GEOLOGICAL STRUCTURE AND ORE MINERALOGY OF THE KHAKANDZHA SILVER AND GOLD DEPOSIT (NORTHEASTERN RUSSIA)

Abstract

Some specific features of the geological structure and ore mineralogy of the Khakandzha silver and gold deposit are discussed. The main ore masses are accumulated in complex, gently inclined, veined metasomatic zones. The ore zones are localized near the neck of a paleovolcano. The deposit was formed during several phases of volcanic activity. Orebodies are enclosed in both hydrothermally altered rocks and in only slightly altered ones. Manganese minerals are common in the primary ores and in the oxidized zone at the upper and intermediate levels. The geological structure of the deposit depends on the volcanic structure. The paleovolcano was formed in the Late Cretaceous within the Okhotsk branch of the Okhotsk-Chukotka volcanic belt. The stratigraphic sequence of the paleovolcano consists mainly of coarse pyroclastics (agglomerate breccias, psephitic tuffs, and ignimbrites) and several rhyolite, rhyodacite, and dacite bodies extruded at various times. The extensive metasomatic alteration of the wall rocks is due to the saturation of magmas with gases and fluids, especially near the volcanic neck. The metasomatically altered rocks and ore mineralization are concentrated in the most permeable zones of the felsic volcanic sequence. Two gold- and silver-bearing mineral complexes occur at the deposit: quartz-adularia and quartz-rhodonite-carbonate. Gold and silver minerals were deposited during the latest ore stages in the above-mentioned mineral assemblages. The common sequence of mineral deposition with decreasing temperature was disturbed by the rejuvenation and deposition of high-temperature quartz and carbonate assemblages. The economic ores of the Khakandzha ore field are hosted by the dacite and rhyodacite extrusive bodies and fluidal lava flows. Within these structures, hydrothermal solutions mixed with meteoric waters. The ore bodies leave zones of metasomatic alteration somewhere at the periphery of the deposit. The veinlet-disseminated ores belong to the porphyry type. According to biostratigraphic and isotopic geochronological dating of the Amkinsk Sequence, rhyolite, dacite, and rhyodacite volcanic bodies were hot at the time of ore deposition. The temperature gradient between the magmatic bodies and the host pyroclastic rocks favors the filtration of hydrothermal fluids into the water-saturated stratified cool pyroclastic rocks.