| In the western margin of Yangtze Block, there distributed a large triangular area of170,000km2in the border region among SW Sichuan, NE Yunnan and NW Guizhou (SYG) province, and more than400Pb-Zn deposits and mineralization occurrences have been discovered. They are characterized by irregular vein-type ore bodies with epigenetic hydrothermal filling characteristics, simple mineralogy, and high Pb+Zn grades of ores usually associated with Ag, Ge, Ga, In and Cd. Most of the Pb-Zn ore bodies are mainly hosted in Neoproterozoic to Permian carbonate rocks, and controlled by the tectonic belts, particularly the anticline with NE-trending and NW-trending faults. As result, a Pb-Zn metallogenic domain or belt was defined. All the known large or superlarge deposits are mainly hosted within the carbonaceous sedimentary rocks of Neoproterozoic, Cambrian, Devonian, Carboniferous and Upper Permian strata, but only little occurrences hosted within Ordovician and Silurian strata have been discovered. In fact, there are much early Paleozoic strata outcrops and coverage area under thick late Paleozoic strata in SYG. For decades, the prospecting potential in these areas has been expected.The Yuele lead-zinc ore field is located in the north of NE Yunnan rich Pb-Zn metallogenic belt of SYG domain, which is a blank area of Pb-Zn ores exploration with little surficial outcrops. The recent exploration works (including the ground survey, tectonic-lithofacies-alteration mapping, geochemical exploration, exploration engineering and etc.) has gained many discoveries. It is showed that, the shallow occurrences of vein-type lead-zinc-copper ore bodies hosted in the Silurian carbonate are is relatively clear, all come in the transtensional faults and/or fractures and/or the junctions of different extending faults. And in the short axis anticline zone underlying Middle-Late Ordovician strata, there are often appeared salt tectonics, accompanying stratiform lead-zinc deposit. All these information shows there is a great exploring potential for mineral resources. The deep changing of different types of ores and the spatial distribution law of stratiform ore bodies, and the prospecting direction is unknown, brought much difficult for the exploration prospecting progress as soon as possible. Meanwhile, the occurrence and stable extension of different thickness of carbonaceous shale, mudstone and limestone of ore formation and overlying strata, and also is abundant karst fissure groundwater, take noticeable interference to the interpretation of the surveying data, and limit the prospecting effect of many geophysical methods. Therefore, this research selected the Audio frequency megnetotelluric method, and used the new method of data processing and interpretation for the concealed ores prediction application of the Yuele lead-zinc ore field, and provide the geophysical basis for the mineral resources exploration and prospecting for this mining and other analogous areas.The research topic is come from the cooperation project "The Ore-Forming Model and the Comprehensive Information Prospecting Model of Yuele Lead-Zinc Ore Field, Daguan County, Zhaotong City, NE Yunnan". In this paper, based on the systematically collecting and collating the completed funding and literature, the regional geology and deposit geology and the characteristics of lead-zinc (-copper) ore deposit has been summarized, and the genesis model has been established. According to the electrical parameters determination results of different types of rocks specimens collected from the ground survey and the exploratory boreholes and channels, their electrical characteristics has been identified respectively. Through the method validity test above the known ore body and structure, the geological and geophysical model has been preliminary set up. For the field surveying Audio frequency megnetotelluric (electromagnetic) data on the favorable metallogenic area, the high precision data processing and two-dimensional inversion has been carried out. According to the inversion results of the geological interpretation, the prospecting prediction targets have been put forward. During this study,18lines of Audio frequency megnetotelluric (electromagnetic) profile sun eying have been completed, including13lines of EH4instrument surveying with total length of7580meters (measured in April2011), and5lines of GMS-07e instrument surveying with total length of5,7200meters (measured in December2012). And the total physical surveying points were up to450, including226points of EH4instrument, and224points of GMS-07e instrument.Through this study, the understanding and achievements are as follows:1. The regional metallogenic geological conditions and characteristics of Yuele lead-zinc deposit has been summarized. This ore field is located in the northern segment of NE Yunnan rich Pb-Zn metallogenic belt of SYG Pb-Zn metallogenic domain with great potential. In the ore field and close area, only little Pb-Zn ores outcrops have been discovered. The output characteristics showed that the shallow vein-type Cu-Pb-Zn ore bodies were hosted in the Ordovician and Silurian carbonate. The main structure framework of this ore field is nearly SN-trending, the main fold is NW-trending Longjinguo Fold, with NE-and NEE-trending secondary folds and NWW-, NW-, NNW-, NE-, and NEE-trending faults and layer faults. The No. F1fault and No. F2fault are the ore-controlling structures. The secondary NW-trending fault and NEE-trending fold all controlled the ore-formation. NNW-trending fault, NE-trending fracture and the shear fracture zone between the formations controlled the outputs and the formation of the ore bodies. The shallow Pb-Zn-Ag orebodies predominantly output as steeply inclined veins and stockwork veins. They strictly controlled by the NW-trending, NNE-trending transtensional faults. Partially, the stratiform ore bodies were hosted in the interlayer fracture zone. Deep layered, layer-like stockwork output (Pb-Ag) Zn ore bodies were strictly controlled by the salt layers and the underlying carbonate strata. The vein-type and veinlet-type Cu-Pb-Ag ore bodies output on the plate of the No. F1fracture in the west area of this ore field. It predominantly output as steeply dipping and stockwork vein, and strictly controlled by the SN-trending and NWW-transtensional faults and joints. The ore hosted strata are mainly Lower Ordovician Shizipu Formation (O2s), Lower Silurian Huanggexi Formation (Sih), and Middle Silurian Daluzhai Formation (S2d). The lithology is mainly limestone, argillaceous limestone, dolomitic limestone and containing quartz sandstone and gypsum stone. They belong to a sedimentary formation of Early Paleozoic shore-shallow marine clastic rocks-carbonate. The main ore minerals are galena, sphalerite, pyrite and a small amount of bornite and chalcopyrite, etc. Gangue minerals are mainly barite, calcite, dolomite, and a small amount of quartz, fluorite, gypsum, etc. Local oxidation of the ore commonly contain malachite, azurite and calamine. The ore mainly have the vein, stockwork and euhedral subhedral struture, and have the euhedral, subhedral, anhedral account the residual and accountable dissolution texture. The mineralization can be divided into diagenesis and hydrothermal ore-forming hydrothermal mineralization, and the hydrothermal ore-forming hydrothermal mineralization can be divided into Cu-Pb sulfide stage, Pb-Zn-Ag sulfide stage and carbonate stage. The wall rock alteration includes carbonate (mainly calcitization and a small amount of dolomitization), baritization, pyritization and a small amount of silicification, fluoritization, plasterization and etc. They have clear zonation characteristics both in horizontal and vertical direction.2. The metallogenic model of the Yuele lead-zinc (-copper) deposit has been established, which provided a guiding prospecting model for the geophysical exploration of this ore field. All the controlled ore-bodies is featured by the Ordovician and Silurian carbonate-hosted, three kinds of mineralization type overlay, associated with salt tectonic related to the thrust fold structure and post-filled epigenetic mineralization. There are three kinds of mineralization type, that is the upper steeply dipping vein (or veinlet) in shape Pb-Zn (-Ag) ore-bodies controlled by overlying salt tectonics along the anticlinal wing, the lower stratiform or stratiform-like or veinlet in stratiform in shape Pb-Zn (-Ag) ore-bodies controlled by underlying salt tectonics along the anticlinal wing, and the upper steeply dipping vein(or veinlet) in shape Cu-Pb-Ag ore-bodies controlled by underlying salt tectonics along the anticlinal core respectively. They are spatially superimposed with the similar geological characteristics, and made up the mineralization pattern of "multiple veins and layers overlying". They are all the product of the same metallogenic system under the unified fluid evolution. The output characteristics showed that the shallow vein-type lead-zinc-copper ore bodies were hosted in the Silurian carbonate, all come in the transtensional faults and/or fractures and/or the junctions of different extending faults. And in the short axis anticline zone underlying Middle-Late Ordovician strata, there are often appeared salt tectonics, accompanying stratiform lead-zinc deposit. This deposit is a sediment-hosted polymetallic deposit controlled by salt tectonics related to the thrust fold structure, belong to the low temperature epigenetic hydrothermal fluids filling vein-type deposit, and exhibit many similar geological and geochemical features with the MVT deposits all over the world relatively. The ore-bodies exposed indicate that there are favourable metallogenic geological conditions, and its prospecting potential is great in Yuele ore field and its surrounding areas. Lithology, fracture and multi-stage fluid migration are the key controlling factors of alteration and mineralization zoning, which have great geological significance of ore-forming fluid’s migration, concentration and enrichment. So, this understanding can guide the future mineral exploration effectively.3. The electrical characteristics of different types of rocks were summarized, which provides important physical conditions for the next geophysical exploration, abnormal inference and interpretation.4. On the basis of the experimental test approach to the known ore body and structures, the geological and geophysical model has been established. It provided a model for the periphery abnormal inference, the data interpretation and the prospecting prediction foe this ore field.5. Through the data processing, two dimensional inversion, comprehensive analyzing and interpretation of the electromagnetic surveying data, the extension direction, the delay situation and the spatial distribution of the ore-bearing No. F5fault was framed. The results have pointed out the future prospecting direction for the mining company.6. After this study, the prospecting predictions of concealed ores have been put forward, including FP1, FP2, Fp3, F2, F3, F4and F5. These target regions can be taken as the future exploration area for mining company, and also provided the important geophysical basis for the next exploratory deployment plan.7. This paper presented a successful example using magnetotelluric method to prospect the concealed lead-zinc ores in Yuele ore field, Daguan County, NE Yunnan province, China. After comparing the interpretation results of magnetotelluric data with the exploratory data, it is found that there is a distinct difference in resistivity and polarizability between orebody and lower-and upper-stratum, and ore-bearing faults and non-ore-bearing faults. The results showed that magnetotelluric method is helpful to identify the concealed lead-zinc ores under the similar geological conditions for other prospecting areas, and has extensive application value. |
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