Oxygen isotope studies of wolframite in tungsten ore deposits of south China

Based on the oxygen isotopic compositions of 133 wolframite samples and 110 quartz samples collected from 30 tungsten ore deposits in south China, in conjunction withδD values and other data, these deposits can be divided into four types.

Thermodynamic analysis of ore-forming conditions and sulfur isotopic systematics of Dachang ore field

On the basis of ore-forming periods and stages of the Dachang ore field, the pH and conditions and the S isotopic systematics during ore formation have been thcrmodynamically treated in this paper. Calculations show a progressively decreased pH, an increased oxidation regime and an intensified activity of sulfur from the early to the late stage. Owing to the unreliability of inferring the S source from δ³⁴S_min, has been calculated using the Ohmoto’s model. Results indicate that the δ³⁴ _min frequency distribution is more concentrated than that of δ³⁴S_min and the peak value shifts to negative region by 2.5%. The sulfur in the whole ore field seems to be of multiple source, i.e., different deposits have their own S sources. But the S isotopic composition pertaining to each stage is nearly constant, suggesting that the ore-forming system be open to sulfur and the supply of sulfur be sufficient. The conclusions deduced from calculations are supported by many lines of geological evidence.     

Genesis and Geological—Geochemical Characters of the ushan Gold Deposit,Shandong,China

The Rushan gold deposit, explored in recent years in the Jiaodong area, Shandong Province, is a quartz vein-type gold deposit hosted in granite. The temperature of its major mineralization episode is between 220°C and 280°C. The salinity of the ore-forming fluid is 5 % to 9% NaCl equivalent, with H₂O and CO₂ as the dominant gas constituents. The fluid is rich in Na⁺, Ca²⁺ and Cl⁻, but relatively impoverished in K⁺ and F⁻, characterized by either Ca²⁺ > Na⁺ > K⁺ (in three samples) or Na⁺ > Ca²⁺ > K⁺ (in six samples). Hydrogen and oxygen isotopes in the ore-forming fluid are highly variable with δ¹⁸ ranging between − 7.70‰ and 5. 97‰ and between − 128‰ and − 71‰. The possibility of lamprophyre serving as the source of gold can be excluded in view of its low gold content on the order of 2.5 × 10⁻⁹. Rb-Sr isochron ages of the deposit and the host Kunyushan granite are ( 104.8 ± 1.5) Ma and 134.6 Ma respectively with the respective initial Sr ratios of 0. 71307 and 0.7096. It is considered that the emplacement of the lamprophyre under a tensile environment had provided sufficient heat energy to facilitate deep circulation of meteoric water by which ore metals were extracted from the Kunyushan granite through long-term water-rock reaction. This project was financially supported by the National Natural Science Foundation of China.     

Genesis and geological and geochemical characteristics of Qixia gold deposit, Shandong, China

The Qixia gold deposit is one of the important quartz vein-type deposits hosted in metamorphic rocks in the east of Shandong, China. Compositionally the wolframite which is associated with gold mineralization in the deposit is rich in iron, but poor in manganese, showing that this mineral crystallized from hydrothermal solution at low temperature. The temperatures at the main metallogenic stage of the Qixia gold deposit are within the range of 160–270°C. The gaseous phases in fluid inclusions are dominated by H₂O and CO₂, while Na⁺, Ca²⁺ and Cl⁻ are the major species in the fluid phase with K⁺ and F⁻ present in relatively small amounts. The salinities of fluids are 4.2 wt% –8.0 wt% NaCl equiv. Meanwhile, the fluid is characterized by either Ca²⁺ < Na⁺ < K⁺ (in five samples) or Na⁺ < Ca²⁺ < K⁺ (in two samples), quite similar to the composition of ore-forming fluid derived from meteoric water. Primary data on the hydrogen and oxygen isotopic compositions of the ore-forming fluid fall within a wide range: = − 56‱ − 95‱. and = − 3. 6‱ − 4. 5‱ relative to SMOW. These isotopic values fully reflect the distribution features of meteoric water which has exchanged with the metamorphic rocks of the Jiaodong Group at different temperatures and W/ R ratios, and the metallogenic process is characterized by low W/R ratios. The Rb-Sr isochron age of the Qixia gold deposit is 125.8 ± 1.7 Ma, with (⁸⁷Sr/⁸⁶Sr)_i being 0.7168, and the time interval between the gold deposit and its country rocks (granite or metamoprhic rocks) is very large. The formation of the Qiaxia gold deposit is directly related to the evolution of geological history in eastern Shandong, and a genetic model in which the deep convective circulating meteoric water reacts with the country rocks can be used to explain the metallogenic mechanism of the deposit. This project was financially supported by the National Natural Science Foundation of China (No. 49000020).     

The graphite-COH fluid equilibrium in P,T, f_{O_2 } space

The oxygen fugacity ( ) of a C-O-H fluid in equilibrium with graphite has been determined in the range 10–30 kbar by equilibrating solid -buffer assemblages in graphite capsules containing C-O-H fluid. By using different buffers (Fe_xO-Fe₃O₄, Ni-NiO, Co-CoO, Mo-MoO₂), the of the graphite-saturated fluid is bracketed within a narrow range. This technique produces a calibration for the imposed on a sample contained within a graphite capsule. To achieve a thermodynamically-invariant system at fixed P and T, the was imposed on the system with an external buffer and the double-capsule technique. The experiments were performed in solid-media, high pressure apparatus with 19 mm tale-pyrex assemblies. A series of experiments at 10, 15, 20, 25, and 30 kbar, 800–1600° C, with imposed by the Fe₂O₃-Fe₃O₄-H₂O equilibrium were conducted. The experimental results have been fitted to the following equation:

Study on the origin of the Hongshan brecciated copper deposit in Huichang County,Jiangxi Province,China

The Hongshan copper deposit is a typical cryptoexplosive breccia-type deposit, which occurs in a metamorphic rock series of the Mesoproterozoic Taoxiyuan Formation. Orebodies are distributed inside and outside porphyry-cryptoexplosive breccia pipes. The isotope geochemistry of the deposit is consistent with the origin of porphyry breccia: the δ18OH2O values ranging from 1.2‰ to 6.1‰ and the δ34S values varying from 0 to 2.5‰. 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb ratios of pyrite, which coexists with ore minerals, indicate it was derived from the orogenic belt. Thermodynamic analysis indicates that the main metals were deposited largely as a result of the decreasing of proton concentrations associated with H2S and CO2 exsolution during explosion and temperature dropping. Based on K-Ar dating of quartz coexisting with ore minerals, the age of mineralization was estimated to be 97.1–98.8 Ma, which suggests that mineralization occurred between the Early and Late Cretaceous. According to the relevant information obtained, a diagenetic and metallogenic pattern in the area has been presented in this paper.     

Sulfur Isotope Fractionation in Magmatic Systems：Models of Rayleigh Distillation and Selective Flux

The effect of Rayleigh distillation by outgassing of SO2 and H2S on the isotopic composition of sulfur remaining in silicate melts is quantitatively modelled.A threshold mole fraction of sulfur in sulfide component of the melts is reckoned to be of critical importance in shifting the δ^34S of the melts mith respect to the original magmas.The partial equilibrium fractionation in a magmatic system is evaluated by assuming that a non-equilibrium flux of sulfur occurs between magmatic volatiles and the melts,while an equilibrium fractionation is approached between sulfate and sulfide within the melts.The results show that under high fo2 conditions,the sulfate/sulfide ratio in a melt entds to increase,and the δ^34S value of sulfur in a solidified rock might then be shifted in the positive direction.This may either be due to Rayleigh outgassing in case the mole fraction of sulfide is less than the threshold,or due to a unidirectional increase in δ^34S value of the sulfate with decreaing temperature,Conversely,at low fo2,the sulfate/sulfide ratio tends to decrease and the δ^34S value of total sulfur could be driven in the negative direction,either because of the Rayleigh outgassing in case the mole fraction of sulfide is greater than the threshold,or because of a unidirectional decrease inδ^34S value of the sulfide.To establish isotopic equilibrium between sulfate and sulfide,the HM,QFM or WM buffers in the magmatic system are suggested to provide the redox couple that could simultaneously reduce the sulfate and oxidize the sulfide.CaO present in the silicatte Melts is also called upon to participate in the chemical equilibrium between sulfate and sulfide,Consequently,the δ^34S value of an igneous rock could considerably deviate from that of its original magma due to the influence of oxygen fugacity and temperature at the time of magma solidification.     

P total,P_{H_2 O} and the occurrence of cummingtonite in volcanic rocks

The phenocryst assemblage of cummingtonite, orthopyroxene, quartz, titanomagnetite and ilmenite in rhyolites of New Zealand has been used to calculate P _total and . The values of P _total and depend strongly upon whether an ideal mixing, or an ordered, model is used for the solid-solutions, but in both cases P _total.The rhyolite magma contained over 9 per cent water (by weight) when the cummingtonite phenocrysts precipitated, and possibly as much as 12 per cent, so that it is surprising that one of these rhyolites is a coherent lava. The calculated values of P _total and are very sensitive to uncertainty in both the composition of the solid-solutions and temperature. Calculations show that >0.7–0.8 P _total for cummingtonite to precipitate in rhyolites, and that iron-rich olivine and cummingtonite could only exist in rhyolites over a small temperature range at a pressure near 5 kilobars. Hornblende phenocrysts co-existing with fayalitic olivine in rhyolites accordingly have a very low activity of Mg₇Si₈O₂₂(OH)₂.     

A radiometric study of polymetamorphism in the Bamble region,Norway

Metamorphism-induced parent-daughter isotopic rearrangement yields information concerning the nature and duration of metamorphism in the Bamble Area, Southern Norway. The thermal maximum of the Bamble Sveconorwegian metamorphism was reached at 1160–1200 m.y. ago, according to zircon and sphene U-Pb, and Rb-Sr whole rock results. Dating of post-kinematic pegmatites suggests that the major kinematic episodes took place not much more than 100 m.y. and that not until more than 200 m.y. after the thermal maximum had uplift and cooling resulted in closure of the K-Ar system in micas. Petrological considerations together with radiometric data on the Levang Gneiss Dome suggest that Rb-Sr whole rock samples show open and closed system behaviour under similar temperatures but possibly different . Variable recrystallization of zircon in the Levang Gneiss Dome (when taken with accompanying radiometric U-Pb data) appears to substantiate the idea that when = P _solid, resetting of both the U-Pb systems in zircon and Rb-Sr whole rock systems is greatly facilitated.     

Petrology, geothermobarometry and C-O-H-S fluid compositions in the environs of Rampura-Agucha Zn-(Pb) ore deposit, Bhilwara District, Rajasthan

The massive Zn-(Pb) sulfide ore body at Rampura-Agucha in Bhilwara district, Rajasthan, occurs within graphitic metapelites surrounded by garnet-biotite-sillimanite gneiss containing concordant bodies of amphibolite. These rocks and the sulfide ores have been studied to estimate the pressure, temperature and fluid composition associated with upper amphibolite facies metamorphism. Geothermobarometric calculations involving garnet-biotite and garnet-hornblende pairs, as well as sphalerite-hexagonal pyrrhotite-pyrite and garnet-plagioclase-sillimanite-quartz assemblages indicate that the most pervasive P-T condition during peak of regional metamorphism was 650°C and 6 kb, and was attained between the first and second deformations in the region. Some temperature-pressure estimates also cluster around 500°C–5.1 kb which probably represent retrograde cooling during unloading. Consideration of devolatilization equilibria in the C-O-H-S system at the pervasive metamorphic conditions mentioned above shows that the metamorphic fluid was H₂O-rich ( ) but also had a substantial component of . and were the other important phases in the fluid. CO (X_CO = 0.002) and were the minor phases in the fluid. It is probable that a part of this aqueous fluid was consumed by re-/neocrystallization of hydrous silicate phases like chlorite during the retrogressive metamorphic path, so that fluid entrapped in quartz below 450°C was rendered CO₂-rich (Holleret al 1996).     

Geochemistry and origin of gas pools in the Gaoqing-Pingnan fault zone,Jiyang Depression

In the surroundings of the Gaoqing-Pingnan fault zone are developed quite a number of gas reservoirs. Based on gas compositions, they can be divided into two groups, i.e., CO2 and CH4. Their composition and isotope geochemistry were dealt with in this study. The CO2 contents range from 60.72%–99.99%, the δ13CCO2 values from -3.41‰– -9.8‰, and the 3He/4He ratios from 4.35×10-6–6.35×10-6 (i.e. R/Ra=4.45–4.35). Based on the data on composition and isotope geochemistry, deep geological background, deep faults and volcanic rocks, it is shown that CO2 ,distributed in the Gaoqing area, mostly originated from mantle-source inorganic matter which is associated with magmatic rocks. The favorable tectonic environment for the formation of CO2 reservoirs is the rift, which is related to great fault-magmatic activity, the formation of CO2 gas pools and their space-time correlation to the most recent magmatic activities. Hydrocarbon gas pools occur in the Huagou area. The CH4 contents are within the range of 88.83%–99.12%, and the δ13CCH4 values, -44.7‰– -54.39‰. This indicates that the hydrocarbon gas resulted from the decomposition of oil-type gas at high temperatures. Volcanic rocks in the CO2 gas pool-and CH4 gas pool-distributed areas show significant differences in Fe2O3 and FeO contents. This has proven that the hydrocarbon gas may have resulted from various chemical reactions. Magmatic activities are the primary reason for the distribution of CO2 and CH4 gas pools in the Gaoqing-Pingnan fault zone.     

Carbon and hydrogen isotopic composition and generation pathway of biogenic gas in China

The carbon and hydrogen isotopic composition of biogenic gas is of great importance for the study of its generation pathway and reservoiring characteristics. In this paper, the formation pathways and reservoiring characteristics of biogenic gas reservoirs in China are described in terms of the carbon and hydrogen isotopic compositions of 31 gas samples from 10 biogenic gas reservoirs. The study shows that the hydrogen isotopic compositions of these biogenic gas reservoirs can be divided into three intervals: δ>−200‰, −250‰<δ<-−200‰ and δ<−250‰. The forerunners believed that the main generation pathway of biogenic gas under the condition of continental fresh water is acetic fermentation. Our research results showed that the generation pathway of biogenic gas under the condition of marine facies is typical CO₂- reduction, the biogenic gas has heavy hydrogen isotopic composition: its δ values are higher than −200‰; that the biogenic gas under the condition of continental facies also was generated by the same way, but its hydrogen isotopic composition is lighter than that of biogenetic gas generated under typical marine facies condition: −250‰<δ<−200‰, the δ values may be related to the salinity of the water medium in ancient lakes. From the relevant data of the Qaidam Basin, it can be seen that the hydrogen isotopic composition of biogenic methane has the same variation trend with increasing salinity of water medium. There are biogenic gas reservoirs formed in transitional regions under the condition of continental facies. These gas reservoirs resulted from both CO₂-reduction and acetic fermentation, the formation of which may be related to the non-variant salinity of ancient water medium and the relatively high geothermal gradient, as is the case encountered in the Baoshan Basin. The biogenic gas generating in these regions has light hydrogen isotopic composition: δ<−250‰, and relatively heavy carbon isotopic composition. There is a fairly strong negative correlation between the carbon isotopic composition and the hydrogen isotopic composition. The generation mechanism and pathway of carbon, and the hydrogen isotopic composition of biogenic gas may be used to ascertain whether biogenic gas samples from the natural world are of industrial utilization value. In general, the biogenic gas formed by way of acetic fermentation is not propitious to the formation of gas reservoirs.     

Partition of Ni between olivine and sulfide: equilibria with sulfide-oxide liquids

The partition of Ni between olivine and monosulfide-oxide liquid has been investigated at 1300–1395° C, =10^–8-9–10^–6.8, and =10^–2.0–10^–0.9, over the composition range 20–79 mol. % NiS. The product olivine compositions varied from Fo98 to Fo59 and from 0.06 to 3.11 wt% NiO. The metal/sulfur ratio of the sulfide-oxide liquid increases with increase in , decrease in , and increase in NiS content. The Ni/Fe exchange reaction has been perfectly reversed using natural olivine and pure forsterite as starting materials. The FeO and NiO contents of olivine from runs equilibrated at the same and form isobaric distributions with NiS content, which, to a first approximation, are dependent at constant temperature and total pressure on a variable term, –0.5 log ( / ). The Ni/Fe distribution coefficient (K _D3) exhibits only a weak decrease from 35 to 29 with increase in from the IW buffer to close to the FMQ buffer. At values higher than FMQ, the sulfide-oxide liquid has the approximate composition (Ni,Fe)₃±_xS₂K _D358. The present K _D3 vs O/(S+O) data define a trend which extrapolates to K _D320 at 10 wt% oxygen in the sulfide-oxide liquid. The compositions of olivine and Ni-Cu sulfides associated with early-magmatic basic rocks and komatiites are consistent, at 1400° C, with a value of -log ( / ) of about 7.7, which is equivalent to 0.0 wt% oxygen in the hypothesized immiscible sulfide-oxide liquid. Therefore, K _D3 would not be reduced significantly from the 30 to 35 range for sulfide-oxide liquids with low oxygen contents.     

One hundred years of rapakivi granite

Summary Rapakivi granites, recently redefined as A-type granites showing rapakivi texture at least in the larger batholiths, occur on all continents and presumably represent the most voluminous continental silicic intraplate magmatism on Earth. Most of the rapakivi granites are Proterozoic (mainly 1.0 to 1.7 Ga) but also Archean (2.8 Ga) and Phanerozoic (0.05 to 0.4 Ga) are known. The magmatic association is bimodal comprising anorthosite to gabbro, diabase, minor Fe-enriched intermediate rocks, and monzonite, beside granite; mingling of silicic and mafic magmas is typical. Geochemically and otherwise, rapakivi granites show the characteristics of the Phanerozoic A-type granites, except that they encompass relatively few peralkaline rocks and that they may occur as very large (up to 40,000 km²) batholiths. Some of the rapakivi granite complexes host important Sn-polymetallic and Fe-Cu deposits.The rapakivi granites crystallized from relatively hot, restite-poor magmas at low (epizonal-subvolcanic) pressure, , and . Mineral assemblages are indicative of a multiphase crystallization history; the conspicuous mantling of the perthite ovoids with plagioclase can be explained by changes in magma composition and/or, P, T, and affecting the stabilities of feldspars. The isotopic composition of rapakivi granites is generally compatible with a lower crustal protolith. The latter could have been either a melt-depleted residue or otherwise relatively anhydrous igneous or metaigneous rock. Melting of the protolith commenced under vapor-absent conditions and was induced by heat from the contemporaneous mantle-derived mafic magmas. The widespread rapakivi granite magmatism in the Middle Proterozoic may have been related to the establishment of a major continental mass (supercontinent).

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Einhundert jahre rapakivi-granit

Zusammenfassung Rapakivi-Granite sind A-Typ Granite mit Rapakivi Texturen, die zumindest in den größeren Batholiten zu erkennen sind. Sie kommen auf alien Kontinenten vor and stellen wahrscheinlich das umfangreichste Beispiel kontinentalen sauren Intraplate-Magmatismus dar. Die meisten Rapakivi-Granite sind proterozoisch (1.0 bis 1.7 Ga), jedoch sind auch archaische (2.8 Ga) and phanerozoische (0.05 bis 0.4 Ga) Beispiele bekannt. Die magmatische Assoziation ist bimodal and umfaßt Anorthosit bis Gabbro, Diabas, in kleinerem Umfang Fe-angereicherte intermediäre Gesteine and Monzonit, zusätzlich zu Granit. Das gemeinsame Auftreten von Silizium-reichen and mafischen Magmen ist typisch. Die geochemischen Charakteristika der Rapakivi-Granite entsprechen phanerozoischen A-Typ Graniten mit der Ausnahme, daß sie relativ wenige peralkaline Gesteine umfassen and dab sie als sehr große (bis zu 40.000 km²) Batholithe vorkommen können. Einige Rapakivi-Granite führen wichtige Zinn-polymetallische and Fe-Cu Lagerstätten.Die Rapakivi Granite kristallisierten aus einem relativ heißen, Restit-armen Magma bei niedrigem (epizonalem bis subvulkanischem) Druck, and . Mineralassoziationen weisen auf eine vielphasige Kristallisationsgeschichte hin; die auffallenden Umwachsungen von Perthit-Ovoiden mit Plagioklas konnen durch Änderungen in der Magmenzusammensetzung and/oder von P, T and erklärt wurden, die die Stabilitäten der Feldspate beeinflussen. Die Isotopen-Zusammensetzung der Rapakivi Granite entspricht im allgemeinen einem tieferen Krusten-Protolith. Der letztere kann entweder ein an Schmelze verarmtes Residuum oder auch ein relativ wasserarmes, magmatisches oder metamagmatisches Gestein gewesen sein. Schmelzen des Protoliths begann in Abwesenheit von volatilen Phasen and wurde durch Wärmezufuhr von gleichaltrigen mafischen Magmen, die aus dem Mantel stammen, herbeigeführt. Der weit verbreitete Rapakivi Granit-Magmatismus im mittleren Proterozoikum dürfte mit der Bildung eines Superkontinentes in Beziehung zu setzen sein.

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With 11 figures     

An almandine garnet isograd in the Rogers Pass area,British Columbia: The nature of the reaction and an estimation of the physical conditions during its formation

In the Rogers Pass area of British Columbia the almandine garnet isograd results from a reaction of the form: 5.31 ferroan-dolomite+8.75 paragonite+4.80 pyrrhotite+3.57 albite+16.83 quartz+1.97 O₂=1.00 garnet+16.44 andesine+1.53 chlorite+2.40 S₂+1.90 H₂O+10.62 CO₂. The coefficients of this reaction are quite sensitive to the Mn content of ferroan-dolomite.Experimental data applied to mineral compositions present at the isograd, permits calculation of two intersecting P, T equilibrium curves. P=29088–39.583 T is obtained for the sub-system paragonite-margarite (solid-solution), plagioclase, quartz, ferroan-dolomite, and P=28.247 T–14126 is obtained for the sub-system epidote, quartz, garnet, plagioclase. These equations yield P=3898 bars and T=638° K (365° C). These values are consistent with the FeS content of sphalerite in the assemblage pyrite, pyrrhotite, sphalerite and with other estimates for the area.At these values of P and T the composition of the fluid phase in equilibrium with graphite in the system C-O-H-S during the formation of garnet is estimated as: bars, bars, bars, bars, bars, bars, bars, bars, , bars, bars.     

A case study of the amount and distribution of heat and fluid during metamorphism

The volume of fluid and amount of heat involved in a portion of a metamorphic event around three synmetamorphic granitic stocks has been quantitatively estimated using mineral composition and modal data from carbonate rocks. Values of volumetric fluid-rock ratios range, with respect to a reference zoisite isograd, from 0.001 to 0.434. Amounts of heat involved range from –25 to 134 cal/cm³ rock. Contours of constant fluid-rock ratio and of constant amount of heat are generally concentric about the granitic stocks indicating that the stocks are foci of high heat and fluid fluxes during metamorphism. In addition, contours of fluid-rock ratios and amount of heat outline NE-SW-trending channelways of high fluid and heat fluxes that alternate with regions of lower fluid and heat fluxes. The NE-SW-trending vertical bedding and schistosity in the area — of premetamorphic origin — probably constrained fluid and heat transfer to occur preferentially in NE-SW directions. Large values of heat involved in metamorphism are strongly correlated with large fluid-rock ratios, suggesting that fluids are an important carrier of heat during metamorphism. Configurations of mapped isograds in the area mimic the patterns of contours of constant fluid-rock ratio and of heat content, indicating that configurations of isograds may contain useful information about regional patterns of heat and fluid transfer during metamorphism.Notation T Last temperature recorded by metacarbonate rocks (°C) - P Lithostatic pressure (bars) - Pi Partial pressure of component i (bars) - of last fluid in equilibrium with carbonate rocks during metamorphism - R 1.987 cal/bar-degree - K _s Activity constant for an assemblage of solid mineral phases - In Natural logarithm - c _v Volumetric heat capacity (cal/cm³-degree) - Q Heat added to or subtracted from a rock during metamorphism in the zoisite zone (kcal/100 cm³ rock; cal/cm³ rock) - Q{ibrxn} Heat added to or subtracted from a rock due to mineral reactions during metamorphism in the zoisite zone (kcal/100 cm³ rock; cal/cm³ rock) - Std. Dev. Standard Deviation - Average of fluid in equilibrium with carbonate rocks during their metamorphism in the zoisite zone - of fluid in equilibrium with carbonate rocks at the zoisite isograd - T Temperature at the zoisite isograd (°C) - X _i,j Mole fraction of component i in phase j - H _i Molar enthalpy of reaction i at 0 bars pressure - ¯V _i Change of molar volume due to reaction _ii - _i Measure of progress of reaction i - V Change in rock volume due to fluid-rock reactions - iV Initial rock volume before metamorphism within the zoisite zone - ¯V _s,i Change in molar volume of solid minerals due to reaction i Component notation an CaAl₂Si₂O₈ Phase notation Pl Plagioclase - Am Amphibole - Cc Calcite - Qz Quartz - Di Diopside - Zo Zoisite - Ga Garnet - Bi Biotite - Kf Microcline - Mu Muscovite     

The role of oxidation-reduction and C-H-O fluids in determining melting conditions and magma compositions in the upper mantle

High pressure experimental studies of the melting of lherzolitic upper mantle in the absence of carbon and hydrogen have shown that the lherzolite solidus has a positive dP/dT and that the percentage melting increases quite rapidly above the solidus. In contrast, the presence of carbon and hydrogen in the mantle results in a region of ‘incipient’ melting at temperatures below the C,H-free solidus. In this region the presence or absence of melt and the composition of the melt are dependent on the amount and nature of volatiles, particularly the CO₂, H₂O, and CH₄ contents of the potential C-H-O fluid. Under conditions of low (IW to IW + 1 log unit atP ∼ 20–35kb), fluids such as CH₄+H₂O and CH₄+H₂ inhibit melting, having a low solubility in silicate melts. Under these conditions, carbon and hydrogen are mobile elements in the upper mantle. At slightly higher oxygen fugacity (IW+2 log units,P∼20–35 kb) fluids in equilibrium with graphite or diamond in peridotite C-H-O are extremely water-rich. Carbon is thus not mobile in the mantle in this range and the melting and phase relations for the upper mantle lherzolite approximate closely to the peridotite-H₂O system. Pargasitic amphibole is stable to solidus temperatures in fertile lherzolite compositions and causes a distinctive peridotite solidus, the ‘dehydration solidus’, with a marked change in slope (a ‘back bend’) at 29–30kb due to instability of pargasite at high pressure. Intersections of geothermal gradients with the peridotite-H₂O solidi define the boundary between lithosphere (subsolidus) and asthenosphere (incipient melt region). This boundary is thus sensitive to changes in [affecting CH₄:H₂O:CO₂ ratios] and to the amount of H₂O and carbon (CO₂, CH₄) present. At higher conditions (IW + 3 log units), CO₂-rich fluids occur at low pressures but there is a marked depression of the solidus at 20–21 kb due to intersection with the carbonation reaction, producing the low temperature solidus for dolomite amphibole lherzolite (T∼925°C, 21 to >31kb). Melting of dolomite (or magnesite) amphibole lherzolite yields primary sodic dolomitic carbonatite melt with low H₂O content, in equilibrium with amphibole garnet lherzolite. The complexity of melting in peridotite-C-H-O provides possible explanations for a wide range of observations on lithosphere/asthenosphere relations, on mantle melt and fluid compositions, and on processes of mantle metasomatism and magma genesis in the upper mantle.     

Johillerit,Na(Mg,Zn)3 Cu(AsO4)3, ein neues Mineral aus Tsumeb,Namibia

Zusammenfassung Die chemische Analyse des neuen Minerals Johillerit mit der Elektronenmikrosonde ergab: Na₂O 5,4, MgO 18,3, ZnO 5,4, CuO 15,8 und As₂O₅ 55,8, Summe 100.7%. Aus diesem Ergebnis wurde die idealisierte Formel Na(Mg, Zn)₃ Cu(AsO₄)₃ abgeleitet. Johillerit ist monoklin mit der RaumgruppeC2/c. Die Gitterkonstanten sind:a=11,870 (3),b=12,755 (3),c=6,770 (2) , =113,42 (2)°,Z=4. Die stärksten Linien des Pulverdiagramms sind: 4,06 (5) (22 ), 3,50 (4) (310), 3,25 (8) (11 ), 2,75 (10) (330, 240), 2,64 (5) (311, 13 , 40 ), 1,952 (4) (13 , 35 ), 1,682 (4) (20 , 460), 1,660 (5) (40 , 71 , 550, 64 ), 1,522 (4) (442, 153, 13 ). Es bestehen enge strukturelle Beziehungen zwischen Johillerit und O'Danielit, Na(Zn, Mg)₃H₂(AsO₄)₃, sowie einigen synthetischen. Verbindungen.Johillerit ist violett durchscheinend. Die Spaltbarkeit nach {010} ist ausgezeichnet und nach {100} und {001} gut.H (Mohs)3.D=4,15 undD _X =4,21 g·cm^–3. Das Mineral ist optisch zweiachsig positiv, 2V80 (5)°. Die Werte der Lichtbrechung sindn =1,715 (4),n =1,743 (4) undn =1,783 (4). Die Auslöschung istn b und auf (010)n c16°. Johillerit ist stark pleochroitisch mit den AchsenfarbenX=violett-rot,Y = blauviolett undZ = grünblau. Das neue Mineral kommt in radialstrahligen Massen gemeinsam mit kupferhaltigem Adamin und Konichalcit in zersetzem Kupfererz von Tsumeb, Namibia, vor. Die Benennung erfolgte nach Prof. Dr.J.-E. Hiller (1911–1972).

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Johillerite, Na(Mg, Zn) ₃ Cu(AsO ₄ ) ₃ , a new mineral from Tsumeb, Namibia

Summary Electron microprobe analysis of the new mineral johillerite gave Na₂O 5.4, MgO 18.3, ZnO 5.4, CuO 15.8, and As₂O₅ 55.8, total 100.7%. From this result, the ideal formula is given as Na(Mg, Zn)₃ Cu(AsO₄)₃. Johillerite crystallizes monoclinic,C2/c. The unit cell dimensions are:a=11.870(3),b=12.755 (3),c=6.770 (2) , =113.42 (2)°,Z=4. The strongest lines on the X-ray powder diffraction pattern are: 4,06 (5) (22 ), 3,50 (4) (310), 3,25 (8) (11 ), 2,75 (10) (330, 240), 2,64 (5) (311, 13 , 40 ), 1,952 (4) (13 , 35 ), 1,682 (4) (20 , 460), 1,660 (5) (40 , 71 , 550, 64 ), 1,522 (4) (442, 153, 13 ). There is a close relationship between johillerite, o'danielite, Na(Zn, Mg)₃H₂(AsO₄)₃, and some synthetic compounds. Johillerite is violet in colour, transparent. Cleavage is {010} perfect, {100} and {001} good.H (Mohs)3.D=4.15 andD _X =4.21 g·cm^–3. The mineral is optically biaxial positive, 2V80 (5)°. The refractive indices are:n =1.715 (4),n =1.743 (4),n =1.783 (4). The extinction isn b and on (010)n c16°. Strongly pleochroic with axial coloursX=violet-red,Y=bluish violet andZ=greenish blue. The new mineral was found in radiated masses together with cuprian adamite and conichalcite in an oxidized copper ore from Tsumeb, Namibia. It is named in honour of Prof. Dr.J.-E. Hiller (1911–1972).

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Mit 1 Abbildung     

Isotope geochemistry of ore fluids for the Dongsheng sandstone-type uranium deposit, China

The Dongsheng sandstone-type uranium deposit is one of the large-sized sandstone-type uranium deposits discovered in the northern part of the Ordos Basin of China in recent years. Geochemical characteristics of the Dongsheng uranium deposit are significantly different from those of the typical interlayered oxidized sandstone-type uranium ore deposits in the region of Middle Asia. Fluid inclusion studies of the uranium deposit showed that the uranium ore-forming temperatures are within the range of 150–160℃. Their 3He/4He ratios are within the range of 0.02–1.00 R/Ra, about 5–40 times those of the crust. Their 40Ar/36Ar ratios vary from 584 to 1243, much higher than the values of atmospheric argon. The δ18OH2O and δD values of fluid inclusions from the uranium deposit are -3.0‰– -8.75‰ and -55.8‰– -71.3‰, respectively, reflecting the characteristics of mixed fluid of meteoric water and magmatic water. The δ18OH2O and δD values of kaolinite layer at the bottom of the uranium ore deposit are 6.1‰ and -77‰, respectively, showing the characteristics of magmatic water. The δ13CV-PDB and δ18OH2O values of calcite veins in uranium ores are -8.0‰ and 5.76‰, respectively, showing the characteristics of mantle source. Geochemical characteristics of fluid inclusions indicated that the ore-formation fluid for the Dongsheng uranium deposit was a mixed fluid of meteoric water and deep-source fluid from the crust. It was proposed that the Jurassic-Cretaceous U-rich metamorphic rocks and granites widespread in the northern uplift area of the Ordos Basin had been weathered and denudated and the ore-forming elements, mainly uranium, were transported by meteoric waters to the Dongsheng region, where uranium ores were formed. Tectonothermal events and magmatic activities in the Ordos Basin during the Mesozoic made fluids in the deep interior and oil/gas at shallow levels upwarp along the fault zone and activated fractures, filling into U-bearing clastic sandstones, thus providing necessary energy for the formation of uranium ores.     

Thermodynamic mixing properties of Mg(Al,Cr)2O4 spinel crystalline solution at high temperatures and pressures

Three Al-Cr exchange isotherms at 1,250°, 1,050°, and 796° between Mg(Al, Cr)₂O₄ spinel and (Al, Cr)₂O₃ corundum crystalline solutions have been studied experimentally at 25 kbar pressure. Starting from gels of suitable bulk compositions, close approach to equilibrium has been demonstrated in each case by time studies. Using the equation of state for (Al, Cr)₂O₃ crystalline solution (Chatterjee et al. 1982a) and assuming that the Mg(Al, Cr)₂O₄ can be treated in terms of the asymmetric Margules relation, the exchange isotherms were solved for Δ G ^*, and . The best constrained data set from the 1,250° C isotherm clearly shows that the latter two quantities do not overlap within three standard deviations, justifying the choice of asymmetric Margules relation for describing the excess mixing properties of Mg(Al, Cr)₂O₄ spinels. Based on these experiments, the following polybaric-polythermal equation of state can be formulated: , P expressed in bars, T in K, G _m ^ex and W _G,i ^Sp in joules/mol. Temperature-dependence of G _m ^ex is best constrained in the range 796–1,250° C; extrapolation beyond that range would have to be done with caution. Such extrapolation to lower temperature shows tentatively that at 1 bar pressure the critical temperature, T _c, of the spinel solvus is 427° C, with dT_c/dP≈1.3 K/kbar. The critical composition, X _c, is 0.42 , and changes barely with pressure. Substantial error in calculated phase diagrams will result if the significant positive deviation from ideality is ignored for Al-Cr mixing in such spinels.