Geology and Genesis of the Natalka Gold Deposit,Northeast Russia

The Natalka lode gold deposit, also known as the Matrosov mine, is located in the Magadan region of northeastern Russia at 61° 39′ N, 147° 50′ E. The deposit was discovered in 1943 and production started in 1945. The mine has produced more than 75 metric tons of gold, with an average grade 4 g/metric ton (mt), and has reserves of about 450 mt.

The Natalka deposit occurs along the southwestern flank of the Yana-Kolyma metallogenic belt and is confined to the major, NW-trending Tenka fault. The deposit is hosted by Upper Permian carbonaceous sediments, subjected to greenschist metamorphism. The ore zones occur along a Z-shaped, strike-slip fault zone that extends for about 12 to 13 km. In plan view, the ore zones are about 5 km long and 100 to 200 m wide in the northwest portion, 350 to 400 m wide in the central portion, and 600 m wide in the southeast portion of the deposit.

The main ore minerals are arsenopyrite and pyrite, which comprise about 95% of the sulfides, along with subordinate pyrrhotite, Co-Ni sulfarsenides, sphalerite, chalcopyrite, galena, native gold, ilmenite, and rutile. Scheelite, tetrahedrite, bournonite, boulangerite, and stibnite occur locally. The major gangue mineral is quartz, with subordinate carbonates, feldspars, chlorite, sericite, kaolinite, montmorillonite, and barite. The total sulfide content of the ore zones ranges from 1 to 3%, and in places up to 5%. Native gold occurs as large individual grains ranging from 0.1 to 2.0 mm in diameter, or as fine disseminations in arsenopyrite. The average gold fineness is 750 to 790.

Fluid inclusion studies reveal homogenization temperatures of 150° to 360° C, with mainly liquid and as much as 5% vapor. Two temperature peaks of 280° to 320° C and 180° to 240° C occur in many samples. The δ³⁴S composition of sulfides in orebodies ranges from ?6.3 to ?2.4 per mil and approximates that of sedimentary rock-hosted pyrite. The δ³⁴S values of the ore solutions are interpreted as having been close to that of the sulfide minerals. The δ¹⁸O composition of ore quartz ranges from 13.9 to 14.1 per mil. The calculated δ¹⁸O composition for the ore fluid ranges from 7.1 to 7.3 per mil at 300° C. The δ¹⁸O values of oxygen indicate a quite homogeneous fluid of metamorphic origin.

The sulfur, arsenic, and gold in the ore deposit were mobilized during metamorphism that included transformation of pyrite to pyrrhotite. The PT conditions for this reaction are estimated at about 400°C and 2.5 kbar, approximately at the biotite isograd. Associated decarbonatization and dehydration reactions produced much of the ore fluid. The interaction of ore-fluid sulfur with Fe-bearing silicate and oxide minerals probably caused deposition of sulfide minerals and gold.