Abstract
The Chuxiong basin, located in southwest China, is well known as a mineralization area of red-bed type copper deposits in China. These deposits are characterized by mineral zoning, which is especially true for the Dayao deposits. The mineral zoning is consistent for both horizontal and vertical zoning; from the base (center) of the ore body to the top (outermost), the mineral zones are from hematite, chalcocite, chalcocite + bornite, and bornite + chalcopyrite to pyrite. We studied the mineral zoning in detail using a thermodynamic phase diagram method, such as logfO2-logfS2, pH-logfO2, and pH-Eh, and discussed the constraints on the order of the minerals precipitation under different physiochemical conditions. It is indicated that changes in temperature have little effect on pH and Eh in the formation of minerals. S2− is stable only below 473 K, and the forming temperature of chalcocite must be below 473 K. In this paper, we also explain the mineral zoning formation mechanism and propose that the main controlling factor of mineral zoning is pH. Because this mineral zoning is widespread in sediment-hosted deposits, studies on this mechanism can considerably promote better understanding of the genesis of ore deposits in order to guide the exploration.
Highlights
Sediment-hosted stratiform copper deposit (SSC), concerned by many researchers [1,2,3,4,5,6,7,8,9,10,11], occurs in many places around the world [7, 12,13,14,15]
Among the various physical and chemical conditions, pH is the vital factors controlling the order of different sulfide precipitation and explaining mineral zoning
The calculations of the Eh, pH, log fS2, and log fO2 within the principal metallogenic temperature of the Dayao redbed type copper deposits and the analysis of phase diagrams resulted in the following conclusions: (1) The zoning pattern and mineral assemblage shown in the phase diagram are in accordance with the macroscopic and microscopic geological features of the deposits
Summary
Sediment-hosted stratiform copper deposit (SSC), concerned by many researchers [1,2,3,4,5,6,7,8,9,10,11], occurs in many places around the world [7, 12,13,14,15]. Mineral zonation both laterally and vertically from pyrite to chalcopyrite to bornite to chalcocite to hematite is characteristic of SSC copper deposits [11, 22]. The same mineral zoning is found in red-bed deposits widely, one of the basic broad categories in SSC deposits [1, 6, 22, 30,31,32]
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