Geological,Geochemical Characteristics And Genesis Of The Laerma Gold Deposit In Gansu Province

The Laerma gold deposit is located on the western section of the Bailongjiang duplex anticline in the south asian belt of the western Qinling mountains.The ore-bearing rock series is a set of black carbon-silicon clastic rocks of the cambrian Taiyangding group.The ore body is deposited in the siliceous rock interlayer fracture zone in the anticline of the Erdu formation.Based on the existing research results and field geological work,we conducted researches on mineralogy,fluid geochemistry,isotope geochemistry,etc.,and the following conclusions were drawn based on the regional tectonic evolution background.?1?Mineralogy studies have shown that the ore and mineral combination is complex.In addition to pyrite,stibnite,tetrahedrite,cinnabar,tiemannite,realgar,orpiment,natural gold,quartz and so on.These ores generally show automorphic granular texture,xenomorphic granular texture,metasomatic texture,annulus texture,framboidal texture,crush texture,etc.Massive structure,disseminated structures,brecciated structure,vein-meshvein structure and so on,they are main ore structures.?2?The temperature measurement results of quartz and barite fluid inclusions show that the deposit is mainly composed of liquid-rich inclusions and containing a small amount of gas-rich inclusions.The fluids are medium to low temp?163.8³48.6??and low salinity?3.4¹2.7wt%NaCleqv?,characterized by low pressure?13.6³5.8MPa?.and it is accompanied by the effect of reducing temperature and pressure during the ore-forming process.?3?The wide?³⁴S value?-28.3³2.4‰?of the hydrothermal mineral in the mineralization period indicates that the sulfur comes from the wall rock and stratigraphy,and the difference of?³⁴S between the pyrite core and the pyrite margin(the?³⁴S value of the core and outer band of one ring-banded pyrite are 5.4 and 10.0,while the?³⁴S value of the core and outer band of another ring-banded pyrite are 7.8 and 4.2)indicates that it has undergone multiple hydrothermal processes.The isotope composition of Pb:²⁰⁶Pb/²⁰⁴Pb=16.532³7.266,²⁰⁷Pb/²⁰⁴Pb=15.257¹6.870,²⁰⁸Pb/²⁰⁴Pb=36.703³9.555,the isotopic composition is extremely uneven,and ²⁰⁶Pb/²⁰⁴Pb varies greatly indicates that the formation contains more radioactive causes Pb,and it is consistent with the characteristics of radioactive uranium anomalies in the uranium belts and uranium mining area.Combine²⁰⁷Pb/²⁰⁴Pb—²⁰⁶Pb/²⁰⁴Pb projection relationship and show that it is mainly from ore-bearing rocks.The ore formed after the ore-bearing rocks were subjected to ore-containing hydrothermal fluids during the ore-forming period still retains the characteristics of the high uranium and low thorium/plumbum system of surrounding rocks,not from another geological system.Hydrogen and oxygen isotope studies indicate that the hydrothermal metallogenic fluid supplies groundwater replenished by atmospheric precipitation.When the mineral-containing hydrothermal fluid circulates convection several times and rises to the near surface,the abrupt changes in physical and chemical conditions lead to the accumulation and precipitation of gold elements.The direct reasons for the precipitation of gold are the decrease in sulfur fugacity and the increase in oxygen fugacity,the mineralization mechanisms of ore-forming fluids are fluid mixing,fluid boiling and water-rock reaction.?4?The Laerma gold deposit is formed by groundwater recharged by atmospheric precipitation and formed by hydrothermal mineralization with surrounding rock formations.It's genetic type belongs to the layer-controlled hydrothermal transformation type gold deposit.