Study On Geology,Source Of Ore-Forming Materials And Genesis Of Baguamiao Gold Deposit,Shaanxi Province

The Baguamiao gold deposit discovered in the early 1980s is located in the northern of Pb-Zn multi-metal ore-field in the Fengtai basin,Qinling orogen.It is one of the largest gold deposits in Shaanxi Province,with proved Au reserves of about 106 t.Orebodys which are stratiform and lentoid are controlled by NWW-trending Baguamiao-Xiaoliyuan-Yangchanggou brittle-ductile shear zone.Its dominant country rock is low-grade metamorphosed muddy clastic rock and carbonate rocks of Upper Devonian Xinghongpu Formation.Silicification,carbonatization,pyritization and pyrrhotitezation are well developed well in the altered wall rocks.The ore-forming process of the Baguamiao gold deposit was composed of three main ore-forming periods,including?1?strata-bounded pyrrhotite-quartz vein period,?2?NE-oriented pyrite-quartz vein period,and?3?fracture-hosted sulfide-calcite vein period.The more detailed stages are divided based on mineralogical characteristics,the strata-bounded pyrrhotite-quartz vein period includes four stages of euhedral pyrite-quartz stage?I?,pyrrhotite-native gold-calcite stage?II?,biotite stage?III?and chlorite-epidote stage?IV?,whiletheNE-orientedpyrite-quartzveinperiodincludeseuhedral pyrrhotite-pyrite-coarse-grained quartz stage?V?,anhedral pyrite-electrum-fine-grained quartz stage?VI?,anhedral pyrrhotite-native gold-calcite stage?VII?and biotite stage?VIII?.The stage II,VI and VII are the main gold mineralization stages.The main auriferous mineral of stage II and stage VII is the native gold,but the auriferous mineral of stage VI is the electrum,they are all in the form of intergranular gold.Fluid inclusion,oxygen and hydrogen isotope analyses of quartz and In-suit S isotope analyses of sulfide in country rock and different ore-forming stages were carried out in this work,in order to constrain the source of ore-forming materials,gold mineralization mechanism and genesis of the Baguamiao gold deposit.The fluid inclusion data show that the the mineralization depth is 0.72km³.47km,and the ore-forming fluid of Baguamiao gold deposit is characterized by middle-low temperature?140?²00??,middle-low salinity?5.5wt%NaCl¹3.5wt%NaCl?and low density?0.89g/cm³¹.01g/cm³?.Besides,temperature and salinity decrease with the mineralization.The oxygen and hydrogen isotope data indicate that the ore-forming fluid has a multi-source,that of strata-bounded pyrrhotite-quartz vein period was mixed magmatic water and organic water(?D=-92.8‰^-82.0‰;?¹⁸O_H2O=3.68‰⁶.38‰),while that of NE-oriented pyrite-quartz vein period was organic water and meteoric water(?D=-96.2‰^-84.6‰;?¹⁸O_H2O=0.36‰².86‰).The?³⁴S values of pyrrhotite without alteration in low-grade metamorphosed wall rock are concentrated in11.6‰¹3.0‰,which are between the?³⁴S data of original pyrite from 3.3‰to 16.0‰in previous publication.And those of sulfide in ore vary from 2.5‰to 15.9‰,indicating that the sulfur source is generally characterized by the contribution of both stratum sulfur and magmatic sulfur.The?³⁴S values of stage I euhedral pyrite are 3.6‰⁴.0‰,which implies that the sulfur source is main magmatic sulfur in early ore-forming system.The?³⁴S values of sulfide increase gradually with mineralization,suggesting that more stratum sulfur is involved.Gold in hydrothermal fluid of Baguamiao gold deposit was estimated to be transportated as Au?HS?₂^-.The fluid inclusion data show that the changes of ore-forming temperature is slight.The changes of sulfide?³⁴S values and the mineral assemblage of different stages indicate that oxygen fugacity of ore-forming fluid decreased slightly in early ore-forming period.Combining with the mineral association from stage I to II?pyrite?pyrrhotite?,stage V to VII?pyrrhotite+pyrite?pyrite?pyrrhotite?and the changes of physic-chemical conditions in the different stages,we speculate that the decrease of oxygen fugacity led to the gold precipitation in stage II,sulfidation induced the gold precipitation in stage VI,while interfusing of magmatic water is the main mineralization mechanism in stage VII.The ore-bearing strata,ore controlling structure,temperature and salinity of ore-forming fluid,ore-forming depth and S isotope composition of the Baguamiao gold deposit are very similar to the typical orogenic gold deposits all over the world.Thus,we propose that the Baguamiao gold deposit is an epizonal orogenic gold deposit.