Study On Characteristics And Its Prediction Of Carlin-type Gold Ore Belt In Sichuan-Shannxi-Gansu Region

The district of Sichuan-Shanxi-Gansu is one of the most important districts of primary gold deposit-mainly is Carlin type gold deposit-in China. Yangshan gold deposit is the largest gold deposit with the type of Carlin-Carlin-like gold deposit, with the total resources3O8t that have been probed. There are also other large-scale or super large-scale gold deposits in the district, such like Shuajingsi, Lianhecun, Gala, Jinmuda, etc..., which show the good prospecting outlook of this district. The disquisitive area of the paper locates passive epicontinent of Gutetisi eastern margin, in environment of gulf of stable continental margin or closed bay turbidity basin. Gold accumulates primarily in turbidite system of deposit, which is main source bed of Carlin type gold deposit in the district and had the microthermal dynamic metamorphism during Indo-Chinese epoch. Within the district, magmatic activities which are tie up closely with the gold and polymetallic mineralization act frequently and mostly intruded along the marginal fracture zone of the structure becoming a notable tectonic magmatic rock belt. The geotectonic background of gold mineralization is bidirection-contraction orogenic movement, forms ductile and brittle fractures which control the distributing of gold deposit and provide the main thermodynamic of mineralization. the paper shows Carlin-Carlin-like gold deposit in Sichuan-Shanxi-Gansu has close relationship with orogeny which is different from traditional by research its metallogenic regularity and genesis; aim at research special plateau geography position, tough work condition and brittle ecological environment, by using remote sensing, geochemical exploration and geophysical prospecting, getting effective prospecting methods, and receiving fine prospecting results, as below:Ⅰ. Broke through traditional concept of Carlin-Carlin-like type, Carlin type gold deposit of Sichuan-Shanxi-Gansu has close relationship with orogeny, and formed during continental-continental collision orogeny.In space:gold deposit formed in orogenic environment; deposit was controlled by structure, which produced in secondary or fault plume.In time:orogeny and regional gold mineralization were in same time.Source bed:tectonic environment controlled the distribution of source bed, lava basin/flysch basin of ocean/rift valley controlled gold source bed and host construction.Ⅱ. Gold deposit in the area mainly is structural ore-control; regional fault structure controlled the distribution of magmatic belt and Carlin type gold belt. The gold ore body lays in plagioclase granite porphyry vein or the contact zone of porphyry vein and psammite-silty phyllite. Ore body was hosted in shear zone, and controlled by structure.Gold ore body controlled by fault structure strictly, mainly hosted in interlayer shear zone that occurred along the strata; veins in plane is smooth wave, in profile is vein occurrence;Ⅲ.Had close relationship with granite vein magmatic rock:the gold ore body lays in plagiogranite-porphyry vein or the contact zone of porphyry vein and psammite-silty phyllite, mineralization age is close to the intrusive age of vein rock, metallogenic relation of both appears; porphyry vein with characteristic of syn-collisional granite; gold appeared in disseminated rock, but also in visible gold. Au, Ag, As, Sb, Hg, Bi give priority to the element combination, among which Au/Ag takes a goodⅣ. Ore-forming fluid of gold deposit mainly was meteoric water, but not magmatic fluid; Ore-forming fluid were all belong to middle-low temperature hydrothermal, the range of temperature was from105to340℃, whole temperature was between100and250℃. Salinity of ore-forming was low which range was from0.5to15.0(NaClWB%), and mainly between6and9(NaClWB%); the density range of ore-forming fluid was0.7～1.02g/cm3, pressure range was50～1136.15x105Pa, which conformed that deposit was formed in different depth range; pH value was between5.16and7.2, in neutral to weak acid or weak alkaline environment; EH value was between-0.27to-1.12, the range of1gfs2was between-13and-25.02, low fs2、fo2of ore-forming in most of gold deposit show that mineralization always happened in reducing environment; Contents of fluid were same, cations were K+、Ca2+、Na+and little Mg2+、Li+, and so on; anions were HCO3-or SO2-, and with little F-and CI-; gas component were H2O and CO2, CO2of fluid inclusion could reach in7.3and21.5mol%, and also contain CO、CH4and H; ore liquid belonged to the system of K+-Na+-Ca2+-HCO3--(SO42-)·Cl-. Gold in the areas associated with all kinds of sulfide that mainly was pyrite, metal sulfide components are pyrite, stibnite, realgar and arsenopyrite; lacked of metal oxide; formed in reducing environment, with characteristic of mantle origin sulfure.Ⅴ. By research of regional typical gold deposit, showed that gold deposits of the area were controlled by change of brittle-ductile deformation strictly, and gold deposited while deformation happened. Gold mainly hosted in change part of interlayer shear zone that occurred along the strata; intersection part of tow group of fault; conjugation part of fragmentation fissure (conjugation joints) and structural lens.Veins in plane is smooth wave, in profile is vein occurrence. According to regional metallogenic rule, regional mineralization had the characteristic of belts in transverse, sections in portrait and layers in vertical. The paper built the regional metallogenic modal to direct prospecting.Ⅵ. Combined metallogenic characteristic and condition of regional gold deposit, used integration of predicting method, formed a suit of methods-remote sense, geochemical exploration and geophysical prospecting that fit for Chuanxibei plateau. Methods combination meant that selected area by remote sense and geochemical exploration in regional, checked by geochemical exploration in target area, orientated by geophysical in diggings. Predicting the regional prospecting direction by using GIS, found7prospecting target. It gave the right direction of regional prospecting.Ⅶ. Checked by regional prediction and target area, found new gold deposit in Dailong, put in reserves3214Kg primary, which confirmed that the methods was effect in Chuanxibei. It also confirmed that region had better prospecting potential.