Brief Study On The Fluid Inclusion Characteristics Of Lala Copper-gold Deposit In Huili County Of Sichuan Province

Lala copper-gold deposit, located in the Lushui village of Sichuan Province, is a large-super large copper-gold polymetallic deposit in the low temperature metallogenic domainin of Sichuan –Yunnan-Guizhou which is famous in China. Its geotectonic attribution belongs to Kang-Yun basement complex belt of the secondary tectonic units under Yangtze landmass of First-order tectonic units. Kang-Yun basement complex belt(Pt1-2、Pt3)is equal to the traditional sense of the kang yunnan earth’s axis. Anling River- Lvzhi River fracture and Chuxiong foreland basin is bounded on the west, and the east side is Ganluo- Shijia- Dongchuan – Cheng River fracture(roughly equivalent to Xiao-Jiang fracture).It is characterized by a wide range of proterozoic basement metamorphic rock series exposed in the Yangtze landmass and sedimentary cover extensive development in upper paleozoic erathem and mesozoic- Cenozoic erathem. Kang-Yun basement complex have strong deformations, and often appear mylonite(mylonitization), and developed a series of anticline and syncline structure. There are three phase of the main structural feature: ①Ribbon or sporadic: Foliation formed by early bedding shearing and mylonite exposed ribbon or sporadic in some parts; ②Nearly east-west foliation(local) and nearly east-west width slow fold, At the same time, developed the nearly east-west interval strong chopping physical and chemical zone at the same time; ③East-west tension and extrusion formed the north-south left thrust fault zone and a series of secondary fault(such as edge dam, Gua-da-gou fault zone), and accompanied by the formation of the Da-Du river gold mine, etc. Many paleoproterozoic basic- ultrabasic intrusive body are exposed in Huili-Dongchuan-Wu Ding area, which is covered by Nanhuan System unconformity. The predecessor has achieved that metagabbro Sm–Nd isochron age is 1151~857Ma and Rb-Sr isochron age is 1954~1211Ma. While unstratified rock mass with gabbro,diabase gabbro,diabase(porphyrite) as the main body,stratified rock mass can be made of gabbro, norite, pyroxenite, olivine-gabbro, pyroxene peridotite and etc. that formed“rhythm” structure and has the high ratio of magnesium-iron(average 6.59).Intrusive rocks of proterozoic located in the Mi-Yi saddle back area that is in the western margin of Yangtze landmass m easy pass area. Main lithological character is Granite-Tonalite that belong to shell type granite with high Si rich K features, and acquired that SHRIMP zircon U-Pb age is 1014±8Ma, and the product of collision tectonics between Yangtze landmass and Huaxia landmass.The regional geology and structural properties make attribution to the formation of Lala copper-gold polymetallic deposit and settle very favourable conditions of metallogenic geology, structure and rock.Lala copper-gold polymetallic deposit occurs in epizonal metamorphic volcanic-sedimentary rock formation in LuoDang form of palaeoproterozoic Hekou Group. Predecessors of Lala copper-gold polymetallic deposit conducted a large number of geological exploration and scientific research, but have divided for a long time on the issue of deposit genetic type. Because of this, this paper is to fluid inclusion research of Lala copper-gold deposit as the subject. This paper focuses on observation and study on the fluid inclusions in the ore microscopy petrographic characteristics from the in-depth observation of macro geological characteristics of ore deposit and ore system sampling. Through testing the inclusions homogenization temperature and freezing point, and calculating the salinity, density, pressure and mineralization degree of inclusions, we reveal Fluid sources, the nature and evolution of ore-forming fluid,and The migration mechanism. This provides more rich fluid inclusion data to further understand and know the cause of Lala copper-gold deposit formation, and then give technical support for the deep and periphery prospecting of deposits. The research work of this paper has made the following main achievements and innovative knowledge.(1)Through observing the ore fluid inclusions slice of Lala copper-gold deposit on microscope, we find that the types of fluid inclusions are fluid inclusions, the pure liquid inclusions, gas inclusion and contained mineral inclusions, while pure gas inclusions are rarely seen. By testing homogenization temperature and freezing point of widely distributed gas-liquid phase fluid inclusion, determination of freezing point, and calculated the salinity, density and pressure of inclusions.At last roughly calculated the mineralization depth.(2)Homogenization temperature measurement result of inclusion set within the range of 110~403 ° C, while the average temperature is 161° C, and it shows the process of fluid temperature change from low to medium. Previous measured that burst temperature range of Lala copper-gold deposit is 250~480 ° C by applying the method of burst, which is wide; chalcopyrite, bornite, pyrite, iron, dolomite are showed blowout twin peaks in different degrees, and explosion temperature of ore-bearing rock is in conformity with the calculated temperature of the metamorphic facies of. These may show the burst characteristic that the formation of ore deposit is related to metamorphic recrystallization.(3)According to conversion of inclusions freezing point,the salinity W(NaCl) range between 2.07~27.86 wt %, and most concentrate in the range of 13 to 24 wt %. Results show that the ore-forming fluid has high salinity and high salt ore-forming fluid properties.(4)Using the relation equation about inclusions homogenization temperature, salinity and density, we calculated that scope of fluid density of Lala copper-gold deposit is from 0.966 to 1.687 g/cm3 and the average is 1.152 g/cm3, which shows that the ore-forming fluid density range is wide. Metallogenic pressure of Lala copper-gold deposit is 4.97~124.27 MPa by stress calculation formula. If only considering lithostatic pressure, metallogenic depth of Lala copper-gold deposit is in roughly0.19-4.6 km range by convering metallogenic pressure into metallogenic depth.(5)With depth analysis we found that the inclusion temperature mainly concentrated in two interval: 110 ° C to 200 ° C and 200 ° C to 374 ° C, which can be explained that the deposit formation has experienced two major metallogenic period. The temperature evolution from the Medium to low, show that ore-forming fluid ha s the characteristics of multiphase multi-stage form evolution, the original high tempe rature and high salinity of fluid with new fluid, resulting in a decline in temperature a nd salinity of fluid, the ore-forming environment transition from relative to relative ox idation,We infer the possible main metallogenic period is regional metamorphic fluid mineralization period, and the period can be roughly divided into four metallogenic stages: The ore-forming temperature of The phase I is in 430~490 ° C, and the best temperature is at 460 ° C to 480 ° C. The ore-forming temperature of The phase II is in 350°C~450°C. The ore-forming temperature of The phase III is between 250°C to 350°C, and peak metallogenic temperature is 320 ° C. The ore-forming temperature of The phase IV is in 110°C~200°C. Salinity data and its wide range suggest the Lala copper ore have high salinity of ore-forming fluid property, and reflect at the same time that the new fluid may join in the late mineralization, which make a direct effect that different fluid mixed in different proportion caused the large variation of the salinity.It also tally with the characteristic that the formation temperature and pressure, of ore deposit range widely.(6)To sum up, this paper suggest characteristics of fluid inclusions in Lala copper-gold polymetallic deposit reveals that the ore-forming fluid has the characteristics of multiphase and multi-stage forming evolution, and it’s may contribute to determine that the genetic typy of the Lala copper-gold deposit is Iron Oxide Copper-Gold deposit(IOCG).