| 其他摘要 | The Laochang large silver ploymetallic deposit is located in the center of the Lancang sub-falt basin of the Changning-Menglian rift, which belongs to the southern segment. Its one of the most important metallogenic segment in the Nujiang-Lancangjiang-Jinshajiang metallogenic belt. Geotectonically, it is situated at the eastern margin of Baoshan-Danbang micro-Landmass and the western edge of the Lanping-Simao basin and the Lincang block. It belongs to the east Tethys’ tectonic domain, which is a part of suture zone between Gondwanaland and Eurasia.
The ore deposit has a long mining history, which can be date back to the Yongle 2year of the Ming Dynasty about 605 years ago. Our ancestors exploited sulfide ores to extract silver and abandoned a large quantity of slag with high content lead. The highest annual output of silver is 0.3 million tales during the mining history. The high content lead slags were the main exploiting objects, and a preliminary prospection for the deep primary ore bodies had been done after China's Liberation in 1949. Since 1990, the systematic exploration on the deep primary ore bodies have been developed and those are main exploration targets for the mine. It is characterized by the unique mineralization geology, the silver enrichment（average 629.46× 10-6 in lead-zinc ores and 2069.07×10-6 in galena）, the abundance of associated elements（In, Se, Te, Ga, Cd, Bi）, the large reserves and content higher Sn. It is one of the most typical Pb-Ag deposit types in the southern segment of Nujiang-Lancangjiang-Jinshajiang metallogenic belt, and is the only large scale silver ploymetallic deposit in the Changning-Menglian rift. It has significant research values, so, many geologists were attracted by it.
Also the deposit geology, the metallogenic conditions and the ore controlling factors and so on have been researched, but those are still unsystem and controversial in terms of sources of the ore-forming material and fluid, the ore genesis, the metallogenic geodynamics background and the relations between the deposit and granite porphyry, etc. The detailed studies on the regional geology and the local geology of the Laochang deposit have been carried out in this study. We systematic researched the mineralogy, the petrology, the geochronology and the ore deposit geochemistry by using many modern analysis and testing technologies. Then, we found the ore forming materials and fluids, revealed the metallogenic geodynamics background, discussed the ore genesis and preliminary established the genetic model for metallogenesis of the deposit. Based on those, several conclusions can be drawn from this study are as follows:
1. We ascertained the material composition and the formation stage of sulfides. The sphalerites are characterized by content high iron（average11.51% in the early stage and average 8.41% in the late stage, respectively） and associate many trace elements. The early stage sphalerites enrich Fe, In but the Cu, Cd, Mn enrich in the late stage sphalerites. The galena is the main carrier mineral of Ag, the early stage galena enrich Ag, Te, Bi and content Se, Cu. The middle stage mainly enrich Ag, but in the late stage, it distinguished between the two stages with enrichment As and Bi. Except the tennantite, the compositions of other sulfide minerals are relative simple.
2. The characteristics of associate elements in ores and its sulfide minerals were revealed. The contents associate elements are very high, average grade is Cd 4293.19×10-6, In 555.37×10-6 in sphalerite and Te 143.81×10-6 in galena, respectively. The others elements（As、Se、Bi、Ga） also have very higher content. Based on those, we briefly assessment the metal reserve of Ga970t, Cd18400t, In1300t, Se600t, Te1500t, Bi12800t, As43100t. The sphalerite is the main carrier of Ga、In、Cd, the galena mainly enrich Te and Bi, the Se occurrence in the lead-zinc and pyrite ores.
3. We obtained the precise date of geochronology about ore deposit at the first time. Employing ultra-low procedural blank single grain sphalerite-pyrite Rb-Sr method, single grain sphalerite-pyrite yield an isochron age （45±3.6）Ma with initial 87Sr/86Sr ratios 0.70977±0.00034. This mineralization age is consist with diagenesis age and implication that the close relation of deposit formation and granite porphyry intrusion, the deposit formation also is the southern segement of Three-river metallogenic belt responsed to the Himalayan collision and mineralization orogenesis.
4. The systematic works of geology, geochemistry about granite porphyry have been carried out. The research show that the granite porphyry have high SiO2, K2O , rich alkalis, poor Na2O, low MgO, CaO, FeO, P2O5, rock type is peraluminous granite. Its geochemistry shows that rich LREE, Eu negative anomaly, almost no Ce anomaly, enrichment in LILE. The relative high initial strontium value, low εNd and high Nd model age and all of those geochemistry are similar to the enrich alkalic porphyry. Its formation may contribute to remelting of thickening low crust, and the mantle material mixed with the magma during the low crust melting, we have proved it formation in syn-collision tectonic environment.
5. Discussed the geochemical characteristics of ore-bearing formation , the ores and their sulfide minerals in the deposit. We have analyzed systematically major elements, trace elements and rare earth elements of the sedimentary host carbonate rocks. By comparing preceding characteristics of the Laochang Ag polymetallic deposit with corresponding values of typical hot-water sedimentary rocks and modern sea-floor hydrothermal sediments. We suggest that the volcanic exhalative sedimentary evidences for the ore-bearing formation, meanwhile, the trace elements and rare earth elements of the ores and its sulfide minerals also prove that extensive hot-water exhalative sedimentary activities took place in the Laochang mining district in early carboniferous.
6. Revealed the sources of ore-forming materials and fluid. The ore-forming elements and geochemistry of the deposit show that the ore-forming materials are from multiple sources. Pb isotopic compositions indicate that the base metals may derive from different strata by leaching. The S isotopic compositions show that the seawater leached underlying volcanic strata and the seawater sulfate reduction was the main source of sulfur for sulfide, of course, the post granite porphyry magma hydrothermal also may provide some sulfur. In addition, the characteristics of C, H, O isotopic compositions, trace elements and rare earth elements of ores and their sulfide minerals indicate that the early stage ore-forming fluid mainly came from deep volcanic hydrothermal but the late stage ore-forming fluid mainly sourced granite porphyry magamatic hydrothermal.
7. Preliminary established a genetic model for metallogenesis of the deposit. The Laochang deposit is the production after long-term many geological processes, it has characteristics of multiple sources of ore-forming materials and several ore-forming stage. It experienced volcanic exhalative sedimentary mineralization in early carboniferous and granite porphyry magamatic hydrothermal superimposition mineralization in Himalayan period. The exhalative hydrothermal sedimentary mineralization, which formed the massive sulfide deposit with dominant pyrite. Then the enrich alkalis granite porphyry magamatic hydrothermal carried abundance Cu, Mo, Ag, Pb, Sn ore-forming elements and superposed on the former ore-bodies, at last, the deposit formation. To sum up, the laochang Ag ploymetallic deposit belongs to volcanic exhalative sedimentary+magamatic hydrothermal superimposition origin. |
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