| Located at the conjunction site of Yangtze block’s western margin and Sanjiang Tethyan orogen, the stratiform-like Pb-Zn-Ag polymetallic mineral deposit in the outer zone of Beiya Au ore is hosted in the medium-southern segment of the alkali-rich porphyry belt formed by the Jinshajiang-Ailaoshan large strike-slip tectonic activity. Based on the previous researchers’ works and after systematical field geology investigation researching on this deposit, ore deposit geochemistry characteristics including rare earth elements of calcite, trace elements of sphalerite, S-Pb isotope of some sulfides as well as fluid inclusions trapped in the calcite and quartz are detailedly studied in this paper, by which the metallogenesis, ore genesis, metallogenic model are discussed and then some main views can be obtained as follows:The fabrics, paragenesis assemblages, cutting or interposing correlation of the minerals are observed in detail, by which the ore-forming period of the deposit can be divided into two ones: ①Hydrothermal Metallogenic period is subdivided into two stages, one is Quartz-Sulfide stage, in which galena, sphalerite, pyrite, chalcopyrite, quartz, calcite, chlorite as main minerals are formed, with a small amount of natural gold, pyrrhotite and magnetite; another one is Carbonate stage, during which calcite and a little dolomite, pyrite, siderite are present.② Supergene period consist primarily of cerussite, anglesite, masicotite, limonite and so on.Sulfur isotope compositions of ores range from -2.4%o to +2.8%o, indicating that sulfur in the metallogenic fluids of the deposit has the same source from deep mantle or magma with the alkali-enriched porphyry body; and Pb isotope data plotted in the lead tectonic model figure and Δβ-Δγ genetic classification diagram show that the ore-forming material is derived from crust-mantle mixture.Rare earth elements geochemistry features of the ore-stage calcite suggest that the deposit is hydrothermal in origin within the oxidized environment. Sphalerite is mainly enriched in Cd, Cu, Pb, Sb, and depleted in In, Ga, Ge, T1, which is similar to the trace elements characteristics of sphalerite in the typical hydrothermal deposit; and its Ge/In and Ga/In ratio is lower than 0.1 and 1 respectively, implying that the metallogenic temperature is within the medium-low range, identical to the microthermometry results from fluid inclusions.The type of fluid inclusions trapped within the calcite or quartz vein in the deposit is consistent with that in the quartz phenocryst of alkali-rich porphyry, porphyry-type mineralization quartz vein and skarn minerals, while their homogeneous temperature and salinity are somewhat lower than the latter. That demonstrates that the ore-forming fluid is the evolutionary products of alkali-rich porphyry hydrothermal solution in different stages, concordant with the fluid derivation of porphyry- and skarn-type deposit.On the basis of a combination among the geological characteristics of mineral deposit, metallogenic material source, ore-forming fluid provenance and its evolution, and metallogenesis condition analysis, the metallogenetic model of the stratiform-like hydrothermal Pb-Zn-Ag polymetallic mineral deposit in the outer zone of Beiya Au ore is preliminarily established.It is comprehensively thought that as the external zoned products of Beiya alkali-rich porphyry Au polymetallic metallogenic system, the stratiform-like Pb-Zn-Ag polymetallic mineral deposit belongs to Himalayan alkali-rich porphyry-skarn hydrothermal-type polymetallic deposit in terms of genetic type. |
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