Abstract:Tinggong is a large porphyry Cu deposit located in the western segment of the Gangdise porphyry Cu belt in southern Tibet. It contains 1.49 Mt of Cu with an average grade of 0.362% and 0.04 Mt of Mo with an average grade of 0.018%. The deposit consists of both collisional and post-collisional stage porphyry-type Cu systems, which are genetically associated with the early Eocene coarse-and fine-grained granite and the Miocene monzogranite porphyry stock, respectively. It is the most important deposit in the Gangdise porphyry Cu belt, but geology of the deposit remains poorly constrained. In this paper, the authors conducted a detailed study of geological features of the deposit, so as to ① document the geological characteristics (e.g., alteration and mineralization) of the deposit; ② investigate intrusive sequences and magma source of main intrusions in the deposit; ③ investigate tectonic control of intrusion emplacement, alteration and mineralization; ④ track fluids evolution process; and ⑤ determine genesis of the deposit. On such a basis, the authors formulated a genetic model for the Tinggong deposit.The authors surveyed the petrography-alteration of the Tinggong copper deposit, and exactly determined the distribution of formations and magmatites. In combination with zircon U-Pb isotope dating results of other researchers, the authors built up a detailed series of magma evolution. It is shown that there existed two peaks of magmatic events, i.e., K-feldspar granite in Eocene (~50 Ma) and porphyritic monzonite granite, tonalite porphyry and diorite porphyry in Miocene (13~17 Ma).Through field and lab research, the authors have come to the conclusion that there occured two phases of mineralization events in the Tinggong deposit, i.e., copper mineralization in Eocene (~50 Ma) and Cu-Mo mineralization in Miocene (~15 Ma). Moyite formed in Eocene, which has a particular texture, i.e., it has no classical porphyritic texture but shows unique fine grain-like pegmatite texture. This suggests the existence of sudden cooling of the ore-bearing magma, which might have been caused by the sudden escape of fluids. There also exist large quantities of micro graphic texture, quartz eyes and some other textures. These special textures imply that the magma was rich in water and the fluid was once in a saturation state. Porphyritic monzonite granite was associated with Cu-Mo mineralization in Miocene.The alteration zoning of the deposit is similar to that of other porphyry deposits in Gangdise. In term of time, the alteration was potassic alteration in early period, propylitization occurred in a transitional phase, followed by phyllic alteration and argillization. In term of space, from the core to the outer part there are potassic alteration, phyllic alteration, and propylitization. The latest argillization exhibits patch or banded form, superimposed upon other alterations of earlier time. The authors described all kinds of alterations, mineral assemblages and associated veins in detail and dealt with the evolution process of the fluid as well as the source and accumulation mechanism of metals.

Abstract:Bangpu in Tibet is a large but poorly studied Mo-rich (~0.089%), and Cu-poor (~0.32%) porphyry deposit formed in a post-collisional tectonic setting. The deposit is located in the Gangdise porphyry copper belt (GPCB), and formed at the same time (~15.32 Ma) as other deposits within the belt (12~18 Ma), although it is located further to the north and has a different ore assemblage (Mo-Pb-Zn-Cu) compared with other porphyry deposits (Cu-Mo) in this belt. Two distinct mineralization events have been identified in the Bangpuhyry deposit, which are porp Mo-(Cu) and skarn Pb-Zn mineralization. According to detailed drilling catalog and study of mineral assemblages and vein intersection relationship in 11 drill holes, veins in the Bangpu deposit can be divided into three types, which are A, B, D veins respectively. Also, minerals such as garnet and epidote are also been detected which can help to invert fluid evolution process in the skarn mineralization period. Along the circular alteration zonation from a central potassic alteration zone through silicification and phyllic alteration zones to an outermost propylitic alteration zone, argillic alteration is present as patches within the phyllic and propylitic alteration zones. Skarn alteration is identified by the presence of typical skarn minerals within the surrounding rocks. A-type veins contain fluid inclusions that generally homogenize between 320℃ and 550℃, and have w(NaCl_eq) values that are generally split between salinities of 17.0%~22.0% (gas-liquid two-phase inclusions) and 30.8%~67.2% (crystal-bearing inclusions); fluid inclusions within B-type veins generally homogenize at temperatures between 380℃ and 550℃, and have w(NaCl_eq) values that are clustered within three distinct intervals, i.e., 1.6%~10.1%, 23.2%~24.5% (gas-liquid two-phase inclusions), and 30.8%~67.2% (crystal-bearing inclusions); fluid inclusions within D-type veins have homogenization temperatures and salinities of 213~450℃ and 7.3%~11.6%, respectively. Fluid inclusion microthermometry indicates that the ore-forming fluids related to the porphyry mineralization evolved from high-temperature and high-salinity fluids to low-temperature and low-salinity fluids. Intense boiling occurring in A, B veins indicates that pressure fluctuation contributed a lot to the deposition of Mo and Cu. 16 quartz samples related to porphyry mineralization have the following H-O isotope data: δD_V-SMOW=-185.8‰~-107.1‰,δ¹⁸O_V-SMOW=9.5‰~14.5‰, while δD_V-SMOW=-184.7‰ ~-126‰,δ¹⁸O_V-SMOW=3.9‰~12.9‰ for garnet, epidote, quartz, and calcite related to skarn mineralization. δ¹⁸O_V-SMOW = -1.6‰ ~10.4‰, δC_V-PDB=-6.5‰ ~-3.4‰ for calcite in porphyry period and δ¹⁸O_V-SMOW=1.8‰~11.9‰, δC_V-PDB=-5.1‰~4.6‰ for calcite in skarn period. All the C-H-O isotope data demonstrate two processes of fluid evolution in the Bangpu porphyry-skarn deposit, i.e., dehydration-degassing process and atmospheric precipitation addition process.

Abstract:The Yulong porphyry Cu-Mo deposit in eastern Tibet is one of the largest copper deposits in China. The formation of the deposit has been regarded to be related to a monzogranilic composite intrusion in the center of the deposit, but the genetic relationship between mineralization and the composite intrusion remains poorly constrained. Based on field geological mapping and drill hole logging along No. 8 section, the authors identified a suite of granite porphyry stocks from the composite pluton. The stocks include abundant quartz-feldspar A-type veins, and some quartz shows unidirectional solidification texture and pegmatite/aplite texture. These characteristics indicate that the magma was once water-saturated. Additionally, high-grade (>0.6%) copper mineralization is distributed closely around the granite stocks, with high-temperature A-type veins within the stock, and meso-temperature quartz-sulfide D-type veins far away from the stock. Hydrothermal alteration surrounding the deposits is generally characterized by concentric zones from an inner potassic zone outward to phyllic and argillic alteration zones, and an outer propylitic and skarn zone. The potassic alteration zone well overlaps the granite porphyry stocks. Based on above observation, the authors hold that the granite porphyry stocks are ore-forming intrusions at Yulong.

Abstract:In order to trace the origin of ore-forming fluid during the complicated mineralization and alteration process of the Zhushahong porphyry copper-gold deposit, the authors analyzed H and O isotopes of quartz from different alteration and mineralization assemblages to evaluate the source and evolution of the ore-forming fluid. Some conclusions have been reached: ① δ¹⁸O and δD values of water show balance with A-type veins (+6.0‰~+11.2‰, -90‰~-101‰),quartz in phyllite (A1), Qtz-kfs veins (A3), B-type veins, straight quartz veins (B1), and Qtz-Moly-Py±Anhy±Cpy veins (B2),suggesting that all the ore-forming fluids were derived from magma; ② δ¹⁸O and δD values of water show balance with the B1-type veins (+6.6‰ and -101‰), implying that fluids inducing propylitic alteration were dominated by the fluid separated from magmatic vapor during exsolution with low density, low salinity and high temperature; ③ Considerable variations of δ¹⁸O in Qtz-Py-Cpy veins (B3) (+4.1‰~+6.0‰) suggest that minor amounts of meteoric water and regional metamorphic water were partially involved in the magmatic water during transforming of K-silicate alteration to feldspar-destructive alteration; ④ During the formation of D-type veins, meteoric water and underground water were infused into the ore-forming fluid-system and mixed with magmatic water (δ¹⁸O:+2.8‰~+4.9‰). Feldspar-destructive alteration which overprinted most of the potassic alteration and part of the propylitic alteration was triggered by the reaction of mixed water with porphyries and phyllite. It is thus held that K-silicate alteration and propylitic alteration accompanying with A-type veins and early B-type veins were caused by the action of magmatic fluid. Meteoric water began being infused into the alteration-mineralization system from the transformation period. However, feldspar-destructive alteration was triggered by the mixed fluid of magmatic water and meteoric water.

Abstract:Molybdenite occurring widely in metal deposits has been considered to be the most ideal target for directly dating deposits due to its high content of Re and Os and low content of common Os. However, it is very difficult to collect enough pure molybdenite grains for dating in many types of ore deposits due to its fine-grained mode of occurrence and low content. As we know, the Re and Os are chalcophile and siderophile elements, and they would be concentrated mainly in sulfide minerals rather than in silicate mineral crystals. So it is theoretically feasible to determine Re-Os age directly by using molybdenite-bearing whole rock samples. To examine this idea, the authors chose age well-constrained Dexing porphyry copper orefield as a case study. In the study, five molybdenite-bearing rock samples were collected from drill holes in the Zhushahong deposit for Re-Os isotopic analysis. As a result, the five samples yielded a high-quality isochron age of 172.6 ±2.6 Ma, which is the same with previously reported Re-Os age of the deposit obtained by using pure molybdenite samples. This shows that directly dating molybdenite-bearing wholerock samples is not only feasible but also reliable. Additionally, the Re-Os age of molybdenite the authors reported in the study is in accordance with the zircon U-Pb age of ore-forming granodiorite porphyry, implying that the mineralization and the porphyry intrusion are genetically related to each other. The previous Rb-Sr ages and K-Ar ages of the deposit do not coincide with the Re-Os age and the U-Pb ages, which might be caused by the late hydrothermal alteration and thermal events that damaged or reset the Rb-Sr and K-Ar isotopic systems.

Abstract:The Mujicun porphyry copper deposit is located in northern Taihang Mountains of North China Craton (NCC). The precise SHRIMP U-Pb dating and Hf isotope of diorite porphyry show that diorite porphyry formed at 142.0±1.5 Ma. The diorite porphyry has the εHf(t) value of -21.2~-17.9, corresponding to t^C_DM model ages of 2.3~2.5 Ga and having similar Hf isotopes to the gabbrodiorite with εHf(t) of -19.6~-15.3 in Wanganzhen complex. Re-Os isotopic dating technique for molybdenites collected from the porphyry type ores in the Mujicun porphyry Cu deposit was applied to confirm the age of mineralization. The Re-Os dating of five molybdenite samples from porphyry-type ore yielded Re-Os model ages ranging from 144.5 to 145.5 Ma, with an average of 144.8 ±1.4 Ma, and the isochron age is 145.3 ± 2.2 Ma, representing the metallogenic age of the Mujicun porphyry copper deposit. The interaction of crust-mantle materials is crucial to the formation of the Mujicun porphyry copper deposit. Mesozoic lithospheric extension might have induced the melting of the enriched lithospheric mantle in response to the asthenosphere uprising for generating these intermediate-acidic rocks in the region.

Abstract:In the compressive environment, intrusion-related Pb-Zn mineralization includes porphyry, crytoexplosive breccia, skarn, manto, and vein types. However, examples of all of them occurring in the same mineralization system are less known. Narusongduo is a superlarge Pb-Zn deposit in the Gangdise metallogenic belt, Tibet. Except porphyry type, all Pb-Zn mineralization types mentioned above have been found in this deposit, which makes this deposit an excellent case to study the scientific question mentioned above. In this study, the authors present a preliminary description about the four Pb-Zn mineralization types and oxygen, hydrogen and sulfur isotopic analyses, and establish a descriptive and genetic model that involves all the mineralization types in an independent Pb-Zn mineralization system related to magmatism. The Pb-Zn mineralization in the Narusongduo deposit has close relationship with the quartz syenite porphyry. Four Pb-Zn mineralization types have been found, i.e., crytoexplosive breccia type formed in the tuff strata of the Dianzhong Formation in Paleocene, skarn type formed in the contact between limestone of the Xiala Formation in Permian and quartz syenite porphyry, manto type formed in the contact between sandstone and slate in the Angjie Formation in Permian and tuff in the Dianzhong Formation, and vein type formed in pre-existing fractures in tuff and sandstone-slate strata. Epidotization, chloritization, silicification, sericitization and carbonation are main alteration types for the four mineralization types, galena and sphalerite are ore minerals, and pyrite, chalcopyrite, quartz, sericite and calcite are main gangue minerals. In accordance with the order of cyrtoexplosive type, skarn type, manto type and vein type, the host rock alteration becomes weaker, the epidote, chlorite, quartz, sericite and chalcopyrite decrease and eventually disappear, and the carbonate increases. These characteristics indicate a cooling evolution in a unified mineralization system. As for the fluids of crytoexplosive breccia type and skarn type Pb-Zn mineralization, the δD_V-SMOW values have large ranges (-177‰~-118‰ and -164‰~-139‰, respectively) and the δ¹⁸O_V-SMOW values have narrow ranges (-2.76‰~3.29‰ and -5.46‰~-4.58‰, respectively). These data suggest that the ore fluids were derived from magmatic hydrothermal fluid which had undergone degasification in an open system and was mixed with meteoric water. The δ³⁴S values vary in a narrow range and become larger in order of cyrtoexplosive type (4.15‰), skarn type (7.92‰), manto type (8.49‰) and vein type (8.80‰), which indicates that the sulfur came from magmatic hydrothermal fluid which had undergone H₂S degasification in an open system, and the degasification degree became stronger and stronger. The chemical geology of the quartz syenite porphyry is similar to that of porphyry hosting the Lengshuikeng porphyry Pb-Zn deposit, so the authors infer that some porphyry-type Pb-Zn mineralization might have also occurred in the depth of the Narusongduo porphyry. On such a basis, an independent Pb-Zn mineralization system, which includes all the Pb-Zn mineralization types related to magmatism occurring in compressed environment, has been built. The model can be described as follows: With the subduction and revolution of the Neo Tethys oceanic crust, the mantle wedge and the overlying crust became melted, forming the Narusongduo porphyry magma with unique chemical geology. With the magma ascending, H₂O and H₂S became degassing, the fluid became exsolved, oxidized magmatic fluid with oxidized S and metals was concentrated in the center of the magma, and reduced magmatic fluid with reduced S and metal was concentrated on the top of the magma. With further ascending, the temperature of the magma decreased, the magma crystallized, SO₂ became hydrolytic, and the porphyry Pb-Zn mineralization occurred. When the magma ascended into the tuff strata, the compact rocks caused the pressure of the hydrothermal fluid to be higher than the rocks, which led to the crytoexplosion of the tuff, and the crytoexplosive type Pb-Zn mineralization occurred. At the same time, with the stronger H₂S degasification, magmatic fluid rich in metal and sulfur also moved to the host rocks far away from the magma, and then skarn-type, manto-type and vein-type Pb-Zn mineralization occurred in different locations with different lithologies and structures.

Abstract:In the magma-related deposits, the ore-forming plutons generally experienced high-degree evolution and differentiation, so its petrologic and geochemical characteristics can only reflect the final state of magma evolution. The evolution of ore-hosted magma and the geochemical behavior of main metallogenic elements in the evolution remain unclear. However, the study of contemporaneous and sympatric magmatic rocks provides a potential solution for this question. Here the authors made a case study of the northern Gangdise Pb-Zn ore belt in Tibet formed in the early period of continental collision. Based on the statistical study of lots of published geochemical analyses of magmatic rocks from the Gangdise belt, this paper presents the evolution of magma and its control of metallogenesis. The results suggest that, besides the crystallization differentiation of magma itself, crustal contamination was another factor controlling the evolution from basic magma to acid magma during differentiation. It is also found that most of the magmatic rocks formed in the early period of continental collision are less oxidized and less hydrous; nevertheless, when magma evolved to the middle and late period, the oxidation of magma increased with progressive evolution. Mantle-derived magmas are enriched in S and H₂O, which experienced underplating at the base of Zn-enriched crust materials and induced partial melting. Crust-derived magmas experienced high-degree evolution, and finally resulted in the formation of the northern Gangdise Pb-Zn ore belt.

Abstract:Lying in the western segment of Lunggar-Gongbo'gyamda arc back faulted-uplifted belt of the Lhasa terrane, the Lunggar iron deposit in Coqen County of Tibet is one of the important ore deposits in the Mesozoic skarn iron belt in the western part of Gandese. Field and indoor studies show that the Lunggar iron deposit is a skarn deposit related to coarse-grained monzogranite. In this study, LA-ICP-MS and SHRIMP zircon U-Pb dating of the metallogenic rock mass in the Lunggar iron deposit was conducted to probe into the ore-forming epoch of this deposit. The analytical result shows that the coarse-grained adamellite has an age of 115.5±2.1 Ma, which can approximately represent the ore-forming age of iron. The Lunggar iron deposit and the Nyicung iron deposit on the eastern side of the Lunggar iron deposit have rock-forming and ore-forming ages of 110~116 Ma, and the Luobule iron deposit between them have rock-forming and ore-forming ages of 111.3±1.6 Ma. These iron deposits are located in the same tectonic unit, have similar ore-forming characteristics and ore-forming as well as rock-forming ages, and constitute a EW-trending early Cretaceous skarn iron metallogenic belt which possibly extends westward and includes such skarn iron deposits as Bangbuqing, Longrenla and Chajiasi. In this belt, there are also late Cretaceous skarn copper-gold deposits such as the Ri'a copper-gold deposit whose ore-forming age is 87Ma. Therefore, this ore-forming belt should have the same or similar geological settings and tectonic-magmatic evolution processes. The iron-forming magmatic activity of the Lunggar iron deposit and even of the whole ore-forming belt might have to do with the underbow process of the plate, the copper- and gold-forming magmatic activity might have been related to the faulting and separating process of the oceanic crust, and the intrusion of the quartz diorite might have occurred between them.