| The Changning-Menglian Late Palaeozoic Volcanic Zone in the southwestern Yunnan is geotectonically located in the Baoshan-Lancang Geodome System of the Southwestern Yunnan Geodepression Region, and has, at least .undergone geosyncline, platform, and diwa stages, and the Late Palaeozoic volcanics of the Volcanic Zone were formed in the epicontinental rift circumstances at platform stage.The Laochang silver polymetallic ore deposit and the Laochang lateritic Ag-Mn ore deposit are representative ore deposits of the Changning-Menglian Ag-Pb-Zn-Cu Metallogenic Belt, where the metallogeneses formed with the geotectonic evolution of crust. In Middle-Late Proterozoic, The zone was at geosyncline stage, and the initial Ag-Pb-Zn source bed represented by the Langchang Group and Ximeng Group metamorphics. In Palaeozoic, the zone was at platform stage, and the evolution period of the epicontinental rift in Late Palaeozoic was the key period of ore-forming processes in the zone. During Devonian through early Early Carboniferous, terrigenous fragmental rocks were deposited in the rift graben, So came into being the first source bed. The culmination of the development of the rift, i.e., late Early Carboniferous, was the most important tnetallogenic epoch of the region, during which formed the most important second source bed (i.e., the Lower Carboniferous volcanics ) and chief ore deposits (i.e., the volcanogenic silver polymetallic orebodies of the lower portion of the Laochang silver polymetallic ore deposit, and the Tongchangjie cupreous pyrite deposit). During Middle-Late Carboniferous to Permian, the rifting was relatively weak, yet post-volcanic hot water activities were rather frequent, and the third source bed (i.e. ,C2+3-P| carbonate rocks) which is thousands of meters thick was formed, locally (such as in the Laochang mining district) there came into being stratoid and lenticular Ag-Pb-Zn orebodies hosted by C2f3 carbonate rocks. Since Late Triassic, The region had entered diwa stage.Because of the intrusion of (intermediate) acid magma of the Yanshan Cycle and its metal logeneses of superimposion and reformation,the Laochang polygenetic compound Ag-Pb-Zn-Cu polymetallic ore deposit came into being in the end. In Quarternary,The Ag-Mn source bed of C2+3-P, carbonate rocks undergoes lateritization, accompanying the formation of lateritic Ag-Mn ore deposits represented by the Laochang deposit.The Laochang silver polymetallic ore deposit is of "five-storey" mineralizing structure: in the uppermost portion are near-surface veinlike and sackform Ag-Pb-Zn orebodies hosted by C2+r PI carbonate rocks, below which are stratoid and lenticular Ag-Pb-Zn orebodies hosted by lower horizon of C2+3 limy dolomite, in the middle are stratoid and lenticular Ag-Pb-Zn orebodies with accompanying copper (and minor pyrite orebodies) occurring in C, volcanics and interstratal fault between C2)3 and C,, below which are stratoid and lenticular Ag-Pb-Zn orebodies with accompanying copper (and a few pyrite orebodies, and copper orebodies), at deepare veinlike copper orebodies and copper mineralization. There are granite porphyry veins [of which Rb-Sr isotope age is about 50Ma and (87Sr/86Sr)0 is 0.711267], skarns, skarnized volcanics, skarnized marbles, skarnized silicolites and hornstonized slates in the deep of the Laochang mining district. Some typical minerals which are associated with intermediate-acid magmatic hydrothermal mineralization, such as molybdenite, bismuthinite.fluorite have also been found in the mining district. Isotope study indicates that ore sulfur is mainly derived from magma, ore lead is crust-mantal mixture lead, ore fluid is mixture water which is made up of magmatic water and sea water or meteoric water. In a word, the Laochang silver polymetallic ore deposit is typical polygenetic compound ore deposit, of which the mineralizations include volcanic exhalative sedimentation, post-volcanic exhalative sedimentation, (intermediate) acid magmatic hydrothermal superimposition and reformation,the chief ore-controlling fac...
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