Iron Oxide Copper-gold and Related Mineralisation of the Siberian Craton, Russia
Part I - Iron Oxide Deposits in the Angara and Ilim River Basins, South-Eastern Siberia
by
Serguei G. Soloviev, International GeoSol Consulting Inc., Calgary, Alberta, Canada and Centre for Russian and Central EurAsian Mineral Studies (CERCAMS), Natural History Museum, London, UK.
in Porter, T.M. (Ed), 2010 - Hydrothermal Iron Oxide Copper-Gold and Related Deposits: A Global Perspective, Advances in the Understanding of IOCG Deposits; PGC Publishing, Adelaide. v. 4, pp. 495-514.
ABSTRACT
The Angara and Ilim River Basins in Siberia incorporate several tens of significantiron oxide concentrations known as the Angara-Ilim type deposits. They are associated with mafic igneous suites distinguished within the Permian-Triassic Siberian traps, and are represented by mineralised sub-vertical breccia pipes, likely diatremes-maars, extending for >1.5-2 km downward. These breccia pipes intersect tholeitic, calc-alkaline, mafic (dolerite) sills but incorporate younger basaltic dykes and stocks, which are possibly alkalic and exhibit a shoshonitic affinity. Gradual upward transition from massive basalts through porphyritic to vuggy and foamy varieties and finally to "tuffisites" cementing explosive breccias are observed.
Two episodes of brecciation, hydrothermal alteration and mineralisation, divided by emplacement of basaltic dykes, are distinguished. Specifically, the first (early) episode is expressed as brecciation of dolerites and sedimentary host rocks, followed by hydrothermal alteration of the breccias (prograde magnesian and calciferous skarn to retrograde and hydrosilicate alteration) and mineralisation, including abundant magnetite. The second (late) episode occurred after, or contemporaneously with, the emplacement of basaltic dykes, associated "tuffisite" and intense "re-brecciation" includes the formation of numerous massive magnetite, magnetite-apatite, and magnetite-calcite veins.
The deposits are characterised by abundant magnesian and calciferous skarns, with varying pyroxene/garnet ratios, intense retrograde and hydrosilicate (mostly chlorite-serpentine) alteration. All these assemblages include magnetite, although it is especially abundant in association with chlorite and serpentine forming brecciated, disseminated and massive ores. Late massive magnetite (+apatite, calcite) veins crosscut the early assemblages and often contain "oolite" (concentric, spherulitic, ball-like) magnetite aggregations as well as magnetite-halite accumulations. Enrichment in sulphides (chalcopyrite, pyrite) is observed in the uppermost parts of some deposit.
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