Geology and geochemistry of telluride-bearing Au deposits in the Pingyi area, Western Shandong, China

Summary Telluride-bearing gold deposits of the Pingyi area, western Shandong, China, are located on the southeastern margin of the North China Craton. There are two main types of deposits: (i) mineralized cryptoexplosive breccia, e.g., Guilaizhuang; and (ii) stratified, finely-disseminated mineralization hosted in carbonate rocks, e.g., Lifanggou and Mofanggou deposits. In Guilaizhuang, the cryptoexplosive breccia is formed within rocks of the Tongshi complex and Ordovician dolomite. The mineralization is controlled by an E–W-trending listric fault. Stratified orebodies of the Lifanggou and Mofanggou deposits are placed along a NE-trending, secondary detachment zone. They are hosted within dolomitic limestone, micrite and dolomite of the Early-Middle Cambrian Changqing Group. The mineralization in the ore districts is considered to be related to the Early Jurassic Tongshi magmatic complex that formed in a continental arc setting on the margin of the North China Craton. The host rocks are porphyritic and consist predominantly of medium- to fine-grained diorite and pyroxene (hornblende)-bearing monzonite. SHRIMP U–Pb zircon dating of diorites give a ²⁰⁶Pb/²³⁸U weighted mean age of 175.7 ± 3.8 Ma. This is interpreted as representing the crystallization age of the Tongshi magmatic complex. Considering the contact relationships between the magmatic and host sedimentary rocks, as well as the genetic link with the deposits, we conclude that this age is relevant also for the formation of mineralization in the Pingyi area. We hence consider that the deposits formed in the Jurassic. The principal gold minerals are native gold, electrum and calaverite. Wall-rock alteration comprises pyritization, fluoritization, silicification, carbonatization and chloritization. Fluid inclusion studies indicate that all the analyzed inclusions are of two-phase vapor–liquid NaCl–H₂O type. Homogenization temperatures of the fluid inclusions vary from 103 °C to 250 °C, and the ice melting temperatures range from −2.5 °C to −13.5 °C, corresponding to a salinity range of 4.65 to 17.26 wt.% NaCl equiv. The δ³⁴S values of pyrite associated with gold mineralization exhibit a narrow range of −0.71 to + 2.99‰, implying that the sulfur was probably derived from the mantle and/or dioritic magma. The δ¹³C_PDB values of the fluid inclusions in calcite range from −7.3 to 0.0‰. The δ¹⁸O_SMOW values of vein quartz and calcite range from 11.5 to 21.5‰, corresponding to δ¹⁸O_fluid values of −1.1 to 10.9‰; δD values of the fluid inclusions vary between −70 and −48‰. The isotope data for all three deposits suggest mixing of ore-forming fluids derived from the mantle and/or magma with different types of fluids at shallow levels. Pressure release and boiling of the fluids, as well as fluid-rock interaction (Lifanggou and Mofanggou) and mixing of magmatically-derived fluids with meteoritic waters (Guilaizhuang) played an important role in the ore-forming processes.     

Gold ore-forming fluids of the Tanami region, Northern Australia

Fluid inclusion studies have been carried out on major gold deposits and prospects in the Tanami region to determine the compositions of the associated fluids and the processes responsible for gold mineralization. Pre-ore, milky quartz veins contain only two-phase aqueous inclusions with salinities ≤19 wt% NaCl eq. and homogenization temperatures that range from 110 to 410°C. In contrast, the ore-bearing veins typically contain low to moderate salinity (<14 wt% NaCl eq.), H₂O + CO₂ ± CH₄ ± N₂-bearing fluids. The CO₂-bearing inclusions coexist with two-phase aqueous inclusions that exhibit a wider range of salinities (≤21 wt% NaCl eq.). Post-ore quartz and carbonate veins contain mainly two-phase aqueous inclusions, with a last generation of aqueous inclusions being very CaCl₂-rich. Salinities range from 7 to 33 wt% NaCl eq. and homogenization temperatures vary from 62 to 312°C. Gold deposits in the Tanami region are hosted by carbonaceous or iron-rich sedimentary rocks and/or mafic rocks. They formed over a range of depths at temperatures from 200 to 430°C. The Groundrush deposit formed at the greatest temperatures and depths (260–430°C and ≤11 km), whereas deposits in the Tanami goldfield formed at the lowest temperatures (≥200°C) and at the shallowest depths (1.5–5.6 km). There is also evidence in the Tanami goldfield for late-stage isothermal mixing with higher salinity (≤21 wt% NaCl eq.) fluids at temperatures between 100 and 200°C. Other deposits (e.g., The Granites, Callie, and Coyote) formed at intermediate depths and at temperatures ranging from 240 to 360°C. All ore fluids contained CO₂ ± N₂ ± CH₄, with the more deeply formed deposits being enriched in CH₄ and higher level deposits being enriched in CO₂. Fluids from deposits hosted mainly by sedimentary rocks generally contained appreciable quantities of N₂. The one exception is the Tanami goldfield, where the quartz veins were dominated by aqueous inclusions with rare CO₂-bearing inclusions. Calculated δ ¹⁸O values for the ore fluids range from 3.8 to 8.5‰ and the corresponding δD values range from −89 to −37‰. Measured δ ¹³C values from CO₂ extracted from fluid inclusions ranged from −5.1 to −8.4‰. These data indicate a magmatic or mixed magmatic/metamorphic source for the ore fluids in the Tanami region. Interpretation of the fluid inclusion, alteration, and structural data suggests that mineralization may have occurred via a number of processes. Gold occurs in veins associated with brittle fracturing and other dilational structures, but in the larger deposits, there is also an association with iron-rich rocks or carbonaceous sediments, suggesting that both structural and chemical controls are important. The major mineralization process appears to be boiling/effervescence of a gas-rich fluid, which leads to partitioning of H₂S into the vapor phase resulting in gold precipitation. However, some deposits also show evidence of desulfidation by fluid–rock interaction and/or reduction of the ore-fluid by fluid mixing. These latter processes are generally more prevalent in the higher crustal-level deposits.     

The Rožná uranium deposit (Bohemian Massif, Czech Republic): shear zone-hosted, late Variscan and post-Variscan hydrothermal mineralization

Three major mineralization events are recorded at the Rožná uranium deposit (total mine production of 23,000 t U, average grade of 0.24% U): (1) pre-uranium quartz-sulfide and carbonate-sulfide mineralization, (2) uranium, and (3) post-uranium quartz-carbonate-sulfide mineralization. (1) K–Ar ages for white mica from wall rock alteration of the pre-uranium mineralization style range from 304.5 ± 5.8 to 307.6 ± 6.0 Ma coinciding with the post-orogenic exhumation of the Moldanubian orogenic root and retrograde-metamorphic equilibration of the high-grade metamorphic host rocks. The fluid inclusion record consists of low-salinity aqueous inclusions, together with H₂O-CO₂-CH₄, CO₂-CH₄, and pure CH₄ inclusions. The fluid inclusion, paragenetic, and isotope data suggest that the pre-uranium mineralization formed from a reduced low-salinity aqueous fluid at temperatures close to 300°C. (2) The uraniferous hydrothermal event is subdivided into the pre-ore, ore, and post-ore substages. K–Ar ages of pre-ore authigenic K-feldspar range from 296.3 ± 7.5 to 281.0 ± 5.4 Ma and coincide with the transcurrent reorganization of crustal blocks of the Bohemian Massif and with Late Stephanian to Early Permian rifting. Massive hematitization, albitization, and desilicification of the pre-ore altered rocks indicate an influx of oxidized basinal fluids to the crystalline rocks of the Moldanubian domain. The wide range of salinities of fluid inclusions is interpreted as a result of the large-scale mixing of basinal brines with meteoric water. The cationic composition of these fluids indicates extensive interaction with crystalline rocks. Chlorite thermometry yielded temperatures of 260°C to 310°C. During this substage, uranium was probably leached from the Moldanubian crystalline rocks. The hydrothermal alteration of the ore substage followed, or partly overlapped in time, the pre-ore substage alteration. K–Ar ages of illite from ore substage alteration range from 277.2 ± 5.5 to 264.0 ± 4.3 Ma and roughly correspond with the results of chemical U–Pb dating of authigenic monazite (268 ± 50 Ma). The uranium ore deposition was accompanied by large-scale decomposition of biotite and pre-ore chlorite to Fe-rich illite and iron hydrooxides. Therefore, it is proposed that the deposition of uranium ore was mostly in response to the reduction of the ore-bearing fluid by interaction with ferrous iron-bearing silicates (biotite and pre-ore chlorite). The Th data on primary, mostly aqueous, inclusions trapped in carbonates of the ore substage range between 152°C and 174°C and total salinity ranges over a relatively wide interval of 3.1 to 23.1 wt% NaCl eq. Gradual reduction of the fluid system during the post-ore substage is manifested by the appearance of a new generation of authigenic chlorite and pyrite. Chlorite thermometry yielded temperatures of 150°C to 170°C. Solid bitumens that post-date uranium mineralization indicate radiolytic polymerization of gaseous and liquid hydrocarbons and their derivatives. The origin of the organic compounds can be related to the diagenetic and catagenetic transformation of organic matter in Upper Stephanian and Permian sediments. (3) K–Ar ages on illite from post-uranium quartz-carbonate-sulfide mineralization range from 233.7 ± 4.7 to 227.5 ± 4.6 Ma and are consistent with the early Tethys-Central Atlantic rifting and tectonic reactivation of the Variscan structures of the Bohemian Massif. A minor part of the late Variscan uranium mineralization was remobilized during this hydrothermal event.     

Gold mineralization in the Mazowe area, Harare-Bindura-Shamva greenstone belt, Zimbabwe: II. Genetic relationships deduced from mineralogical, fluid inclusion and stable isotope studies, and the Sm-Nd isotopic composition of scheelites

In the Mazowe area some 40 km NW of Harare in Zimbabwe, gold mineralization is hosted in a variety of lithologies of the Archean Harare-Bindura-Shamva greenstone belt, in structures related to the late Archean regional D2/3 event. Conspicuous mineralzogical differences exist between the mines; the mainly granodiorite-hosted workings at Mazowe mine are on pyrite-rich reefs, mines of the Bernheim group have metabasalt host rocks and are characterized by arsenopyrite-rich ores, and Stori's Golden Shaft and Alice mine, both in metabasalts, work sulfide-poor quartz veins. In contrast to the mineralogical diversity, near-identical fluid inventories were found at the different mines. Both H₂O-CO₂-CH₄ fluids of low salinity, and highly saline fluids are present and are regarded to indicate fluid mixing during the formation of the deposits. Notably, these fluid compositions in the Mazowe gold field markedly contrast to ore fluids “typical” of Archean mesothermal gold deposits on other cratons. Stable isotope compositions of quartz from the various deposits (δ¹⁸O=10.8 to 13.2‰ SMOW), calcite (δ¹⁸O=9.5 to 11.9‰ SMOW and δ¹³C=−3.2 to −8.0‰ PDB), inclusion water (δD=−28 to −40‰ SMOW) and sulfides (δ³⁴S=1.3 to 3.2‰ CDT) are uniform within the range typical for Archean lode gold deposits worldwide. The fluid and stable isotope compositions support the statement that the mineralization in the Mazowe gold field formed from relatively reduced fluids with a “metamorphic” signature during a single event of gold mineralization. Microthermometric data further indicate that the deposits formed in the PT range of 1.65–2.3 kbar and 250–380 °C. Ages obtained by using the Sm/Nd and Rb/Sr isotope systems on scheelites are 2604 ± 84 Ma for the mineralization at Stori's Golden Shaft mine, and 2.40 ± 0.20 Ga for Mazowe mine. The Archean age at Stori's is regarded as close to the true age of gold mineralization in the area, whereas the Proterozoic age at Mazowe mine probably reflects later resetting. Received: 30 September 1998 / Accepted: 17 August 1999     

四川盆地北缘马元铅锌矿床与古油藏关系研究

马元铅锌矿床是四川盆地北缘重要的铅锌矿床之一,赋矿地层为震旦系灯影组,野外可见大量的沥青与铅锌矿共生/伴生,镜下见早期沥青与闪锌矿、方铅矿共生,沥青的生物标志物与郭家坝组烃源岩极为相似。物质来源方面,铅锌成矿主要的物质来源和两期油气成藏的物质来源均为下寒武统郭家坝组;时间关系方面,铅锌成矿的共生矿物Rb-Sr同位素等时线年龄为(486.7±3.1)Ma,仅可能与第一期古油藏形成、破坏有关。闪锌矿和重晶石包裹体中含有CH4和少量H2S、沥青,表明铅锌成矿流体中富含有古油藏裂解产物。生烃、排烃、初次运移萃取出"双源层"中的铅锌元素,第一期古油藏裂解和短暂TSR作用产生的还原性硫为铅锌络合物沉淀提供了物质基础,第一期古油藏最终完全破坏残留下早期沥青。     

Constraints on the genesis of gold mineralization at the Homestake Gold Deposit, Black Hills, South Dakota from rhenium–osmium sulfide geochronology

The Homestake gold deposit, located in the Black Hills, South Dakota, USA, is one of the largest known hydrothermal gold deposits globally, with total mining production exceeding 40 Moz Au. Rhenium–osmium geochronology of ore-associated arsenopyrite and pyrrhotite was performed in an effort to delineate the timing of gold mineralization in relation to known tectonothermal events in the northern Black Hills. Arsenopyrite yields a rhenium–osmium (Re–Os) age of 1,736 ± 8 Ma (mean squared weighted deviation = 1.6), consistent with existing age constraints for gold mineralization, whereas Re–Os pyrrhotite data are highly scattered and do not yield a meaningful mineralization age. This is taken to indicate that the Re–Os arsenopyrite chronometer is robust to at least 400°C, whereas the Re–Os pyrrhotite chronometer is likely disturbed by temperatures of 300–350°C. The Re–Os arsenopyrite age and initial Os ratio (0.28 ± 0.15) are interpreted to indicate that gold was introduced at ca. 1,730 Ma, coincident with the onset of exhumation of crustal blocks and, possibly, the earliest intrusive phases of Harney Peak granite magmatism. New in situ U–Pb monazite analyses from an aplite dike in the east-central Black Hills indicate that granite magmatism was a protracted event, persisting until at least ca. 1,690 Ma.     

Mineralogical and fluid inclusion studies of low-sulfidation epithermal veins at Osilo (Sardinia), Italy

Several precious metal-bearing, low sulfidation epithermal veins occur in the rolling topography of the Osilo area, northern Sardinia. The Sa Pala de Sa Fae and the Sa Pedra Bianca veins were subject to intense diamond drilling exploration in the mid 1990 s. The veins extend for 1–3 km, dip steeply, and range from 1 to 10 m in width. High K-calc-alkaline volcanic deposits containing plagioclase phenocrysts (along with lesser pyroxene, amphibole, magnetite, olivine and sanidine) form the main host rocks. Gold grades in drill intersections range from <0.1 to <20 ppm, with silver-gold ratios of around 4 to 7. Mineralogical studies show a systematic distribution of three hydrothermal mineral assemblages. At distances >50 m from the vein, the assemblage albite + Fe-chlorite + illite + pyrite (± montmorillonite ± calcite ± K-feldspar) prevails regionally, and its formation is attributed to minor metasomatism of the country rock involving the addition of water, carbon dioxide and hydrogen sulfide. At distances <10 m from the vein, the assemblage quartz + K-feldspar + pyrite ± illite dominates, forming an alteration envelope that cross cuts regional alteration. Quartz and K-feldspar increase in abundance towards the vein. Quartz is the main vein mineral, and it displays a range of morphologies and textures including crustiform colloform banding, quartz pseudomorphs of platy calcite, breccias and coarse euhedral crystals. Electrum and argentite which are the main gold and silver minerals deposited during the early stages of vein mineralization with rhomb-shaped crystals of K-feldspar (adularia). Pyrite, plus lesser marcasite, arsenopyrite, stibnite and sphalerite, are the other sulfide phases in veins. Kaolinite ± halloysite ± jarosite form a late assemblage overprinting earlier hydrothermal alteration. It is mostly restricted to shallow depths of a few meters, except near veins. Most of this assemblage likely formed from weathering and oxidation of sulfides. Microthermometric measurements were made on quartz-hosted, two-phase (liquid + vapor) inclusions, containing ∼75% liquid; mean homogenization temperatures (∼750 measurements) range from 220 to 250 °C, and ice-melting temperatures (∼550 measurements) range from 0.0 to −2.3 °C. The presence of co-existing vapor-rich and liquid rich inclusions, with quartz pseudomorphs of platy calcite, indicate that boiling conditions existed. Slight vapor-bubble expansion of a few fluid inclusions subjected to crushing experiments indicates inclusion fluids contained variable but low concentrations of dissolved gas. This study shows that gold-silver mineralization formed in subvertical channels from ascending solutions at 250 °C at around 300 to 450 m below the paleo-water table in a typical low-sulfidation epithermal environment. Hydrothermal solutions that produced vein mineralization and related alteration were dilute (<4.1 equivalent wt.% NaCl and <4 wt.% CO₂), near neutral pH, reduced and, at times, boiling. Received: 19 May 1998 / Accepted: 8 March 1999     

The Malanjkhand copper ( + molybdenum) deposit, India: mineralization from a low-temperature ore-fluid of granitoid affiliation

  Copper and subordinate molybdenum mineralization at Malanjkhand occurs within a fracture-controlled quartz-reef enclosed in a pink granitoid body surrounded by grey-granitoids constituting the regional matrix. Sulfide-bearing stringers, pegmatites with only quartz + microcline and sulfide disseminations, all within the pink-granitoid, represent other minor modes of occurrences. Despite this diversity in mode of occurrence, the mineralogy of ores is quite consistent and conform to a common paragenetic sequence comprising an early `ferrous' stage of precipitation of magnetite (I) and pyrite (I) and, the main-stage chalcopyrite mineralization with minor sphalerite, pyrite (II), magnetite (II), molybdenite and hematite. Both stages witnessed continuous precipitation of quartz ± microcline ± (chlorite, biotite and epidote). The enclosing pink-granitoid and the regional grey-granitoids display alteration features such as saussuritization of plagioclase, breakdown of hornblende and chloritization of biotite on a regional scale, indicating interaction with a pervasive fluid. Quartz and microcline precipitation mostly restricted within the pink granitoid, postdates this alteration. Four types of primary inclusions were encountered in quartz from ore samples: (1) type-I – aqueous-biphase(L + V) inclusions, the commonest variety in all ore types; (2) type-II – aqueous-carbonic(L_aq + L_carb ± V_carb); (3) type-III – pure-carbonic(L_carb ± V_carb) – type-II and III being restricted to stringer and pegmatitic ores, and (4) rare polyphase (L_aq + V_aq + calcite/gypsum) inclusions. Quartz in granitoids contain primary type-I inclusions only. Type-I inclusions from ore samples furnish a temperature range (after a rough pressure correction to the T _H  -maxima of 140–180 °C) of 150–275 °C and a moderately low salinity of 4–12 wt.% NaCl equivalent. This is inferred to represent the signature of the major component (F2) of the ore fluid. A few type-I inclusions of higher T _M (up to 380 °C) and low salinity and density represent the other (F1) identifiable component of the ore fluid present in low proportion. The T _H  -maxima and the total range in salinity of type-I inclusions in quartz from granitoids are strikingly similar to those from the ore samples. Composition of syn-ore chlorites furnished a temperature range of 185–327 °C, which conforms to the fluid inclusion microthermometric data. Pressure estimates using standard fluid inclusion geobarometric methods, vary from 550 to 1790 bar in the stringer ores. Observed temperature-salinity/density relationships are best explained by a two-stage evolution model of the ore fluid: the first stage witnessed mixing of the two components, F1 and F2 in unequal proportion, bringing about mineralization. The second stage of evolution was marked by the separation of a carbonic component on continued sulfide precipitation and attendant increase in salinity of the fluid. The F1 component emerged as a distinct, heated and (CO₂ + S)-charged entity due to steam-heating and contamination of the early-ingressed F2 fluid at the fracture zone. The pervasive fluid phase in the surrounding granitoids contributed the F2 component. Received: (10 August 1994), 15 August 1995 / Accepted: 12 January 1996     

The evolution of the hydrothermal IOCG system in the Mantoverde district,northern Chile: new evidence from microthermometry and stable isotope geochemistry

New microthermometric data combined with stable isotope geochemistry and paragenetic relationships support a previously suggested cooling–mixing model for the iron oxide–copper–gold mineralization in the Mantoverde district. Fluid inclusions show characteristics of a CO₂-bearing aqueous NaCl ± CaCl₂ salt system. The evolution of the Mantoverde hydrothermal system is characterized by (1) an early hypersaline, high to moderate temperature fluid; (2) a moderate saline, moderate temperature fluid; and (3) a low saline, moderate to low temperature fluid. Early magnetite formation took place at median temperatures of 435.0°C, whereas hematite formed at median temperatures of 334.4°C. The main sulfide mineralization texturally post-dates the iron oxides and occurred before late-stage calcite, which developed at a median temperature of 244.8°C. Boiling occurs only locally and is of no relevance for the ore formation. The microthermometric and stable isotope data are supportive for a fluid cooling and mixing model, and suggestive for a predominantly magmatic–hydrothermal fluid component during the iron oxide and main sulfide mineralization. Thereafter, the incursion of a nonmagmatic fluid of ultimately meteoric or seawater gains more importance.     

Fluid inclusion characteristics of mesothermal gold deposits in the Xiaoqinling district, Shaanxi and Henan Provinces, People's Republic of China

Fluid inclusions were studied in quartz samples from early (stage I) gold-poor quartz veins and later (stage II) gold- and sulphide-rich quartz veins from the Wenyu, Dongchuang, Qiangma, and Guijiayu mesothermal gold deposits in the Xiaoqinling district, China. Fluid inclusion petrography, microthermometry, and bulk gas analyses show remarkably consistent fluid composition in all studied deposits. Primary inclusions in quartz samples are dominated by mixed CO₂-H₂O inclusions, which have a wide range in CO₂ content and coexist with lesser primary CO₂-rich and aqueous inclusions. In addition, a few secondary aqueous inclusions are found along late-healed fractures. Microthermometry and bulk gas analyses suggest hydrothermal fluids with typically 15–30 mol% CO₂ in stage I inclusions and 10–20 mol% CO₂ in stage II inclusions. Estimates of fluid salinity decrease from 7.4–9.2 equivalent wt.% NaCl to 5.7–7.4 equivalent wt.% NaCl between stage I and II. Primary aqueous inclusions in both stages show consistent salinity with, but slightly lower Th _total than, their coexistent CO₂-H₂O inclusions. The coexisting CO₂-rich, CO₂-H₂O, and primary aqueous inclusions in both stage I and II quartz are interpreted to have been trapped during unmixing of a homogeneous CO₂-H₂O parent fluid. The homogenisation temperatures of the primary aqueous inclusions give an estimate of trapping temperature of the fluids. Trapping conditions are typically 300–370 °C and 2.2 kbar for stage I fluids and 250–320 °C and 1.6 kbar for stage II fluids. The CO₂-H₂O stage I and II fluids are probably from a magmatic source, most likely devolatilizing Cretaceous Yanshanian granitoids. The study demonstrates that gold is largely deposited as pressures and temperatures fall accompanying fluid immiscibility in stage II veins. Received: 15 May 1997 / Accepted: 10 June 1998     

Mineralogical, fluid inclusion, and stable isotope constraints on mechanisms of ore deposition at the Samgwang mine (Republic of Korea)—a mesothermal, vein-hosted gold–silver deposit

The Samgwang mine is located in the Cheongyang gold district (Cheonan Metallogenic Province) of the Republic of Korea. It consists of eight massive, gold-bearing quartz veins that filled NE- and NW-striking fractures along fault zones in Precambrian granitic gneiss of the Gyeonggi massif. Their mineralogy and paragenesis allow two separate vein-forming episodes to be recognized, temporally separated by a major faulting event. The ore minerals occur in quartz and calcite of stage I, associated with fracturing and healing of veins. Hydrothermal wall-rock alteration minerals of stage I include Fe-rich chlorite (Fe/(Fe+Mg) ratios 0.74-0.81), muscovite, illite, K-feldspar, and minor arsenopyrite, pyrite, and carbonates. Sulfide minerals deposited along with electrum during this stage include arsenopyrite, pyrite, pyrrhotite, sphalerite, marcasite, chalcopyrite, galena, argentite, pyrargyrite, and argentian tetrahedrite. Only calcite was deposited during stage II. Fluid inclusions in quartz contain three main types of C–O–H fluids: CO₂-rich, CO₂–H₂O, and aqueous inclusions. Quartz veins related to early sulfides in stage I were deposited from H₂O–NaCl–CO₂ fluids (1,500–5,000 bar, average 3,200) with T _htotal values of 200°C to 383°C and salinities less than about 7 wt.% NaCl equiv. Late sulfide deposition was related to H₂O–NaCl fluids (140–1,300 bar, average 700) with T _htotal values of 110°C to 385°C and salinities less than about 11 wt.% NaCl equiv. These fluids either evolved through immiscibility of H₂O–NaCl–CO₂ fluids as a result of a decrease in fluid pressure, or through mixing with deeply circulated meteoric waters as a result of uplift or unloading during mineralization, or both. Measured and calculated sulfur isotope compositions (δ³⁴S_H2S = 1.5 to 4.8‰) of hydrothermal fluids from the stage I quartz veins indicate that ore sulfur was derived mainly from a magmatic source. The calculated and measured oxygen and hydrogen isotope compositions (δ¹⁸O_H2O = −5.9‰ to 10.9‰, δD = −102‰ to −87‰) of the ore-forming fluids indicate that the fluids were derived from magmatic sources and evolved by mixing with local meteoric water by limited water–rock exchange and by partly degassing in uplift zones during mineralization. While most features of the Samgwang mine are consistent with classification as an orogenic gold deposit, isotopic and fluid chemistry indicate that the veins were genetically related to intrusions emplaced during the Jurassic to Cretaceous Daebo orogeny.     

The timing of epigenetic gold mineralization on the Baie Verte Peninsula, Newfoundland, Canada: new evidence from Re–Os pyrite geochronology

The absolute timing of epigenetic mineralization, including most types of gold deposits, is difficult to resolve due to the absence of suitable minerals in veins and replacement zones. However, gold is commonly closely associated with pyrite and arsenopyrite, which may be amenable to Re–Os geochronology, providing sufficient Re and Os are present within them. This short paper outlines the use of this method to date two gold deposits in Newfoundland using pyrite. Although the Os contents of the pyrites are extremely low (≪0.1 ppb), the Os is almost exclusively radiogenic ¹⁸⁷Os, and data are amenable to model age calculations, as used in Re–Os molybdenite dating. The pyrites from these deposits correspond to low-level highly radiogenic sulphides, as defined by other studies. The Stog’er Tight and Pine Cove gold deposits yield mean Re–Os model ages of 411 ± 7 Ma (n = 4) and 420 ± 7 Ma (n = 5), respectively, which agree with isochron regression of ¹⁸⁷Os against ¹⁸⁷Re. The Re–Os age for Stog’er Tight is within uncertainty of a previous U–Pb age from ‘hydrothermal’ zircon (420 ± 5 Ma) in spatially related alteration. A latest Silurian–earliest Devonian age for the mineralization is consistent with indirect age constraints from some other gold deposits in central Newfoundland and suggests a broad temporal link to the mid-Silurian Salinic Orogeny. However, the gold mineralization appears to be younger than most plutonic activity associated with this event. The results illustrate the potential value of Re–Os pyrite geochronology in understanding the temporal framework of epigenetic mineralization, especially if future improvements in analytical precision and reductions in procedural blanks allow wider application to material with similarly low Re and Os concentrations.     

Archaean gold mineralization synchronous with late cratonization of the Western Dharwar Craton,India: 2.52 Ga U–Pb ages of hydrothermal monazite and xenotime in gold deposits

New zircon U–Pb ages for a felsic volcanic rock (2,588 ± 10 Ma) and an intrusive granite (≥2,555 ± 6 Ma) in the Gadag greenstone belt in the Western Dharwar Craton, southern India, are similar to dates for equivalent rocks in the Eastern Dharwar Craton and indicates docking of the two cratons prior to this time. The zircons in the intrusive granite are strongly overprinted, and coexisting titanites yielded two different age populations: the dominant group gives an age of 2,566 ± 7 Ma, interpreted as the emplacement age, whereas the minor group gives an age of 2,516 ± 10 Ma, reflecting a hydrothermal overprint. In situ U–Pb dating of monazite and xenotime in gold reefs of the Gadag (2,522 ± 6 Ma) and Ajjanahalli (2,520 ± 9 Ma) gold deposits reveal a previously undated episode of gold mineralization at 2.52 Ga, substantially younger than the 2.55 Ga Hutti deposit in the eastern Dharwar Craton. The new dates confirm that both the younger greenstone belts and lode gold mineralization in the Dharwar Craton are about 100–120 My, younger than in other well-dated Archaean cratons. Although gold mineralization across the craton postdates most of the magmatic activity and metamorphism at upper crustal levels, widespread thermal reworking of the lower-middle crust, involving partial melting, metamorphism, and lower crustal granitoid intrusion, occurred concurrently with gold mineralization. It is likely that the large-scale hydrothermal fluid flow that produced widespread gold deposition was also part of this tectono-thermal event during the final stages of cratonization of the Dharwar Craton in southern India.     

The La Unión Au ± Cu prospect,Camagüey District,Cuba: fluid inclusion and stable isotope evidence for ore-forming processes

The Camagüey district, Cuba, is known for its epithermal precious metal deposits in a Cretaceous volcanic arc setting. Recently, the La Unión prospect was discovered in the southern part of the district, containing gold and minor copper mineralization interpreted as porphyry type. Mineralization is hosted in a 73.0 ± 1.5 Ma calc–alkaline I-type oxidized porphyry quartz diorite intrusive within volcanic and volcaniclastic rocks of the early Cretaceous Guáimaro Formation. The porphyry is affected by propylitic alteration and crosscut by a network of quartz and carbonate veinlets and veins. Chlorite, epidote, sericite, quartz, and pyrite are the main minerals in the early veins which are cut by late carbonate and zeolite veins. Late barite pseudomorphously replaces pyrite. Gold is associated with pyrite as disseminations in the altered quartz diorite and in the veins, occurring as inclusions or filling fractures in pyrite with 4 g/t Au in bulk samples, and up to 900 ppm Au in in pyrite. Fluid inclusion and oxygen isotope data are consistent with a H₂O–NaCl–(KCl) mineralizing fluid, derived from the quartz diorite magma, and trapped at least at 425°C and 1.2 kbar. This primary fluid unmixed into two fluid phases, a hypersaline aqueous fluid and a low-salinity vapor-rich fluid. Boiling during cooling may have played an important role in metal precipitation. Pyrite δ³⁴S values for the La Unión prospect range between 0.71‰ and 1.31‰, consistent with a homogeneous magmatic sulfur source. The fluids in equilibrium with the mineralized rocks have estimated δ¹⁸O values from 8‰ to 11.8‰, calculated for a temperature range of 480–505°C. The tectonic environment of the La Unión prospect, its high gold and low copper contents, the physical–chemical characteristics of the mineralizing fluids and the isotopic signature of the alteration minerals and fluids indicate that the La Unión gold mineralization is similar to the porphyry gold type, even though the ore-related epidote–chlorite alteration can be classified as propylitic and not the classic potassic and/or phyllic alteration. The low copper contents in the prospect could be due to a mineralizing fluid previously saturated in copper, which is indicated by trapped chalcopyrite crystals in high-temperature fluid inclusions. The low-temperature paragenesis, represented by carbonate, zeolite and barite, indicates epithermal overprint. The study shows the potential for other gold porphyry-type deposits in the Cretaceous volcanoplutonic arc of Cuba.     

Fluid inclusion microthermometry and the P-T evolution of gold-bearing hydrothermal fluids in the Niuxinshan gold deposit, eastern Hebei province, NE China

Niuxinshan is a typical example of the numerous mesothermal gold deposits formed during Mesozoic tectono-magmatic reactivation of the Archean North China Craton in eastern Hebei province. Gold occurs in quartz-sulfide lodes in Archean amphibolites and also in greisen zones in the Mesozoic Niuxinshan granite stock. Four mineralization stages can be recognized from early to late: (1) quartz-K-feldspar, (2) quartz-pyrite, (3) quartz-polysulfide, and (4) quartz-carbonate. Gold mineralization mainly occurs in stages 2 and 3. Fluid inclusions in quartz and fluorite from greisen zones in the Niuxinshan granite, and inclusions in vein quartz and sphalerite from stages 1 to 3 in the amphibolites, have been studied by microthermometry. Three compositional types of inclusions are recognized: type 1 (Tp1) are H₂O-CO₂-bearing inclusions and include primary (Tp1-P) and secondary (Tp1-S) inclusions. These are found in quartz and fluorite from the greisen zones as well as in vein quartz and sphalerite from stages 1 to 3. The Tp1-P inclusions are considered to represent the gold-bearing hydrothermal fluids. Type 2 (Tp2-S) are secondary H₂O-CO₂ + solid phase inclusions in fluorite from the greisen zones. Type 3 (Tp3-S) are secondary aqueous inclusions with a solid phase which coexist with the Tp2-S in fluorite from the greisen zones. The Tp1-P inclusions show variable V_CO2 (commonly 0.3 to 0.6) and X_CO2 values (mainly 0.1 to 0.4). The salinities of inclusions cluster around 3 to 11 wt.% NaCl equivalent and their homogenization temperatures to the liquid phase (Th(L)) fall dominantly in the range of 260 to 360 °C. The compositional variations of inclusions in stage 1 probably result from exsolution of magmatic fluids at various stages; immiscibility or boiling of the fluids can be ruled out. The compositional variations of inclusions in the greisen zones and in vein stages 2 and 3 are attributed to cooling, mixing (dilution), and necking-down of the fluids. The Tp1-S and Tp2-S inclusions show salinities of 3 to 6 wt.% NaCl equivalent and X_CO2 values of 0.04 to 0.17. Th(L) clusters at 240 to 260 °C. The Tp3-S inclusions have salinities of 3 to 6 wt.% NaCl equivalent and Th(L) of 170 to 240 °C. Isochoric reconstructions, combined with oxygen and sulfur isotope geothermometry of mineral pairs, give trapping P-T conditions for the gold-bearing fluids. The greisen zones formed at 310 to 460 °C and 1.3 to 3.7 kbar; stage 1 veins at 300 to 430 °C and 1.2 to 3.7 kbar; stage 2 veins at 290 to 380 °C and 1 to 3 kbar; stage 3 veins at 250 to 350 °C and 1 to 3 kbar. H₂O-CO₂ fluids with low to moderate salinities and moderate to high densities (0.66 to 1.01 g/cm³) dominated at early mineralization stages, and evolved towards H₂O-richer and CO₂- and less saline fluids through time. The retrograde P-T evolution probably resulted from regional uplift and cooling of gold-bearing hydrothermal fluids. The gold bisulfide complex was dominant in the fluids during mineralization and gold deposition was mainly induced by decreases of temperature and pressure, as well as destabilization of the bisulfide complex during sulfidization of wall rocks. Received: 16 March 1998 / Accepted: 11 January 1999     

Archean high-Mg monzodiorite–syenite,epidote skarn,and biotite–sericite gold lodes in the Granny Smith–Wallaby district,Australia: U–Pb and Re–Os chronometry of two intrusion-related hydrothermal systems

The Granny Smith (37 t Au production) and Wallaby deposits (38 t out of a 180 t Au resource) are located northeast of Kalgoorlie, in 2.7 Ga greenstones of the Eastern Goldfields Province, the youngest orogenic belt of the Yilgarn craton, Western Australia. At Granny Smith, a zoned monzodiorite–granodiorite stock, dated by a concordant titanite–zircon U–Pb age of 2,665 ± 3 Ma, cuts across east-dipping thrust faults. The stock is fractured but not displaced and sets a minimum age for large-scale (1 km) thrust faulting (D2), regional folding (D1), and dynamothermal metamorphism in the mining district. The local gold–pyrite mineralization, controlled by fractured fault zones, is younger than 2,665 ± 3 Ma. In augite–hornblende monzodiorite, alteration progressed from a hematite-stained alkali feldspar–quartz–calcite assemblage and quartz–molybdenite–pyrite veins to a late reduced sericite–dolomite–albite assemblage. Gold-related monazite and xenotime define a U–Pb age of 2,660 ± 5 Ma, and molybdenite from veins a Re–Os isochron age of 2,661 ± 6 Ma, indicating that mineralization took place shortly after the emplacement of the main stock, perhaps coincident with the intrusion of late alkali granite dikes. At Wallaby, a NE-trending swarm of porphyry dikes comprising augite monzonite, monzodiorite, and minor kersantite intrudes folded and thrust-faulted molasse. The conglomerate and the dikes are overprinted by barren (<0.01 g/t Au) anhydrite-bearing epidote–actinolite–calcite skarn, forming a 600-m-wide and >1,600-m-long replacement pipe, which is intruded by a younger ring dike of syenite porphyry pervasively altered to muscovite + calcite + pyrite. Skarn and syenite are cut by pink biotite–calcite veins, containing magnetite + pyrite and subeconomic gold–silver mineralization (Au/Ag = 0.2). The veins are associated with red biotite–sericite–calcite–albite alteration in adjacent monzonite dikes. Structural relations and the concordant titanite U–Pb age of the skarn constrain intrusion-related mineralization to 2,662 ± 3 Ma. The main-stage gold–pyrite ore (Au/Ag >10) forms hematite-stained sericite–dolomite–albite lodes in stacked D2 reverse faults, which offset skarn, syenite, and the biotite–calcite veins by up to 25 m. The molybdenite Re–Os age (2,661 ± 10 Ma) of the ore suggests a genetic link to intrusive activity but is in apparent conflict with a monazite–xenotime U–Pb age (2,651 ± 6 Ma), which differs from that of the skarn at the 95% confidence level. The time relationships at both gold deposits are inconsistent with orogenic models invoking a principal role for metamorphic fluids released during the main phase of compression in the fold belt. Instead, mineralization is related in space and time to late-orogenic, magnetite-series, high-Mg monzodiorite–syenite intrusions of mantle origin, characterized by Mg/(Mg + Fe_TOTAL) = 0.31–0.57, high Cr (34–96 ppm), Ni (22–63 ppm), Ba (1,056–2,321 ppm), Sr (1,268–2,457 ppm), Th (15–36 ppm), and rare earth elements (total REE: 343–523 ppm). At Wallaby, shared Ca–K–CO₂ metasomatism and Th-REE enrichment (in allanite) link Au–Ag mineralization in biotite–calcite veins to the formation of the giant epidote skarn, implicating a Th + REE-rich syenite pluton at depth as the source of the oxidized hydrothermal fluid. At Granny Smith, lead isotope data and the Rb–Th–U signature of early hematite-bearing wall-rock alteration point to fluid released by the source pluton of the differentiated alkali granite dikes.     

Relative timing of deformation and two-stage gold mineralization at the Hutti Mine, Dharwar Craton, India

Gold mineralization at Hutti is confined to a series of nine parallel, N–S to NNW–SSE trending, steeply dipping shear zones. The host rocks are amphibolites and meta-rhyolites metamorphosed at peak conditions of 660±40°C and 4±1 kbar. They are weakly foliated (S₁) and contain barren quartz extension veins. The auriferous shear zones (reefs) are typically characterized by four alteration assemblages and laminated quartz veins, which, in places, occupy the entire reef width of 2–10 m, and contain the bulk of gold mineralization. A <1.5 m wide distal chlorite-sericite (+biotite, calcite, plagioclase) alteration zone can be distinguished from a 3–5 m wide proximal biotite-plagioclase (+quartz, muscovite, calcite) alteration zone. Gold is both spatially and temporally associated with disseminated arsenopyrite and pyrite mineralization. An inner chlorite-K-feldspar (+quartz, calcite, scheelite, tourmaline, sphene, epidote, sericite) alteration halo, which rims the laminated quartz veins, is characterized by a pyrrhotite, chalcopyrite, sphalerite, ilmenite, rutile, and gold paragenesis. The distal chlorite-sericite and proximal biotite-plagioclase alteration assemblages are developed in microlithons of the S₂–S₃ crenulation cleavage and are replaced along S₃ by the inner chlorite-K-feldspar alteration, indicating a two-stage evolution for gold mineralization. Ductile D₂ shearing, alteration, and gold mineralization formed the reefs during retrograde evolution and fluid infiltration under upper greenschist to lower amphibolite facies conditions (560±60°C, 2±1 kbar). The reefs were reactivated in the D₃ dextral strike-slip to oblique-slip environment by fault-valve behavior at lower greenschist facies conditions (ca. 300–350°C), which formed the auriferous laminated quartz veins. Later D₄ crosscutting veins and D₅ faults overprint the gold mineralization. The alteration mineralogy and the structural control of the deposit clearly points to an orogenic style of gold mineralization, which took place either during isobaric cooling or at different levels of the Archean crust. From overlaps in the tectono-metamorphic history, it is concluded that gold mineralization occurred during two tectonic events, affecting the eastern Dharwar craton in south India between ca. 2550 – 2530 Ma: (1) The assemblage of various terranes of the eastern block, and (2) a tectono-magmatic event, which caused late- to posttectonic plutonism and a thermal perturbation. It differs, however, from the pre-peak metamorphic gold mineralization at Kolar and the single-stage mineralization at Ramagiri. Notably, greenschist facies gold mineralization occurred at Hutti 35–90 million years later than in the western Dharwar craton. Editorial handling: G. Beaudoin     

Sediment-hosted disseminated gold mineralisation at Zarshuran, NW Iran

Mineralisation at the Zarshuran, NW Iran, occurs on the flank of an inlier of Precambrian rocks hosted in black silty calcareous and carbonaceous shale with interbedded dolomite and limestone varying in thickness from 5 to 60 m and extending along strike for approximately 5–6 km. Two major, steeply dipping sets of faults with distinct trends occur in the Zarshuran: (1) northwest (310–325) and (2) southwest (255–265). The main arsenic mineralisation occurs at the intersection of these faults. The mineral assemblage includes micron to angstrom-size gold, orpiment, realgar, stibnite, getchellite, cinnabar, thallium minerals, barite, Au-As-bearing pyrite, base metal sulphides and sulphosalts. Hydrothermal alteration features are developed in black shale and limestone around the mineralisation Types of alteration include: (1) decalcification, (2) silicification, (3) argillisation, (4) dolomitisation, (5) oxidation and acid leaching and (6) supergene alteration. The early stage of mineralisation involved removal of carbonates from the host rocks, followed by quartz precipitation. The main stage includes massive silicification associated with argillic alteration. In the late stage veining became more dominant and the main arsenic ore was deposited along fault cross cuts and gouge. These characteristics are typical of Carlin-type sediment-hosted disseminated gold deposits. The early stage of mineralisation contains only two-phase aqueous fluid inclusions. The main stage has two groups of three-phase CO₂-bearing inclusions with minor CH₄ ± N₂, associated with high temperature, two-phase aqueous inclusions. During the late stage, fluids exhibit a wide range in composition, salinity and temperature, and CH₄ becomes the dominant carbonic fluid with minor CO₂ associated with a variety of two-phase aqueous fluid inclusions. The characteristics of fluids at the Zarshuran imply the presence of at least two separate fluids during mineralisation. The intersections of coexisting carbonic and aqueous inclusion isochores, together with stratigraphic and mineral stability evidence, indicate that mineralisation occurred at 945 ± 445 bar and 243 ± 59 °C, implying a depth for mineralisation of at least 3.8 ± 1.8 km (assuming a lithostatic pressure gradient). Fluid density fluctuations and the inferred depth of formation suggest that the mineralisation occurred at the transition between overpressured and normally pressured regimes. Geochronologic studies utilising K/Ar and Ar/Ar techniques on hydrothermal argillic alteration (whole rock and separated clay size fractions) and on volcanic rocks, indicates that mineralisation at Zarshuran formed at 14.2 ± 0.4 Ma, and was contemporaneous with nearby Miocene volcanic activity, 13.7 ± 2.9 Ma. It is proposed that mineralisation was the result of the infiltration of hydrothermal fluids containing a magmatic gas component, and that it was localised in the Zarshuran Unit because of the redox boundary that it provided and/or because it lay between an overpressured region at depth and a zone of circulating, hydrostatically pressured fluids above. Received: 10 December 1997 / Accepted: 5 March 1999     

Structural controls,temperature–pressure conditions and fluid evolution of orogenic gold mineralisation at the Betam mine,south Eastern Desert,Egypt

The Betam gold deposit, located in the southern Eastern Desert of Egypt, is related to a series of milky quartz veins along a NNW-trending shear zone, cutting through pelitic metasedimentary rocks and small masses of pink granite. This shear zone, along with a system of discrete shear and fault zones, was developed late in the deformation history of the area. Although slightly sheared and boudinaged within the shear zone, the auriferous quartz veins are characterised by irregular walls with a steeply plunging ridge-in-groove lineation. Shear geometry of rootless intra-folial folds and asymmetrical strain shadows around the quartz lenses suggests that vein emplacement took place under a brittle–ductile shear regime, clearly post-dating the amphibolite-facies regional metamorphism. Hydrothermal alteration is pervasive in the wallrock metapelites and granite including sericitisation, silicification, sulphidisation and minor carbonatisation. Ore mineralogy includes pyrite, arsenopyrite and subordinate galena, chalcopyrite, pyrrhotite and gold. Gold occurs in the quartz veins and adjacent wallrocks as inclusions in pyrite and arsenopyrite, blebs and globules associated with galena, fracture fillings in deformed arsenopyrite or as thin, wire-like rims within or around rhythmic goethite. Presence of refractory gold in arsenopyrite and pyrite is inferred from microprobe analyses. Clustered and intra-granular trail-bound aqueous–carbonic (L_CO2 + L_aq ± V_CO2) inclusions are common in cores of the less deformed quartz crystals, whereas carbonic (L_CO2 ± V_CO2) and aqueous H₂O–NaCl (L + V) inclusions occur along inter-granular and trans-granular trails. Clathrate melting temperatures indicate low salinities of the fluid (3–8 wt.% NaCl eq.). Homogenisation temperatures of the aqueous–carbonic inclusions range between 297 and 323°C, slightly higher than those of the intra-granular and inter-granular aqueous inclusions (263–304°C), which are likely formed during grain boundary migration. Homogenisation temperatures of the trans-granular H₂O–NaCl inclusions are much lower (130–221°C), implying different fluids late in the shear zone formation. Fluid densities calculated from aqueous–carbonic inclusions along a single trail are between 0.88 and 0.98 g/cm³, and the resulting isochores suggest trapping pressures of 2–2.6 kbar. Based on the arsenopyrite–pyrite–pyrrhotite cotectic, arsenopyrite (30.4–30.7 wt.% As) associated with gold inclusions indicates a temperature range of 325–344°C. This ore paragenesis constrains f _S2 to the range of 10⁻¹⁰ to 10^−8.5 bar. Under such conditions, gold was likely transported mainly as bisulphide complexes by low salinity aqueous–carbonic fluids and precipitated because of variations in pH and f _O2 through pressure fluctuation and CO₂ effervescence as the ore fluids infiltrated the shear zone, along with precipitation of carbonate and sericite. Wallrock sulphidation also likely contributed to destabilising the gold–bisulphide complexes and precipitating gold in the hydrothermal alteration zone adjacent to the mineralised quartz veins.     

Isotope geochemistry and fluid inclusion study of skarns from Vesuvius

Summary We present new mineral chemistry, fluid inclusion, stable carbon and oxygen, as well as Pb, Sr, and Nd isotope data of Ca-Mg-silicate-rich ejecta (skarns) and associated cognate and xenolithic nodules from the Mt. Somma-Vesuvius volcanic complex, Italy. The typically zoned skarn ejecta consist mainly of diopsidic and hedenbergitic, sometimes “fassaitic” clinopyroxene, Mg-rich and Ti-poor phlogopite, F-bearing vesuvianite, wollastonite, gehlenite, meionite, forsterite, clinohumite, anorthite and Mg-poor calcite with accessory apatite, spinell, magnetite, perovskite, baddeleyite, and various REE-, U-, Th-, Zr- and Ti-rich minerals. Four major types of fluid inclusions were observed in wollastonite, vesuvianite, gehlenite, clinopyroxene and calcite: a) primary silicate melt inclusions (T_HOM = 1000–1050 °C), b) CO₂ ± H₂S-rich fluid inclusions (T_HOM = 20–31.3 °C into the vapor phase), c) multiphase aqueous brine inclusions (T_HOM = 720–820 °C) with mainly sylvite and halite daughter minerals, and d) complex chloride-carbonate-sulfate-fluoride-silicate-bearing saline-melt inclusions (T_HOM = 870–890 °C). The last inclusion type shows evidence for immiscibility between several fluids (silicate melt – aqueous chloride-rich liquid – carbonate/sulfate melt?) during heating and cooling below 870 °C. There is no evidence for fluid circulation below 700 °C and participation of externally derived meteoric fluids in skarn formation. Skarns have considerably variable ²⁰⁶Pb/²⁰⁴Pb (19.047–19.202), ²⁰⁷Pb/²⁰⁴Pb (15.655–15.670), and ²⁰⁸Pb/²⁰⁴Pb (38.915–39.069) and relatively low ¹⁴³Nd/¹⁴⁴Nd (0.51211–0.51244) ratios. The carbon and oxygen isotope compositions of skarn calcites (δ¹³C_V-PDB = −5.4 to −1.1‰; δ18O_V-SMOW = 11.7 to 16.4‰) indicate formation from a ¹⁸O- and ¹³C-enriched fluid. The isotope composition of skarns and the presence of silicate melt inclusion-bearing wollastonite nodules suggests assimilation of carbonate wall rocks by the alkaline magma at moderate depths (< 5 km) and consequent exsolution of CO₂-rich vapor and complex saline melts from the contaminated magma that reacted with the carbonate rocks to form skarns. Received March 1, 2000; revised version accepted November 2, 2000