Erdenetiin Ovoo Porphyry Copper-Molybdenum Deposit in Northern Mongolia

Erdenetiin Ovoo, the largest porphyry copper-molybdenum deposit in Mongolia (1.78 Gt @ 0.62% Cu, 0.025% Mo), is exploited by the Erdenet mine. It is located within the Orkhon-Selenge volcano-sedimentary trough which was developed on an active continental margin. The geodynamic evolution of the trough involves an early intra-continental stage, comprising rifting of a shallow continental shelf, accompanied by the emplacement of sub-aerial Permian mafic and felsic, and Triassic mafic volcanics. The Permian volcanics are predominantly alkali-rich trachyandesites, occurring as interlay ered flows and pyroclastics of the Khanui Group, overlying a Vendian (late Neoproterozoic) to early Cambrian basement with Palaeozoic (Devonian) granitoid intrusions, and Carboniferous sediments. Plutons, ranging in composition from diorite to granodiorite, quartz syenite and leucogranite intrude the Permian volcanic succession and exhibit similar compositional trends as the host volcanics. This suggests the intrusions are related to, and possibly coeval with, the volcanic rocks. The Triassic Mogod Formation volcanics, which are largely composed of trachyte flows, trachyandesite and basaltiotrachyandesite, directly overlie the Permian sequence. Early Mesozoic porphyritic subvolcanic and hypabyssal intrusions, which are genetically associated with the trachyandesite volcanics, are related to a continental collisional setting. These include syn-mineral granodiorite-porphyry intrusions which form shallow bodies, occurring as elongated dykes or small, shallow stocks. These porphyries vary from quartz diorite through granodiorite to granite in composition. They are characterised by porphyritic textures (up to 40% phenocrysts) with plagioclase phenocrysts set in a fine-grained groundmass of K feldspar, and are found in the core of the hydrothermal systems, where they are associated with high-grade ore.

Three principal alteration zonations are developed within the Erdenet deposit (Kominek et ah, 1977, Khasin et ah, 1977), from the core to the periphery, namely: i) sericitic (quartz-sericite) and late siliceous, ii) intermediate argillic (chlorite-sericite), and iii) propylitic (chlorite and epidote-chlorite).

\ Researchers have progressively evolved an understanding of the paragenseis of mineralisation at Erdenet \ jjnce the mid 1970s. The most recent observations (Gavrilova et ah, 1990) suggest: i) pre-ore quartz-sericite; followed by the ore stages of ii) quartz-chalcopyrite-pyrite; iii) quartz-pyrite-molybdenite-chalcopyrite; iv) quartz-chalcopyrite-tennantite; v) quartz-pyrite-galena-sphalerite; vi) over-printing bornite-chalcocite-covellite; and post-ore vii) gypsum-calcite with pyrite. The first two ore stages are dominated by vein stockworks, while the succeeding three phases are localised by dykes and associated fracturing. All of these phases however, overprint, and largely obliterate, an earlier weak potassic alteration with associated chalcopyrite. The early potassic phase occurs as secondary biotite and magnetite, followed by pink feldspar veining, and is only encountered as remnants in the less fractured, deeper, central sections of the deposit.

The hypogene stage is characterised by chalcopyrite, bornite, covellite and minor chalcocite, and the oxidation stage by Cu carbonates, oxides, phosphates and sulphates, native Cu and ferrimolybdite. The oxide zone overlies a 30 to 300 m thick supergene enrichment blanket where secondary chalcocite replaces hypogene chalcopyrite and bornite-covellite assemblages in stockworks and sheeted veins.

The proposed magmatic model for the Erdenet district involves multiple partial melting of upper mantle material, a changing level of melting and vertical movement of the magma chamber. In the late crustal phase, large chambers of dioritic magma formed, cooling and fractionating to produce the full spectrum of subvolcanic and shallow porphyries ranging in composition from diorite to granite.