Alexander S. Borisenko

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.

Spatial-temporal and genetic relationships between gold and antimony mineralization at gold-sulfide deposits of the Ob-Zaisan folded zone

The Ob-Zaisan folded zone is a fragment of a single structure composed of Paleozoic sedimentary and volcanogenic rocks (mainly black shale), which was formed at the margin of the Siberian continent and features a common set of magmatic complexes and mineral systems. However, there are some differences that determine the specific geological and metallogenic features of the Irtysh-Zaisan and Kolyvan-Tomsk fragments of the Ob-Zaisan folded zone. In the gold deposits of the West Kalba and Kolyvan-Tomsk auriferous belt, the main gold-sulfide mineralization is controlled by zones of shearing and dynamic metamorphism in carbonaceous carbonate-terrigenous rocks. This type of mineralization was formed in tectonic blocks in a compressional setting. Antimony mineralization is characterized by brecciated textures and the vein-like morphology of ore bodies, reflecting extensional tectonics. At some deposits (Zherek, Mirazh, Dalny), Sb mineralization is spatially separated from the main gold-sulfide ores and shows cross-cutting relations to the principal ore-controlling structures. In other gold deposits, stibnite is spatially associated with disseminated gold-sulfide ores and forms mineral assemblages with Ni, Co, Au, Pb, and Fe (Alimbet, Zhanan, Legostaevskoe, Semiluzhenskoe, and Kamenskoe deposits).This study reveals no direct correlation between Au and Sb in gold-sulfide ores of these deposits. SEM analysis indicated the absence of free gold in stibnite veins. However, atomic absorption and electron microprobe analysis indicated the presence of “invisible gold” from a few ppm to several tens of ppm in the stibnite. High gold contents in the gold-sulfide ores overprinted by antimony mineralization (Suzdalskoe, Zhanan, and Legostaevskoe deposits) can be explained by the processes of regeneration and redeposition.The results of microstructural observations, isotope geochronology, studies of mineral assemblages and fluid inclusions in the ores from gold deposits of the Ob-Zaisan folded zone suggest that antimony mineralization was formed at gold-polysulfide stage, which was separated from the early ore pyrite-arsenopyrite stage by a 30 Ma time gap. It can be assumed that the essentially stibnite mineralization was formed at a separate stage and was separated from the gold-polysulfide mineralization by a 7 Ma interval of tectonic activity. Our Ar-Ar data on sericite from ore samples, combined with U-Pb data on zircons from igneous rocks and previous data from the literature show that there were two major stages of ore formation: the Early Permian (282-270 Ma) and the Early Triassic (250-240 Ma). Most researchers suggest that these stages of mineralization can be related to the epochs of intraplate magmatism that led to the formation of the Tarim (280 Ma) and Siberian (250 Ma) large igneous provinces. These global geological events are generally connected with the influence of Tarim and Siberian mantle plumes.

Thermodynamic modeling of REE behavior in oxidized hydrothermal fluids of high sulfate sulfur concentrations

Thermodynamic calculations using the HCh software were made for mineral equilibriums including REEs in the fluoride–sulfide–chloride–carbonate–sulfate–system in the presence of Na, Ca, and P with fluids of various acidities–alkalinities [11]. The obtained thermodynamic characteristics of thenardite allowed us to carry out the calculations for this phase under complicated hydrothermal conditions simulating the presence of oxidized fluids at 500–100°C and 2000–125 bar. Among other solid phases, REEs–fluorite, monazite, and REE–F–apatite were formed as CaF2–(Ln,Y)F3, LnPO4, and Ca5(PO4)3F–(Ln,Y)3(PO4)3 ideal solid solutions, respectively, where Ln is La, Ce, Pr, Nd, Sm, Eu, and Gd. Xenotime, anhydrite, elemental sulfur, and calcite were found as well.

Effect of the presence of As, Bi, and Te on the behavior of Pt metals during fractionation crystallization of sulfide magma

The results of study of the chemical and phase compositions of three Ag-, Au-, platinum group element-, As-, Bi-, and Te-bearing samples, which are the product of directed crystallization of the Cu–Fe–Ni sulfide melt, are presented. One sample contained only metals, the second one was additionally doped with As, and the third one was enhanced in As, Bi, and Te. The mechanism of formation of the minerals of the platinum group metals during fractionation crystallization of multicomponent sulfide melts was proposed on the basis of interpretation of experimental data.

Permian – Triassic Metallogeny

Three mineralization complexes were formed during the Permian – Triassic period in the following order: PGE-Cu-Ni and V-Ti – Fe of magmatic origin; Au – sulfide and Sn – sulfide. The first ore complex including magma-origin Cu-Ni-(Pt) mineralization is related to ultramafic - mafic magmatic differentiation in structures of Song Da, Song Hien rifts (such as Ban Phuc, Ban Khoa, Suoi Cun blocks, etc.) as well as being connected to layered gabbro – peridotite intrusions in folded structures surrounding the Song Chay anticlinoria (such as Nui Chua and Khao Que blocks). Cu-Ni-(PGE)-type mineralization is found in Ta Khoa – and Suoi Cun-type mafic – ultramafic, while Ti-Fe-(V) (mainly) ore type and Cu-Ni-(PGE) are characterized for Nui Chua-type ultramafic intrusion. Radiometric age determined for Ta Khoa ultramafic magmas is 257–270 Ma, for Suoi Cun is 260 Ma and for Nui Chua mafic magmas is 251 Ma; the ages are also applied for the related ore formation, implying that the mineralization of Cu-Ni-(PGE) and Ti-Fe-(V) in geological structures in northern Vietnam was associated with Permian – Triassic tectono-magmatic period. Besides, associated with Cu-Ni-(PGE) ore type is highly potential Ni-Co-As hydrothermal mineralization (discovered in inner contact zones in Ban Phuc block within the Song Da structure). Au-sulfur-type ore complex is widely developed in various structures in northern Vietnam; Representatives of Sn-sulfur-type ore complex are located to the southern margin of Song Hien rift and Lo Gam belt.

Permian – Triassic Pluton – Volcanic Magmatic Associations in the Song Hien Structure, Northeast Vietnam

The Song Hien Basin in northeast Vietnam is linear- shaped, running in the northwest – southeast direction from Vietnam-China border to the easternmost of Lang Son province; with a total length about 200 km the structure divides the Ha Lang belt to the northeast and Phu Ngu belt in the south and southwest. Permian – Triassic magmatism comprises pluton-volcanic felsic and mafic bimodal magmatic associations, including basalt, gabbrodolerite – dolerite (and lherzolite-gabbronorite); rhyodacite – rhyolite and granite. These magmatic associations are spatially, temporally and genetically related. Detailed mineralogical, geochemical and chemically mineralized studies have been conducted intensively on the Song Hien magmatism; among these, the mafic and ultramafic magmas have been described separately while gabbrodolerites were grouped to pluton-volcanic basaltoidic associations. Formation ages of lhrzolite and gabbro-dolerite are 260–262 Ma, while rhyolite – 246 Ma. Geochemical and isotopic data show genetic relationship between basalts and gabbro-dolerite and rhyolite and porphyric granite.

Metallogeny in the Cenozoic

The Cenozoic mineralization in structures associated with the Red River shear zone (RRSZ) in northern Viet Nam includes the following mineralized ore types TR- (U-Th)-Pb-Zn and TR- (U-Th) –Ba-F, Au-Cu in the Song Da rift zone. The origin of the mineralizations is spatially and (possibly) temporally related to the formation of Paleogene potassic mafic – ultramafic and silisic associations. Molybdenum –precious metals (Mo-W-Cu-Au) in the along with Cu-Au mineralization in the Phan Si Pan uplift are related to Paleogene granitoidic magmatism in the Ye Yen Sun complex. Beside, Cu-(Au – REE) mineralizationsin the Phan Si Pan Uplift are described.

An Overview on the Structures, Tectonics and Magmatic Activities in North Vietnam

Reconstruction of paleo-geodynamic framework of Vietnam in the relation to major tectonic events in southeast Asia reveals that Paleozoic structures in Northern Vietnam (from northeastern Song Ma suture zone northward) were developed in the southern and southeastern margins of the Yangtze Block. Northern Vietnam is divided into two major folding systems, based on their positions relative to the Song Hong fault zone.