δ18O Values Of Quartz Research Articles

Relative importance of Asian-dust-derived micas estimated from oxygen isotopic analysis of fine quartz on specific retention of radiocesium in Andosols in Tohoku Region, Japan

ABSTRACT Andosols are the soil type that potentially has the highest radiocesium (RCs) transfer risk due to their deficiency in micaceous minerals (micas) that can strongly retain RCs in their interlayer sites. However, an exogeneous source of micas as a component of Asian dust may have contributed largely to increase the RCs retention ability for Andosols in Tohoku Region, Japan. We investigated 29 pasture soils with andic soil properties that were collected at various places across this region to clarify the provenance of micas whether they originated from Asian dust or local lithological materials. Mica content in bulk size fraction (<2.0 mm) positively and strongly correlated with the factor related to felsic elemental composition obtained by principal component analysis (r = 0.90, p < 0.01). Fine micas in the <20 µm fraction (Mica<20 µm) accounted for a large fraction (72 ± 16%) of the total mica contents and positively correlated with the content of fine quartz derived from Asian dust (Qzad_<20 µm) (r = 0.86, p < 0.01), which were distinguished from fine quartz derived from igneous bedrock materials (Qzig_<20 µm) by using the δ18O values for quartz isolated from the <20 µm fraction. Furthermore, Mica<20 µm correlated positively with radiocesium interception potential (RIP) of this fraction. These results for the first time demonstrated the importance of Asian-dust-derived micas relative to those from another local sources on the RCs retention abilities in Andosols in Tohoku, Japan.

Discriminating Superimposed Alteration Associated with Epigenetic Base and Precious Metal Vein Systems in the Rouyn-Noranda Mining District, Quebec; Implications for Exploration in Ancient Volcanic Districts

Abstract The Rouyn-Noranda mining district of Quebec contains 20 Cu-Zn (±Au ±Ag) volcanogenic massive sulfide (VMS) deposits, including the giant and gold-rich Quemont and Horne deposits. Mineralized epigenetic veins are also present, but their origin and relative timing remain enigmatic. The nature and extent of their alteration signatures and the effect of their superposition on district-scale alteration patterns is unknown. The VMS-related quartz-sulfide Cu-Zn-Ag veins have δ18Oquartz values of 8.5 ± 0.8‰, reflecting δ18Ofluid compositions of –0.4 to 3.1‰ (250°–350°C) that are typical of Archean seawater. They are associated with a proximal Fe-rich chlorite alteration and marginal spotted sericite-chlorite alteration with whole-rock δ18O values of 2.9 to 5.9‰ and are interpreted to have formed within the structurally controlled discordant upflow zones of a VMS hydrothermal system. Younger gold-bearing quartz-carbonate veins were emplaced along mechanical anisotropies created by mafic dikes during north-south compression and the formation of regional E-trending faults, folds, and cleavage. They are characterized by δ18Oquartz values of 11.3 ± 0.8‰, reflecting δ18Ofluid compositions of 2.4 to 5.9‰ (250°–350°C), typical of a metamorphic fluid, possibly mixed with a lower δ18O upper crustal fluid. They are associated with ankerite, calcite, muscovite, chlorite, albite, and quartz ± hematite alteration with whole-rock δ18O values of 5.8 to 10.3‰. Chemical abrasion-isotope dilution-thermal ionization mass spectrometry (CA-ID-TIMS) U-Pb zircon ages for two tonalite intrusions constrain the maximum age of the Cu-Zn-Ag veins to 2697.6 ± 0.7 Ma and the minimum age to 2695.3 ± 1.0 Ma, which is also the maximum age of the gold quartz-carbonate veins. Superposition of alteration related to the gold quartz-carbonate veins on previously chlorite- and sericite-altered rocks has resulted in mixed alteration signals with whole-rock δ18O values of ~6 to 8‰ that have perturbed and masked regional alteration patterns related to older VMS mineralization, such as those found in the Quemont and Horne deposits. These results indicate that defining alteration vectors in camps that have superimposed hydrothermal systems requires full consideration of the hydrothermal history of the camp, and if such constraints are lacking, whole-rock δ18O values should not be used as a stand-alone exploration method.

The gold mineral systems of the West Qinling orogen, central China: New insight from the Baguamiao gold deposit

The Baguamiao gold deposit is the first large deposit discovered in the West Qinling orogen in central China in the 1990 s. However, the occurrence of invisible gold in the deposit has not been reported. There is also no consensus on whether magmatic activity is involved in the gold mineralization, the style of mineralization, and the mechanism of gold precipitation. In this study, the in-situ trace elements, and chemical mapping of pyrite and pyrrhotite in the Baguamiao gold deposit were carried out for the first time. These data combined with the C-H-O isotope data of quartz, ankerite and calcite were used to determine mineralization process. In the Baguamiao gold deposit, five types of pyrite (Py0 to Py4) and pyrrhotite (Po0 to Po4) were identified, corresponding to the sedimentary-diagenetic period and four stages of mineralization (Stage I to Stage IV), respectively. From stage I to stage IV, gold mineralization in pyrite and pyrrhotite occurs as either visible or invisible crystals. The latter occurs dominantly as lattice-bound solid solutions (Au+) and is related to the content of Cu and Te. Co and Ni contents and Co/Ni ratios indicate that pyrite and pyrrhotite are magmatic-hydrothermal origin and affected by fluid-rock interaction. The change of Se and Te concentrations in pyrite and pyrrhotite indicates that oxygen fugacity (ƒO2) was increased from stage I to stage II, and then decreased during stage III and stage IV. In addition, the δ18O values of quartz, calculated δ18OH2O values of the ore-forming fluids and δ13C values range from 16.9 to 20.3 ‰, 7.7 to 12.9 ‰, and − 19.3 to + 0.2 ‰, respectively. Such isotopic signature records typical characteristics of orogenic gold deposit. The ore-forming fluid is a mixed hydrothermal solution involving metamorphic water, magmatic water, and meteoric water, and mixed with marine carbonate and sedimentary organic matter. Gold was precipitated by phase separation, fluid-rock interaction, and fluid mixing. Combined with the tectono-magmatic evolution, metamorphic deformation, and gold mineralization age of the West Qinling orogen, it can be concluded that with the closure of the Mianlue portions of the East Paleo-Tethys Ocean, the Baguamiao gold deposit was formed in the post-collisional extensional environment between the South China Block and the North China Block in the Late Triassic and is an orogenic gold deposit reference to the crustal continuum model.

Oxygen and radiogenic isotopes require different contamination paths in the Azufre volcano, Central Andes, northern Chile

Azufre Volcano (21°47′S, 68°15′W), located in the Altiplano-Puna Volcanic Complex of the Andean Central Volcanic Zone (CVZ), is formed by two coalescent edifices (Northern and Southern) constructed between 120 and 1300 kaExtensive hydrothermal alteration occurred within a host sequence of andesites/dacites (120–700 ka in age) in the Northern edifice. The silica content of the erupted lavas of this volcano range from ∼60% m/m (andesite) to ∼67% m/m (dacite). These lavas have 87Sr/87Sr ratios ranging from 0.7066 to 0.7074, 143Nd/144Nd ratios ranging from 0.51229 to 0.51239, and quartz δ18O values ranging from 7.9 to 9.8‰. The original magmas had δ18O values from 7.4 to 9.4‰ (average ∼8.0‰) assuming that Δquartz-melt = 0.5‰.A three-stage magmatic evolution is proposed for Azufre. In the first stage, a mantle-derived magma (87Sr/86Sr = 0.70376; δ18O ∼5.7‰) ascended to plagioclase-dominated felsic crustal levels (25–50 km depth), assimilating up to 21% of crustal material. This generated a sub-arc parental melt with 87Sr/86Sr ∼0.705 and δ18O ∼6.5‰. The second stage involves contamination of the parental magma by assimilation of up to 25% of crust with higher 87Sr/86Sr ratios (0.714) and δ18O (6.4–11.8‰) at upper crustal levels (10–25 km depth). This upper crust corresponds to partially molten zones which coincides with the location of the Altiplano-Puna Magma Body. The generated melts have 87Sr/86Sr ratios up to ∼0.7075 and δ18Omelt 8.5–9.4‰, which fractionates high-δ18O quartz (9.0–9.8‰). The final stage occurred at shallow crustal levels (<10 km depth) where the magmas underwent fractional crystallization during assimilation of crustal material, which due to high-temperature hydrothermal alteration had low δ18O value (δ18Omelt 7.4–8.5‰). During this stage, high-δ18O quartz of the previous stage remains as antecrysts, coexisting with the low-δ18O quartz (δ18Oquartz 7.9–9.0‰) fractionated at shallow depths.

Isotope Geochemistry of the Heihaibei Gold Deposit within the Kunlun River Area in the Eastern Kunlun Orogen in Northwest China and Its Metallogenic Implications

The Heihaibei gold deposit is located in the Eastern Kunlun Orogen in Northwest China. The gold mineralization here occurs predominantly in quartz veins within faulted granite zones. The sulfide mineral assemblage is dominated by pyrite and arsenopyrite, with minor chalcopyrite, galena, sphalerite, tetrahedrite, and micro-native gold. Weak alterations in Heihaibei granites include silicification and sericitization, with minor chloritization and carbonatization. The measured δDH2O and δ18Oquartz values of quartz in auriferous quartz veins range from −104.2‰ to −81.1‰ and +9.2‰ to +13.9‰, respectively. The δ34S values of sulfides in auriferous quartz veins range from +7.60‰ to +8.65‰, and the lead isotope compositions of sulfides in ores range from 18.7219 to 19.0007 for 206Pb/204Pb, 15.6959 to 15.7062 for 207Pb/204Pb, and 37.7359 to 38.8055 for 208Pb/204Pb. The Pb isotope compositions of potassic feldspars from Heihaibei granites vary from 18.3532 to 19.4864 for 206Pb/204Pb, 15.6475 to 15.6812 for 207Pb/204Pb, and 37.1750 to 38.4598 for 208Pb/204Pb. Collectively, the isotope (H, O, S, and Pb) geochemistry suggests that the ore-forming fluid was a special metamorphic water evolved from the deep slab-derived fluids, and the sulfur and lead were predominantly sourced from such metamorphic fluids, and from the deep parts of the Heihaibei granites. Therefore, the Heihaibei gold deposit can be classified as an orogenic gold deposit, which is closely associated with the subduction of the Paleo-Tethys oceanic plate, and even the final closure of this ocean by the Later Triassic.

OXYGEN ISOTOPE SYSTEM OF LEUCOGRANITES OF THE OMSUKCHAN TROUGH (NE RUSSIA): CONDITIONS AND MECHANISMS OF WATER-ROCK INTERACTION

The oxygen isotopic composition of minerals (Qtz, Zrn, Kfs) and bulk samples of Late Cretaceous porphyritic leucogranites of the Omsukchan trough, which are associated with the formation of tin and Sn-polymetallic mineralization, was studied. The δ18O values of bulk samples of leucogranites decrease from +9.7‰ (outside the Dukat ore field) to 10.6‰ (within the ore field), and the light isotope characteristics are mainly an attribute of feldspars and correspond to isotope exchange with meteoric fluid (δ18О = –15‰), over a wide range of T at limited fluid-to-rock ratios (W/R from 0.9–1.6 to 0.3–0.1). Unlike feldspars, the δ18O values of quartz and zircon are partially changed, in the former, probably, by the diffusion mechanism, and in the later, by the dissolution-precipitation one.

Geochronology (U-Pb and Re-Os) and H[sbnd]O[sbnd]S isotopes reveal ore fluid origin and crucial role of meteoric water for Laozuoshan quartz-vein-hosted gold metallogeny in Jiamusi, NE China

The Laozuoshan auriferous sulfide-quartz veins, which are hosted in the granodiorite, monzogranite, and quartz diorite, are the only Early Cretaceous gold deposit discovered in the central Jiamusi Massif, but its origin and ore-forming processes remain unclear. Here, we integrate mineralogy, HOS isotopes, and U-Pb and Re-Os geochronology to unravel the formation of the Laozuoshan quartz-vein gold mineralization. Zircon and molybdenite from the monzogranite wallrock and gold ores yield weighted mean U-Pb and isochron Re-Os dates of 259.9 ± 2.1 Ma and 103.3 ± 2.3 Ma, respectively. Our radiometric dating suggests that the Laozuoshan quartz-vein gold mineralization was coeval with the quartz diorite (105–103 Ma) but after the monzogranite emplacement. Quartz grains from the auriferous veins of different stages yielded scattered δ18O values (5.4 to 13.0 ‰), with the δD values (fluid inclusions) ranging from –99.9 to –74.9 ‰. The δ18O values of quartz (10.9–13.3 ‰) and feldspar (9.4–11.0 ‰) from the unaltered wallrocks are similar to those (δ18Oquartz = 11.2–13.2 ‰ and δ18Ofeldspar = 10.0–10.4 ‰) from the potassic alteration halos. Meanwhile, the sericite-altered rocks have δ18O values of 12.4–12.9 ‰ (quartz) and 3.4–5.2 ‰ (sericite). Sulfides from the gold ores yielded decreasing δ34S values from 5.3 to 6.9 ‰ (early stage), through 1.0–3.5 ‰ (intermediate stage), to –4.9 to –4.4 % (late stage). The isotope data reflect a largely magmatic source for the early ore fluids, whilst the sulfur was possibly derived from the Mashan Group migmatite. The results, in combination with geologic evidence, indicate that the Laozuoshan quartz-vein gold mineralization was spatially, temporally, and genetically related to the Early Cretaceous quartz diorite, and thus can be classified as intrusion-related, magmatic-hydrothermal-vein type. In the ore-forming processes, the ascending Early Cretaceous magma may have acquired most of their sulfur and gold through assimilation of the Mashan Group migmatite. The early ore fluids derived from the quartz diorite magma may have initially focused into narrow cracks/fractures in the quartz diorite margin and then flow along the fracture system. Subsequently, meteoric water incursion led to temperature drop, boiling, oxygen fugacity increase, and chemical exchange of the resultant fluids, which facilitated gold scavenging by bismuth-rich melts and rapid precipitation as arsenopyrite-hosted electrum and gold-bismuth mineral eutectic assemblage. This study highlights the crucial role of meteoric water in formation of the magmatic-hydrothermal-vein type gold deposit.

Chlorite chemistry, H-O-S-Pb isotopes and fluid characteristics of the Yuhai Cu-Mo Deposit in Eastern Tianshan: Implications for porphyry copper mineralization and exploration

The Yuhai porphyry Cu-Mo deposit, located in the Eastern Tianshan metallogenic belt, northwest China, is a new discovery in the eastern section of Dananhu-Tousuquan arc, hosted mainly by quartz diorite and granodiorite intrusions. Based on alteration and mineralization assemblages as well as crosscutting relationships, three stages of hydrothermal activities have been identified: 1) early mineralization stage characterized by potassic alteration and minor Cu-Mo mineralization; 2) main mineralization stage with phyllic and propylitic alterations and extensive Cu-Mo precipitation; and 3) late mineralization stage forming barren quartz and minor calcite, chlorite, and epidote minerals. Sulfur isotope compositions of the Yuhai chalcopyrite, pyrite, bornite and molybdenite record a narrow range of −3.4 to +0.4 ‰, indicating a deep source of magmatic sulfur. Lead isotope data of sulfides from our previous study display consistent 208Pb/204Pb and 207Pb/204Pb ratios, but distinguishable 206Pb/204Pb ratios from the Yuhai intrusions analysed in this work. Combined sulfur and lead isotopic signatures, we suggest that the ore-forming components (metals and sulfur) were derived from a mantle-derived magmatic reservoir with some crustal components. The δ18O values of quartz, chlorite, calcite, and sericite samples separated from different mineralization stages range from 1.5 to 8.2 ‰ (n = 11), with calculated δ18OH2O values of −2.2 to 4.5 ‰, and the δD values of fluids range from −92.7 to −68.7 ‰ (n = 4), indicating the ore-forming fluids are composed of magmatic fluid and meteoric water. Mineral chemistry of chlorite, fluid inclusions microthermometry, and petrographic evidences reveal that the ore-forming fluids evolved from a high fO2 to relative weak oxidized condition with decreasing temperature, and copper was largely precipitated at temperatures of <320 °C in a weakly oxidized system during main stage. All these observations demonstrate that Yuhai was formed by hydrothermal fluids related to the diorite magma, and that the fO2 and temperature decreasing plays a critical role for abundant depositions of Cu and Mo. The Paleozoic diorite complex formed in a subduction setting is suggested as a significant exploration target for porphyry-style Cu mineralization in the Eastern Tianshan.

Fluid origin and critical ore-forming processes for the giant gold mineralization in the Jiaodong Peninsula, China: Constraints from in situ elemental and oxygen isotopic compositions of quartz and LA–ICP–MS analysis of fluid inclusions

The Jiaodong Peninsula, China, is one of the largest gold provinces in the world. During a short time interval in the Early Cretaceous, over 5000 t of gold were accumulated there. The mechanisms controlling the focused Au deposition and the origins of massive amounts of auriferous fluids are still controversial. Quartz–sulfide lode mineralization (Linglong-type) and disseminated/stockwork mineralization (Jiaojia-type) are the two major mineralization styles in the Jiaodong Peninsula. Herein, we conducted in situ textural, elemental and oxygen isotopic analyses using scanning electron microscopy (SEM), laser ablation-inductively coupled plasma-mass spectrometry (LA–ICP–MS) and secondary ion mass spectrometry (SIMS) on quartz from different hydrothermal stages and depths for three representative Linglong-type (Linglong deposit) and Jiaojia-type (Xiadian and Jiangjiayao deposits) gold deposits in the northwestern Jiaodong Peninsula, with the aim to constrain the ore-forming processes. The LA–ICP–MS analysis of individual fluid inclusions was also conducted to unravel the origin of the auriferous fluids. The results show that from the pre-mineralization to the ore-forming stages in the Linglong-type mineralization, the homogenization temperatures and salinities of fluid inclusions and the δ18Oquartz values change slightly, but the Al concentrations of quartz decrease continuously. Combined with the coexistence of CO2- and H2O-rich fluid inclusion endmembers, these trends indicate the increase of pH induced by CO2 degassing (fluid–fluid unmixing), which led to focused Au deposition. The quartz grains from the Jiaojia-type mineralization typically show a homogeneous texture with well-correlated lithophile elements (e.g., K, Al and Rb), which are distinct from those of the Linglong-type that are characterized by oscillatory zoning with much higher Al concentrations and δ18Oquartz values. These phenomena indicate that enhanced fluid–rock interaction occurred in the Jiaojia-type mineralization. In addition, the δ18Oquartz values show an increase from deep to shallow depths. In combination with thermodynamic modelling, it suggests decreasing fluid–rock interaction with decreasing depth. The above results corroborate that multiple mechanisms led to the efficient Au deposition in the Jiaodong Peninsula. Fluid–fluid unmixing typically occurred in the Linglong-type mineralization, while fluid–rock interaction was dominant in the Jiaojia-type mineralization. The different mechanisms might render mining challenging due to the changeable locations of favorable Au unloading.The LA–ICP–MS results of fluid inclusions suggest that the ore-forming fluids are metamorphic fluids rather than magmatic–hydrothermal fluids. In combination with the isotopic compositions of ores showing considerable mantle affinities and large-scale mafic underplating occurring beneath the study region, we propose a new genetic model that accounts for the origin of the auriferous fluids. The new model invokes the devolatilization of previously emplaced H2O- and Au-rich amphibole cumulates through amphibolite- to granulite-facies metamorphism in the middle–lower crust. It suggests an alternative way to generate the ‘orogenic-like’ gold deposits.

Isotope Geochemistry of the Shenshuitan Gold Deposit within the Wulonggou Gold Field in the Eastern Kunlun Orogen, Northwest China: Implications for Metallogeny

The Shenshuitan gold deposit is located within the Eastern Kunlun Orogen in northwest China. The gold mineralization here occurs primarily within the ductile fault XI. The sulfide mineral assemblage is dominated by pyrite and arsenopyrite, with minor pyrrhotite, chalcopyrite, galena, and sphalerite. Host rocks predominantly consist of Ordovician silicic slate and late Silurian granites, and their alterations include silicification and sericitization. The measured δD and δ18O values of quartz and sericite range from −113.9‰ to −93.1‰ and from 4.6‰ to 12.0‰, respectively. Bulk and in situ δ34S values of sulfides range from −7.3‰ to +9.6‰ and from −3.92‰ to 11.04‰, respectively. Lead isotope compositions of sulfides show 206Pb/204Pb ratios from 18.071 to 19.341, 207Pb/204Pb ratios from 15.530 to 15.67, and 208Pb/204Pb ratios from 37.908 to 38.702. Collectively, the isotope (H, O, S, and Pb) geochemistry suggests that the ore-forming fluids were of a metamorphic origin mixed with meteoric water and that the sulfur and lead were sourced from a mixture of host rocks and original ore-forming metamorphic fluids. Lastly, this deposit can be classified as an orogenic gold deposit associated with the final collision between the Bayan Har–Songpanganzi Terrane and the Eastern Kunlun Orogen during the Later Triassic.

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.

Fluid circulation in the South Tibetan Detachment System: Evidence from fluid inclusions and oxygen isotope data of quartz veins in the Ramba Dome, North Himalayan Gneiss Domes

Detachment faults are sites of intensive fluid–rock interactions. Here, we report fluid inclusion and oxygen isotope data for quartz veins in the Ramba Dome in the North Himalayan Gneiss Domes, with an aim to constrain the origin and circulation of crustal fluids associated with the South Tibetan Detachment System (STDS). Microthermometric data for fluid inclusions in quartz indicate that the fluids were aqueous and CO2 − H2O ± CH4 ± N2‐bearing with low to moderate salinities (0.60–11.80 wt% eq. NaCl). The entrapment conditions are 295–410°C and 98–135 Mpa, indicating a forming‐depth of 8–10 km. Oxygen isotopic compositions (δ18O) of quartz measured in situ by secondary ion mass spectrometry and bulk by the BrF5 method show limited variations in individual quartz veins, but δ18Oquartz values vary from 12.07 to 18.16‰ (V‐SMOW) among veins. The corresponding δ18Ofluid values range from 7.71 to 13.80‰, based on equilibrium temperatures obtained from fluid inclusions. From the footwall to the detachment zone, δ18Ofluid values exhibit a broadly decreasing trend and indicate that the STDS dominated the fluid flux pathway in the crust, with more contributions of meteoric water in the detachment zone. We further quantified the contribution of meteoric fluids to 8–27% using a binary end‐member mixing model. These data imply that the fluids were predominantly metamorphic/magmatic in origin, and were mixed with infiltrating, isotopically light, meteoric water during extensional detachment shearing of the STDS. The meteoric water can infiltrate from the surface to 8–10 km depth.

Quartz oxygen isotopes from Tick Hill area in Mount Isa Inlier: indication of a regional fluid overprint

At the Tick Hill gold deposit, δ18Oquartz data for the mineralised lithologies and surrounding rocks are similar and fall within a narrow range of 10.5–13.7‰ V-SMOW. The highly mineralised quartzo-feldspathic mylonite has quartz δ18O (δ18Oquartz) values of 11.3–13.6‰, which are similar to values for the surrounding rocks both locally and regionally, i.e. δ18Oquartz by itself does not provide a useful exploration tool. The δ18Oquartz values from the Tick Hill area most likely reflect the late Isan hydrothermal overprint at 1525–1520 Ma. The origin of the altering fluids is unclear, as the δ18Oquartz values overlap with reported δ18O values calculated for both metamorphic and igneous fluids. When combining the δ18Oquartz results with δ18Ocalcite results available from the literature, a temperature of 350–550 °C was calculated, which is consistent with observed alteration assemblages associated with gold mineralisation. KEY POINTS The δ18Oquartz values (10.5–13.7‰ V-SMOW) of Au-rich quartz–feldspar mylonite are indistinguishable from the altered host rocks both local and regional. The narrow range of δ18Oquartz values for the rock units in the Tick Hill area and Mary Kathleen Domain most likely reflect a regional fluid overprint. The post-mineralised quartz–calcite veins yield higher δ18Oquartz values (14.1–17‰), possibly reflecting (partial) re-equilibration of minerals locally formed in late cataclastic fault rocks.

Early Neoproterozoic Gold Deposits of the Alto Guaporé Province, Southwestern Amazon Craton, Western Brazil

Abstract The Alto Guaporé gold province, southwestern Amazon craton, contains gold deposits that have been mined since the beginning of the 18th century and these deposits, together, have modern-day, pre-mining gold resources of at least 1.8 Moz. The ore is associated with quartz vein systems along the southeastern part of the Aguapei belt, a ~35-km-wide and ~500-km-long, NNW-trending shear zone formed due to reactivation of a terrane-bounding suture. The Aguapei belt evolved by ca. 1150 to 1100 Ma rifting and deposition of siliciclastic sediments in an aulacogen basin, followed by deformation and low-grade metamorphism of the sedimentary sequences during 1100 to 900 Ma terrane collision along the craton margin. The deformation was characterized by a compressional regime until ca. 950 Ma and transition to a transpressional setting during the final 50 m.y. The gold deposits are hosted in a variety of structures that are second-order to the main Aguapei shear zone. The Ernesto and Pau-a-Pique deposits are located ~40 km apart and at jogs along the Aguapei belt. They are marginal to pre-ore igneous rocks, with Ernesto hosted in the basal part of the metasedimentary Fortuna Formation that overlies tonalite and Pau-a-Pique at the contact between metasedimentary rocks and diorite. Three deformational phases comprise the compressional (D1 to D2) to transpressional (D3) tectonic events. In the Pau-a-Pique deposit and the deeper level of the Ernesto deposit, the ore-bearing veins are bedding parallel and follow D2 strike-slip and reverse fault zones, respectively. However, the veins formed during D3 reactivation of the older structures by an array of oblique accommodation faults. In contrast, ores at shallower levels of Ernesto, both in discordant and bedding-parallel veins, are hosted within a ~20-m-thick rigid metaconglomerate with associated dilation due to the structural complexity as sedimentary rocks of the Aguapei Group were folded around the dome-shaped roof of the pre-ore tonalite. The ores in both deposits, as well as in many other deposits of the province, are characterized by disseminated and vein-hosted pyrite. Gold occurs mainly as inclusions in the pyrite, with other hydrothermal phases comprising muscovite, Fe-Ti oxides, and minor apatite, chalcopyrite, and galena. Fluid inclusion data, coupled with stable isotope geochemistry and geothermometry, indicate that gold precipitated from a low-salinity, CO2-rich fluid at ~300°C and ~2.5 kbar. The source for the fluid and gold was the interbedded pelites during devolatilization of the Aguapei Group sequence. The aqueous-carbonic fluid inclusions and the narrow range of δ18O values of quartz (12 ± 1‰) from many auriferous veins from the central part of the province represent a regional ore-forming fluid. The broad range of δD for hydrous minerals (–116 to –55‰) reflects influx of small amounts of meteoric water into the steeply dipping shear zones during postgold exhumation. The 40Ar/39Ar geochronology from hydrothermal muscovite indicates a widespread hydrothermal event along the belt between 928 and 920 Ma. Collectively, the geological, geochronological, and geochemical data suggest that metamorphic fluids migrated laterally into and then upward along the Aguapei belt and deposited gold in lower-order structures where strain gradients existed between lithounits. The province has many characteristics of large orogenic gold provinces worldwide and represents a highly prospective and underexplored target region for early Neoproterozoic gold, a time period that generally is not well endowed in gold ores.

Geochronology and Sr-Nd-Pb-Hf-O isotope geochemistry of Miocene intrusive rocks from Tsushima Islands, Japan: Constraints on petrogenesis and tectonic setting

We present geochemical and Sr-Nd-Pb-Hf-O isotopic data and zircon U-Pb ages for Miocene intrusive rocks (granitoids, mafic microgranular enclaves (MMEs), and diabases) from Tsushima Islands between Kyushu (Japan) and southeastern Korea. A granite yielded an age of 16.23 ± 0.16 Ma. The granitoids are mostly I-type, but are transitional between metaluminous and peraluminous compositions. All samples exhibit enrichment in LREE and LILE (e.g., K, Rb, and Pb), and depletion in HFSE (e.g., Nb, Ta, and Ti), implying formation in a subduction-related tectonic setting. However, on tectonic discrimination diagrams, data for all samples plot between the orogenic and anorogenic fields, indicating that Tsushima Islands were in a transitional tectonic regime from subduction to continental extension at ca. 16 Ma. The δ18Oclinopyroxene values of a diabase range from 5.65‰ to 5.84‰. The Sr-Nd-Pb isotopic compositions of the diabases (87Sr/86Sr = 0.70423–0.70481; εNd = −0.8 to +3.6; 206Pb/204Pb = 18.27–18.40) are within the range of Miocene Eoil basalts from the Cenozoic Yangnam Basin in southeastern Korea, indicating the mantle source was a metasomatized mantle wedge. The granitoids are characterized by more enriched isotopic compositions than the diabases (87Sr/86Sr = 0.70611–0.70756; εNd = −6.0 to −3.8; 206Pb/204Pb = 18.43–18.55), and the MMEs have an intermediate isotopic composition. δ18Oquartz values of the granitoids vary from 8.68‰ to 9.25‰, and zircons from a granite have εHf(t) values of −1.7 ± 1.2. The MMEs may have formed by partial melting of amphibolitic lower crust, whereas the granitoids possibly had a hybrid source consisting of amphibolite and graywacke.

Tracking fluid mixing in epithermal deposits – Insights from in-situ δ18O and trace element composition of hydrothermal quartz from the giant Cerro de Pasco polymetallic deposit, Peru

Ore precipitation in mineral deposits formed in the upper parts of a porphyry system, at shallow crustal level (< 1.5 km), such as epithermal Au-Ag-(Cu)-(As) and polymetallic deposits is often triggered by fluid cooling and/or mixing between fluids from different sources. Commonly, in such deposits, two main fluid sources are identified a deeply sourced magmatic fluid and a shallow meteoric water stored in a surficial aquifer. Oxygen and hydrogen isotope compositions of gangue and alteration minerals using conventional bulk isotopic methods support the existence of mixing between these two fluid types. However, bulk isotope analysis provides only limited information on the exact mixing mechanisms and on the changing proportions of the involved fluids. Due to their high spatial resolution, SIMS in-situ oxygen isotope and LA-ICP-MS trace element analyses, in transects across growth zones of single crystals are adequate tools to trace the dynamics of this fluid mixing. In this study, in-situ SIMS oxygen isotope and LA-ICP-MS trace element analyses were performed on 10 selected quartz crystals from the giant Cerro de Pasco porphyry-related epithermal polymetallic deposit in central Peru. The results, combined with previous microthermometric and LA-ICP-MS fluid inclusion studies on the same or equivalent crystals, allow quantifying and documenting the mixing between different types of fluids that formed the large Cerro de Pasco epithermal polymetallic deposit. The δ18Oquartz values range between 4‰ and 20‰ and display variations up to 11.5‰ inside single crystals that cannot be only, nor mainly ascribed to fluid temperature changes. Rather, these variations record variable mixing proportions of a rising moderate-salinity magmatic fluid with a δ18OH2O around 10‰ and a low-salinity fluid with a δ18OH2O between 0 and 4‰, the latter stored below the paleo-water table. Each analyzed quartz crystals also display important variation of their trace element content, with Al (43 to 2098 ppm), Li (0.7 to 18 ppm), Ge (1.1 to 24ppm) and Ti (0.8 to 10 ppm). These variations do not systematically correlate with oxygen isotope compositions. This suggests that quartz trace element content is controlled by a complex interplay of fluid composition, temperature, pressure, and growth rate. Application of published Ti-in-quartz geothermometers on quartz grains from which the precipitation temperature is well constrained by fluid inclusion microthermometry, shows that it can lead to overestimation or underestimation of precipitation temperatures by more than 50 °C.The obtained δ18Oquartz patterns measured along profiles in the studied quartz crystals, and less clearly the in-situ trace element compositions, reveal abrupt changes and suggest that mixing between magmatic and surface-derived low-salinity fluids was not a continuous process. It rather took place through the influx of multiple short-lived pulses of magmatic fluid into the surface-derived low-salinity fluid surface aquifer.

Triple Oxygen (δ18O, Δ17O), Hydrogen (δ2H), and Iron (δ56Fe) Stable Isotope Signatures Indicate a Silicate Magma Source and Magmatic-Hydrothermal Genesis for Magnetite Orebodies at El Laco, Chile

Abstract The Plio-Pleistocene El Laco iron oxide-apatite (IOA) orebodies in northern Chile are some of the most enigmatic mineral deposits on Earth, interpreted to have formed as lava flows or by hydrothermal replacement, two radically different processes. Field observations provide some support for both processes, but ultimately fail to explain all observations. Previously proposed genetic models based on observations and study of outcrop samples include (1) magnetite crystallization from an erupting immiscible Fe- and P-rich (Si-poor) melt and (2) metasomatic replacement of andesitic lava flows by a hypogene hydrothermal fluid. A more recent investigation of outcrop and drill core samples at El Laco generated data that were used to develop a new genetic model that invokes shallow emplacement and surface venting of a magnetite-bearing magmatic-hydrothermal fluid suspension. This fluid, with rheological properties similar to basaltic lava, would have been mobilized by decompression-induced collapse of the volcanic edifice. In this study, we report oxygen, including 17O, hydrogen, and iron stable isotope ratios in magnetite and bulk iron oxide (magnetite with minor secondary hematite and minor goethite) from five of seven orebodies around the El Laco volcano, excluding San Vicente Bajo and the minor Laquito deposits. Calculated values of δ18O, Δ17O, δD, and δ56Fe fingerprint the source of the ore-forming fluid(s): Δ17Osample = δ17Osample – δ18Osample * 0.5305. Magnetite and bulk iron oxide (magnetite variably altered to goethite and hematite) from Laco Sur, Cristales Grandes, and San Vicente Alto yield δ18O values that range from 4.3 to 4.5‰ (n = 5), 3.0 to 3.9‰ (n = 5), and –8.5 to –0.5‰ (n = 5), respectively. Magnetite samples from Rodados Negros are the least altered samples and were also analyzed for 17O as well as conventional 16O and 18O, yielding calculated δ18O values that range from 2.6 to 3.8‰ (n = 9) and Δ17O values that range from –0.13 to –0.07‰ (n = 5). Bulk iron oxide from Laco Norte yielded δ18O values that range from –10.2 to +4.5‰ (avg = 0.8‰, n = 18). The δ2H values of magnetite and bulk iron oxide from all five orebodies range from –192.8 to –79.9‰ (n = 28); hydrogen is present in fluid inclusions in magnetite and iron oxide, and in minor goethite. Values of δ56Fe for magnetite and bulk iron oxide from all five orebodies range from 0.04 to 0.70‰ (avg = 0.29‰, σ = 0.15‰, n = 26). The iron and oxygen isotope data are consistent with a silicate magma source for iron and oxygen in magnetite from all sampled El Laco orebodies. Oxygen (δ18O Δ +4.4 to –10.2‰) and hydrogen (δ2H ≃ –79.9 to –192.8‰) stable isotope data for bulk iron oxide samples that contain minor goethite from Laco Norte and San Vicente Alto reveal that magnetite has been variably altered to meteoric values, consistent with goethite in equilibrium with local δ18O and δ2H meteoric values of ≃ –15.4 and –211‰, respectively. The H2O contents of iron oxide samples from Laco Norte and San Vicente Alto systematically increase with increasing abundance of goethite and decreasing values of δ18O and δ2H. The values of δ2H (≃ –88 to –140‰) and δ18O (3.0–4.5‰) for magnetite samples from Cristales Grandes, Laco Sur, and Rodados Negros are consistent with growth of magnetite from a degassing silicate melt and/or a boiling magmatic-hydrothermal fluid; the latter is also consistent with δ18O values for quartz, and salinities and homogenization temperatures for fluid inclusions trapped in apatite and clinopyroxene coeval with magnetite. The sum of the data unequivocally fingerprint a silicate magma as the source of the ore fluids responsible for mineralization at El Laco and are consistent with a model that explains mineralization as the synergistic result of common magmatic and magmatic-hydrothermal processes during the evolution of a caldera-related explosive volcanic system.

The nature of subduction system in the Neoarchean: Magmatic records from the northern Yangtze Craton, South China

The Neoarchean is an important era during which plate tectonics began to operate widely on Earth. The style of plate tectonics in the Neoarchean should be different but poorly constrained. Here we report geochemical studies on felsic to mafic igneous rocks from the northern Yangtze Craton, which record a subduction at ca. 2.65–2.70 Ga with distinctive magmatic and geodynamic features from modern subduction system. The metaigneous rocks of felsic composition, trondhjemite gneisses (TTG), from the northern margin show high sodic contents, positive Eu anomalies, high (La/Yb)cn and Sr/Y ratios, low Yb and Y contents. The metaigneous rocks of mafic-intermediate composition from the northern margin are mainly calc-alkaline and show less fractionated REE patterns. They have similar zircon U-Pb ages of 2651 ± 30 Ma to 2703 ± 9 Ma, similar chondritic zircon εHf(t) values and whole-rock εNd(t) values of −1.6 to 3.4. Their zircon δ18O values are mostly 4.6–6.6‰, similar to or slightly higher than normal mantle values. Additionally, they all record a metamorphic event at ca. 2.47–2.50 Ga. The potassic granites from the interior are A-type in geochemistry and show similar zircon U-Pb ages and εHf(t) values to the metaigneous rocks in the northern margin but obviously lower zircon δ18O values from 3.6‰ to 5.4‰. The two types of rocks exhibit quartz δ18O values of 12.0–15.2‰ and 3.5–8.2‰, respectively. The trondhjemite gneisses and calc-alkaline metaigneous rocks were derived from partial melting of the subducted crust and the metasomatized mantle wedge, respectively. The A-type granites are the products of high temperature magmatism in an extensional setting in response to the contemporaneous subduction. This demonstrates the development of a subduction system in the northern Yangtze Craton during 2.65–2.70 Ga. The contemporaneous development of both slab-derived and mantle wedge-derived magmatism is suggested as a feature of the Neoarchean subduction system that is distinguished from the modern ones. The lack of TTG magmatism at 2.50–2.65 Ga but the reburst of TTG and metamorphism at 2.47–2.50 Ga imply that the subduction in the Neoarchean is intermittent.

A trace metal, stable isotope (H, O, S), and geochronological (U-Pb titanite) characterization of hybridized gold orebodies in the Missanabie-Renabie district, Wawa subprovince (Canada)

Mineralized shear zones in the Archean Missanabie-Renabie gold district (~ 1.1 Moz Au; Wawa, Ontario, Canada) locally define composite orebodies that record three hydrothermal events: (1) a pre-orogenic Au1 event (pre-D1 and pre-prograde-metamorphic); (2) a syn-orogenic, post-peak-metamorphic Retrograde event (syn-D3); and (3) a late syn-to post-orogenic Au2 event (late syn- to post-D4). Genetic considerations indicate the orebodies are hybrids with early intrusion-related (Au1) and later orogenic (Retrograde + Au2) events. Pearson product-moment correlation coefficients (log10) of whole-rock and LA-ICP-MS pyrite trace metal datasets distinguish Au1 from Au2 mineralization by Au-Ag, Au-Bi, and Au-Te correlations > 0.7 (p < 0.05) in the former, irrespective of sample medium and analytical method. An Au-Mo correlation in whole rock data (0.58–0.76; p < 0.05) further distinguishes Au1 from Au2 and supports an independently inferred intrusion-related origin for Au1. Sulfur isotope data is similar for both Au1 and Au2 pyrite with average δ34S values of − 5.5‰ ± 0.2‰ (1σ) and − 3.5‰ ± 0.3‰ (1σ) and average Δ33S values of 0.4‰ ± 0.1‰ (1σ) and − 0.3‰ ± 0.2‰ (1σ), respectively. SIMS δ18Oquartz values for the Au1, Retrograde, and Au2 events largely overlap and, like δ18Ocarbonate values of previous studies, tend to be lower than values typical of Archean gold deposits. The results of this study suggest that correlation coefficients in trace metal datasets are useful in discriminating and characterizing different gold events. Caution is emphasized with the use of S- and O-isotope datasets for these purposes. The presence of low δ18O values in vein quartz and carbonate is best explained by an 18O-depleted fluid formed during the Retrograde hydrothermal event. The latter is inferred at 2580 ± 21 Ma based on U-Pb geochronlogy of hydrothermal titanite, and relates to deformation and metamorphism in the nearby, amphibolite- to granulite-grade Kapuskasing metamorphic belt. Geochronological and geochemical evidence suggest that the 18O-depleted fluid may have formed via the devolatilization of biotite-bearing granitoids during deep-crustal metamorphism.

Geological and geochemical constraints on the genesis of the Bengge gold deposit in the Yidun Terrane, SE Tibet

The Bengge gold deposit is one of a small number of deposits in the Zhongdian-Muli region, which is part of the southern segment of the Yidun Terrane in the eastern Tibetan Plateau. The deposit is hosted in a Late Triassic alkaline igneous complex comprising mainly biotite–pyroxene syenite and quartz syenite, emplaced into Triassic slate. These syenites show phyllic alteration with mineral association of quartz–sericite–pyrite–carbonate. The main gold mineralization occurs in fault zones as gold–sulfide–quartz veins, in altered syenite as telluride in veinlets, and as fissure-filling gold in disseminated pyrite. Four stages of veins are recognized: sheeted K-feldspar–quartz–magnetite veins (dikes) (stage I), quartz–carbonate–K-feldspar veins with sericite–quartz–pyrite alteration haloes (stage II), polymetallic sulfide–telluride–sericite–quartz veins with pervasive phyllic alteration (stage III), and quartz–carbonate–stibnite veins (stage Ⅳ). The stage II and III veins are closely related to gold mineralization.40Ar/39Ar dating of hydrothermal sericites from the phyllic alteration zone, which are intergrown with gold-bearing pyrite and telluride, yielded an age of ~126 Ma, indicating the mineralization event occurred in the Cretaceous extensional setting. Raman spectroscopy and microthermometry study for fluid inclusions in quartz from stage II revealed heterogeneous trapping of low-salinity (0.14–6.63 wt% NaCl equivalent) and mesothermal (270–300 °C) CO2–H2O–NaCl ore-forming hydrothermal fluids. The calculated fluid-trapping pressures range mainly from 750 to 850 bar. δ18O values of quartz from the gold mineralization stage (stage II) vary between 15.21‰ and 19.35‰. The δ18O values for waters in equilibrium with quartz vary from 6.96‰ to 10.73‰, which are consistent with values for magmatic waters. We propose that the auriferous fluids were derived from an underlying intrusion during the Cretaceous. The geological characteristics, low salinity, and mesothermal CO2–H2O–NaCl fluids, and the magmatic source of the ore-forming fluids, all support the hypothesis that the Bengge deposit is an intrusion-related gold deposit.