Stages Of Hydrothermal Activity Research Articles

Formation and evolution of multistage hydrothermal fluids in the Baishitouwa quartz–wolframite vein‐type deposit in the southern Great Xing'an Range tungsten belt, NE China: Constraints from individual fluid inclusion LA‐ICP‐MS analysis

Revealing hydrothermal evolution from the early oxide to late carbonate stages for quartz–wolframite vein‐type deposits is essential for understanding the ore‐forming process. In this study, we choose the Baishitouwa tungsten polymetallic deposit located in the southern Great Xing'an Range tungsten belt as a case study, and present detailed deposit geology and in situ fluid inclusion (FI) analyses including microthermometry, laser Raman spectra, and LA‐ICP‐MS microanalysis to address this issue. Four stages of hydrothermal activity were identified: (1) quartz–wolframite (I), (2) quartz–wolframite (II)–pyrite–chalcopyrite, (3) quartz–polymetallic sulphides, and (4) quartz–carbonate. Four types of FIs were recognized: CO2‐rich, CO2‐bearing, liquid‐rich, and brine inclusions. Microthermometric data showed that the homogenization temperatures and salinities from the early to late stages are 380–460°C, 7.4–17.3, and 29.3–43.2 wt% NaCl equiv., 300–390°C and 7.1–17.0 wt% NaCl equiv., 220–320°C and 2.7–8.1 wt% NaCl equiv., and 150–250°C and 0.5–4.8 wt% NaCl equiv., respectively, suggesting a decreasing trend. Geochemically, all stage fluids contained high Rb and Mn concentrations, high Rb/Na, Cs/Na, Li/Na, K/Na, Rb/Sr, low K/Rb, and consistent Cs/Rb and Cs/(Na + K) ratios, indicating that the mineralizing fluids originated from a common source—an underlying, geochemically uniform, and highly fractionated granitic magma. Fluid immiscibility and cooling are the main mechanisms for wolframite precipitation, whereas greisenization is subordinate; the incursion of meteoric water into the hydrothermal system initiated at the sulphide stage, and fluid mixing is the dominant mechanism for sulphide precipitation.

Fluid inclusion evidence for hydrothermal evolution of the Sadaigoumen porphyry Mo deposit on the northern margin of the north China Craton

The Sadaigoumen porphyry Mo deposit is located on the northern margin of the North China Craton (NCC). In this study, in situ fluid-inclusion analyses including microthermometry, laser Raman spectroscopy, and LA–ICP–MS microanalysis, as well as Quartz H–O isotopes are present to precisely constrain the hydrothermal evolution of the Sadaigoumen deposit. Five stages of hydrothermal activity are identified, with Stage 1 characterized by the development of magnetite with rare sulfide minerals; Stage 2 by K-feldspar as the typical vein mineral; Stage 3 by molybdenite mainly in veins; Stage 4 by the development of fluorite–quartz and quartz–pyrite veins; and Stage 5 by barren carbonate–quartz veins. Fluid inclusion types were distinguished based on petrographic observations and microthermometric analysis, and fluid compositions were detected by LA–ICP–MS. Microthermometric analyses indicate decreasing homogenization temperatures in primary fluid inclusions from Stage 1 to Stage 3. Intermediate-density fluids of stages 1 and 2 are characterized by high homogenization temperatures (420 °C–470 °C), moderate salinities (6.3–11.2 wt%), and high Mo contents (52–541 ppm). Hydrothermal evolution during Mo mineralization in Stage 3 is characterized by immiscible inclusion assemblages that formed through phase separation from intermediate-density fluid. Fluid inclusions of Stage 3 were trapped at pressures fluctuating around the two-phase fluid surface, leading to at least two fluid immiscible phases (373 °C–414 °C and 311 °C–341 °C). The Mo contents of fluid inclusions vary greatly in Stage 3, with Mo tending to be enriched in brine. In stages 4 and 5, fluids display characteristics that indicate mixing with meteoric water, with homogenization temperatures (<299 °C) and salinities (<5.8 wt%) decreasing together with Mo content. The Cs/(Na + K + Mn + Fe) ratios of fluid inclusions are stable from stages 1 to 4, indicating a constant supply of ore-forming fluids from an underlying, geochemically uniform magma chamber. The fluid inclusion data suggest that Mo and Cu in the initial exsolved fluids of the Sadaigoumen porphyry Mo deposit display different migration and precipitation progresses, which is of great significance to the study of the delicate ore-forming progress of porphyry Mo (–Cu) deposits in the northern NCC.

Early Paleozoic Orogenic Gold Deposit in the Cathaysia Block, China: A first example from the Shuangqishan Deposit

The Shuangqishan gold deposit (>20t gold) is located in a hitherto poorly-documented gold province in the southeastern Cathaysia Block, South China. It is hosted by Neoproterozoic amphibolite-facies metamorphic rocks and consists of quartz-sulfide veins with distinct alteration haloes. Three stages of hydrothermal activity have been identified, and gold is mainly associated with veins of Stage II (quartz-pyrite±chalcopyrite). Gold occurs both as refractory gold hosted by pyrite and free gold enclosed in, or filling, microfractures of pyrite and quartz. U-Pb isotope dating of hydrothermal apatite from the Substage IIa veins yielded an age of 436.8 ± 7.6 Ma. The ore fluid was of carbonic-aqueous composition and low oxygen fugacity. Total homogenization temperatures in Substage IIa and IIb range from 270° to 310°C and 250° to 295°C, respectively. Variable CO2/H2O ratios indicate fluid unmixing under fluctuating pressures. Stage I quartz veins show evidence of ductile deformation and have δ18Ofluid and δD values of 10.3 to 11.2‰ and −67to −66‰, respectively, consistent with a metamorphic fluid. Stage II veins have slightly higher δ18O values of 10.9 to 13.7‰, with calculated δ18Ofluid values of 4.8 to 8.0‰. The 3He/4He ratios of pyrite are elevated (0.06–0.23 Ra) relative to crustal He reservoirs (<0.01 Ra). Fluid inclusions in quartz yielded δ13CCO2values of −23.0to −11.4‰ (V-PDB), reflecting variable contributions of organic carbon. The sulfide δ34SV-CDT values range from −4.7to −1.0‰, consistent with a deep-seated sulfur source. The Pb isotope compositions of pyrite (206Pb/204Pb=17.141–17.871, 207Pb/204Pb=15.484–15.570) are more radiogenic than those of unaltered host rocks. All available data demonstrate that the Shuangqishan deposit is the first report orogenic gold deposit formed during the late Wuyi-Yunkai intracontinental orogeny in the Cathaysia Block.

Hydrothermal alteration characteristics of the Chating Cu-Au deposit in Xuancheng City, Anhui Province, China: Significance of sericite alteration for Cu-Au exploration

The Chating Cu-Au deposit in Xuancheng City, Anhui Province, is characterized by porphyry-style mineralization with intensive hydrothermal alteration. Short-wave infrared reflectance spectroscopy was used to systematically analyze the alteration of drill cores from two cross sections. Eleven alteration minerals—sericite, smectite, kaolinite, dickite, chlorite, epidote, gypsum, carbonate, iron oxide, saponite, and prehnite—were identified, and the relative abundances of individual minerals at various depths were calculated by depth of absorption peak. In these minerals, iron oxides and gypsum are probably related to supergene process and other alteration minerals are hydrothermal alteration minerals. The mineral assemblages and their spatial distributions indicate that the alteration patterns in this Cu-Au deposit were controlled mainly by water–rock interaction of the magmatic–hydrothermal process, resulting in a typical porphyry-style hydrothermal mineralization system. A saponite–smectite–K-feldspar–anhydrite (gypsum) alteration assemblage is located mainly in the host quartz diorite porphyry, and is minor in the wall rocks of limestone and a post-mineralization diorite porphyry. A sericite–chlorite alteration assemblage was widely developed in the quartz diorite porphyry, and the intensity of alteration decreases gradually outward from the center to the margins of the deposit. A kaolinite–calcite alteration assemblage is mainly located on the top of the quartz diorite porphyry and in the surrounding limestone. The physiochemical conditions of the hydrothermal fluid, such as temperature and pH, control the compositional variations of specific alteration minerals. The most remarkable feature is a subtle change in octahedral Al cation content of sericite, as identified by the change in wavelength of sericite Al-OH absorption peak position. Muscovite is the main component of sericite, with a trace amount of phengite. The Cu-Au grades are negatively correlated with the wavelength of sericite Al-OH absorption peak position, because high ore grades are associated with relatively shorter Al-OH wavelengths, corresponding to a high Al/Si ratio in sericite (i.e. towards muscovite). This correlation suggests that the ore metal tends to be enriched and precipitated in an acidic environment of the alteration system. In addition, a lateral variation across the deposit in Al-OH wavelength indicates that the acidity of the hydrothermal fluids decreases gradually from the center of the deposit outward to the wall rocks, whereas the variation of sericite Al-OH wavelength with depth suggests an increase in fluid pH with decreasing depth. The alteration patterns imply that during mineralization the ore-forming fluids probably migrated from the southwest in the deep part of the porphyry system towards the northeast near the surface. Based on the spectral alteration mapping, it is concluded that there were at least three stages of hydrothermal activity in the Chating mining district: an early potassic–skarn hydrothermal alteration stage, an intermediate propylitic–sericite medium-low temperature alteration stage, and a late kaolinite–dickite epithermal stage. The sericite alteration stage is temporally related to Cu-Au mineralization, leading to the formation of the main orebody at Chating. As the current exploration is still focused in the sericite alteration zone, it is inferred that deep exploration has great potential in the Chating district.

Genesis of the Linghu Au deposit in Xiaoqinling Region, Henan province, China: Constraints from fluid inclusions and isotope systematics

The Linghu Au deposit is located within the northeastern part of the Xiaoqinling region in the southern margin of the North China Craton. Field investigations, cross-cutting relationships, and mineral paragenetic associations have identified three stages of hydrothermal activity. To determine the origin and evolution of ore-forming fluids, and better understand genesis of the Linghu Au deposit, we have undertaken a series of studies including fluid inclusion and He–Ar–H–O–S–Pb isotope systematics. Fluid inclusion study indicates that the early stage of gold mineralization was characterized by a high-temperature and low-salinity H2O–CO2–NaCl hydrothermal fluids that have δD values of – 82.0 to – 78.6‰ and δ18OH2O values of 5.3 to 6.4‰. The intermediate stage of gold mineralization involved moderate temperature H2O–CO2–NaCl hydrothermal fluids with δD and δ18OH2O values of – 92.7‰ to – 87.9‰ and – 1.4‰ to 0‰, respectively. Pyrite samples from this mineralization stage have 3He/4He and 40Ar/36Ar ratios of 0.26–0.31 Ra and 2834.7–4300.6, respectively. The last mineralization stage involved (low-temperature and low-salinity NaCl–H2O system of hydrothermal fluids with relatively lower δD values of – 93.1 to – 92.3‰ and δ18OH2O values of – 5.7 to – 5.2‰. Moreover, sulfides in ores yielded δ34S values of – 3.6 to 5.9‰, with an average of 2.65‰. The sulfides have 206Pb/204Pb ratios of 17.101–17.804, 207Pb/204Pb ratios of 15.438–15.554, and 208Pb/204Pb ratios of 37.438–38.654. The results of fluid inclusions and H–O, S–Pb, and He–Ar isotopes indicate that hydrothermal fluids associated with the early stage of mineralization originated from magmatic fluids, with sulfur and lead being derived from both units of the Taihua Group and a Late Triassic magmatic–hydrothermal system. All of these data indicate that the Linghu gold deposit formed as a result of magmatic–hydrothermal mineralizing processes.

Geology of the Souter Head subvolcanic complex, Aberdeenshire, Scotland: an Ordovician granite-related Mo–(Bi–As–Au) system

ABSTRACTAn Ordovician subvolcanic intrusive complex hosted by Neoproterozoic metasediments crops out at Souter Head about 6km S of Aberdeen, Scotland. The complex is composed mainly of two-mica red granite and breccia with minor dykes of pegmatite, quartz porphyry, felsite and dolerite, and widespread quartz veining, hydrothermal alteration and minor molybdenite mineralisation. Anomalous levels of bismuth (Bi), arsenic (As) and gold (Au) occur in quartz–pyrite veins. The complex has been mapped and the major- and minor-element geochemistry, including rare-earth elements of intrusives and mineralisation, has been determined. These data reveal a complex tectonic, intrusive and hydrothermal history. The intrusives are peraluminous and magnetite-, muscovite- and garnet-bearing. The youngest member, a quartz porphyry, is highly fractionated. There are two stages of hydrothermal activity: the first is linked to the explosive release of volatiles from a granite cupola and breccia formation; and the second, widespread quartz veining. Mo is associated with both stages, and Bi–As–Au anomalies are found in late quartz–pyrite veins. The mineralisation is classified as a granite-related vein-type Mo system. The unique preservation, in the Grampian terrane, of an Ordovician subvolcanic complex may be attributed to pre-Devonian movements on the nearby Dee fault and possibly also the collapse of the magma chamber following the explosive release of volatiles. The combination of large size, poor exposure and abundant multi-stage hydrothermal activity suggests that there is potential for further Mo and possibly Au mineralisation in this complex. Further mineralisation of this style may be present in the NE Grampian terrane.

Two-Stage Late Jurassic to Early Cretaceous Hydrothermal Activity in the Sakar Unit of Southeastern Bulgaria

Southeastern Bulgaria is composed of a variety of rocks from pre-Variscan (ca. 0.3 Ga) to pre-Alpine sensu lato (ca. 0.15 Ga) time. The Sakar Unit in this region comprises a series of granitoids and gneisses formed or metamorphosed during these events. It is cut by a series of post-Variscan hydrothermal veins, yet lacks pervasive Alpine deformation. It thus represents a key unit for detecting potential tectonism associated with the enigmatic Cimmerian Orogenic episode, but limited geochronology has been undertaken on this unit. Here we report age constraints on hydrothermal activity in the Sakar Pluton. The investigated veins contain mainly albite–actinolite–chlorite–apatite–titanite–quartz–tourmaline–epidote and accessory minerals. The most common accessory minerals are rutile and molybdenite. Apatite and titanite from the same vein were dated by U–Pb LA–ICP-MS geochronology. These dates are interpreted as crystallization ages and are 149 ± 7 Ma on apatite and 114 ± 1 Ma on titanite, respectively. These crystallization ages are the first to document two stages of hydrothermal activity during the late Jurassic to early Cretaceous, using U–Pb geochronology, and its association with the Cimmerian orogenesis. The Cimmerian tectono-thermal episode is well-documented further to the east in the Eastern Strandja Massif granitoids. However, these are the first documented ages from the western parts of the Strandja Massif, in the Sakar Unit. These ages also temporally overlap with previously published Ar–Ar and K–Ar cooling ages, and firmly establish that the Cimmerian orogeny in the studied area included both tectonic and hydrothermal activity. Such hydrothermal activity likely accounted for the intense albitization found in the Sakar Unit.

Zonation in primary geochemical haloes for orogenic vein-type gold mineralization in the Quartz Ridge prospect, Sukari gold mine area, Eastern Desert of Egypt

The Quartz Ridge prospect is one of >100 gold occurrences distributed in the Neoproterozoic basement rocks of the Eastern Desert of Egypt. The mineralization is hosted in quartz veins along NE-trending shear zones, and their associated altered wallrocks. Opaque minerals include pyrite and arsenopyrite with less abundant chalcopyrite, galena, and sphalerite. Gold occurs as tiny inclusions and as microfracture-filling grains mainly in arsenopyrite and in deformed quartz. Sericitization, carbonatization, and silicification are the main alteration signs accompanying ore formation. The chemical analysis of 640 core samples from four selected boreholes from the study area was used for characterizing the primary geochemical haloes and their zonation developed around detected subsurface mineralized zones. Statistical analysis of the geochemical data applying techniques based on median absolute deviation (MAD) values and log-ratio transformation (CLR) was carried out to address the problems of outliers and closed compositional data, whereas statistical factor analysis (FA) combined with enrichment factor (EF) associations were used to investigate the multi-element associations describing the gold mineralization and its primary dispersion haloes. Mineralization (Au-Ag-As-Sb ± S), alteration (Ba ± Cu ± Mo) and wall rock (Ni-Cr ±Co) associations can be inferred directly by factor analysis study. However, the dispersion of trace elements around mineralized zones can better be described by their enrichment factors calculated for data subsets. More than one stage of hydrothermal activity has been suggested and believed to be responsible for precipitating ore metals and their associated elements. Molybdenum and copper were found to be good indicators for upper zones, while tungsten and zinc are good indicators for the lower or down-dip zones. The current study manifests the success of using statistical procedures based on data transformation for compositional data analysis (i.e. centered log-ratios) coupled with enrichment factors in characterizing the primary geochemical haloes and their zoning which can be effectively used to follow similar occurrences of orogenic hydrothermal gold deposits.

He–Ar–S Isotopic Compositions of Polymetallic Sulphides from Hydrothermal Vent Fields along the Ultraslow-Spreading Southwest Indian Ridge and Their Geological Implications

Noble gases have become a powerful tool to constrain the origin and evolution of ore-forming fluids in seafloor hydrothermal systems. The aim of this study was to apply these tracers to understand the genesis of newly discovered polymetallic sulphide deposits along the ultraslow-spreading Southwest Indian Ridge (SWIR). The helium, argon, and sulphur isotope compositions of metal sulphide minerals were measured for a number of active/inactive vent fields in the Indian Ocean. The helium concentrations and isotopic ratios in these ore samples are variable (4He: 0.09–2.42 × 10−8 cm3STP∙g−1; 3He: 0.06–3.28 × 10−13 cm3STP∙g−1; 3He/4He: 1.12–9.67 Ra) and generally greater than the modern atmosphere, but significantly lower than those in massive sulphides from the fast-spreading East Pacific Rise (EPR), especially for three Cu–Fe-rich samples from the ultramafic-hosted Tianzuo and Kairei vent fields. On the contrary, most of the SWIR sulphide deposits have somewhat higher 40Ar/36Ar ratios of trapped fluids (ranging from 290.6 to 303.4) when compared to the EPR ore samples. Moreover, the majority of sulphide minerals from the Indian Ocean have much higher δ34S values (3.0‰–9.8‰, ~5.9 on average, n = 49) than other basaltic-hosted active hydrothermal systems on the EPR. Overall, these He–Ar–S results are well within the range of seafloor massive sulphide deposits at global sediment-starved mid-ocean ridges (MORs), lying between those of air-saturated water (ASW) and mid-ocean ridge basalt (MORB) end members. Therefore, our study suggests that the helium was derived mainly from the MORB mantle by degassing during the high-temperature stage of hydrothermal activity, as well as from a mixture of vent fluids with variable amounts of ambient seawater during either earlier or late-stage low-temperature hydrothermal episodes, whereas the argon in ore-forming fluids trapped within sulphide minerals was predominantly derived from deep-sea water. Additionally, relatively high δ34S values exhibit a great estimated proportion (up to nearly 40%) of seawater-derived components. In summary, sub-seafloor extensive fluid circulation, pervasive low-temperature alteration, shallow seawater entrainment, and mixing processes, may make a larger contribution to the SWIR hydrothermal ore-forming systems, compared to fast-spreading centres.

Geochemistry of ore-forming fluids, Deh-Salm uneconomic porphyry Mo system, SW Nehbandan, Iran: a fluid inclusion investigation

ABSTRACTBased on the fluid inclusion and rock sample geochemistry in Deh-Salm hydrothermal system, it is shown that uneconomic molybdenite mineralization occurred in both stockwork- and limited skarn-type alterations. Homogenization temperatures and salinities based on microthermometry data are presented for fluid inclusions from hydrothermal quartz associated with pyrite, galena, sphalerite and rare molybdenite mineralization. The data indicate that ore mineral precipitation occurred within a temperature range of 250° to > 490°C from moderate to high-salinity (9 to > 42 wt% NaCl equivalent) aqueous fluid. Two stages of hydrothermal activity characterized by high-salinity fluids (> 30 wt% NaCl equivalent) are identified; 1) relatively high-temperatures (350° to > 490°C); 2) lower temperatures (170° to 250°C). The high-salinity, high-temperature stage is interpreted to be the evidence for resurgent boiling. Dilution of these fluids by convicting meteoric water resulted in low to moderate salinity fluids, which dominated the inclusion population in quartz hosted crystals. The high-salinity, low-temperature fluid inclusions identify the residual fluids derived from boiling which happened as a result of a change in the pressure regime from dominantly lithostatic to hydrostatic conditions. Condensation and infiltration of ground waters caused major chemical variations, increasing the hydrostatic pressure, and the acid-washed processes on the system. The intense acid-washed and the intrusive rock depth is suggested to be the main reasons caused an uneconomic metal deposition in Deh-Salm system.

Metallogenic age and hydrothermal evolution of the Jidetun Mo deposit in central Jilin Province, northeast China: Evidence from fluid inclusions, isotope systematics, and geochronology

The Jidetun deposit is a large porphyry Mo deposit that is located in central Jilin Province, northeast China. The Mo mineralization occurs mainly at the edge of porphyritic granodiorite, as well as the adjacent monzogranite. Field investigations, cross-cutting relationships, and mineral paragenetic associations indicate four stages of hydrothermal activity. To determine the relationships between mineralization and associated magmatism, and better understand the metallogenic processes in ore district, we have undertaken a series of studies incluiding molybdenite Re–Os and zircon U–Pb geochronology, fluid inclusions microthermometry, and C–H–O–S–Pb isotope compositions. The molybdenite Re–Os dating yielded a well-defined isochron age of 168.9±1.9Ma (MSWD=0.34) that is similar to the weighted mean 206Pb/238U age of 173.5±1.5Ma (MSWD=1.8) obtained from zircons from the porphyritic granodiorite. The results lead to the conclusion that Mo mineralization, occurred in the Middle Jurassic (168.9±1.9Ma), was spatially, temporally, and genetically related to the porphyritic granodiorite (173.5±1.5Ma) rather than the older monzogranite (180.1±0.6Ma). Fluid inclusion and stable (C–H–O) isotope data indicate that the initial H2O–NaCl fluids of mineralization stage I were of high-temperature and high-salinity affinity and exsolved from the granodiorite magma as a result of cooling and fractional crystallization. The fluids then evolved during mineralization stage II into immiscible H2O–CO2–NaCl fluids that facilitated the transport of metals (Mo, Cu, and Fe) and their separation from the ore-bearing magmas due to the influx of abundant external CO2 and heated meteoric water. Subsequently, during mineralization stage III and IV, increase of pH in residual ore-forming fluids on account of CO2 escape, and continuous decrease of ore-forming temperatures caused by the large accession of the meteoric water into the fluid system, reduced solubility and stability of metal clathrates, thus facilitating the deposition of polymetallic sulfides.

Origin of ore-forming fluids of the Haigou gold deposit in the eastern Central Asian Orogenic belt, NE China: Constraints from H-O-He-Ar isotopes

The Haigou lode deposit contains 40t of gold at an average grade of 3.5g/t, and is one of the largest deposits in the Jiapigou gold belt located along the eastern segment of the northern margin of the North China Craton. The deposit comprises 15gold-bearing quartz veins hosted in a Carboniferous monzonite–monzogranite stock. Cretaceous dikes consisting of diorite, diabase, and granodiorite porphyries are well developed in the deposit. The diorite porphyry dikes (130.4±6.3Ma) occur together with gold-bearing quartz veins in NNE- and NE-striking faults. Gold-bearing quartz veins crosscut the diorite porphyry dikes, and the veins are in turn crosscut by E–W-striking 124.6±2.2Ma granodiorite porphyry dikes. The mineralization mainly occurs as auriferous quartz veins with minor amounts of sulfide minerals, including pyrite, galena, chalcopyrite, and molybdenite. Gold occurs as either native gold or calaverite. Common gangue minerals in the deposit include quartz, sericite, and calcite. The deposit is characterized by various types of hydrothermal alteration, including silicification, sericitization, chloritization, potassic alteration, and carbonatization. Three stages of hydrothermal activity have been recognized in the deposit: (1) a barren quartz stage; (2) a polymetallic sulfide (gold) stage; (3) a calcite stage. Fluid inclusions in hydrothermal pyrites have 3He/4He ratios of 0.3 to 3.3 Ra and 40Ar/36Ar ratios of 351 to 1353, indicating mixing of fluids of mantle and crustal origin. Hydrothermal quartz yielded δ18O values of −1.3‰ to +7.2‰ and δD values of fluid inclusions in the quartz vary between −80‰ and −104‰. These stable isotope data also suggest mixing of magmatic and meteoric fluids. Noble gas and stable isotopic data suggest that the ore fluids have a predominant mantle source with a significant crustal component. Based on the spatial association of gold-bearing quartz veins with early Cretaceous intrusions, and the H–O–He–Ar isotopic data from the deposit and intrusions, we propose that gold mineralization in the Haigou deposit was formed in an extensional setting and was related to the early Cretaceous, mantle-derived intrusions.

Hydrothermal evolution and ore genesis of the Beiya giant Au polymetallic deposit, western Yunnan, China: Evidence from fluid inclusions and H–O–S–Pb isotopes

The Beiya porphyry–skarn gold-polymetallic deposit is one of the largest gold deposits in China, and also contains significant amounts of silver and base metals. The estimated reserve at the end of 2015 was 130million metric tonnes (Mt) of ores, grading 2.42g/t Au, 0.48wt.% Cu, 25.5wt.% Fe, 38.85g/t Ag, 1.24wt.% Pb, and 0.53wt.% Zn. The deposit is located in the Jinshajiang–Ailaoshan porphyry–skarn Cu–Mo–Au metallogenic belt in western Yunnan province, southwest China. Skarn alteration and mineralization are spatially and temporally associated with Eocene monzogranite porphyries, which were emplaced into Triassic carbonates. Field evidence and petrographic observations indicate there are four stages of hydrothermal activity, i.e. prograde (Stage I), iron oxide (Stage II), sulfide (Stage III) and carbonate stage (Stage IV). Stage I is characterized by the formation of garnet and pyroxene by high-temperature (398–560°C) hypersaline (48.81–67.24wt.% NaCl equiv.) hydrothermal fluids with H-O isotopic compositions that are similar to typical magmatic fluids. These fluids were most likely generated by the separation of brine from a silicate melt. In Stage II garnet and pyroxene was replaced by epidote, amphibole, chlorite, quartz and magnetite. Associated hydrothermal fluids are preserved in coexisting L-type and V-type fluid inclusions and are characterized by anomalously low δD values (approximately −80‰), and lower δ18O than Stage I fluids. The decrease in ore fluid δ18OH2O values with time coincided with marked decreases in fluid salinity and temperature. Based on the fluid inclusion and stable isotopic data, we suggest that the ore fluid evolved by boiling of the magmatic brine. Stage III is characterized by the development of pyrite, chalcopyrite, galena, and sphalerite. Fluids inclusions associated with Stage III display and continuous range of salinities, with the majority being low-salinity inclusions (1.28–3.54wt.% NaCl equiv.). These fluids yield lower δ18OH2O values (−0.85 to 5.52‰) and moderate δD values (−78.6 to −88.6‰). Altogether, these data indicate that Stage III fluids originated from the mixing of residual Stage II fluids with meteoric water. The sulfur isotope (δ34S) values of Stage III sulfides cover a narrow range of −4.2 to 1.2‰ (mean=−1.1‰), and boiling also occurred at low temperatures (165–347°C) during Stage III. In Stage IV, only L-type fluid inclusions are present, which yielded the lowest homogenization temperatures (157–272°C) and the lowest salinities (1.06–12.20wt.% NaCl equiv.). The H–O isotope data indicate that the ore-forming fluids were dominated by magmatic water in Stage I and Stage II, and that the magmatic water gradually mixed with circulating meteoric water during Stage III and Stage IV. This conclusion is further supported by the isotopic composition of lead of the ores, which is similar to that of the monzogranite porphyries, but differs significantly from Permian basalts and Triassic wall rocks.

The Muruntau gold deposit (Uzbekistan) – A unique ancient hydrothermal system in the southern Tien Shan

The Muruntau gold deposit in the Central Kyzylkum, Uzbekistan is one of the largest single gold deposits worldwide. Data available from the literature are reviewed with the aim to (1) integrate the present knowledge on this unique deposit from Russian and English literature; (2) show the considerable progress made in the understanding of the genesis of the Muruntau deposit during the last decades; and (3) point to problems still open for future research. Deposit formation occurred through a multi-stage process involving sedimentation, regional metamorphism including thrusting, magmatism with formation of hornfels aureoles and several stages of hydrothermal activity. According to recent knowledge, synsedimentary or pure metamorphic formation of gold mineralization seems unlikely. The role of granite magmatism occurring roughly within the same time interval as the main hydrothermal gold precipitation remains uncertain. There are no signs of interaction of matter between the magma(s) and the hydrothermal system(s). On the other hand, there was an intense, high-temperature (above 400 °C) fluid – wall rock interaction resulting in the formation of gold-bearing, cone-like stockworks with veins, veinlets and gold-bearing metasomatites. Several chemical and isotope indicators hint at an involvement of lower-crustal or mantle-related sources as well as of surface waters in ore formation. Deposit formation through brecciation involving explosion, hydrothermal or tectonic breccias might explain these data. Further investigations on breccia formation as well as on the exact timing of relevant sedimentary, metamorphic, magmatic and hydrothermal events are recommended.

Iron transport and deposition mechanisms in the Taochong iron-rich skarn deposit, Middle–Lower Yangtze Valley, Eastern China

The Taochong skarn iron deposit is one of 38 iron prospects in the Fanchang iron ore district of the Middle–Lower Yangtze River Valley, one of the most important economic mineral districts in China. Orebodies are hosted mainly within the middle–upper Carboniferous Huanglong dolomitic limestone. In this deposit, two types of ore (types 1 and 2) are recognized based on the mineral assemblage. The type 1 ore is dominant throughout the deposit and consists mainly of hematite, magnetite, quartz, and calcite. This ore is characterized by euhedral and skeletal textures, and massive, brecciated, comb-like, and vein structures. The type 2 ore is composed of hematite, garnet, pyroxene, actinolite, chlorite, quartz, and calcite, and has a relict metasomatic texture and disseminated structure. Field evidence and petrographic observations indicate that three stages of hydrothermal activity (i.e., the skarn, iron oxide, and carbonate stages) contributed to the formation of the Taochong iron deposit. The iron oxide stage can be further divided into early and late oxide stages, which saw the formation of the type 2 and type 1 ores, respectively.Fluid inclusion and stable isotope data indicate that the skarn formed at depths of ~2km under lithostatic pressure of ~500bars, from high-temperature (~500°C), acidic, saline (up to ~50wt.% NaCl equiv.), and iron-rich fluids that had chemical and isotopic compositions (δ18Owater=4.5‰–11.7‰) similar to those of magmatic fluid replacing a certain amount of the Huanglong dolomitic limestone. The iron ores formed under hydrostatic conditions after fracturing of the early skarn. Fluid inclusions within quartz in equilibrium with hematite in the type 2 ores yields homogenization temperatures of 372–390°C and salinities of 31–39wt.% NaCl equiv., whereas fluid inclusions within quartz in equilibrium with hematite in the type 1 ores has lower homogenization temperatures (280–320°C) and variable salinities (0–40wt.% NaCl equiv.). During the formation of the type 1 ores, fluids contained moderate concentrations of ions, variable δ13C values (−5.3‰ to +5.8‰), and relatively convergent δ18O values (1.7‰–4.5‰). The temporal decrease in the concentration of ions and δ18O values in the fluid inclusions is matched by marked decreases in fluid salinity and temperature, suggesting that the fluids of the oxide stage were mixtures of magmatic brine and meteoric water. The coexistence of saline and vapor-rich fluid inclusions as internal trails or clusters within individual quartz grains of the late oxide stage, with contrasting homogenization characteristics (for the liquid and vapor) at similar temperatures, strongly suggests that fluid boiling occurred during the late oxide stage.Our estimates of the physico-chemical conditions during ore formation, together with fluid inclusion data and the results of previous experimental studies, indicate that iron in the Taochong deposit was transported mainly as FeCl4–, along with minor amounts of other chloride complexes (e.g., FeCl2+ and FeCl2(H2O)4+). The precipitation of hematite followed the breakdown of the iron chloride complexes, probably induced by a decrease in temperature and ligand (chloride) ion activity, and an increase in pH. Based on the results, we infer that deposition of the type 2 ore was caused by the replacement of garnet and pyroxene by a retrograde alteration assemblage, which was likely induced by fluid–rock interaction and the mixing of magmatic brine and meteoric water. The type 1 ore was deposited in open spaces, probably induced by fluid boiling.

Multifractal distribution of geochemical elements in Dexing porphyry copper deposit, Jiangxi Province, China

Hydrothermal mineralization can cause enrichment or depletion of elements in ore bodies and surrounding country rocks. Local metal-enriched rocks can become an ore body that can be mined at an economic profit. Therefore, understanding the spatial distribution of elements caused by mineralization is crucial for mineral exploration and resource assessment. In this study, a total of 24 representative samples were collected from different veins within the Tongchang ore body of the Dexing porphyry Cu–Mo Deposit. The objectives of this work were firstly to identify different vein types and secondly to evaluate the potential of multifractal parameters, constructed using the so-called method of moments, to characterize the spatial and temporal distribution patterns of concentration of elements on mineral surfaces, which may provide insight into hydrothermal mineralization and its effect on spatial and temporal distribution of element concentration. Based on the analytical results and previous investigations, the vein systems within the deposit can be divided into four main types as follows: (1) early quartz veins (A-type veins) related to K-feldspar alteration; (2) quartz-pyrite-molybdenite-chalcopyrite veins (B-type veins) related to chlorite+illite alteration; (3) quartz-pyrite-chalcopyrite veins (D-type veins) related to late phyllic (muscovite+quartz) alteration; and (4) the latest carbonate-sulphate-oxide veins (H-type veins). The multifractal distributions of concentration of chemical elements on the surface of pyrite crystals selected from veins of different types were investigated using digital images generated by means of ESEM. To achieve our goal, the distribution patterns of element concentration values of eight elements (Cu, Fe, Mn, Ni, P, S, Si and Ti) on the pyrite surface selected from B-type, D-type and H-type veins and veinlets were represented by multifractal spectra. Several multifractal parameters were calculated; however, the most significant ones are ∆α and ∆αL. These parameters are found to be higher in the D-type veins related to late phyllic alteration and mineralization in comparison with other veins and vein systems, indicating that the ore minerals containing Cu and Fe sulphides are highly enriched in D-type veins. This result is consistent with previous research on the area as well as new field observations, chemical and petrographic analysis carried out for this study. Therefore, the multifractal parameters, particularly ∆α and ∆αL, appear to be promising to discriminate between mineralized veins developed during different stages of hydrothermal activity which led to development of the porphyry system at Tongchang ore body.

HYDROTHERMAL EVOLUTION IN THE HOIDAS LAKE VEIN-TYPE REE DEPOSIT, SASKATCHEWAN, CANADA: CONSTRAINTS FROM FLUID INCLUSION MICROTHERMOMETRY AND EVAPORATE MOUND ANALYSIS

The Hoidas Lake rare earth element (REE) deposit, located in northern Saskatchewan, Canada, is a structurally controlled vein-type LREE deposit with allanite-(Ce) and fluorapatite as the main REE carriers. The mineralized veins cut Archean and Paleoproterozoic orthogneisses and supracrustal rocks of the southern Rae Subprovince. The paragenesis includes hyalophane-bearing pegmatite dikes, REE-mineralized allanite-, diopside-, hornblende-, hyalophane-, and titanite-bearing veins, and later breccia veins that contain several stages of apatite. The mineralized veins record hydrothermal alteration with nucleation of monazite and REE-carbonate inclusions in apatite and allanite, respectively. Barren quartz-, carbonate-, and hematite-rich veins represent the final stage of hydrothermal activity. Samples from the hyalophane-bearing pegmatites, apatite breccia veins, and quartz-carbonate veins, used for fluid inclusion petrography and microthermometry, indicate four distinct fluid inclusion assemblages (FIAs): (1) carbonic inclusions showing LCO2 (liquid CO2) + VCO2 (CO2 vapor) phase composition with 20–40 vol.% VCO2 at 0 °C; (2) aqueous inclusions showing L (liquid H2O) + V (H2O vapor) phase composition with 90–100 vol.% V at 20 °C; (3) aqueous inclusions showing L + V + H (halite) phase composition with 15 vol.% V at 20 °C; and (4) aqueous L + V inclusions with 15–20 vol.% V at 20 °C. Type 1 inclusions homogenize to liquid CO2 with Th(CO2) from 3.3 to 30.5 °C. Type 2 V-rich inclusions have high salinities and contain salts other than NaCl. Type 3 L-V-H inclusions have different homogenization behavior in quartz of the hyalophane-bearing pegmatites (L+V+H → L+V → L) compared to quartz-carbonate veins (L+V+H → L+H → L), and Th (total homogenization) values range from 180 to 315 °C with salinities of 30–40 wt.% eq. NaCl. In type 4 inclusions Th ranges from 90 to 290 °C, but for specific samples and FIAs (fluid inclusion assemblages) there is a more limited spread (from 5 to 25 °C). Salinities range from 8 to 24 wt.% eq. NaCl, and the inclusions have variable Na/(Na + Ca). Evaporate mound analysis shows average normalized (to 100%) cation contents of 48% Na, 24% Ca, 6% K, 5% Ba, 4% Mn, 2% Fe, 2% Mg, and 9% Sr for quartz-hosted inclusions in quartz-carbonate veins, and 61% Na, 32% Ca, and 7% K for quartz-hosted inclusions in hyalophane-bearing pegmatite dikes. The thermometric and chemical data suggest that evolution of the Hoidas Lake mineralization involved two fluid types with early entrapment of a carbonic fluid followed by introduction of a mixed Na-Ca-K-(Ba-Mn-Mg-Fe-Sr) aqueous fluid with variable salinities that was responsible for the late alteration of the mineralized veins. Furthermore, the inclusion data provide constraints on entrapment temperature (<310 °C) and also indicate that the pressure was transient (0.5 to 2 kbars) based on the homogenization temperature data for the carbonic (type 1) and aqueous L-V-H (type 3) inclusions.

Ore genesis and hydrothermal evolution of the Huanggang skarn iron–tin polymetallic deposit, southern Great Xing'an Range: Evidence from fluid inclusions and isotope analyses

The southern Great Xing'an Range is one of the most important metallogenic belts in northern China, and contains numerous Pb–Zn–Ag–Cu–Sn–Fe–Mo deposits. The Huanggang iron–tin polymetallic skarn deposit is located in the Sn-polymetallic metallogenic sub-belt. Skarns and iron orebodies occur as lenses along the contact between granite plutons and the Lower Permian Huanggangliang Formation marble or Dashizhai Formation andesite. Field evidence and petrographic observations indicate that the three stages of hydrothermal activity, i.e., skarn, oxide and sulfide stages, all contributed to the formation of the Huanggang deposit.The skarn stage is characterized by the formation of garnet and pyroxene, and high-temperature, hypersaline hydrothermal fluids with isotopic compositions that are similar to those of typical magmatic fluids. These fluids most likely were generated by the separation of brine from a silicate melt instead of being a product of aqueous fluid immiscibility. The iron oxide stage coincides with the replacement of garnet and pyroxene by amphibole, chlorite, quartz and magnetite. The hydrothermal fluids of this stage are represented by L-type fluid inclusions that coexist with V-type inclusions with anomalously low δD values (approximately −100 to −116‰). The decrease in ore fluid δ18OH2O values with time coincides with marked decreases in the fluid salinity and temperature. Based on the fluid inclusion and stable isotopic data, the ore fluid evolved by boiling of the magmatic brine. The sulfide stage is characterized by the development of sphalerite, chalcopyrite, fluorite, and calcite veins, and these veins cut across the skarns and orebodies. The fluids during this stage are represented by inclusions with a variable but continuous sequence of salinities, mainly low-salinity inclusions. These fluids yield the lowest δ18OH2O values and moderate δD values ( −1.6 to −2.8‰ and −101 to −104‰, respectively). The data indicate that the sulfide stage fluids originated from the mixing of residual oxide-stage fluids with various amounts of meteoric water. Boiling occurred during this stage at low temperatures.The sulfur isotope (δ34S) values of the sulfides are in a narrow range of −6.70 to 4.50‰ (mean=−1.01‰), and the oxygen isotope (δ18O) values of the magnetite are in a narrow range of 0.1 to 3.4‰. Both of these sets of values suggest that the ore-forming fluid is of magmatic origin. The lead isotope compositions of the ore (206Pb/204Pb=18.252–18.345, 207Pb/204Pb=15.511–15.607, and 208Pb/204Pb=38.071–38.388) are consistent with those of K-feldspar granites (206Pb/204Pb=18.183–18.495, 207Pb/204Pb=15.448–15.602, 208Pb/204Pb=37.877–38.325), but significantly differ from those of Permian marble (206Pb/204Pb=18.367–18.449, 207Pb/204Pb=15.676–15.695, 208Pb/204Pb=38.469–38.465), which also suggests that the ore-forming fluid is of magmatic origin.

Origin and evolution of hydrothermal fluids in the Taochong iron deposit, Middle–Lower Yangtze Valley, Eastern China: Evidence from microthermometric and stable isotope analyses of fluid inclusions

The Middle–Lower Yangtze River Valley is one of the most important metallogenic belts in China, hosting numerous Cu–Fe–Au–Mo deposits. The Taochong deposit is located in the northern part of the Fanchang iron ore district of the Middle–Lower Yangtze River metallogenic belt. The Fe-orebody is hosted by Middle Carboniferous to Lower Permian limestones. Skarns and Fe-orebodies occur as tabular bodies along interlayer-gliding faults, at some distance from the inferred granitic intrusions. Field evidence and petrographic observations indicate that the three stages of hydrothermal activity—the skarn, iron oxide (main mineralization stage), and carbonate stages—all contributed to the formation of the Taochong iron deposit. The skarn stage is characterized by the formation of garnet and pyroxene, with high-temperature, hypersaline hydrothermal fluids with isotopic compositions similar to those of typical magmatic fluids. These fluids were probably generated by the separation of brine from a silicate melt instead of the product of aqueous fluid immiscibility. The iron oxide stage coincides with the replacement of garnet and pyroxene by actinolite, chlorite, quartz, calcite and hematite. The hydrothermal fluids at this stage are represented by saline fluid inclusions that coexist with vapor-rich inclusions with anomalously low δD values (−66‰ to −94‰). The decrease in ore fluid δ18Owater with time and decreasing depth is consistent with the decreases in fluid salinity and temperature. The fluid δD values also show a decreasing trend with decreasing depth. Both fluid inclusion and stable isotopic data suggest that the ore fluid during the main period of mineralization was evolved by the boiling of various mixtures of magmatic brine and meteoric water. This process was probably induced by a drop in pressure from lithostatic to hydrostatic. The carbonate stage is represented by calcite veins that cut across the skarn and orebody, locally producing a dense stockwork. This observation indicates the veins formed during the waning stages of hydrothermal activity. The fluids from this stage are mainly represented by a variety of low-salinity fluid inclusions, as well as fewer high-salinity inclusions. These particular fluids have the lowest δ18Owater values (−2.2‰ to 0.4‰) and a wide of range of δD values (−40‰ to −81‰), which indicate that they were originated from a mixture of residual fluids from the oxide stage, various amounts of meteoric water, and possibly condensed vapor. Low-temperature boiling probably occurred during this stage.We also discuss the reasons behind the anomalously low δD values in fluid inclusion water extracted by thermal decrepitation from quartz at high temperatures, and suggest that calcite data provide a possible benchmark for adjusting low δD values found in quartz intergrown with calcite.

Mineralogy and Mineral Chemistry of the Cretaceous Duolong Gold‐Rich Porphyry Copper Deposit in the Bangongco Arc, Northern Tibet

AbstractThe Early Cretaceous Duolong gold‐rich porphyry copper deposit is a newly discovered deposit with proven 5.38 Mt Cu resources of 0.72% Cu and 41 t gold of 0.23 g t−1 in northern Tibet. Granodiorite porphyry and quartz diorite porphyrite are the main ore‐bearing porphyries. A wide range of hydrothermal alteration associated with these porphyries is divided into potassic, argillic and propylitic zones from the ore‐bearing porphyry center outward and upward. In the hydrothermal alteration zones, secondary albite (91.5–99.7% Ab) occurs along the rim of plagioclase phenocryst and fissures. Secondary K‐feldspar (75.1–96.9% Or) replaces plagioclase phenocryst and matrix or occurs in veinlets. Biotite occurs mainly as matrix and veinlet in addition to phenocryst in the potassic zone. The biotite are Mg‐rich and formed under a highly oxidized condition at temperatures ranging from 400°C to 430°C. All the biotites are absent in F, and have high Cl content (0.19–0.26%), with log (XCl/XOH) values of −2.74 to −2.88 and IV (Cl) values of −3.48 to −3.35, suggesting a significant role of chloride complexes (CuCl2‐ and AuCl2‐) in transporting and precipitating copper and gold. Chlorites are present in all alteration zones and correspond mainly to pycnochlorite. They have similar Fe/(Fe+Mg), Mn/(Mn+Mg) ratios, and a formation temperature range of 280–360°C. However, the formation temperature of chlorite in the quartz‐gypsum‐carbonate‐chlorite vein is between 190°C and 220°C, indicating that it may have resulted from a later stage of hydrothermal activity. Fe3+/Fe2+ ratios of chlorites have negative correlation with AlIV, suggesting oxygen fugacity of fluids increases with decreasing temperature. Apatite mineral inclusions in the biotite phenocrysts show high SO3 content (0.44–0.82%) and high Cl content (1–1.37%), indicating the host magma had a high oxidation state and was enriched in S and Cl. The highest Cl content of apatite in the propylitic zone may have resulted from pressure decrease, and the lowest Cl content of apatite in the argillic zone may have been caused by a low Cl content in the fluids. The low concentration of SO3 content in the hydrothermal apatite compared to the magmatic one may have resulted from the decrease of oxygen fugacity and S content in the hydrothermal fluid, which are caused by the abundant precipitation of magnetite.