Study On Mesozoic Gold Metallogenic System,Northern Heihe,Heilongjiang Province

Study area is located in the north-eastern margin of the Central Asian Orogenic Belt(northern Heihe,Heilongjiang province),which is a key gold metallogenic belt in NE China.This study is focused on gold metallogenic system,and established Late Mesozoic gold metallogenic model in northern Heilongjiang province,based on the theory of the metallogenic system.The details are following:(1)Classifying the type of the gold metallogenic system,and finding out mineral assemblages:the study area includes orogenic and epithermal metallogenic systems.Typically,Sb-Au deposit is related to orogenic events,including stibnite,famatinite,tetrahedrite,boulangerite,freibergite,native gold and minor andorite in the deposit.In addition,epithermal deposits consist of Te-Au deposits(e.g.,Sandaowanzi and Beidagou)and Ag-Au deposits(e.g.,Shangmachang,Tianwangtaishan and Pangkaimen).Tellurides and native gold are common in the Te-Au deposits,whereas argentite,polybasite and electrum are enrichment in the Ag-Au deposits.(2)Constraining the relationship between tectonic-magmatic activity and gold mineralization,and discussing regional tectonic evolution:In the Jurassic,regional tectonic events included the Paleo-Pacific Plate subduction and the Mongolia-Okhotsk orogenesis.Till the Late Jurassic,the tectonic evolution was featured by the continuous subduction of the Paleo-Pacific Plate with Mongolia-Okhotsk Ocean closure,and Late Jurassic gold event related to orogenesis.In the Early Cretaceous,gold mineralization is related to magmatic activity with a superposition extension.The geochemical and isotopic results indicate that primary melts of Late Mesozoic magmatic rocks have features by partial melting of the continental crust.The adakitic rocks may have been the products of the thickened lower crustal delamination in NE China.(3)Orogenic gold mineralization:the deposits are hosted by Precambrian-Paleozoic two-mica schist and marble or Devonian meta-sedimentary rocks.The mineralization ages are concentrated on Late Jurassic(ca.153 Ma).Sulfur and lead isotopes imply a deep metamorphic source.Ore-forming fluids consist of CO₂-H₂O-Na Cl±CH₄/H₂S,and featured by low temperature and salinity(112?353°C;1?12wt.%Na Cl eq.).The mineralization depth is about 6.7 km.Hydrogen and oxygen isotopes indicate that the ore-forming fluids were originated from metamorphic fluids and mixed with meteoric water.Gold and antimony in the deposits are predominated by Au(HS)₂^-and HSb₂S₄^-with an alkaline condition.(4)Epithermal gold mineralization:the deposits are hosted by Early Cretaceous trachyandesite and dacite.The mineralization ages are concentrated on Early Cretaceous(ca.115?120 Ma).Sulfur,lead and carbon isotopes imply a mantle source.Ore-forming fluids consist of CH₄-CO₂-H₂S,and featured by low temperature and salinity(175?225°C;1?10wt.%Na Cl eq.),and meteoric water.The mineralization depth is<1 km.Gold and silver are predominated by Au HS⁰ and Ag Cl₂^-with an acidic condition in the Au-Ag deposits,whereas gold,silver and tellurium are predominated by Au(HS)^2-,Ag(HS)^2-and Te₂^2-with an alkaline in the Au-Ag-Te deposits.(5)Established regional gold metallogenic model:the compressive tectonic setting changed to extension during Late Jurassic-Early Cretaceous with large scale gold mineralization.Ore-forming materials are extracted from deep-seated metamorphic rocks and migrated by metamorphic fluids due to Late Jurassic tectonic deformation.Gold and antimony depositions are related to fluids mixing with p H,f S₂and f O₂ decreasing.During Early Cretaceous,the lower crustal delamination and the subsequent asthenospheric upwelling during the intra-continental extension bring the mantle materials to shallow crust.Gold,silver and tellurium depositions are related to the fluids boiling and p H neutralization.(6)Established regional model of formation-change-preservation and prospecting model for gold deposits:the preserved conditions of the orogenic gold deposits are adverse,but these deposits are extended in the deep with ductile shear zones and thrust faults.The preserved conditions of the epithermal gold deposits are advantageous,and these deposits are rarely changed by the tectonic-magmatic activity during Cenozoic.In the tellurium-bearing gold deposits,mineral spatial distribution consists of silver-bearing minerals in the upper parts of the ore bodies,gold-,silver-and tellurium-bearing minerals in the middle parts of the ore bodies,and sulfides in the lower parts of the ore bodies,which can be better used to evaluate the new discovered deposits.The intersection of NE-and NW-trending structures or the magmatic activity zones are favorable for ore formation.