CHROMITE MAPPING USING INDUCED POLARIZATION METHOD BASED ON SPREAD SPECTRUM TECHNOLOGY

Chromites mapping using geophysical methods proved difficulty during the past several decades, because most Chromites are feature as podiform deposits which always pinch out and reappear in the same survey line. Several ground-based geophysical methods including: Microgravity, Magnetic and Controlled Source Audio MagnetoTelluric (CSAMT) have been used for chromites mapping. However, the result did not satisfy the need of the mining geologists. A successful case history of chromites mapping using induced polarization method based on spread spectrum technology in Luobusa Ophiolite, Southern Tibet, is presented in this paper. Wireless Sensor Network (WSN) based electromagnetic system (SinoCGI) is used for the data acquisition, which is its first time using in China. Luobusa chromites mine is one of the chromites deposits with maximum mineral production in China. But drilling and ground-based geophysical exploration did not meet the good deposit for sustainable yield in the past several years. Experiment measurement in the lab indicates that the chromites samples and its host rocks sample characterize by different rock apparent resistivity and chargeability. Based on this, we conduct induced polarization method based on spread spectrum technology to map the potential favorite deposit. We carried out around 500 IP scanning stations and three 2-D IP sounding profiles in the working area with acreage of 0.5 square kilometer. The favorite chromites deposits which features as conduct and low polarizability has been mapped. Four boreholes have been drilled to verify the ore delineation by IP method, three of them met chromites, the other one met chromites mineralization. The result gives a new recognition on geophysical methods to chromites mining geologists in China. Introduction Chromite exploration is challenging (Mohanty et al., 2011). It is more difficult to explore podiform chromites, because this kind of ore body produces only very weak geophysical response. Luobusa chromites deposit is a typical podiform chromites lie in southern Tibet of China. Traditional geophysical methods, such as gravity, magnetic method and CSAMT, failed in exploring chromites in Luobusa, because of small podiform ore body and complex geological structure. In order to map the concealed ores, a distributed high precision 2D/3D IP instrument based on wireless sensor network (WSN), spread spectrum technology, and relative IP phase was developed. This new IP instrument can acquire high quality IP data with more than 150 channels simultaneously. We tested the new IP instrument in Luobusa, and got a satisfied result. Background and geological setting About 50 years ago, a herdsman in Luobusa, which is located in southern Tibet, found some chromites and told government agency. This revealed the biggest chromites deposit in China. About 500 boreholes were drilled, and the deposit is larger than 5,000,000 tons. However, it was very difficult SAGEEP 2013 Denver, Colorado USA http://www.eegs.org to find new ore body by drilling in recent years. Many geophysical methods were used in this area, but drillings suggested by geophysical methods met no ore body. Luobusha chromites deposit (Figure 1) is located in Luobusha town, Qusong county, Shannan district, Tibet. It is in the southern of Gangdisi Mountains and Nien-ch'ing-t'ang-ku-la Mountains, and is between the downstream and midstream of Yarlung Zangbo River with longitude as 92.15-92.24 degree, latitude as 29.18-29.23 degree. Luobusa is located in collision area between Gangdisi block and Himalaya block(Figure 2). The collision generated a long and deep fault. Ultra-mafic magma went up from the fault and brought chromites deposit in ophiolite. Figure 1: Location of Luobusha Chromites Deposit. Red rectangular shows the location of working area. Figure 2: Geological map of Luobusa and neighbor area (after WENJI BAI et al. 2000) SAGEEP 2013 Denver, Colorado USA http://www.eegs.org