The problem of large and super-large gold deposits attracts a constant interest of economic geologists, being a major topic of the various symposiums and monographs in the last decades. Many gold giants were discovered in Russia and neighboring former USSR countries during the last century. However, the known publications have a deficit of the joint single maimer detail description of these gold giants in the comparison with the worldwide analogs.
Gold giants are unevenly distributed in the global scale and in the geological history. The most productive metallogenic epochs (and provinces) are:
- Archean maximum (2700-2600 Ma) including the formation of the green schist belts and associated deposits of the gold-sulfide-quartz and gold-quartz types (e.g., Abi-tibi province in Canada and the Yilgarn block in Australia) as well as the gold-uranium Witwatersrand deposit (South African province);
- relatively low productive Proterozoic maximum (1600 Ma), nevertheless including the gold-iron quartzite Homestake deposit and gold-(arsenic)-sulfide Olimpiada deposit;
- Devonian maximum (400-380 Ma) represented by gold-quartz Bendigo deposit (West Australian province, Ural province) and gold-polysulfide-quartz Vasilkovskoye (Kazakhstan province) deposits;
- Devonian-Carbonaceous maximum (320-280 Ma) represented by gold-quartz Muruntau deposit (Western Uzbekistan province), gold-(arsenic)-sulfide Sukhoy Log and Kumtor deposits, as well as the earliest large gold-silver Kubaka deposit;
- Late Jurassic-Cretaceous maximum (130-80 Ma) represented by gold-quartz deposits and gold-(arsenic)-sulfide Mayskoye, Nezhdaninskoye deposits (Northeast Russia province), gold-silver Kuranakh deposit (Aldan province), and some gold-silver deposits of the Circum Pacific belt;
- Tertiary-Quaternary maximum (20-0 Ma) represented by gold-silver Porgera, Hishikari, Pueblo Viejo, Round Mountain and many other deposits (Circum-Pacific provinces). Circum Pacific belt concentrates various deposits (gold-quartz, gold-silver, gold-(ar-senic)-sulfide types), which were formed in some discreet periods. Sometimes in the same period, the different deposit types were formed in different parts of the Pacific belt simultaneously (e.g. in the Late Cretaceous, gold-silver deposits of the Okhotsk-Chuckchee province in the west versus gold-quartz deposits of the British Columbia in the east). Comparison between the various parts of the Circum Pacific demonstrates the high potential of the Okhotsk-Chuckchee province.
In the geological history, maximums of the gold mineralization coincide with the riftogenic impulses (widening of the Earth) and maximums of a coal accumulation. Large gold deposits often demonstrate the sulfide and oxide (quartz) deposit pairs (e.g. Lebedinoye and Kuranakh, Muruntau and Daugyztau, Leadwill and Cripple Creek deposits. Giant gold deposits are explained as the large historical-geological anomalies, forming during the long temporal periods. In many cases, formation of giant deposits includes the long pre-ore (so cold ore-preparing) period, when gold is concentrated in the host rocks as a barren impregnation. Analysis of the gold giants geological settings shows that the deposits were formed in the definite long-developed energetic centers («focuses» of the endogenous (magmatic, metamorphogenic, hyd-rothermal-sedimentary, etc.) activity).
Gold giants are usually multi-stage deposits, they begin the ore-type series in the ore districts. The mineralogical-geochemical assemblages of the giant deposits embrace the variety of the compositions of many smaller nearby satellite deposits. Zones of the dispersed suffidization arc widespread in the various sedimentary and igneous rocks. However, especially in the black shales, they have a regional scale abundance being an important source of the gold (as well as PGE, silver, tin, tungsten) for the giant deposits. The large and super-large gold deposits in the black shales commonly include the first stages (e.g., hyd-rothermal-sedimentary, carbon-hydrogen-fluid) with the relatively barren dispersed metasomatic ores and the second stages (metamorphogenic or magmato-genic-hydrothermal) with more concentrated vein-type ores. Ore districts with the giant deposits commonly are characterized by the thorough (multi-stage) gold-bearing activity, including the primary and secondary inputs of gold, as well as the re-generation periods. Multi-stage origin of the ore matter sources is an important criterion for the gold deposit evaluation.
Structural factors determine the positions of 60% of the gold giants. Other 40% of the deposits are controlled by the coincidence of the structural, magmatic, and lithological factors.
Main structural types of the giant gold deposits include:
- long, wide intervals of the ore-bearing faults, schistosity and brecciation zones, as well as their intersections and conjugations (Sukhoy Log, Olimpiada, Nezhdaninskoye, Kumtor, Bakyrchik, Amantaitau, Daugyztau, Goldstrike, Comstock, Getchell, Homestake, Morrow Value, Ashanti, Prestea and others);
- long and wide ore-bearing fracture systems consisting of the numerous common faults grouped «a la echelone» (Muruntau, Natalka, Kalgurly, Hollinger-Mclntair, Dome);
- long bundles and zones consisted of the fractures and sub-parallel faults, including those in the incompetent rocks (Mayskoye, Kochbulak, Cripple Creek, Silverton-Telluride and others).
The first type criteria include the bends and splits of the regional and local faults, and more rarely the large lens-like tectonic blocks — in homogenous host rocks. Intersections of the ore-bearing faults and small intrusive stocks or horizons of the relatively incompetent rocks — in inhomogenous host rocks. Inherited cleavage zones in the axes of the compressed folds in any host rocks.
The second type criteria include the ore-bearing fracture systems along the crests of the compressed folds, between the sub-parallel branches of the ore-bearing faults.
The third type criteria are represented by bends and splits of the ore-bearing fault and their intersections with those of other orientation and the favorable horizons and the fragile block-plates, including the small intrusive stocks, dykes; at the epithermal deposits, the ore-bearing faults are superimposed on the local volcano-tectonic structures.
The differences of the physical-chemical conditions of the large and small deposits were studied on the fluid inclusion data. Epithermal (or gold-silver) deposits of the widespread adularia-quartz type are formed at the depth down to 1 km, at the relatively low pressures, in the open hydrothermal system, at the middle temperature (190 -330°C) and low salinity (0-7,5 wt.% NaCl-equiv.) due to the mixing of the magmatic and meteoric fluids. Less widespread gold-(copper)-porphyry and alunite-quartz types are also formed at the low pressures, but higher temperatures (130-540°C) and higher salinity (20-40 wt.% NaCl-equiv.), that is connected with the participation of the magmatic fluids together with the mixed ones.
All epithermal deposits are characterized by the instability of the ore-forming process: high temperature gradients, inter-stage temperature inversions, local drops and highs of pressure, and boiling of the fluids.
Mesothermal (gold-quartz) deposits are formed at the depth of 1-5 km, in the closed hydrothermal system, at the average pressure of 1 kbar, middle temperature (200-300°C) and middle salinity (<4 wt.% NaCl-equiv.). Hypothermal (pre-Cambrian gold-sulfide-quartz deposits) are formed at the depth of 5-10 km, in the closed hydrothermal system, at the average pressure of 2 kbar, high temperature (100-500°C) and high salinity (0-28 wt.% NaCl-equiv.).
Mesothermal and hypothermal deposits are alike by the higher stability of the ore-forming process and the higher, comparing with the epithermal type, role of the COr There are the differences between the large and small deposits within the each class.
The giant deposits are characterized by the higher amplitude of the temperature and salinity, which are supposed due to the numerous mineralization stages and the longer term of the deposit formation.