Geological Characteristics And Metallogenic Prognosis In Deep And Periphery In Shanmen Silver Deposit Of Siping City, Jilin Province

Abstract: Shanmen Silver ore field is geotectonically located in the south part of Daheishan rise in Changbai mountain umbo within the active continental margin to the east part of Eurasia plate. And it also situated to the south margin of North China plate and to Northwest side of Songliao rift. The direction of major regional structural line strikes from NE to NNE, and Siping-Dehui and Yitong-Yilan discordogenic faults are distributed within the both sides of Daheishan rise.The Shanmen silver deposit is divided into five ore blocks, i. e. Zhangjiatun, Longwang, Wolong, Yingpan and Gudong ore blocks. Geological exploration was finished in Wolong ore block, and 1200 tons silver reserves were submitted. Detailed survey was finished in Longwang ore block, and 800 tons silver reserves were submitted.Major strata crops out in the mine area are Cambrain-Ordovician Xibao'an and Huangyingtun formations, which belong to a series of metamorphic marine volcanic-sedimentary rock systems. The major lithologies of Huangyingtun formation consist of metamorphic packsand, marble with interbedded metamorphic siltstone, rhyolite and dacite etc. Alteration is developed in this formation, which was major ore source of Au, Ag, Pb and Zn mineralization. Moreover there are the upper Jurassic, the lower Cretaceous volcanic-clastic rock systems in fault basin.Magmatism was strong and frequent in the mine area, whose ages from Caledon to Yanshanian. The intrusive rocks from Caledon to Hercynian consist of ultrabasic rock, basic rock and intermediate-acid rock etc. The Indosinian-Yanshanian magmatic rocks are related close to mineralization. The Indosinian medium-fine grained quartz diorite is major wall rock of ore bodies, and the Yanshanianintermediate-acid, intermediate-basic dykes have close spatio-temporal relation with mineralization.The predecessors considered that dominant ore-controlling factors were Huangyingtun formation and Indosinian quartz diorite in Shanmen silver deposit, because the major ore bodies in Wolong ore block are situated in the contact zone between Huangyingtun formation marble and quartz diorite. But we thought that the northeast fault structure is the most important ore-controlling factor because of the following reasons: (1) the occurrence of ore body is controlled by northeast structure; (2) there is abnormal contact zone between the Indosinian quartz diorite and the medium Ordovician Huangyingtun formation; (3) Northeast mineralization and alteration are located in the superposed belt between the ductile shear zone and later brittle structure; (4) some ore bodies are limited in quartz diorite or adamellite by means of deep project verification, which are not controlled by contact zone, and their occurrence are controlled by northeast structure without reference to the wall rock of ore bodies. Moreover the polygenous activities of structures caused polyphase and multistage metallogenesis.Ore bodies shows stratiform, nervation and podiform. They strikes northeast ca 30°±, dipping northwest with 20°~60° angle. The length of a single ore body is 100-3 00m, and average thickness is 3~4m, the average grade of Ag is 240 xlO'6 and thatofAuisl.SxlO"6.Ore belongs to sulphide type, which consist of pyrite, galenite? sphalerite, chalcopyrite etc , and the average content of metal sulphide is 3.08%. Gangue mineral are dominant quartz. The type of argentiferous minerals are very complicated, including common native silver, argentite, acanthite, aerosite, Ag- Zn tetrahedrite, with minor pearceite, allargentum, and cyprargyrite. And electrum is major auriferous mineral. Ore texture and structure are also very various. The assemblages of paragenesis of minerals are relatively simple, including quartz-argentiferous sulphide, quartz sulfosalt-Au, Ag minerals-sulphide, quartz-sericite-sulphide assemblages. Major metallogenic stage can be divided into five phases, i. e. quartz-sericite-pyrite, grey quartz- argentiferous sulphide, nervation white quartz vein-argentiferous sulphide, dolomite vein-silver-copper mineralization and carbonate-quartz phases. According to researching fluid inclusions, it was thought that the source of metallogenic fluid came from magmatic and meteoric water, while the source of metallogenic material came from major magma with minor wall rock (Paleozoic strata) based on analysis of C, Pb, S isotope and REE.Shanmen Silver deposit belongs to epithermal deposit controlled by fault structure, the following factors played key role in mineralization: (1) long activities of Yilan-yitong discordogenic fault and its marginal faults; (2) felsic magmaticacitivities related to mineralization in the mine area; (3) Paleozoic calcareous-argilaceus- carbonaceous fine grained clastic rock with interbedded carbonate are good wall rock condition; (4) entrapment condition of hangingwall is also important factor; (5) the difference of chemical nature from hangingwall to footwall is important factor of mineral matter precipitation.The descriptive model of Shanmen silver deposit: (1) the deposit is located in the secondary fault belt within the both sides of deep fault in the rise near Mesozoic rift. (2) ore bodies are controlled by NE fault structures. (3) ductile shear zone develops in the mine area, which superposed multistage brittle fault, multiphase dyke intruded into such faults. (4) the host rock is dominant carbonaceous fine grain clastic block with carbonate. (5)alkaline igneous dyke develops in the mine area. (6) platy similar bedded silicification belt occurs in the mine area, which include breccia and multistage silicification veins. (7) apparent carbonaceous enrichment occurs in the enriched part of ore bodies. (8) the alteration mineral assemblage consists of quartz+illite (sericite) +carbonate. (9) reddened plagioclase occurred commonly in wall rock. (10) host rock cropped out only in thrust terrane. (11) normal fault dipping in high angle developed in the mine area.The prospecting model in order to delineate ore fields was made based on synthesis of all synthetic prospecting information from geology, geophysics and geochemistry. This model are : (1) the gradient zone of linear gravitational anomaly anear positive value. (2) regional linear magnetic anomaly. (3) low resistance and high polarizability area which superposed the above gravity and magnetism anomaly. (4) matched anomalies of stream sediment. Indicator elements of long distance are Sb, As, Ag, and indicator elements of short range are Pb, Zn, Au. Anomalies arrayed and superposed by zoning sequence, and indicator elements of long distance lie in the outer side. (5) Primary halo of indicator elements are Sb, As, Ag, Cu, Pb, Zn, Au by zoning sequence. The enriched zone where Ag abundance in native rock is 3-10 times than Clarke value. (6) the source area of water systems where electrum+metal sulphide assemblage occurred in panning concentrate survey. (7) faulted zone interpreted by remote sensing where superposed by the above anomalies. (8) the join place of two sets of faults interpreted by aerial magnetometer where superposed by the above anomalies. (9) the alteration fault found on the ground where superposed by the above anomalies, and indicator alteration is reddened plagioclase. (10) the place where Au, Pb, Zn, Cu mineralization spots were found in the above anomalies.The prospecting model of ore bodies in prospecting ore field are: (1) the superposed part of indicator elements anomalies in primary halo interface. (2) the anomaly area of soil geochemical prospecting delineated with 0.22 *10'6 Ag. (3) alteration zoning including reddened zone, illite- hydromica zone, pyritization...