Genesis of the koka gold deposit in northwest eritrea, ne africa: Constraints from fluid inclusions and C-H-O-S isotopes

Abstract

The Koka gold deposit is located in the Elababu shear zone between the Nakfa terrane and the Adobha Abiy terrane, NW Eritrea. Based on a paragenetic study, two main stages of gold mineralization were identified in the Koka gold deposit: (1) an early stage of pyrite-chalcopyrite- sphalerite-galena-gold-quartz vein; and (2) a second stage of pyrite-quartz veins. NaCl-aqueous inclusions, CO 2 -rich inclusions, and three-phase CO 2 -H 2 O inclusions occur in the quartz veins at Koka. The ore-bearing quartz veins formed at 268 °C from NaCl-CO 2 -H 2 O(-CH 4 ) fluids averaging 5 wt% NaCl eq. The ore-forming mechanisms include fluid immiscibility during stage I, and mixing with meteoric water during stage II. Oxygen, hydrogen, and carbon isotopes suggest that the ore-forming fluids originated as mixtures of metamorphic water and magmatic water, whereas the sulfur isotope suggests an igneous origin. The features of geology and ore-forming fluid at the Koka deposit are similar to those of orogenic gold deposits, suggesting that the Koka deposit might be an orogenic gold deposit related to granite.