G. G. Pavlova

Silver Deposits of the Pamir Region, Tajikistan: Metallogeny, Mineralogy, and Genesis

Silver deposits of the South Pamir region of Tajikistan occur as siderite veins with sulfosalts and sulfides, and formed during a late Mesozoic-early Alpine cycle of rifting and orogenesis. Three mineral-geochemical types of ore deposits that occur in several ore districts are recognized: Ag-Sb, Ag-Pb, and Ag-Hg. The Ag vein deposits all occur in terrigenous-carbonate sedimentary rocks and in intrusive granites. The veins are controlled by late Mesozoic faults, and are the products of the last endogenous event in the region. The most significant group of Ag-Sb vein deposits is located in the Bazardara ore district. Three stages of mineralization are recognized in this region: siderite-tetrahedrite, quartz-tetrahedrite, and quartz-galena-freibergite. Most of the veins were deposited during the first stage. Tetrahedrite is the prevailing and most economically important mineral, and contains 0.2 to 15.5% Ag. Associated freibergite contains from 19.5 to 32% Ag. Fluid-inclusion studies indicate that Ag-Sb ore deposits formed over a temperature interval of 280-100°C and pressure of 750-450 bars. Ore deposits were formed from saline solutions (up to 38%) that in late stages were diluted (1.5%). Considerable fluctuations apparently occurred in the associated regional hydrothermal system. Temperature gradients ranged from 8-10°C/100 meters to 14-15°C/100 meters. Temperatures and concentrations of hydrothermal solutions significantly decreased from the main ore-controlling fault toward the north and south flanks of the ore districts.

Forming conditions and age of native silver deposits in Anti-Atlas (Morocco)

663 Mercury bearing silver and Ag and Hg intermetal lides are described in primary and oxidized ores from different type ore deposits: Ni–Co–Ag–Bi–U, mer cury, polymetallic, Ag–Sb, Ag–Hg, and others. At the same time, the economic concentrations of these min erals are recorded only in ores of the Ni–Co–Ag–Bi– U and Ag–Hg deposits. The Ag–Hg deposits repre sent a peculiar separate type of silver mineralization, which distinctly differs from other types by the domi nant role of native Hg silver (main ore mineral) and the presence of different sulfides and Ag sulfasalts. There are several ore areas with such mineralization: Anti Atlas in Morocco, Verkhoyansk region in Yaku tia, the Altai–Sayan region, and others. Anti Atlas, which hosts three known deposits of the Ag–Hg type (Imiter, Igoudran, Zgounder) and several ore occur rences including those in the ore field of the Bou Azzer Co deposit [1–3], is most famous among them. The Imiter deposit, development of which started as early as in the 8th century and continues at present, is the largest of these deposits. The Imiter and Igourdan deposits are located among Middle Proterozoic terrigenous rocks repre sented by sandstones, siltstones, and black shales, which are intruded by Neoproterozoic gabbro, diorite, and granodiorite bodies. They are unconformably overlain by the Upper Neoproterozoic effusive sequence composed of andesites, rhyolites, and ign imbrites with dikes and stocks of andesites, dolerites, rhyolites, and granite porphyries. The Neoproterozoic igneous rocks in the ore field of the Imiter deposit host age and type variable mineralization: small occur rences of Mo greisen, Au–Ag and polymetallic Ag bearing, Co–As, and barite ores. The Neoproterozoic rocks are overlain by Cambrian platform strata with post Ordovician dolerite, monzonite, and K microsy enite dikes. The biotite and K feldspar 40Ar–/39Ar ages of microsyenites determined at the Analytical Center of the Institute of Geology and Mineralogy (Novosi birsk) are estimated to be 204.5 ± 2.5 and 199.5 ± 2.4 Ma, respectively (Fig. 1).

The role of chloride-carbonate melts in the formation of sideritic carbonatites of the KARASUg FE-F-REE deposit (Tyva Republic, Russia)

The authors have studied melt-fluid and fluid inclusions in quartz and fluorite of sideritic and ankerite-calcitic carbonatites of the Karasug ore field, as well as melt inclusions in apatite from granosyenites. The content of salt and fluid components in brine-melt inclusions was evaluated on the basis of the thermodynamic data, the calculations of volumes and densities of the solid phases, a solution of about 50% concentration, and the gas phase of the inclusions, as well as the results of the LA-ICP-MS analysis. The content of salt phases, the solution, and the gas phase amounted to 85–70, 10–25, and about 5% of the inclusion substance, respectively. The total percentage of salt and fluid components (H2O and CO2) amounted to 90–80 and 10–20 wt %, respectively. The fraction of a carbonate constituent in the inclusions was as high as 45–50% and over in ankerite-calcite carbonatites and about 15 wt % in sideritic carbonatites. The 117.2 ± 1.3 Ma age of these carbonatites by 40Ar/39Ar, along with other datings for this area, shows that their formation was associated with a manifestation of the Cretaceous alkaline-mafic magmatism (117–120 Ma). The presented model of the formation of carbonatites is in agreement with the sequence of the development of magmatic processes and mineralization in this area. The model is also confirmed by the results of the studies of melt and fluid inclusions in minerals.

In-bearing potential of tin‒sulfide mineralization in ore deposits of the Russian Far East

The increased demand for indium has made it necessary to revise prospects of In-bearing tin ore deposits in the Russian Far East on the basis of geological data and results of recent analytical methods (X-ray fluorescence with synchrotron radiation, atomic absorption, and ICP-MS). The average In contents in ores of the Tigrinoe and Pravourmiiskoe deposits vary from 55 to 70 ppm, which allows tin ore deposits with Sn‒sulfide mineralization to be considered as quite promising with respect to In production from ores of Russian deposits. By their estimated In reserves, the Tigrinoe and Pravourmiiskoe deposits may be attributed to large ore objects.