δ18Ofluid Values Research Articles

Origin and evolution of the ore-forming fluids in the southern Abbas Abad iron skarn deposit, NE Isfahan, Central Iran: Insights from geology, fluid inclusions, and C[sbnd]O isotopes

The southern Abbas Abad iron skarn deposit is located in the western part of the Central Iran zone, SW edge of the Urumieh˗Dokhtar magmatic arc. The skarn alteration occurred mainly at the contact of the Abbas Abad gabbro and Triassic Nayband Formation. The Triassic Nayband Formation in the southern Abbas Abad area consists of limestone, shale, siltstone, and marl with interlayers of tuff. The Abbas Abad gabbro is calc˗alkaline with adakitic signatures and continental arc setting affinity. Four iron ore bodies occur along the NW˗SE trending normal fault along the southern margin of Abbas Abad gabbro within sedimentary host rocks of the Nayband Formation. Three main paragenetic stages of skarnification and ore deposition have been recognized: i) prograde stage, ii) retrograde stage, and iii) supergene stage. Spinel, garnet, and pyroxene formed during the prograde stage. Epidote, tremolite˗actinolite, calcite and ore minerals, such as magnetite, hematite-I, pyrite, and minor chalcopyrite formed the retrograde stage assemblage. Magnetite is the main economic ore mineral, which is diverse in terms of fabric and texture, including massive, disseminated, needle˗like, blade˗like, prismatic, crustification, and cubes. Stable isotope data from Abbas Abad iron skarn deposit indicated that the interaction of non-magmatic brine (connate) fluids with Triassic carbonate host rocks at high temperature and its mixing with meteoric water has led to changes in the isotopic composition of the mineralizing fluid. Subsequently, garnet and magnetite with relatively high estimated fluid δ18O values (average 12.3 and 14.6 ‰, respectively) formed from this modified fluid. The estimated δ18O values of fluid in equilibrium with calcite decreased as a result of the greater influx of the meteoric water into the hydrothermal system in the late stages of ore formation. Fluid inclusion data from this iron skarn deposit are consistent with minor boiling during the prograde stage, although cooling and dilution seem to be principally responsible for Fe precipitation. The petrogenetic model proposed for the Southern Abbas Abad iron skarn deposit suggests that during the Eocene a gabbroic stock intruded the sedimentary rocks of the Nayband Formation and acted as a heat source for the non-magmatic brine (connate and meteoric) fluids convection cells, then reacted with cooler Triassic carbonate rocks. In this model, leaching of the gabbroic rock provided the iron, whereas carbon came from degassing and dissolution of carbonate rocks during contact metamorphism-metasomatism.

Genesis of the Shibaogou Mo–Pb–Zn deposit in the Luanchuan ore district, China: Constraints from geochronology, fluid inclusion, and H–O–S–Pb isotopes

The Shibaogou deposit is located in the Luanchuan ore district within the East Qinling orogenic belt (EQOB), central China, which is a newly discovered Mo–Pb–Zn skarn deposit. The skarn and Mo–Pb–Zn ore bodies are mostly hosted in the contact zones between the Shibaogou porphyritic granite and carbonaceous sedimentary rocks from the Luanchuan and Guandaokou sets. A study combined of geochronology, fluid inclusion (FI), and stable isotopes was performed to constrain the mineralization age, source of ore materials, and the origin and evolution of the ore-forming fluids and their relationship with the subduction of the Paleo-Pacific Plate. The mineralization process includes skarn and quartz–sulfide episodes, which has four stages: skarn (I), quartz–molybdenite (II), quartz–galena–sphalerite (III), and quartz–calcite (IV). Molybdenite Re-Os dating suggests that the deposit was formed in the Late Jurassic (147.4 ± 7.2 Ma). Reportedly, there are five primary types of fluid inclusions: L-type, V-type, H-type, S-type, and C-type. In the skarn stage, coexisting H-type (35.58 wt%–46.05 wt% NaCl equiv.) and low-salinity V-type (0.35 wt%–5.7 wt% NaCl equiv.) fluid inclusions show similar homogenization temperatures, which suggests that fluid boiling occurred at 513–550°C and 580–650 bar (2.19–2.45 km). In the quartz–molybdenite stage, the homogenization temperatures of L-type, V-type, minor H-type, and S-type fluid inclusions indicate continued fluid boiling at 324–387°C and 180–250 bar (0.49–0.94 km). In the quartz–galena–sphalerite stage, a fewer number of coexisting V-type and L-type fluid inclusions in quartz shows different salinities with similar homogenization temperatures, indicating that they are trapped at 303–347°C and <150 bar in the boiling process (<0.56 km hydrostatic depth). The minor primary L-type fluid inclusions that have lower salinities of 0.88 wt%–11.34 wt% NaCl equiv were observed in quartz and calcite in the quartz–calcite stage; in addition, their homogenization temperatures are 103–247°C (typical post-ore conditions). This study found that the ore-forming fluids at the Shibaogou deposit were dominantly magmatic water at the early stage, with input of atmospheric water during fluid evolution, with δ18Ofluid values from −1.168‰ to 8.997‰ and δ18Dfluid values from −106.5‰ to −79.9‰, based on the O and H isotope data from garnet, quartz, and calcite. Furthermore, the S isotopic compositions were measured ranging from 0.8‰ to 14.7‰, and it demonstrated that the ore-forming fluid was mainly derived from magmatic sources. The relatively homogeneous Pb isotopic compositions are similar to those of Shibaogou granite porphyry, which demonstrated that the ore-forming materials were mainly derived from magmatic sources. Molybdenite was precipitated as a result of fluid–rock interactions and fluid boiling, and the galena and sphalerite were precipitated as a result of the decreasing temperature. The subduction of the Paleo-Pacific plate has a critical impact on the complex evolution of ore formation in the Shibaogou skarn deposit in EQOB.

Geology, geochemistry, fluid inclusions, and H–O–C–S–Pb isotope constraints on the genesis of the Atash-Anbar epithermal gold deposit, Urumieh–Dokhtar magmatic arc, central-northern Iran

The newly discovered Atash-Anbar gold deposit, with ∼2 Mt of ore grading 2.13 g/t Au (locally up to 14 g/t), is located in the Buin-Zahra Range, central-northern Iran. Hosted by Middle Eocene (ca. 39.0 Ma) volcanic-subvolcanic rocks, the Au-polymetallic orebodies occur as NW− and NE − trending quartz-sulfide veins that are structurally controlled by NW-trending faults. The Middle Eocene host rocks are characterized by high-K calc-alkaline, peraluminous signatures, and moderately Eu negative anomalies (Eu/Eu* = 0.2–0.6). Four primary paragenetic stages of veining have been recognized: (I) smoky-grey quartz-chalcopyrite stage, (II) grey-white quartz-polysulfide stage, (III) white-pinkish barite-sulfide stage, and (IV) late quartz-carbonate stage. Fluid inclusion investigations coupled with laser Raman analyses indicate that the ore-forming fluids were formed in a NaCl − H2O ± CO2 system with two types of FIs: two phase, liquid-rich inclusions (type I) and two phase, vapor-rich inclusions (type II). The primary coexisting types I and II inclusions are observed in gold-bearing ore-stage II, sharing similar homogenization temperatures in the range of 233–286 °C and 243–281 °C, but contrasting salinity values of 10.7–14.8 and 14.6–15.5 wt% NaCl equivalent, respectively. Fluid inclusion examinations show that fluid phase separation occurred concurrently with gold precipitation during the ore-stage II. The δ18Ofluid values calculated from the δ18OSMOW and δDSMOW values of the inclusions in quartz veins are 4.9–11.1 ‰ and −84.6 ‰ to −68.3 ‰, respectively. δ13CPDB and δ18OSMOW values of Fe-dolomite are −6.5 ‰ to −5.0 ‰ and 3.1 ‰–4.9 ‰, respectively. H–O–C isotope data indicate magmatic origin of initial ore-forming fluids with minor addition of meteoric water through time. The δ34S values (−3.1 ‰ to −1.2 ‰, avg. = −2.0 ‰) of the ore-stage II sulfides suggest that sulfur comes from a homogeneous magmatic source. The Pb isotopic compositions of sulfides (207Pb/204Pb = 15.420–15.525, 206Pb/204Pb = 18.423–18.536, 208Pb/204Pb = 37.135–38.208) indicate that the Pb in the Atash-Anbar gold deposit is a mixture of crust and mantle components. We conclude that (1) the Atash-Anbar is an IS epithermal system that (2) resulted from Eocene subduction of the NeoTethys Ocean plate beneath the central Iran plate and thus (3) ore-forming fluid and its metal components (i.e., Ag, Pb, and Zn) were emanated through crystallization of the final phase of a granite porphyry intrusion.

Multiple fluid sources in skarn systems: Oxygen isotopic evidence from the Haobugao Zn-Fe-Sn deposit in the southern Great Xing’an Range, NE China

Abstract Diverse fluid sources and complex fluid flow paths in skarn systems appear to be well documented. Nevertheless, in situ microanalysis of oxygen isotopes by secondary ion microprobe (SIMS) in skarn minerals can provide further high spatial resolution information on this complexity and the formation of skarns and associated ore deposits. In this study, we investigated the Haobugao skarn Zn-Fe-Sn deposit (0.36 M tonnes Zn) in the southern Great Xing’an Range, northeast (NE) China, and the associated Early Cretaceous Wulanba biotite granite. Based on drill hole logging, four early skarn phases are recognized: proximal red-brown garnet-hedenbergite exoskarn, central green garnet exoskarn, light brown garnet-diopside exoskarn, and distal pyroxene skarn. Oxygen isotope analyses of garnet, pyroxene, and other minerals from skarn, oxide, and quartz-sulfide stages were carried out using SIMS to determine the origin and evolution of the skarn-forming hydrothermal system. Garnet from exoskarn has a much wider range in δ18OVSMOW, between –8.1 and +6.0‰, than other stages and minerals. The estimated δ18O values of fluids in equilibrium with the Haobugao skarn vary widely from –5.1‰ to +8.9‰, suggesting that the skarn formed via episodic flux of magmatic fluid and meteoric water. Low δ18O values of cassiterite and quartz from quartz-sulfide stage rocks are +1.2 to +3.6‰, and +5.7 to +5.9‰, respectively, indicating significant contributions of meteoric water during deposition of Pb-Zn sulphides. Therefore, meteoric fluids were periodically present throughout most of the stages of skarn formation at Haobugao.

Source and Evolution of Subduction–Related Hot Springs Discharged in Tengchong Geothermal Field, Southwest China: Constrained by Stable H, O, and Mg Isotopes

The hydrothermal system plays a crucial role in material and energy cycling between the lithosphere and hydrosphere. In general, seafloor hydrothermal systems are one of important Mg sinks, but the situation may not be the same as it is in terrestrial hydrothermal systems. In addition, the behavior of Mg isotopes during hydrothermal circulation is still unclear. Thus, in this study, we determined the Mg isotopic compositions of the hydrothermal fluids discharged in the Tengchong region to understand better the fate of Mg in the continental hydrothermal system. The δ2H and δ18O values of the Tengchong hydrothermal fluids indicate that the recharge water sources are primary from meteoric water and influenced by the evaporation process. In contrast, the subduction–related volcanic water input is limited, except in for the Rehai area. The Mg in most of the samples is contributed by percolated meteoric water. The measured δ26Mg values range from –0.969 to 0.173‰, which are enriched in light Mg compared to the volcanic rocks of Tengchong. Combined with the precipitation dissolution of carbonate, we calculated the δ26Mg value for the endmember fluid before precipitation, which shows that the process of carbonate precipitation changes the Mg isotope of the fluid, substantially. The Shiqiang (SQ) vent is unique among all of the samples, characterized by an extremely a high δ26Mg value and Mg concentration, and it is estimated that it could have been mixed with an upper crustal material. This also reveals the diversity of the hydrothermal fluid material sources in the subduction zone.

Constraining diagenesis within shallow water carbonate environments: Insights from clumped and sulfur isotopes

The geochemical records of shallow-water carbonate sediments can be influenced by both meteoric and marine diagenesis, where the contribution from each can be difficult to constrain using traditional δ18O and δ13C values. Here sediment cores from Enewetak Atoll have been utilized as a case study, in which clumped isotope-derived temperatures and calculated δ18Ofluid values are combined with the δ34S values of the carbonate-associated sulfate, to differentiate the types of diagenetic alteration which have influenced the geochemical record, thus providing further insights into the extent and timing of carbonate alteration. While sediments at Enewetak Atoll have undergone repeated periods of subaerial exposure, calculated δ18Ofluid values indicate that the amount of meteoric alteration is reliant on the availability of meteoric fluids, as the presence of exposure surfaces alone does not imply that extensive diagenetic alteration has occurred. Additionally, in the deepest zone of the Enewetak cores, it could be determined that the sediments were initially exposed to meteoric fluids, resulting in partial alteration, and later affected by burial and circulation of deep ocean water, resulting in recrystallization within the section, uniformly overprinting the temperature and δ18Ofluid values. The results of this study indicate that by utilizing systematics such as δ34SCAS, ∆47, and δ18Ofluid values, in addition to the more traditional proxies, a detailed understanding of the diagenetic processes which have altered shallow water sediments can be better constrained, allowing for more accurate interpretations of ancient sedimentary records.

Metallogenic model of the Fengyan stratabound Zn-Pb deposit in Fujian of southeastern China: Constraints from fluid inclusions, C[sbnd]O[sbnd]S-Pb isotopes, and pyrite chemistry

The Fengyan stratabound Zn-Pb deposit (total resources: 24.7 Mt of ore @ 4.09 % Zn and 1.92 % Pb) is hosted by Neoproterozoic marble and metavolcanic rocks in the Meixian ore field of southeastern China. Hitherto, the genesis of this deposit has been a matter of debate, including a Neoproterozoic volcanogenic massive sulfide genesis (VMS) or an Early Cretaceous magmatic-hydrothermal origin. Based on fluid inclusion data from various hydrothermal stages, stable isotope data of hydrothermal minerals, and in-situ trace element analyses of poly-stage pyrite (Py1-3), the deposit can be classified as a distal skarn-type. A complex paragenesis reflects four hydrothermal stages: stage I (pyroxene + garnet + Py1), stage II (epidote + hematite + magnetite + Py2), stage III (quartz + sphalerite + galena + Py3a + Py3b), and stage IV (calcite). The prograde skarn comprises andradite-rich garnet (on average Ad57Gr34Py9) and pyroxene (on average Hd58Di10Jo32) with high Mn contents. Fluid inclusion data reveal salinity and temperature ranges for the prograde and retrograde skarn fluids as 11.5 to 14.6 wt% NaCl equiv. at 278 to 338℃ and 7.5 to 11.2 wt% NaCl equiv. at 235 to 275℃, respectively. The entrapment pressure is calculated as 130 bars. The sulfide ore minerals of stage III were precipitated at lower temperatures (186 to 259℃) from aqueous fluids with lower salinities (<6.6 wt% NaCl equiv.). Late-stage calcite was deposited from a more dilute (<2.2 wt% NaCl equiv.) and cooler (<200℃) fluid with extremely depleted δ18Ofluid values of −10.1 to −2.0 ‰ (V-SMOW), consistent with a meteoric origin. A mean δ34SV-CDT of 1.7 ± 0.9 ‰ of sulfide minerals points to a magmatic sulfur source. Lead isotopic compositions of sphalerite separates (206Pb/204Pb = 18.295–18.381, 207Pb/204Pb = 15.644–15.662, and 208Pb/204Pb = 38.494–38.724) are similar to those of the Yanshanian granitic bodies in the region but different from those of the ore-hosting marble and metavolcanic rocks. Calcite in the late stage has δ13CV-PDB values of −5.4 to −1.5 ‰, consistent with a predominant magmatic origin. Variations of Co, Ni, Se, and As in pyrite of different generations are interpreted to reflect changes in temperature during hydrothermal evolution. The Co/Ni ratios in all generations of pyrite, mainly between 1 and 10, indicate a magmatic-hydrothermal source. Overall, our new data indicate progressive mixing of a granite-derived magmatic fluid with meteoric waters, as is expected in a distal skarn deposit.

The mineralization of Daxiao carbonate-hosted Pb-Zn deposit, northeast Yunnan province, SW China: Constraints from Rb-Sr isotopic dating and H[sbnd]O[sbnd]S-Pb isotopes

The Sichuan-Yunnan-Guizhou (SYG) Pb-Zn metallogenic province located in western Yangtze Block contains more than 400 carbonate-hosted Pb-Zn deposits. Uncertain mineralization age and ore-forming material source causing debates on the genesis of these carbonate-hosted Pb-Zn deposits. To address this issue, the Daxiao Pb-Zn deposit was chosen as a case study. The orebodies of Daxiao deposit are hosted in Mesoproterozoic Heishan formation argillaceous dolomite interbedded with slate. Three main mineralization stages have been identified, including syn-sedimentary stage, hydrothermal stage and supergene stage. The ore minerals consist of galena, sphalerite, chalcopyrite, argentite, smithsonite, anglesite and cerussite in the deposit. The early hydrothermal stage sphalerite yielded a Rb-Sr isochron age of 223.5 ± 2.9 Ma, consistent with the spike of Pb-Zn mineralization in SYG metallogenic province. Quartz in hydrothermal stage yields δDSMOW values of −69.6 to −65.8 ‰ and calculated δ18OFluid values of + 3.4 – +5.6 ‰, showing an affinity of metamorphic water. While the quartz in supergene stage yields δDSMOW values of −68.3 to −65.8 ‰ and calculated δ18OFluid values of −4.1 to −1.7 ‰, indicating abundant basinal brine or meteoric water had mixed into ore-forming fluid during its evolution from hydrothermal stage to supergene stage. Sulfur isotopic composition (δ34S = +6.3 – +19.1 ‰) of different stage sulfides reveal that the ore-forming sulfur originated from sulfates within the wall rocks, and mainly reduced by Thermal-chemical reduction (TSR). Moreover, the 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb ratios of early hydrothermal stage sphalerite are 18.282 – 18.312, 15.603 – 15.681and 38.287 – 38.311, respectively. Those of late hydrothermal stage galena are 18.315 – 18.491, 15.676 – 15.721 and 38.314 – 38.326, respectively. All of them plot in the overlapping area of Emeishan flood basalts, Kunyang and Huili groups and Sinian to Triassic strata, indicating that these wall rocks jointly contributed the ore-forming metals for these Pb-Zn deposits in SYG metallogenic province. Hence, a metallogenic model for Daxiao and other carbonate-hosted Pb-Zn deposit in SYG metallogenic province was proposed: In the thrustfold systems caused by the subduction of Paleo-Pacific and the closure of Jinshajiang-Ailaoshan Paleo-Tethys ocean in Late Triassic to Early Jurassic. The fluids originated from metamorphic water migrated along deep faults and extracted ore-forming materials from wall rocks (including Proterozoic basement rocks, Sinian to Triassic sedimentary wall rocks and Permian Emeishan flood basalts), and then precipitated ores in the secondary thrust faults or interlayered fractures within these thrustfold systems.

Origin of the Bada porphyry Cu–Au deposit, eastern Tibet: Geology and isotope geochemistry (C–O–S–Pb) constraints

Bada is a newly discovered Cu–Au deposit located in the south part of the Yulong porphyry Cu (–Mo–Au) belt (YPCB), eastern Tibet. The Cu–Au mineralization, mainly hosted by the alkali-rich quartz monzonite porphyry (QMP), with minor by the fine-grained monzonitic porphyry (MP) and surrounding strata of the Nanxin Formation. The hydrothermal alteration assemblages recognized at Bada mainly include potassic, phyllic, propylitic, and argillic. The Cu–Au mineralization occurs within the potassic and phyllic alteration zones. Three main metallogenic stages have been identified: stage I ore-barren quartz ± dolomite ± K-feldspar ± pyrite veins, stage IIa dolomite ± quartz + pyrite veins, stage IIb dolomite ± quartz + polymetallic sulfide veins, and stage III dolomite ± calcite veins. The dolomite δ13CPDB values from stage I to III range from –2.2 – –1.8 ‰, –6.8 – –3.4 ‰, to –4.2 – 1.9 ‰. Their δ18OSMOW values vary from 7.7 – 9.8 ‰, 6.5 – 11.9 ‰, to 7.8 – 11.4 ‰, respectively. The corresponding calculated δ18Ofluid values ranging from 6.0 to 6.3 ‰ for stage I, and –1.1 to 4.3 ‰ for stage II, and –1.7 to 1.9 ‰ for stage III. The C–O isotopic compositions imply that the ore-forming fluids were dominated by magmatic water with the input of meteoric water. The narrow δ34S value range of sulfides (1.43 to 6.61 ‰, mean 4.64 ‰) reflects a predominantly magmatic origin for the sulfur. The Pb isotopic compositions of most sulfide samples coincide with the alkali-rich porphyries in the region, indicating that the alkalic porphyries provided the main source of Pb. Combined with the zircon U–Pb age of the ore-bearing porphyry (ca. 35.0 Ma), the Bada deposit is defined as a porphyry Cu–Au deposit in a late-collisional setting. The study of the genesis of Bada provides a better understanding of the Cu–Au mineralization potential of the southern YPCB.

High temperature wall-rock alteration zoning in the Sanjin deposit, Hishikari gold mine, Japan: Implication for exploration of mature mining districts

The Sanjin deposit is one of three major ore zones hosted by Pleistocene quartz-adularia veins being mined at the Hishikari low-sulfidation epithermal gold mine, which has produced more than 242 t of gold at an extraordinary average grade since 1985. Hydrothermal alteration zoning of the Sanjin deposit was examined with respect to mineralogy, geochemistry, fluid inclusion microthermometry, and oxygen and hydrogen isotopes. Clay minerals are dominated by interstratified chlorite-smectite (C/S) and interstratified illite-smectite (I/S) with <20% smectite. Epidote and prehnite are recorded for the first time at Hishikari in the southeastern part of the Sanjin deposit, typically coexisting with chlorite. Trapping temperatures of fluid inclusions from associated ore zone quartz veins typically range from 195 to 230 °C, with higher temperatures prevalent in the southeastern part of the Sanjin deposit. The calculated fluid δ18O and δ2H values from clay minerals and quartz cannot be explained by a simple water-rock interaction or a simple fluid mixing model, since variable isotopic exchange temperature and endmembers have to be considered. This suggests that both water-rock interaction and mixing of fluids occurred between dynamically variable end members during mineralization. The average estimated formation temperature of chlorites in the Sanjin deposit using chlorite geothermometry is 233 ± 19 °C, in agreement with the highest temperature zone in Hishikari, estimated by homogenization temperature of fluid inclusions of ore veins. In addition, the estimated formation temperatures of chlorite in epidote- and/or prehnite-rich altered rocks are higher (avg. 240 ± 17 °C) than those in epidote- and/or prehnite-poor altered rocks (avg. 216 ± 9 °C). Thus, the chlorite-epidote/prehnite assemblage can be an index of a high temperature alteration zone in the Sanjin deposit. Considering the position of the paleo-water table of the three ore zones, these factors are consistent with the formation of the Sanjin deposit proximal to the upflow zone responsible for gold mineralization at Hishikari. We suggest that our approach could be utilized to understand the thermal structure of epithermal gold system, which may be important to explore for blind veins at mature mining districts.

Genesis of the Talate Pb–Zn (–Fe) deposit in the Altay, Xinjiang, NW China: Evidence from fluid inclusions and stable isotopes

The Talate Pb–Zn (–Fe) deposit, located in the Kelan volcanic-sedimentary Basin, Chinese Altay (part of Central Asian Orogenic Belt, NW China), is hosted in the second unit of the lower member of the Lower Devonian Kangbutiebao Formation. This deposit has undergone two periods, including submarine exhalative-sedimentary period (I), and magmatic hydrothermal (skarn) period (II), and period II can be further divided into three stages: prograde skarn stage (II-1), retrograde skarn stage (II-2), and quartz–sulfide stage (II-3). Electron microprobe analyses data show that garnet from II-1 stage is consists of solid solutions of almandine, spessartine, pyrope, and grossularite, with small fraction of andradite and uvarovite. Chlorite forming temperature ranging from 163 to 169 °C. L-type fluid inclusions in period I sphalerite has two total homogenizing temperature of 259 °C and 286 °C, respectively. The ore-forming fluids in the II-1 and II-2 stage belong to the H2O–NaCl system with moderate to high temperatures (mainly at 206–368 °C and 202–416 °C, respectively) and moderate–low salinities (7.17–17.34 wt% NaCl eqv. and 6.45–19.21 wt% NaCl eqv., respectively). The II-3 stage ore-forming fluids belong to H2O–CO2 (±CH4/N2)–NaCl system, their temperature and salinity varies widely (193–445 °C, 0.42–37.40 wt% NaCl eqv., respectively). The garnet and quartz samples from II-1 and II-3 stages yield δDV-SMOW and δ18Ofluid values (–79.8 to –64.9‰ and 4.14 to 6.17‰, respectively) and (–80.7 to –67.0‰ and –0.15 to 3.05‰). The microthermometry data combined with stable isotope data indicate ore-forming fluids in II-1 and II-3 stages are derived from primary magmatic water, but in II-3 stage is mixed with more meteoric water. In addition to meteoric water, it is supplied by carbon-rich, high-temperature and high-salinity fluid in II-3 stage. The δ34S values of the period I sulfides range from –11.7‰ to 2.4‰. Sulfur source in Talate deposit derived from the deep magmatic hydrothermal fluid. The Talate Pb–Zn (–Fe) deposit belongs to the volcanogenic massive sulfide (VMS) + skarn type, thus a genetic model is proposed to explain the metallogenic process of the Talate deposit.

Millennial pulses of ore formation and an extra-high Tibetan Plateau

Abstract Quantifying the rhythms and rates of magmatic-hydrothermal systems is critical for a better understanding of their controls on ore formation and the dynamics of magmatic reservoirs that feed them. We reconstructed the evolution of ore-forming fluids using hydrothermal quartz from the 17.4 Ma Zhibula skarn, Tibet. Ion probe analysis reveals sharp and dramatic changes in quartz δ18O values between 5‰ and −9.3‰, with fluid δ18O values varying between 2.8‰ and −18.2‰, which are best explained by transient meteoric water incursion into a hydrothermal system dominated by magmatic fluids. Two pulses of magmatic fluids and a meteoric water incursion event are inferred, which operated at the millennium scale (760−1510 yr) as constrained by the aluminum diffusion chronometer. Our results indicate that magmatic reservoirs are likely water unsaturated for most of their lifetime (&amp;gt;105−106 yr), with transient and episodic fluid exsolutions (~103 yr) being driven by magma replenishment or crystallization-induced water saturation. With focused and efficient metal deposition, multiple pulses of metalliferous fluids favor the formation of giant deposits with high grade. Meteoric water δ18O values (−25.4 ± 2.3‰) derived from Zhibula quartz further suggest a paleo-elevation of 5.9 ± 0.3 km; this transient early Miocene surface uplift plausibly was due to break-off of the oceanic slab attached to the Indian Plate. Our research highlights that ubiquitous hydrothermal quartz in orogenic belts can probe the dynamics of magmatic-hydrothermal systems and also quantify paleo-elevations, which has significant tectonic implications.

“Roll-Front” Mass Transfer of Carbonate Cations in Carlin-Type Gold Deposits: Insights from UV-Fluorescent Calcite Veins

Abstract Hydrothermal fluids dissolve and precipitate vast quantities of carbonate during the formation of Carlin-type gold deposits. This mass transfer results in extensive volumes of decarbonatized rock and hydrothermal calcite with distinct δ18O and δ13C signatures, occurring as both veins and wall-rock pseudomorphs. However, the processes which cause the fluids to switch between carbonate-dissolving and carbonate-precipitating are not well understood. Here, we present a model for the formation of ore-stage calcite through the pseudomorphic replacement of pre-existing calcite based on a detailed study of UV-fluorescent (UVF) calcite veins from the Nadaleen trend Carlin-type gold deposits in Yukon. The UV fluorescence in the veins is a visual indicator of their high-Mn, low-Fe chemistry, which we use to develop our model. The proximity of UVF calcite veins to ore-stage alteration and the realgar within the veins indicate that they formed during Au mineralization. Vein orientation and crosscutting relationships with faults and dikes suggest they formed by replacing pre-existing calcite veins that developed during folding. Cathodoluminescence responses, showing the selective replacement of pre-ore calcite by Mn-rich calcite, support this hypothesis. We propose that differences in Mn, Sr, and Mg content between pre-existing calcite and UVF calcite drive this replacement reaction as the fluids approach calcite saturation. UVF calcite veins have fluid-dominated δ13C and δ18O compositions, suggesting that the fluids dissolved fluid-buffered hydrothermal carbonate upstream along the flow path. In our model, unreactive and high-permeability decarbonatized rock allows upstream fluids to reach reactive calcite at the edge of the dissolution zone, where they replace the calcite with ore-stage calcite pseudomorphs. Over several iterations of this process, the fluids repeatedly precipitate and dissolve ore-stage calcite, moving it downstream as a roll front, which concentrates Mn at the edge of the dissolution zone. We suggest that this mass transfer also occurs at the deposit scale to form zoned carbonate alteration along the flow path. Additionally, we present carbonate clumped isotope thermometry results that indicate UVF calcite veins formed at temperatures of ~140°C, although higher temperatures of up to 300°C are possible if the veins exhumed slowly after their formation. Calculated fluid δ13C and δ18O values at these temperatures suggest a meteoric fluid source, although the isotopic data are not inconsistent with a magmatic source if fluid temperatures were ≥200°C because less isotopic fractionation occurs at higher temperatures.

Variability in effective moisture inferred from inclusion fluid δ18O and δ2H values in a central Sierra Nevada stalagmite (CA)

The oxygen isotopic composition of stalagmites is widely used to infer regional changes in terrestrial surface temperatures and precipitation dynamics. The stalagmite δ18O values, however, record the influence of multiple environmental conditions (e.g., temperature, precipitation source and amount) as well as in-cave physicochemical processes and possible disequilibrium precipitation effects. The δ18O and δ2H values of fluids entombed in stalagmites as inclusions have the potential to be robust proxies of paleo-precipitation δ18O and δ2H. Here we analyze the inclusion-fluid δ18O and δ2H values for a stalagmite from a central Sierra Nevada foothill cave, McLean's Cave, to reconstruct changes in effective moisture (precipitation – evaporation) in the region over the last deglaciation (20–13 ka). The results demonstrate high variability in inclusion-fluid δ18O and δ2H values and further suggest that the δ18O and δ2H values have several intervals driven by disequilibrium dis oxygen isotopic fractionation. These findings demonstrate that effective moisture was likely lower during past warm periods in comparison to some colder periods, consistent with other stalagmite calcite-based paleoclimate records from the southwestern United States.

Metallogeny of the Late Jurassic Qiucun epithermal gold deposit in southeastern China: Constraints from geochronology, fluid inclusions, and H-O-C-Pb isotopes

On the basis of zircon and apatite U-Pb dating, H-O-C-Pb isotopes, and fluid inclusion data, a metallogenic model is established for the Qiucun gold deposit (gold resource > 10 t, average grade: 4.9 g/t) in the Dehua goldfield in the Mesozoic Coastal Volcanic Belt (southeastern China). Three hydrothermal stages can be distinguished within the Late Jurassic volcanic host rocks: Stage I, dominated by quartz-pyrite, Stage II with a quartz-sulfide assemblage, and Stage III reflected by quartz-calcite. The ore-hosting volcanic rocks were formed at 170.2 ± 0.9 Ma. Distinctly younger U-Pb ages of 162.5 ± 1.0 Ma and 160.5 ± 6.0 Ma obtained on syn-ore hydrothermal zircon and apatite, respectively, clearly indicate that gold mineralization postdates the volcanic host rocks. Fluid inclusion data reveal that the ores were precipitated from low-temperature (275 to 170 °C), low-pressure (<55 bar), and low-salinity (<5 wt% NaCl equiv.) solutions, with a systematic decrease in temperature and pressure from stages I to III. The δ34SV-CDT ratios for gold ores vary between −3.2 and 0.6 ‰. Sulfide separates yielded 206Pb/204Pb ratios of 18.368 to 18.504, 207Pb/204Pb ratios of 15.685 to 15.718, and 208Pb/204Pb ratios of 38.792 to 39.063. These data suggest S and Pb derivation from the host volcanic rocks and perhaps deeper-seated crustal materials. Calcite from Stage III has δ13CV-PDB values of −2.9 to −2.7 ‰ and δ18OV-SMOW values of 8.2 to 10.8 ‰. Calculated δ18Ofluid (−7.6 to −2.7‰) and δDfluid (−119 to −49‰) values document a largely meteoric nature of the mineralizing fluid. All data together point at a Late Jurassic low-sulfidation epithermal gold mineralization with metal remobilization from host volcanic rocks and basement.

Au-rich bimodal-mafic type volcanogenic massive sulphide deposit associated with Jurassic arc volcanism from the Central Pontide (Kastamonu, Turkey)

The central part of the Pontide Orogenic Belt, located in northern Anatolia, is a segment of the Alpine-Himalayan orogeny. Çangaldağ Metamorphic Complex (CMC) occurs within this segment which consists of ensimatic island arc volcanics and deep sea sediments which occasionally cut cross by mafic sills/dykes. They were exposed to metamorphism under the greenschist facies conditions and remain in tectonic contact with each other. The Say yayla mineralization is one of the recently discovered volcanogenic massive sulfide (VMS) deposits in the CMC with a remarkable Au content. The mafic and felsic metavolcanic rocks are the host-rocks of the Say Yayla mineralization and the ages determined by U-Pb zircon grains of these felsic volcanic rocks range from 170 Ma to 165 Ma indicating Middle Jurassic time.At the 0.1% cut-off grade, 7.5 million tons of resource has been calculated with 0.75 ppm Au, 0.65 wt% Cu, and 0.35 wt% Zn grades. The Say Yayla mineralization consists of massive and semi-massive sulfide minerals and a mineral assemblage represented by predominantly pyrite and chalcopyrite and trace amounts of sphalerite, tennantite and galena. Average base and precious metal grades of the zones occur with >1 wt% Cu are 2 wt% for Cu, 0.7 wt% for Zn, 0.04 wt% for Pb and 1.5 g/t for Au.The sulfur isotope values (δ34S VCDT) range from 4.9‰ to 6.7‰ for pyrite grains and 4.2‰ was obtained from the one sphalerite sample. δ18O values for the quartz from quartz-rich ore samples were 10.6 ‰ and 11.7‰ δ18O (VSMOW)qtz. δ18O (VSMOW)fluid values for the ore-forming fluids vary between 0‰ and 2.4‰. Lead isotope compositions of the pyrite minerals were 18.148–18.150 (206Pb/204Pb), 15.548–15.550 (207Pb/204Pb) and 38.078–38.083 (208Pb/204Pb)The geological, geochemical, and isotopic data of the ore and host-rocks reveal that the mineralization is a bimodal-mafic type (Noranda type), which is classified into Cu-Zn group of volcanogenic massive sulfides. Geochemical and geochronological data from the wall rocks further show that mineralization was developed within an ensimatic island arc environment during the Middle Jurassic.

Mineralogy, Fluid Inclusions, and Oxygen Isotope Geochemistry Signature of Wolframite to Scheelite and Fe,Mn Chlorite Veins from the W, (Cu,Mo) Ore Deposit of Borralha, Portugal

Scheelitization of Mn-bearing wolframite, scheelite, quartz, and Fe,Mn-chlorite veins was identified in the W, (Cu,Mo) ore deposits of Borralha, by optical microscopy, electron-microprobe analysis, and stable isotope geochemistry. Fluid inclusions derived scheelite crystallization temperature was compared with the oxygen isotope temperature estimated. Scheelite was formed mainly during stage I from a low salinity aqueous-carbonic fluid dominated by CO2, where the homogenization temperature (Th) decreased from 380 °C to 200 °C (average of 284 °C). As temperature decreased further, the aqueous-carbonic fluid became dominated by CH4 (Stage II; (average Th = 262 °C)). The final stage III corresponds to lower temperature mineralizing aqueous fluid (average Th = 218 °C). In addition, salinity gradually decreased from 4.8 wt.% to 1.12 wt.%. The δ18OFluid values calculated for quartz-water and wolframite-water fractionation fall within the calculated magmatic water range. The ∆quartz-scheelite fractionation occurred at about 350–400 °C. The ∆chlorite-water fractionation factor calculated is about +0.05‰ for 330 °C, dropping to −0.68‰ and −1.26‰ at 380 °C and 450 °C, respectively. Estimated crystallizing temperatures based on semi-empirical chlorite geothermometers range from 373 °C to 458 °C and 435 °C to 519 °C. A narrower temperature range of 375 °C to 410 °C was estimated for Fe,Mn-chlorite crystallization.

Textures and chemical compositions of the Narm iron oxide-apatite deposit in Kuh-e-Sarhangi District (Central Iran): Insights into the magmatic-hydrothermal mineralization

The Narm deposit is located in the Kuh-e-Sarhangi district which is a main part of the most significant Iranian iron mineralization belt, the Kashmar-Kerman Tectonic Zone (KKTZ) in Central Iran. The Narm deposit comprises an estimated total of ∼ 135000 tons of iron ore with an average grade of ∼ 55% Fe and is hosted in Early Cambrian volcano sedimentary rocks of the Rizu formation. Ore occurrences in this deposit consist of lens-shaped magnetite ore bodies, magnetite-apatite-actinolite veins and locally rare brecciated dolomite with magnetite clasts. Magnetite, pyrite, chalcopyrite and specularite associated with apatite, actinolite, biotite and carbonate minerals form the primary main mineral assemblage which is accompanied by hematite and goethite as the secondary minerals. Magnetite as the most current mineral of Narm deposit reveal the magmatic to hydrothermal evolution of mineralization. Magmatic magnetite minerals (Mag I) are dark-gray inclusion-rich magnetite spatially correlated with high temperature Ca-Fe alteration. The brighter inclusion-free hydrothermal magnetite groups (Mag II and Mag III) form during the temperature decreasing of the mineralizing fluid. According to the magnetite chemistry examination, most magnetite fall into the field for magnetite from iron-oxide apatite (IOA) deposits. Apatite minerals with F/Cl > 2, belong geochemically to the fluorapatite type. In addition to the primary dolomite, there are some hydrothermal Fe-rich dolomites and Mn bearing ones, indicating the hydrothermal fluid playing the important role for Fe-rich mineralization. In respect to fluid evolution, fluid inclusion analysis of calcite and apatite minerals form the magnetite paragenesis assemblage represent homogenization temperature range for fluid between 325 and 557℃. The salinity of fluid varied from 7.7 to 11.6 wt% NaCl equivalent and a cooling trend with the dominant chlorine complex as an agent for deposition of the Fe-rich ores. The geochemical characteristics of the δ18Ofluid values of magnetite (from + 6.1 to + 10.4‰) and δ18Ofluid values of actinolite (from + 7.7 to + 12.5‰) represent the magmatic-hydrothermal (δ18Ofluid > + 0.9 ‰) formation process. The iron rich Al-clinochlore composition from the alteration zone indicates a temperature range between 250 and 330℃ which points to a temperature reduction of hydrothermal fluids in this mineralizing zone. The integrated geochemical data from this investigation, including mineral chemistry, microthermometry of fluid inclusions and oxygen isotope data all reveal a magmatic-hydrothermal genesis for this deposit.

Triple oxygen isotope systematics of diagenetic recrystallization of diatom opal-A to opal-CT to microquartz in deep sea sediments

The oxygen-18 isotopic composition (δ18O) of silica preserved in oceanic sediments is an important archive of Earth’s temperature and/or seawater δ18O from the Archean to present. Recent advances in high-precision measurements of both δ18O and δ17O values have been used to provide additional constraints on what the oxygen isotopic composition of chert reflects about past conditions. Here, we examine the effects on the triple oxygen isotopic composition of chert that occurs during transformation and recrystallization of biogenic opal-A to opal-CT to microquartz in deep sea sediments. We studied late Miocene to present samples from the Sea of Japan at ODP Site 795 and measured biogenic diatom opal-A, opal-CT, microquartz chert, and ‘altered’ opal-A samples—previously measured for δ18O values only—for both δ18O and δ17O values. We find that δ18O decreases and Δ′17O increases (where Δ′17O = δ17O – 0.528 × δ18O) with depth, coincident with the conversions from diatom opal-A to opal-CT to microquartz. Silica samples deviate from the trend expected for triple oxygen isotopic equilibrium with modern seawater. To explain these data, we developed a model that shows that local temperature gradients and pore fluid δ18O profiles in combination lower the measured opal-CT and microquartz δ18O values and raise the Δ′17O values relative to the initial opal-A, but deviate from triple oxygen isotopic equilibrium with seawater. We find that a steeper local temperature gradient and a larger influence of hydrothermal alteration of basalt at the base of the sediment column (which lowers pore fluid δ18O values) in the past explain both the measured δ18O and Δ′17O values of the opal-CT and microquartz.These data and our modeling show that the transformation of opal-A to microquartz in marine sediments at elevated temperatures and in the presence of lowered pore water δ18O values leads to an array that falls below the theoretical triple oxygen isotope line of equilibrium for SiO2 with modern seawater. Further, our model indicates that opal-CT and microquartz do form in triple oxygen isotopic equilibrium with pore fluids that are offset in their triple oxygen isotopic composition compared to seawater due to fluid-rock alteration of igneous rocks at the base of the sediment column. The diagenetic processes taking place in the Japan Sea do not explain a large portion of the existing Archean to present triple oxygen isotope chert data, which likely require either changes in the oxygen isotopic composition of the source water (i.e., ocean water) and/or alteration by meteoric fluids. Further, our data demonstrate that the triple oxygen isotopic composition of preserved chert need not represent surface conditions, but instead may reflect processes that occur in subsurface sediments at elevated temperatures and with modified pore fluid oxygen isotopic compositions.

Intrusion-hosted gold deposits of the southeastern East Sayan (northern Central Asian Orogenic Belt, Russia)

Our paper reviews a series of relatively low-grade gold deposits and prospects hosted by granitoid intrusions within the southeastern East Sayan in the Altaid Belt (northern Central Asian Orogenic Belt, Russia). The studied deposits/prospects are characterized by the presence of Te-, Sb- and Bi-bearing minerals. However, they differ in their main gold-associated mineral assemblages, which permits to distinguishing four mineralization assemblages: 1) gold-telluride; 2) gold-tetradymite; 3) gold-stibnite; and 4) gold-bismuth sulfosalt. The gold-telluride mineralization (1) is represented by the Tainskoye deposit, and the Khoringolskoye and Sagangolskoye prospects which are defined by paragenesis of native gold with Au-Ag-Bi-Pb-Ni tellurides, such as hessite, petzite, altaite, wehrlite, calaverite, melonite, altaite, and tellurobismuthite. The gold-tetradymite mineralization (2) which is represented by the Konevinskoye deposit, is hosted by the Saylag granitoid pluton, and contains tetradymite as one of the principal minerals associated with native gold. The gold-stibnite mineralization (3), represented by the Tumannoye prospect, is also hosted in a granitoid complex consisting of granodiorite and leucogranite. The gold-bismuth-sulfosalt mineralization (4) is documented at the Pogranichnoye prospect, where native gold is associated with the bismuth-bearing minerals: bismuthinite, galeno-bismuthite, lillianite, and native bismuth. Estimated δ18O values of ore-forming fluids from intrusion-hosted deposits range from 5.7 to 7.4‰, indicative of a magmatic origin; δ34S values of sulfides vary from −4.2 to 4.5‰, which corresponds to magmatic sulfur. The geological setting, element geochemistry of host granitoids, stable isotope ratios, and mineralogy of the studied deposits are consistent with a magmatic origin of the mineralizing fluids.The studied deposits are spatially associated with either Neoproterozoic or Early Paleozoic granitoids, belonging to the two main orogenic stages of the East Sayan geodynamic evolution. At ∼850 Ma, during the Neoproterozoic stage, deposits with dominant gold-telluride assemblages, formed in association with granitoids characterized by geochemical features of island arc granites. During 458–439 Ma, in the Early Paleozoic stage, gold-tetradymite, gold-stibnite, gold-telluride and gold-bismuth-sulfosalt assemblages were formed that are spatially associated with orogenic granites with different geochemical compositions. Most gold-bearing mineral assemblages are intersected by post-mineral Late Paleozoic dykes. The origin of these different gold-sulfide-telluride assemblages is explained with their genetic association with granitoid intrusions of different ages and compositions.