Geological Structure and Ore Mineralogy of the Juliette Gold-Silver Deposit, Northeast Russia

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Abstract
Julietta is a rich epithermal gold-silver deposit of the low-sulfidation, adularia-sericite type, located in the Cretaceous Okhotsk-Chukchi volcanic-plutonic belt 250 km northeast of Magadan. The deposit was discovered in 1989 by a regional soil geochemical survey in an area previously considered barren on the basis of a regional stream-sediment survey. The deposit has not been completely explored, but presently is in the feasibility stage; proven reserves are 26 metric tons of Au (grades averaging 23 g/t) and the Au/Ag ratio is about 1:10. The deposit occurs on the periphery of a large volcanic-tectonic depression. Host andesite, andesite-basalt lava, corresponding subvolcanic bodies, and tuff are cut by Early Cretaceous quartz diorite stocks. Six vein zones occur in tensional and compression fissures. Ore shoots and smaller bonanzas comprising most of the gold reserves are located in flexures of the ore-host fissures. Ore mineralization was preceded by intense voluminous propylitization and linear sericitization (sericite + quartz + pyrite + ankerite). Orebodies occur within the low-temperature propylite (pyrite + calcite + quartz + chlorite + hydromica). Colloform-crustiform banded textures are commonly observed in the ore. Most of the ore minerals occur within thin, cyclically repeated, fine-grained bands of a hydromica-carbonate-adularia-quartz aggregate. Ore-bearing, fine-grained bands probably formed by periodic fracturing of the veins, whereas barren bands were deposited in relatively quiet conditions. “Micro-stalactites” and other gravitational textures demonstrate that minerals grew in open spaces. Ore-host structures gradually opened during mineralization. Gangue minerals are primarily quartz, various carbonates (calcite, dolomite, Fe-dolomite [Mg:Fe>2:1], parankerite [Mg: Fe = 2:1], ankerite [Mg: Fe = 1:1], and mesitite [Mg: Fe = 1:1]), and minor hydromica and adularia. Major ore minerals include pyrite, sphalerite, chalcopyrite, galena, tetrahedrite, silver sulfosalts, native gold, and custelite (Au: Ag = 9: 1). Ore mineralization occurred in two stages-an early, post-volcanic stage and a late, post-granitoid stage. The early stage contains most of the precious metals and includes two substages-(1) gold-polymetallic (200 to 260° C) and (2) gold-silver-sulfosalt (90 to 200° C). The late stage also includes two substages-(1) carbonate-rhodonite-quartz (260 to 380° C) and (2) postore quartz-carbonate. Fluid-inclusion homogenization temperatures demonstrate complex temperature zoning. Fluid composition was mainly aqueous, with Cl−, HCO3−, Na+, K+, Ca2 +, and a salinity less than 4 to 9%. The isotopic age of the deposit is 136 ± 3 Ma by the Rb-Sr method on adularia. The 87Sr/86Sr ratio is about 0.7075 ± 0.0005, indicating a mixed crust-mantle source of the vein matter. Chloride complexes transported gold and silver. The gas composition of the fluid suggests a near-surface, “closed” paleohydrothermal system. A major ore-forming factor could have been high seismic activity related to intrusion of the subvolcanic bodies. Breccias and multiphase veinlets may be related to relatively large-magnitude earthquakes, whereas cyclically banded ores may reflect local pH variations caused by smaller earthquakes.