Gangdese Porphyry Copper Belt Research Articles

Subduction-related Late Triassic Luerma porphyry copper deposit, western Gangdese, Tibet, China: Evidence from geology, geochemistry, and geochronology

Due to the difference of lithospheric structure between the eastern and western segment of Gangdese metallogenic belt, it is generally believed that porphyry copper deposits are mainly developed in the east. However, the newly discovered of Luerma porphyry copper deposit in the western segment of Gangdese provides an excellent research object for us to supplement this understanding. The molybdenite re-Os isotope dating of two samples from quartz veinlet ores (ca. 213.1 ± 5.2 Ma and ca. 212.0 ± 3.7 Ma), combined with the LA-ICP-MS zircon U-Pb dating of three ore-bearing quartz monzonite porphyry samples (ca. 212.7 ± 0.9 Ma, 213.1 ± 1.1 Ma, and ca. 211.8 ± 1.8 Ma), indicates a new generation of Late Triassic Luerma copper porphyry-type mineralization events in the western Gangdese magmatic belt. The Luerma porphyry is a strongly peraluminous shoshonite series rock, is rich in silicon total alkalis, potassium and aluminum, has low contents of calcium, magnesium, and iron. It is enriched in light rare earth elements and large ion lithophile elements, relatively depleted in heavy rare earth elements and high field strength elements, and has negative Eu anomalies and no Ce anomalies. The porphyry exhibits relatively low (87Sr/86Sr)i values (0.705617 on average), high εNd(t) values (0.63 on average), high εHf(t) values (11.98 on average), and young two-stage Hf model ages. The Luerma ore-related porphyries are most likely generated dominantly from partial melting of juvenile crust that formed by metasomatized mantle during the subduction of the NeoTethys Ocean to the Lhasa terrane and underwent crystal fractionation during the process. The deposit is currently the oldest and westernmost porphyry deposit in the Gangdese porphyry copper belt, providing evidence that porphyry copper mineralization in this belt began in at least the Late Triassic.

Fluid evolution of the Zhunuo porphyry Cu-Mo deposit in Gangdese Tract, Tibet, China: A fluid-inclusion investigation

The Zhunuo porphyry Cu-Mo deposit in the western part of south Gangdese porphyry copper belt (GPCB), Tibet, is a large porphyry system. The Cu-Mo mineralization of this deposit is mainly associated with a weakly aluminous I-type Miocene granite porphyry pluton that formed in a post-collisional extensional tectonic setting. Field observations and petrographic studies demonstrate that emplacement of the pluton took place in several intrusive pulses, each with associated hydrothermal activities. Early hydrothermal alteration produced a potassic assemblage, overprinted by later phyllic alteration. At least three main stages of mineralization have been identified: (1) early A stage quartz + K-feldspar + minor sulfide veins; (2) middle stage B1-subtype veins composed of quartz + molybdenite + sporadic sulfide and B2-subtype quartz + pyrite + chalcopyrite veins; and (3) late stage pyrite + quartz D veins.Three types of fluid inclusions (FIs) in quartz were identified in the early and middle stages (i.e., CO2 bearing-aqueous vapor (V)-type and liquid (L)-type, and solid (S)-type); only aqueous L-type FIs were observed in the late stage minerals. S-type FIs contain variable solid particles, including halite, calcite, anhydrite (or gypsum), and chalcopyrite, hematite, and an unidentified transparent crystal. Only halite was dissolved during heating. Halite-bearing S-type FIs were mainly homogenized by halite dissolution at 300–470°C (S2-type), corresponding to salinities of 38.9–56.3 wt% NaCl equiv.; and minor halite bearing S-type FIs were homogenized to liquid at 360–520°C via vapor disappearance (S1-type), with salinities of 31.9–56.7 wt% NaCl equiv. Other FIs in minerals of A, B1, B2 and D veins were homogenized at temperatures of 350–550°C, 303–500°C, 305–460°C, and 250–347°C with salinities of 3.4–20.9, 2.6–22.4, 4.8–21.8, and 4.7–18.2 wt% NaCl equiv., respectively. These data suggest that the ore fluids forming the Zhunuo deposit changed from high-temperature, low-moderate salinity, CO2-bearing magmatic fluids to low-temperature, low-salinity and CO2-poor meteoric fluids. Phase separation and cooling caused the precipitation of the abundant chalcopyrite in the middle stage B2-subtype veins. The molybdenite mineralization was caused mainly by the decrease of pressure due to the release of CO2, and by phase separation of fluid in the middle stage as recorded in the B2-subtype veins. This hydrothermal ore-forming system is different from other magmatic-hydrothermal systems in GPCB, but is similar to the typical Cu-Mo porphyry systems, which were initially CO2-bearing as indicated by the presence of abundant CO2-bearing, low-moderate salinity FIs and calcite-bearing S-type FIs.

Apatite and zircon compositions for Miocene mineralizing and barren intrusions in the Gangdese porphyry copper belt of southern Tibet: Implication for ore control

Gangdese porphyry copper belt in southern Tibet of China contains many Miocene fertile and barren adakite-like intrusions. Their magmatic oxidation state and volatile concentrations were poorly constrained. Here, we present apatite and zircon composition data for the Miocene fertile intrusions in the Qulong, Jiru, and Zhunuo porphyry copper deposits, and the coeval barren porphyries at Yare, Mayum, and Lasa in the Gangdese belt. Magmatic zircons of intrusions from the six locations share similar ΔFMQ (FMQ represents the reference buffer fayalite + magnetite + quartz) values, indicating both fertile and barren rocks have similar magmatic oxidation state. Variations of apatite Cl concentrations and characteristics of S- and Cl-rich core to abruptly S- and Cl-poor rim at Qulong, Jiru, Zhunuo, Yare, and Mayum are indicative of fluid exsolution from vapor-saturated magma. In contrast, fluid exsolution was not recognized from the available apatite composition data at Lasa which are characterized by constant or increasing apatite Cl concentrations with decreasing F/Cl ratios from core to rim. Some apatites at Lasa have high SO3 concentrations at the core compared with the rim, which would be attributed to early crystallization from S-rich magma followed by crystallization of sulfate minerals in a vapor-undersaturated magma. Some apatites from the fertile intrusions at Qulong, Jiru, and Zhunuo have lower Cl concentrations than those from the barren rocks at Yare, Mayum, and Lasa, which is consistent with the observation from the calculated melt Cl concentration. In addition, the SO3 concentrations at the core of apatites from all the fertile rocks are overlapping with those from all the barren rocks. Therefore, enrichment of Cl and S alone in melts may not be critical to forming economic porphyry copper mineralization in the Gangdese belt. Cautions must be taken when using compositions of detrital zircon and apatite grains to evaluate metallogenic potential during regional geochemical surveys.

Apatite fission track thermochronology of Cenozoic igneous rocks from porphyry copper deposits in the Gangdese belt of southern Tibet: Implications for cooling history and ore preservation

Gangdese of southern Tibet is the largest porphyry copper belt in China. Although ore-forming processes have been extensively concerned, the post-mineralization ore preservation state was poorly focused on. Here, we reported apatite fission track (AFT) data for three porphyry copper deposits in the belt (Zhunuo, Jiru and Dabu) to reveal the cooling and exhumation history and evaluate ore preservation. The AFT ages are of 13.5 ± 2.2 to 9.4 ± 1.2 Ma (2σ) for the Miocene monzogranite and granite porphyry, 5.9 ± 1.4 to 3.5 ± 0.8 Ma (2σ) for the Eocene quartz porphyry and rhyolite, and 32.6 ± 8.8 Ma (2σ) for the Eocene rhyolite at Zhunuo. Combined with the thermal history modelling results, these AFT ages and the elevations of samples revealed two rapid cooling episodes at ~10 Ma and ~4 Ma, respectively, in the Zhunuo deposit. Available AFT data from both sides of a nearly N-trending fault at Zhunuo indicated the eastern part would likely have been uplifted relative to the western part where orebodies occurred. The missing portion of orebodies in the eastern part should be located at ca. 600–1200 m above the present surface, all or part of the displaced orebodies would have been eroded at Zhunuo. Our new AFT ages at Jiru (15.4 ± 3.4 to 8.4 ± 1.4 Ma; 2σ) and Dabu (16.4 ± 2.4 to 11.9 ± 3.4 Ma; 2σ), together with the previous low-temperature geochronological data, indicated that Gangdese porphyry copper belt may have undergone similar cooling and exhumation history during ~15–10 Ma.

A Downgoing Indian Lithosphere Control on Along-Strike Variability of Porphyry Mineralization in the Gangdese Belt of Southern Tibet

Abstract The E-trending Gangdese porphyry copper belt in southern Tibet is a classic example of porphyry mineralization in a continental collision zone. New zircon U-Pb geochronological, zircon Hf-O, and bulk-rock Sr-Nd isotope data for the Miocene mineralizing intrusions from the Qulong, Zhunuo, Jiru, Chongjiang, and Lakange porphyry copper deposits and Eocene igneous rocks from the western Gangdese belt, together with literature data, show that both Paleocene-Eocene igneous rocks and Miocene granitoids exhibit coupled along-arc isotopic variations, characterized by bulk-rock ɛNd(t) and zircon ɛHf(t) values increasing from ~84° to ~92°E and then decreasing toward ~95°E. These are interpreted to reflect increasing contributions of subducted Indian continental materials from ~92° to ~84°E and from ~92° to ~95°E, respectively. The Miocene mineralizing intrusions were derived from subduction-modified Tibetan lower crust represented isotopically by the Paleocene-Eocene intrusions, with contributions from Indian plate-released fluids and mafic melts derived from mantle metasomatized by subducted Indian continental materials. Involvement of isotopically ancient Indian continental materials increased from east (Qulong) to west (Zhunuo), which is interpreted to reflect an increasingly shallower angle of the downgoing Indian slab from east to west, consistent with geophysical imaging. Exploration of Gangdese Miocene porphyry copper deposits should focus on the Paleocene-Eocene arc where the subarc mantle was mainly enriched by fluids from the subducted Neo-Tethyan oceanic slab. Neodymium-Hf isotope data for mineralizing igneous rocks from porphyry copper deposits globally show no obvious correlations with Cu endowment. Although Nd-Hf isotopes are useful for imaging lithospheric architecture through time, caution must be taken when using Nd-Hf isotopes to evaluate the potential endowment of porphyry copper deposits, because other factors such as tectonic setting, crustal thickening, magma differentiation, fluid exsolution, and ore-forming processes all play roles in determining Cu endowments and grades.

Formation and evolution of multistage ore-forming fluids in the Miocene Bangpu porphyry–skarn deposit, Southern Tibet: Insights from LA–ICP–MS trace elements of quartz and fluid inclusions

Abstract The large Bangpu Mo–Cu–Pb–Zn deposit, is a Mo-rich (~0.089%), Cu-poor (~0.32%) porphyry–skarn deposit located in the northeastern Gangdese porphyry copper belt (GPCB), Southern Tibet. In this study, LA–ICP–MS data for fluid inclusions and quartz formed during different stages of the mineralization are used to (1) constrain changes in fluid composition during mineralization; (2) infer the origin, transport pathways, and conditions of ore precipitation; and (3) explore the influence of magma chamber on fluid exsolution and shallow mineralization. The Cu content of the vapor-rich (VL) inclusions is higher than that of the halite-bearing (LVH) inclusions, while the Mo, Pb, and Zn contents of the VL and LVH inclusions are similar, indicating that Mo, Cu, Pb, and Zn had a common source, which is inferred to be the intermediate-level magma chamber. The metal contents of fluid inclusions in the early stage potassic alteration-related vein (A vein) and later stage sericitic alteration-related vein (B vein) are similar but variable, implying the intermediate-level magma chamber was too small to form a large stable fluid system. The enrichment of Mo, Pb, and Zn in the Bangpu deposit relative to other porphyry deposits in the GPCB is related to the participation of ancient crustal material. A comparison of our results with those from super-large porphyry Cu deposits elsewhere indicates that metal-rich fluids are not the sole factor to form ore deposits, and the variability of metal contents in the early exsolved fluids is a better predictor of mineralization potential than absolute metal concentrations.

Ore genesis of skarn mineralization in continental collision orogens: A case study from the Pusangguo Co-bearing Cu–Pb–Zn deposit in Tibet

The Gangdese belt in southern Tibet, is renowned for hosting numerous collision-related porphyry Cu-Mo deposits. The recent discovery of Pusangguo Co-bearing Cu–Pb–Zn skarn deposits is the first reported Co-bearing deposit in Gangdese belt as well as in Tibet. Researches on the geological and metallogenic characteristics of Pusanguo indicate that this Co-bearing Cu-Pb-Zn deposit was formed in a collisional setting. We document mineralogy, fluid inclusion studies, and H–O–S–Pb isotope systematics of the skarns and ores of the Pusangguo deposit. Intense endoskarn alteration, dominated by green and pink epidote and quartz, is widely disseminated throughout the host granodiorite as well as in veinlets. Exoskarn alteration is characterized by the presence of prograde grandite (andradite32–100 and grossular0–68) with minor pyroxene (diopside0–46, hedenbergite38–74, and johannsenite1–62). Retrograde epidotes from both the endoskarn and exoskarn show a narrow range of Fe3+/(Fe3+ + Al) ratios (0.25–0.34). Ore minerals include chalcopyrite, sphalerite and galena, with lesser pyrite, pyrrhotite, bornite, Co minerals and falkmanite.Four paragenetic stages of skarn formation and ore deposition are recognized in the Pusangguo Cu–Pb–Zn deposit: prograde (stage I), retrograde (stage II), early sulfide (stage III), and late sulfide (stage IV). Fluid inclusions in all stages are dominated by two-phase liquid-rich (type L) inclusions. During the major phase of mineralization (stage III), fluid inclusions formed at low temperatures (180–240 °C) and with low salinities (0.5–8.6 wt% NaCl equiv.). This is in contrast to other skarn deposits within the Gangdese porphyry copper belt (e.g., the Zhibula, Jiama, and Bangpu deposits) that formed from highly saline fluids at high temperatures. Hydrogen-Oxygen isotopic values obtained from garnet, epidote, quartz and calcite are δDH2O = −179‰ to − 213‰ and δ18OH2O = −9.1‰ to 9.3‰, whereas S-Pb isotopic values for sphalerite, chalcopyrite, pyrite and galena are δ34S = −1.0‰ to 1.2‰, estimated δ34Sfluid = −0.05‰. Such data are indicative of ore-forming fluids and metals that originated from a magmatic–hydrothermal source related to granodiorite emplacement during the Miocene post-continental collision. Extensive mixing between the magmatic-hydrothermal and meteoric fluids led to the co-precipitation of Cu, Pb and Zn minerals from low-salinity fluids at relatively low temperatures. The Pusangguo Co-bearing Cu-Pb-Zn skarn deposit shows consistent features with other (porphyry-related) skarn deposits in the Gangdese belt, except for relatively low temperature and salinity. The research is of great significance for understanding cobalt as an associated mineral in the Gangdese sulfide skarn deposit in Tibet.

Geochemistry and zircon trace elements composition of the Miocene ore‐bearing biotite monzogranite porphyry in the Demingding porphyry Cu‐Mo deposit, Tibet: Petrogenesis and implication for magma fertility

Demingding is a poorly studied post‐collisional porphyry Cu‐Mo deposit, located in the eastern part of the Gangdese porphyry copper belt. In this study, we present LA‐ICP‐MS zircon U‐Pb dating, whole‐rock geochemistry, and zircon trace elemental data for the mineralization related biotite monzogranite porphyry in Demingding. Zircon U‐Pb dating shows that the weighted mean 206Pb/238U ages of the Miocene biotite monzogranite porphyry is 20.36 ± 0.46 Ma (MSWD = 2.6, n = 21). The Miocene biotite monzogranite porphyry is characterized by high SiO2, K2O, and Al2O3 contents and shows adakite‐like signatures. These rocks have high Sr/Y ratios, fractionated REE patterns with low HREE abundances, enrichment in LILE compared with HFSE, and have (87Sr/86Sr)i values of 0.7059 to 0.7062, εNd(t) values of −2.35 to −1.67 and (206Pb/204Pb)i ratios of 18.50–18.55. These features are similar to other Miocene adakite‐like intrusions in the Gangdese belt. We propose that the biotite monzogranite porphyry shares the same petrogenesis with other Miocene ore‐bearing adakite‐like intrusions in the Eastern Gangdese belt and was derived from remelting of the subduction‐modified, eclogitized Tibetan lower crust and mixing with hydrous mafic magmas from metasomatized Tibetan mantle. The zircons from biotite monzogranite porphyry have only minimal negative Eu anomalies (EuN/EuN* > 0.3), and exhibit large ratios of Ce4+/Ce3+(average value of 113), 10,000*(EuN/EuN*)/Y (3.94–8.14, >1), Ce/Nd (9–58, Average value of 30), (Ce/Nd)/Y (0.008–0.127, >0.003), and low value of Dy/Yb (0.15–0.24, <0.3), indicating that the biotite monzogranite porphyry is relatively hydrous and oxidized. The biotite monzogranite porphyry, therefore, is believed to be a relatively fertile intrusion with certain exploration potential.

Hydrothermal evolution and ore precipitation of the No. 2 porphyry Cu–Au deposit in the Xiongcun district, Tibet: Evidence from cathodoluminescence, fluid inclusions, and isotopes

The No. 2 porphyry Cu–Au deposit in the Xiongcun district forms part of the Gangdese porphyry copper belt of Tibet. This study clarifies the fluid evolution and ore precipitation mechanisms of the magmatic-hydrothermal system that formed the deposit by combining the results of fluid inclusion petrography, microthermometry, scanning electron microscope-cathodoluminescence imaging, and stable isotope analyses of multiple quartz-dominated veins. Four distinct sets of quartz-dominated veins and eight generations of quartz (Q1–Q8) within the veins were identified, namely from early to late, early barren quartz veins (V1) comprised of equigranular CL-bright Q1 quartz, quartz–pyrite–chalcopyrite ± magnetite veins (V2) containing early equigranular CL-bright Q1 quartz and later CL-gray or CL-dark Q2–3 (Q2 + Q3) quartz grains intergrown with Cu–Fe sulfides, quartz–molybdenite ± pyrite ± chalcopyrite veins (V3) comprised of quartz grains (Q4–Q6) characterized by subhedral comb-like textures with well-developed oscillatory growth zoning, and late quartz veins (V4) containing CL-gray Q7 quartz with euhedral growth zones and later CL-dark Q8 quartz. Fluid inclusions in these veins can be classified into six types: B40, B40H, B80, B15H, B15H+, and B15. The B40 and B40H inclusions; B80 inclusions; and B15, B15H, and B15H+ inclusions contain vapor bubbles occupying 30–60 vol%, >70 vol%, and 5–25 vol%, respectively; in addition, B15H, B15H+, and B40H inclusions contain transparent daughter minerals (halite, and in some cases, sylvite). Raman spectra showed that CO2 is only present in B40 and B80 inclusions. In the Q1 quartz in V1 and V2 veins, the dominant type of inclusions is B40, which have homogenization temperatures (Th) of 278 °C–389 °C (peaking at 310 °C–330 °C) and salinities of 2.0–12.8 wt% NaCl equiv. In Q2–3 quartz in V2 veins, all inclusion types were observed. These were trapped at temperatures of approximately 325 °C–340 °C and had salinities of 1.8–69.7 wt% NaCl equiv. In V3 veins, only B15H and B15 inclusions were observed, which had Th values of 171 °C–320 °C (peaking at 240 °C–260 °C) and 173 °C–317 °C (peaking at 210 °C–230 °C) and salinities of 28.7–33.9 wt% NaCl equiv and 5.6–23.1 wt% NaCl equiv, respectively. In V4 veins, only B15 inclusions were observed. Primary B15 inclusions in V4 veins yielded Th values of 166 °C–229 °C (peaking at 170 °C–200 °C) and salinities of 5.1–17.1 wt% NaCl equiv. The 3He/4He and 40Ar/36Ar ratios of the fluid inclusions exhibited the ranges of 0.08–0.84 Ra and 451.3–1567.8, respectively, and the δ18Ofluid and δDfluid values varied from −3.2‰ to 5.8‰ and −92.8‰ to −80.3‰, respectively. By integrating all results from the fluid inclusion, cathodoluminescence, and isotopic analyses, we conclude that the initial ore-forming fluids of the No. 2 deposit were low-salinity, CO2-rich single-phase fluids of magmatic origin. Subsequently, fluid immiscibility developed in the initial ore-forming fluids, generating hypersaline liquid and low-salinity vapor phases and leading to the separation of a CO2 phase plus chalcopyrite precipitation from the fluids. As meteoric water was injected into the hydrothermal system, the ore-forming fluids gradually evolved to become meteoric water-dominated, low temperature, low-salinity, and CO2-poor; in addition, fluid-cooling due to the meteoric water input resulted in molybdenite precipitation.

Mineral chemistry of magmatic and hydrothermal biotites from the Bangpu porphyry Mo (Cu) deposit, Tibet

The Bangpu deposit, located in the eastern part of the Gangdese porphyry copper belt, Tibet, is a porphyry Mo (Cu) deposit in a continental collisional orogenic belt. The Mo (Cu) mineralization occurs mainly in monzogranite porphyry, whereas the Cu (Mo) mineralization occurs mainly in diorite porphyry. Most magmatic and hydrothermal biotites from the Bangpu deposit are Mg-rich, plotting in the phlogopite and Mg-biotite compositional fields. The Bangpu magmatic biotite has high BaO (>0.1 wt%) and TiO2 (>3 wt%), while the hydrothermal biotite has low BaO (<0.1 wt%) and TiO2 (<3 wt%). Hydrothermal biotite from the Bangpu Mo (Cu) mineralization system has higher Al2O3, SiO2, MgO, K2O, Na2O and F contents and lower TiO2, FeOtot, MnO and Cl contents than hydrothermal biotite from the Cu (Mo) mineralization system. Crystallization temperatures range from 757 °C to 827 °C and 687 °C to 740 °C for magmatic biotite in monzogranite porphyry and biotite monzogranite, respectively. Hydrothermal biotite geothermometry yields a temperature range from 362 °C to 547 °C for the Mo (Cu) mineralization system and 302 °C to 410 °C for the Cu (Mo) mineralization system. Calculated oxygen fugacity, relative F and Cl contents, and fugacity ratios indicate that the Cu (Mo) and Mo (Cu) mineralization systems formed from different magmatic-hydrothermal fluids. The Mo (Cu) mineralization system formed from F-rich high oxygen fugacity (above NNO) magmatic-hydrothermal fluids favorable for extracting Mo from the melt and transporting Mo, whereas the Cu (Mo) mineralization system formed from Cl-rich high oxygen fugacity (above NNO) magmatic-hydrothermal fluids favorable for extracting Cu from the melt and transporting Cu. The Bangpu porphyry Mo (Cu) deposit is the result of two different superimposed magmatic-hydrothermal fluids.

Zircon U-Pb, Molybdenite Re-Os and Quartz Vein Rb-Sr Geochronology of the Luobuzhen Au-Ag and Hongshan Cu Deposits, Tibet, China: Implications for the Oligocene-Miocene Porphyry–Epithermal Metallogenic System

The Gangdese metallogenic belt in Tibet is an important copper and iron polymetallic, metallogenic belt in western China. The Luobuzhen epithermal Au-Ag and Hongshan porphyry Cu deposits, as two new discovery deposits in the last few years, are located in the western Gangdese metallogenic belt. In this paper, we present quartz vein Rb-Sr isochron, zircon U-Pb and molybdenite Re-Os ages for a better understanding of the minerallogenetic epoch of the deposits. Geochronological data show that the Rb-Sr isochron age of a quartz vein in a Luobuzhen Au-Ag deposit is 21.1 ± 1.8 Ma (MSWD (mean standard weighted deviation) = 0.19), zircon U-Pb ages from diorite and granodiorite porphyry in Hongshan Cu deposit are 50.0 ± 0.4 Ma (MSWD = 0.94) and 23.7 ± 0.1 Ma (MSWD = 0.73), respectively, and a Re-Os isochron age of molybdenite in Hongshan Cu deposit is 23.0 ± 2.0 Ma (MSWD = 0.014). These data suggest that the Luobuzhen epithermal Au-Ag and Hongshan porphyry Cu deposits formed at ca. 23–21 Ma, which were controlled by the same magmatic hydrothermal events. Formation of both the Luobuzhen and Hongshan deposits were obviously earlier than the Miocene porphyry metallogenetic events in the Gangdese porphyry copper belt.

Age and genesis of the Pusangguo skarn Cu-dominated polymetallic deposit, Gangdese metallogenic belt, Tibet

The Pusangguo skarn Cu-polymetallic deposit is a newly discovered deposit in the western Gangdese porphyry copper belt (GPCB). The orebodies are hosted along the contact between limestone of the Early Cretaceous Takena Formation and a biotite granodiorite intrusion. The mineralization is associated with the biotite granodiorite intrusion and is divided into four-stages: (1) a prograde skarn stage; (2) a pre-ore retrograde skarn stage; (3) a quartz-sulfides stage; and (4) a post-ore carbonate stage. These skarns have the characteristics of both calcareous and manganese skarns. The mineral compositions of the garnets and pyroxenes indicate that the ore-forming environment gradually evolved from a high-temperature, oxidative and alkaline environment into a low-temperature and reductive environment. The sulfur and lead isotopic compositions of the ore sulfides suggest a magmatic source for the sulfur and a crust-dominated source for the ore-forming metals. The hydrogen and oxygen isotopic compositions of the quartz and garnets in the ores indicate that the ore-forming fluids may have originated from a mixture of magmatic and meteoric fluids. The geochronological data show that both the magmatism (14.8 ± 0.4 Ma) and mineralization (13.2 ± 0.7 Ma) occurred during the Miocene in a post-collisional tectonic setting. The ore-forming biotite granodiorite is a high-K calc-alkaline I-type granite and has an adakitic geochemical affinity similar to the Gangdese Miocene adakites, which were generated from thickened lower crust in a post-collisional tectonic setting. The low positive εHf(t) values (0 to +2.6), young two-stage Hf model zircon ages, and whole-rock geochemical and lead isotopic compositions indicate that the magma of the ore-related biotite granodiorite may have originated from the thickened lower crust in the Lhasa terrane.

Apatite and zircon geochemistry of Jurassic porphyries in the Xiongcun district, southern Gangdese porphyry copper belt: Implications for petrogenesis and mineralization

The Xiongcun district, a unique area where oxidized porphyry copper-gold deposits (OPCD) and reduced porphyry copper-gold deposits (RPCD) coexist, is located in the central part of the Gangdese porphyry copper belt (GPCB) in Tibet, China, and hosts the only known mineralization related to the Neo-Tethyan oceanic subduction. Apatite and zircon major and trace elements, in situ oxygen isotopes in zircon from fertile quartz diorite porphyry (QDP) of the No. I deposit, fertile hornblende quartz diorite porphyry (HQDP) of the No. II deposit, and barren hornblende quartz diorite porphyry with coarse quartz eyes (QQDP) in the Xiongcun district were determined by EPMA, LA-ICP-MS, and SHRIMP II ion microprobe to investigate the petrogenesis, magma oxidation state, and mineralization differences between the No. I and No. II deposits, as well as the potential of apatite as a metallogenic indicator. The results from this study show that a combination of REE, Ti, oxygen isotopes, and the Th/U ratios in zircon and REE as well as Sr in apatite can be used to track the magma compositions and crystallization histories in porphyry systems. Multiple indicators, including the apatite SO3 content, apatite Mn oxybarometer, and zircon Ce4+/Ce3+ ratio, indicate that (1) the parental magmas of fertile QDP and HQDP containing Cu and Au mineralization were more reduced than that of barren QQDP; (2) despite barren QQDP having higher oxygen fugacity than QDP and HQDP, the lower Cu and water contents in the magma result in a lower potential for ore formation; and (3) the similar oxidation states of QDP and HQDP show that the mineralization differences between RPCD (No. I deposit) and OPCD (No. II deposit) were not caused by the oxidation states of the magmas but were likely caused by the different processes of hydrothermal fluid evolution. Apatite grains from fertile porphyries (QDP and HQDP) have much higher Cl/F ratios than those from barren porphyry (QQDP), indicating that the Cl/F ratio of apatite can be used as a proxy for porphyry Cu-Au mineralization related to the Jurassic porphyry intrusions in the Gangdese porphyry copper belt.

Variation of copper isotopes in chalcopyrite from Dabu porphyry Cu-Mo deposit in Tibet and implications for mineral exploration

The study presents copper (Cu) isotope data of mineral separates of chalcopyrite from four drill core samples in the Miocene Dabu porphyry Cu-Mo deposit formed in a post-collisional setting in the Gangdese porphyry copper belt, southern Tibet. Copper isotope values in hypogene chalcopyrite range from –1.48‰ to +1.12‰, displaying a large variation of up to 2.60‰, which demonstrates Cu isotope fractionation at high-temperature during hydrothermal evolution. The majority of measured chalcopyrite isotopic compositions show a gradual increasing trend from –1.48‰ to +1.12‰ with the increase of drilling depth from 130m to 483m, as the alteration assemblages change from potassic to phyllic. Similarly, the other δ65Cu values (δ65Cu=((65Cu/63Cu)sample/(65Cu/63Cu)standard−1)×1000) of the chalcopyrite show a gradual increasing trend from −1.48‰ to +0.59‰ with the decrease of drilling depth from 130m to 57m, as the alteration assemblages change from potassic, phyllic, through argillic to relatively fresh. These observations suggest a genetic link between Cu isotope variation and silicate alteration assemblages formed at different temperatures, indicative of a Rayleigh precipitation process resulting in the large variation of δ65Cu values at Dabu. In general, samples closest to the center of hydrothermal system dominated by high-temperature potassic alteration are isotopically lighter, whereas samples dominated by low-temperature phyllic alteration peripheral to the center are isotopically heavier. The predicted flow pathways of hydrothermal fluids are from No. 0 to No. 3 exploration line, and the lightest δ65Cu values are the most proximal to the hydrothermal source. Finally, we propose that the northwest side of the No. 0 exploration line has high potential for hosting undiscovered orebodies. The pattern of Cu isotope variation in conjunction with the features of silicate alteration in porphyry system can be used to trace the hydrothermal flow direction and to guide mineral exploration.

Geology, fluid inclusion and isotope constraints on ore genesis of the post-collisional Dabu porphyry Cu–Mo deposit, southern Tibet

The Dabu Cu-Mo porphyry deposit is situated in the southern part of the Lhasa terrane within the post-collisional Gangdese porphyry copper belt (GPCB). It is one of several deposits that include the Qulong and Zhunuo porphyry deposits. The processes responsible for ore formation in the Dabu deposit can be divided into three stages of veining: stage I, quartz–K-feldspar (biotite)±chalcopyrite±pyrite, stage II, quartz–molybdenite±pyrite±chalcopyrite, and stage III, quartz–pyrite±molybdenite. Three types of fluid inclusions (FIs) are present: liquid-rich two-phase (L-type), vapor-rich two-phase (V-type), and solid bearing multi-phase (S-type) inclusions. The homogenization temperatures for the FIs from stages I to III are in the ranges of 272–475°C, 244–486°C, and 299–399°C, and their salinities vary from 2.1 to 49.1, 1.1 to 55.8, and 2.9 to 18.0wt% NaCl equiv., respectively. The coexistence of S-type, V-type and L-type FIs in quartz of stage I and II with similar homogenization temperatures but contrasting salinities, indicate that fluid boiling is the major factor controlling metal precipitation in the Dabu deposit. The ore-forming fluids of this deposit are characterized by high temperature and high salinity, and they belong to a H2O–NaCl magmatic–hydrothermal system. The H–O–S–Pb isotopic compositions indicate that the ore metals and fluids came primarily from a magmatic source linked to Miocene intrusions characterized by high Sr/Y ratios, similar to other porphyry deposits in the GPCB. The fluids forming the Dabu deposit were rich in Na and Cl, derived from metamorphic dehydration of subducted oceanic slab through which NaCl-brine or seawater had percolated. The inheritance of ancient subduction-associated arc chemistry, without shallow level crustal assimilation and/or input of the meteoric water, was responsible for the generation of fertile magma, as well as CO2-poor and halite-bearing FIs associated with post-collisional porphyry deposits. The estimated mineralization depths of Qulong, Dabu and Zhunuo deposits are 1.6–4.3km, 0.5–3.4km and 0.2–3.0km, respectively, displaying a gradual decrease from eastern to western Gangdese. Deep ore-forming processes accounted for the generation of giant-sized Qulong deposit, because the exsolution of aqueous fluids with large fraction of water and chlorine in deep or high pressure systems can extract more copper from melts than those formed in shallow systems. However, the formation of small-sized Dabu deposit can be explained by a single magmatic event without additional replenishment of S, metal, or thermal energy. In addition, the ore-forming conditions of porphyry Cu–Mo deposits in GPCB are comparable to those of porphyry Cu±Au±Mo deposits formed in oceanic subduction-related continental or island arcs, but differ from those of porphyry Mo deposit formed in the Dabie-Qinling collisional orogens. The depth of formation of the mineralization and features of primary magma source are two major controls on the metal types and ore-fluid compositions of these porphyry deposits.

Origin of the Newly Discovered Zhunuo Porphyry Cu-Mo-Au Deposit in the Western Part of the Gangdese Porphyry Copper Belt in the Southern Tibetan Plateau, SW China

The newly discovered Zhunuo porphyry Cu-Mo-Au deposit is located in the western part of the Gangdese porphyry copper belt in southern Tibet, SW China. The granitoid plutons in the Zhunuo region are composed of quartz diorite porphyry, diorite porphyry, granodiorite porphyry, biotite monzogranite and quartz porphyry. The quartz diorite porphyry yielded zircon U-Pb ages of 51.9±0.7 Ma (Eocene) using LA-ICP-MS, whereas the diorite porphyry, granodiorite porphyry, biotite monzogranite and quartz porphyry yielded ages ranging from 16.2±0.2 to 14.0±0.2 Ma (Miocene). Cu-Mo-Au mineralization is mainly hosted in the Miocene granodiorite porphyry. Samples from all granitoid plutons have geochemical compositions consistent with high-K calc-alkaline series magmatism. The samples display highly fractionated light rare-earth element (REE) distributions and heavy REE distributions with weakly negative Eu anomalies on chondrite-normalized REE patterns. The trace element distributions exhibit positive anomalies for large-ion lithophile elements (Rb, K, U, Th and Pb) and negative anomalies for high-field-strength elements (Nb and Ti) relative to primitive mantle-normalized values. The Eocene quartz diorite porphyry yielded ∊Nd(t) values ranging from –3.6 to –5.2, (87Sr/86Sr)i values in the range 0.7046–0.7063 and initial radiogenic Pb isotopic compositions with ranges of 18.599–18.657 206Pb/204Pb, 15.642–15.673 207Pb/204Pb and 38.956–39.199 208Pb/204Pb. In contrast, the Miocene granitoid plutons yielded ∊Nd(t) values ranging from –6.1 to –7.3 and (87Sr/86Sr)i values in the range 0.7071–0.7078 with similar Pb isotopic compositions to the Eocene quart diorite. The Sr-Nd-Pb isotopic compositions of the rocks are consistent with formation from magma containing a component of remelted ancient crust. Zircon grains from the Eocene quartz diorite have ∊Hf(t) values ranging from −5.2 to +0.9 and two-stage Hf model ages ranging from 1.07 to 1.46 Ga, while zircon grains from the Miocene granitoid plutons have ∊Hf(t) values from −9.9 to +4.2 and two-stage Hf model ages ranging from 1.05−1.73 Ga, indicating that the ancient crustal component likely derives from Paleo- to Mesoproterozoic basement. This source is distinct from that of most porphyry Cu-Mo-Au deposits in the eastern part of the Gangdese porphyry copper belt, which likely originated from juvenile crust. We therefore consider melting of ancient crustal basement to have contributed significantly to the formation Miocene porphyry Cu-Mo-Au deposits in the western part of the Gangdese porphyry copper belt.

Zircon U–Pb and Molybdenite Re–Os Ages of the Lakange Porphyry Cu–Mo Deposit, Gangdese Porphyry Copper Belt, Southern Tibet, China

AbstractThe Lakange porphyry Cu–Mo deposit within the Gangdese metallogenic belt of Tibet is located in the southern–central part of the eastern Lhasa block, in the Tibetan Tethyan tectonic domain. This deposit is one of the largest identified by a joint Qinghai–Tibetan Plateau geological survey project undertaken in recent years. Here, we present the results of the systematic logging of drillholes and provide new petrological, zircon U–Pb age, and molybdenite Re–Os age data for the deposit. The ore‐bearing porphyritic granodiorite contains elevated concentrations of silica and alkali elements but low concentrations of MgO and CaO. It is metaluminous to weakly peraluminous and has A/CNK values of 0.90–1.01. The samples contain low total REE concentrations and show light REE/heavy REE (LREE/HREE) ratios of 17.51–19.77 and (La/Yb)N values of 29.65–41.05. The intrusion is enriched in the large‐ion lithophile elements (LILE) and depleted in the HREE and high field‐strength elements (HFSE). The ore‐bearing porphyritic granodiorite yielded a Miocene zircon U–Pb crystallization age of 13.58 ± 0.42 Ma, whereas the mineralization within the Lakange deposit yielded Miocene molybdenite Re–Os ages of 13.20 ± 0.20 and 13.64 ± 0.21, with a weighted mean of 13.38 ± 0.15 Ma and an isochron age of 13.12 ± 0.44 Ma. This indicates that the crystallization and mineralization of the Lakange porphyry were contemporaneous. The ore‐bearing porphyritic granodiorite yielded zircon εHf(t) values between −3.99 and 4.49 (mean, −0.14) and two‐stage model ages between 1349 and 808 Myr (mean, 1103 Myr). The molybdenite within the deposit contains 343.6–835.7 ppm Re (mean, 557.8 ppm). These data indicate that the mineralized porphyritic granodiorite within the Lakange deposit is adakitic and formed from parental magmas derived mainly from juvenile crustal material that partly mixed with older continental crust during the evolution of the magmas. The Lakange porphyry Cu–Mo deposit and numerous associated porphyry–skarn deposits in the eastern Gangdese porphyry copper belt (17–13 Ma) formed in an extensional tectonic setting during the India–Asia continental collision.

Geochronology and petrogenesis of Miocene granitic intrusions related to the Zhibula Cu skarn deposit in the Gangdese belt, southern Tibet

The Zhibula Cu skarn deposit (19.5Mt ore @ 1.64%), near the Qulong porphyry Cu–Mo deposit, is located in the Gangdese porphyry copper belt in southern Tibet. The deposit is a typical metasomatic skarn that is related to the interaction of magmatic-hydrothermal fluids and calcareous host rocks. Stratiform skarn orebodies are mainly distributed in the contact between tuff and marble in the lower part of the Jurassic Yeba Formation. Endoskarn zonations for an outward trend are observed in the granodiorite, which grade from a fresh granodiorite to a weakly chlorite-altered granodiorite, a green diopside-bearing granodiorite, and a dark red-brown garnet-bearing granodiorite. The Zhibula granodiorite and monzogranite have similar secondary ion mass spectrometry (SIMS) zircon U–Pb ages of 16.9±0.3Ma and 17.0±0.2Ma, respectively. They exhibit different fractional crystallization from granodiorite (SiO2=64.8–69.3wt.%) to monzogranite (SiO2=72.3–76.8wt.%). Both the granodiorite and monzogranite are characterized by high Al2O3 (12.6–16.7wt.%) and K2O (1.5–5.5wt.%) contents, high Sr/Y (35–151) and La/Yb (19–48) ratios, and variable MgO (0.16–3.91) and Mg# (31–61) values. They display features of enrichment in large ion lithophile elements (LILEs, e.g., Rb, Ba, Sr, and K), depletion in high field strength elements (HFSEs, e.g., Nb, Ta, Ti, and P), and moderate negative Eu anomalies (δEu=0.58–0.98). They show restricted in situ zircon Hf isotopic compositions (+6.7 to +8.8; only one sample is +4.5) and consistent δ18O values (+6.0‰ to +6.6‰). The geochemical data indicated that the Miocene Zhibula granitic intrusions formed by the magma that were characterized by high Sr/Y ratios and were derived from the partial melting of the thickened juvenile lower crust, which may have been metasomatized by the slab melts during subduction of the Neo-Tethyan oceanic crust and were induced by the convective removal of the thickened lithosphere. In addition, the Zhibula Cu skarn deposit and the Qulong Cu–Mo deposit form a significant porphyry–skarn ore system.

REVIEW OF THE PORPHYRY ORE DEPOSITS IN THE TETHYAN OROGENIC BELT-A CASE STUDY OF THE BANGPU MO-CU PORPHYRY ORE DEPOSITS, TIBET, CHINA

Ponte Academic JournalSep 2016, Volume 72, Issue 9 REVIEW OF THE PORPHYRY ORE DEPOSITS IN THE TETHYAN OROGENIC BELT-A CASE STUDY OF THE BANGPU MO-CU PORPHYRY ORE DEPOSITS, TIBET, CHINAAuthor(s): H. A. Ahmed ,Chang-Qian Ma, (a,b) Chang-Qian Ma, (a) Lian-Xun Wang, (a) Mukhtar Habib, (c) Kabir Shola Ahmed (c)J. Ponte - Sep 2016 - Volume 72 - Issue 9 doi: 10.21506/j.ponte.2016.9.13 Abstract:Abstract The occurrence of the post-collisional Bangpu Mo-Cu porphyry ore deposits has been reviewed. The Cenozoic deposit is located within the Gangdese Porphyry Copper Belt (GPCB) in Tibet, part of Himalayan (Tethyan) Orogenic Belt, with an estimated resource of over 800,000 and 200,000 tons of Mo and Cu respectively, and an approximate grades of 0.089% Mo and 0.32% Cu. The Mo reserves within the Bangpu deposit accounts for about 60% of the known Molybdenum reserve in Tibet. The mineralization is hosted by Mid- Miocene porphyritic monzogranites, diabase and diorite porphyry and is controlled by extensional structures and are distributed along E-W fault and fracture zones. Isotopic and fluid inclusion analyses suggest that the ore fluids might have been derived from degassed mafic magmas that originate from an upwelling asthenospheric mantle. The Redox state of the magma could be viewed as an important factor in the formation of Mo rich and Cu poor in the study area. Several studies have indicated that the Bangpu porphyry Mo-Cu deposit was generated by post-collisional crustal extension that took place during the Indo-Asian collision that started at 65 Ma. In future, a rigorous fluid-rock interaction studies and determination of Redox state of magma and emplacement mechanisms in the region would give a better understanding of the genetic model that resulted in the formation of Mo-Cu deposits in the area and perhaps a great potential for discovery of additional new deposits in the Tibetan sector of the Tethyan Orogenic Belt. Keywords: Tethyan Orogenic Belt, Bangpu, porphyry Mo-Cu ore deposits, Gangdese, Magmatic degassing. Download full text:Check if you have access through your login credentials or your institution Username Password

Alteration and mineralization at the Zhibula Cu skarn deposit, Gangdese belt, Tibet

The Zhibula Cu skarn deposit contains 0.32Mt. Cu metal with an average grade of 1.64% and is located in the Gangdese porphyry copper belt in southern Tibet. The deposit is a typical metasomatic skarn that is related to the interaction of magmatic–hydrothermal fluids and calcareous host rock. Stratiform skarn orebodies occur at the contact between tuff and marble in the Lower Jurassic Yeba Formation. Alteration zones generally grade from a fresh tuff to a garnet-bearing tuff, a garnet pyroxene skarn, and finally to a wollastonite marble. Minor endoskarn alteration zonations are also observed in the causative intrusion, which grade from a fresh granodiorite to a weakly chlorite-altered granodiorite, a green diopside-bearing granodiorite, and to a dark red-brown garnet-bearing granodiorite. Prograde minerals, which were identified by electron probe microanalysis include andradite–grossularite of various colors (e.g., red, green, and yellow) and green diopside. Retrograde metamorphic minerals overprint the prograde skarn, and are mainly composed of epidote, quartz, and chlorite. The ore minerals consist of chalcopyrite and bornite, followed by magnetite, molybdenite, pyrite, pyrrhotite, galena, and sphalerite. Three types of fluid inclusions are recognized in the Zhibula deposit, including liquid-rich two-phase inclusions (type L), vapor-rich two-phase inclusions (type V), and daughter mineral-bearing three-phase inclusions (type S). As the skarn formation evolved from prograde (stage I) to early retrograde (stage II) and later retrograde (stage III), the ore-forming fluids correspondingly evolved from high temperature (405–667°C), high salinity (up to 44.0wt.% NaCl equiv.), and high pressure (500–600bar) to low-moderate temperature (194–420°C), moderate-high salinity (10.1–18.3 and 30.0–44.2wt.% NaCl equiv.), and low-moderate pressure (250–350bar). Isotopic data of δ34S (−0.1‰ to −6.8‰, estimated δ34Sfluids=−0.7‰), δDH2O (−91‰ to −159‰), and δ18OH2O (1.5‰ to 9.2‰) suggest that the ore-forming fluid and material came from magmatic–hydrothermal fluids that were associated with Miocene Zhibula intrusions. Fluid immiscibility likely occurred at the stage I and stage II during the formation of the skarn and mineralization. Fluid boiling occurred during the stage III, which is the most important Cu deposition mechanism for the Zhibula deposit.