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Ore-forming Fluid Study Of The Dasuji Porphyry Molybdenum Deposit In Zhuozi County, Inner Mongolia

Posted on:2016-08-04Degree:MasterType:Thesis
Country:ChinaCandidate:H WuFull Text:PDF
GTID:2180330461495666Subject:Mineralogy, petrology, ore deposits
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The Dasuji deposit of Inner Mongolia, located in the Liangcheng uplift, is a newly discovered large porphyry Mo deposit in the western segment of the northern margin of the North China Craton. The orebodies of the deposit dominantly occur within the Indosinian quartz porphyry and syenogranite porphyry. The ore-forming process at the deposit can be divided into three stages: an early quartz–potassium feldspar–pyrite±molybdenite, a middle quartz–sericite–molybdenite–pyrite stage, and a late quartz–carbonate–pyrite±siderite±sphalerite±galena stage.Through LA-ICP-MS zircon U-Pb dating, we got the emplacement ages of 234±3Ma, 230±5Ma and 224±4Ma for quartz porphyry, syenogranite porphyry and granite porphyry, respectively. The Re-Os isochron age of five molybdenite samples is 223.5±5.5 Ma, which is similar with the emplacement age of granite porphyry, indicting that this deposit formed in the Indosinian and that the ore-forming rock is granite porphyry. Quartz porphyry, syenogranite porphyry and granite porphyry all have high contents of Si O2, Al2O3 and total alkali(K2O+Na2O), belonging to high-K calc-alkaline series. The contents of total rare earth elements(REEs) for the three types of rocks range from 48.2 ppm to 527 ppm, and their δEu values range from 0.15 to 0.93. The three rocks are all enriched in Rb, K, Th, Nb and Hf, but depleted in Ba, Sr, P and Ti. Quartz porphyry and granite porphyry are highly fractionated A2-type granite, but syenogranite porphyry is typical I-type granite. The εHf(t) values of zircons from the three types of rocks range from-21.0 to-7.9; their Hf two-stage model ages(TDM2(Hf)) range from 2585 to 1763 Ma. Through the study of Hf isotope, we conclude that the quartz porphyry is most likely formed by partial melting of a Paleoproterozoic crust, the syenogranite porphyry is most likely formed by partial melting of a Paleoproterozoic lower crust with a small amount of Mesoproterozoic crust, and that the granite porphyry is mainly formed by partial melting of a Paleoproterozoic crust with a small amount of Neoarchaean crust.Five types of fluid inclusions have been distinguished in quartz veins, including liquid-rich, gas-rich, H2O–CO2, daughter mineral-bearing and pure gas inclusions. The homogenization temperatures and salinities of the early stage inclusions vary from 340°C to 430 °C and 3.4 wt% to 15.9 wt% Na Cl eqv, respectively. The homogenization temperatures and salinities of the middle stage inclusions vary from 280 °C to 380 °C and 0.9 wt% to 38.3 wt% Na Cl eqv, respectively. The homogenization temperatures and salinities of the late stage inclusions vary from 202 °C to 280 °C and 1.2 wt% to 11.7 wt% Na Cl eqv, respectively. Both early- and middle-stage fluids are immiscible fluid, and the multiepisode fluid boiling is the dominant mechanism for mineral deposition. The H and O isotopes indicate the ore-forming fluid derived from magmatic water. The S isotope suggests the ore-forming material originated from granitic magma. The Pb isotope suggests the ore-forming material came from lower crust.
Keywords/Search Tags:molybdenite Re-Os dating, fluid inclusion, petrogeochemistry, porphyry Mo deposit, Dasuji, Inner Mongolia, the northern margin of the North China Craton
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