Evolution of the Garmab-e-Paein native copper-rich volcanogenic massive sulfide deposit from northeast Iran: Insights from sulfur isotope and chlorite chemistry

Abstract

The Garmab-e-Paein Cu-Ag volcanogenic massive sulfide (VMS) deposit occurs as stratiform and stratabound orebody within a specific stratigraphic horizon in a Late Cretaceous volcano-sedimentary sequence of the Sabzevar zone, northeast Iran. The host rocks to mineralization are andesitic-trachyandesitic volcanic and volcaniclastic rocks. Based on textural and mineralogical studies, the VMS mineralization is comprised of four ore facies: 1) vein-veinlets (stringer) containing chalcopyrite, pyrite and minor magnetite, 2) massive ore, dominated by pyrite and minor chalcopyrite, 3) bedded ore containing laminated pyrite, and 4) exhalite containing Fe-Mn oxide-hydroxides such as hematite, psilomelane and pyrolusite. Wall rock alteration styles are chloritic, minor silicic and secondary argillic. There is a distinct metal zonation in the massive sulfide orebody; Au, Pb, As, and Ag contents and the Cu/Zn ratio increase vertically from the stringer to the massive facies, but decrease laterally from the stringer to the bedded facies. Zinc and Mn increase from the stringer to the bedded ore facies. Sulfur isotope values for pyrite range from −1.7 to +5.1‰, with average +1.4‰. The δ34S values of massive ore facies (−1.7 and +2.7‰) increase downward toward the stockwork (+2.2 to +4.3‰) and laterally outward toward the bedded (+5.1‰) zones. The near-zero sulfur isotope values indicate that most of the sulfur was derived from leaching of underlying host volcanic rocks by hydrothermal fluids circulating in the high temperature reaction zone.Extensive disseminations and vein-veinlets of native copper mineralization formed within the hanging wall Late Cretaceous volcanic rocks and younger Paleocene conglomerates. The native copper mineralization is accompanied by chlorite and zeolite alteration, and both formed after formation of the Garmab-e-Paein VMS deposit during diagenesis, burial metamorphism and uplift. The deposit is now potentially economic due to the native copper mineralization.Two generations of chlorite are observed in the deposit: Chl-I is accompanied pyrite and chalcopyrite, in the VMS mineralization system, and the second Chl-II occurs along with zeolite and native copper mineralization. The EPMA results show that Chl-I is more Fe-rich than Chl-II.