Chunji Xue

Geochronology and Geochemistry of the Ore-Forming Porphyries in the Lailisigao'er-Lamasu Region of the Western Tianshan Mountains, Xinjiang, NW China: Implications for Petrogenesis, Metallogenesis, and Tectonic Setting

Porphyry-type Cu (Mo, Zn) deposits have been discovered along the late Paleozoic Kokirqin arc in the western Tianshan Mountains of China, part of the Central Asian Orogenic Belt. The deposits include the Lailisigao’er Mo-Cu deposit, the 3571 Cu deposit, and the Lamasu Cu-Zn deposit. The ore-forming porphyries from these three deposits are predominantly intermediate-felsic and belong to calc-alkaline and transitional series. Laser ablation–ICPMS zircon U-Pb dating on ore-forming porphyries from the Lailisigao’er and 3571 deposits yields ages of Ma and Ma, respectively. The trace element compositions of these porphyries from these three deposits are similar to those formed in a continental arc setting and are characterized by enrichment of large ion lithophile elements and depletion of high field strength elements and heavy rare earth elements ) coupled with slightly negative Eu anomalies. These rocks also show high (87Sr/86Sr)t (0.70722–0.71028) and low ϵNd(t) values (−3.71 to +0.17), coupled with depletion of Ba relative to Th and elevated Th/Ce, Nb/Y, and Th/Yb ratios, suggesting that the porphyry magma originated from a partial melting of subducted sediments, mixed with minor melts produced by partial melting of mantle wedge components and involvement also of lower continental crust during emplacement. These three deposits belong to the first metallogenic group in the Chinese Tianshan, which formed from the Middle Devonian to the early Carboniferous in a continental arc environment related to a subducted oceanic slab; this group is distinguishable from a second group that formed in the Permian during a late collisional stage, in which regional collisional compression changed to extension.

An overpressured fluid system associated with the giant sandstone-hosted Jinding Zn-Pb deposit, western Yunnan, China

The Jinding Zn-Pb deposit is hosted in continental clastic rocks in the Meso-Cenozoic Lanping-Simao Basin in western Yunnan. The deposit has been compared to SEDEX, MVT, and sandstone-hosted types (SST), all assuming intra-basinal origins for the ore-forming fluids. The driving forces of fluid flow and the pressure system have not been systematically investigated. This paper presents field and fluid-inclusion evidence to show that the Jinding mineralizing system was strongly overpressured. The common occurrence of CO2-rich fluid inclusions and the high fluid pressures suggest a mineralizing system markedly different from most other sediment-hosted base metal deposits including SEDEX, MVT and SST. Numerical modeling of basinal fluid flow indicates that sediment compaction alone in the Lanping-Simao Basin cannot produce any significant overpressure. Tectonic thrusting can significantly increase fluid pressure, but is not enough to cause the high fluid overpressure indicated by fluid inclusions. Input of extra-basinal fluids (e.g. deepsourced CO2) into the basin may have contributed to the development of the overpressured mineralizing system.

Oceanic Subduction-continental Collision Transition and Epithermal Gold Mineralization: Insight From Geochronological Study of the Tawuerbieke Deposit, Tulasu Ore Cluster, Western Tianshan

Epithermal gold deposits form from hydrothermal systems at shallow depth and low temperature, and commonly develop in compressional setting at active volcanic arcs or extensional environment at back-arc basins both in oceanic subduction geodynamic setting, e.g., the circle Pacific region (Simmons et al., 2005). In addition, epithermal Au ± Ag deposits in the Alpine-Himalayan belt appeared to be formed during continental collision (e.g., Chah Zard in central Iran; Duolong in Tibet of SW China). Currently, there is no consensus on the tectonic setting of epithermal Au deposits in the Central Asian Orogenic Belt (CAOB), possibly due to the multiple subduction–collision and amalgamation processes of the Paleo-Asian ocean during Neoproterozoic to Paleozoic. The western Tianshan of the Southwestern CAOB, represents an important porphyry Cu ± Au ± Mo and epithermal Au belt that extends for more than 2500 km from Uzbekistan to NW China. The Tulasu area, is known to be an important epithermal gold ore cluster in north Chinese western Tianshan, and hosts several important epithermal Au deposits or prospects, e.g. Axi, Jingxi–Yelmend, Tawurbieke, and Qiabukanzhuota. In this study, based on a systematic field investigation, we carried out zircon SHRIMP U–Pb studies on magmatic rocks in the Tulasu area, together with new Re–Os pyrite age of the Tawuerbieke gold deposit, in an attempt to detect the geodynamic setting of epithermal gold system in the CAOB.

The Fluid Dynamic Process of Large-scale Mineralization in the Lanping Basin, Yunnan, SW China: Evidence from Fluid Inclusions and Basin Fluid Modeling

Abstract The Lanping basin, Yunnan, SW China, is known for the giant Jinding Zn-Pb deposit and the newly-discovered Baiyangping Cu-Co-Ag super-large deposit. With a reserve of ∼ 200 Mt ore grading 6.08% Zn and 1.29% Pb (i.e., a metal reserve of ∼ 15 Mt) hosted in Cretaceous and Tertiary terrestrial rocks, the Jinding deposit is the largest Zn-Pb deposit in China, and also the youngest sediment-hosted and the only continental sediment-hosted super-large Zn-Pb deposit in the world. Differing from the known major types of sediment-hosted Zn-Pb deposits in the world, including SST, MVT and Sedex, the Jinding deposit represents a new type of sediment-hosted Zn-Pb deposits. Most previous studies assumed that the mineralizing fluids were derived from within the basin and the fluid flow was driven by topographic relief under a hydrostatic regime. However, the observations of hydraulic fractures and fluid inclusion data indicate that the mineralizing fluid system was strongly over-pressured. The study of fluid inclusions in sphalerites and associated gangue minerals (quartz, celestite, calcite, and gypsum) shows that homogenization temperatures cluster around 110–150°C, with salinities of 1.6–18.0 wt% NaCl equivalent. The fluid temperature increases with the decrease of salinities during the main ore stages, and there is a systematic westward decrease in temperature and increase in salinity in the Jinding ore district. Fluid pressures as high as (513–1364) × 105 Pa are indicated by CO2-rich fluid inclusions. The basin fluid dynamic modeling results indicate that the overpressures could not have been produced by normal sediment compaction, and the overpressure related to the thrusting may not be enough to explain the high fluid pressures indicated by fluid inclusions. The injection of mantle-derived fluids is likely responsible for the building-up of the high overpressures. The mixing of two types of fluids in a structural-lithologic trap may have been the key dynamic process for the large-scale mineralization in the Lanping basin: one was a mantle-derived fluid enriched in metals and CO2 with higher temperature and lower salinity, and the other was H2S-rich saline formation water with lower temperature and higher salinity. The special hydrodynamic regime and potential contribution of mantle-derived fluids to the mineralizing system distinguish Jinding from other known sedimentary basin-related Pb-Zn deposits.

Mineralization stages and fluid processes in the giant jinding deposit, western Yunnan, China

Jinding is the largest Zn-Pb deposit in China, and also the yougest sediment-hosted super-large Zn-Pb deposit in the world. The Jinding mineralization processes are discussed in this paper based on the investigations of the mineral intergrowth. The hydrothermal mineralization experienced three stages: the stage of quartz sphalerite-galena, the stage of sphalerite-galena-celestite and the stage of galena-calcite-celestite-gypsum. Fluid inclusions in sphalerite and associated gangue minerals show that the homogenization temperatures range from 54° to 309°C and cluster around 110°∼150°C, with salinities of 1.6∼18.0 wt% NaCl equivalent. The temperature increased and the salinity decreased during the early stage, the temperature was stable and the salinity decreased during the main stage, and both temperature and sality decreased during the final stage. This temperature-salinity trend probably resulted from mixing of a high-temperature, low-salinity fluid with a low-temperature, high-salinity fluid during mineralization.

Geological Controls of Orogenic Gold Mineralization at Zarmitan, Uzbekistan Tianshan

The Zarmitan deposit is located in the central Kyzylkum-Nuratau gold metallogenic province of the South Tianshan Hercynian thrust-fold-belt, north Nuratau mountain of western Uzbekistan. The Zarmitan deposit represents one of the best explored examples of gold deposit hosed in narrow quartz veins in granites and metasedimentary rocks, and is estimated to contain as much as 314 t Au at an average grade of 9.8 g/t (Abzalov, 2007). Previous studies on the Zarmitan deposit emphasized on gold mineralization in host granitoids and other geological characteristics, and classified it as an intrusion-related gold system during the orogenesis of the South Tianshan (Abzalov, 2007; Graupner et al., 2010). However, significant gold enrichment is confined to hydrothermal altered fracture zone only. This paper reviews the regional geological setting of the Zarmitan deposit, with an emphasis on the geological controls of gold mineralization, especially the possible links between metasedimentary strata, magmatic intrusions, and structural activities, and their roles in the formation of the worldclass Zarmitan deposit.