Mineralization Of Sedimentary-rock-hosted Lead-zinc Deposits Of The Lanping-simao Basin In Southern Part Of Sanjiang Area

The area of " Three Rivers" named by the flow of Nujiang, Lancang and JinshaRiver, including western Yunnan, eastern Tibet, western Sichuan, and southernQinghai, is located on the eastern and northern margin with a curved shape. Lanping-Simao Basin, located on the southern section of " three rivers" area and held by theJinsha in the west and the Lancang River suture zone in the east, tends to annihilate inthe north and a broom-like spread in the south. The Cenozoic tectonic overthrust iswell developed and lots of carbonate-hosted lead-zinc ore deposites yield out inLanping-Simao basin. The futher study on carbonate-hosted lead-zinc ore depositesyield out in Lanping-Simao basin is of great significance for the entire " three rivers"with base metal mineralization, as well as enriching and improvingsedimentary-rock-hosted lead-zinc ore-type forming theory since the causes of thosedeposits is still controversial and that type of deposite in Simao basin is rarely studiedbefore.A lot of extensive fieldwork and systemic laboratory experiments testing andanalysis have been done on these typical sedimentary-rock-hosted lead-zinc depositsincluding each Pb-Zn deposit in Baiyangping ore district, Jinding Pb-Zn deposit,Hexi–Sanshan Pb-Zn deposit, Caizidi Pb-Zn deposit, Laojunshan Pb-Zn deposit inLanping basin and Luoboshan Pb-Zn deposit, Changdong Pb-Zn deposit, Yitian Pb-Zndeposit in Simao Basin to identify the basic geological characteristics ofsedimentary-rock-hosted lead-zinc deposits in southern section of Sanjiang area,clarify the form of control to the lead-zinc deposits and ore bodies by the Cenozoicoverthrust tectonic and its associated secondary structure system in the basin, explorethe fabric and ore mineral assemblages type; determine the property of theore-forming fluids and mineral sources into; propose that the sedimentary-rock-hostedlead-zinc deposits in Lanping-Simao basin can be attributed to the MVT Pb-Zn typedeposits and Simao Basin is the southern extension of the Sanjiang MVT-like orezone, establish the model of MVT-like deposit in Lanping-Simao Basin, summedup the prospecting ideas and indicate the direction of this type of deposit.The sedimentary-rock-hosted Pb-Zn deposit in Lanping-Simao basin isepigenetic deposit, significantly controled by structure such as the main thrust faultand its secondary fracture and strike-slip fault formed in a thrust stage, salt domestructure result from napping, reopened preexisting thrust fault during the Cenozoicregional strike-slip stage, tension fracture and karst cave collapse structures formed during transient stress relaxation stage or regional tension after regional thrust, inaddition, layered or quasi-lamellar mineralization appeares in individual depositbecause of the metasomatic interaction between ore fluid and surrounding rock. Thehost rocks are mainly of carbonate rock and calcareous sandstone and The orescontain galena, sphalerite, and calcite alone or their assemblages in terms of replacingcarbonates and cements of breccias, and cementing breccias.Barite, celestite, andfluorite occur in some deposits. In addition to galena and sphalerite, pyrite,tetrahedrite, baumhauerite etc also appears.Fluid inclusion studies indicate that theore-forming temperature ranges from110～210℃with a peak at110～150℃.Salinity ranges from3.7～24.17wt%(NaCleq) with a average of17.28wt%(NaCleq).The ore forming fluid is brine in the basin of low temperature and high salinity. Singlemineral trace and rare earth elements research results of hydrothermal mineralizationperiod show that the evolution process of ore-forming fluid undergoes graduallydecreased temperature.The ore-forming fluid is of reducibility and middle-lowtemperature. C-O isotope studies show that carbon mainly comes from dissolution ofcarbonate rocks in the formation as well as sedimentary organic matterdehydroxylation contribution in some deposits. The ore-forming fluid mainly comesfrom basin brine in addition to precipitation.S isotopic compositions show that sulfurmay have multiple sources. Sulfur mainly comes from thermochemical sulfatereduction effect or organic matter decomposition. Bacteria reduced sulfur contributeobviously to the ore-forming in partial deposits.Sr isotope and Pb isotope tracerstudies show that ore-forming material, has nothing to do with magmatism, totallycomes from sedimentary strata or basin basement. Study on the metallogenicchronology shows regional mineralization occurred in the41～29Ma, correspondingto the transition stage of late India-Eurasia collision.A consolidated model of sedimentary-rock-hosted type Pb-Zn metallogenicseries in Lanping-Simao Basin is established: sedimentary-rock-hosted type Pb-Zndeposits in Lanping-Simao Basin are the results of the activities of brine. A strike-slipbasin rich of brine is formed in the interior of the continent becase of the India-Eurasia collision, brine infiltrates into the deep and then heated to uplift, migratingfrom both sides of the basin to lateral low-lying terrain. Fluid migrate along varioussedimentary strata and extract ore-forming elements, precipitating to mineralization infavorable position finally.