On the Relationship Between Gold–Silver and Mercury Mineralization at the Oganchinskoye Deposit

Yu. M. Shchepot'ev (TsNIGRI)

In world practice, numerous cases are known of the spatial association of gold–silver and mercury mineralization in Meso-Cenozoic volcanic belts (Lindgren, 1935; Helke, 1946; Sidorov, 1966, etc.). Clarifying the relationships between these types of mineralization plays a major role in understanding questions of ore formation in these structures and has important metallogenic significance. However, only isolated cases of the observed relationships have so far been described in detail. The difficulty here lies in the fact that, even with a close structural and temporal connection, gold and mercury usually form separate, independent objects.

In the Central Kamchatka and East Kamchatka ore zones, in individual ore nodes of Kamchatka, the structural confinement of gold–silver and mercury mineralization to the same deep-seated faults is observed on a small scale (Vlasov, Vasilevsky, 1964). In addition to the common structural position, a closeness in the time of ore manifestation of both is established (the end of the Neogene), but, on the other hand, within most deposits gold–silver and mercury mineralization are spatially separated, the latter more often occupying a peripheral position relative to the former. Similar spatial relations of the two types of mineralization under consideration are observed in the Northeast of the USSR, Japan, America, and Transylvania. Age relationships of gold–silver and mercury mineralization were observed by the author at the Oganchinskoye deposit of near-surface genesis.

The Oganchinskoye gold-ore deposit is located in the field of development of basic and intermediate volcanics of the Anavgai series ($Pg_3—N_1$), intruded by pre-Upper Miocene granitoids, as well as by dikes and extrusions of Pliocene andesite-dacites. The ore bodies occur in quartz–hydromica metasomatites and are represented by a series of quartz and quartz–carbonate veins. Formation of the ore bodies proceeded in several stages, characterized by definite mineral associations (from early to late): pyrite—with saccharoidal quartz and some amount of finely dispersed gold in pyrite; gold (productive)—with vari-grained quartz, hydromica, and isolated grains of simple sulfides; telluride–

sulfide with fine-grained quartz, carbonate, and hydromica, with low gold contents; metacinnabar–quartz–fluorite with chalcedony-like quartz; and cinnabar. A chalcopyrite–galena–sphalerite association with amethyst-like quartz, superimposed on the gold and telluride–sulfide associations, is rarely observed. The amount of ore minerals is insignificant—0.1–1%. The structures and textures of the ores are typical of near-surface deposits: brecciated, metacolloidal, massive, etc.

Mercury mineralization is observed in quartz–hydromica metasomatites and in some veins (veins Nos. 2, 3, 4). In the veins, gold-bearing quartz is cut by thin (up to 10 cm) veinlets of dark-gray chalcedony-like quartz with fine disseminations of metacinnabar, fluorite, and pyrite. Sometimes, in ore breccia, these minerals cement the gold-bearing quartz. Fluorite and metacinnabar crystallize as small (0.01–0.5 mm) grains, often of irregular shape. More rarely, fluorite displays a colloform structure with zonal flow forms. Pyrite usually forms idiomorphic simple forms, at times with distinct zoning. Cinnabar is observed as thin crusts composed of small crystals in cracks and cavities on the surface of dark-gray chalcedony-like and fine-grained gold-bearing quartz. In addition, cinnabar develops after metacinnabar. The deposition of mercury mineralization in the final stage of the hydrothermal process was also noted by G. F. Loughlin and coauthors (1937) at the Cripple Creek gold-ore deposit.

The observed separation in the hydrothermal process of the metacinnabar–fluorite association and cinnabar is quite regular, since fluorite and metacinnabar form at low temperatures and pressures and, for example, from acidic solutions, whereas cinnabar forms from alkaline solutions (Saukov, 1946; Ramdohr, 1962).

At the deposit, mercury contamination of minerals of purely gold-ore associations is observed, up to an admixture of mercury in gold (tenths of a percent), and gold and silver contamination of the mercury mineralization, which indicates their close connection in the ore-forming process. A. A. Sidorov (1966) also noted, in Chukotka gold–silver ore occurrences, an admixture of mercury in native gold and silver sulfosalts up to tenths of a percent.

The observed connection between gold and mercury in the ore process makes it possible, under the conditions of Kamchatka, to use dispersion halos of mercury, as a highly mobile element, in prospecting for gold–silver deposits, especially those not yet exposed by erosion. This prospecting indicator is used quite effectively in America and Japan (Kishimoto et al., 1964).

REFERENCES

Klasov G. M., Vasilevskii M. M. Hydrothermally altered rocks of Central Kamchatka, their ore potential and regularities of spatial distribution. Izd-vo “Nedra,” 1964.

Loughlin G. F., Koschmann A. H., Tunell D., Ksanda K. J. Study of the paragenesis and hypogene gold and silver telluride ores at Cripple Creek, Colorado, USA. International Geological Congress. Proceedings of the XVII Session, vol. V, Moscow, 1937 (Abstracts).

Lindgren V. Mineral Deposits. Issue III. ONTI NKTP USSR, translated from English, 1935.

Ramdohr P. Ore Minerals and Their Intergrowths. Foreign Literature Publishing House, Moscow, 1962.

Saukov A. A. Geochemistry of mercury. Transactions of GIN, issue 78, mineralogical-geochemical series, No. 17, 1946.

Sidorov, A. A. Gold–Silver Mineralization of Central Chukotka. Nauka Publishing House, 1966.

Helke, A. Young Volcanic Gold–Silver Ore Deposits of the Carpathian Arc. All-Union Chamber of Commerce, Moscow, 1946.

Kishimoto, F.; Takasimo, K.; Kato, K. On dispersed mercury in the wall rocks of a gold–silver deposit. Okuchi, Kagoshima Prefecture. Bull. Geol. Surv. Japan, 15, No. 1, 1964.