Metallogenic Charcteristics And Genesis Of Au-Hg-Sb Ore Deposits In Jinlongshan District In Zhen’an, Shanxi

The metallogenic belt is located in Zhenan-Xunyang basin of Indosinian fold belts of the southernQinling mountains in Qinling fold system, the south side of Zhen-Banyan fault, the south wing ofSongshuling-Zaoshutan secondary anticline, Ding-Ma Hg-Sb ore belt of south Qinling Pb,Zn,Au,Sb,Hgmetallogenic belt.The principal ore-host stratas are medium-and thick-bedded limestone of YanglinggouFormation of Middle Devonian, the Upper Devonian Nanyangshan Formation silt (calcareous) shale withmedium-and thin-bedded limestone, Lower Cambrian Yuanjiagou Formation thick-and-medium beddedlimestone, siliceous limestone.By this work, combinied with field investigation, the following results are obtained:(1) Au-Hg-Sb deposit was formed in the transitional period of compressional to extensionalThere is a clear thrusted nappe structural system in Qiuling deposit area. Contrasted the3anticlinestructures where3ore blocks hosted in Jinlongshan ore area, Jinlongshan anticline and Yaojian anticlineare all with closed linear characteristics. Only the anticline core of Qiuling ore area is wide. It caused theoutcrop of ore-bearing strata with the width of up to800meters. Structural interpretation holds ore-bearingbed was folded. Closed and compressiona small fold and foliation substitution are common in ore-bearingstrata. Analysis suggests Qiuling anticline has transverse folds superposed.Affected by the thrust structure from north to south, the original east-west linear anticline wastransversely bended. It caused the original nearly upright axial plane to fall to the north. Acted with furtherthrust in the late Indosinian period, the Nanyangshan stratas in the core are thickened. The limestonestreching and fracturing in Carboniferous period beneath wing of the anticline resulted broken anticlinewith type “A” superimposed folds.A huge thrust nappe structure made a way for hydrothermal solution moveing; the ore-formingmaterial of the underlying formations activated and migrated up, accumulated in secondary structures andfavorable lithologics.(2) Ore-forming materials originated from stratas and deep-seated crustThrough analysis of the regional metallogenetic geological setting, the epimetamorphicvolcanic-sedimentary rock series of southen Qinling base are with high Au-bearing content. The “blackrock series” overlying sedimentary rocks is with Au-bearing content3.45times, Ag-bearing content3.42times, Hg-bearing content4.44times, Sb-bearing content2.76times higher than Clark values of crust. Itmay be the source of gold mineralization material. The regional Au-bearing content of upper DevonianNanyangshan Formation is not high. Mineralization occurred mostly in Nanyangshan Formation, obviouslycontrolled by different lithologic section of Nanyangshan Formation. And, the carbonaceous andargillaceous content is higher in Nanyangshan Formation. They are strongly capable of absorbing gold. Inthe late stage, the tectonic and hydrothermal action makes further enrichment of gold.Isotope geochemical indexes indicate: δ34S variation is wide(-4.23×10-3～19.76×10-3), the mean valueis9.5×10-3, its range reachs23.99×10-3. Except a negative value samples, the rest are all positive value.The sulfur isotope in sulfide occurs in the shape of buttress. This indicates complexity of the sulfur source.δ34S is concentrated between10×10-3-17×10-3, enriched strongly into sulfur. The sulfur mainly comes from the seawater. These show that sulfur in ore mainly comes from rock formations of sulfur-bearingsedimentation interbeds.At the same time, in mineralization process of hydrothermal stage, the deep originsulfur was also incorperated in the ore formation.Pb isotope geochemical characteristics: Pb in gold deposit may be derived from sedimentaryformations, deep origin fluids in orogeny, etc. Metallogenic materials derived from stratas and deep sourcematerials.(3) The mineral assemblage feature and its meaningThe ore mineral assemblage is given priority to with arsenic, As-bearing pyrite, antimonite, which isshown as low temperature mineral assemblage. Ore-forming element assemblage is Au-As-Hg-Sbcombination of low temperature. Hydrothermal alteration mineral assemblage is silicification, clayization,carbonatization, chloritization. Based on fluid inclusion homogenization temperature is between Celsiusdegree130-220, it can be inferred that gold mineralization stage is mostly concentrated between Celsiusdegree140-220, antimony stage140~220, and carbonatization stage130~180. Arsenic, As-bearing pyriteand antimonite in ore are closely coexisted. According to the temperature-pressure experiment that Fe-As-Ssystem can come into being such above mineral assemblage, its temperature is below281Celsius degree.Therefor, mineral assemblage characteristics and fluid inclusion homogenization temperature indicateore-foming temperature is below281Celsius degree, between130~220℃. From the main metallogenicperiod to late, homogenization temperature gradually reduced. At the same time, it shows that themetallogenic fluid is with characteristic of meduim-low temperature. The result is basically consistent withmicro fine disseminated gold deposit.The evolution series of mineral coexisting-assemblage are: arsenopyrite-arsenic pyrite,-pyrite,pyrite-stibnite-cinnabar. Above mineral combination contains large amounts of arsenic mineral, so theyrelates closely to gold mineralization. At the same time, in alkali ore-forming solution, gold solubilityincreases with the increase of content of As. Therefore, gold-arsenic complex is likely to be the mainmigration form of Au in ore-forming hydrothermal solution.(4) Geochemical characteristicsThere are obvious1:10000soil geochemical characteristics in the metallogenic belt. It correspondswith the orebody better. Through the analysis of characteristics of the soil anomalies: anomalies aroundYaojian-Guloushan are controlled by NWW,NW and NE direction intersection of structures.