Intergrated Geophysical Explozation And Its Application In Conceal Ore-bodies Prospecting In The Laba Porphyry Molybdenum (-copper)Ore Field, Shangri-la County, NW Yunnan Province, China

Porphyry molybdenum deposit is the most important source of molybdenum metal, is also the main molybdenum deposit type in China. There distributed a large number of copper-lead-zinc-iron-siver-polymetallic deposits and mineralization occurrences in the Xiaozhongdian-Lijiang region along the western margion of the Yangtze Block. For decades, the prospecting potential in these areas has been expected in the mining industry, but some large scale deposits have not been found for several decades. In recent years, the Laba molybdenum (-copper) deposits has been discovered as a superlarge scale porphyry type deposits in Laba-Tongchanggou area, Shangri-la county, northwestern Yunnan province, and the prospecting potential is great. The shallow vein-type molybdenum (-copper-lead-zinc) mineralization usually located in the secondary fractures and their junctions in various directions. And the deep mineralization occurred in monzonitic granite porphyries and diorite porphyries intruded into the Middle Triassic carbonate rocks, and form a connected set of deposits combinations. It includes porphyry-type ore deposits within the rock mass, skarn-type ore deposits in the contact zone and hydrothermal vein-and veinlet-type ore deposit in the outer contact zone. These also confirmed constantly by the recent exploratory work. As the important ore blocks, the (332+333+334) class resource reserves of634700tons of molybdenum metal has been gained, and accompanying amount of33700tons of copper metal, and14.7tons of gold metal. The exploration prospecting and related research work is still ongoing, and the resource reserves increase gradually. The estimated molybdenum metal is expected to exceed1.5million tons.In this exploratory area, owing to the thick overburden of superficial Quaternary eluvium and diluvial sediments, no clear deep sedimentary formation and tectonic features, and the buried depth and the morphology of the ore-bearing magmatic rocks, the prospecting effectiveness of the exploration deployment is poorer. To reduce the target area and effectively guide the further mineral exploration and evaluation, the intergrated geophysical surveying including Induce Polarization (IP) method, Magnetic (MT) method and Electrical Magnetotellurics (EH4) method has been carried out in the Laba mining district. Meanwhile, a new data processing method has been put forward to improve effectively the reliability and effectiveness of data processing and interpretation. The results show that the intergrated geophysical method for ores prospecting has high accuracy, and coincided highly with the geological phenomenon exposed by the exploratory drill holes. It can further narrowed the prospecting target areas, and provide the geophysical basis for the concealed ore prediction and the deployment of the mineral resources exploration for this mining and other analogous in similar areas. Through this study, the understanding and achievements are as follows:(1) Through the physical property parameters measurement of different types of ore rocks and wall rocks outcrops collected from the Laba ore field, the results show that the polarizability of the ore properties is entirely different from that of all kinds of surrounding rocks. So, three profile measurement using IP method was applied to carry out in the ore field with weak and/or thin covering layer. The ore anomalies have been determined, which shows that the IP method has more superiority applied in the weak covered areas and good ore-prospecting results.(2) One section of EH4measuring work has also been carried out in this time, and the standard signal receiver capture frequency if from100KHZ to11.7Hz. The measuring results is consistent with the IP method, and shows more deep information. The results are high accurate indicator.(3) Based on the above work, to comprehensively evaluate and market the prospecting target areas of the mining area, the systematicly magnetic scanning measurement has been carried out, and a new data processing method has been put forward. During the magnetic data processing, the3D Euler analysis, and the derivative horizontally and vertical (Werner, Hilbert) using Fast Fourier Transform filter (FFT) was applied for rectangular blocks. And the FFT transform window is10meters. The calculated universal frequency energy was used to predict the depth and intensity of the apparent remanence magnetization of source (Hilbert). Analytical signal of spectrum frequency of the intensity signal derivative determine and mark the top of the strength blocks in the study area. Using the Blakely algorithm to automatically find peaks in the gridded analytic signal data with square least method of FFT filter along three directions, and determine the depth and size of the remanence magnetization peaks. Source Parameter Imaging (SPI) and Keating Magnetic correlation coefficients for Kimberlite Exploration was used to interpret the results. The remanence anomaly is caused by the magmatic rocks intrued into the Middle-Triassic limestone, and the small anomalies indicates their located depth is not great. A number of positions and drilled boreholes have been identified, the drilling results specify the interpretation by magnetic method with very high accuracy. As a result, it can determine the shape and burial depth of the concealed ore-bearing intrusion bodies. The result shows that magnetic method is helpful to identify porphyry molybdenum (-copper) orebodies under this geological condition.(4) By comparing and comprehensive analysising on the measurement data from the line20measuring position by all the three kinds of geophysical exploration methods, the results showed that measuring position of EH4method is bigger than that of IP method, and the anomaly characteristics are very similar. However, only partial resistivity inosculation have apparent differences. From the data analysis results, the same block of abnormal residual magnetism of magnetic susceptibility value is from0.024to0.033cgs, otherwise the corresponding apparent resistivity of abnormal body is from800to2000Ω.m. Comparing the data interpretation results of the three methods, the abnormal features of the middle section is considerable. On account of the high cost of IP method and EH4method, and the implementation of difficult conditions, MT method has obvious advantages of the lower measurement cost, more quickly implementation analysis and better applicated effect.(5) Based on the analysis and interpretation results of the MT method test studying, the prospecting target areas have been put forward. After comparing the four drilling verification results drilled in line20and line24, the ore-prospecting results of the MT surveying data interpretation are well coincided with the exploratory data. And the granite diorite and porphyry diorite are the molybdenum (copper)-bearing bodies, their occurring depth is buried below the ground from50meters to700meters. At the same time, the limestone of T2b1and T2b2is obviously more conducive to mineralization than the limestone of T2b3. Comparing the results of geophysical anomalies and the drilling geological phenomenon, the interpretation accuracy is up to85%.(6) As such, this paper presented a successful example using intergrated geophysical method to prospect the concealed porphyry molybdenum (-copper) ores in Laba ore field, Shangri-1a county, northwestern Yunnan province, China. And the MT method is more helpful to identify the ore-bearing porphyries under the similar geological conditions for other prospecting areas. Through the deepening researching, the MT method has extensive application value.