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Characteristics and Genesis of Lalongla MVT-Like Deposit in Changdu Region, Tibet
详细信息   
摘要
Changdu region, located in the northeastern margin of Tibetan collisional orogenic belt, is the important part of Southwest ‘Sanjiang’ metallogenic belt. Carbonate-hosted Pb-Zn deposits controlled by thrust nappe structures are abundant and with plenty reserves in this region. Lalongla deposit is located in the southwest of the most famous carbonate-hosted Pb-Zn ore concentration area in Changdu region and is a typical representative. The Pb-Zn mineralization of Lalongla deposit is born in the hanging wall of a thrust fault and ore bodies, mainly produced as lens, distribute along a facies transition between mud shale in Jiapeila Formation and limestone in Bolila Formation in Late Triassic. Breccia and mesh-vein are main ore structures and galena, sphalerite and smithsonite are main ore minerals. The ore-control factor and ore occurrence in this deposit represent a new carbonate-hosted deposit type in ‘Sanjiang’ metallogenic belt and then it is named as Lalongla type. The main ore-forming process in this deposit can be divided into Period I (shortened form of sulfide period) and Period II (shortened form of sulfide-carbonate period), between which a compressing deformation occurred. Two kinds of fluid inclusion including LV inclusion rich in liquid and LV inclusion rich in CO2 and CH4 occur in Period I. A microthermal testing work suggests two kinds of hydrothermal fluids in this period, which are with low temperature (130~140℃), high salinity (23%~24% NaCleqv) and middle to high density (1.10~1.12g·cm-3) nature and with middle to low temperature (170~180℃), high salinity (23%~24% NaCleqv) and middle to low density (1.06~1.08g·cm-3) nature, respectively. For the two periods, the liquid parts of hydrothermal fluids both belong to Ca2+-Mg2+-Na+-K+-SO42--Cl--F--NO3- system and the ion contents, H-O isotope composition (-137‰~-110‰ for δDV-SMOW and -2.92‰~13.42‰ for δ18OV-SMOW), C-O isotope composition of calcite (0.9‰~7.2‰ for δ13CV-PDB and 9.1‰~26.5‰ for δ18OV-SMOW) are all similar to each other, suggesting two resources, including low-temperature, high salinity basin brine and middle-temperature, high-salinity regional fluid composed by evaporated-concentrated seawater cocooned in strata, metamorphic water released by metamorphic rock and the meteoric water, for the hydrothermal fluids. The sulfur isotope composition for the two Periods is still similar. The δ34S values of sulfide are negative (-24.7‰~-11.5‰), of barite are positive (11.3‰~22.9‰) and of gypsum (2.0‰~4.7‰) are between the two former, suggesting reducing S comes from biological sulfate reduction which derives from the basinal brine descending from overlying Paleogene basin and evaporated-concentrated seawater cocooned in strata, and mixing of two kinds of fluid, which are local fluid rich in reducing sulfur and regional fluid rich in metal, causes the deposition of mineralized minerals. The Pb isotope composition of galena in the two periods are nearly the same and the ranges of 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb are 18.8646~18.8835, 15.6619~15.6677 and 38.9404~38.9796, respectively, proving that Pb isotope comes from metamorphic basement and limestone (and clastic rock) caprock distributed in the orogenic belt. By comparing Lalongla deposit with carbonate-hosted deposits in ‘Sanjiang’ belt, the authors finally point out that Lalongla deposit could be classified as MVT-like deposit controlled by thrust nappe and its moralization model can be explained as followings: Structural trap formes in the carbonate formation due to thrust nappe. The basinal brine in Paleogene basin descends, accumulates, and generates the local fluid reservoir rich in H2S accompanied by the biological sulfate reduction. Regional fluid released by the compression deformation migrates along the main detachment zone of thrust nappe, leachs ore-forming minerals in strata and generates allochthonous fluid rich in metallic elements. Along extended faults by final stretch of local thrust faults regional fluid ascends to the interface between limestone and mud shale, the favorable space, mixes with local fluid and leads to the uninstall mineralization of metal materials and the lens ore-bodies along the facies transition between different rocks finally form.

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