Prospecting Practice Of Gold-Copper Deposits In Shijie Province Of Myanmar And The Prospecting Methods In Volcanic Areas

The Union of Myanmar, located in the west of Indochina Peninsula, is surrounded by Bangladesh in the west, India in the northwest, China in the north and northeast, Laos in the east and Thailand in the southeast. In Myanmar, there are nearly N-S-trending mountains and hills, which belong to eastern Himalayan orogenic system; they suddenly bend towards west in northmost of Myanmar that might be caused by the drift of Indian sub-continent toward Eurasia. The orogeny has been in progress since Early Tertiary. Myanmar can be divided into three terranes, from east to west, they are Shan State-West Malaysia-Sumatra terrane, Basin terrane in central Myanmar and Arakan Yoma terrane in the west. These terranes are bounded by faults which were formed by the subduction between Indian plate and Eurasian plate.Gaoling gold-copper deposit in Sagaing in Myanmar is located within Cenozoic andesite, about200km northwest of Mandalay. The orebodies, occurring in the east edge of the middle part of Basin terrane in central Myanmar, belong to volcanic-hydrothennal gold-copper deposit. According to the analysis and research on the geological characteristics of metallization, it can be found that the ore-bearing stratum (ore-bearing rock series) is continental andesite. The orebodies, situated in structural fissures in andesite, were formed by self-alteration of andesite in structural fissures of volcanic rocks-hydrothermal reformation in the later stage of sub-volcanic rocks. The stratified, stratoid or lenticular orebodies extends about1000 m and300m along strike and dip directions respectively; the boundaries between orebodies and wall rocks are clear. The dip of stratified orebodies is22°and the dip angle is82°, their occurrences are nearly upright or in very high angle. Within the orebodies there occurred kaoline-bearing argillizatized belts which were seen as interlayers, their thicknesses show the characteristics of branch, compounding, pinch-out and reappearance along the strike directions of orebodies; the wall rocks are of light color pyritized and silicified andesite of which the volumic mass is lighter than ores. The ores show grayish-black color, the volumic mass is large that is up to4～5g/cm3. The ores are massive, the ore-forming elements are mainly of copper, followed by gold and sulfur; the grade of copper is higher while that of gold is lower, thus, these ores are gold-bearing copper-rich ores. The prospecting criteria of this deposit are the geophysical characteristics of low resistance and high polarization.According to the study on the geological characteristics of metallization of Gaoling gold-copper deposit, combined with the collected regional geological data of Myanmar, the main conclusions and understanding in this paper are as follows:(1) Metallogenic epoch:since the determined age of andesite samples is32-35Ma (Eocene～Oligocene) by K-Ar isotopic method, it can be concluded that the outcropped andesite in the ore area was originated from the volcanic eruption during Himalayan epoch. The orebodies are situated in Tertiary andesite, and no other sedimentary rocks or older volcanic rocks can be found around, therefore, it can be inferred that this gold-copper deposit was formed in Eocene～Oligocene.(2) Source of metallogenic materials:δ34S values of andesite in Gaoling gold-copper deposit are-0.50‰～1.30‰with the average of+0.4‰; the measured δ34S values of massive sulfide ores are-0.14‰～1.20‰with the average of+0.53‰, the variations of δ34S values of both wall rocks and ores are lower than3‰, their averageδ34S values are between+0.4‰and+0.53‰that is close to meteorite (0.2‰), thus, it can be concluded that the metallogenic materials of Gaoling gold-copper deposit were derived from upper mantle and erupted with volcanic rocks.(3) Deposit genesis:the comprehensive analysis on above measured isotopic data and regional geological data of metallogenic belt shows that Gaoling gold-copper deposit occurs near the deep and great fault, the metallogenic materials of gold and copper gushed up from upper mantle along this fault, then erupted andesite within surface continental interactive marine-continental facies (many continental plant fossils could be found on the earth’s surface-continental eruption is predominant) and formed vein-like, stratoid or lenticular gold-copper hydrothermal orebodies in the structural fissures in andesite. The intrusion of later diorite porphyrite provided the already formed orebodies with heat and some metallogenic materials, made these orebodies re-enrich, and finally formed the rich gold-copper orebodies.(4) Prospecting methods in volcanic region:generally, the volcanic rocks must be close to a deep and great fault; prospecting (such as copper-nickel, gold and copper ores) within the volcanic eruption regions in island arcs and in suture zones of continental crust collision is worthy of notice.The specific steps are as follows:(1) Geophysical aerial survey should be carried out firstly in the target area. Then, the methods of starting regional geophysical exploration should be selected among aeromagnetic survey, airborne electromagnetic survey, airborne gravimetry and airborne radioactivity survey based on the main differences of physical properties of the geological bodies in the project area; then regional geophysical anomalies should be delineated initially with the help of interpretations of satellite remote sensing images; finally, assisted by field geological verification of surface anomaly areas, geological boundaries and anomaly range should be divided.(2) Geochemical exploration should be carried out section by section within the above anomaly areas in order to find mineralization anomalies, including dispersion train survey along the first-order water system or heavy concentrate survey and reconnaissance along water systems.(3) When the mineralization anomalies were found on the earth surface within volcanic rocks, artificial exhumation perpendicular to the anomaly strikes should be carried out for surface orebodies, fracture zones or altered zones. Electrical method, gravity, magnetic survey and soil geochemical profiling are recommended in delineating and tracing concealed orebodies or mineralized belts. Eventually, the purposes are to expose orebodies or mineralized bodies through geological mapping and light mountain engineerings.