Geochemical effects of endoskarn formation in the Mazraeh Cu–Fe skarn deposit in northwestern Iran

Abstract

The Mazraeh Cu–Fe skarn deposit, located in the Ahar region of northwestern Iran, is currently being mined and contains reserves of about 400,000 tons grading 1.2 wt% Cu. The deposit contains both exoskarn formed within the calcareous wall rock and endoskarn formed within the associated granodiorite. Mineralogically, there are three types of skarn at Mazraeh: (1) pyroxene exoskarn; (2) garnet endoskarn and exoskarn; and (3) secondary epidote skarn. Locally, gradational alteration of the Mazraeh granodiorite to garnet endoskarn is present over a width of one metre. Mass-balance calculations show that garnetization of the Mazraeh granodiorite was the result of interaction with a hydrothermal solution enriched in Si, Fe 2+ , Fe 3+ , Mn, Ca, Mg, P, Ag, Cu, Zn, La, Pb, Cd, Mo, and Y. These elements are thought to have originated from the magma. The calcareous wall rocks (limestone and marble) were the main source of Ca and Mg within the endoskarn. This hydrothermal alteration resulted in an average 157% volume gain in the affected wall rocks. The dilatent structures and open spaces were filled by paragenetically younger minerals, especially magnetite, pyrite, chalcopyrite, bornite, covellite, chalcocite, epidote, calcite and quartz, during the late stage of alteration. During the endoskarnification process, primary biotite, hornblende and titanite in the Mazraeh granodiorite have been replaced by chlorite and rutile, and plagioclase and K-feldspar have been replaced by garnet. These reactions have leached (removed) K, Na, Ba, Rb and possibly Sr and added Si, Fe, Mn, Ca, Mg, P, Ag, Cu, Zn, La, Pb, Cd, Mo, W and Y. Orthogonal jointing in the Mazraeh pluton played a significant role in the channelling and migration of the hydrothermal solutions in the pluton and development of the endoskarn. Although most of the copper and iron were precipitated in the exoskarn due to reaction between hydrothermal solutions and marble and limestone, locally significant skarn mineralization is also developed in the pluton. Although significant sulphide mineralization was not precipitated during garnet endoskarn formation, porosity and permeability created by this process served as the sites of sulphide mineral precipitation during late stages of skarn formation. Thus, the exploration for, and delineation of garnet endoskarn may result in the discovery of ore. Field guides for the exploration of garnet endoskarn include: (1) its high physical relief due to abundant garnet which is highly resistant to surface weathering processes; (2) predominance of red-brown andraditic garnet species over grossular-rich ones; and (3) local development of secondary malachite and azurite after primary Cu and Fe sulphides.