The Characteristics Of Ore-forming Fluids And Mineralization Of Dehelongwa Copper Deposit, Tongren, Qinghai Province

The Dehelongwa copper Deposit is geotectonically located in the west section of western Qinling fold belt and Tongde-Zeku early indosinian orogenic subzone. The ore-bearing strata are mainly the lower Triassic epimetamorphic clastic rocks as well as carbonate formation. The major mineralized intrusive rocks are indosinian intermediate acidity and i-type granite. The copper ore-bodies mainly controlled by NW structure are typically located at the contact zone between the lower Triassic clastic rocks and intrusions as vein or lay-like shape. The main ore mineral compositions are copper, pyrite, natural gold, molybdenite, etc., showing interstitial-like, emulsion-like, radial textures and banded-massive, disseminated, net-veined, brecciated structures as well. Based on the mineral veins interspersed features, two mineralization stages such as quartz-copper and arsenopyrite were recognised.Three types of inclusions hosted in quartz and calcite were found in the ore-bodies, i.e., liquid-rich two-phased aqueous solution inclusion(typeⅠ), vapor-rich aqueous solution inclusion(typeⅡ), aqueous solution inclusion with daughter mineral (typeⅢ). Type I inclusions are homogenized to liquid at the temperatures of 187-413℃and the corresponding fluid's calculated salinities vary from 1.98 to 18.95(wt% NaCl equiv.). TypeⅡinclusions are homogenized to vapor at the temperatures from 350℃to 413℃and the corresponding fluid's calculated salinities are 5.32-9.84(wt% NaCl equiv.). TypeⅢinclusions can be furtherly classified to two subtypes (Ⅲa,Ⅲb) according to the dissolution of daughter minerals before or after vanish of gas bubbles during the heating process. TypeⅢa inclusions have liquid/vapor homogenized temperatures of 276℃to 520℃, whose daughter minerals melted between 192℃to 491℃, calculating salinities from 31.46 to 57.05 wt% NaCl equiv.. TypeⅢb inclusions'homogenization temperatures (daughter minerals melting temperatures) vary from 250℃to 500℃, calculating salinities from 32.95 to 59.76 (wt% NaCl equiv.). their liquid/vapor homogenization temperatures are 195-324.9℃.Based on the analyses, three types of ore-forming fluids had been deduced, that is, high salinity magmatic hydrothermal fluid A, medium-high salinity boiling hydrothermal fluid B, low-medium salinity fluid C. These fluids were all originated from different evolutional stages of the primitive fluid formed after the period of magma. According to the lowest boiling temperature of fluid B(350-370℃)and the calculated pressure(160-221 bar), the depth of mineralization (1.6-2.2Km) was deduced, implying intermediate-depth metallogenetic condition. Additionally, the calculated results of both sulfur fugacity and oxygen fugacity reveal that the metallogenetic ambient is relatively reducing.The geotectonic setting of this deposit most likely is active continental margin. Fluid inclusions and S isotopes indicate that mineralization elements were mainly sourced from the deeply intrusive magma and slightly from the wall rocks. Comprehensively, it is suggested that mass ore-forming minerals were carried by high salinity fluids formed after he period of magma, and precipitated while salinity, temperature, and pressure of fluids decline due to boiling and mixture of fluids. Based on the above researches, a metallogenic model was established.