The Lunnyi gold-silver deposit (Northeast Russia): Geological setting and mineral composition of ores

The Lunnyi gold-silver deposit is situated within the Okhotsk-Chukotsk marg inal-continental volcanic belt. It is confined to a long-lived intrusion-do me edifice, which is complicated by a volcano-plutonic depression in the so uth. The deposit area is composed of sandy-argillaceous terrigenous-marine sediments, Early Cretaceous rhyolites and siltstones with coal interbeds, E arly-Late Cretaceous andesites, and Late Cretaceous rhyolites. The dome edi fice includes a large polyphase intrusive massif framed by satellites and n umerous concentrically situated dikes of andesites, diorite porphyries, and rhyodacites. The intrusion-dome uplift is separated into blocks by a serie s of large sublatidutional and northeastern faults. The northeast trending feather faults are ore-hosting in many cases. The deposit was formed during two megastages: I early (postvolcanic); and II late (postintrusive) stages . The granite intrusion separates these megastages. The isotope age of the postintrusive megastage determined using the Rb-Sr technique is 87 +/- 2 Ma . Ore-hosting rocks underwent regional metasomatic alteration (propylitizat ion), pre-ore beresitization (quartz-sericite-carbonate-pyrite rocks), and synore metasomatic alteration with the formation of quartz-feldspar rocks. Ore bodies are vein zones including one or several main veins and a halo of the veinlet-disseminated mineralization. Both of them have nearly equal th ickness. The gold-silver mineralization is related to gold-quartz-selenide and quartz-silver-sulfide stages of the first megastage and quartz-rhodonit e-carbonate and quartz-silver-polymetallic stages of the second megastage. The textural studies suggest that the ore deposition occurred mainly in ope n spaces; therefore, the rhythmical-banded, crustified, and breccia fabrics prevail. More then 70 mineral species, including 18 silver and silver-bear ing minerals, have been found in the deposit. Sulfides and silver sulfosalt s, silver-bearing tetrahedrite-tennantite minerals, and the Au-Ag mineral s eries predominate among silver minerals. The Ag-Cu, As-Sb, and S-Se isomorp hous substitutions are widespread in the silver-bearing minerals. Within ea ch megastage, a regular decrease in fineness of native gold from electrum t o kustelite (up to native silver during the second megastage) and a synchro nous increase silver content in silver-bearing tetrahedrite up to freibergi te is observed. The Au/Ag ratio varies from 1 : 170 to 1 : 1900. A substant ial portion of ore mineralization is concentrated in ore shoots. The deposi t contains large silver reserves and small gold reserves.