DURATION OF GOLD MINERALIZATION FORMATION IN COPPER-SKARN DEPOSITS AND ITS СORRELATION WITH DEVELOPMENT OF GRANITOID MAGMATISM

(ALTAI-SAYAN FOLD AREA)

Gaskov I.V., Borisenko A.S., Babich V.V.

Institute of Geology and Mineralogy SB RAS, Novosibirsk, Russia, gaskov@uiggm.nsc.ru

Both the data on isotope geochemistry and geochronology of rocks and geological data on the gap in the processes of ore genesis obtained in recent decades allowed to get evidence of greater time span in formation of ore deposits as previously thought (Rundquist, 1997). This fact may also be referred to the formation of gold mineralization at many copper-skarn deposits at the Altai-Sayan fold area (ASFA).

Gold-bearing copper-skarn deposits of the ASFA occur rather widely in many ore regions: Gorny Altai (Sinyukhinskoe, Murzinskoe, Choiskoe), Kuznetsky Alatau (Natalievskoe, Fedorovskoe), Gornaya Shoriya (Maisko-Lebedskoe), and Tuva (Tardan, Khopto). Most of the deposits are of economical value. Skarn-forming processes in these deposits are related to formation of the Low- and Middle Paleozoic gabbro-diorite-plagiogranite magmatism of various composition - the Tannuol, Yugalin (Sinyukha), and Ust-Belov intrusive complexes (Shokal’sky et al., 2000). Formation of gold mineralization has more intricate and prolonged character as compared to skarn process. Gold mineralization in many deposits is represented by products of multistage ore process that differ in mineral composition and spatial position. Two types of gold mineralization are distinguished in these deposits: the first type is spatially and genetically associated with skarn and aposkarn formations (gold, magnetite, pyrite, chalcopyrite, bornite, and chalcosine), while the second one is spatially isolated from skarns and represented by middle- and low-temperature association of sulfides and tellurides (gold, bismuthine Bi₂S₃, cinnabar HgS, aikinite CuPbBiS₃, emplectite CuBiS₂, polybasite Ag₁₆Sb₂S₁₁, tetradimite Bi₂Te₂S, ingodite Bi₂TeS, hessite Ag₂Te, heidleite Bi₂T and others). This mineralization is generally developed along the crush zones of rocks being various by composition, including sedimentary, magmatic, and skarn ones.

However the contribution of every stage into the formation of the commercial gold-ore mineralization is different at various deposits. Some deposits (Sinyukhinskoe) are dominated by the development of the early high-temperature formations, while other deposits (Choiskoe) are characterized by the late low-temperature mineral associations, and at the third ones (Tardan, Murzinskoe) of wide occurrence are the early and late auriferous ores.

The available age determinations of the Tardan and Murzinskoe deposits provide insight into the understanding the sequence of their formation ande duration of ore formation.

Skarn mineralization in the Tardan deposit has formed at the contact of diorites and carbonate rocks due to intrusion of the Kopto-Baisyut massif. The age determination of the massif using Ar-Ar dating of biotite yielded ages of 485,74,4 Ma corresponding to Lower Ordovician. Of similar ages are skarns, magnetite ores, and post-skarn hydrothermal gold-sulfide mineralization (pyrite, chalcopyrite, pyrrhotine, bornite, gold) spatially and genetically related to skarn-magnetite bodies. The high-fineness gold (820-990‰) with elevated copper impurity deposited in this stage. The intrusion of dike and stock-like small bodies of granitoids took place after the formation of skarn and post-skarn mineralizations as evidenced by their Ar-Ar dating. The age of granite dikes is 484,24,3 Ma.

The next stage of mineralization occurred with significant gap from skarn process. Gold-sulfide mineralization deposited in the linear tectonic crush zones superimposed on all types of skarns and magmatic rocks. Within these zones along with gold-sulfide mineralization, there formed a wide range of metasomatites (beresites, propylites, argillizites, etc.). The Ar-Ar dating of sericite in ore-bearing quartz of beresites yielded ages of 4816,1 Ma. The gold fineness of this stage varies greatly from 440 to 820‰. Gold contains Ag (8-45%) and Hg- (0,3-3,65%) as admixtures.

Skarn mineralization at the Murzinskoe deposit has formed on the contact of granodiorite body related to the Ust-Belov gabbro-diorite-granite complex. U-Pb dating indicates the age of the Ust- Belov massif as 364 8 Ma (Shokal'sky et al., 2000). The age of skarn and aposkarn gold-copper mineralization is close to this value. Deposition of hydrothermal veinlet-impregnated sulfide mineralization (chalcopyrite, pyrite, bornite, sphalerite, gold) was accompanied by the development of middle temperature metasomatic minerals - chlorite, actinolite, epidote, quartz as well as thick and extended zones of quartz-tourmaline metasomatites.

After the deposition of hydrothermal-metasomatic formations within the system of steeply dipping sublatitudinal fractures, the intrusion of diabase dikes and diabase porphyrites cutting these earlier formations takes place. The mineralized zones of crush with quartz-carbonate veins and low-temperature gold-telluride-sulfide mineralization form in the final stage of endogenous ore process. This mineralization was accompanied by low-temperature hydrothermal alteration of host rocks (argillization, adularization, propylitization). The age of this late gold mineralization by Ar-Ar dating of sericite from quartz-carbonate gold-ore veins with sulfides equaled 3588 Ma.

Thus, the data on the Tardan and Murzinskoe gold-skarn deposits suggest that formation of economical gold mineralization had a great time span (5-6 Ma) and proceeded in step-by-step manner. This fact is in good agreement with the development of compositionally complex ore-bearing magmatic complexes. The three complexes (Tannuole, Ust-Belov, and Yugalin (Sinyukha)) are characterized by differentiated rock series starting from gabbro and ending by granites. The specific feature of these magmatic formations is the increase in total alkalinity, in particular, potassium content with the increasing rock acidity. Plagiogranites are widespread acid formations in these complexes. We conclude that there is a distinct relationship between the formation of skarn type gold mineralization and complexes characterized by complex history of mantle-crust development and differed by elevated alkalinity of acid phases.

References

D.V. Rundquist. Time factor during the formation of hydrothermal deposits: periods, epochs, and stages of ore formation// Geology of Ore Deposits. 1997. vol. 39, N1, P.11-24.

S.P. Shokalsky, G.A. Babin, A.G.Vladimirov, S.M. Borisov. Correlation of magmatic and metamorphic complexes in the Altai-Sayan Fold Area. Novosibirsk, 2000, 187p.