotopes of gangue minerals indicate that two dominant distinct fluids involving the deposit formation. They include (1) a low-temperature (130–140 °C), high-salinity (23–24 wt.% NaCl equivalent) basinal brine containing Na⁺–K⁺–Mg^2 +–Ca^2 +–Cl⁻ ions and abnormally high SO₄^2 − concentrations, which probably derived from Tertiary basins in the regional district, and (2) a low- to moderate-temperature (170–180 °C) and moderate- to high-salinity (19–20 wt.% NaCl equivalent) metamorphic fluid containing Na⁺–K⁺–Mg^2 +–Cl^––SO₄^2 − ions and abnormally high F⁻ and organic matter concentrations, that probably formed during regional metamorphism. Some evaporated seawaters and meteoric fluids were also identified in mixtures with these two dominant fluids. The Pb–Zn mineralization at Mohailaheng formed during three distinct stages, consistent with the regional tectonic history. The first stage involved the formation of favorable lithological and structural traps at Mohailaheng during regional thrusting, leading to the migration of compressed metamorphic waters at depth along a detachment zone, sequestering metals from sediments within the region. Basinal brines at the surface also began to infiltrate down along the secondary faults, dissolving gypsum from the underlying sediments. The second stage was associated with the cessation of thrusting and the onset of strike-slip movements along these thrust faults. Metamorphic fluids containing high concentrations of halogen ions, organic gases, and metals ascended into the structural traps at Mohailaheng along the reactivated thrust faults, causing fluorite, calcite, and some sulfide precipitation. Then, basinal brines rich in SO₄^2 − quickly descended into the structural traps along the reactivated faults, causing reduction of SO₄^2 − by organic matter, and producing significant amounts of H₂S. The reduced sulfur then reacted with the metals in the fluids, causing significant sulfide precipitation. The third stage was associated with metal-depleted fluids, which only resulted in the precipitation of calcite from the diluted basinal brines. Combining these findings with research results on other fluorite-rich carbonate-hosted Pb–Zn deposits in the Himalayan–Zagros orogenic system suggests that this type of carbonate-hosted Pb–Zn deposits can also be classified as Mississippi Valley-type (MVT) deposits, and that the origin of the fluorite in these deposits may be related to multiple hydrothermal fluids involved in the mineralization evolution.     

Uranium–lead ages of apatite from iron oxide ores of the Bafq District,East-Central Iran

Iron oxide–apatite (IOA) deposits, often referred to as Kiruna-type iron ore deposits, are known to have formed from the Proterozoic to the Tertiary. They are commonly associated with calc–alkaline volcanic rocks and regional- to deposit-scale metasomatic alteration. In the Bafq District in east Central Iran, economic iron oxide–apatite deposits occur within felsic volcanic tuffs and volcanosedimentary sequences of Early Cambrian age. In order to constrain the age of formation of these ores and their relationship with the Early Cambrian magmatic event, we have determined the U–Pb apatite age for five occurrences in the Bafq District. In a ²⁰⁶Pb/²³⁸U vs. ²⁰⁷Pb/²³⁵U diagram, apatite free of or poor in inclusions of other minerals plots along the Concordia between 539 and 527 Ma with four out of five samples from one deposit clustering at the upper end of this range. For this deposit, we interpret this cluster to represent the age of apatite formation, whereas the spread towards younger ages may reflect either minor Pb loss or several events of IOA formation. Apatite with inclusions of monazite (±xenotime) yields disturbed systems with inclusions having developed after formation of the iron ore–apatite deposits, possibly as late as 130–140 Ma ago. Obtained apatite ages confirms that (IOA) and the apatite-rich rocks (apatites) of the Bafq district formed coevally with the Early Cambrian magmatic (-metasomatic) events.     

Origin of the early Permian Wajilitag igneous complex and associated Fe–Ti oxide mineralization in the Tarim large igneous province,NW China

The Wajilitag igneous complex is part of the early Permian Tarim large igneous province in NW China, and is composed of a layered mafic–ultramafic intrusion and associated syenitic plutons. In order to better constrain its origin, and the conditions of associated Fe–Ti oxide mineralization, we carried out an integrated study of mineralogical, geochemical and Sr–Nd–Hf isotopic analyses on selected samples. The Wajilitag igneous rocks have an OIB-like compositional affinity, similar to the coeval mafic dykes in the Bachu region. The layered intrusion consists of olivine clinopyroxenite, coarse-grained clinopyroxenite, fine-grained clinopyroxenite and gabbro from the base upwards. Fe–Ti oxide ores are mainly hosted in fine-grained clinopyroxenite. Forsterite contents in olivines from the olivine clinopyroxenite range from 71 to 76 mol%, indicating crystallization from an evolved magma. Reconstructed composition of the parental magma of the layered intrusion is Fe–Ti-rich, similar to that of the Bachu mafic dykes. Syenite and quartz syenite plutons have ε_Nd(t) values ranging from +1.4 to +2.9, identical to that for the layered intrusion. They may have formed by differentiation of underplated magmas at depth and subsequent fractional crystallization. Magnetites enclosed in olivines and clinopyroxenes have Cr₂O₃ contents higher than those interstitial to silicates in the layered intrusion. This suggests that the Cr-rich magnetite is an early crystallized phase, whereas interstitial magnetite may have accumulated from evolved Fe–Ti-rich melts that percolated through a crystal mush. Low V content in Cr-poor magnetite (<6600 ppm) is consistent with an estimate of oxygen fugacity of FMQ + 1.1 to FMQ + 3.5. We propose that accumulation of Fe–Ti oxides during the late stage of magmatic differentiation may have followed crystallization of Fe–Ti-melt under high fO₂ and a volatile-rich condition.     

The role of evaporites in the formation of magnetite–apatite deposits along the Middle and Lower Yangtze River,China: Evidence from LA-ICP-MS analysis of fluid inclusions

Numerous magnetite–apatite deposits occur in the Ningwu and Luzong sedimentary basins along the Middle and Lower Yangtze River, China. These deposits are located in the contact zone of (gabbro)-dioritic porphyries with surrounding volcanic or sedimentary rocks and are characterized by massive, vein and disseminated magnetite–apatite ± anhydrite mineralization associated with voluminous sodic–calcic alteration. Petrologic and microthermometric studies on multiphase inclusions in pre- to syn-mineralization pyroxene and garnet from the deposits at Meishan (Ningwu basin), Luohe and Nihe (both in Luzong basin) demonstrate that they represent extremely saline brines (~ 90 wt.% NaCl_equiv) that were trapped at temperatures of about 780 °C. Laser ablation ICP-MS analyses and Raman spectroscopic studies on the natural fluid inclusions and synthetic fluid inclusions manufactured at similar P–T conditions reveal that the brines are composed mainly of Na (13–24 wt.%), K (7–11 wt.%), Ca (~ 7 wt.%), Fe (~ 2 wt.%), Cl (19–47 wt.%) and variable amounts of SO₄ (3–39 wt.%). Their Cl/Br, Na/K and Na/B ratios are markedly different from those of seawater evaporation brines and lie between those of magmatic fluids and sedimentary halite, suggesting a significant contribution from halite-bearing evaporites. High S/B and Ca/Na ratios in the fluid inclusions and heavy sulfur isotopic signatures of syn- to post-mineralization anhydrite (δ³⁴S_Anh = + 15.2 to + 16.9‰) and pyrite (δ³⁴S_Py = + 4.6‰ to + 12.1‰) further suggest a significant contribution from sedimentary anhydrite. These interpretations are in line with the presence of evaporite sequences in the lower parts of the sedimentary basins.The combined evidence thus suggests that the magnetite–apatite deposits along the Middle and Lower Yangtze River formed by fluids that exsolved from magmas that assimilated substantial amounts of Triassic evaporites during their ascent. Due to their Fe-oxide dominated mineralogy, their association with large-scale sodic–calcic alteration and their spatial and temporal associations with subvolcanic intrusions we interpret them as a special type of IOCG deposits that is characterized by unusually high contents of Na, Ca, Cl and SO₄ in the ore-forming fluids. Evaporite assimilation apparently led to the production of large amounts of high-salinity brine and thus to an enhanced capacity to extract iron from the (gabbro)-dioritic intrusions and to concentrate it in the form of ore bodies. Hence, we believe that evaporite-bearing sedimentary basins are more prospective for magnetite–apatite deposits than evaporite-free basins.     

Geological and isotopic evidence for magmatic-hydrothermal origin of the Ag–Pb–Zn deposits in the Lengshuikeng District,east-central China

The Lengshuikeng ore district in east-central China has an ore reserve of ～43 Mt with an average grade of 204.53 g/t Ag and 4.63 % Pb?+?Zn. Based on contrasting geological characteristics, the mineralization in the Lengshuikeng ore district can be divided into porphyry-hosted and stratabound types. The porphyry-hosted mineralization is distributed in and around the Lengshuikeng granite porphyry and shows a distinct alteration zoning including minor chloritization and sericitization in the proximal zone; sericitization, silicification, and carbonatization in the peripheral zone; and sericitization and carbonatization in the distal zone. The stratabound mineralization occurs in volcano-sedimentary rocks at ～100–400 m depth without obvious zoning of alterations and ore minerals. Porphyry-hosted and stratabound mineralization are both characterized by early-stage pyrite–chalcopyrite–sphalerite, middle-stage acanthite–native silver–galena–sphalerite, and late-stage pyrite–quartz–calcite. The δ³⁴S values of pyrite, sphalerite, and galena in the ores range from ?3.8 to +6.9‰ with an average of +2.0‰. The C–O isotope values of siderite, calcite, and dolomite range from ?7.2 to ?1.5‰ with an average of ?4.4‰ (V-PDB) and from +10.9 to +19.5‰ with an average of +14.8‰ (V-SMOW), respectively. Hydrogen, oxygen, and carbon isotopes indicate that the hydrothermal fluids were derived mainly from meteoric water, with addition of minor amounts of magmatic water. Geochronology employing LA–ICP–MS analyses of zircons from a quartz syenite porphyry yielded a weighted mean ²⁰⁶Pb/²³⁸U age of 136.3?±?0.8 Ma considered as the emplacement age of the porphyry. Rb–Sr dating of sphalerite from the main ore stage yielded an age of 126.9?±?7.1 Ma, marking the time of mineralization. The Lengshuikeng mineralization classifies as an epithermal Ag–Pb–Zn deposit.     

Platinum-group minerals from the Jinbaoshan Pd–Pt deposit,SW China: evidence for magmatic origin and hydrothermal alteration

The Jinbaoshan Pt–Pd deposit in Yunnan, SW China, is hosted in a wehrlite body, which is a member of the Permian (∼260 Ma) Emeishan Large Igneous Province (ELIP). The deposit is reported to contain one million tonnes of Pt–Pd ore grading 0.21% Ni and 0.16% Cu with 3.0 g/t (Pd + Pt). Platinum-group minerals (PGM) mostly are ∼10 μm in diameter, and are commonly Te-, Sn- and As-bearing, including moncheite (PtTe₂), atokite (Pd₃Sn), kotulskite (PdTe), sperrylite (PtAs₂), irarsite (IrAsS), cooperite (PtS), sudburyite (PdSb), and Pt–Fe alloy. Primary rock-forming minerals are olivine and clinopyroxene, with clinopyroxene forming anhedral poikilitic crystals surrounding olivine. Primary chromite occurs either as euhedral grains enclosed within olivine or as an interstitial phase to the olivine. However, the intrusion has undergone extensive hydrothermal alteration. Most olivine grains have been altered to serpentine, and interstitial clinopyroxene is often altered to actinolite/tremolite and locally biotite. Interstitial chromite grains are either partially or totally replaced by secondary magnetite. Base-metal sulfides (BMS), such as pentlandite and chalcopyrite, are usually interstitial to the altered olivine. PGM are located with the BMS and are therefore also interstitial to the serpentinized olivine grains, occurring within altered interstitial clinopyroxene and chromite, or along the edges of these minerals, which predominantly altered to actinolite/tremolite, serpentine and magnetite. Hydrothermal fluids were responsible for the release of the platinum-group elements (PGE) from the BMS to precipitate the PGM at low temperature during pervasive alteration. A sequence of alteration of the PGM has been recognized. Initially moncheite and atokite have been corroded and recrystallized during the formation of actinolite/tremolite, and then, cooperite and moncheite were altered to Pt–Fe alloy where they are in contact with serpentine. Sudburyite occurs in veins indicating late Pd mobility. However, textural evidence shows that the PGM are still in close proximity to the BMS. They occur in PGE-rich layers located at specific igneous horizons in the intrusion, suggesting that PGE were originally magmatic concentrations that, within a PGE-rich horizon, crystallized with BMS late in the olivine/clinopyroxene crystallization sequence and have not been significantly transported during serpentinization and alteration.     

The characteristics of minerals in civil coal and its combustion products in Xuanwei County, Yunnan, China

Background:The combustion of bituminous coal resulted in the high incidence of lung cancer of peasants in Xuanwei and Fuyuan counties of Yunnan Province because of particulate matter (PM) absorbing a great deal of heavy metal elements and organic matter s…     

A laser ablation ICP-MS study of platinum-group and chalcophile elements in base metal sulfide minerals of the Jinchuan Ni–Cu sulfide deposit,NW China

The paper presents concentrations of the platinum-group and chalcophile elements in the base metal sulfides (BMS) from the Jinchuan Ni–Cu sulfide deposit determined by laser ablation-inductively coupled plasma-mass spectrometry. Mass balance calculations reveal that pentlandite hosts a large proportion of Co, Ni and Pd (> 65%), and that pentlandite and pyrrhotite accommodate significant proportions of Re, Os, Ru, Rh, and Ag (~ 35–90%), whereas chalcopyrite contains a small amount of Ag (~ 10%) but negligible platinum-group elements. Iridium and Pt are not concentrated in the BMS and mostly occur in As-rich platinum-group minerals. The enrichments of Co, Ni, Re, Os, Ru, and Rh in pentlandite and pyrrhotite, and Cu in chalcopyrite are consistent with the fractionation of sulfide liquid and exsolution of pentlandite and pyrrhotite from the mono-sulfide solid solution (MSS). The Ir-bearing minerals exsolved from the MSS, depleting pentlandite and pyrrhotite in Ir, whereas sperrylite exsolved from the residual sulfide liquid on cooling. Diffusion of Pd from residual sulfide liquid into pentlandite during its exsolution from the MSS and crystallization of Pt-bearing minerals in the residual sulfide liquid resulted in the enrichment of Pd in pentlandite and decoupling between Pd and Pt in the Jinchuan net-textured and massive ores.     

The origin and response of zircon in eclogite to metamorphism during the multi-stage evolution of the Huwan Shear Zone,China: Insights from Lu–Hf and U–Pb isotopic and trace element geochemistry

The Huwan Shear Zone (HSZ) is an eclogite bearing transpressive wrench zone located along the Shangdan Suture that juxtaposes the Paleozoic Qinling and Mesozoic Hong'an–Dabie orogenic terrains. The region preserves a complex history that bridges the gap between adjacent orogenic terrains. Simultaneous in-situ trace element, U–Th–Pb and Lu–Hf‐isotope analysis of zircon grains from samples of the Xiongdian and Sujiahe eclogite identify a late Carboniferous to early Permian period of high pressure metamorphism, ca. 283 to 306 Ma. Zircon grains are observed to respond to metamorphic overprint via a two stage process: (1) An initial prograde stage of fluid catalyzed interface coupled dissolution–reprecipitation, involving exsolution of a non-ideal solid solution thorite (ThSiO₄) end member and loss of highly incompatible components (LREE and Pb), (2) A second stage of coupled zircon dissolution, coarsening, and new rim growth in equilibrium with garnet at high pressure conditions.We identify Proterozoic whole rock Sm–Nd and zircon grain Lu–Hf isotopic evidence which challenges the traditional interpretation that the Xiongdian and Sujiahe eclogite formed in response to early Paleozoic mantle melting and oceanic crust generation. We argue the Huwan Shear Zone contains no conclusive evidence of early/middle Paleozoic oceanic crust, but rather Proterozoic crustal components analogous to those found in the Northern Qinling Terrain and associated with formation of the Shangdan Suture. We present a simpler geodynamic model involving continuous convergence and accretion of terrains onto the southern margin of the North China Block during the Paleozoic Qinling and Mesozoic Dabie orogenies.     

The origin of the world class tin-polymetallic deposits in the Gejiu district,SW China: Constraints from metal zoning characteristics and 40Ar–39Ar geochronology

The Gejiu tin-polymetallic deposits in the Western Cathaysia Block of South China comprise the world's largest primary tin district, with a total resource of approximately 300 million metric ton ores, at an average grade of 1 wt percent Sn. Tin polymetallic mineralization occurs in five deposits and has four ore types, i.e., greisen, skarn, stratabound cassiterite-sulfide (mostly oxidized) and vein type ore. In each deposit the orebodies typically occur in an extensive hydrothermal system centered on a shallow Late Cretaceous granitoid cupola. Metal zoning is well developed both vertically and horizontally over the entire district, from W + Be + Bi ± Mo ± Sn ores inside granite intrusions, to Sn + Cu-dominated ores at intrusion margins and farther out to Pb + Zn deposits in the surrounding host carbonate. This zoning pattern is similar to that of other hydrothermal deposits in other parts of the world, indicating a close genetic relationship between magmatism and mineralization. In this paper, we dated thirteen mica samples from all types of mineralization and from the five deposits in the Gejiu district. The ages range from 77.4 ± 0.6 Ma to 95.3 ± 0.7 Ma and are similar to the existing zircon U–Pb age of the granitic intrusions (77.4 ± 2.5–85.8 ± 0.6), indicating a genetic relationship between the mineralization and the intrusions. Geological characteristics, metal zoning patterns and new geochronological data all indicate that the tin-polymetallic ores in the Gejiu district are hydrothermal in origin and are genetically related to the nearby granitic intrusions. It is unlikely that the deposits are syngenetic, as has been proposed in recent years.     

Zircon LA-ICP-MS U-Pb dating and Sr-Nd isotope study of the Guposhan granite complex, Guangxi, China

Zircon U-Pb dating by the LA-ICP-MS method was applied to determining the ages of different units of the Guposhan granite complex, among which the East Guposhan unit is 160.8±1.6 Ma, the West Guposhan unit is 165.0±1.9 Ma, and the Lisong unit is 163.0±1.3 Ma in age. Much similarity in ages of the three units has thus proved that the whole Guposhan granite complex was formed in the same period of time. They were the products of large-scale granitic magmatism through crust-remelting in the first stage of the Middle Yanshanian in South China. However, the three units have differences both in petrology and in geochemistry. Besides the differences in major, trace and rare-earth elements, they are distinct in their Rb-Sr and Sm-Nd isotopic compositions. The East Guposhan unit and Lisong unit and its enclaves have a similar (87Sr/86Sr)i value of 0.7064 with an average of εNd(t)=-3.03, indicating that more mantle material was evolved in the magma derivation; whereas the West Guposhan unit has a higher (87Sr/86Sr)i value of 0.7173 but a lower εNd(t) value of -5.00, and is characterized by strong negative Eu anomalies and higher Rb/Sr ratios, suggesting that its source materials were composed of relatively old crust components and new mantle-derived components. In addition, an inherited zircon grain in the East Guposhan unit (GP-1) yielded a 206Pb/238U age of 806.4 Ma, which is similar to the ages of the Jiulin cordierite granite in northern Jiangxi and of the Yinqiao migmatic granite in Guangxi in the HZH granite zone. All this may provide new evidence for Late Proterozoic magmatism in the HZH granite zone.     

Mechanism of mineralization in the Changjiang uranium ore field,South China: Evidence from fluid inclusions,hydrothermal alteration,and H–O isotopes

The Changjiang uranium ore field, which contains >10,000 tonnes of recoverable U with a grade of 0.1–0.5%, is hosted by Triassic two-mica and Jurassic biotite granites, and is one of the most important uranium ore fields in South China. The minerals associated with alteration and mineralization can be divided into two stages, namely syn-ore and post-ore. The syn-ore minerals are primarily quartz, pitchblende, hematite, hydromica, chlorite, fluorite, and pyrite; the post-ore minerals include quartz, calcite, fluorite, pyrite, and hematite. The fluid inclusions of the early syn-ore stage characteristically contain O₂, and those of the late syn-ore and post-ore stage contain H₂ and CH₄. The fluid inclusions in quartz of the syn-ore stage include H₂O, H₂O–CO₂, and CO₂ types, and they occur in clusters or along trails. Homogenization temperatures (T_h) for the H₂O–CO₂ and two-phase H₂O inclusions range from 106 °C to >350 °C and cluster in two distinct groups for each type; salinities are lower than 10 wt% NaCl equiv. The ore-forming fluids underwent CO₂ effervescence or phase separation at ∼250 °C under a pressure of 1000–1100 bar. The U/Th values of the altered granites are lowest close to the ore, increase outwards, but subsequently decrease close to unaltered granites. From the unaltered granites to the ore, the lowest Fe₂O₃/FeO values become lower and the highest values higher. The REE patterns of the altered granites and the ores are similar to each other. The U contents of the ores show a positive correlation with total REE contents but a negative correlation with LREE/HREE ratios, indicating the pitchblende is REE-bearing and selectively HREE-rich. The δEu values of the ore show a positive correlation with U contents, indicating the early syn-ore fluids were oxidizing. The δCe values show a negative correlation, indicating the later mineralization environment became reducing. The water–rock interactions of the early syn-ore stage resulted in oxidization of altered granites and reduction of the ore-forming fluids, and it was this reduction that led to the uranium mineralization. During alteration in the early syn-ore stage, the oxidizing fluids leached uranium from granites close to faults, and Fe₂O₃/FeO ratios increased in the alteration zones. The late syn-ore and post-ore alteration decreased the Fe₂O₃/FeO ratios in the alteration zones. The δ¹⁸O_W–SMOW values of the ore-forming fluids range from −1.8‰ to 5.4‰, and the δD_W–SMOW values range from −104.4‰ to −51.6‰, suggesting meteoric water. The meteoric water underwent at least two stages of water–rock interaction: the first caused the fluids to become uranium-bearing, and the second stage, which was primarily associated with ore-bearing faults, led to uranium deposition as pitchblende, accompanied by CO₂ effervescence.     

K–Ar ages of plutonism and mineralization at the Shizhuyuan W–Sn–Bi–Mo deposit,Hunan Province,China

The Qianlishan granite complex, situated 16 km southeast of Chenzhou City, Hunan Province, China, hosts the Shizhuyuan W–Sn–Bi–Mo deposit. This complex, which intruded the Protozoic metasedimentary rocks and the Devonian clastic sedimentary and carbonate rocks, consists of mainly medium- to coarse-grained biotite granites and minor amounts of fine-grained biotite granite in addition to granite and quartz porphyry. K–Ar ages suggest three episodes of plutonism: the medium- to coarse-grained biotite granite (before 152 Ma), the fine-grained biotite granite (137 Ma), and the granite porphyry (129–131 Ma). Muscovite ages of the greisen are 145–148 Ma, suggesting that the W–Sn–Bi–Mo mineralization was related to the main, medium- to coarse-grained biotite granites. The K–Ar age of the hydrothermal vein mineralization is 92 Ma and is probably related to the porphyries.     

The origin of Ag–Au–S–Se minerals in adularia-sericite epithermal deposits: constraints from the Broken Hills deposit, Hauraki Goldfield, New Zealand

The 7.1 Ma Broken Hills adularia-sericite Au–Ag deposit is currently the only producing rhyolite-hosted epithermal deposit in the Hauraki Goldfield of New Zealand. The opaque minerals include pyrite, electrum, acanthite (Ag₂S), sphalerite, and galena, which are common in other adularia-sericite epithermal deposits in the Hauraki Goldfield and elsewhere worldwide. Broken Hills ores also contain the less common minerals aguilarite (Ag₄SeS), naumannite (Ag₂Se), petrovskaite (AuAgS), uytenbogaardtite (Ag₃AuS₂), fischesserite (Ag₃AuSe₂), an unnamed silver chloride (Ag₂Cl), and unnamed Ag?±?Au minerals. Uytenbogaardtite and petrovskaite occur with high-fineness electrum. Broken Hills is the only deposit in the Hauraki Goldfield where uytenbogaardtite and petrovskaite have been identified, and these phases appear to have formed predominantly from unmixing of a precursor high-temperature phase under hypogene conditions. Supergene minerals include covellite, chalcocite, Au-rich electrum, barite, and a variety of iron oxyhydroxide minerals. Uytenbogaardtite can form under supergene and hypogene conditions, and textural relationships between uytenbogaardtite and associated high-fineness electrum may be similar in both conditions. Distinguishing the likely environment of formation rests principally on identification of other supergene minerals and documenting their relationships with uytenbogaardtite. The presence of aguilarite, naumannite, petrovskaite, and fischesserite at Broken Hills reflects a Se-rich mineral assemblage. In the Hauraki Goldfield and the western Great Basin, USA, Se-rich minerals are more abundant in provinces that are characterized by bimodal rhyolite–andesite volcanism, but in other epithermal provinces worldwide, the controls on the occurrences of Se-bearing minerals remain poorly constrained, in spite of the unusually high grades associated with many Se-rich epithermal deposits.     

Submicron-sized fluid inclusions and distribution of hydrous components in jadeite,quartz and symplectite-forming minerals from UHP jadeite–quartzite in the Dabie Mountains,China: TEM and FTIR investigation

Fluid inclusions and clusters of water molecules at nanometer-to submicron-scale in size have been investigated using transmission electron microscopy (TEM) and Fourier transform infrared spectroscopy (FTIR) in jadeite, quartz and symplectite aegirine–augite, albite, taramite and magnetite corona minerals from ultrahigh-pressure (UHP) jadeite–quartzite at Shuanghe, the Dabie Mountains, China. Fluid inclusions from 0.003 μm to 0.78 μm in size occur in jadeite and quartz crystals, and a small number of fluid inclusions from 0.001 μm to 0.25 μm have also been detected in symplectite-forming minerals. Most of the fluid inclusions have round or negative crystal morphology and contain aqueous fluids, but some contain CO₂-rich fluids. They are usually connected to dislocations undetectable at an optical scale. The dislocations represent favorable paths for fluid leakage, accounting for non-decrepitation of most fluid inclusions when external pressure decreased at later stages, although there was partial decrepitation of some fluid inclusions unconnected to defect microstructures resulting from internal overpressure. Non-decrepitation and partial decrepitation of fluid inclusions resulted in changes of original composition and/or density. It is clear that identification of hidden re-equilibration features has significant implications for the petrological interpretation of post-peak metamorphic processes. Micro-FTIR results show that all jadeite and quartz samples contain structural water occurring as hydroxyl ions (OH⁻) and free water (H₂O) in the form of clusters of water molecules. The H₂O transformed from OH⁻ during exhumation and could have triggered and enhanced early retrograde metamorphism of the host rocks and facilitated plastic deformation of jadeite and quartz grains by dislocation movement, and thus the H₂O released during decompression might represent early-stage retrograde metamorphic fluid. The nominally anhydrous mineral (NAM) jadeite is able to transport aqueous fluids in concentrations of at least several hundred ppm water along a subduction zone to mantle depths in the form of clusters of water molecules and hydroxyl ions within crystals.     

A multi-analytical study of the crystal structure of unusual Ti–Zr–Cr-rich Andradite from the Maronia skarn, Rhodope massif, western Thrace, Greece

Unusual Ti–Cr–Zr-rich garnet crystals from high-temperature melilitic skarn of the Maronia area, western Thrace, Greece, were investigated by electron-microprobe analysis, powder and single-crystal X-ray diffraction, IR, Raman and Mössbauer spectroscopy. Chemical data showed that the garnets contain up to 8 wt.% TiO₂, 8 wt.% Cr₂O₃ and 4 wt.% ZrO₂, representing a solid solution of andradite (Ca₃Fe³⁺ ₂Si₃O₁₂ ≈46 mol%), uvarovite (Ca₃Cr₂Si₃O₁₂ ≈23 mol%), grossular (Ca₃Al₂Si₃O₁₂ ≈10 mol%), schorlomite (Ca₃Ti₂[Si,(Fe³⁺,Al³⁺)₂]O₁₂ ≈15 mol%), and kimzeyite (Ca₃Zr₂[Si,Al₂]₃O₁₂ ≈6 mol%). The Mössbauer analysis showed that the total Fe is ferric, preferentially located at the octahedral site and to a smaller extent at the tetrahedral site. Single-crystal XRD analysis, Raman and IR spectroscopy verified substitution of Si mainly by Al³⁺, Fe³⁺ and Ti⁴⁺. Cr³⁺ and Zr⁴⁺ are found at the octahedral site along with Fe³⁺, Al³⁺ and Ti⁴⁺. The measured H₂O content is 0.20 wt.%. The analytical data suggest that the structural formula of the Maronia garnet can be given as: (Ca_2.99Mg_0.03)_Σ=3.02(Fe³⁺ _0.67Cr_0.54Al_0.33Ti_0.29Zr_0.15)_Σ=1.98(Si_2.42Ti_0.24Fe_0.18Al_0.14)_Σ=2.98O₁₂OH_0.11. Ti-rich garnets are not common and their crystal chemistry is still under investigation. The present work presents new evidence that will enable the elucidation of the structural chemistry of Ti- and Cr-rich garnets.     

Fractional crystallization and the formation of thick Fe–Ti–V oxide layers in the Baima layered intrusion,SW China

The Baima layered intrusion is located in the central part of the Emeishan Large Igneous Province (ELIP). The N–S striking intrusion is ~ 24 km long and ~ 2 km thick and dips to the west. Based on variations in modal proportions and cumulus mineral assemblages, the intrusion from the base to the top is simply subdivided into a lower zone (LZ) with most of the economic magnetite layers, and an upper zone (UZ) with apatite-bearing troctolite and gabbro. The rock textures suggest crystallization of the Fe–Ti oxide slightly later than plagioclase (An_67-54) but relatively earlier than olivine (Fo_74-55), followed by clinopyroxene and finally apatite.Relatively low olivine forsterite content and abundant ilmenite exsolution lamellae in clinopyroxene indicate that the Baima parental magma is a highly evolved Fe–Ti-rich magma. Via MELTS model, it demonstrates that under a closed oxygen system, extensive silicate mineral fractionation of a picritic magma might lead to Fe and Ti enrichment and oxygen fugacity elevation in the residual magma. When such Fe–Ti-rich magma ascends to the shallower Baima intrusion, the Fe–Ti oxides may become an early liquidus phase. Well-matched olivine and plagioclase microprobe data with the results of MELTS calculation, combined with relatively low CaO content in olivine (0.02–0.08 wt.%) indicate that wall-rock contamination probably plays a weak role on oxygen fugacity elevation and the early crystallization of Fe–Ti oxides. Several reversals in whole-rock chromium and plagioclase anorthite contents illustrate that multiple recharges of such Fe–Ti-rich magma mainly occurred along the lower part of the Baima magma chamber. Frequent Fe–Ti-rich magma replenishment and gravitational sorting and settling are crucial for the development of thick Fe–Ti oxide layers at the base of the Baima layered intrusion.