Ore-forming Fluid And Metallogenic Mechanism Study Of The Nibao Gold Deposit,Guizhou Province

The "Golden Triangle" Carlin-type gold deposit in Yunan-Guizhou-Guangxi area coincides with the traditional Youjiang basin,which is located in the structural triangle area sandwiched by several tectonic units such as Cathaysia Block,Yangtze block,Simao block and North Vietnam block.According to the occurrence of ore bodies,the Carlin-type gold deposits in this area can be divided into three types:stratabound type,fault-controlled type and composite type.The Nibao gold deposit is a composite gold deposit dominated by fauk-controlled type orebody and supplemented by stratabound type orebody located in platform area in the southwest Guizhou province.Fault-controlled type orebody is mainly developed in the low angle thrust fault,which ore-hosting rocks is mainly Calcareous-bearing pyroclastics of second member of Late Permian Emeishan Fm.Stratabound type orebody are mainly above fault-controlled type orebody which developed in the core of dome structure in the Late Permian Emeishan Fm and Dachang Beds.Many scholars have done a lot of research on the Nibao gold deposit since the discovery of this gold deposit,however,there are great controversies about the determination of ore-bearing lithology,the division of ore-bearing strata and the genetic connection between the ore deposit and the Late Permian Emeishan Fm,besides,the research work on the relationship of the ore-forming fluid between stratabound type orebody and fault-controlled type orebody,the migration path of the ore-forming fluid,the law of element migration during hydrothermal alteration and mineralization,and the metallogenic mechanism of the deposit is still insufficient.Therefore,based on the 1:10000 tectonic-alteration mapping of the Nibao gold deposit,this paper focuses on the fluid inclusion microthermometry,laser Raman spectroscopy analysis,stable isotope analysis and rock geochemical analysis,and inquire into the characteristics of element migration during mineralization and metallogenic mechanism,and the main achievements are as follows:(1)The ore-bearing lithology is determined as(sedimentary)volcanic breccias tuff,tuffaceous fine conglomerate and sandstone base on the field geological survey,drilling core cataloging,and indoor light and thin slice microscopic examination.On this basis,the ore-bearing strata in the mining area are redivided into Middle Permian Maokou Fm,Dachang Beds(informal stratigraphic unit),Late Permian Emeishan Fm and Longtan Fm from old to new.(2)The genetic relationship of the ore-forming fluid between stratabound type orebody and fault-controlled type orebody in the mining area is identified,and proved that the ore-forming fluids of the two types orebody have the same properties and sources,and have similar evolutionary processes.This two types of orebodies are demonstrated to be formed in the same metallogenic system from the perspective of the ore-forming fluid.The ore-forming fluid belongs to medium to low temperature(180?240?),low salinity(generally<7%NaCleqv),and medium to low density fluid,which is derives from metamorphic fluid that formed by mixing of construction water,buried metamorphic water,meteoric water and sea water,and carbon mainly derives from dissolution of marine carbonate rocks.(3)The genetic relationship between the ore deposit and the Late Permian Emeishan Fm is identified,it is considered that the mineralization in this area is closely related to the Emeishan Fm.The Emeishan Fm is an important material basis for the formation of the Nibao gold deposit,which is rich in Fe,Cu,Zn,Ti and P,and other trace elements.By contrast,there is multilayered bioclastic limestone developed in the mining area,though they have been strong decarbonation and silicification,they can not form orebodies due to the lack of sufficient Fe.(4)The law of element migration during mineralization is identified,and the main precipitation mechanism of Au is also discussed During the metallogenic process,elements such as Au,As,Hg,Sb,TI and S moved into orebody,and elements such as Na2O,Li,CaO and MgO moved out from orebody.Sulfidation is the principal mechanism of Au precipitation of the Nibao gold deposit.(5)The migration path of the ore-forming fluid in the mining area is summarized.The ore-forming fluid moved upwards along the deep fault-Panjia zhuang fault zone and then entered the secondary fault(F1 fault).The fluid was changed to move along the unconformable to the core of anticline when it arrived at the intersection part of F1 fault and the unconformable surface of the towing fold of the upper plate.After that,it also can move to other parts through the steep shear fracture near the core of the anticline.