An analysis and measurement of joints developed in typical orefields of the Gejiu tin-copper polymetallic ore concentration area, Yunnan Province

Abstract：Gejiu is one of the largest polymetallic tin ore concentration areas in the world and belongs to a granite-related magmatic-hydrothermal system. Its ore-controlling structural characteristics are very obvious. In this paper, the authors selected the joints developed in the Songshujiao orefield, Laochang orefield and Kafang orefield as the research objects, measured and statistically analyzed the modes of occurrence of joints developed in tunnels in combination with surface, and made a comparative analysis of the rock-controlling structures and ore-controlling structures. Statistics show that the variation regularity of the predominant joints within the orefields has several characteristics from north to south and in vertical direction： 1) the main predominant ore-hosting joints are in NE, NW, NS and EW directions, especially the NW-trending joints constitute an important ore-hosting structure in recent exploration and development of the western segment of the ore concentration area. 2）The predominant directions of the joints developed in granite are different in the north and south parts of this mining area. The directions of joints in the north part are single and concentrated, being mainly NW306°-312° and NE45°, while orientations of the joints in the south part are complex and are mainly NE50°, N-S, EW, NW312°-324° and WNW280°. 3) Statistical measurements and analyses of the joints in the carbonate rocks suggest that the joint development in the north segment is relatively simple, while that in the south part of the mining area is complex. 4) The joints in the Songshujiao orefield show some regularity in the vertical direction. From surface to the depth, the orientation of joints in carbonate strata is NE50° in the shallow part and NS-striking in the depth; the orientation of joints in the granite is mainly NW306°-312°in the shallow part and NE45° in the depth. The regional stress field of middle-late Yanshanian metallogenic period in Gejiu area was NW-SE trending compressive stress and then converted into the NS trending reverse shear stress. The newly discovered NW-trending ore-hosting joints constitute an important structure in the west segment, and the scales, advantages and origins of the ore-hosting joints in the north segment and the south segment of the mining area are obviously different. These phenomena deserve much attention in the further exploration and prospecting work. The NE and NS trending joints connected the granite body with carbonate rocks and served as the channels of heat, water and ore source. The results obtained by the authors will provide geological structural evidence for the relationship between ore field structures and mineralization and can guide further ore-prospecting work in the depth and the outskirts of the ore district.