Paper Information

Journal:   AMIRKABIR   Winter 2004 , Volume 15 , Number 57-D; Page(s) 192 To 203.


Author(s):  HEZARKHANI A.*
* Department of Mining, Metallurgy and Petrolum Engineering, Amirkabir University

The Raigan porphyry system that intruded Eocene volcano-sedimentary rocks is hosted in a diorite/granodioritic to quartz-monzonitic stock. Copper mineralization is esentialy associated with potassic and less sericitic alteration. Based on the fluid inclusion mineral paragenesis and Field relationships, it is concluded that the main copper mineralization at Raigan system occurred in the temperature range of 350-550 °c (with the mode of 450 °C).
These data also indicate that the hydrothermal system involved both magmatic and meteoric waters, and boiled extensively. In this research, thermodynamic data are used to delineate the stability fields of alteration and ore assemblages as a function of fs2,fo2, aCr and pH (in the fIXed temperature of 460 °c and the pressure of 550 bars) for the chalcopyrite dissolution and deposition. During potassic alteration (> 550 °C), copper solubility is calculated to have been >100,000 ppm, whereas the copper content of the initial fluid responsible for ore deposition is estimated from fluid inclusion data to have been 1200-2000 ppm. This indicates that the fluid was initially undersaturated with respect to chalcopyrite, which agrees with the observation that veins formed at T>500 ° C and contain rarely chalcopyrite. Copper solubility drops rapidly with decreasing temperature, and at 400 °c is approximately 1000ppm, within the range estimated from fluid inclusion data. At the Raigan porphyry system, the temperature never drops to less than 400 °c degree before all the copper separates from the fluid. These calculations are consistent with observations that the bulk of the chalcopyrite deposited at Raigan system, is hosted by veins formed at approximate temperatures more than 450 ± 60 °c, which according to the previous measurements, it is impossible to have a copper deposit. Other factors that may reduce chalcopyrite solubility are increase in pH, decrease in fo2 and a cr. Based on the calculations of this paper, it is proposed that the Raigan porphyry system formed partly in high temperature, high pH, and decrease in aCr that accompanied boiling. The combination of these factors in high temperatures could seriosly prevent the Raigan system to produce an economic porphyry copper deposit.

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