A Study On The Mineralization And Metallogenic Model Of Pb-Zn-Ag Polymetallic Ore Concentration Area In Cangyuan-Nanla, Yunnan And Jinchang, Myanmar

Jinla (Jingchang-Nanla) Zinc-Silver enrichment region stepping across China-Burma board is a fundamental prospecting target for Zinc and Sliver polymetallic deposit. Tectonically, this region belongs to Cangyuan-Ximeng bundle of fold lying in the west of Changning-Menglian fold belt of Gangdese-Nyaingentanglha fold system, southern part of Sanjiang River. This region is characterized by its complex tectonic settings and favorable minerogenetic conditions, which is one of the most important Pb-Zn-Ag-Au-Cu resources-rich areas in southern part of Sanjiang River. Based on the theory of"magma-fluid-metallogenic system", the author combined various subjects involving geology, petrology, stratigraphy, structural geology, study of mineral deposits, geophysics and geochemistry, basically verified the spatial distribution of main ore deposits, the geological and physical chemistry metallogenic conditions, and the metallogenesis and ore-controlling factors of the research region, and evaluated the metallogenic potential for Pb-Zn-Ag-Au-Cu.This study distribution and characteristic of magmatic rock.For the first time, this study successfully separated and picked out Zircon from granitoids closely related to metallogenesis of this region, yielding ages of 40.3Ma, 40Ma and 40.9Ma, respectively, testifying that the timing of magmatism related to metallogenesis is defined mainly in the Himalayan.The systematic research on the geochemical characteristics of the ore district shows that the ore-forming fluid is mainly NaCl-H2O system, followed by NaCl-CO2-H2O system, and the homogenization temperature and salinity of fluid inclusions vary in a relatively wide range, with a peak temperature in a range of 240～320℃and corresponding salinity of 0-20wt% NaCl-equivalent, which falls into the category of intermediate to low temperature-intermediate to low salinity ore-forming fluid. The ore-forming depth lies in a range of 350-2700m, which is a depth of intermediate-low temperature and salinity epithermal deposits. The gas composition mainly consists of H2O and CO2;positive ion in the liquid is mainly composed of Na+,K+ and negative ion is dominated by SO₄^2- belonging to Cl--Na+-Ca²⁺ and SO₄^2--Na+-Ca²⁺ type;most ratios of SO₄^2- and Cl1- are higher than 0.5, showing that the ore-forming fluid is dominant by underground hot brine, with mixing of magmatic fluid. Isotopic data of C-H-O indicates thatδDH2O‰varies in a relatively small range (-103‰-120‰), and shows no significant correlation with temperature, confirming that the ore-forming fluid is a mixture of meteoric water and deep-derived fluids. From Jingchang to Nanla, the REE patterns shift from weak LREE enrichment to strong LREE enrichment. The trace elements are mainly incompatible elements,and the results reveal that the ore-forming fluids derive form upper mantle. Theδ18O of fluids (-9.4‰～8.8‰)fully demonstrate ore-forming fluids are characterized by mixing of meteoric water and magmatic water. The sulfur isotopic data shows a heavy sulfur dominating trend. Sulfur is mainly derived from a deep source, and the Lead isotopic composition is relatively homogeneous, which is resulted from a crust-mantle mixing source. Ore-forming material and fluids of this region are derived from the ancient metamorphic strata and deep magma.This study systematically summarized the geological condition and characteristics and discussed the ore-forming phases and metallogenesis of the ore district. The mineralization can be classified into five categories:Epithermal metasomatic-filling dolomite type, taconite-skarn type, structural fracture zone type, SEDEX type, and porphyry copper type. The ore district experienced Tethys and collisional ore-forming phases, and can be divided into there ore-forming systems:the first one is a Pb-Zn-Ag ore system occurring in the oceanic carbonate rocks of Yungou set,Ximeng group; the second one is a Pb-Zn-Ag-Cu-S ore system related to the Early Carboniferous oceanic basic-intermediate volcanic rocks; the last one is a Pb-Zn-Ag-Cu-Mo-Au ore system associated with the Himalayan granite porphyry and granodiorite porphyry.This study initially set up an ore-forming model for the Jinla Pb-Zn-Ag polymetallic deposit. During the Proterozoic epoch of Tethys phase, this region was a geocynclinic sedmentary area, with strong basic-intermediate oceanic volcanic eruptive activities, forming a nearly huge thick Proterozoic Wangya set, Ximeng group volcanic rocks, which constitutes the primary ore source of Pb-Zn-Ag-Au polymetallic deposit. Meanwhile, some part of the region underwent strong sea bottom volcanic eruptions, which gave rise to the deposition of metallic minerals, forming SEDEX type polymetallic depostis. During Jinningian to Caledonian, this region was subject to regional metamorphism and structural movement, which made the Proterozoic strata deformed, and the involvement of hot metamorphic water accelerated the regional metamorphism and structural movement. In the Early Carboniferous, the Tethys Ocean was under an early development phase, belonging to shallow ocean-deep ocean basin condition. With the aggravating activity of the Changning-Menglian deep fault, and cutting into the upper mantle, large-scale volcanic eruptions emerged and produced the sea bottom VMS type polymetallic deposit in Laochang, Lanchang area. During the Late Variscan to the Indosinian, as a consequence of the close of Tethys Ocean, the intracontinental evolution stage began and both the strong activity of Nanding River deep fault and the forming of intrusion of granites not only enhanced the enrichment of Pb-Zn-Ag-Au in the initial ore-forming source, but also provide the migrating path of ore-forming fluids due to the secondary structural faults and fissures of Nanding River deep fault. Penetrating cycling water continuously exchanged with deep magmatic fluids, bringing ore-forming material to the fluids. The Himalayan structural movements caused the fold uplift of this region and brought force the Jingchang-Nanla anticline. Owing to the intensified pulse activities of the consanguinity magma during this phase, various scales of granite porphyry stocks were formed. The hydrothermal and deep cycling of ore-forming fluids driven by the magmatic activities stimulated the extraction, precipitation and enrichment of ore-forming material, and ultimately generated Pb-Zn-Ag polymetallic ore bodies which mainly occurred in the structural fracture zone of inner granite contact zone, faults and interlayer weak zones of wall rock, structural weak zone of the contact zone between intrusions and wall rock.