FORMATION CONDITIONS OF GOLD-SILVER DEPOSITS IN THE NORTHERN OKHOTSK REGION, RUSSIA

Abstract

The paper considers the results of geochemical studies of gold-silver ore-magmatic systems and deposits in two of the largest ore districts of the northern Okhotsk region: the Evensk (Turomchinsk ore-magmatic system, the Dal'nee and Kvartsevoe deposits) and Omsukchan (the Dukat deposit) districts. New data on behavior of ore and alkaline elements and fluid inclusions are presented. Two genetic types of hydrothermal gold-silver ore-forming systems (volcanic and volcano-plutonic) are distinguished. The main peculiar feature of volcanic gold-silver ore-magmatic systems is their single-megastage development and bimetal pattern. The volcano-genic trend of magmatism is prevalent. The andesite-granodiorite volcano-plutonic rock association specialized for Au, Ag, and K is a potentially ore-bearing one. The Turomchinsk ore-magmatic system, where the hydrothermal process begins with formation of zones with disseminated sulfide mineralization and extended multicomponent low-contrast geochemical fields, is a typical example of such systems. The main peculiarity of volcano-plutonic gold-silver ore-magmatic systems is their multimegastage and polymetallic mode, as well as their long-term and intricate development, which can be exemplified by the Dukat ore-magmatic system. There, two stages can be distinguished in the magmatic and hydrothermal activity related to ore-bearing volcano-plutonic associations: andesite-granodiorite (the early stage) and rhyolite-granite-leucogranite (the late, terminal stage). The last stage is distinctly specialized for F, B, Ag, Sn, Mo, W, and Pb. In the Dukat system, Sn-Ag and Sn-rare metal mineralization is intensely developed, along with Ag-Pb and Au-Ag ores. Nevertheless, general regularities in formation of ores and related geochemical fields are preserved. The formation of Au-Ag ores in the multimegastage polymetallic volcano-plutonic Dukat ore-forming system and the single-megastage compositionally relatively simple volcanic Turomchinsk system occurred in similar environments: (1) under close temperatures (355-205°C for the Evensk group of deposits and 340-175°C for the Dukat deposit) and pressures (170-20 bar and 135-70 bar, respectively); (2) from low-concentration fluids of similar salt composition (NaCl, KCl, CaCl 2, and MgCl 2)-3.9-0.4 and 6.4-0.4 wt % for the Evensk group of deposits and the Dukat deposit, respectively; (3) at depths less than 1000 m; and (4) in a hydrothermal structure less than 1.5 km long. Mineral-forming solutions that formed Au-Ag and Ag ores substantially differ in composition from magmatic and postmagmatic fluids, which can be explained by dilution of the latter by meteoric waters.