Study On Metalliferous Deposits Mineralization In Southern Qilian Orogenic Belt, Qinghai Province

Southern Qilian area is located in the northern margin of Chaidamu basin, rich in endogenous metal mineral, and it is one of the important polymetallic ore belts in central orogenic belt. In this paper, the latest orogenic tectonic theory will be severed as the guiding ideology. Redefining on geodynamic evolution of southern Qilian area comes to the conclusion that it has passed complex geological evolution with the characteristics of ploycycle multiple orogeny. The orogenic belt mainly experienced Mesoproterozoic continental nucleus forming,Neoproterozoic ancient land divergence and limited ocean basin stretching,oceanic basin among the micro-continental blocks starting closing in Caledonian Tethys tectonic domain that leading to arc-continent and continent-continent collision and fitting, the oblique subduction of CaiBei ocean in Late Caledonian,in addition to,scissors-type closing tectonics,switching into intracontinental orogeny period in Variscan,forming Zongwulong accretionary wedge orogeny in Late Variscan-Indosinian and the NE tectonic evolution in Meso- Cenozoic.There is a coupling relationship between mineral deposits and geodynamic evolution in Southern Qilian area. Metallogenic epoch of orogenic gold deposits were consistent with the tectonic evolution of Chaibei belt,Lajishan belt,Danghenanshan belt and Zongwulong belt,with a tendency of progressively changing newer from north to south. The gold deposits in Chaibei belt were superimposed with multi-metallogenic stage in Late Caledonian-Indosinian. Gold deposits in Danghe south mountain belt doesn't display any features of the multi metallogenic stages. Mineralization in Zongwulkong belt had a close relationship with late Variscan–Indo magma-tectonic activities. Low-temperature hydrothermal vein-type deposits only produced in Chaibei belt in early Paleozoic, and not yet be found in the Lajishanbelt, Danghenanshan and Zongwulong paleozoic rift belts;however,the skarn-type gold deposits only be found in the Lajishan belt,in addition,limited by marine volcanic rocks ,which rich in calcium.Ore-bearing strata in Southern Qilian area were mainly Precambrian sedimentary (Wandonggou group),early Paleozoic marine volcanic sedimentary formation (Tanjianshan group), Ordovician shallow marine-fluvial clastic formation (Yanchiwan group), Early Paleozoic medium-basic marine sedimentary volcanic lava formation (Liudaogou group) and Neopaleozoic flysch formation. In despite of different rock lithology and age,gold deposits in this area were of similar geological-geochemical characteristics and genesis, satisfying the definition of orogenic gold deposits. C,H,O,S isotope studies showed that metallogenic material were mainly from the upper mantle and host rocks,the ore-forming fluids were mainly meteoric water,magmatic water and metamorphic water,as well as the features of deep-seated derived sulfur and multi-stage metallogenic evolution .A large number of fluid inclusion geochemical studies showed that fluid inclusions in quartz were not well developed in this area. There are three types,namely,CO₂-containing fluid inclusions, CO₂-H₂O fluid inclusions and H₂O fluid inclusions. Among which, gas-liquid two-phase fluid inclusions were well developed,the ore-forming fluid was of a low temperature, low salinity and low-density H₂O-CO₂-NaCl system and the H₂O-NaCl brine system, showing a hypabyssal metallogenic environment.According to the studies on tectonic environment, gold deposit characteristics, mineralization, genetic types, metallogenic epoch in Southern Qilian region,we can come to the conclusion that these orogenic gold deposits were significant unique: (1) ploycycle multiple compound orogeny as metallogenic background; (2) mineralization occurs in hypabyssal system (<6Km), even in a super hypabyssal environment; (3) Ore-forming fluid is entirely consistent with H₂O-CO₂ -NaCl system and H₂O-NaCl system ,which shows the fluid characteristic of epizonal subgroup of orogenic gold deposit; (4)showing regional differences in mineralization, generally getting newer from northern to southern, and even more obvious characteristics of multi-stage mineralization; (5) deposits formed with a characteristic of multi-phase and multi-stage activity; (6)the main ore type is alterated rock type, only individual deposits were the quartz vein-type and not constitute an orebody alone; (7)The metallogenic epoch is Paleozoic, showing a larger time span that is different from Mesozoic mineralization with smaller time scale in orogenic belts.By recognizing the typical VMS deposits in Southern Qilian area, combined with studies on deposit geology, mineralization, geochemical characteristics and ore-forming temperature, pressure conditions and mineralization depths, this paper determined a new deposit type as the low-temperature hydrothermal vein-type deposits in Chaibei belt, as skarn-type gold deposits in Lajishan belt.In this paper, five regional metallogenic mineralization in southern Qilian area were summarized by studies on typical endogenous metal deposits: (1) orogenic gold metallogenic mineralization related to late Caledonian- Variscan granitic intrusive activities, produced in the green schist metamorphism strata, NW large-scale shear zone-controlled; (2)orogenic gold metallogenic mineralization associated with Caledonian granitic rock, produced in neritic facies flysch formation of Middle Ordovician; (3)orogenic gold deposit metallogenic mineralization related to the Variscan marine volcanic-sedimentary formation; (4)skarn-type gold metallogenic mineralization related to the Caledonian granodiorite pluton,produced in marine volcanic formation of late Cambrian; (5) epithermal-mesothermal metallogenic mineralization related to the Caledonian granodiorite hidden pluton,produced in marine volcanic formation of Early Paleozoic.The author researched the metallogenic mineralization and assessed metallogenic potentials of these metallogenic belts in Southern Qilian belt,while summarizing prospecting indicators of main deposit types in the areas. According to previous research results,then combining field studies and indoor work of involving deposits (points),five prospecting areas were classified into A,B and C classes,and were evaluated briefly in this paper.