Yu. G. Safonov

Gold-bearing reefs of the Witwatersrand Basin: A model of synsedimentation hydrothermal formation

The current concepts concerning the genesis of the unique ore-bearing reefs of the Witwatersrand Basin and its gold resource potential are considered. The results of microscopic examination of ore from the Black, Ventersdorp Contact, Carbon Leader, and Vaal reefs, as well as of thermobarometric study of quartz, are presented. A model of synsedimentation hydrothermal origin of the reefs in the process of evolution of primary colloidal-disperse systems is substantiated on the basis of these results and the data published by other authors. The formation of these systems is related to the periodic gain of deep ore-bearing gas-saturated fluids. The gold mineralization was formed under conditions of partially closed systems, where various mineral-forming processes developed (metasomatism, crystallization of true solutions and gels, gel metasomatism, dispersion of crystalline phases, segregation of mineral particles, formation of early minerals, etc.). New data on REE specialization of ore-bearing fluids are discussed. The specific features of the gold, carbonic, and uranium mineralization of the intracratonic basin are emphasized.

Lead isotopic composition from data of high-precession MC-ICP-MS and sources of matter in the large-scale Sukhoi Log noble metal deposit, Russia

The lead isotopic composition of 33 sulfide samples from orebodies of the Sukhoi Log deposit was studied by high-precession MC-ICP-MS with a precision of ±0.02% (±2SD from 120 analyses of the SRM 981 standard sample). The deposit is located in the Bodaibo gold mining district in the northern Baikal-Patom Highland. Gold mineralization is hosted in Neoproterosoic black slates. Variations of lead isotope ratios of the Sukhoi Log sulfides are generally typical of Phanerozoic deposits and ore fields. They are significant for 206Pb/204Pb (17.903–18.674), moderate for 208Pb/204Pb (37.822–38.457), and relatively narrow for 207Pb/204Pb (15.555–15.679). In the Pb-Pb isotope diagrams, the data points of pyrite and galena constitute a linear trend. The points corresponding to pyrite from metasomatic ore occupy the left lower part of the trend. Galena from late gold-quartz veins shows more radiogenic Pb, and corresponding data points are located in the upper part of the trend. According to the Stacey-Kramers model, the end points of the trend, which is regarded as a mixing line, have μ2 = 9.6 and μ2 = 13.2 and model Pb-Pb ages 455 and 130 Ma, respectively. The isotope characteristics of ore lead, their relationships in pyrite and galena, and the mixing trend of Pb isotopic compositions are clearly tied to two Paleozoic stages in the formation of the Sukhoi Log deposit (447 ± 6 and 321 ± 14Ma) and testify to the leading role of crustal sources, which are suggested as being the Neoproterozoic black-shale terrigenous-carbonate rocks.

Topical issues of the theory of gold deposit formation

The issues of the theory of gold deposit formation are considered. These issues were crucial in the works by N.V. Petrovskaya. The systematics of gold deposits based on stable mineral assemblages and mineral and geochemical types of ores and their families characteristic of certain groups of gold deposits is consonant with contemporary analogue classifications of ore deposits, which, however, require improvement by means of developing their genetic version. Petrovskaya’s proposed subdivision of gold deposits into families distinguished by depth of their formation is such a version, because in Petrovskaya’s understanding, the factor of depth integrates genetic features of deposits formed at shallow, medium, and great depths. According to recent data, the depth interval of localization of ore deposits is estimated at 15–20 km instead of 6–10 km suggested earlier; thus, the intervals of medium and great depths increase. Attention is called to the Petrovskaya’s ideas of stadial progressive development of ore formation with change in pyrite-arsenopyrite mineralization by base-metal and telluride mineralization along with advanced and concomitant deposition of quartz. The stadial formation of shallow-seated deposits and colloidal mineral formation therein, as well as the concept of gel metasomatism and mechanisms of accompanying destruction of preceding mineral assemblages developed by Petrovskaya, are considered and their implications are illustrated by specific deposits. Petrovskaya’s views on sources of ore matter and the polygenetic character of gold and Au-bearing deposits are discussed.

Structure of ore fields and deposits: Current status and outlook for further development

The current status of investigations of the structure of ore fields and deposits in Russia is considered. The importance of traditional lines of research in this field and the great body of information acquired is emphasized. The changes in ideas of geodynamic and tectonic crustal evolution have made it necessary to alter concepts of the geological and structural position of ore fields and deposits. New challenges are caused by these changes and progress in the knowledge of ore deposits and the depth of their formation, as well as geodynamic, tectonophysical, and hydrodynamic processes in ore-bearing crustal blocks. The results of hydrodynamic modeling of deposit formation controlled by deeply penetrating fault zones are discussed. Various relationships between hydrodynamic and structural factors in ore formation are considered.

A New Type of Gold-Platinum-Copper Mineralization in Northern Transbaikalia

The northern Transbaikal region situated at thesouthwestern margin of the Aldan Shield incorporates aunique ore district (Fig. 1). Giant ore deposits, such asUdokan (copper, silver), Chinei (iron, titanium, vana-dium, copper, and noble metals), and Katun (rare metaland rare earth elements), are located 30–100 km southof the Novaya Chara station along the Baikal AmurMainline [2].Despite unfavorable natural mining–geological andtechnological features of the Udokan copper deposit, itsexploitation in the nearest future is inevitable [7].Therefore, solution to problems related to enhancementof the efficiency of raw mineral mining in the depositand ore district has become a crucial issue. This districtis characterized by juxtaposition of two large deposits(Udokan and Chinei). Moreover, these deposits aresuperlarge (giant) objects with respect to resources ofmajor metals (Cu and V). This fact is of paramount sig-nificance in the strategy of economic development ofnorthern Transbaikalia. Extraction of associated orecomponents and elucidation of new types of complexores are essential for enhancement of the efficiency offuture mining–metallurgical plants in the region. In thisrespect, assessment of the genesis, abundance, andscale of gold–platinum–copper mineralization, whichwas first found in the Udokan–Chinei ore district, isvery important.Quartz veins with noble metals and copper weredetected in the nearest framing of the Chinei Massif andthe Pravoingamakit deposit. The Pravoingamakitdeposit was exploited for cupriferous sandstones by theUdokan Expedition in the 1960s. However, we scruti-nized ores of this deposit in 2004–2006 and revealedthat its structure is more complicated and differs signif-icantly from that of the standard cupriferous sandstonedeposit (Udokan). Orebodies of the Pravoingamakitdeposit are hosted in terrigenous–carbonate rocks ofthe middle section of the Chitkanda subformation of theLower Proterozoic Udokan Formation. The discontinu-ous cupriferous horizon is traced as a NW-striking (dipazimuth 350, dip angle 65°–70°) zone extending overnearly 10 km on the surface and 400–500 m along thedip. Economic grade mineralization is developed over4.5 km. The deposit includes orebodies of two types:(1) veins and lenses of milky white massive quartz withstringers and patches of sulfides (Fig. 2); (2) echelon ofmassive sulfide bodies surrounded by dissemination ofpyrite and chalcopyrite. Quartz veins (0.3–1 m thick)extend along the strike over a few tens of meters. Sul-fide bodies are 3–5 m thick and 300–440 m long. Theorebodies are characterized by significant content of Cu(0.47–2.5 wt %) and very high variation of the Cu/Niratio (from 10 to 700 in various sectors).Ores are represented by the pyrite–chalcopyritevarieties with typical stringer and breccia structures(Fig. 2). The maximal concentration of Ni in quartzveins is related to high contents of nickel minerals (mil-lerite and pentlandite). The quartz vein ore is alsoenriched in noble metals (g/t): Pt 0.1–2.2, Pd 0.9–6.2,and Au 0.1–0.4. The ore includes fine (up to 10 µm)inclusions of clausthalite Pb

Deposition of finely disseminated gold mineralization in black shales: A hypothesis of microstructural control

The deposition of finely disseminated gold in the deposits hosted in black shales is considered. It is suggested that gold deposition is controlled by microstructure of pore space in host rocks. The pore space structure of tight shales indicates that most pore volume is occupied by nanopores with hundredths of micrometers in characteristic dimension. The balance calculations show that deposition of native gold in nanopore channels of filtration is hampered by shortage of number of atoms necessary to overcome a nucleation threshold of the future gold crystal in the pore volume. When ore-transporting solution meets on its way the cavities (pores, micro- and macrofractures), whose volume is sufficient to overcome the nucleation threshold, the excess content of ore component, which exceeds equilibrium concentration, is released with formation of crystallization centers and further precipitation of gold. The conditions of ore deposition are exemplified in the reference Sukhoi Log deposit hosted in black shales. On the basis on the PT conditions of ore deposition and physical features of fluid heat and mass transfer, it is suggested that ore disseminations were deposited at the early high-temperature stage under a fluid pressure close to lithostatic and at a host rock permeability markedly exceeding its present-day value.

Participation of hydrothermal fluids in the formation of deposits of the proterozoic Udokan-Chineysky ore system

Numerous Cu deposits of the Udokan-Chineysky ore district (Udokan deposit of cupriferous sandstones, deposits of the Chineysky pluton, etc.), which occur in sedimentary and igneous rocks, are similar in many genetic features. One of these features is the similar parameters of fluid inclusions (hydrothermal fluids) in quartz of the Udokan deposit, rich chalcopyrite ores from the Rudnyi (Rudnyie deposit, and veins of the Pravoingamakitsky deposit studied previously. Fluid inclusions have the same range of high salinity and homogenization temperatures. These studies combined with geological data allow us to consider the formation of contact sulfide ores from the Chineisky pluton and Pravoingamakitsky and Udokan deposits as a common compound fluid-magmatic ore-generating system. The sedimentary rocks primarily enriched in chalcopyrite, pyrite, and pyrrhotite of the entire section of the Udokan complex served as the host media for the hydrothermal fluids and deposits of various scales.

Academician Nikolai Pavlovich Laverov, eminent scientist and geologist celebrates his 80th birthday

Original Russian Text

F. I. Wolfson: Scientist, teacher, and patriot

Following a brief biography, the scientific, administrative, and educational activity of F.I. Wolfson is described. Also considered are the principal lines of his research: the study of the structures of ore fields and deposits, the geology of uranium deposits, the theory of hydrothermal ore formation, and the history of the study of ore deposits. The immense contribution made by Wolfson to the development of the mineral resources of the country (uranium, nonferrous metals, etc.) is shown and the continuing relevance of his theoretical and methodological works in the field of geology of ore deposits is emphasized.