The Varvarinskoye Complex Copper–Gold Deposit in Northern Kazakhstan: Mineral Types and Ore Composition

Abstract

The Varvarinskoye copper–gold deposit, large in terms of Au and small in terms of Cu, was discovered in 1979 in the western part of the Torgai trough in Kazakhstan. It is composed of Devonian volcanosedimentary and intrusive rocks. Ore zones with a thickness from a few meters to a few tens of meters have been identified against a background of disseminated ore mineralization. They are composed of densely disseminated (porphyry) up to massive ores. The ore zones are localized in the thermal metamorphism band of volcanosedimentary and intrusive rocks, while disseminated mineralization is widespread beyond it. Ten ore mineralization types can be distinguished. The Au–Cu mineralization type is common in volcanosedimentary rocks and diorites, while the Au–Cu–Ni ores occur in serpentinites formed after thermally metamorphized intrusive rocks. Densely disseminated and massive magnetite ores are confined to skarns. Sulfide and sulfide–arsenide mineralization can also be observed. Each type of Au–Cu mineralization (pyrrhotite, chalcopyrite–marcasite–pyrite, pyrite, and gold–sulfide–arsenopyrite) can be correlated to an analog of Au–Cu–Ni mineralization (pentlandite–pyrrhotite, chalcopyrite–pentlandite–pyrite, pentlandite–pyrite, and gold–nickeline–gersdorffite). This indicates the uniform ore-forming process in serpentinites and volcanosedimentary rocks with diorites and borrowing of ore material from host rocks, at least, partially. The gold–chalcopyrite mineralization type, synchronous to the thermal metamorphism and analogous to mineralization of the alkali stage at porphyry copper deposits, has no analog in the Au–Cu–Ni mineralization. Gold is present in all mineralization types, but gold–chalcopyrite and sulfide–arsenide are the most enriched in Au. Eighty ore minerals have been found at the studied deposit. Pyrrhotite disulfidization and chalcopyrite pyritization are universal in occurrence. These processes likely led to the formation of chalcopyrite–marcasite–pyrite and chalcopyrite–pentlandite–pyrite mineralization after pyrrhotite and pentlandite–pyrrhotite types, respectively. Invisible gold present in chalcopyrite was enlarged during these processes.