Copper deposition by fluid mixing in deformed strata adjacent to a salt diapir, Dongchuan area, Yunnan Province, China

Abstract

The Dongchuan area is the largest of five copper mining areas in a north-trending rift containing sandstone, shale, and carbonate rocks of the Kunyang Group of middle Proterozoic age. In the Dongchuan area, the group is about 18,000 m thick. Three formations, about 1,200 m thick, in the lower part of the group contain the copper deposits. They are, from younger to older: the Heishan Formation, mainly carbonaceous slate; the Laoxue Formation, mainly stromatolitic dolostone; and the Yinmin Formation, chiefly purple shale and sandstone. These formations were intruded by mafic igneous rocks. Structurally below the Yinmin Formation is a thick breccia consisting of clasts of Yinmin-type shale and sandstone, diabase, and other rocks. The three ore-producing formations and the breccia are exposed in a band of steeply dipping strata extending across the Dongchuan area.Most of the copper ore occurs in the lowermost dolostones of the Laoxue Formation. Stratabound ores characteristically occur as disseminations of sulfides along stromatolitic algal layers. Locally, high-grade vein-type ores crosscut the dolostone. Both ore types contain bornite, chalcopyrite, chalcocite, and other minerals. Fluid inclusions in quartz veinlets contemporaneous with copper mineralization contain liquid, vapor, and daughter minerals NaCl, CaCl 2 , KCl, and BaCl 2 . They homogenize between 200 degrees and 280 degrees C.We believe that the breccia was produced by tectonically controlled salt diapirs from which the salt was removed by hydrothermal leaching, and that the copper ores are associated with a thermal anomaly that resulted from this salt diapirism and contemporaneous diabase intrusion. Hot brines circulating upward along the flanks of the diapirs leached copper from the oxidized Yinmin Formation and carried it to the base of the overlying Laoxue Formation. Sulfide-rich fluids were formed by reaction of evaporite sulfates with decayed algal material in the dolostones. Copper sulfide deposition in the dolostones resulted from mixing of these fluids. Continued circulation leached all of the remaining salt from the diapir, and the overlying and surrounding rocks collapsed to form the breccia.