Uranium in Phosphorites

The uranium concentration in phosphorites on continents and modern seafloor varies from 0.nto n· 10²ppm (average 75 ppm). The average uranium concentration is 4–48 ppm in Precambrian and Cambrian deposits, 20–90 ppm in Paleozoic and Jurassic deposits, 40–130 ppm in Late Cretaceous–Paleogene deposits, 30–130 ppm in Neogene deposits, and 30–110 ppm in Quaternary (including Holocene) deposits. On the whole, the variation range is almost similar for phosphorites of different ages. The U/P₂O₅ratio in phosphorites ranges from less than unity to 24 · 10^–4(average 3.2 · 10^–4). Major phosphorite deposits of the world with ore reserves of approximately 250 Gt (or 58 Gt P₂O₅) contain up to 19 Mt of uranium. Uranium is present in phosphorites in the tetra- and hexavalent, i.e., U(IV) and U(VI) forms, and their ratio is highly variable. At the early diagenetic stage of the formation of marine phosphorites in a reductive environment, U(VI) diffuses from the near-bottom water into sediments. It is consequently reduced and precipitated as submicroscopic segregations of uranium minerals (mainly uraninite) that are probably absorbed by phosphatic material. During the subsequent reaction between phosphorites and aerated water and the weathering in a subaerial environment, uranium is partly oxidized and lost. The uranium depletion also occurs during catagenesis owing to a more complete crystallization of calcium phosphate and replacement of nonphosphatic components.     

Porphyry Cu–Au and associated polymetallic Fe–Cu–Au deposits in the Beiya Area, western Yunnan Province, south China

The Alkaline porphyries in the Beiya area are located east of the Jinshajiang suture, as part of a Cenozoic alkali-rich porphyry belt in western Yunnan. The main rock types include quartz-albite porphyry, quartz-K-feldspar porphyry and biotite–K-feldspar porphyry. These porphyries are characterised by high alkalinity [(K₂O + Na₂O)% > 10%], high silica (SiO₂% > 65%), high Sr (> 400 ppm) and ⁸⁷Sr/⁸⁶Sr (> 0.706)] ratio and were intruded at 65.5 Ma, between 25.5 to 32.5 Ma, and about 3.8 Ma, respectively. There are five main types of mineral deposits in the Beiya area: (1) porphyry Cu–Au deposits, (2) magmatic Fe–Au deposits, (3) sedimentary polymetallic deposits, (4) polymetallic skarn deposits, and (5) palaeoplacers associated with karsts. The porphyry Cu–Au and polymetallic skarn deposits are associated with quartz–albite porphyry bodies. The Fe–Au and polymetallic sedimentary deposits are part of an ore-forming system that produced considerable Au in the Beiya area, and are characterised by low concentrations of La, Ti, and Co, and high concentrations of Y, Yb, and Sc.The Cenozoic porphyries in western Yunnan display increased alkalinity away from the Triassic Jinshajiang suture. Distribution of both the porphyries and sedimentary deposits in the Beiya area are interpreted to be related to partial melting in a disjointed region between upper mantle lithosphere of the Yangtze Plate and Gondwana continent, and lie within a shear zone between buried Palaeo-Tethyan oceanic lithosphere and upper mantle lithosphere, caused by the subduction and collision of India and Asia.     

Sn-polymetallic greisen-type deposits associated with late-stage rapakivi granites, Brazil: fluid inclusion and stable isotope characteristics

Tin-polymetallic greisen-type deposits in the Itu Rapakivi Province and Rondônia Tin Province, Brazil are associated with late-stage rapakivi fluorine-rich peraluminous alkali-feldspar granites. These granites contain topaz and/or muscovite or zinnwaldite and have geochemical characteristics comparable to the low-P sub-type topaz-bearing granites. Stockworks and veins are common in Oriente Novo (Rondônia Tin Province) and Correas (Itu Rapakivi Province) deposits, but in the Santa Bárbara deposit (Rondônia Tin Province) a preserved cupola with associated bed-like greisen is predominant. The contrasting mineralization styles reflect different depths of formation, spatial relationship to tin granites, and different wall rock/fluid proportions. The deposits contain a similar rare-metal suite that includes Sn (±W, ±Ta, ±Nb), and base-metal suite (Zn–Cu–Pb) is present only in Correas deposit. The early fluid inclusions of the Correas and Oriente Novo deposits are (1) low to moderate-salinity (0–19 wt.% NaCl eq.) CO₂-bearing aqueous fluids homogenizing at 245–450 °C, and (2) aqueous solutions with low CO₂, low to moderate salinity (0–14 wt.% NaCl eq.), which homogenize between 100 and 340 °C. In the Santa Bárbara deposit, the early inclusions are represented by (1) low-salinity (5–12 wt.% NaCl eq.) aqueous fluids with variable CO₂ contents, homogenizing at 340 to 390 °C, and (2) low-salinity (0–3 wt.% NaCl eq.) aqueous fluid inclusions, which homogenize at 320–380 °C. Cassiterite, wolframite, columbite–tantalite, scheelite, and sulfide assemblages accompany these fluids. The late fluid in the Oriente Novo and Correas deposit was a low-salinity (0–6 wt.% NaCl eq.) CO₂-free aqueous solution, which homogenizes at (100–260 °C) and characterizes the sulfide–fluorite–sericite association in the Correas deposit. The late fluid in the Santa Bárbara deposit has lower salinity (0–3 wt.% NaCl eq.) and characterizes the late-barren-quartz, muscovite and kaolinite veins. Oxygen isotope thermometry coupled with fluid inclusion data suggest hydrothermal activity at 240–450 °C, and 1.0–2.6 kbar fluid pressure at Correas and Oriente Novo. The hydrogen isotope composition of breccia-greisen, stockwork, and vein fluids (δ¹⁸O_quartz from 9.9‰ to 10.9‰, δD_H2O from 4.13‰ to 6.95‰) is consistent with a fluid that was in equilibrium with granite at temperatures from 450 to 240 °C. In the Santa Bárbara deposit, the inferred temperatures for quartz-pods and bed-like greisens are much higher (570 and 500 °C, respectively), and that for the cassiterite-quartz-veins is 415 °C. The oxygen and hydrogen isotope composition of greisen and quartz-pods fluids (δ¹⁸O_qtz-H2O=5.5–6.1‰) indicate that the fluid equilibrated with the albite granite, consistent with a magmatic origin. The values for mica (δ¹⁸O_mica-H2O=3.3–9.8‰) suggest mixing with meteoric water. Late muscovite veins (δ¹⁸O_qtz-H2O=−6.4‰) and late quartz (δ¹⁸O_mica-H2O=−3.8‰) indicate involvement of a meteoric fluid. Overall, the stable isotope and fluid inclusion data imply three fluid types: (1) an early orthomagmatic fluid, which equilibrated with granite; (2) a mixed orthomagmatic-meteoric fluid; and (3) a late hydrothermal meteoric fluid. The first two were responsible for cassiterite, wolframite, and minor columbite–tantalite precipitation. Change in the redox conditions related to mixing of magmatic and meteoric fluids favored important sulfide mineralization in the Correas deposit.     

华北克拉通南缘卢氏多金属矿集区硫化物Rb-Sr定年及地质意义

华北克拉通南缘秦岭成矿带发育大量金矿、钼矿及铅锌多金属矿床。卢氏多金属矿集区位于东秦岭成矿带,主要矿床有夜长坪钼钨矿、八宝山铁铜矿、楼房银铜矿、柳关铅锌矿等。其中楼房银铜矿为热液脉状多金属矿床,矿床赋存于太华群角闪斜长片麻岩中,矿体受构造蚀变破碎带控制,矿床中划分出两个成矿阶段:石英-黄铁矿-黄铜矿组合和石英-黄铁矿-方铅矿-闪锌矿-方解石组合,其中前者是铜成矿阶段,后者为铅锌成矿阶段。柳关铅锌矿为矽卡岩矿床,矿体产于官道口群白云岩与花岗斑岩岩体或隐爆角砾岩接触矽卡岩化带内,矿床划分出两个成矿阶段:透辉石-透闪石-阳起石-石榴石-磁铁矿组合和方铅矿-闪锌矿-黄铁矿-绿帘石-蛇纹石-石英-方解石组合,前者为磁铁矿成矿阶段,后者是铅锌成矿阶段。金属硫化物定年结果表明,楼房银铜矿黄铜矿Rb-Sr等时线年龄为127. 8±3. 1Ma(2σ,MSWD=1. 1),初始87Rb/86Sr为0. 710998±0. 000068;柳关铅锌矿黄铁矿Rb-Sr等时线年龄为124. 8±1. 6Ma(2σ,MSWD=1. 4),初始87Rb/86Sr为0. 711074±0. 000064。研究表明卢氏多金属矿集区内热液多金属矿床形成于早白垩世,其形成与区内早白垩世岩浆活动有关。综合区域地质研究,区内多金属矿床形成于早白垩世与克拉通破坏有关的构造环境。     

Mineral assemblages of the Francisco I. Madero Zn–Cu–Pb–(Ag) deposit, Zacatecas, Mexico: Implications for ore deposit genesis

The Francisco I. Madero deposit, central Mexico, occurs in the Mesozoic Guerrero Terrane, which hosts many ore deposits, both Cretaceous (volcanogenic massive sulfides) and Tertiary (epithermal and skarn deposits). It is hosted by a 600 m-thick calcareous-pelitic unit, of Lower Cretaceous age, crosscut by porphyritic dikes that strike NW–SE. A thick felsic volcanic Tertiary sequence, consisting of andesites and rhyolitic ignimbrites, unconformably overlies the Cretaceous series. At the base, the mineralization consists of several mantos developed within calcareous beds. They are dominantly composed of sphalerite, pyrrhotite and pyrite with minor chalcopyrite, arsenopyrite and galena. At the top of the orebody, there are calcic skarns formed through prograde and retrograde stages. The resulting mineral assemblages are rich in manganoan hedenbergite (Hd_75–28Di_40–4Jh_40–20), andraditic garnets (Adr_100–62Grs_38–0), epidote (Ep_95–36Czo_60–5Pie_8–0), chamosite, calcite and quartz. The temperature of ore deposition, estimated by chlorite and arsenopyrite geothermometry, ranges from 243° to 277 °C and from 300° to 340 °C, respectively. The pressure estimated from sphalerite geobarometry averages 2.1 kbar. This value corresponds to a moderately deep skarn and agrees with the high Cu content of the deposit. Paragenesis, P–T conditions and geological characteristics are compatible with a distal, dike-related, Zn skarn deposit. Its style of mineralization is similar to that of many high-temperature carbonate replacement skarn deposits in the Southern Cordillera.     

Raman spectroscopy and heat capacity measurement of calcium ferrite type MgAl2O4 and CaAl2O4

Raman spectroscopy of calcium ferrite type MgAl₂O₄ and CaAl₂O₄ and heat capacity measurement of CaAl₂O₄ calcium ferrite were performed. The heat-capacity of CaAl₂O₄ calcium ferrite measured by a differential scanning calorimeter (DSC) was represented as C_P(T)=190.6–1.116 × 10⁷T^–2 + 1.491 × 10⁹T^–3 above 250 K (T in K). The obtained Raman spectra were applied to lattice dynamics calculation of heat capacity using the Kieffer model. The calculated heat capacity for CaAl₂O₄ calcium ferrite showed good agreement with that by the DSC measurement. A Kieffer model calculation for MgAl₂O₄ calcium ferrite similar to that for CaAl₂O₄ calcium ferrite was made to estimate the heat capacity of the former. The heat capacity of MgAl₂O₄ calcium ferrite was represented as C_P(T)=223.4–1352T ^–0.5 – 4.181 × 10⁶T ^–2 + 4.300 × 10⁸T ^–3 above 250 K. The calculation also gave approximated vibrational entropies at 298 K of calcium ferrite type MgAl₂O₄ and CaAl₂O₄ as 97.6 and 114.9 J mol^–1 K^–1, respectively.     

Siderite mineralization of the Gemericum superunit (Western Carpathians, Slovakia): review and a revised genetic model

The Gemericum is a segment of the Variscan orogen subsequently deformed by the Alpine–Carpathian orogeny. The unit contains abundant siderite–sulphide and quartz–antimony veins together with stratabound siderite replacement deposits in limestones and stratiform sulphide mineralization in volcano-sedimentary sequences. The siderite–sulphide veins and siderite replacement deposits of the Gemericum represent one of the largest accumulations of siderite in the world, with about 160 million tonnes of mineable FeCO₃. More than 1200 steeply dipping hydrothermal veins are arranged in a regional tectonic and compositional pattern, reflecting the distribution of regional metamorphic zones. Siderite–sulphide veins are typically contained in low-grade (chlorite zone) sedimentary, volcano-sedimentary or volcanic Lower and Upper Paleozoic rocks. Quartz–antimony veins are hosted by higher-grade units (biotite zone). Siderite–sulphide veins are dominated by early siderite followed by a complex set of stages, including quartz–sulphide (chalcopyrite, tetrahedrite), barite, tourmaline–quartz, and sulphide-remobilization stages. The temporal evolution of these stages is difficult to study because of the widespread and repeated tectonic processes, within-vein replacement and recrystallization. Siderite–sulphide veins show considerable vertical (up to 1200 m) and lateral (up to 15 km) extent, and a thickness typically reaching several metres. Carbonate-replacement siderite deposits of the Gemericum are hosted by a Silurian limestone belt and are similar to stratabound siderite deposits of the Eastern Alps (e.g., Erzberg, Austria).Based on a review of geological, petrological and geochronological data for the Gemericum, and extensive stable and radiogenic isotope data and fluid inclusion data on hydrothermal minerals, the siderite–sulphide veins and siderite replacement deposits are classified as metamorphogenic in a broad sense. The deposits were formed during several stages of regional crustal-scale fluid flow. Isotope (S, C, Sr, Pb) fingerprinting identifies the metamorphosed rock complexes of the Gemericum as a source of most components of hydrothermal fluids. Fluid inclusion and stable isotope data evidence the participation of several contrasting fluid types, and the existence of contrasting P–T conditions during vein evolution. A high-δ¹⁸O, medium- to high-salinity, H₂O-type fluid is the most important component during siderite deposition, whereas H₂O–CO₂-type fluid inclusion containing dense liquid CO₂ and corresponding to minimal pressures between 1 and 3 kbar were found in a younger tourmaline–quartz stage. Younger quartz–ankerite(±siderite)–sulphide stages are characterized by high-salinity (17 to 35 wt.% NaCl equivalent) and low-temperature (T_h=90 to 180 °C) H₂O-type fluids.The vein deposits are interpreted as a result of multistage hydrothermal circulation, with Variscan and Alpine mineralization phases. Based on available indirect data, the most important mineralization phase was related to regional fluid flow during the uplift of a Variscan metamorphic core complex, producing siderite–sulphide (±barite) mineralization, while tourmaline–quartz stage and sulphide remobilization stages are related to Alpine processes. Two phases of vein evolution are evident from two groups of ⁸⁷Sr/⁸⁶Sr isotope ratios of Sr-rich, Rb-poor hydrothermal minerals: 0.71042–0.71541 in older barite and 0.7190–0.7220 in late-stage celestine and strontianite.     

Geochemistry of the kaolin deposits of Swat (Pakistan)

Kaolin deposits of the Swat District in Pakistan are indicated to have derived by hydrothermal alteration of more feldspathic parts of felsic intrusives, which occur enclosed in orthoamphibolites and orthogneisses of the Cretaceous Kohistan Island Arc terrane. These latter “country rocks” formed under epidote–amphibolite conditions that prograde northwards to amphibolite facies, and locally manifest slight metamorphic differentiation. The felsic intrusives exhibit a general decrease in siliceous character from west to east, but are less siliceous than most hosts of world kaolins. They are composed of chemically allied quartz diorite, tonalite, trondhjemite and pegmatoids, which evolved mainly by an orthomagmatic crystal fractionation. These parental rocks are calc-alkaline in nature, and kaolinization has proceeded in Ca-richer environment. This is in variance with the occurrence of most known kaolin deposits over potassic granites or rhyolites. Ca-metasomatism of the “host rocks” is in evidence. Kaolin formation by a supergene process is not displayed.The raw kaolin with contained unaltered plagioclase is characterized by a rather low silica (46.54–50.93%) and potash (<1%), and high alumina (23.54–26.77%), Fe₂O₃ (1.73–5.45%) and lime (8.13–16.93%) content. Kaolinization proceeded with a decrease in SiO₂ and concomitant increase in Al₂O₃. The same trend is followed with fineness of grain size of washed fractions, in resemblance to other known kaolin deposits of primary as well as secondary origin.     

Relationship between Se/S and sulfur isotope ratios of hydrothermal sulfide minerals

The pH and f_{O 2} dependences of the [Se^2–]/[S^2–] ratio in chloride solutions at 100°, 200° and 300°C are predicted thermodynamically. Under the high f_{O 2} conditions where sulfate species are dominant in solution, the [Se^2–]/[S^2–] ratio always increases with increasing pH and/or f_{O 2}. Under the low f_{O 2} conditions where sulfide species are dominant in solution, the pH and f_{O 2} dependences of the [Se^2–]/[S^2–] ratio are seriously affected by the presence of native selenium. With native selenium present, the [Se^2–]/[S^2–] ratio decreases with increasing f_{O 2}, but almost independent of pH in geologically important pH regions. When native selenium is absent, the [Se^2–]/[S^2–] ratio is solely a function of pH and independent of f_{O 2}. Combining the above with the pH and f_{O 2} dependences of ³⁴S value of aqueous sulfur species, we discuss the possible influences of the pH and f_{O 2} of ore-forming solutions on the relationship between the Se/S ratio and ³⁴S value of hydrothermal sulfide minerals. The results are applied to some Japanese sulfide ore deposits.     

Geochemistry and evolution of ore-forming fluids of the Yueshan Cu–Au skarn- and vein-type deposits, Anhui Province, South China

The Yueshan mineral belt is geotectonically located at the centre of the Changjiang deep fracture zone or depression of the lower Yangtze platform. Two main types of ore deposits occur in the Yueshan orefield: Cu–Au–(Fe) skarn deposits and Cu–Mo–Au–(Pb–Zn) hydrothermal vein-type deposits. Almost all deposits of economic interest are concentrated within and around the eastern and northern branches of the Yueshan dioritic intrusion. In the vicinity of the Zongpu and Wuhen intrusions, there are many Cu–Pb–Zn–Au–(S) vein-type and a few Cu–Fe–(Au) skarn-type occurrences.Fluid inclusion studies show that the ore-forming fluids are characterised by a Cl⁻(S)–Na⁺–K⁺ chemical association. Hydrothermal activity associated with the above two deposit types was related to the Yueshan intrusion. The fluid salinity was high during the mineralisation processes and the fluid also underwent boiling and mixed with meteoric water. In comparison, the hydrothermal activity related to the Zongpu and Wuhen intrusions was characterised by low salinity fluids. Chlorine and sulphur species played an important role in the transport of ore-forming components.Hydrogen- and oxygen-isotope data also suggest that the ore-forming fluids in the Yueshan mineral belt consisted of magmatic water, mixed in various proportions with meteoric water. The enrichment of ore-forming components in the magmatic waters resulted from fluid–melt partitioning. The ore fluids of magmatic origin formed large Cu–Au deposits, whereas ore fluids of mixed magmatic-meteoric origin formed small- to medium-sized deposits.The sulphur isotopic composition of the skarn- and vein-type deposits varies from − 11.3‰ to + 19.2‰ and from + 4.2‰ to + 10.0‰, respectively. These variations do not appear to have been resulted from changes of physicochemical conditions, rather due to compositional variation of sulphur at the source(s) and by water–rock interaction. Complex water–rock interaction between the ore-bearing magmatic fluids and sedimentary wall rocks was responsible for sulphur mixing. Lead and silicon isotopic compositions of the two deposit types and host rocks provide similar indications for the sources and evolution of the ore-forming fluids.Hydrodynamic calculations show that magmatic ore-forming fluids were channelled upwards into faults, fractures and porous media with velocities of 1.4 m/s, 9.8 × 10^− 1 to 9.8 × 10^− 7 m/s and 3.6 × 10^− 7 to 4.6 × 10^− 7 m/s, respectively. A decrease of fluid migration velocity in porous media or tiny fractures in the contact zones between the intrusive rocks and the Triassic sedimentary rocks led to the deposition of the ore-forming components. The major species responsible for Cu transport are deduced to have been CuCl, CuCl₂⁻, CuCl₃²⁻ and CuClOH, whereas Au was transported as Au₂(HS)₂S²⁻, Au(HS)₂⁻, AuHS and AuH₃SiO₄ complexes. Cooling and a decrease in chloride ion concentration caused by fluid boiling and mixing were the principal causes of Cu deposition. Gold deposition was related to decrease of pH, total sulphur concentration and fO₂, which resulted from fluid boiling and mixing.Geological and geochemical characteristics of the two deposit types in the Yueshan mineral belt suggest that there is a close genetic relationship with the dioritic magmatism. Geochronological data show that the magmatic activity and the mineralisation took place between 130 and 136 Ma and represent a continuous process during the Yanshanian time. The cooling of the intrusions and the mineralisation event might have lasted about 6 Ma. The cooling rate of the magmatic intrusions was 80 to 120 °C my^− 1, which permitted sufficient heat supply by magma to the ore-forming system.     

Tin mineralization in the Dajing tin–polymetallic deposit, Inner Mongolia, China

Dajing is a large-scale tin–polymetallic deposit that hosts the largest tin mine in North China. It is a hydrothermal vein-type deposit containing Sn, Cu, Pb, Zn, Ag, and minor components Co and In. The deposit consists of more than 690 veins hosted within Upper Permian sedimentary rocks.Three mineralization stages and six ore types are recognized with cassiterite constituting the dominant tin mineral. The SnO₂ content of cassiterite increases in the sequence of mineralization stages shear-deformation→cassiterite–quartz→cassiterite–sulfide (or chalcopyrite–pyrite) stage, while the content of FeO, TiO₂, Nb₂O₅, Ta₂O₅, and In₂O₅ tends to decrease with increases in NiO and Ga₂O₅. It is considered that the negative correlation between SnO₂ and FeO, Nb₂O₅, Ta₂O₅, and In₂O₅ results from elemental substitutions. The early stage cassiterite is much richer in Ta and the later stage cassiterite is much poorer in Ti and Fe than is usual in hydrothermal vein type tin deposits. This is interpreted to indicate that the component of early stage cassiterite reflects a granitic magma source while the composition of later stage cassiterite has a more obvious strata source. The compositional variation of cassiterite corresponds to decreasing crystallization temperatures within each stage and between sequential stages with time. The characteristics of REE in cassiterite from two stages are in accord with that of subvolcanic rocks and the Linxi formation. It suggests that tin transported during the cassiterite–quartz stage may have originated from subvolcanic dikes (e.g., dacite porphyry), while in the cassiterite–sulfide stage, tin may have been derived from wallrock (e.g. siltstone) of the Upper Permian-age Linxi Formation.     

Gas maturity and alteration systematics across the Western Canada Sedimentary Basin from four mud gas isotope depth profiles

Carbon isotope and molecular compositions of Mississippian to Upper Cretaceous mud gases have been examined from four depth profiles across the Western Canada Sedimentary Basin (WCSB). The profiles range from the shallow oil sands in the east (R₀ = 0.25) to the very mature sediments in the overthrust zone to the west (R₀ = 2.5). In the undisturbed WCSB, δ¹³C₁–δ¹³C₂ and δ¹³C₂–δ¹³C₃ cross-plots show three maturity and alteration trends: (1) pre-Cretaceous gas sourced from type II kerogen; (2) Cretaceous Colorado Group gas; and (3) Lower Cretaceous Mannville Group biodegraded gas. A fourth set of distinctly different maturity trends is recognized for Lower Cretaceous gas sourced from type III kerogen in the disturbed belt of the WCSB. Displacement of these latter maturity trends to high δ¹³C₂ values suggests that the sampled gas was trapped after earlier formed gas escaped, probably as a result of overthrusting. Unusually ¹³C-enriched gas (δ¹³C₁ = −34‰, δ¹³C₂ = −13‰, and δ¹³C₃ = 0‰), from the Gething Formation in the disturbed belt, is the result of late stage gas cracking in a closed system. In general, gas maturity is consistent with the maturity of the host sediments in the WCSB, suggesting that migration and mixing of gases was not pervasive on a broad regional and stratigraphic scale. The ‘Deep Basin’ portion of the WCSB is an exception. Here extensive cross-formational homogenization of gases has occurred, in addition to updip migration along the most permeable stratigraphic units.     

Two magma series and associated ore deposit types in the Permian Emeishan large igneous province, SW China

The Late Middle Permian ( 260 Ma) Emeishan large igneous province in SW China contains two magmatic series, one comprising high-Ti basalts and Fe-rich gabbroic and syenitic intrusions, the other low-Ti basalts and mafic–ultramafic intrusions. The Fe-rich gabbros are spatially and temporally associated with syenites. Each series is associated with a distinctive type of mineralization, the first with giant Fe–Ti–V oxide ore deposits such as Panzhihua and Baima, the second with Ni–Cu–(PGE) sulfide deposits such as Jinbaoshan, Limahe and Zhubu. New SHRIMP zircon U–Pb isotopic data yielded 263 ± 3 Ma for the Limahe intrusion, 261 ± 2 Ma for the Zhubu intrusion and 262 ± 2 Ma for a syenitic intrusion. These new age dates, together with previously reported SHRIMP zircon U–Pb ages, suggest that all these intrusions are contemporaneous with the Emeishan flood basalts and formed during a major igneous event at ca. 260 Ma.The oxide-bearing intrusions have higher Al₂O₃, FeO (as total iron) and total alkalis (Na₂O + K₂O) but lower MgO than the sulfide-bearing intrusions. All intrusions are variably enriched in LREE relative to HREE. The oxide-bearing intrusions display positive Nb- and Ti-anomalies and in certain cases negative Zr–Hf anomalies, whereas the sulfide-bearing intrusions have obvious negative Nb- and Ti-anomalies, a feature of crustal contamination. Individual intrusions have relatively small ranges of Nd(t) values. All the intrusions, however, have Nd(t) values ranging from − 3.9 to + 4.6, and initial ⁸⁷Sr/⁸⁶Sr ratios from 0.7039 to 0.7105. The syenites have very low MgO (< 2 wt.%) but highly variable Fe₂O₃ (2.5 to 13 wt.%) with initial ⁸⁷Sr/⁸⁶Sr ratios ranging from 0.7039 to 0.7089. Magmas from both series could have derived by melting of a heterogeneous mantle plume: the high-Ti series from a Fe-rich, more fertile source and the low-Ti series from a Fe-poor, more refractory source. In addition, the low-Ti series underwent significant crustal contamination. The two magma series evolved along different paths that led to distinct mineralization styles.     

The Daduhe gold field at the eastern margin of the Tibetan Plateau: He, Ar, S, O, and H isotopic data and their metallogenic implications

The Daduhe gold field comprises several shear-zone-controlled Tertiary lode gold deposits distributed at the eastern margin of the Tibetan Plateau. The deposits are hosted in a Precambrian granite–greenstone terrane within the Yangtze Craton. The gold mineralization occurs mainly as auriferous quartz veins with minor sulphide minerals. Fluid inclusions in pyrite have ³He/⁴He ratios of 0.16 to 0.86 R_a, whereas their ⁴⁰Ar/³⁶Ar ratios range from 298 to 3288, indicating a mixing of fluids of mantle and crust origins. The δ³⁴S values of pyrite are of 0.7–4.2‰ (n = 12), suggesting a mantle source or leaching from the mafic country rocks. δ¹⁸O values calculated from hydrothermal quartz are between − 1.5‰ and + 6.0‰ and δD values of the fluids in the fluid inclusions in quartz are − 39‰ and − 108‰. These ranges demonstrate a mixing of magmatic/metamorphic and meteoric fluids. The noble gas isotopic data, along with the stable isotopic data suggest that the ore-forming fluids have a dominantly crustal source with a significant mantle component.     

S- and O-isotopic character of dissolved sulphate in the cover rock aquifers of a Zechstein salt dome

Sulfur and O isotope analyses of dissolved SO₄ were used to constrain a hydrogeological model for the area overlying the Gorleben–Rambow Salt Structure, Northern Germany. Samples were collected from 80 wells screened at different depth-intervals. The study area consists of a set of two vertically stacked aquifer systems. Generally, the isotope data show a good spatial correlation, outlining well-defined groundwater zones containing SO₄ of characteristic isotopic composition. Highly saline waters from deeper parts of the lower aquifer system are characterized by rather constant SO₄ isotopic compositions, which are typical of Permian Zechstein evaporites (δ³⁴S=9.6–11.9‰; δ¹⁸O=9.5–12.1‰). Above this is a transition zone containing ground waters of intermediate salinity and slightly higher isotopic values (average δ³⁴S=16.6‰; δ¹⁸O=15.3‰). The confined groundwater horizon on the top of the lower aquifer system below the low permeable Hamburg Clays is low in total dissolved solids and is characterized by an extreme ³⁴S enrichment (average δ³⁴S=39.1‰; δ¹⁸O=18.4‰), suggesting that bacterially mediated SO₄ reduction is a dominant geochemical process in this zone. Two areas of distinct isotopic composition can be identified in the shallow ground water horizons of the upper hydrogeological system. Sulfate in groundwaters adjacent to the river Elbe and Löcknitz has a typical meteoric isotopic signature (δ³⁴S=5.2‰; δ¹⁸O=8.2‰), whereas the central part of the area is characterized by more elevated isotopic ratios (δ³⁴S=12.7‰; δ¹⁸O=15.6‰). The two major SO₄ pools in the area are represented by Permian seawater SO₄ and a SO₄ of meteoric origin that has been mixed with SO₄ resulting from the oxidation of pyrite. It is suggested that the S-isotope compositions observed reflect the nature of the SO₄ source that have been modified to various extent by bacterial SO₄ reduction. Groundwaters with transitional salinity have resulted from mixing between brines and low-mineralized waters affected by bacterial SO₄ reduction.     

Zoning in the Carboniferous-Lower Permian Cracow epithermal vein system, central Queensland, Australia

Four epithermal vein deposits (i.e. Dawn, Central Extended, Rose's Pride and Klondyke) in the Cracow gold field, central Queensland were investigated in terms of paragenesis, mineralogy, vein textures, fluid inclusions and stable isotopes. The Cracow epithermal field is confined to an area approximately 6 by 5 kilometers. All the deposits are hosted by the massive Camboon Andesite of Upper Carboniferous to Lower Permian age, occur as open-space vein fillings, and have similar paragenesis. However, significant variations in mineralogy, textures of quartz and adularia, and fluid geochemistry were found for a main mineralisation stage (Stage II) of each individual deposits. At Rose's Pride and Klondyke, basemetal sulphides are virtually absent, but significant amounts of calcite and quartz with minor adularia are widely distributed. Replacement textures are distinct, and mineralisation temperature is less than 220 °C and salinity less than 0.2 wt%. The ¹⁸O values of quartz and calcite range from –2.65 to –2.06 and from –6.66 to –6.34%. respectively, and calculated ¹⁸O_H2_O value is about –17%. which represents a nearly unshifted palaeo-meteoric water. Gold mineralisation is best developed at Central Extended among the studied deposits, where patches rich in electrum are often observed in polished thin sections and where gold grades exceeding 10 g/t are frequently indicated by assays. Base-metal sulphides are only present locally and rarely exceed 5 volume percent of the vein samples. Quartz is the dominant gangue mineral, but significant amounts of rhombic adularia and chlorite are widely distributed. Various primary and recrystallisation textures possibly inherited from silica gel are well developed and widespread. At individual sites where crustiform bands developed from both walls of a fissure, temperatures could drop sharply from 275 °C to less than 220 °C. The ore-forming fluid at Central Extended, compared with that at Rose's Pride and Klondyke, was isotopically shifted from meteoric water with ¹⁸O_H2_O value of –13.5 calculated in equilibrium with quartz ( ¹⁸O values of –3.09 to –1.44%.). The orebodies at Dawn are rich in base-metal sulphides which are commonly coarse-grained and form up to 20 volume percent of the vein materials. Quartz is the predominant gangue mineral, and commonly shows a coarse comb texture. The ore-forming fluid was 275 ± 10 °C and low salinity (0.4 to 0.7 wt%). The ¹⁸O values of quartz range from –3.97 to –3.22%., and calculated ¹⁸O_H2_O value is about –12, indicating large isotopic shifts from palaeo-meteoric water. A depth zoning in typical boiling epithermal systems, corresponding to different fluid compositions, wall rock permeability and boiling behaviors, was invoked to explain different characteristics of these selected epithermal veins.     

Influence of localised igneous activity on cleat dawsonite formation in Late Permian coal measures, Upper Hunter Valley, Australia

Stable (δ¹³C and δ¹⁸O) and radiogenic ⁸⁷Sr/⁸⁶Sr isotopic data have been used to investigate the origin of cleat dawsonite (NaAlCO₃(OH)₂) in the Late Permian Wittingham Coal Measures of the Upper Hunter region in the Sydney Basin, New South Wales. The δ¹³C_PDB values have a narrow range (− 1.7‰ to + 2.4‰), with an average of + 0.3‰, suggesting a magmatic source for the carbon. In contrast, δ¹⁸O_SMOW values have a wide range (+ 13.6‰ to + 19.8‰), and decrease systematically with decreasing distance from a major intrusion. This systematic variation reflects establishment of localised hydrothermal cells. Water–rock interaction between fluids associated with these hydrothermal cells, and Rb-poor volcaniclastic detritus in the coal measures, produced mantle-like ⁸⁷Sr/⁸⁶Sr (0.705032 to 0.706464) in the dawsonite.     

The role of phosphorus in rhyolitic liquids as determined from the homogeneous iron redox equilibrium

The redox ratio of iron is used as an indicator of solution properties of silicate liquids in the system (SiO–Al₂O₃–K₂O–FeO–Fe₂O₃–P₂O₅). Glasses containing 80–85 mol% SiO₂ with 1 mol% Fe₂O₃ and compositions covering a range of K₂O/Al₂O₃ were synthesized at 1400°C in air (fixed fO₂). Variations in the ratio FeO/FeO_1.5 resulting from the addition of P₂O₅ are used to determine the solution behavior of phosphorus and its interactions with other cations in the silicate melt. In 80 mol% SiO₂ peralkaline melts the redox ratio, expressed as FeO/FeO_1.5, is unchanged relative to the reference curve with the addition of 3 mol% P₂O₅. Yet, the iron redox ratio in the 85 mol% SiO₂ potassium aluminosilicate melts is decreased relative to phosphorus-free liquids even for small amounts of P₂O₅ (0.5 mol%). The redox ratio in peraluminous melts is decreased relative to phosphorus- free liquids at P₂O₅ concentrations of 3 mol%. In peraluminous liquids, complexing of both Fe⁺³–O–P⁺⁵ and Al⁺³–O–P⁺⁵ occur. The activity coefficient of Fe⁺³ is decreased because more ferric iron can be accommodated than in phosphorus-free liquids. In peralkaline melts, there is no evidence that P⁺⁵ is removing K⁺ from either Al⁺³ or Fe⁺³ species. In chargebalanced melts with 3 mol% Fe₂O₃ and very high P₂O₅ concentrations, phosphorus removes K⁺ from K–O–Fe⁺³ complexes resulting in a redox increase. P₂O₅ should be accommodated easily in peraluminous rhyolitic liquids and phosphate saturation may be suppressed relative to metaluminous rhyolites. In peralkaline melts, phosphate solubility may increase as a result of phosphorus complexing with alkalis. The complexing stoichiometry may be variable, however, and the relative influence of peralkalinity versus temperature on phosphate solubility in rhyolitic melts deserves greater attention.     

Correlation of irradiation-induced yellow color with the O<Superscript>−</Superscript> hole center in tourmaline

Tourmaline with the general formula XY₃Z₆(BO₃)₃Si₆O₁₈(OH,O)₃(OH,F) and the trigonal space group R3m (C_3v⁵) is known as a gemstone with great variety of colors. Some color centers are related to transition metal ions, and others to electron or hole traps. In this paper we report on a combined study using electron paramagnetic resonance (EPR), electron nuclear double resonance (ENDOR), and the optical detection of EPR (ODEPR) on a yellow color center produced by -irradiation in colorless Li-bearing elbaite tourmaline from Brazil. The color center is an O^– hole trap center, which is stabilized within the plane spanned by three Y sites, and is located in the structural channels formed by Si₆O₁₈. We suggest that two of the Y sites are substituted by ²⁷Al and the other by ^6,7Li. During the irradiation process atomic hydrogen H⁰ is also produced, which shows the same thermal stability as the hole center (250 °C). Therefore, we assign H⁰ to be the local charge compensator for the hole trap. From the ODEPR measurements we conclude that the yellow color is caused by the O^– hole center. The large negative isotropic Al superhyperfine interaction of the O^– hole trap center is consistent with a calculation of the transferred hyperfine interactions by exchange polarization supporting the proposed defect model of an O^– at the O₁ sites, whereby the O^– is relaxed into the plane formed by three Y ions.     

Megacrysts and xenoliths in kimberlite,Elliott County,Kentucky: A mantle sample from beneath the Permian Appalachian Plateau

Two kimberlite pipes in Elliott County contain rare ultramafic xenoliths and abundant megacrysts of olivine (Fo_85–93), garnet (0.21–9.07% Cr₂O₃), picroilmenite, phlogopite, Cr-poor clinopyroxene (0.56–0.88% Cr₂O₃), and Cr-poor orthopyroxene (<0.03–0.34% Cr₂O₃) in a matrix of olivine (Fo_88–92), picroilmenite, Cr-spinel, magnetite, perovskite, pyrrhotite, calcite, and hydrous silicates. Rare clinopyroxene-ilmenite intergrowths also occur. Garnets show correlation of mg (0.79–0.86) and CaO (4.54–7.10%) with Cr₂O₃ content; the more Mg-rich garnets have more uvarovite in solution. Clinopyroxene megacrysts show a general decrease in Cr₂O₃ and increase in TiO₂ (0.38–0.56%) with decreasing mg (0.87–0.91). Clinopyroxene megacrysts are more Cr-rich than clinopyroxene in clinopyroxene-ilmenite intergrowths (0.06–0.38% Cr₂O₃) and less Cr-rich than peridotite clinopyroxenes (1.39–1.46% Cr₂O₃). Orthopyroxene megacrysts and orthopyroxene inclusions in olivine megacrysts form two populations: high-Ca, high-Al (1.09–1.16% CaO and 1.16–1.18% Al₂O₃) and low-Ca, low-Al (0.35–0.46% CaO and 0.67–0.74% Al₂O₃). Three orthopyroxenes belonging to a low-Ca subgroup of the high-Ca, high-Al group were also identified (0.86–0.98% CaO and 0.95–1.01% Al₂O₃). The high-Ca, high-Al group (Group I) has lower mg (0.88–0.90) than low-Ca, low-Al group (Group II) with mg=0.92–0.93; low mg orthopyroxenes (Group Ia) have lower Cr₂O₃ and higher TiO₂ than high mg orthopyroxenes (Group II). The orthopyroxene megacrysts have lower Cr₂O₃ than peridotite orthopyroxenes (0.46–0.57% Cr₂O₃). Diopside solvus temperatures indicate equilibration of clinopyroxene megacrysts at 1,165°–1,390° C and 1,295°–1,335° C for clinopyroxene in clinopyroxene-ilmenite intergrowths. P-T estimates for orthopyroxene megacrysts are bimodal: high-Ca, high-Al (Group I) orthopyroxenes equilibrated at 1,165°–1,255° C and 51–53 kb (± 5kb) and the low-Ca, low-Al (Group II) orthopyroxenes equilibrated at 970°–1,020°C and 46–56 kb (± 5kb). Garnet peridotites equilibrated at 1,240°–1,360° C and 47–49 kb. Spinel peridotites have discordant temperatures of 720°–835° C (using spinel-olivine Fe/Mg) and 865°–1,125° C (Al in orthopyroxene).Megacrysts probably precipitated from a fractionating liquid at >150 km depth. They are not disaggregated peridotite because: (1) of large crystal size (up to 1.5 cm), (2) compositions are distinctly different from peridotite phases, and (3) they display fractionation trends. The high mg, low T orthopyroxenes and the clustering of olivine rims near Fo_89–90 reflect liquid changes to higher MgO contents due to (1) assimilation of wall-rock and/or (2) an increase in Fe³⁺/Fe²⁺ and subsequently MgO/FeO as a result of an increase in f _o.