同位素地质新进展：技术、方法、理论与应用

近两年来，我国同位素地质在基础地质和实验方法研究等方面取得了很多成果，在国际交流和青年人才培养方面也有可喜进展。为了促进同行交流与展示最新研究成果，并计划将于两年后召开的第八届全国同位素地质年代学、同位素地球化学学术讨论会等事宜，中国地质学会同位素地质专业委员会提出建议，并经过中国地质学会批准     

稳定同位素地球化学研究新况

近年来稳定同位素地球化学进展显著。同位素测试方面的主要进展表现为：①离子探针质谱及激光制样系统的迅速发展和在同位素分析中广泛应用；②同位素测量仪器的自动化和电脑化；③分析方法和结果标准化；④新的同位素分析方法的开拓。同位素分馏机制方面最突出的进展是对与质量无关的同位素分馏的研究。已发现这种分馏在大气化学反应中和前太阳系阶段起重要作用。同时对热力学和动力学同位素分馏研究正进一步系统化。同位素应用方面，地球表面圈层研究受到更多注意。与资源与环境问题直接相关的研究更受到特别重视。对硼、硅、氯和锂等同位素新方法的地质应用也进展突出。     

同位素示踪技术在地质研究中的某些应用

文中介绍了同位素示踪技术的基本原理,回顾了利用氢氧同位素示踪成矿溶液来源,研究水岩反应所取得的重要成果和最新进展;提出水岩相互作用是一个连续的演化过程,可分三个阶段,不同阶段的交换温度、水／岩比及体系的平衡、封闭情况是不一样的。以太古宙条带状硅铁建造为例,介绍了硅同位素示踪技术的新进展,证明了太古宙条带状硅铁建造为海底喷气成因,条带状构造与海底热液的周期性喷发有关。文中还介绍了氦同位素在示踪幔源组份方面的最新进展,指出我国东部油气区天然气中的氦、氩、二氧化碳等有相当一部分来自地幔;根据太平洋中部多金属结核的氦同位素组成和分布特征,提出成矿物质主要来源于海底热液活动,结核的一个圈层可能代表了一次大的海底热液活动。     

山东蓬家夼金矿硫铅碳氧同位素地球化学

位于山东胶莱盆地东北缘的蓬家夼金矿，受基底元古宙荆山群变质杂岩中的低角度层间滑动断层控制，金矿化类型属蚀变构造碎裂－角砾岩型。同位素地球化学研究表明，蓬家夼金矿硫同位素组成与胶东其它典型金矿相似，以富集^34S为特征，矿石硫一般稍高于老地层和中生代花岗岩的δ^34S值，反映大气降水循环淋滤作用使硫同位素发生了一定程度的分馏。铅同位素组成范围变化大，表现为异常铅特征。蓬家夼金矿床碳酸盐矿物碳氧同位素组成不同于胶东金青顶、三山岛等金矿，介于岩浆碳酸岩与荆山群大理岩范围之间，说明蓬家氚金矿的碳质来源于沉积碳酸盐岩和深源热液的混合。矿床地球化学特征显示了成矿物质的多源性，这与该矿床形成时所处的构造边缘环境有关。     

北秦岭东段峡河岩群中斜长角闪岩Sm—Nd同位素年龄

北秦岭造山带东段的牌楼沟地区，峡河岩群中的斜长角闪岩测得全岩Ｓｍ－Ｎｄ同位素等时线年龄为１６０５±７６Ｍａ，钕同位素初台比值为εＮｄ＝＋５．３，表明峡河岩群变质岩系形成于中元古代。斜长角闪岩来源于亏损的地幔源区。     

粘土矿物稳定同位素地球化学研究现状

本文对有关粘土矿物稳定同位素地球化学目前主要的研究成果和研究过程中存在的障碍作了较为系统的总结分析,主要内容包括：提取单矿物时杂质的处理和分离方法、粘土矿物氢氧同位素组成和同位素分馏机制以及分馏系数方程等。重要的是,论述了进行上述各方面研究时还存在的问题,希望能为有关研究人员提供一定的研究思路,以提高我国在相近领域研究的学术水平。     

同位素地球化学发展趋势

本文回顾了近年来国际国内同位素地球化学发展的趋势,指出了国内同位素地球化学研究的新的生长点、研究思路和研究方向,展望了国内同位素地球化学发展前景。     

滇黔交界地区玄武岩铜矿同位素地球化学特征

对滇黔交界地区玄武岩铜矿进行了同位素地球化学示踪。铜矿石中沥青艿δ^13CPDB为-33.1‰～-30.9‰，炭质δ^13CPDB为-23.2‰～-20.2‰；方解石的δ^13CPDB一般为-13.5‰～-19.4‰，δ^13CPDB一般为19.0‰～23.5‰；石英流体包裹体水泐为-69‰～-89‰，δ^18O石英SMOW为15.7‰～17.4‰，与其平衡的流体的δ^18O水SMOW为2.2‰～3.9‰；各种矿物的铅同位素组成一般为^206Pb/^204 17.855～18.923，^207Pb/^204 Pb 15.503～15.694，^208Pb/^204Pb 38.293～39.036；辉铜矿δ^34SCDT为19.2‰和20.7‰。这些特征综合显示峨眉山玄武岩铜矿的成矿作用与盆地流体的对流循环及从玄武岩中萃取成矿物质有关，有机质对成矿流体的还原和对成矿物质的吸附作用可能是成矿的重要机制。     

吉黑东部花岗岩类的稳定同位素组成

吉黑东部花岗岩类分布广泛，形成历史漫长，岩石类型齐全。按成因类型划分，以I型为主，A型次之，S型很少，M型极少。其稳定同位素组成独特，大多数岩体锶初始值低，氧同位素正常或低，铅同位素以低^204Pb和高^206Pb/^204Pb、^207Pb/^204Pb、^208Pb/^204Pb为主，钕初始值高，说明其基底岩石成熟度低，地幔物质混入量较多。这些特点明显区别于华北和华南的花岗岩。     

水同位素技术示踪地下水活动

水同位素技术已成为研究地下水的重要手段，有助于从宏观和微观上阐明地下水活动机理。本文分别从环境同位素和人工放射性同位素示踪地下水活动，阐述水同位素技术在解决地下水资源问题所起的独特的作用。     

Activity variations attending tungsten skarn formation,Pine Creek,California

An integrated geochemical analysis of the well-exposed Pine Creek, California tungsten skarn deposit has been undertaken to evaluate changes in chemical gradients across various lithologies. Thermodynamic calculations using available experimental and thermodynamic data allow limits to be assigned to the activities of important chemical components in the metasomatic environment. Quantifiable changes in “non-volatile” component activites (CaO, MgO, Al₂O₃, Fe₂O₃, WO₃) and in fugacities (O₂, F₂) have been traced across the system. The activities of Al₂O₃, Fe₂O₃ and WO₃ generally increase from the marble (<10², <10⁻⁶, <10⁻⁵ respectively), through the outer skarn zone and into the massive garnet skarn (10^−1.7±0.3, 10^−3.4±0.4, 10^−4.8±0.1) While CaO and MgO activities decrease for the same traverse from 10⁻⁵ and 10^−2.1±1 respectively, to <10^−5.7 and <10⁻³. Calculated oxygen fugacities are 10^−23.5+1.0 at T=800 K (527° C), about one log unit below QFM, and more reducing than that required by Mt-Py-Po. The high variance of the garnet-pyroxene-quartz assemblages adds sufficient uncertainty to the calculated activities for individual specimens that only the large-scale trends survive the small-scale scatter. None of the chemical variables emerge as major independent or controlling factors for the mineralogy or phase compositions. Changes in the activity of one component may be offset by compensatory changes in another resulting in an environment that, while different from Pine Creek, could still host scheelite mineralization. Mass balance calculations indicate that the exposed endoskarn cannot have supplied the necessary chemical components to convert the country rock to skarn.     

Constraints on the mineralization processes of the Makeng iron deposit,eastern China: Fluid inclusion,H–O isotope and magnetite trace element analysis

The Makeng iron deposit is located in the Yong’an-Meizhou depression belt in Fujian Province, eastern China. Both skarn alteration and iron mineralization are mainly hosted within middle Carboniferous-lower Permian limestone. Five paragenetic stages of skarn formation and ore deposition have been recognized: Stage 1, early skarn (andradite–grossular assemblage); Stage 2, magnetite mineralization (diopside–magnetite assemblage); Stage 3, late skarn (amphibole–chlorite–epidote–johannsenite–hedenbergite–magnetite assemblage); Stage 4, sulfide mineralization (quartz–calcite–fluorite–chlorite–pyrite–galena–sphalerite assemblage); and Stage 5, carbonate (quartz–calcite assemblage). Fluid inclusion studies were carried out on inclusions in diopside from Stage 2 and in quartz, calcite, and fluorite from Stage 4.Halite-bearing (Type 1) and coexisting two-phase vapor-rich aqueous (Type 3) inclusions in the magnetite stage display homogenization temperatures of 448–564 °C and 501–594 °C, respectively. Salinities range from 26.5 to 48.4 and 2.4 to 6.9 wt% NaCl equivalent, respectively. Two-phase liquid-rich aqueous (Type 2b) inclusions in the sulfide stage yield homogenization temperatures and salinities of 182–343 °C and 1.9–20.1 wt% NaCl equivalent. These fluid inclusion data indicate that fluid boiling occurred during the magnetite stage and that fluid mixing took place during the sulfide stage. The former triggered the precipitation of magnetite, and the latter resulted in the deposition of Pb, Zn, and Fe sulfides. The fluids related to magnetite mineralization have δ¹⁸O_fluid-VSMOW of 6.7–9.6‰ and δD of −96 to −128‰, which are interpreted to indicate residual magmatic water from magma degassing. In contrast, the fluids related to the sulfide mineralization show δ¹⁸O_fluid-VSMOW of −0.85 to −1.04‰ and δD of −110 to −124‰, indicating that they were generated by the mixing of magmatic water with meteoric water. Magnetite grains from Stage 2 exhibit oscillatory zoning with compositional variations in major elements (e.g., SiO₂, Al₂O₃, CaO, MgO, and MnO) from core to rim, which is interpreted as a self-organizing process rather than a dissolution-reprecipitation process. Magnetite from Stage 3 replaces or crosscuts early magnetite, suggesting that later hydrothermal fluid overprinted and caused dissolution and reprecipitation of Stage 2 magnetite. Trace element data (e.g., Ti, V, Ca, Al, and Mn) of magnetite from Stages 2 and 3 indicate a typical skarn origin.     

Geochemistry and mineralization age of magnesian skarn-type iron deposits of the Janggun mine, Republic of Korea

The Janggun iron deposits, Republic of␣Korea, occur as lens-shaped magnesian skarn, magnetite and base-metal sulfide orebodies developed in the Cambrian Janggun Limestone Formation. Mineralization stage of the deposits can be divided into two separate events. The skarn stage (107 Ma) consists of magnetite, pyrrhotite, base-metal sulfides, carbonates and magnesian skarn minerals. The hydrothermal stage (70 Ma) consists of base-metal sulfides, native bismuth, bismuthinite, tetrahedrite, boulangerite, bournonite and stannite. Mineral assemblages, chemical compositions and thermodynamic considerations indicate that formation temperatures, −log fs₂ and −log fo₂ values of ore fluids from the skarn stage were 433 to 345 °C, 8.1 to 9.7 bar and 29.4 to 31.6 bar, and the hydrothermal stage was 245 to 315 °C, 10.4 to 13.2 bar and 33.6 to 35.4 bar, respectively. Thermochemical considerations indicate that the XCO₂ during magnesian skarnization ranged from 0.06 to 0.09, and the activity of H⁺ presumably decreased when the fluids equilibrated with host dolomitic limestone which resulted in a pH change from about 6.1 to 7.8, and decreases in fo₂ and fs₂. The δ³⁴S values of ore sulfides have a wide range from 3.2 to 11.6 ‰ (CDT). Calculated ³⁴SH_{2 S} values of ore fluids are 2.9 to 5.4 ‰ (skarn stage) and 8.7 to 13.5 ‰ (hydrothermal stage). These are interpreted to represent an initial deep-seated, igneous source of sulfur which gave way to influence of oxidized sedimentary sulfur to hydrothermal stage. The δ¹³C values of carbonates in ores range from −4.6 to −2.5 ‰ (PDB). It is likely that carbon in the ore fluids was a mixture of deep-seated magmatic carbon and dissolved carbon of dolomitic limestone. The δ¹⁸OH_{2 O} and δD values (SMOW) of water in the ore fluids were 14.7 to 1.8 and −85 to −73 ‰ during the skarn stage and 11.1 to −0.2 and −87 to −80 ‰ in the hydrothermal stage. Received: 5 March 1997 / Accepted: 28 August 1997     

Geochemical Characteristics,Origin, and Evolution of Ore‐Forming Fluids of the Khut Copper Skarn Deposit,West of Yazd in Central Iran

The Khut copper skarn deposit is located at about 50 km northwest of Taft City in Yazd province in the middle part of the Urumieh‐Dokhtar magmatic arc. Intrusion of granitoid of Oligocene–Miocene age into carbonate rocks of the Triassic Nayband Formation led to the formation of marble and a calcic skarn. The marble contains high grade Cu mineralization that occurs mainly as open space filling and replacement. Cu‐rich sulfide samples from the mineralized marble are also anomalous in Au, Zn, and Pb. In contrast, the calcic skarn is only weakly anomalous in Cu and W. The calcic skarn is divided into garnet skarn and garnet–pyroxene skarn zones. Paragenetic relationships and microthermometric data from fluid inclusions in garnet and calcite indicate that the compositional evolution of skarn minerals occurred in three main stages as follows. (i) The early prograde stage, which is characterized by Mg‐rich hedenbergite (Hd_53.7Di_42.3–Hd_86.1Di_9.5) with Al‐bearing andradite (69.8–99.5 mol% andradite). The temperature in the early prograde skarn varies from 400 to 500°C at 500 bar. (ii) The late prograde stage is manifested by almost pure andradite (96.2–98.4 mol% andradite). Based on the fluid inclusion data from garnet, fluid temperature and salinity in this stage is estimated to vary from 267 to 361°C and from 10.1 to 21.1 wt% NaCl equivalent, respectively. Pyrrhotite precipitation started during this stage. (iii) The retrograde stage occurs in an exoskarn, which consists of an assemblage of ferro‐actinolite, quartz, calcite, epidote, chlorite, sphalerite, pyrite, and chalcopyrite that partially replaces earlier mineral assemblages under hydrostatic conditions during fracturing of the early skarn. Fluids in calcite yielded lower temperatures (T < 260°C) and fluid salinity declined to ～8 wt% NaCl equivalent. The last stage mineralization in the deposit is supergene weathering/alteration represented by the formation of iron hydroxide, Cu‐carbonate, clay minerals, and calcite. Sulfur isotope data of chalcopyrite (δ³⁴S of +1.4 to +5.2‰) show an igneous sulfur source. Mineralogy and mineral compositions of the prograde assemblage of the Khut skarn are consistent with deposition under intermediately oxidized and slightly lower fS₂ conditions at shallow crustal levels compared with those of other typical Fe‐bearing Cu–Au skarn systems.     

Isotopic constraints on fluid evolution and precipitation mechanisms for the Boléo Cu–Co–Zn district, Mexico

Stable and radiogenic isotope composition of stratiform Cu–Co–Zn mineralization and associated sedimentary rocks within the Boléo district of the Miocene Santa Rosalía basin, Baja California Sur, constrains the evolution of seawater and hydrothermal fluids and the mechanisms responsible for sulfide and oxide deposition. Stable isotope geochemistry of limestone and evaporite units indicates a strong paleogeographic influence on the chemistry of the water column. Near-shore limestone at the base of the Boléo Formation is characterized by modified marine carbon (δ ¹³C_PDB=−6.0 to +4.4‰) and oxygen (δ ¹⁸O_SMOW=+19.5 to +26.2‰) isotope composition due to the influx of ¹³C- and ¹⁸O-depleted fluvial water. Sulfate sulfur isotope composition (δ ³⁴S_CDT=+17.21 to +22.3‰ and δ ¹⁸O_SMOW=+10.7 to +13.1‰) for basal evaporite and claystone facies are similar to Miocene seawater. Strontium isotopes are less radiogenic than expected for Miocene seawater due to interaction with volcanic rocks. Low S/C ratios, high Mn contents and sedimentological evidence indicate the basin water column was oxidizing. The oxygenated basin restricted sulfide precipitation to within the sedimentary pile by replacement of early diagenetic framboidal pyrite and pore-space filling by Cu–Co–Zn sulfides to produce disseminated sulfides. Quartz–Mn oxide oxygen isotope geothermometry constrains mineralization temperature between 18 and 118°C. Sulfur isotopes indicate the following sources of sulfide: (1) bacterial sulfate reduction within the sedimentary pile produced negative δ ³⁴S values (<−20‰) in framboidal pyrite; and (2) bacterial sulfate reduction at high temperature (80–118°C) within the sedimentary pile during the infiltration of the metal-bearing brines produced Cu–Co–Zn sulfides with negative, but close to 0‰, δ ³⁴S values. Isotope modeling of fluid-rock reaction and fluid mixing indicates: (1) sedimentary and marine carbonates (δ ¹³C=−11.6 to −3.2‰ and δ ¹⁸O=+19.0 to +21.8‰) precipitated from basin seawater/pore water that variably mixed with isotopically depleted meteoric waters; and (2) hydrothermal calcite (δ ¹³C=−7.9 to +4.3‰ and δ ¹⁸O=+22.1 to +25.8‰) formed by dissolution and replacement of authigenic marine calcite by downward-infiltrating metalliferous brine and brine-sediment exchange, that prior to reaction with calcite, had mixed with isotopically depleted pore water. The downward infiltration of metalliferous brine is inferred from lateral and stratigraphic metal distributions and from the concentration of Cu sulfides along the upper contact of pyrite-bearing laminae. The co-existence and textural relationships among framboidal pyrite, base metal sulfides, carbonate and Mn–Fe oxides (including magnetite) within mineralized units are consistent with carbonate replacement and high-temperature bacterial reduction within the sedimentary pile occurring simultaneously below a seawater column under predominantly oxygenated conditions.     

Genesis of the Saishitang skarn type copper deposit,West Qinling,Qinghai Province: Evidence from fluid inclusions and stable isotopes

The Saishitang skarn type copper deposit, located in the southeast part of the Dulan–Ela Mountain Triassic volcanic–magmatic arc and forearc accretionary wedge, belongs to the Tongyugou–Saishitang tin–copper polymetallic ore field in West Qinling, Qinghai province. Based on the contact/crosscutting relationships, mineral associations and mineralization characteristics, hydrothermal fluid evolution can be divided into three stages: skarn (I), quartz sulfide (II) and polymetallic sulfide-bearing quartz–calcite vein (III). The quartz sulfide stage (II) can be further divided into a massive sulfide stage (II-1) and a layered sulfide stage (II-2). This paper presents detailed analysis of fluid inclusions, H–O, S and Pb isotope compositions of rock samples from each of the above three stages as well as analysis of fluid inclusions from quartz diorite. The homogenization temperature, salinity, density and pressure of fluid inclusions in quartz diorite and typical transparent minerals showed a tendency of gradual decline in these evolutionary stages. The ore-forming fluid can be classified as a Na⁺–Ca^2 +–SO₄^2 − Cl⁻ system with a minor proportion of a Na⁺–Ca^2 +–NO₃⁻ SO₄^2 − system, which likely resulted from mixing of magmatic and formation water. The H–O isotope composition indicates that the proportion of formation water increased during the ore-forming process, and meteoric water was mixed in the late quartz–calcite vein stage. The δ³⁴S (CDT) values (− 6.45–5.57‰) and Pb isotope compositions show that the ore-forming materials were mainly derived from magmatic fluid. Ore-forming fluid was boiling during the main ore-forming stage (II-1) due to pressure decrease. Consequently, the physical and chemical conditions (i.e., pH, Eh, f_O2, f_S2) changed, and metallic elements (including Cu) in the fluid could no longer exist in the form of complexes and precipitated from the fluid. According to the integrated analysis of ore features, mineral associations, alteration characteristics, ore-forming environment and fluid evolutionary process, it is concluded that the Saishitang deposit is a typical skarn deposit.     

Origin of the Zálesí U–Ni–Co–As–Ag/Bi deposit,Bohemian Massif,Czech Republic: fluid inclusion and stable isotope constraints

The Zálesí vein-type deposit is hosted by Early Paleozoic high-grade metamorphic rocks on the northern margin of the Bohemian Massif. The mineralization is composed of three main stages: uraninite, arsenide, and sulfide. The mineral assemblages formed at low temperatures (~80 to 130°C, locally even lower) and low pressures (<100 bars). The salinity of the aqueous hydrothermal fluids (0 to 27 wt.% salts) and their chemical composition vary significantly. Early fluids of the oldest uraninite stage contain a small admixture of a clathrate-forming gas, possibly CO₂. Salinity correlates with oxygen isotope signature of the fluid and suggests mixing of brines [δ ¹⁸O around +2‰ relative to standard mean ocean water (SMOW)] with meteoric waters (δ ¹⁸O around −4‰ SMOW). The fluid is characterized by highly variable halogen ratios (molar Br/Cl = 0.8 × 10⁻³ to 5.3 × 10⁻³; molar I/Cl = 5.7 × 10⁻⁶ to 891 × 10⁻⁶) indicating a dominantly external origin for the brines, i.e., from evaporated seawater, which mixed with iodine-enriched halite dissolution brine. The cationic composition of these fluids indicates extensive interaction of the initial brines with their country rocks, likely associated with leaching of sulfur, carbon, and metals. The brines possibly originated from Permian–Triassic evaporites in the neighboring Polish Basin, infiltrated into the basement during post-Variscan extension and were finally expelled along faults giving rise to the vein-type mineralization. Cenozoic reactivation by low-salinity, low-δ ¹⁸O (around −10‰ SMOW) fluids of mainly meteoric origin resulted in partial replacement of primary uraninite by coffinite-like mineral aggregates.     

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。     

Fluid Inclusions,Stable Isotopes,and Geochronology of the Haobugao Lead‐Zinc Deposit,Inner Mongolia,China

The Haobugao deposit, located in the southern segment of the Great Xing'an Range, is a famous skarn‐related Pb‐Zn‐(Cu)‐(Fe) deposit in northern China. The results of our fluid inclusion research indicate that garnets of the early stage (I skarn stage) contain three types of fluid inclusions (consistent with the Mesozoic granites): vapor‐rich inclusions (type LV, with V_H2O/(V_H2O + L_H2O) < 50 vol %, and the majority are 5–25 vol %), liquid‐rich two‐phase aqueous inclusions (type VL, with V_H2O/(V_H2O + L_H2O) > 50 vol %, the majority are 60–80 vol %), and halite‐bearing multiphase inclusions (type SL). These different types of fluid inclusions are totally homogenized at similar temperatures (around 320–420°C), indicating that the ore‐forming fluids of the early mineralization stage may belong to a boiling fluid system. The hydrothermal fluids of the middle mineralization stage (II, magnetite‐quartz) are characterized by liquid‐rich two‐phase aqueous inclusions (type VL, homogenization temperatures of 309–439°C and salinities of 9.5–14.9 wt % NaCl eqv.) that coexist with vapor‐rich inclusions (type LV, homogenization temperatures of 284–365°C and salinities of 5.2–10.4 wt % NaCl eqv.). Minerals of the late mineralization stage (III sulfide‐quartz stage and IV sulfide‐calcite stage) only contain liquid‐rich aqueous inclusions (type VL). These inclusions are totally homogenized at temperatures of 145–240°C, and the calculated salinities range from 2.0 to 12.6 wt % NaCl eqv. Therefore, the ore‐forming fluids of the late stage are NaCl‐H₂O‐type hydrothermal solutions of low to medium temperature and low salinity. The δD values and calculated δ¹⁸O_SMOW values of ore‐forming fluids of the deposit are in the range of ?4.8 to 2.65‰ and ?127.3‰ to ?144.1‰, respectively, indicating that ore‐forming fluids of the Haobugao deposit originated from the mixing of magmatic fluid and meteoric water. The S‐Pb isotopic compositions of sulfides indicate that the ore‐forming materials are mainly derived from underlying magma. Zircon grains from the mineralization‐related granite in the mining area yield a weighted ²⁰⁶Pb/²³⁸U mean age of 144.8 ±0.8 Ma, which is consistent with a molybdenite Re‐Os model age (140.3 ±3.4 Ma). Therefore, the Haobugao deposit formed in the Early Cretaceous, and it is the product of a magmatic hydrothermal system.     

The origin of the Tongkeng-Changpo tin deposit,Dachang metal district,Guangxi, China: clues from fluid inclusions and He isotope systematics

Tongkeng-Changpo is the largest tin deposit within the giant Dachang polymetallic tin ore field in Guangxi, southern China, which is part of a large skarn system associated with Cretaceous granitoids. The Tongkeng-Changpo mineralization consists of veins and stockworks in the upper levels and replacement stratiform orebodies (mantos) at lower levels. Based on textural relationships, three major mineralizing stages can be recognized: stage I with cassiterite, sulphides, stannite, tourmaline, and quartz; stage II with cassiterite, sulphides, sulphosalts, quartz, and calcite; and stage III with calcite as the main phase. The study of fluid inclusions has shown that there are two main fluid types: CO₂ and NaCl-H₂O. Homogenization temperatures are 270 to 365°C, 210 to 240°C, and 140 to 190°C for stages I, II, and III, respectively. Salinities range from 1 to 7 wt.% NaCl equiv. in the early ore stage and 3 to 10 wt.% NaCl equiv. in the late stages. Laser Raman Spectroscopy indicates that the inclusion fluids in stages I and II were of carbono-aqueous composition, with minor amounts of CH₄ and H₂S, whereas those in stage III were aqueous. Helium isotopic analyses of inclusion fluids indicate that the ³He/⁴He ratios in the ore veins are in between 1.2 to 2.9 Ra (Ra = 1.4 × 10⁻⁶, modern atmospheric ratio), and range from 1.6 to 2.5 Ra in the stratiform orebodies. This range of ³He/⁴He ratios is significantly higher than that of crustal fluids (0.01–0.05 Ra). The similar characteristics of fluid inclusions and their He isotopic composition, as well as age constraints, indicate that the ore veins and stratiform orebodies of the Tongkeng-Changpo deposit formed from the same hydrothermal system, likely related to granite intrusions of the Mesozoic Yanshanian tectono-thermal event. In addition, the high R/Ra ratios indicate a mantle contribution in the ore fluids.