Sulfur–selenium isomorphous substitution and morphotropic transition in the Ag3Au(Se,S)2 series

Gold–silver sulfoselenides of the series Ag3AuSexS2–x (x = 0.25; 0.5; 0.75; 1; 1.5) were synthesized from melts on heating stoichiometric mixtures of elementary substances in evacuated quartz ampoules. According to X-ray single-crystal analysis, compound Ag₃Au₁Se_0.5S_1.5 has the structure of gold–silver sulfide Ag3AuS2 (uytenbogaardtite) with space group R3c. The volume of this compound is 1.5% larger than that of the sulfide analog. According to powder X-ray diffraction, compounds Ag₃AuSe_0.25S_1.75 and Ag₃AuSe_0.75S_1.25 also show trigonal symmetry. Compounds Ag₃AuSeS and Ag₃AuSe_1.5S_0.5 are structurally similar to the low-temperature modification of gold–silver selenide Ag₃AuSe₂ (fischesserite) with space group I4132. These data suggest the existence of two solid solutions: petzite-type cubic Ag₃AuSe₂–Ag₃AuSeS (space group I4132) and trigonal Ag₃AuSe_0.75S_1.25–Ag₃AuS₂ (space group R3c).It was found that fischesserite from the Rodnikovoe deposit (southern Kamchatka) contains 3.5–4 wt.% S. At the Kupol deposit (Chukchi Peninsula), fischesserite contains up to 2.5 wt.% S and uytenbogaardtite contains up to 5.3 wt.% Se. At the Ol’cha and Svetloe (Okhotskoe) deposits (Magadan Region), uytenbogaardtite contains up to 0.5 and 1.8 wt.% Se, respectively. Literature data on the compositions of silver–gold selenides and sulfides from different deposits were summarized and analyzed. Analysis of available data on the S and Se contents of natural fischesserite and uytenbogaardtite confirms the miscibility gap near composition Ag3AuSeS.     

Peculiarities of formation of the Devonian Au-Ag epithermal mineralization in pipe orebodies

The formation of pipe orebodies of the Yunoe deposit was caused by thermal metamorphism related to intrusions of postmineral basaltic dikes and characterized by heating to a temperature above 500°C, which is evident from index minerals. Based on thermodynamic calculations, the model elaborated suggested that intense heating of pyritized rhyolitic necks resulted in S loss and increase in the S concentration in the hydrothermal system that was favorable for uytenbogaardtite crystallization. The results of thermobarogeochemical investigations are consistent with this model. They show the possibility of fast cooling of the oreforming fluid in subsurface conditions or upon mixing with cold meteoric waters that probably caused the formation of ore mineralization with uytenbogaardtite, acanthite, and native gold. In particular, the results confirm a rise in salinity with a decrease in temperature in fluid inclusions.     

Peculiarities of Ore Formation at the Teploe Ag–Au Deposit,Northeast Russia

The mineralogical–geochemical and thermobarogeochemical features of the Teploe Ag–Au epithermal deposit are considered. The dissolution and redeposition of ore minerals are a result of thermal metamorphism. The redistribution of Ag leads to the formation of lenaite and exsolution structures in minerals of the Cu–Ag–S system; abundant stromeyerite, jalpaite, and mckinstryite compose a significant amount of ores. The atypical physicochemical parameters of the formation of ores include high (for epithermal mineralization) temperatures and low salinity and density of the fluid typical of a dry vapor. The results of fluid inclusion study ascribe the Teploe deposit to an intermediate class of epithermal deposits.     

The age of HP metasomatism in shear zones during collision-related metamorphism in the Lapland Granulite Belt: The U/Pb SHRIMP II dates on zircons from sillimanite-hypersthene rocks of the Porya Guba nappe

Doklady Earth Sciences - This study presents the first U/Pb SHRIMP II dates on zircons from metasomatic sillimanitehypersthene rocks within the Porya Guba nappe of the Lapland Granulite Belt. The...     

Rb-Sr age of metasomatism and ore formation in the low-temperature shear zones of the Fenno-Karelian CRATON,Baltic Shield

Manifestations of the main types of metasomatites (beresite, propylite, listwaenite, aceite, and gumbeite) were identified in the shear zones of the Fenno-Karelian craton on the basis of the previously proposed systematics of metasomatic facies. These metasomatites were formed in shear stress environments, which determined their morphological features, in particular, finely banded texture. Comparatively low-temperature conditions of infiltration process and salting out effect (reduction of CO₂ solubility with increasing salt content) lead to the heterogenization of fluid into two phases: aqueous salt solution and almost pure CO₂. This results in more aggressive and mobile behavior of the fluid, and, correspondingly, more intense differentiation of the matter and contrast in metasomatic banding. Relations between metasomatic parageneses indicate an evolution trend of the processes from propylite, beresite and listwaenite to alkaline varieties and their repeated manifestation in the same shear zones. The results of Rb-Sr isochron dating of ore metasomatites from eight deposits and occurrences of the Fenno-Karelian craton (more than 100 samples of rocks and minerals) confirm previous assumptions. In general obtained data show that the shear zones controlling the distribution of the studied occurrences operated as fluid pathways during a long time period, up to 200 Ma, after the Svecofennian orogeny completion and did not show any correlation with Paleoproterozoic and Neoarchean magmatism. Rb-Sr isotope data on the metasomatites indicate three peaks of the post-Svecofennian metallogenic activity: 1700–1780, 1600–1650, and 1400–1500 Ma. Since the studied ore deposits were formed within tectonic structures, which evolved on the Archean crust and have a long prehistory, and fluid flows were subjected to intensive contamination by ancient crustal material, a relatively high initial Sr isotope ratios of formed ore-metasomatic systems were developed. High variablity of this value in the studied rocks ((⁸⁷Sr/⁸⁶Sr)₀ from 0.706 to 0.750) is related to the heterogeneity of crustal protolith and to the relative storage and manifestation of the juvenile component of the fluid, which was responsible for the metasomatic transformation of the Archean and Paleoproterozoic rocks and ore-deposit formation.