Abstract

The Pliocene Cirotan gold deposit provides an exceptional example of mineralized breccias, and in particular cockade breccias. Analysis of these breccias, approached through both field observation and image analysis on photographs, made it possible to determine their method of graded formation. The mineralized structure, as much a 25 m thick in places, is a right-lateral strike-slip fault that evolved to a normal fault at the end of the system's development. It consists of juxtaposed breccia bodies separated by major slip, or "septa," which formed at a major inflection of the host fault. The breccia bodies all show a similar internal organization. From footwall to hanging wall, one passes from a siliceous breccia with angular clasts to a thick infill of cockade breccia showing a reverse grading with small centimetre-size cockades (aspect ratios of 1:10) passing to larger (up to 1 m) more equant cockades (aspect ratios 1:3) in contact with the hanging wall. The different stages of cockade formation demonstrate genesis in an open milieu through rolling-accretion and collapse of the clasts, which show signs of mechanical attrition. The large cockades are formed of an agglomeration of several small cockades. Ore deposition occurred during a pulsating process that was active throughout the complex evolution of the structure. The final ore stage, filling the intercockade spaces, marked the end of the structure's evolution. High fluid pressures and the local structural context combined to maintain open the hydrothermal channelways, enabling cockade growth and collapse. The process of breccia formation at Cirotan was therefore exclusively of hydrothermal origin in an active tectonic context. It represents a self-organizing system within a fault zone.

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