The Geochemical Characteristics Of The Ore-Forming Fluids And Metallogenic Genesis Discussion Of The Porphyry Molybdenum (copper)deposit Of Tongcun In Kaihua County Of Zhejiang Province

Samples systematically collected from the Songjizhuang, the Tongcun and the Huangbaikeng of the Tongcun District are detailed studied in this article. The fluid inclusions contained in the samples are mainly the NaCl-H₂O systems except ones from Hole ZK005, which are CO₂-bearing NaCl-H₂O systems. The microthermalmometric method is applied to calculate the salinity, density and homogenization pressure, and the salinities of the Songjiazhuang, Tongcun and Huangbaikeng districts are 3.06¹2.51%NaCl, 4.03¹2.96%NaCl and 4.96¹1.93%NaCl, repectivly, with homogenization temperatures of 132.5³00℃, 158⁴12℃and 181³46℃. A software is designed for CO₂-H₂O system with salinity less than 6%NaCl, which can calculate the composition characteristics of this system. According to the data obtained above, the features of ore-forming fluids in these three areas are widely compared internally, and externally with other four typical porphyry Mo deposits. Based the contrast results, we find that: （1） the ore-forming fluids of these three areas are belong to one large metallogenetic fluid system, in which the fluids of the Songjiazhuang and Huangbaikeng districts represent the late and early stages respectively; （2） two suites of ore-forming fluids are considered to exist in the Songjiazhuang district, which mixed in a certain position, likely in the Hole ZK005; （3） the CO₂-bearing system in the Tongcun Distirct contains CH₄ prevalently with extremely low salinity, which is distinct from that in typical porphyry Mo deposits. The isochors of the ore-forming fluid are compiled using the parameters calculated above. The decrepitation temperature is considered to be the upper limit of metallogenetic temperature, which can be used to estimate the upper limit of the metallogenetic pressure intersecting with the isochors. With this principle, the upper limits of the metallogenetic pressure of the ore-forming fluids represented by the samples from ZK001, ZK004 and ZK005 are 1800²800bar, 1900²550bar and 600²700bar, respectively . Basing on the geostress gradient of 265bar/km, the maximal metallogenic depths of the fluids above are 6.8¹0.6km, 7.2⁹.6km and 2.3¹0.2km, respectively.A“Three-stage Fluid Metallogenic Model”is established in this article, which supposes that the CO₂ contained in fluid is derived from the reaction of 3CH₄+4SO₂=3CO₂+4H₂S+2H₂O, between CH₄ and SO₂ which primarily exist in fluid, which takes place when the chemical balance is destroyed by the mixing process of two kinds of fluid. According to this model and the maximal depth calculated above, the most optimal section with fluid immiscibility of this metallogenic system is supposed to be eroded away, while the fluid mixing area is likely to be reserved in the distric which can be defined by the“CO₂ fluid aureole”for further prospection...