The Metallogenic Environments And Metallogenic Mechanism Of Gold Deposits In Southwestern Tianshan Mountains

The Southern Tianshan antimony-mercury-gold metallogenic belt in Central Asia is one of the important gold belts in the world, hosts decade world-class, superlarge and large gold deposits. The Southwestern Tianshan Mountains in Xinjiang are its eastward extension, where severa gold deposits (occurrence) discovered recently highlight the exploration potential of this area. The gold deposits in the Southwestern Tianshan Mountains in Xinjiang are not compare with the Southern Tianshan in Central Asia on the amount and resource, so study the metallogenic regularities of gold deposits in the Southwestern Tianshan Mountains is the"hot spots"studied by the economic geologists. Based on the detailed field investigation and previous researches, many methods, such as rock geochemistry analyses, fluid inclusions study, stable isotopes, chronology and synthetically comparative study etc are used in this study.This Ph.D thesis divided the gold deposit type, summarized the metallogenic regularities of gold deposits, studied the geological characteristics of representative gold deposit and metallogenic mechanism, and discussed the relation between geodynamics evoluton of the Southwestern Tianshan Mountains and gold mineralization, and compared with the Southern Tianshan gold metallogenic belt in Central Asia. The main advancements achieved from the thesis are as followings: Gold deposits in the Southwestern Tianshan Mountains may be classified as follows: gold deposit related to the ductile shear zone (orogenic gold), gold deposit related to the intrusion, quartz-barite vein type gold deposit and epithermal gold deposit. Among which orogenic gold deposit is important type of gold deposit, the quartz-barite vein type gold deposit is a rare new gold type. This paper summarized the metallogenic regularities of gold deposits in the Southwestern Tianshan Mountains. The gold deposits are controlled by geotectonic units and faults, also are controlled by strata and lithology. The gold deposits related to the ductile shear zone are mainly distributed along the deep faults, are controlled by brittle-ductile shear zone. The host rocks of the Muruntau type gold deposits are carbonaceous phyllites, metasandstone and metasilstone, and are characterized by turbidity current deposition. The magmatic activity restricted the gold mineralization, provided ore-forming materials, ore-forming fluids in some degree, and acted as"engines"which drive the hydrothermal solution to form a conductive circulation system.Based on the comparisons with other types of deposits, this paper propose that the Bulong gold deposit is representative of a rare quartz-barite vein type gold deposit, studies systematically geological and geochemical characteristics and discusses ore-forming process of the Bulong gold deposit. Homogenization temperatures of fluid inclusions vary from 159 to 390oC, with Th during the main stage of gold mineralization ranging from 200 to 340oC. Salinity ranges from 2.42 to 19.29 wt.% NaCl equiv. Ore fluid belong to a CO₂-H₂O-NaCl system. Theδ³⁴S values of pyrite associated with gold mineralization range from 14.6 to 19.2‰and those of barite from 35.0 to 39.6‰, indicating that the sulfur was probably derived from the sedimentary host rocks. Helium and argon isotope compositions of fluid inclusions suggest that the ore fluids of the Bulong gold deposit were mainly derived from the crust and were possibly mixed with a small amount of mantle component during the metallogenic process. Theδ¹³CPDB values of fluid inclusions in vein quartz from the Bulong gold deposit define a narrow range of–4.6 to–1.4‰. Theδ¹⁸OSMOW values of vein quartz range from 17.2 to 21.1‰, with correspondingδ18Ofluid values of 6.7 to 14.7‰, andδ^D values of fluid inclusions of between–70 and–55‰. The combined isotopic data imply that the ore-forming fluids of the Bulong gold deposit were mainly derived from basinal fluids, with some minor contributions from magmatic fluids and meteoric water, and the carbon in the ore fluids was mainly derived from marine carbonate rocks. Changes in physico-chemical conditions of ore fluids, fluid compositions and immiscibility of ore fluids played important roles in ore-forming processes of the Bulong gold deposit.The Sawayaerdun gold deposit occurs in Upper Silurian and Lower Devonian epimetamorphic carbonaceous turbidite, and the host rocks are carbonaceous phyllites, metasandstone and metasilstone. Gold mineralized zones and orebodies are controlled by brittle-ductile shear zone. The ore-forming fluids belong to a medium-to low-temperature (78～355℃), low- to medium-salinity(2.57～22.10%) H₂O–NaCl–CO₂–CH₄ system. Gold and antimony mineralizations are closely related to the variations of CO₂ and CH₄ contents of the ore-forming fluids. Theδ¹³CPDB values of siderite, dolomite and siderite range from–10.5 to–4.98‰, indicating that carbon was derived from a mixture of marine carbonates and the mantle and was affected by low-temperature alteration. The ⁴⁰Ar/³⁹Ar step heating age for whole-rock samples of sericite-quartz altered rock from brittle-ductile shear zone supports that gold mineralization formed in the Triassic. Remobiliation, concentration and precipitation of gold are closely associated with ductile shear zone evoluton in terms of space and time. The Sawayaerdun gold deposit resembles the Muruntau gold deposit in Uzbekistan and Kumtor gold deposit in Kyrgyzstan in respect to geological setting, geological characteristics of ore deposits and metallogenic mechanism. Nevertheless, the Sawayaerdun gold deposit has some special features. The ores were deposited at shallow levels and at medium-low temperatures and gold is associated with antimony.Dashankou gold deposit occurs in Upper Silurian–Lower Devonian (carbonaceous ) fine-grained clastic rocks. Gold mineralization is controlled by the ductile shear zone. Orebodies distributed within dacite porphyry dyke and diorite porphyrite dyke or contact zone between the dyke and wall rock. Systematic analyse of the major elements and rare earth elements (REE) of the dyke, sandstone, altered rock and quartz-carbonate vein suggest that dacite porphyry dyke and diorite porphyrite dyke were derived from a common magma chamber. Characteristics of rare earth elements imply that the ore-forming fluids of the Dashankou gold deposit were mainly derived from volatile component-richment magmatic fluids. Theδ13CPDB values of ankerite range from–2.8 to–3.3‰,δ18OV-SMOW values of 14.1 to 14.5‰, indicating that carbon was derived from a mixture of marine carbonates and the mantle. Theδ18OSMOW values of quartz range from 15.9 to 18.6‰, with correspondingδ18Ofluid values of -1.14 to 10.9‰, andδD values of fluid inclusions of between–51 and–74‰. The combined isotopic data imply that the ore-forming fluids of the Dashankou gold deposit were mainly derived from magmatic fluids, with some minor contributions from meteoric water. The Dashankou gold deposit is controlled by ductile shear metallogenic system, ductile shear structure and magmatic activity are mainly ore-controlling factors.The gold mineralization of the Southwestern Tianshan Mountains is markedly controlled by the regional geodynamics evoluton. The age of gold mineralization in the Southwestern Tianshan Mountains is mainly Permian to Late Triassic, in a few cases Late Devonian–Carboniferous. The gold deposits formed in the Permian occurring in an environment of regional extension during post-collisional stage. The gold deposits formed in the Triassic, were controlled by the Indosinian movement. Based on the comparisons with the Southern Tien Shan gold metallogenic belt in Central Asia, indicating that the highlight the exploration potential of this area, though there are differences in the regional geological setting.