The Sukhoi Log gold deposit is centrally located in the Lena goldfield region, approximately 850 km NE from the city of Irkutsk, and is hosted in Upper Proterozoic marine sandstone, carbonaceous slate and phyllite, metamorphosed to low greenschist facies in an outlying part of the major Akitkan Foldbelt. The disseminated pyritic tabular orebody has no outcrop, is defined solely by assay grades and is located in the axial zone of a large, near-isoclinal, reclining anticline. Highest ore grades occur in pyritic black shale beds, especially where they cross the axial zone and include two elongate higher grade (4–9 ppm gold) cylindrical zones, termed ore pillars, along the gently plunging anticlinal crest. The anticline is exposed E-W over a length of 3 km and plunges at approximately 10° degrees NW. The axial plane and orebody dip 15° N, and the latter is open to depth beyond 400 m. Three phases of syn- and post-metamorphic mesoscopic folding developed characteristic structures. The first two (F1 and F2) are congruent with the anticline and localized quartz-pyrite-gold veinlet mineralization in F1 axial plane cleavages (S1), in narrow, spaced axial zones of small non-penetrative folds (F2), in irregular disseminated zones in shale, and in small irregular clusters (stringers) of quartz-carbonate veins. A third phase generated scattered kink-fold bands (F3) and irregular crumpled zones without mineralization. A later episode of transgressive mesothermal quartz-vein mineralization developed many low-grade auriferous veins that have been the main sources of the extensive alluvial gold deposits. The mineralization assemblage is consistently quartz-pyrite-gold carbonate with minor base and platinum group metals. Pyrite is widely distributed in black shale throughout the deposit, at between 2 and 5 percent, and in lesser amounts in a disseminated envelope around the orebody. In the outer parts of the orebody and in the enclosing mineralization envelope it contains gold of higher fineness (900–920), whereas clustered and veinlet pyrite is more common in the interior productive zone, with gold of lower fineness (840–880). The deposit was intensively explored from October 1971 to December 1977. The orebody at a cutoff grade of 1 ppm gold, is an elongate irregular planoconvex sheet up to 140 m thick, dipping NNE at 15° to 30°, with a length of 2.2 km, a down dip width of more than 500 m, and is probably open to depth. It contains 384 million tonnes with an average grade of 2.5 to 2.7 grams per tonne. Additional resources include 165 million tonnes at 2.0 to 2.3 ppm in a low grade, possible pit extension, and 205 million tonnes at 0.8 ppm in the mineralization envelope.
Sequences of pyrite-gold paragenesis, of textural and structural changes, and limited isotope data indicate that four distinct stages of mineralization occurred during MOR subduction and Riphean closure of the major Proterozoic Akitkan Foldbelt.
