NaCl Equiv Research Articles

Genesis of the Hajibolagh-Zalibolagh Cu-(Ag) intermediate-sulfidation epithermal deposit, Urumieh-Dokhtar magmatic arc, Iran: Evidence from ore geology, fluid inclusions, and stable isotopes

The Hajibolagh-Zalibolagh deposit located northeast of the city of Saveh in the central part of the Urumieh-Dokhtar magmatic arc of Iran, demonstrates vein and breccia vein style copper-(silver) mineralization hosted by Eocene volcano-sedimentary rocks. The main mineralization stage can be divided into four phases, respectively (1) jasperoid + quartz veins, chalcopyrite, and pyrite; (2) quartz veins along with chalcopyrite, bornite, chalcocite, arsenopyrite, and pyrite; (3) quartz-barite veins along with chalcopyrite, bornite, chalcocite and pyrite; and (4) veins of quartz, calcite, hematite, and oligist along with a small number of copper sulfides. The main stages of mineralization are cut by barren post-ore quartz and calcite veinlets. As a result of supergene processes, minerals are oxidized, and malachite, azurite, chrysocolla, chalcocite, covellite, goethite, and hematite are formed, presenting an important exploration guide. The hydrothermal alteration from the center of the vein-veinlets to the outside includes, respectively, silicification, sericitization, intermediate argillic, and propylitic. The average salinity and homogenization temperature (Th) of fluid inclusions is 14.2 wt% NaCl equiv. and 288 °C, respectively. The coexistence of the vapor phase, liquid-rich phase, and halite-bearing fluid inclusions is a sign of boiling in this hydrothermal system, which has contributed to copper saturation. Sulfur isotope values for pyrite, chalcopyrite, chalcocite, and bornite are mainly from –0.16 to 2.98 ‰, indicating a probable magmatic source for sulfur. Boiling, dilution, and mixing are the mechanisms for the ore deposition and facilitated the mineralization. The Hajibolagh-Zalibolagh deposit shows the highest similarity to intermediate-sulfidation epithermal deposits. The best environment for mineralization and fluid circulation in these epithermal deposits is the intersection of faults with the Eocene subvolcanic rocks.

Fluid source and ore-forming process of the Lishupo Au deposit, NE Hunan: Constraints from fluid inclusions, SIMS oxygen isotope, LA-ICP-MS pyrite trace elements and fsLA-ICP-MS quartz trace elements

The Lishupo Au deposit (4.3 t @ 4.76 g/t) is located in the central Jiangnan Orogen. The processes and origins of regional gold mineralization, as well as the characteristics of the ore-forming fluids, continue to be poorly understood. This study integrates field geology, fluid inclusions, secondary ion mass spectrometry (SIMS) O isotope, femtosecond laser ablation inductively coupled plasma mass spectrometry (fsLA-ICP-MS) trace elements analysis for quartz, and LA-ICP-MS for trace elements study of pyrite to constrain the nature and source of ore-forming fluids, mechanism of gold precipitation, and ore genesis. Field investigations and microscopic observations reveal three mineralization stages at Lishupo: (1) the quartz-pyrite (Qtz1-Py1) stage; (2) the quartz-dolomite-polymetallic sulfides-gold (Qtz2-Py2) stage; and (3) the quartz-carbonate-pyrite (Qtz3-Py3) stage.Cathodoluminescence (CL) observations reveal that Qtz1 and Qtz2 have a homogeneous texture, while Qtz3 exhibits oscillatory zoning. Fluid inclusions in quartz suggest a CO2-H2O-NaCl ± N2 ± CH4 fluid system, which comprise H2O-CO2-NaCl, CO2-rich, and aqueous types. Microthermometry of fluid inclusions indicates low salinity (stage 1: 3.7–7.9 wt% NaCl equiv.; stage 2: 2.2–7.5 wt% NaCl equiv.; stage 3: 1.4–3.9 wt% NaCl equiv.) and medium to low temperature (stage 1: 313–341 °C; stage 2: 209–286 °C; stage 3: 139–198 °C) for ore-forming fluids, consistent with the low Ti content (<10 ppm) found in the quartz. The low Al content (7.99–20.7 ppm) of Qtz1 suggests that the ore-forming fluids were saturated with CO2 and had a near-neutral pH. The Al contents are higher and more variable in Qtz2 (10.7–1959 ppm) and Qtz3 (21.6–713 ppm), which may imply fluctuations in fluid pH. δ18OFluid values for various types of quartz (Qtz1: 9.02–10.95 ‰, Qtz2: 6.30–7.68 ‰, Qtz3: 3.45–3.94 ‰) indicate a metamorphic fluid origin for stages 1 and 2 fluids, which were mixed with the meteoric water during stage 3. The highest concentrations of Au and As in Py2 suggest that the Qtz2-Py2 stage is the primary ore stage. The Co/Ni ratios of all types of pyrite are less than 1, indicating that the ore-forming materials are derived from metamorphic sedimentary rocks. Fluid inclusion microthermometry suggests that fluid immiscibility was responsible for the gold deposition during the Qtz2-Py2 stage. Fluid immiscibility destabilizes Au(HS)2-, leading to the precipitation of native gold. These data strongly suggest that the Lishupo gold deposit is classified as an orogenic-type deposit. Integrating the existing geochronology data and geological context, we emphasize the presence of the Late Triassic orogenic gold deposit in the Jiangnan orogen. This highlights a significant exploration target area within this zone for future gold prospecting.

Improvements in the Quantification of End-Member Vapor Compositions in Fluid Inclusion Boiling Assemblages and Implications for Metal Transport by Low-Density Fluids

Abstract Under conditions typically found in the Earth’s crust, there is a large pressure-temperature-composition range in the H2O-NaCl system where fluids may separate into a low-salinity vapor end member and a high-salinity liquid end member. However, heterogeneous trapping is common during the formation of fluid inclusions in an immiscible fluid system, violating the fundamental assumption of homogeneous entrapment for fluid inclusion microthermometry. This has profound consequences on the composition of these fluid phases and consequently on the formation of ore deposits from hydrothermal systems. At the same time, minor mixture of the high-salinity liquid phase with the low-salinity vapor phase cannot be distinguished from the end-member vapor-rich fluid inclusions by their bubble sizes. Precise determination of the salinities of vapor-rich fluid inclusions using microthermometry is deterred by the very small proportion of the liquid phase and the limitation of the analytical precision. All this will lead to erroneous compositional data from fluid inclusion analysis. We have quantitively calculated the variations of fluid inclusion properties caused by heterogeneous entrapment during phase separation in the H2O-NaCl system and showed that the salinity, and thereby the element contents and element/Na ratios of the vapor-rich fluid inclusions, is significantly changed in heterogeneously trapped fluid inclusions. The addition of 1 vol % of the high-salinity end member to the low-salinity end member results in a salinity change from 0.99 to 4.4 wt % NaCl equiv at the phase separation condition of 800 bar and 750°C. This will have a significant impact when it comes to determining the concentration of elements transported as chlorine complexes and typically leads to an overestimation of the mass transport capability by the vapor phase. Therefore, every effort should be taken during microthermometry to find and measure the least affected vapor-rich fluid inclusions. Our numerical calculations and synthetic fluid inclusions reveal that there is a linear relationship between the salinity and element contents for fluid inclusions at different extents of heterogeneous entrapment. Therefore, linear correction to the measured lowest vapor salinity can obtain a good approximation of the element contents in the end-member vapor phase.

Reconstructing fluid evolution and precipitation mechanism of the Chuankeng skarn copper deposits, China: New insight from microthermometry and LA-ICP-MS analysis of fluid inclusions

Skarn deposits are significant sources of copper, yet there are different understandings on their fluid evolution and precipitation mechanism due to lack of direct constraints on fluid compositions of mineralization process. Here, we present a detailed reconstruction of fluid evolution history from the pre-ore prograde stage through the syn-ore retrograde stage to the post-ore stage of the Chuankeng deposit in the North Wuyi area, South China, based on fluid inclusion microthermometry and LA-ICP-MS analysis of typical minerals from three stages. The pre-ore fluids trapped in pyroxene have higher homogenization temperatures (450–550 ℃), higher salinities (31.9–51.5 wt% NaCl equiv), but lower concentrations of Cu (∼556 ppm) than syn-ore fluids (338–465 ℃, 31.3–38.9 wt% NaCl equiv, ∼3247 ppm Cu). The post-ore fluids show lower temperatures (139-163℃), salinities (1.1–6.5 wt% NaCl equiv), and low concentrations of Cu (<387 ppm). Fluids of all paragenetic stages have high K/Na and Rb/Na ratios, exhibiting signatures of magmatic hydrothermal fluids. Mixing of the magmatic fluids with meteoric waters of lower temperature and salinity occurred at the final post-ore stage, thus it is not related to Cu mineralization. As the temperature descends from 445 ℃ to 340 ℃, the Cu/(Na + K) ratios of syn-ore fluids diminish exponentially, manifesting fluid cooling within a certain temperature may be an important factor controlling Cu mineralization. Concomitantly, a moderate negative correlation between the Ca/(Na + K) ratios and Cu/(Na + K) ratios of syn-ore fluids suggests that fluid-rock interaction do have some impact on copper contents. Additionally, the Cu/(Na + K) ratios of syn-ore fluids in quartz are an order of magnitude higher than those of pre-ore fluids, and it is proposed that during the initial retrograde stage—the onset of mineralization—the magma chamber released fluid pulse with higher copper contents. Based on these findings, the fluids of pre-ore prograde stage and syn-ore retrograde stage are distinct magmatic fluid pulses, and copper-rich fluids, thermal cooling within a specific temperature range and fluid-rock reaction are suggested to be key factors controlling metal enrichment in skarn copper systems.

Genesis of the Mahour Base Metal Deposit, Iran: Constraints from Fluid Inclusions and Sulfur Isotopes

The Mahour base metal deposit is located northeast of Badroud in the middle of the Urumieh–Dokhtar magmatic arc in the Isfahan province of Iran. The main host rocks to the ores are Eocene volcanic and volcaniclastic rocks. Hypogene ore minerals constituting the main ore body are galena, sphalerite, pyrite, and chalcopyrite. In addition to gangue quartz, a variety of supergene minerals comprising gypsum, goethite, hematite, “limonite”, malachite, azurite, covellite, and chalcocite are also present; gangue minerals are quartz, barite, calcite, sericite, and chlorite. Silicification, intermediate argillic, and propylitic are the main wall-rock alteration types. The presence of fluid inclusions with different vapor/liquid ratios in quartz and sphalerite could indicate a boiling process. The primary liquid-rich fluid inclusions suggest that the homogenization temperature was between 107 and 298 °C from fluids with salinities from 1.5 to 13.7 wt.% NaCl equiv. These data suggest that the ore-forming fluids were magmatic with a contribution from meteoric waters. The δ34S values of sulfides range from 1.9 to 3.4‰, those of barite range from 12.1 to 13.2‰, and those of gypsum range from 4.3 to 5.6‰. These data suggest that sulfur was mostly of magmatic origin with a minor contribution from sedimentary rocks. Our data suggest that the boiling of fluids formed an intermediate-sulfidation style of epithermal mineralization for the Mahour deposit.

Precipitation of Ag-Pb-Zn ores of the Poshan vein deposit in the Qinling orogen (central China) by dilution of magmatic hydrothermal fluids

Constraints on the source, nature, and evolution of ore-forming fluids are crucial for better understanding the ore genesis and mechanisms that lead to metal precipitation. Here, we present results of in situ analysis for quartz oxygen isotopes and sulfide sulfur isotopes, combined with apatite U-Pb age and fluid inclusion data, to provide insights into the source and evolution of ore fluids of the Poshan Ag-Pb-Zn vein deposit in the Qinling orogen, central China. Quartz-carbonate-sulfide veins at Poshan are hosted in lower Paleozoic metavolcanic-sedimentary rocks and structurally controlled by fracture zones associated with the Heqianzhuang anticline. The Ag-Pb-Zn mineralization consists of three stages, which are represented by quartz-pyrite-arsenopyrite (stage I), quartz-carbonate-sulfide-silver minerals (stage II), and quartz-calcite ± native silver (stage III). Apatite texturally associated with galena from stage II has a U-Pb age of 125 ± 14 Ma (2σ; mean square of weighted deviates [MSWD] = 0.9), which suggests that the Poshan deposit formed in the Early Cretaceous and thus significantly postdated the Paleozoic and early Mesozoic orogenic deformation of the Qinling orogen. Aqueous-carbonic fluid inclusions from stages I and II have similar total homogenization temperatures of 263−347 °C and calculated salinities of 1.2−5.1 wt% NaCl equivalent, whereas the values for liquid-dominated aqueous inclusions from stage III are 211−245 °C and 0.5−3.0 wt% NaCl equivalent. Five quartz generations are recognized from the three paragenetic stages, including Qz-1 and Qz-2 from stage I, Qz-3 and Qz-4 from stage II, and Qz-5 from stage III. Based on in situ secondary ion mass spectrometry oxygen isotopic analysis of the five quartz generations (Qz-1 to Qz-5), the calculated δ18Ofluid values for the ore fluids are 7.8‰ to 9.2‰ (mean 8.3‰), −3.2‰ to 8.7‰ (mean 5.6‰), −3.5‰ to 7.4‰ (mean 4.6‰), −4.5‰ to 7.6‰ (mean −0.2‰), and −6.4‰ to 1.4‰ (mean −3.8‰). The oxygen isotopic data and trend among the five quartz generations are best interpreted to represent magmatically derived ore fluid being diluted by modified meteoric water to cause mineralization. Fluid mixing is considered to be the principal mechanism that led to Ag-Pb-Zn precipitation at Poshan. Sulfides coexisting with the quartz generations have δ34SV-CDT values of −1.7‰ to 4.4‰, with no systematic variations among different paragenetic stages. These data, combined with apatite U-Pb age and geologic relations, provide additional evidence for a magmatic origin of the Poshan Ag-Pb-Zn deposit, as inferred from the oxygen isotopes. This study highlights the role of fluid dilution in triggering Ag-Pb-Zn deposition, and sheds light on the origin of silver-polymetallic deposits in orogenic belts.

Genesis and evolution of the Natividad Au-Ag(-Ge) epithermal deposit in southern Mexico: Unravelling the final chapters of the Sierra Madre del Sur metallogeny

The final magmatic arc—Miocene in age—of the long-lived Sierra Madre del Sur igneous province in southern Mexico hosts several epithermal deposits that have hitherto received little attention. The Natividad Au-Ag(-Ge) epithermal deposit is one of them and holds a mining record of > 200 years. Here, we present the first petrogenetic and metallogenic study on the Natividad deposit, in which we use a multi-methodological approach to assess the genesis and evolution of the deposit and its associated igneous rocks. Natividad is mostly hosted by Oligocene–Miocene arc-related dacites (U-Pb zircon dates of 22.8–24.7 Ma), whose geochemical features suggest lower-crustal differentiation (i.e., involvement of garnet) at the base of a thickened continental crust. The mineralization occurs in three multi-stage quartz + carbonate veins and consists of Fe-poor sphalerite (<5 mol. % FeS), galena, chalcopyrite, acanthite, and marcasite (sulfide-dominated stage) overprinted by pearceite-polybasite, pyrargyrite-proustite, tetrahedrite-group minerals, electrum, and argyrodite (Ag8GeS6; Ag-sulfosalt-dominated stage). Geothermometry and fluid inclusion data for ore-bearing assemblages reveal fluid temperatures of 170°–400 °C and salinities of 14.6–19.5 wt% NaCl equiv. δ18O from 0.8 ‰ to 7.5 ‰ in mineralizing fluids suggests mixtures of magmatic brines and meteoric waters, while δ13C from –8.4 ‰ to 2.2 ‰ indicates the recycling of organic carbon from the meta-sedimentary basement. Further, δ34S from –3.2 ‰ to –0.3 ‰ in sulfides attest to magmatic S. These results align with an intermediate-sulfidation epithermal model, whereby upwelling magmatic ore-bearing brines precipitated the ores due to conductive cooling, dilution, and, locally, boiling. Our study highlights the metallogenic potential of Miocene magmatism in the eastern Sierra Madre de Sur.

Timing and Mechanism of Ore Precipitation in Porphyry Cu Systems: Insight from LA-ICP-MS Analysis of Fluid Inclusions and In Situ Oxygen Isotope Analysis of Hydrothermal Quartz at Zhunuo Porphyry Cu Deposit, China

Abstract The timing and mechanism of ore precipitation in porphyry copper systems are hot topics and remain controversial. The large Miocene collision-related Zhunuo porphyry Cu deposit in the Gangdese belt of southern Tibet, China, was produced by multistage quartz veining and hydrothermal alteration, accompanied by Cu sulfide precipitation. In this study, we have combined cathodoluminescence (CL) petrography with in situ oxygen isotope analysis and fluid inclusion microthermometry and laser ablation-inductively coupled plasma-mass spectrometer microanalysis to constrain the growth history of individual quartz veins, the source and evolution of the hydrothermal fluids, and the timing and mechanism of ore precipitation at Zhunuo. Early quartz A veins associated with potassic alteration are composed of quartz, K-feldspar, biotite, Cu-Fe sulfides, and pyrite. Quartz B veins are composed of quartz, Cu-Fe sulfides, molybdenite, and pyrite. CL imaging shows that quartz grains in the A and B veins consist of abundant early generation of bright-luminescent quartz (QA and QB) with volumetrically minor later generation of dull-luminescent quartz (QA-crack and QB-crack) occurring in the voids or at the margins of the QA and QB veins with embayed contacts. Cu-Fe sulfides are generally in contact with the dull-luminescent quartz and locally in contact with the bright-luminescent quartz and K-feldspar in the A and B veins or occur as disseminations in the potassic-altered porphyries that have been overprinted by chlorite ± muscovite alteration. QA and QB contain single-phase intermediate-density inclusions and abundant brine and vapor-rich inclusions. A boiling assemblage in QA has a homogenization temperature of ~560°C and trapping pressure of ~530 bar. Two boiling assemblages in QB have homogenization temperatures of ~440°C with trapping pressures of ~230 and ~250 bar, indicating a transition from lithostatic to hydrostatic conditions at a paleodepth of ~2.0 to 2.5 km. QA-crack and QB-crack contain aqueous inclusions with lower homogenization temperatures of 340° to 400°C and salinities of 6 to 12 wt % NaCl equiv. In situ oxygen isotopes indicate that QA and QB have δ18O values of 7.6 to 11.4‰, whereas QA-crack and QB-crack have δ18O values of –7.2 to 6.7‰. Combined with fluid inclusion compositions, we propose that condensation of vapors into brines and mixing with 25 to 60% meteoric water can produce the salinity and oxygen isotopes of fluids that caused the dissolution of the early bright-luminescent quartz and the precipitation of the later dull-luminescent quartz. Zhunuo Cu-Fe sulfides are in contact with the bright-luminescent quartz and cut the growth bands. The dull-luminescent quartz in contact with the Cu-Fe sulfides has oscillatory growth banding. In addition, Cu-Fe sulfides in the A and B veins or in the potassic-altered rocks coexist with chlorite ± muscovite alteration minerals. These geologic observations, together with evidence that there is an abrupt drop in the Cu/(Na + K) ratio by more than one order of magnitude in the aqueous fluids within the dull-luminescent quartz compared to the brines within the bright-luminescent quartz, indicate that precipitation of Cu-Fe sulfides occurred after the potassic alteration stage and during the chlorite ± muscovite alteration stage. Ore precipitation was associated with mixing of magmatic fluids (brines + vapors) with meteoric water, accompanied by fluid cooling, salinity decrease, and retrograde quartz solubility as the rocks transitioned from lithostatic to hydrostatic conditions. During continued cooling in the latest stage, the hydrothermal fluids precipitated D veins that contain abundant pyrite with minor quartz. Quartz in the D veins (QD) contains aqueous inclusions with much lower temperatures and Cu/(Na + K) ratios, indicating that most of the Cu-Fe sulfides have been precipitated during this stage.

The genesis of the Chakabeishan Li-(Be) pegmatite deposit in the northern Tibetan Plateau: Evidence from fluid inclusion and lithium isotope

The Chakabeishan (CKBS) deposit is a newly discovered pegmatite-type lithium-beryllium deposit in the northern Qaidam tectonic belt of the northern Tibet Plateau. In recent years, some studies have discussed the genesis of the deposit, but the topic remains unclear. This study constrains the genesis of CKBS deposit based on detailed field observations and mineralogical studies, and presents fluid inclusion data and Li isotopic compositions of Li-rich pegmatites and Li-poor pegmatites. The mineralization characteristics of CKBS deposit shows a trend from weak Be mineralization to Li mineralization in the south to north direction, which shows the magmatic differentiation feature. Four types of fluid inclusions are identified: (1) triphase crystal-bearing inclusions (type-1), (2) biphase CO2-rich inclusions (type-2), (3) biphase aqueous inclusions (type-3), and (4) monophase liquid inclusions (type-4). The Li-rich pegmatites mainly contain type-1 inclusions and type-2 inclusions, while Li-poor pegmatites contain type-3 inclusions and some type-4 inclusions. Microthermometric and Laser Raman spectroscopy analyses show that Li-rich pegmatites formed in a low to medium temperature (210.5 ∼ 381.3 ℃), low to medium salinity (5.23 ∼ 17.82 % NaCl equiv.) NaCl-H2O-CO2 system, whereas Li-poor pegmatites formed in a low to medium temperature (206.4 ∼ 413.4 ℃), low to medium salinity (1.05 ∼ 10.49 % NaCl equiv.) NaCl-H2O system, which indicates a fluid immiscibility between them. In addition, lithium is greatly enriched in Li-rich pegmatites under the positive effects of CO2. The Li-rich pegmatites display high Li content (8362.4 ∼ 15509.6 ppm) but low δ7Li values (1.61 ∼ 1.80 ‰). In contrast, the Li-poor pegmatites display low Li content (23.9 ∼ 231.9) but high δ7Li values (0.79 ∼ 12.32 ‰). This means that the lithium isotopic fractionation is produced by the melt-fluid immiscibility in CKBS deposit, in which a Li-rich system with rich water and poor silicate and a Li-poor system with poor water and rich silicate were formed during the process. Therefore, the CKBS deposit may have formed under the combined effects of magmatic differentiation, melt-fluid immiscibility, and fluid immiscibility.

Neoproterozoic-Early Cambrian Northern Chah-Farsakh Cu-Zn VMS-type deposit, Iran: Constraints from geology, geochemistry, fluid inclusions and geodynamic

Northern Chah-Farsakh is the first seafloor VMS described in the Torud-Chahshirin metallogenic belt of Iran. The deposit is hosted by Neoproterozoic-Early Cambrian mafic (basaltic andesitic)-siliciclastic rocks in volcano-sedimentary basin. The evolution of the ore-bearing basin is governed by the Proto-Tethys Oceanic crust subduction beneath the Central Iranian microcontinent. Three ore facies can be distinguished based on the ore textures studies: stringer zone ore facies, massive-replacement ore facies, bedded ore and distal facies. Pyrite, chalcopyrite, magnetite and sphalerite are the main minerals, together with minor amounts of pyrrhotite, bornite, tetrahedrite and tennantite. Textures include laminated, banded, replacement, massive, and vein-veinlet texture. There is a distinct metal zonation in the massive sulfide orebody; Gold, Ag, and Zn contents increase vertically from the stringer to the bedded ore facies; Cu and Co increase in the massive ore facies, and Pb is highest in the stringer zone. Ore mineralization in the Northern Chah-Farsakh deposit has emplaced in two stages. The fine-grained ore bands (stage 1) which are intricately interlayered with host rock beds, exhibit sedimentary structure such as laminae and bedding are consistent with a syn-sedimentary origin. Coarser-grained stage 2 ores have massive and vein-veinlet textures that are considered to form by replacement during sub-seafloor fluid flow. Based on SEM-EDS analysis, significant compositional variations were observed in the Py-1b grains. Py-1b is enriched in Pb compared to Py-1a. The elemental distribution map shows that the chalcopyrite (Cpy-2) minerals in Northern Chah-Farsakh deposit do not have variable composition and are homogeneous. Fluid inclusions in quartz minerals rom the footwall stringer zone have trapping temperatures between 203 and 500 °C with salinities (3.39–16 wt% NaCl equiv.) higher than modern seawater. Fluid inclusion studies suggest that the ore-forming fluids were derived from seawater. High level of Se in the massive ore facies of the Northern Chah-Farsakh supports the formation of mineralization from a high-temperature hydrothermal fluid. The Northern Chah-Farsakh Cu-Zn VMS deposit typify the mafic-siliciclastic type (Besshi-type), where the majority of the host stratigraphy is composed of a mafic flow-volcaniclastics and sedimentary rocks.

Nature, source, and evolution of the ore-forming fluids in the Dunbasitao gold deposit, East Junggar, China: Constraints from geology, fluid inclusions, and C-H-O isotopes

The origin of CO2-dominated fluids in orogenic gold deposits is usually mired in controversy. The Dunbasitao gold deposit is the largest among several gold deposits along the Armantai suture zone, East Junggar, China. Orebodies are hosted in volcano-sedimentary rocks of the Lower Carboniferous Jiangbasitao Formation and porphyritic quartz diorite and are controlled by shear zones. Based on the cross-cutting relationship of veins, the hydrothermal process can be divided into the early, middle, and late stages and the gold only occurred in the middle stage. Seven types of fluid inclusions (FIs) were identified in the Dunbasitao deposit: Type-1 FIs (LH2O + LCO2 or LH2O + LCO2 + VCO2) that homogenized to H2O phase or show critical homogenization; Type-2 FIs (LH2O + LCO2 or LH2O + LCO2 + VCO2) that homogenized to CO2 phase; CO2-dominated Type-3 FIs (LCO2 or LCO2 + VCO2); Two-phase aqueous Type-4 FIs (LH2O + VH2O) that homogenized to liquid phase; Type-5 FIs are composed of three phases (LH2O + VH2O + Solid), which homogenized to liquid phase; Type-6 and Type-7 FIs refers to FIs composed of a single liquid or vapor H2O phase, respectively.Microthermometry and laser Raman spectroscopy (LRS) analyses show that the initial ore-forming fluids in the early stage were moderate-Th (326–342 °C), low salinity (2.4 to 4.7 wt% NaCl equiv), and homogenous H2O−CO2−NaCl liquid fluids, containing 13–33 mol% CO2 and a small amount of CH4, H2S, and N2. The calculated δ18OH2O (V-SMOW) values are 7.5–8.5 ‰, δ18DH2O (V-SMOW) values range from −100 to −96 ‰, and δ13Cfluid (PDB) range from −3.8 to −2.5 ‰. In the middle stage, two typical immiscible fluid inclusion assemblages (FIA.18 and FIA. 27) composed of both Type-1 and Type-2 FIs were measured. The salinity of FIA.18 is 3.3 wt% NaCl equiv. and Th is 320 °C. For FIA.27, the salinity is 2.2 wt% NaCl equiv. and Th is 298 °C. The salinities of Type-4 FIAs vary from 5.9 to 14.4 wt% NaCl equiv., significantly higher than Type-1 and Type-2 FIs (1.7 to 3.5 wt% NaCl equiv). The salinities of three isolated Type-5 FIs (halite-bearing) range from 30.3 to 32.9 wt% NaCl equiv. and Th vary from 257 to 277 °C. These facts indicate extreme unmixing between carbonic and aqueous fluids in the middle stage, which led to gold precipitation. The Type-3 FIs represent the salt-free, H2O-lost, CO2 end-member composition, whilst the Type-5 FIs and high-salinity Type-4 FIs represent the highly saline aqueous end-member composition. Late-stage fluids are initially liquid, very low-salinity (1.4 to 2.6 wt% NaCl equiv), and low-Th (176–233 °C) aqueous fluids.Two immiscible carbonic FIAs from the middle stage yield a trapping pressure (Pt) of 86.9–133.9 MPa. Considering the rapid pressure fluctuations during fluid immiscibility, using hydrostatic or lithostatic pressure alone cannot get the real mineralization depth, Therefore, we use the lithostatic pressure for the Pt maximum and hydrostatic pressure for the Pt minimum and calculated a mineralization depth of 5.3–8.9 km, which doesn't represent the change but uncertainty for depth. Combining the characteristics of ore-forming fluids with the hydrothermal alteration and mineral compositions of ores, the Dunbasitao gold deposit is determined as a mesozonal orogenic gold deposit and there is considered to be prospecting potential in the deep.

The formation mechanism of the Xilekuduke porphyry Mo‐Cu deposit, NW China, revealed by the fluid inclusions and H‐O‐S isotopes

AbstractThe Xilekuduke porphyry Mo‐Cu deposit is located in the Altay‐East Junggar region of the Central Asian Orogenic Belt, northwest China. The orebodies occurring as vein type are host within the monzogranite and granite porphyry. Ore minerals include mainly molybdenite, pyrite, and chalcopyrite, whilst the major alteration include potassic, sericite, carbonate, and silicic. Mineralization can be divided into three stages: quartz‐K‐feldspar–polymetallic stage (Stage I), quartz‐polymetallic stage (Stage II), and quartz–calcite–pyrite (minor) stage (Stage III). Three types of fluid inclusion are present in the Mo‐Cu sulfide–calcite–quartz veins: CO2‐bearing (C‐type), aqueous (W‐type), and daughter mineral‐bearing (S‐type). Petrographic and microthermometric analyses of the fluid inclusions yielded homogenization temperatures for Stage I, II, and III to be 402–499°C, 214–391°C, and 136–254°C, respectively, with corresponding salinities of 39.2–59.6, 3.7–44.9 and 4.1–14.4 wt% NaCl equivalent. The δ18OH₂O and δD values of fluid inclusions in quartz are determined to be 5.3–6.0 ‰ and −76 to −60 ‰ (Stage I), 1.7–3.2 ‰ and −96 to −90 ‰ (Stage II), and −2.6 to −2.4 ‰ and −106 ‰ (Stage III), respectively. These results indicate that the primary ore‐forming fluids (stages I and II) were derived from granitic magma and were mixed with meteoric water in stage III. For the sulfide and sulfate (anhydrite), their δ34S values are of 0.4–5.8 ‰, 13.9–14.4 ‰, respectively, also that suggest a magmatic source. Fluid immiscibility, meteoric water interaction, and ore fluid‐wallrock interactions may have been critical for molybdenum precipitation.

Capitana mine, Tignamar district, a unique tin – bismuth concentration in the Chilean Central Andes, 250 km west of the Bolivian Tin Belt: Link to the Proterozoic-Paleozoic basement Belen inlier and the Arequipa-Antofalla terrane, a probable remnant of Laurentia

The Capitana mine, Tignamar District, is a unique Sn-Bi concentration in the highlands of Arica, in the Chilean Central Andes. Tin deposits associated with Triassic to Pliocene age igneous rocks are plentiful in the Tin Belt of Peru, Bolivia, and NW Argentina, but have not been reported in Chile, where contemporaneous rocks formed metallogenic belts rich in Fe, Cu, Mo, Ag and Au. We describe vein ore from Capitana mine, a small but exceptional Sn-bearing Pb-Bi-Ag-Cu-Sb-As epithermal deposit in the Tignamar (or Ticnamar) district in the high Andes of Arica, Chile, ca. 250 km west of the Bolivian Tin Belt. Fluid inclusions in vein quartz trapped a non-boiling, low salinity (<∼4 wt% NaCl equiv.), moderate T fluid (minimum T ∼ 220 °C) prior to metal precipitation. Flashing of this fluid, likely due to rapid changes in confining P, was synchronous with metal deposition; sulfosalt textures show evidence of syn-deformational growth. Although located within a district considered a porphyry copper system of Early Miocene age, it is likely that the Capitana deposit formed later, mainly as a result of regional hydrothermal activity related to compressive and transpressional tectonics as young as Pliocene. Common lead is 206Pb/204Pb 18.18; 207Pb/204Pb 15.61; and 208Pb/204Pb 38.53, significantly less radiogenic than that of Bolivian Sn deposits, and has a Paleozoic model lead age. These values point to affinities with a tectonically-emplaced inlier of crystalline basement of Proterozoic – Early Paleozoic age known as the Belen Metamorphic Complex, and with the Arequipa-Antofalla basement terrane, a probable remnant of Laurentia (North America), left behind during the dismemberment of the Rodinia Supercontinent. The complex mineralogy of Capitana vein, unlike other Chilean deposits, includes various sulphides and sulphosalts with Cu, Fe, Zn, Pb, Bi, Sn, Ag, As, Sb, Ga and In, and is exceptionally rich in U.

Genesis of Pb-Zn deposits in northwestern Guizhou province of China: Constraints from the in situ analyses of fluid inclusions and sulfur isotopes

The northwestern Guizhou ore concentration area on the southwest margin of the Yangtze Block is an important part of the Sichuan-Yunnan-Guizhou Pb-Zn metallogenic province. Several large Pb-Zn deposits (e.g., Shanshulin, Shaojiwan, Yadu, and Tianqiao) are found in the area with similar mineral assemblages, ore-controlling structures, ore-hosting rocks, and genetic types. In this study, fluid inclusion microthermometry and in situ geochemical analysis were employed to determine the nature and compositions of the fluid inclusions trapped in calcite and sphalerite and the sulfur isotopic compositions of sphalerite and pyrite. Microthermometric results show that the ore-forming fluids are moderate to low temperature and salinity. The fluid inclusions in calcite and sphalerite share similar homogeneous temperatures (120 to 200 °C) and salinities (7–14 wt% NaCl equiv.). The in situ LA-ICP-MS analyses reveal that the fluid inclusions have high concentrations of elements such as Na, K, Mg, Ca, Rb, Sr, Ba, Cs, and Li. The ratios of K/Na, Rb/Na, Cs/Na, Ca/Na, and Ba/Na could be comparable to the basinal brines. The trend towards metamorphic water is possibly attributed to the involvement of basement-derived fluids. The in situ δ34S values of sphalerite and pyrite range from 11.3 to 20.2 ‰ and 12.0 to 18.9 ‰, respectively, which indicate that the reduced sulfur is most likely related to the thermochemical reduction of sulfate with multiple sources either from the ore-hosting strata or from Precambrian-Cambrian basement rocks which have high sulfur isotopic compositions. We propose that the fluid mixing is the main mechanism for the formation of Pb-Zn deposits in NW Guizhou.

Overlapping events in the Acoculco geothermal system, Puebla, Mexico

The Acoculco geothermal system records two major hydrothermal events (prograde skarn overlapped by retrograde/”epithermal” to geothermal stages) in association with pre-calderic to calderic events in the eponymous complex, which belongs to the Trans-Mexican Volcanic Belt. The prograde skarn is associated with a hornblende granite (hereby dated at 1.01 ± 0.01 Ma; U-Pb zircon age), whose apex is found at a ∼1600 m depth, and later andesite dykes and sills. Mineralizing fluids in this stage have salinities of fluid inclusions that range between 9.5 wt.% NaCl equiv. and 33 wt.% NaCl and temperatures of homogenization (Th) up to 470 °C that record various process of mineral formation, such as boiling, depressurization, conductive cooling, isothermal mixing, and dilution. The retrograde stage is represented by early “fossil” brines with salinities 1.4 and 7.3 wt.% NaCl equiv. and Th up to 303 °C, and present-day gasothermal fluids that attain stabilized temperatures up to ∼300 °C at a depth of almost 2000 m. Th of early brines are consistently higher than stabilized temperatures at most of the depths that were surveyed, the isotherms that correspond to 250° and 200 °C dropped ca. 500 m from the early retrograde to the gasothermal stage. These features support the hypothesis that the Acoculco geothermal system has entered its senile stage. Despite that, upon a calculated geothermal gradient at ∼150 °C/km for both Th of the early retrograde stage and stabilized temperatures in the EAC-1 well, it is not foreseen a rapid waning of the thermal anomaly and its ultimate cause.Besides the early Ca-silicate assemblages that constitute the skarn, the architecture of alteration assemblages also records the overlapping of shallow on deep hypogene assemblages. The deep hypogene assemblages respond to alkaline to moderately acidic fluids that correspond to the early retrograde (or “epithermal”) stage: propylitic, phyllic and argillic (kaolinite + nontronite + montmorillonite), from bottom to top. Contrastingly, the shallow hypogene assemblages respond to dominantly acidic fluids, and consist of advanced argillic assemblages that may grade upwards into argillic (kaolinite + illite-smectite) or silicic (vuggy or massive to brecciated silica). The shallow hypogene character of advanced argillic assemblages, as due to steam-heated ground environments (as opposed to deep hypogene or magmatic-hydrothermal assemblages), is deduced from (1) the overall tabular shape of their distribution, (2) the mineralogical association and characteristics of rhombohedral alunite within it, and (3) its relationship with overlying silicic alteration that, in turn, do not possess likely characteristics of sinters. Shallow hypogene advanced argillic assemblages are associated with the present-day gasothermal activity due to sealing that was possibly promoted during the early retrograde/”epithermal” activity.

Separation of Au and Sb mineralization in the Qukulekedong intrusion-related deposit, East Kunlun Orogen (NW China): Evidence from fluid inclusions, H–O isotopes, and quartz geochemistry

Separation of Au and Sb mineralization from different hydrothermal stages is a common phenomenon in many Au–Sb deposits, but its cause remains enigmatic. The Qukulekedong intrusion-related Au–Sb deposit in the East Kunlun Orogen has undergone four hydrothermal stages: (1) barren quartz vein, (2) quartz vein with disseminated arsenopyrite–pyrite alteration halo, (3) stibnite–quartz ± native gold vein, and (4) calcite–quartz vein. Stages 2 and 3 formed disseminated Au and vein-type Sb ± Au ore, respectively. Here, internal texture and trace elements in quartz, fluid inclusions (Fls) microthermometry, and H–O isotopes were analyzed to unravel the separation mechanism of the Au and Sb ± Au mineralization at Qukulekedong. Stage 2 quartz has euhedral growth zones with elevated Ti-Al-Li-K-Sb and varying As contents. Stage 2 FIs have medium homogenization temperature (203–307 °C) and salinity (12.2–17.8 wt% NaCl equiv.), and are CH4-rich. They have δ18OH2O = 9.5–10.8 ‰ and δDH2O = −93.4 to −80.8 ‰. In contrast, stage 3 quartz has lower Ti-Al-Li-K and varying Sb contents. Stage 3 FIs show phase separation (coexistence of V-, L- and V-L-type FIs), and are of lower temperature (141–164 °C) and salinity (2.1–10.7 wt% NaCl equiv.), and CO2– and CH4-bearing. They have δ18OH2O = 3.3–5.2 ‰ and δDH2O = − 95.4 to − 89.6 ‰. Considering that the disseminated Au ore is Sb-rich and its pyrite has fine stibnite inclusions, we suggest that the disseminated Au and vein-type Sb ± Au mineralization shared a common Au- and Sb-rich initial magmatic fluids. The disseminated Au mineralization may have formed by fluid–rock interactions. Meanwhile, the vein-type Sb ± Au mineralization was likely caused by the phase separation and cooling led by dilational activities and meteoric water injection.

Sulfur and Carbon–Oxygen Isotopic Geochemistry and Fluid Inclusion Characteristics of the Yolindi Cu-Fe Skarn Mineralization, Biga Peninsula, NW Turkey: Implications for the Source and Evolution of Hydrothermal Fluids

The current study sought to investigate the physiochemical conditions and fluid evolution within the Yolindi Cu-Fe skarn mineralization located in the Biga Peninsula, NW Turkey. This was accomplished through a comprehensive investigation of geological and mineralogical data, along with isotopic analyses of sulfur (δ34S), carbon (δ13C), and oxygen (δ18O) of sulfide and calcite minerals, respectively, as well as fluid inclusion data pertaining to various minerals (e.g., andradite, quartz, and calcite). The Yolindi area features a complex geological framework, including the Paleozoic Kalabak Group (which includes the Torasan, Yolindi, and Sazak formations) and the Triassic Karakaya Complex. These formations were subsequently intruded via Early Miocene Şaroluk granitoids and Hallaçlar volcanics. Skarn formation is zoned into endoskarn and exoskarn types (being categorized into proximal, intermediate, and distal zones), with distinct mineral assemblages indicating concentric and contact metamorphic alteration patterns around the western part of Şaroluk granitoid intrusion in contact with the Torasan formation. The ore mineralogy and paragenesis suggest three distinct stages of evolution: an initial phase of prograde metasomatism characterized by the formation of magnetite and pyrite alongside anhydrous calc-silicate minerals; a subsequent phase of retrograde alteration marked by the formation of epidote, actinolite, and scapolite, accompanied by the occurrence of chalcopyrite and specular hematite; and finally, a post-metasomatic stage involving oxidation processes that led to the development of secondary mineral assemblages containing cerussite, covellite, and malachite. Sulfur isotopes (δ34S) of sulfides from endoskarn (from +0.27 to +0.57‰VCDT) to intermediate exoskarn (from −9.44 to −5.46‰VCDT) zones indicate a diverse sulfur source, including magmatic, sedimentary, and possibly organic matter. δ34S values in hydrothermal fluids suggest a magmatic–hydrothermal origin, with endoskarn and proximal zone fluids showing a slight negative signature and intermediate zone fluids indicating a strong influence from organic-rich or metamorphic sulfur reservoirs. Carbon and oxygen isotopic compositions (δ13C and δ18O) of calcite revealed a progression from marine carbonate signatures in marble samples (from +1.89 to +2.23‰VPDB; from +21.61 to +21.73‰VSMOW) to depleted values in prograde (from −6.0 to +0.09‰VPDB; from +6.22 to +18.14‰VSMOW) and retrograde skarns (from −3.8 to −2.25‰VPDB; from +0.94 to +3.62‰VSMOW), reflecting interactions with high-temperature magmatic fluids and meteoric water mixing. The fluid inclusions in prograde minerals generated under the conditions of fluid boiling exhibited high temperatures, reaching up to 412 °C, and salinities up to 26 wt.% NaCl equivalent. Conversely, the fluid inclusions in retrograde minerals, which were generated due to fluid mixing, exhibited lower temperatures (with an average of 318 °C) and salinities with an average of 4.9 wt.% NaCl equivalent. This indicated that the cooler and more diluted fluids mix with meteoric waters and interact with organic materials in the host rocks. This suggests a multifaceted origin involving various sources and processes. Therefore, this study concluded that the skarn mineralization in the Yolindi area resulted from complex interactions between magmatic, metamorphic, and meteoric fluids, reflecting a dynamic ore-forming environment with implications for the regional metallogeny of Cu-Fe skarn deposits.

Algama Gold Ore Cluster (Aldan–Stanovoi Shield): Mineralogy, Formation Conditions, Sources of Ore Matter, and Age of Mineralization

Abstract —The paper presents results of research into the conditions of formation of ores in the poorly studied Algama ore cluster (Bodorono deposit and Dyvok ore occurrence) located at the junction of the Aldan Shield and the Stanovoi area. We have established that Bi and Se minerals (bismuthinite, lillianite, native bismuth, tellurobismuthite, tetradymite, hedleyite, pilsenite, and laitakarite) are present in the ores of the Bodorono deposit. Two successive productive stages of mineral formation have been distinguished: Au–polymetallic and Au–Bi–Te. The corresponding minerals are products of the evolution of a hydrothermal system, during which a gradual decrease in fluid temperature (from 300 to 145 °C) and salinity (from 5 to 1.9 wt.% NaCl equiv.) took place. The fineness of native gold gradually increases from early (~840‰) to late (~940‰) stages and changes in passing from simple sulfides to sulfosalts. The evolution of the ore system is accompanied by a change in the composition of the vapor phase of fluid inclusions from CH4–CO2 to CO2 with an impurity of N2 and CH4. The results of 40Ar/39Ar dating of pre-ore metasomatites point to ore-forming processes at the Bodorono deposit ca. 150 ± 1.8 Ma. Analysis of the isotopic composition of lead in galena shows the leading role of the ancient crustal source of ore matter. The calculated isotopic composition of oxygen (δ18OH2O) in ore-bearing quartz varies from 1.0 to 7.3‰, which corresponds to an aqueous fluid of a mixed source. The Dyvok ore occurrence differs from the Bodorono deposit in the mineral composition of ores and the physicochemical parameters of ore formation. Four mineral stages have been established within the ore occurrence: gold–arsenopyrite–pyrite–quartz, pyrite–chalcopyrite–sphalerite, quartz–boulangerite, and telluride. The telluride stage is represented by hessite, altaite, volynskite, merenskyite, melonite, and rucklidgeite. Gold-bearing mineralization formed from a fluid of medium salinity (0.9–9.2 wt.% NaCl equiv.) with a predominance of CO2 and an impurity of CH4 in the vapor phase at moderate temperatures (310–360 °C). The calculated values of δ34S and δ18O varied from 2.2 to 3.0‰ and from 0.6 to 12.0‰, respectively. The 40Ar/39Ar age of gold mineralization is 124.0 ± 1.5 Ma, which corresponds to the stage of tectonomagmatic activity in the Aldan Shield.

Evolution of Ore-Forming Fluids at Azegour Mo-Cu-W Skarn Deposit, Western High Atlas, Morocco: Evidence from Mineral Chemistry and Fluid Inclusions

The Azegour Mo-Cu-W skarn deposit, located on the northern side of the Western High Atlas, occurs in lower Cambrian volcanic and sedimentary rocks. The mineralizations are linked to the hydrothermal alterations that affected carbonated layers of the lower Cambrian age during the intrusion of the calc-alkaline hyperaluminous Azegour granite. Four stages of the skarn and ore mineral deposition have been identified as follows. Firstly, (i) the early prograde stage and (ii) the late prograde stage. These prograde stages are characterized by anhydrous minerals (wollastonite, garnets, and pyroxenes) associated with scheelite mineralization. Based on mineral chemistry studies, the early prograde stage is dominated by andradite (Ad72.81–97.07) and diopside (Di61.80–50.08) indicating an oxidized skarn; on the other hand, the late prograde stage is characterized by a high portion of grossular (Gr66.88–93.72) and hedenbergite (Hd50.49–86.73) with a small ratio of almandine (Alm2.84–34.99), indicating “strongly reduced” or “moderately reduced” conditions with low f(O2). The next two stages are (iii) the early retrograde stage and (iv) the late retrograde stage, which contain hydrous minerals (vesuvianite, epidote, chlorite, muscovite, and amphibole) associated with sulfide. Fluid inclusions from pyroxene and quartz (prograde skarn stage) display high homogenization temperatures and high to low salinities (468.3 to &gt;600 °C; 2.1 to &gt;73.9 wt% NaCl equiv.). The boiling process formed major scheelite mineralization during prograde skarn development from dominated hydrothermal magmatic fluid solutions. By contrast, fluid inclusions associated with calcite–quartz–sulfide (retrograde skarn stage) record lower homogenization temperatures and low salinities (160 to 358 °C; 2.0 to 11.9 wt% NaCl equiv.). The distribution of the major inclusions types from the two paragenetic stages are along the trend line of fluids mixing in the salinity–homogenization temperature (magmatic water), illustrating the genesis of ore-forming fluid by mixing with fluids of low temperatures and salinities (metamorphic and meteoric waters).

A transition from porphyry Mo to vein-style Zn-Pb mineralization: Insights from hydrothermal fluid evolution of the Chalukou deposit, NE China

To reveal the hydrothermal evolution from porphyry Mo to vein-style Zn-Pb mineralization, the Chalukou deposit in NE China with these two typical mineralization overprinted is selected for detailed research. From early to late hydrothermal veins, the quartz cathodoluminescence intensities change from bright to dark and the textures also evolve. The Ti concentrations of quartz in these veins change from high to moderate and then to low values. The fluid inclusion petrography, microthermometry and stable isotope analyses indicate intermediate-density magma-derived fluids (δ18Owater = +4.7 to + 6.2‰, δD = −132 to −136‰) exsolved at high temperatures (560° to 600 °C) with low to moderate salinity (2.4 to 8.0 wt% NaCl equiv) containing minor CO2 (<5.7 mol %). Early minor molybdenite occurred under lithostatic conditions at high temperatures (540° to 580 °C) during fluids pH change and early magnetite precipitation. Later main molybdenite precipitation at moderate temperatures (350° to 430 °C), result from fluid cooling and involving of meteoric water under the transition from lithostatic to hydrostatic conditions. The precipitation of pyrite in transitional veins at moderate temperatures (330° to 420 °C), and the late sphalerite-galena from a lower-temperature fluid (180° to 300 °C) is consistent with the cooling of magmatic fluid, and gradually dilution with meteoric water under hydrostatic conditions. It is deduced the transition from porphyry Mo to vein-style Zn-Pb mineralization is likely driven by progressive decrease in temperature of the magmatic fluids and the increasing influx of meteoric water.