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MASS CHANGES ASSOCIATED WITH HYDROTHERMAL ALTERATION ZONES IN THE SAR-CHESHMEH PORPHYRY COPPER DEPOSIT, IRAN

A. Hezarkhani

Abstract

The Sar-Cheshmeh porphyry copper deposit is located in southeastern Iran and is associated with diorite/granodiorite to quartzmonzonite of Miocene age. Copper mineralization was accompanied with both phyllic and potassic alteration. Three main mineralization related alteration episodes (potassic, transition, and phyllic alterations) have been studied in terms of mass transfer and element mobility during the hydrothermal evolution of the Sar-Cheshmeh deposit. In order to illustrate these changes quantitatively, isocon plots (Grant, 1986) have been applied. Isocon plots illustrate that Al, Ti were relatively immobile during alteration, and that mass was essentially conserved alteration (pervasive alteration form). At all stages in the evolution of the hydrothermal system, the volume changes is close to zero. In the potassic alteration zone, there is an obvious enrichment of K, and depletions of Na, Ca, and Fe. These changes were due to replacement of plagioclase and amphibole by Kfeldspar and biotite, respectively. Potassic alteration was associated with large addition of Cu as might be expected from the occurrence of disseminated chalcopyrite in this zone. In the transition alteration zone, Ca, Cu and Fe were added, Na and Mg were relatively unchanged, and K and Ba were depleted. The loss of K and Ba relatively to Na reflects replacement of K-feldspar by albite. Phyllic alteration was accompanied by the depletion of Na, K, Fe and Ba and strong enrichment of Si and Cu. The losses of Na, K and Fe reflect the sericitization of alkali feldspar and destruction of ferromagnesian minerals. The addition of Si is consistent with the widspread silicification, which is a major feature of phyllic alteration and the addition of Cu with mobilization from the both potassic and transition zone which is depleted in this element.

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