Paleoproterozoic Pt-Pd Fedorovo-Pansky and Cu-Ni-Cr Monchegorsk Ore Complexes: Age, Metamorphism, and Crustal Contamination According to Sm-Nd Data

This paper continues the Sm-Nd isotope geochronological research carried out at the two largest Paleoproterozoic ore complexes of the northeastern Baltic Shield, i.e., the Cu-Ni-Cr Monchegorsk and the Pt-Pd Fedorovo-Pansky intrusions. These economically significant deposits are examples of layered complexes in the northeastern part of the Fennoscandian Shield. Understanding the stages of their formation and transformation helps in the reconstruction of the long-term evolution of ore-forming systems. This knowledge is necessary for subsequent critical metallogenic and geodynamic conclusions. We applied the Sm-Nd method of comprehensive age determination to define the main age ranges of intrusion. Syngenetic ore genesis occurred 2.53–2.85 Ga; hydrothermal metasomatic ore formation took place 2.70 Ga; and the injection of additional magma batches occurred 2.44–2.50 Ga. The rock transformation and redeposited ore formation at 2.0–1.9 Ga corresponded to the beginning of the Svecofennian events, widely presented on the Fennoscandian Shield. According to geochronological and Nd-Sr isotope data, rocks of the Monchegorsk and the Fedorovo-Pansky complexes seemed to have an anomalous mantle source in common with Paleoproterozoic layered intrusions of the Fennoscandian Shield (enriched with lithophile elements, εNd values vary from −3.0 to +2.5 and ISr 0.702–0.705). The data obtained comply with the known isotope-geochemical and geochronological characteristics of ore-bearing layered intrusions in the northeastern Baltic Shield. An interaction model of parental melts of the Fennoscandian layered intrusions and crustal matter shows a small level of contamination within the usual range of 5–10%. However, the margins of the Monchetundra massif indicate a much higher level of crustal contamination caused by active interaction of parental magmas and host rock.

The Crystal Structure of Bornite Cu5FeS4: Ordered Fe and Split Cu

The crystal structure of bornite with ideal formula Cu5FeS4 from the Saishitang skarn copper deposit in Qinghai Province, along with bornite from the Yushui spouting hydrothermal copper deposit in Guangdong Province and the Bofang sandstone copper deposit in Hunan Province, has been refined by single-crystal X-ray diffraction with R1 = 0.0259–0.0483 (I > 2σ) and 0.0338–0.1067 for 2732 to 3273 unique reflections. As represented by the Saishitang sample, it is orthorhombic with a Pbca space group and unit cell parameters a = 10.97016(18) Å, b = 21.8803(4) Å, c = 10.9637(2) Å, V = 2631.61(8) Å3 and Z = 16. The structure is composed of sulfur layers parallel to the (0 1 0) lattice plane with interstices occupied by metal atoms. The Fe atoms occupy two tetrahedral sites with full occupancy, but the Cu atoms are all partially distributed over 20 paired sites, split from 10 sites with a distance ranging from 0.24 Å to 0.54 Å. The Fe-S tetrahedra are not split with Fe-S lengths from 2.2609 Å to 2.3286 Å (average 2.2997 Å). The Cu-S lengths in pyramidal triangles are from 2.218 Å to 2.397 Å (average 2.288 Å), whereas the Cu-S tetrahedra are strongly distorted, with great variations in Cu-S lengths from 2.224 Å to 2.604 Å (average 2.391 Å). The orthorhombic unit cell is stacked from 16 1a-type (5.5 Å) cubes; each cube has one tetrahedrally-coordinated Fe atom, five split from 3- to 4-coordinated Cu atoms, and two vacancies, i.e., 5CuIII–IV+FeIV+2[]+4S. The phenomenon of site-splitting of Cu atoms may provide for a more accurate structure of bornite, allowing for a better understanding of its magnetic properties and ore-formation conditions.

“ORE MAGMA”. WHAT IS IT FROM THE STANDPOINT OF THE GRANITE ORE-MAGMATIC SYSTEM?

The concept of “ore magma” remains an obscure hypothesis in ore formation. The article considers the process of natural overgrowth of trivial primary basaltic magma into an ore-bearing granite melting and further into the ore-forming “ore magma” as the concentration of volatile and ore components. The dark side of the problem lies in the fact that during the ore formation the “ore magma” liquates into contrasting phases and leaves practically no traces of itself (with rare exceptions). But the concept of the ore magma has received a logical scientific justification from the standpoint of ore-magmatic systems.

Well-Constrained Mineralization Ages by Integrated 40Ar/39Ar and U-Pb Dating Techniques for the Xitian W-Sn Polymetallic Deposit, South China

Mineralization ages of many mineral deposit types (such as orogenic Au, stratabound Cu, and Mississippi Valley-type Pb-Zn deposits) are still difficult to date by the traditional isotopic chronometry because of the lack of suitable minerals. We have made efforts to establish a widely suitable dating technique to determine ore formation ages using a high-precision 40Ar/39Ar method on ubiquitously present fluid inclusions in quartz, sphalerite, and other nonpotassium minerals from hydrothermal deposits. The Xitian W-Sn polymetallic deposit in central South China contains several minerals suitable for isotopic dating for interchronometer comparison. 40Ar/39Ar laser step heating of 16 micas from ore veins, greisen, and metallogenic granites yields flat age spectra and thus well-defined ore formation ages ranging from 152.4 ± 1.5 (2σ) to 148.1 ± 1.4 Ma with an average of 150.2 ± 0.6 Ma. 40Ar/39Ar progressive crushing of nine quartz samples produces well-defined isochron lines for their primary fluid inclusions corresponding to isochron ages of 153.7–149.9 Ma with an average of 151.6 ± 0.6 Ma. Cassiterites from three hand specimens have weighted mean 206Pb/238U ages of 151.5 ± 1.7 (2σ), 149.7 ± 2.1, and 151.7 ± 2.1 Ma. All these new geochronological dates and previous molybdenite Re-Os ages yield well-constrained mineralization ages of 153–148 Ma for the Xitian W-Sn polymetallic deposit, which also confirms conclusively that the quartz 40Ar/39Ar progressive crushing technique is a feasible, valid dating technique. Furthermore, significant age information on the secondary fluid inclusions is potentially obtained simultaneously by this technique. We expect that this novel dating technique will be widely applied to determine the geologic fluids trapped in minerals during hydrothermal mineralization, hydrocarbon accumulation, metamorphism, tectonic activities, and other geologic processes.

Witwatersrand composite paleoplacers

Background. The Witwatersrand gold province located in South Africa is the richest in the world. The Witwatersrand deposits are composite, where osmirids and diamonds are mined along with gold, silver and uranium. The genesis of the Witwatersrand deposits is controversial. Most local geologists support the hypothesis of the presence of paleoplacer deposits with subsequent metamorphic transformation of ore-bearing conglomerates. In addition, there are aeolian, hydrothermal-sedimentary, hydrothermal-sedimentary-metamorphic and magmatic models of ore formation.Aim. To establish the genesis of the Witwatersrand deposits.Materials and methods. Analysis of published literature and factual data.Results. Osmirids and diamonds are mined along with gold, silver and uranium from the composite Witwatersrand deposits. Such a set of useful components is not known in any of the deposits of magmatic or hydrothermal genesis. Considering the confinement of useful components to conglomerates, the detrital nature of most of the gold grains and the presence of various accompanying minerals in the heavy fraction, characteristic of igneous rocks of felsic, basic and ultrabasic composition, the hypothesis of the primary alluvial nature of the deposits of the Witwatersrand province looks the most reasonable.Conclusions. The factual material indicates an alluvial origin of the Witwatersrand deposits with subsequent metamorphic transformation of ore-bearing reefs. The primary alluvial formation of ore-bearing conglomerates is indicated by the confinement of gold and uranium to channel facies with a tendency to accumulate in the basal horizons of the reefs, the presence of rounded gold particles bearing the traces of transportation in alluvial flows, as well as a set of minerals in the heavy fraction of concentrate, characteristic of placers.

Volcanic and fault-fractured ore-controlling structures and minerals of the Azerbaijan part of the Lesser Caucasus

This article is focused on volcanic and fault-fissure ore-controlling structures of the Lesser Caucasus. The territory of the Lesser Caucasus is dissected by many deep tectonicfaults, along which volatile compound sofpostmagmatic products and water vapor migrate from great depths. The presence of numerous tectonic deep faults, which provide hydrodynamic connection of waters of fracturse, and fracture-veins, determines the complex mineralization of the chemical and gas composition of mineral-thermal waters throughout the studied area. The study of volcano-intrusive and fault-fracture ore-controlling structures, volcanogenic formations, petrochemical composition of rocks and associated mineral deposits and mineral-thermal waters of the Azerbaijan part of the Lesser Caucasus is relevant from a scientific and practical points of view. A detailed study of the modern structure of the Lesser Caucasus shows that transverse rupture faults and deep flexures cut through the folded system. Deposits of various minerals, including mineral and thermal waters, are confined to these ruptured faults. The study revealed that the formation and discharge of numerous outcrops of carbonic mineral and thermal waters occurs mainly in fissure systems, in deep tectonic faults. In the Lok-Karabakh zone, most of the sources of the formation of gold-bearing placers belong to the deposits and manifestations of the gold- quartz-low-sulphide formation. Gosha, Kedabek, Karadag, Chovdar, Dagkesemen, Gyzylbulag and less significant deposits and ore occurrences are located in this zone. The Dashkesan ore region is distinguished by the reserves of iron, alunite and cobalt. The Zaylik alunite deposits are world famous in terms of large reserves. Analysis of geological materials confirms that the origin and distribution of deep tectonic faults plays a major role in the formation of geological, including volcanoplutonic, metamorphic processes and associated ore formation. Currently, there are dozens of variously evaluated and explored gold deposits on the territory of the Republic. The bulk of the primary deposits are located in the Lesser Caucasus part of the Republic.