Mineralization Mechanism And Ore-forming Process Of The Nibao Gold Deposit In Southwestern Guizhou,China

The Nibao gold deposit,as an important part of the Yunnan-Guizhou-Guangxi “Golden Triangle” region,has been found to have both fault-controlled and stratabound gold orebodies,which is considered to be one of the major breakthroughs in gold deposit prospecting in recent years in China.Since the deposit is at the junction of two tectonic units,namely the Yangtze craton and the Cathaysia block,the compression-extension alternation of the taphrogenic troughs at the margins of the blocks was the primary form of tectonic movement.The tectonic style of the mining area is dominated by an anticline-fault combination with extensive deep tectonic development and concealed rock bodies.The Au-As-Sb-Hg combination anomaly shows the superior metallogenic geological setting of this area.Previous research have conducted some fruitful studies with respect to the geological characteristics,ore-controlling factors,ore-forming material source,fluid properties,genesis and other aspects of the deposit,etc.However,there is still a lack of systematic and in-depth scientific studies on the mineralization age,the geodynamic background of the gold deposit,the occurrence form and depositional mechanism of the gold,the mineralization mechanism,the ore-forming process of the gold,and the metallogenic model of the gold.On that account,this paper conducts a systematic study from the perspectives of metallogenic geological setting,geological characteristics,gold occurrence modes,element and isotope geochemistry,fluid inclusions,metallogenic epoch,and so forth.The ore-forming material sources,ore-forming fluid properties and evolution,genesis,controlling factors,and metallogenic conditions of the Nibao gold deposit have been discussed.As a result,the mineralization mechanism and ore-forming process and the deposition of gold in combination with its metallogenic geodynamic setting are revealed,and the metallogenic model of the deposit has been proposed.In particular,an accurate clarification of the metallogenic epochs of those two types of gold ore bodies are not only significant for further understanding of gold mineralization theories,but also can provide a theoretical basis for gold prospecting in the deep and peripheral areas of the deposit.Our research can be summarized mainly as follows:1.Identification of the relationship between the deposit's ore-controlling factors and its ore bodies.The Nibao gold deposit is characterized by ore-control jointly by structure,strata and lithology.Its main ore-controlling structure form is an “anticline-fracture” combination,i.e.,the Erlong Qiangbao anticline and fault F1.The main ore-bearing stratum is the second members of the Longtan Formation,and the main ore-bearing rocks of which are sedimentary tuff and tuff.The deposit has two types of gold ore bodies,namely the strata-bound type and the fualt-controlled type.The various gold ore bodies are all products of the same ore-forming fluid system,except that they manifest different emplacement spaces and ore body forms.To be specific,there is a close relationship between gold mineralization and silicification,pyritization and carbonatization.The hydrothermal mineralization period of the gold deposit consists of three stage: the quartz-pyrite stage,the quartz-arsenian pyrite-arsenopyrite stage,and the quartzcarbonate-clay mineral stage.2.The mineralization age of the gold deposit and the metallogenic geodynamic setting.The ore-forming ages of the strata-bound type and fualt-controlled ore bodies obtained by the quartz Rb-Sr method are 141±2 Ma and 142±3 Ma,respectively.The mineralization age is mainly the Middle-Late Yanshanian,basically corresponding to the mineralization age of gold and antimony and the time limit of magmation in the region.This suggests that the gold and antimony deposits might have been the products of a same geological event,and also it reveals that the metallogenic geodynamic setting of the Nibao gold deposit might have been the extensional setting of the crustal extension stage after orogenesis in the Middle-Late Yanshanian.3.Ore-forming fluids and ore-forming material sources investigation through fluid inclusions and ore deposit geochemistry study.The deposit is a typical medium-low temperature and low-salinity deposit.Its ore-forming fluids have the properties of overpressured fluids,Ca2+-Cl-type fluids rich in H2 O,CO2,CH4,N2 and other gases.Its metallogenic setting is a weak acidic reducing one.The carbon and oxygen isotopes of the calcites reflect that the carbon might have come from mantle-derived magma,mixed with stratum carbon to some extent.The hydrogen and oxygen isotopes of quartz fluid inclusions reveal that the ore-forming fluids might have come from hypomagma,mixed with a large amount of atmospheric precipitation and metamorphic water and showing the characteristic of mixed sources.The in-situ sulfur isotopic composition of zoned pyrites and arsenopyrites reveals that the sulfur in the ore-forming fluids is of a mantle source.4.Ascertaining the occurrence form and depositional mechanism of gold,clarifying its mineralization mechanism and ore-forming process,and developing the deposit's metallogenic model.The main gold-bearing minerals of the deposit are zoned pyrites,fine-grained pyrites and arsenopyrites.The crystallization sequence is as follows: arsenic-poor depositional genesis pyrite(core)? arsenic-rich zoned pyrites and fine-grained pyrites? arsenopyrite.The gold is characterized by a non-uniform distribution and mainly occurs in the form of “invisible” sosoloid gold(Au+1),and a minute quantity may occur in the form of nanoscale native gold(Au0).The ore-forming process of gold can be roughly described as follows: driven by the tectonic activities in the Late Yanshanian,the deep-derived overpressured ore-forming fluids were transported towards the shallow part of the major-deep fracture pathway.When the ore-bearing hydrothermal solution was transported to a certain location where structure pressure was released,the ore-forming fluid volatilized and escaped in large amounts and mixed with the infiltrating atmospheric precipitation due to abrupt changes in the physicochemical conditions of the ore-forming hydrothermal system,thus the fluids immiscibility or mixing occurred.At the same time,Fe-poor and Au-rich fluids(mainly in the forms of Au(HS)0)in ore-forming hydrothermal solution transported and engaged in sulfuration with active irons in the ore-hosting rocks(sedimentary tuff and tuff).This resulted in the formation of gold-rich and arsenic-bearing pyrites and arsenopyrites,as well as massive deposition and concentration of Au,thus gold ore bodies were formed through in-situ deposition in favorable host spaces and lithological combinations.It can thus be held that the immiscibility or mixing action of ore-forming fluids and the water-rock reaction(carbonatization and sulfuration)between fluids and wall rocks constituted the main mechanism of the Au deposition of the Nibao gold deposit.The metallogenic model of the Nibao gold deposit is embodied as a hypomagma model.The formation of the gold deposit can be briefly described as the upwelling of the deep-derived ore-forming fluid along the major deep fractures and its concentration and deposition on ore-hosting rocks in favorable shallow spaces(anticline & fracture).