| Baimachang lead-zinc deposit is located in the Northeast of Yunnan, adjacent to the Maozu giant lead-zinc ore. With the similar regional geological background, tectonic position and geological characteristics, Baimachang ore district has revealed good prospects for finding mineral resources. Owing to the little basic data and the low study of this area, identify the geological and geochemical characteristics, then analyzes its genesis will undoubtedly deepen the understanding of the mineralization of the lead-zinc deposits in Northeast of Yunnan, also promoten the prospecting work. This paper carried out a comprehensive study on the stratum, lithology, tectonic, mineral assemblage, alteration and other geological features, relying on the research of fieldwork and laboratory work. On this basis, this paper firstly conducted geochemical research on the main formation in this ore district with analysis of major elements, trace elements and rare earth elements so as to reveal the relationship among metallogenic environment, stratum and mineralization with geochemical methods, in view of the significant indication of element geochemical characteristics for metallogenic environment. The main research findings and understandings are as follows:(1)The host strata of Baimachang lead-zinc deposit belongs to the upper Dengying formation of upper Sinian-Yuhucun formation of lower Cambrian, the same as Maozu giant lead-zinc ore deposit. Baimachang deposit includes a certain scale of stratiform lead-zinc orebody, which is the main output type in this mining area, and a relatively less scale of vein orebody. The combined characteristics of stratum and lithology in this mining area play an important role in controlling over the lead-zinc mineralization.(2)Field geological survey and microscopic features of zinc blende in bedded orebody did not obviously observe the hydrothermal mineralization. Combined with the field geological investigation, the bedded orebody which is dominated by Zn in Baimachang deposit showed obviously sedimentary genesis. The metallogenic period is Dengying period of late Sinian-Yuhucun period of early Cambrian.(3)Systematic geochemical research on constent elements, trace elements and rare earth elements in this area displayed that stratum, lithology combination enrichment plays an important role in the controlling of various elements of Baimachang lead-zinc deposit.1) Constant element geochemical analysis shows that, Clark values (CK) of CaO, MgO in the formations of mudstone, shale, siltstone are lower than their CK values in carbonate formations, in addition to some individual layer, Clark values (CK) of MgO is less than CaO. CK values of Al2O3, Fe2O3, K2O, Na2O in ore-bearing strata are significantly lower than dead ground.2)Analysis on trace elements shows that:Contents of Zn, Pb, Ba, Cd in ore-bearing strata is significantly enriched, content of Cu element significantly depleted, and contents of associated metals just as Ge, In is low. Contents of trace elements like Nb, Sc, Hf, Rb, Li, Th, U, Sn, Cs, BE, W, Ta, T1, Bi in carbonate-based formations which including Dengying formation, Longwang-miao formation, Xiwangmiao formation, Erdaoshui formation and Dajing formation are very low, shows similar degree of enrichment. On the contrary, contents of those14elements mentioned above in formations of mudstone, shale, siltstone which including Yuhucun formation, Qiongzhusi formation and Hongshiya formation are high.3)The rare earth element analysis showed that in the main ore-bearing formation-Dengying formation, total rare earth is poor, Eu element displays strong positive anomalies and Ce element displays negative anomalies, the severity of the rare earth fractionation obvious characterized, all above shown hydrothermal sedimentary characteristics.(4)Analysis about features of specific elements combinations found, the deposition environment of Baimachang lead-zinc deposit, from Dengying formation of the Upper Sinian (Z2dn)â†'Dajing formation of Middle Ordovician(O2d) evolved from epicontinental sea or lagoon phaseâ†'deep-half deep-sea phaseâ†'epicontinental sea or lagoon phaseâ†'land-sea transition phaseâ†'epicontinental environment or lagoon evolved. Paleoclimate evolved from stable humid climateâ†'wet-dry climateâ†'humid climateâ†'arid climate change the mainâ†'humid climate, ancient oxygen phase has been kept in an oxidizing environment.(5) Study about stable isotope tracering found that the water in Ore-forming fluids mainly comes from precipitation, carbon of the hydrothermal stage experienced marine carbonate rocks carbonate dissolution and may be subject to the impact of the reduction of organic matter off.(6)Since the regional geological movement of Yanshan period and Xishan period has a powerful effect on the whole Yangtza platform, the lead-zinc mineralization of Baimachang mining area in Yunnan Dongtai fold belt has mainly been not only controlled by the combination of stratum and lithology, but also been affected to some degree by the tectonic system, according to the comprehensive analysis of regional tectonic background, regional geological evolution and tectonic features of this mining area.(7)By omprehensive analysis, this paper proposed that Baimachang lead-zinc deposit should be a hot water deposition-latter transformation-type deposit. During Dengying period of the Upper Sinian-Yuhucun period of Lower Cambrian, hot sedimentary lead-zinc deposit formed, which was strictly controlled by the formation and lithology (mainly zinc, associated fluorite and barite ore body), there also Exist the latter transformation of mineralization formed epigenetic hydrothermal vein filling type lead-zinc deposits(mainly lead, associated copper and silver ore body), which controlled by faults and formation-lithology joint. |
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