| Pingchuan iron deposit lies on the western border of Yangze Paraplatform, and is located in the joint of Yanyuan-Lijiang Platform Edge Depression Zone and the western flank of the central Xikang-Yunnan Axis, with a good ore-forming geological background. Pingchuan iron deposit with sub-volcanic hydrothermal type, volcano-sedimentary type and magma differentiation type mineralization, is the typical representative deposit with poor titanium magnetite of southwest China. At present, the ascertain resource is reaching depletion, and the deposit is listed as the crisis mine. Implementing the prospecting breakthrough of the mining area and peripheral area is an outstanding problem need to be solved urgently. Correct understanding the genesis of the ore deposit, systematic summarizing the metallogenic law, and applicating the theory to prospecting and exploration is one of the effective ways to achieve prospecting breakthrough.According to the Pingchuan iron deposit, this article set formation-tectonic-mineralization as the main line, applied geochemistry, geochronology, geochemistry of ore deposit and other methods, defined the metallogenic geological body based on the full and deep understanding of the geological characteristics and previous studies of this deposit. Based on this work, it elaborated the metallogenic law in detail from the aspect of mineralization structure of time and space and the source of ore-forming material, and established the Pingchuan depleted titanium magnetite mineralization series, constructed a metallogenic model, improved the level of the theoretical understanding to the Pingchuan iron ore deposit. This research provides a useful theoretical support to the prospecting breakthrough of the Pingchuan iron deposit. And it’s main research findings are as follows:(1)The magmatite from Hercynian period to Indosinian peride of Pingchuan area have three types, they are gabbro, noritegabbro and lherzolite. The magmatite of this area occurs in the continental intraplate rift, are the product of magmatic homology evolution of the tholeiite series. The magma material may be derived from the partial melting with varying degrees of the upper mantle spinel lherzolite, has experienced the fractional crystallization of the plagioclase, pyroxene and olivine during the evolution process, and has suffered from the contamination of the lithospheric mantle or the crust with varying degrees. The gabbroic magmatite is the ore forming geological body of Pingchuan area, with rich aluminum low calcium, magnesium and titanium, and follow the Fenner differentiation trend in the course of evolution. The enrichment of iron in the late stage of magmatic evolution is conducive to the formation of the low titanium magnetite. And the characteristic of the noritegabbro and lherzolite with rich magnesium determines they are conducive to the formation of copper nickel ore deposit.(2)The iron mineralization and the Hercynian-Indosinian magmatic activity of Pingchuan area have a close temporal relationship. The gabbroic magmatite and the magmatic differentiation type, volcano-sedimentary type iron deposit were formed in the Late Permian Epoch(about260Ma), while the lherzolite and the sub-volcanic hydrothermal type iron deposit were formed in the Early Triassic Epoch(about248～246Ma). Different types of iron deposits of Pingchuan area had an orderly evolution process at the time, and formed the deposits periodically. On the macroscopic scale, the Pingchuan iron deposit experienced magmatic differentiation period, volcano-sedimentary period, sub-volcanic hydrothermal period and epigenetic modification period, four mineralization periods. The first three mineralization periods are the main mineralization period of Dashanshu, Lanzhichang and Kuangshanliangzi ore section respectively. Among them, the sub-volcanic hydrothermal period including phlogopite-serpentine-magnetite stage, magnetite-sulfide-calcite stage, magnetite-carbonate-pyrite stage, siderite-pyrite stage and pyrite-calcite stage.(3)The formation and distribution of Pingchuan iron deposit are restricted by structure and magmatite combined. The Jing River-Qing River fault is the magma and ore transmitting structure, controls the secondary faults, basic-ultrabasic magmatite and iron deposits in the hanging wall to be NNE trending. The secondary Kuangshanliangzi fracture and Xipangou fracture are the main magma-distribution and ore-distribution structure, while the caldera structure, contact belt is the main ore-bearing structure, controls the spatial orientation of the iron ore bodies. The iron ore bodies and the magmatite have a close spatial relationship. Different iron deposits have corresponding ore-bearing formation, the magmatic differentiation type iron deposits are hosted by gabbro formation, the volcano-sedimentary iron deposits are hosted by basalt formation, sub-volcanic hydrothermal type iron deposits are hosted by gabbro formation or the contact belt of the sub-volcanic rock and carbonate rock. Pingchuan iron deposit, after the end of formation in the Early Triassic Epoch, got into the lifting stage in the end of Triassic. The tectonic uplifting and the thinning of the overlying layer occurred in the Cenozoic is the main reason of the exposure of Pingchuan iron deposit.(4)The magnetite of Kuangshanliangzi and Daopingzi, with low titanium, aluminum, and high magnesium, is similar to contact metasomatic type and skarn type magnetite. Trace element analysis shows, iron of Dashanshu ore section originated from the gabbroic magma’s fractional crystallization in the early magma stage, iron of Daopingzi ore section originated from ore-bearing hydrothermal and the precipitation of the gabbro’s alteration, while iron of Kuangshanliangzi and Lanzhichang ore sections have the same source, they all originated from the iron enriched in the late stage of gabbroic magma. The composition of carbon and oxygen isotope of carbonate and sulfur isotope of sulfide of the Kuangshanliangzi and Daopingzi ore sections are analogical, with the δ13CPDB values between-6.0to-1.6per mil, the δ18OSMOW values between15.1and19.7permil, and the δ34S values range from5.1to21.7per mil. The carbon and sulfur are thought to derive from the thermal decomposition of marine carbonate, with a small amount deriving from ore-forming hydrothermal. The magnetite of Kuangchanliangzi and Lanzhichang ore sections has a similar isotopic composition of lead, with the values of206Pb/204Pb between18.934and20.008,207Pb/204Pb range from15.620to15.661, and208Pb/204Pb between38.237and38.301. Lead isotopic composition of Daopingzi ore section’s magnetite are a little different from the two ore sections above, with the values of206Pb/204Pb between17.720and17.874,207Pb/204Pb range from15.491to15.554, and208Pb/204Pb between38.130and38.435, indicating the lead of magnetite of the different ore section derived from the mixing of magma system and formation. The altered limestone of Kuangshanliangzi ore section has significant negative Eu and weakly negative Ce anomaly, and enrichment of High Field Strong Elements of Th, U, Nb, Ta, Zr, Hf, Ti and HREE. The oxygen and hydrogen isotopic composition of magnetite of Daopingzi and Kuangshanliangzi ore sections, with the values of8DH2o between-123and-96per mil and δ18OH2O between13.2and18.1per mil, shows the water of the ore-forming hydrothermal derived from magma system. The magnetite inclusion liquid phase components of Daopingzi and Kuangshanliangzi are rich in Ca2+, Mg2+, Na+, Cl-, and potentially rich in HCO3-or HS-, but the liquid phase concentration of Daopingzi magnetite inclusions is higher than Kuangshanliangzi nearly an order of magnitude. The iron may initially migrates mainly in the form of Fe2+Cl2、Na2[Fe2+Cl4]、Fe2+F2、K2[Fe2+Cl4], and has strong material exchange with carbonate formations, thereby allowing the magnetite has gained more rock information.(5)On the basis of synthesizing all aspects of research content, we put Pingchuan iron deposit into depleted titanium magnetite mineralization series, including Kuangshanliangzi type, Lanzhichang type and Dashanshu type three main occurrence styles. The ore-forming process was thought to be closely related to Hercynian-Indosinian tectonic and magmatic activities, controlling by deep fault and mantle-derived magma, was the product of magma-fluid activities from Late Permian Epoch to Early Triassic Epoch, with an orderly evolution process at the time, and forming the deposits periodically. The ore-forming materials are mainly derived from low titanium basaltic magma system, contaminating by shallow source materials. Each metallogenic type occurs in different lithofacies and formations, with multilayer in the mineralization depth. We established the metallogenic model of Pingchuan iron deposit based on the study of metallogenic series, reflecting the temporal and spatial relationship about the coupling of diagenesis and ore-formation with focus, reflecting the evolution process of the depleted titanium magnetite metallogenic series in this area and it’s metallogenic characteristics vividly. |
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