Genesis of amethyst geodes in basaltic rocks of the Serra Geral Formation (Ametista do Sul,Rio Grande do Sul,Brazil): a fluid inclusion,REE, oxygen,carbon, and Sr isotope study on basalt,quartz, and calcite

In the Ametista do Sul area, Rio Grande do Sul, Brazil, amethyst-bearing geodes are hosted by a ~40- to 50-m-thick subhorizontal high-Ti basaltic lava flow of the Lower Cretaceous Paraná Continental Flood Basalt Province. The typically spherical cap-shaped, sometimes vertically elongated geodes display an outer rim of celadonite followed inwards by agate and colorless and finally amethystine quartz. Calcite formed throughout the whole crystallization sequence, but most commonly as very late euhedral crystals, sometimes with gypsum, in the central cavity. Fluid inclusions in colorless quartz and amethyst are predominantly monophase and contain an aqueous liquid. Two-phase liquid–vapor inclusions are rare. Some with a consistent degree of fill homogenize into the liquid between 95 and 98 °C. Ice-melting temperatures in the absence of a vapor phase between –4 and +4 °C indicate low salinities. Chondrite-normalized REE patterns of calcites are highly variable and show generally no systematic correlation with the paragenetic sequence. The oxygen isotope composition of calcites is very homogeneous (δ¹⁸O_VSMOW=24.9±1.1‰, n=34) indicating crystallization temperatures of less than 100 °C. Carbon isotope values of calcites show a considerable variation ranging from –18.7 to –2.9‰ (VPDB). The ⁸⁷Sr/⁸⁶Sr ratio of calcites varies between 0.706 and 0.708 and is more radiogenic than that of the host basalt (~0.705). The most likely source of silica, calcium, carbon, and minor elements in the infill of the geodes is the highly reactive interstitial glass of the host basalts leached by gas-poor aqueous solutions of meteoric origin ascending from the locally artesian Botucatú aquifer system in the footwall of the volcanic sequence. The genesis of amethyst geodes in basalts at Ametista do Sul, Brazil, is thus considered as a two-stage process with an early magmatic protogeode formation and a late, low temperature infill of the cavity. Editorial handling: A. Cheilletz     

Genesis of the Sb-W-Au deposits at Ixtahuacan,Guatemala: evidence from fluid inclusions and stable isotopes

The Ixtahuacan Sb-W deposits are hosted by upper Pennsylvanian to Permian metasedimentary rocks of the central Cordillera of Guatemala. The deposits consist of gold-bearing arsenopyrite, stibnite and scheelite. Arsenopyrite and scheelite are early in the paragenesis, occurring as disseminations in pyritiferous black shale/sandstone and in argillaceous limestone, respectively. Some stibnite is disseminated, but the bulk of the stibnite occurs as massive stratabound lenses in black shales and in quartz-ankerite veins and breccias, locally containing scheelite.Microthermometric measurements on fluid inclusions in quartz and scheelite point to a low temperature (160–190°C) and low to moderate salinity (5–15 wt% NaCl eq.) aqueous ore fluid. Abundant vapour-rich inclusions suggest that the fluid boiled. Carbon dioxide was produced locally as a result of interaction of the aqueous fluid with the argillaceous limestone. Bulk leaching experiments and SEM-EDS analyses of decrepitated fluid inclusion residues indicate that the ore-bearing solution was NaCl-dominated. The ¹⁸O values of quartz, ankerite and scheelite from mineralized veins range from 19.7 to 20.5, 18.1 to 20.0 and 7.0 to 8.4 respectively. The average temperature calculated from quartz-scheelite oxygen isotopic fractionation is 170°C. The oxygen isotopic composition of the fluid, interpreted to have been in equilibrium with these minerals, ranged from 5.7 to 7.6, and is considered to represent an evolved meteoric water. Diagenetic or syngenetic pyrite has a sulphur isotopic composition of 0.5±0.3 which is consistent with bacterial reduction of sulphate. The ³⁴S values of arsenopyrite and stibnite range from –2.8 to 2.0 and –2.7 to –2.3 respectively, and are though to reflect sulphur derived from pyrite.The Ixtahuacan deposits are interpreted to have formed at low temperature (<200°C) and a depth of a few hundred metres from a low fO₂ (10^–49–10^–57), high pH (7–8) fluid. Arsenic was probably transported as arsenious acid, antimony and gold as thio-complexes and tungsten as the complex HWO ₄ ^– .A model is proposed in which a meteoric fluid, heated by a felsic intrusion at depth, was focused to shallow levels along faults. The interaction of the fluid with pyritiferous beds caused the deposition of arsenopyrite as a result of sulphidation and/or decreasing fO₂; gold probably co-precipitated with As or was adsorbed onto the arsenopyrite. The precipitation of stibnite was caused by boiling. Scheelite deposited in response to the increase in Ca²⁺ activity which accompanied interaction of the ore fluid with the argillaceous limestones.     

Petrography and chemistry of SiO2 filling phases in the amethyst geodes from the Serra Geral Formation deposit,Rio Grande do Sul,Brazil

The filling process of amethyst-bearing geodes from Serra Geral Formation basalts, Brazil, is investigated by different methods performed on the SiO₂ filling phases. Image analysis of quartz–amethyst deposits suggests a single growing mechanism ruled by geometric selection of randomly oriented crystals. Microthermometry of fluid inclusions reveals formation temperature lower than 100 °C, probably lower than 50 °C, and fluid salinity as high as 3 mass% NaCl eq. Composition in REE and trace-elements measured by ICP-MS on acid-digested or laser-ablated samples indicates a common genesis for amethyst, quartz and chalcedony, as well as the absence of significant variations from one geode to another. ⁸⁷Sr/⁸⁶Sr data on chalcedony shows that both the host basalt or the Botucatu sandstone are possible silica sources. These data, combined with thermo-kinetic considerations, permit us to discuss the filling process. We argue in favor of the contribution of a mineralized fluid of hydrothermal origin producing a regional silica source which decreased with time. The observed mineral sequence is related to the depletion of silica in the solution.     

Source and evolution of ore-forming hydrothermal fluids in the northern Iberian Pyrite Belt massive sulphide deposits (SW Spain): evidence from fluid inclusions and stable isotopes

A fluid inclusion and stable isotopic study has been undertaken on some massive sulphide deposits (Aguas Teñidas Este, Concepción, San Miguel, San Telmo and Cueva de la Mora) located in the northern Iberian Pyrite Belt. The isotopic analyses were mainly performed on quartz, chlorite, carbonate and whole rock samples from the stockworks and altered footwall zones of the deposits, and also on some fluid inclusion waters. Homogenization temperatures of fluid inclusions in quartz mostly range from 120 to 280 °C. Salinity of most fluid inclusions ranges from 2 to 14 wt% NaCl equiv. A few cases with T _h=80–110 °C and salinity of 16–24 wt% NaCl equiv., have been also recognized. In addition, fluid inclusions from the Soloviejo Mn–Fe-jaspers (160–190 °C and ˜6 wt% NaCl equiv.) and some Late to Post-Hercynian quartz veins (130–270 °C and ˜4 wt% NaCl equiv.) were also studied. Isotopic results indicate that fluids in equilibrium with measured quartz (d ¹⁸O _fluid ˜–2 to 4‰), chlorites (d ¹⁸O _fluid ˜8–14‰, dD _fluid ˜–45 to –27‰), whole rocks (d ¹⁸O _fluid ˜4–7‰, dD _fluid ˜–15 to –10‰), and carbonates (d ¹⁸O _ankerite ˜14.5–16‰, d ¹³C _fluid =–11 to –5‰) evolved isotopically during the lifetime of the hydrothermal systems, following a waxing/waning cycle at different temperatures and water/rock ratios. The results (fluid inclusions, d ¹⁸O, dD and d ¹³C values) point to a highly evolved seawater, along with a variable (but significant) contribution of other fluid reservoirs such as magmatic and/or deep metamorphic waters, as the most probable sources for the ore-forming fluids. These fluids interacted with the underlying volcanic and sedimentary rocks during convective circulation through the upper crust.     

湘西地区铅锌矿的大范围低温流体成矿作用-流体包裹体研究

对狮子山、茶田、打狗洞、董家河和唐家寨等湘西地区典型铅锌矿床中闪锌矿、方解石及石英等矿物进行了流体包裹体均一温度、盐度和激光拉曼探针成分测定。结果表明,成矿流体温度主要为100~180℃,总盐度一般15%,密度多1g/cm3,成矿压力约340×105~428×105Pa,成矿深度约在1.00~1.55km,是以钠和钙氯化物为主的高浓度溶液,属于低温度、高盐度、高密度的地下热卤水性质的含矿热水溶液。成矿流体离子成分主要为Cl-、Na+、Ca2+、K+、Mg2+,流体氢氧同位素组成表明成矿流体来源与建造水有关,后期可能有雨水和少量变质水的渗入,并使盐度降低。矿物流体包裹体中含有机质,流体包裹体气相成分中CH4普遍存在,还有较强的CO2成分特征峰,表明成矿与有机质相关,处在控矿构造内的容矿层储存有有机质,致使矿床中的硫酸盐硫得以还原为还原硫,促使成矿流体中的铅、锌等组分从络合物中分离、沉淀,继之大量堆积而形成了矿床。相邻的低温成矿域川滇黔地区典型铅锌矿床成矿温度约为150℃~280℃,湘西地区铅锌矿与川滇黔铅锌矿相比,具有相同的低温成矿特征,赋存深度浅,矿床类型均为MVT型,两者可能受控于相同的动力学背景。     

The behavior of lithium and its isotopes in oilfield brines: evidence from the Heletz-Kokhav field, Israel

Twenty-four brine samples from the Heletz-Kokhav oilfield, Israel, have been analyzed for chemical composition and Li isotope ratios. The chemical composition of the brines, together with geological evidence, suggests derivation from (Messinian) seawater by evaporation that proceeded well into the gypsum stability field but failed to reach the stage of halite crystallization. The present salinity of the samples (18-47 g Cl/L) was achieved by dilution of the original evaporitic brine by local fresh waters. Like brines from other sedimentary basins, the Li/Cl ratios in the Heletz-Kokhav samples show a prominent Li enrichment (five-fold to eight-fold) relative to modern seawater. The isotopic ratios of Li, expressed in the δ ⁶Li notation, vary from −26.3 to −17.9‰, all values being significantly higher than that of modern seawater (−32‰) irrespective of their corresponding Li concentration (1.0-2.3 mg/L). The isotopic composition of Li and the Li/Cl ratio in the oilfield brines were acquired in two stages: (a) The original evaporated seawater gained isotopically light Li during the diagenetic interaction between the interstitial Messinian brine and the basin sediments. A parent brine with an elevated Li/Cl ratio was formed. The brine was later diluted in the oilfields. (b) The δ ⁶Li values of the final brines were determined during epigenetic interaction with the Heletz-Kokhav aquifer rocks. At the same time, the Li/Cl ratio inherited from stage (a) remained largely unchanged. This work represents the first use of lithium isotopic composition to elucidate the origin and evolution of formation waters in sedimentary basins.     

Diagenesis of Ordovician carbonates from the north-east Michigan Basin, Manitoulin Island area, Ontario: evidence from petrography, stable isotopes and fluid inclusions

Middle to Late Ordovician subtidal carbonates in the Manitoulin Island area of Ontario are predominantly limestone in composition, but non-ferroan and ferroan dolomite is a common cement as well as a selective or locally pervasive replacement phase. Integration of field, petrographic, geochemical (δ¹³C, δ¹⁸O) and fluid inclusion data indicates that lithification of these carbonates occurred during burial diagenesis, with much of the alteration controlled by regional fracturing and hydrothermal influences. Aqueous (type 1) fluid inclusions in early calcite (pre-dolomite) and dolomite are saline (> 29 wt% NaCl eq.) solutions with Ca and/or Mg in excess of Na and display homogenization temperatures with modes of 95 and 101°C, respectively. These temperatures can be explained by significantly more burial than can be accounted for either by the available stratigraphic information or by an unusually high palaeogeothermal gradient, which also is not well supported. The fluid inclusion temperatures are interpreted to have resulted from hydrothermal fluids which circulated during the burial diagenesis of these strata. Type 1 inclusions in late (post-dolomite) calcite are less saline (<19 wt% NaCl eq.) and have a bimodal distribution of homogenization temperatures with a relatively well defined low temperature peak similar to those in early calcite and dolomite and a broad higher temperature grouping with a mode at 183°C. A small proportion of methane and light hydrocarbon-bearing fluid inclusions (type 2) are present in all stages of carbonate. Dolomitizing fluids were derived from burial compaction of argillaceous sediments in the more central parts of the Michigan Basin and the updip migration of these brines along fractures to the basin margin where the carbonates of the Manitoulin Island area were dolomitized. Alternatively, migration of dolomitizing brines downward from the overlying pervasively dolomitized Silurian sequence into fractures in the Ordovician carbonates may have occurred. Integration of the aqueous fluid inclusion data into the diagenetic history of these carbonates remains equivocal because most of the inclusions are secondary or indeterminate in origin. Nevertheless, high salinities resulting from interaction with evaporitic strata and hydrothermal effects are clearly implicated although the origin of the latter remains unclear. The alteration styles of the Ordovician carbonates in the Manitoulin area are similar to those of Ordovician hydrocarbon reservoirs described from other parts of the Michigan Basin. They indicate that fracture-related diagenesis occurred on a basin-wide scale and that hydrothermal effects were important.     

豫西双河金矿流体包裹体及稳定同位素研究

双河金矿床位于华北克拉通南缘的卢氏金多金属矿集区,为一石英脉型金矿;矿体呈脉状产于中元古代宽坪群石英二云母片岩切层断裂中;金主要产在黄铁矿和多金属硫化物石英/铁白云石脉中。以含金石英矿脉为中心由内到外围岩蚀变主要发育硅化、黄铁矿化及碳酸盐化。流体成矿过程包括早、中、晚3个阶段,分别以无矿白石英、石英-黄铁矿-多金属硫化物-铁白云石组合和石英-方解石组合为标志,矿石矿物主要沉淀于中阶段。双河金矿流体包裹体类型丰富,不同成矿阶段的流体包裹体主要有H_2O-CO_2包裹体、H_2O包裹体、含子晶(NaCl、CaCO_3)包裹体和含C单质包裹体。显微测温学研究表明,成矿早阶段乳白色石英中包裹体类型有H_2O-CO_2包裹体和H_2O包裹体,H_2O-CO_2包裹体均一温度为220～350℃,盐度为3. 89%～16. 55%NaCleqv; H_2O包裹体均一温度介于220～285℃之间,盐度为1. 40%～1. 70%NaCleqv。成矿主阶段烟灰色石英中包裹体类型包括H_2O-CO_2包裹体、H_2O包裹体、含NaCl子晶包裹体和含C单质包裹体,其中H_2O-CO_2包裹体均一温度为189～345℃,盐度为3. 33%～20. 23%NaCleqv; H_2O包裹体的均一温度介于180～348℃之间,盐度为0. 88%～14. 97%NaCleqv;含NaCl子晶包裹体均一温度为210～359℃,盐度为30. 92%～42. 50%NaCleqv。氢氧同位素研究表明成矿流体来自岩浆水与变质水(δ~(18)O_水=5. 3‰～8. 6‰,δD=-72. 6‰～-38. 4‰);热液碳酸盐的δ~(13)C_(V-PDB)值为-7. 5‰～-5. 2‰,δ~(18)O_(V-SMOW)值为14. 7‰～17. 0‰。包裹体及C-H-O同位素的研究表明,流体的沸腾及水岩反应可能是双河金矿金沉淀的主要原因。     

The genesis of calcite in carbonate rocks of the sedimentary basins: Evidence from the stable carbon and oxygen isotopes composition

The distributions of stable carbon and oxygen isotopes in modern and ancient limestones of various types were studied. Carbonate samples from modern sediments were collected in the Black and Barents Seas. Ancient carbonates were represented by Upper Jurassic (Kimmeridgian-Tithonian) limestones from the central part of the West Siberian basin. Carbonate samples include remains of modern and Upper Jurassic fauna, carbonate crust from sediments of the Black Sea, carbonate tube from sediments of the Barents Sea, and Upper Jurassic limestone from the carbonate layer found at top of Abalak, bottom of Bazhenov deposits in the central part of the West Siberian basin. According to the results of the isotope analysis and comparison with modern carbonates, Upper Jurassic limestones of the West Siberian basin belong to the group of methane-derived carbonates and precipitated as a result of anaerobic oxidation of methane (AOM). Fractures in limestones are filled with secondary calcite.     

Origin of brines in the Upper Silesian Coal Basin (Poland) inferred from stable isotope and chemical data

Brines in the Miocene formations of the Upper Silesian Coal Basin have isotopic composition close to SMOW, which identifies them as the connate marine water. However, controversies exist on the origin of brines in the Carboniferous formations. Isotopic and hydrochemical data exclude any relationship to marine water and enrichment by evaporation. The most common brine which occurs at great depths can be identified as the oldest infiltration in a very hot climate (δ¹⁸O ⋟ −2‰, δD ⋟ −20‰, Cl⁻ content 34 to 140 g/L). This brine is free of SO₄²⁻ and U, and rich in Ba²⁺ and²²⁶Ra. Its salinity is probably related to the leaching of evaporites and intensive weathering of rocks during the Rotliegendes.Other brines are difficult to identify because their isotopic contents are within the range of mixing between the oldest brine and the Quaternary waters (δ¹⁸O ⋟ 10‰, δD ⋟ 70‰). Isotopic and hydrochemical data allow identification of several occurrences of brine formed by meteoric water of a warm Tertiary climate, after the last marine transgression in the Tortonian. That brine is rich in SO₄²⁻ and contains moderate contents of²²⁶Ra and U. Its salinity is thought to result from leaching of Miocene evaporites. Two other identified types of brines can be related to some infiltration periods before the last marine transgression. The sources in salinity of these 2 types remain unknown. Mining activity results in a common occurrence of mixed brines. When the Quaternary component dominates, its identification is easy from the isotopic composition, whereas the end brine component can ususally be identified by chosen ion ratios and the presence or lack of sulphates.     

Evaluation of contaminants retention in soils from Viamão District, Rio Grande do Sul State, Brazil

Adsorption is one of the most significant processes in the mobility of soluble pollutants in soils. The aim of this work is to characterize and evaluate the adsorption capacity of soils from Viamão District, Brazil. The studied ions were lead_total, copper_total, sulfate, phosphate, and potassium. The soils were mapped by remote sensing and characterized by granulometrical and mineralogical techniques. The adsorption tests were made by the contact of soil samples with aqueous solutions. The soils adsorption capacity presented the following trend: Pb_total>Cu_total≈PO ₄ ³⁻ >K⁺ ≈SO ₄ ²⁺ . Adsorption in the soils is strongly influenced by clay content. The adsorption of phosphate, copper, and lead was accentuated by the presence of organic matter. Phosphate adsorption was controlled by oxides and organic matter. Both potassium and sulfate showed insignificant adsorption in the studied soils.     

云南麻栗坡南秧田白钨矿床流体包裹体、稳定同位素特征及其成矿意义

对南秧田层状矽卡岩型白钨矿石中石榴石及电气石石英脉型白钨矿石中石英流体包裹体岩相学特征研究表明,与成矿有关的包裹体主要有富液相、富气相和含子晶的流体包裹体3种类型。矽卡岩型矿石中石榴石及共生石英的包裹体均一温度为128~250℃,盐度w(NaCl)=0.7%~8.1%;电气石石英脉型白钨矿石中石英的流体包裹体均一温度范围为181~325℃,盐度w(NaCl)=1.57%~15.76%。两种类型矿石成矿溶液密度为0.75~0.95g/cm³,表明形成这两种类型矿石的成矿流体均属于中温、中-低盐度、低密度的流体。氢氧同位素结果显示其主要来源于地层水(变质水),后期受到了岩浆水的叠加改造作用。硫同位素落入沉积岩、变质岩及蒸发硫酸盐的硫同位素组成范围内。     

Potassic primary melts of vulsini (Roman Province): evidence from mineralogy and melt inclusions

The origin and the relationships between the high potassic (HKS) and potassic (KS) suites of the Roman Comagmatic Province and the nature of their primary magmas have been intensively debated over the past 35 years. We have addressed these problems by a study of mineralogy (olivine Fo_92-87, Cr-spinel and diopside) and melt inclusions in olivine phenocrysts from a scoria sample of Montefiascone (Vulsini area). This rock is considered as one of the most primitive (MgO=13.5 wt%, NiO=340 ppm; Cr=1275 ppm) in the northern part of the Roman Comagmatic Province. The compositions of both the olivine and their melt inclusions are controlled by two main processes. In the case of the olivine Fo<90.5, fractional crystallization (olivine + diopside + minor spinel) was the principal mechanism of the magma evolution. The olivine (Fo_92-90.5) and the Cr-spinel (Cr#=100. Cr/(Cr+Al)=63-73) represent a near-primary liquidus assemblage and indicate the mantle origin of their parental magmas. The compositions of melt inclusions in these olivine phenocrysts correspond to those of poorly fractionated H₂O-rich ( 1 wt%) primary melts (MgO=8.4-9.7 wt%,FeO^total=6-7.5 wt%). They evidence a wide compositional range (in wt%: SiO₂=46.5-50, K₂O=5.3-2.8, P₂O₅=0.4-0.2, S=0.26-0.12; Cl=0.05-0.03, and CaO/Al₂O₃= 0.8-1.15), with negative correlations between SiO₂ and K₂O, Al₂O₃ and CaO, as well as positive correlations between K₂O, and P₂O₅, S, Cl, with nearly constant ratios between these elements. These results are discussed in terms of segregation of various mantle-derived melts. The high and constant Mg# [100.Mg/(Mg+Fe²⁺)] 73-75 of studied melts and their variable Si, K, P, Ca, Al, S contents could be explained by the melting of a refractory lithospheric mantle source, heterogeneously enriched in phlogopite and clinopyroxene (veined mantle source).     

Potassic primary melts of Vulsini (Roman Province): evidence from mineralogy and melt inclusions

The online version of the original article can be found at     

内蒙古东乌旗沙麦钨矿床成矿机制：来自流体包裹体及稳定同位素的证据

内蒙古东乌旗沙麦钨矿床位于中亚巨型缝合带的最东段,矿床内广泛发育云英岩化,钨矿体主要受NW向断裂构造控制,呈黑钨矿-石英脉状产出在燕山期沙麦岩体（岩性主要为黑云母二长花岗岩）和泥盆系围岩地层（岩性主要为角岩化砂岩、砂砾岩）中。成矿阶段从早到晚分别为：石英-黄玉阶段（Ⅰ）、云英岩-黑钨矿阶段（Ⅱ）和晚期硅化阶段（Ⅲ）。本文对矿床流体包裹体及C-H-O-S同位素进行了研究,结果显示：矿床中普遍发育CO2-H2O包裹体及水溶液气液两相流体包裹体。从成矿早期至成矿晚期,矿床流体中富CO2包裹体逐渐减少,CO2含量逐渐降低。成矿早期可见熔体包裹体与流体包裹体共存的现象,属低盐度、富CO2的岩浆来源流体。矿床成矿作用从早到晚,流体包裹体均一温度分别为279~341℃、196~286℃、103~201℃;流体盐度分别为(1.6~9.1)%NaCleq、(3.0~11.1)%NaCleq、(0.9~4.7)%NaCleq。激光拉曼光谱分析结果显示,流体中的气相主要是CO2,部分含有H2O和CH4。成矿流体从早期中高温、中盐度流体向晚期低温、低盐度流体演化,属NaCl-H2O-CO2±CH4热液体系。矿床稳定同位素研究表明,沙麦钨矿床的成矿流体主要来源于岩浆水,并在成矿晚期混入了大气水。矿床硫的来源与早白垩世岩浆-热液系统或深部幔源岩浆相关,并混入了部分地层硫。综合分析认为,沙麦钨矿为一岩浆期后热液型钨矿床。燕山期侵入体从深部携带成矿物质,经历了岩浆分异和流-熔体相互作用等过程,形成区内岩浆-热液成矿系统。流体的不混溶是导致含钨络合物分解乃至最后沉淀成矿的主要原因。     

Origin of sub-lithospheric diamonds from the Juina-5 kimberlite (Brazil): constraints from carbon isotopes and inclusion compositions

Forty-one diamonds sourced from the Juina-5 kimberlite pipe in Southern Brazil, which contain optically identifiable inclusions, have been studied using an integrated approach. The diamonds contain <20 ppm nitrogen (N) that is fully aggregated as B centres. Internal structures in several diamonds revealed using cathodoluminescence (CL) are unlike those normally observed in lithospheric samples. The majority of the diamonds are composed of isotopically light carbon, and the collection has a unimodal distribution heavily skewed towards δ¹³C ~ ?25 ‰. Individual diamonds can display large carbon isotope heterogeneity of up to ~15 ‰ and predominantly have isotopically lighter cores displaying blue CL, and heavier rims with green CL. The light carbon isotopic compositions are interpreted as evidence of diamond growth from abiotic organic carbon added to the oceanic crust during hydrothermal alteration. The bulk isotopic composition of the oceanic crust, carbonates plus organics, is equal to the composition of mantle carbon (?5 ‰), and we suggest that recycling/mixing of subducted material will replenish this reservoir over geological time. Several exposed, syngenetic inclusions have bulk compositions consistent with former eclogitic magnesium silicate perovskite, calcium silicate perovskite and NAL or CF phases that have re-equilibrated during their exhumation to the surface. There are multiple occurrences of majoritic garnet with pyroxene exsolution, coesite with and without kyanite exsolution, clinopyroxene, Fe or Fe-carbide and sulphide minerals alongside single occurrences of olivine and ferropericlase. As a group, the inclusions have eclogitic affinity and provide evidence for diamond formation at pressures extending to Earth’s deep transition zone and possibly the lower mantle. It is observed that the major element composition of inclusions and isotopic compositions of host Juina-5 diamonds are not correlated. The diamond and inclusion compositions are intimately related to subducted material and record a polybaric growth history across a depth interval stretching from the lower mantle to the base of the lithosphere. It is suggested that the interaction of slab-derived melts and mantle material combined with subsequent upward transport in channelised networks or a buoyant diapir explains the formation of Juina-5 diamonds. We conclude that these samples, despite originating at great mantle depths, do not provide direct information about the ambient mantle, instead, providing a snapshot of the Earth’s deep carbon cycle.     

Ore-forming process of the Huijiabao gold district,southwestern Guizhou Province,China: Evidence from fluid inclusions and stable isotopes

The Huijiabao gold district is one of the major producers for Carlin-type gold deposits in southwestern Guizhou Province, China, including Taipingdong, Zimudang, Shuiyindong, Bojitian and other gold deposits/occurrences. Petrographic observation, microthermometric study and Laser Raman spectroscopy were carried out on the fluid inclusions within representative minerals in various mineralization stages from these four gold deposits. Five types of fluid inclusions have been recognized in hydrothermal minerals of different ore-forming stages: aqueous inclusions, CO₂ inclusions, CO₂–H₂O inclusions, hydrocarbon inclusions, and hydrocarbon–H₂O inclusions. The ore-forming fluids are characterized by a H₂O + CO₂ + CH₄ ± N₂ system with medium to low temperature and low salinity. From early mineralization stage to later ones, the compositions of the ore-forming fluids experienced an evolution of H₂O + NaCl → H₂O + NaCl + CO₂ + CH₄ ± N₂ → H₂O + NaCl ± CH₄ ± CO₂ with a slight decrease in homogenization temperature and salinity. The δ¹⁸O values of the main-stage quartz vary from 15.2‰ to 24.1‰, while the δD_H2O and calculated δ¹⁸O_H2O values of the ore-forming fluids range from −56.9 to −116.3‰ and from 2.12‰ to 12.7‰, respectively. The δ¹³C_PDB and δ¹⁸O_SMOW values of hydrothermal calcite change in the range of −9.1‰ to −0.5‰ and 11.1–23.2‰, respectively. Stable isotopic characteristics indicate that the ore-forming fluid was mainly composed of ore- and hydrocarbon-bearing basinal fluid. The dynamic fractionation of the sulfur in the diagenetic pyrite is controlled by bacterial reduction of marine sulfates. The hydrothermal sulfides and the diagenetic pyrite from the host rocks are very similar in their sulfur isotopic composition, suggesting that the sulfur in the ore-forming fluids was mainly derived from dissolution of diagenetic pyrite. The study of fluid inclusions indicates that immiscibility of H₂O–NaCl–CO₂ fluids took place during the main mineralization stage and caused the precipitation and enrichment of gold.     

云南大坪大型金多金属矿床叠加成矿作用地质、流体包裹体和稳定同位素证据

对云南元阳县大坪大型金多金属矿床矿脉地质特征、流体包裹体和稳定同位素的研究表明,矿区内大量的矿脉是叠加成矿的产物,空间上具有并置、侧列（现）、再现等复杂关系,成矿可分石英．黄铁矿（主要形成金矿化）和石英．方铅矿（主要形成铜铅银矿化,并叠加有金矿化）两个阶段共7个亚阶段。石英-黄铁矿阶段流体包裹体以个体小,数量多。杂乱分布为特征,以二相气液包裹体为主,均一温度变化区间为165℃～393℃,平均258℃;流体密度为0．648—0．7984g／cm^3,盐度变化为13,72％～18．96％NaCleqv,平均17．13％NaCleqv,捕获压力为64MPa左右。流体为含CO2的NaCl-H2O体系,在成分上相对富集Na^＋、Cl^-、CH4和N2、NO3^-等组分,从早到晚,具有Na^＋、Ca^2＋、SO4^2-略有升高,而K^＋有所降低的趋势。石英方铅矿阶段流体包裹体大小混杂,杂乱分布或呈线性排列,以CO2包裹体为主,并有二相气液和少量单相包裹体。均一温度177℃～372℃,平均284℃,流体密度为0,7413～O．9518g／cm^3,盐度变化为10．86％～21．26％NaCleqv,平均14．16％NaCleqv,捕获压力为60MPa左右。流体属为NaCl-H2O-CO2体系,在成分上以相对富集Na^＋、K^＋、SO4^2-为特征,部分脉体的流体包裹体中还含有一定量的Ca^2＋、Mg^2＋等离子。矿区不同（亚）阶段矿石氧同位组成总体变化较小（2.65‰～6.0‰）,氢同位素组成变化较大（-120％～-40‰）,其中石英-黄铁矿阶段H、O同位素组成变化均较小,以岩浆源为主,而石英．方铅矿阶段氢同位素组成变化较大,表明有新的流体加入,但也以深源流体为主;石英-方铅矿阶段一个铁白云石的δ^13C值为-4．81％o,而石英．黄铁矿阶段晚期方解石碳同位素组成变化为-2．79‰～-4.34‰,平均-3．75‰。不同矿脉黄铁矿、方铅矿的硫同位素组成变化范围为0．3‰-4.4‰,其中石英-黄铁矿阶段为0.3‰～4．4‰,石英-方铅矿阶段为0.5～4.3‰,总体上与深源（地幔）碳、硫同位素组成基本一致。综合对比研究认为,大坪金多金属矿床为源于深部的多源成矿热液在同一容矿空间充填的结果,叠加成矿作用与区内长期的岩浆活动密切相关。矿床为中-高温热液硫化物．石英薄脉型金多金属矿床。