Composition Of Microinclusions Research Articles

Platinum-Group Minerals in the Placer of the Kitoy River, East Sayan, Russia

The platinum-group minerals (PGM) in placer deposits provide important information on the types of their primary source rocks and ores and formation and alteration conditions. The article shows for the first time the results of a study of placer platinum mineralization found in the upper reaches of the Kitoy River (the southeastern part of the Eastern Sayan (SEPES)). Using modern methods of analysis (scanning electron microscopy), the authors studied the microtextural features of platinum-group minerals (PGM), their composition, texture, morphology and composition of microinclusions, rims, and other types of changes. The PGM are Os-Ir-Ru alloys with a pronounced ruthenium trend. Many of the Os-Ir-Ru grains have porous, fractured, or altered rims that contain secondary PGE sulfides, arsenides, sulfarsenides, Ir-Ni-Fe alloys, and rarer selenides, arsenoselenides, and tellurides of the PGE. The data obtained made it possible to identify the root sources of PGM in the placer and to make assumptions about the stages of transformation of primary igneous Os-Ir-Ru alloys from bedrock to placer. We assume that there are several stages of alteration of high-temperature Os-Ir-Ru alloys. The late magmatic stage is associated with the effect of fluid-saturated residual melt enriched with S, As. The post-magmatic hydrothermal stage (under conditions of changing reducing conditions to oxidative ones) is associated with the formation of telluro-selenides and oxide phases of PGE. The preservation of poorly rounded and unrounded PGM grains in the placer suggests a short transport from their primary source. The source of the platinum-group minerals from the Kitoy River placer is the rocks of the Southern ophiolite branch of SEPES and, in particular, the southern plate of the Ospa-Kitoy ophiolite complex, and primarily chromitites.

Structure and chemical composition of grain boundaries in the magnetic semiconductor GaSb<Mn>

The structure and chemical composition of grain boundaries in GaSb<Mn> magnetic semiconductors have been investigated. We determined that quenching of the GaSb melt with 2% Mn results in the formation of a textured polycrystal (111). The grain boundaries of the texture are formed by split 60 degree dislocations with <110> dislocation lines. Microinclusions based on the ferromagnetic compound MnSb are located on the stacking faults of split dislocations. The chemical compositions of microinclusions differ, but their average composition is close to Mn1.1Sb. The synthesized GaSb<Mn> is a soft ferromagnet with a coercive force of 10 Oe and a magnetic state approaching superparamagnetic

Methodological Aspects of Diamond Diagnostics for Solving of Geological Interpretation Problems

The article reveals the results of the long-term studies for diamond crystals found on the territory of Primorye in placer samples. The crystals’ optical and gemological characteristics, morphology, and the composition of micro inclusions were examined with the microprobe method. Special attention was paid to the study of the photoluminescence spectra of the diamonds, since it is one of the relatively new methods which enables to determine the origin of the diamonds. The rapid development of technologies for the synthetic technical-grade diamonds production has expanded the usage of rough diamonds, including the field of geological exploration. It is widely known that the natural diamonds of technical quality were initially used for making diamond crowns (the end of the XIX century and the beginning of the XX century). Synthetic diamonds are also used to produce crown bits, since the volume of their production is several times higher than the volume of the natural diamonds extraction. The synthetic diamonds properties are similar to those of the natural ones; synthetic diamonds are equal to natural diamonds in abrasive qualities, and the most important thing is that they have a lower processing cost. During the long-term exploitation of drilling crowns, diamond grains may fall out of their seats and be lost. This creates a real risk of natural objects contamination while searching new indigenous diamond deposits. The paper gives an algorithm of research conduction that allows to distinguish between natural and synthetic diamonds, prevent incorrect conclusions and to increase the validity of natural diamond findings in placer deposits.

The compositional peculiarities of microinclusions in diamonds from the Lomonosov deposit (Arkhangelsk province)

The data on the composition of microinclusions in diamonds from the Lomonosov deposit are reported. The studied diamonds include coated (n = 5) and cubic (n = 5) crystals. The determined range of the degree of nitrogen aggregation (4-39 %B1) in diamonds does not support their links with kimberlite magmatism, but their short occurrence in the mantle at higher temperatures is probable as well. The composition of melt/fluid microinclusions in these samples varies from essentially carbonatitic to significantly silicate. It is shown that the content of MgO, CaO, Na2O, Cl and P2O5 decreases with increasing content of silicates and water. Different mechanisms of generation and evolution of diamond-forming media are considered to explain the observed variations.

Состав микровключений в цирконах из пород Березовского мафит-ультрамафитового массива (Восточно-Сахалинская офиолитовая ассоциация)

Состав микровключений в цирконах из пород Березовского мафит-ультрамафитового массива (Восточно-Сахалинская офиолитовая ассоциация)

Protracted fluid-metasomatism of the Siberian diamondiferous subcontinental lithospheric mantle as recorded in coated, cloudy and monocrystalline diamonds

Five typical coated diamonds (from Udachnaya, Yubileynaya, and Aikhal kimberlite pipes) with untypically low microinclusion abundances and four monocrystalline diamonds (Udachnaya, Mir, Nyurbinskaya pipes) that exhibit thin intermediate microinclusion-bearing zones were examined in details for growth structures, characteristic infrared absorption and photoluminescence, and composition of microinclusions. The internal structures of diamonds of both types imply that fluid inclusions entrapment in diamonds does not necessarily relate to the terminal stage of rapid fibrous growth. Instead, nitrogen aggregation state in some diamonds showed that both fibrous coats and inclusion-bearing layers might experience an annealing during mantle residence long enough to pre-date the ultimate kimberlite eruption, whereas the diamonds with internal inclusion-bearing zones also experienced later protracted history of monocrystalline growth. The presence of chloride-carbonate-silicate fluids/melts in monocrystalline diamonds indicate their generation from media generally similar to that observed in some fibrous diamonds. However, the composition of these metasomatizing fluids is different for the mantle beneath Udachnaya (mostly carbonatitic) and other pipes (Aikhal, Yubileynaya, Mir; variable abundance of silicic high-density fluids). The abundance of silica-rich fluids record either a heterogeneous distribution of eclogites in the subcontinental lithospheric mantle, or the operation of silica-rich slab-derived fluids. The inclusion abundance as well as the type of growth (fibrous or monocrystalline) is considered to be controlled by the volume of fluid fluxes; in this case, fluid consumption leads to decreasing growth rates, diminishing inclusion entrainment and stability of layered octahedrons. The detected minor compositional variations of high-density fluids in these diamonds may be due to local scale thermal perturbation in the host source and/or limited chemical heterogeneity of the parental fluid. The high amount of chlorides in high-density fluids from monocrystalline diamonds provide a new evidence for compositions of fluids/melts acting as primary metasomatic agent in the deep mantle of Siberian craton.

Structure and Chemical Composition of Manganese-Doped GaSb Dislocations

Electron microscopic observations showed that the preparation of magnetic semiconductor GaSb〈Mn〉 by melt quenching is accompanied by discrete manganese precipitation on GaSb dislocations, this precipitation generating inclusions sized from a split micrometer to several micrometers. The chemical composition of microinclusions was determined by electron probe microanalysis. The superposition of the magnetic properties of compounds that are formed in the Mn–Sb system determines the magnetic properties of GaSb〈Mn〉.

Carbonate-silicate composition of diamond-forming media of fibrous diamonds from the Snap Lake area (Canada)

This study presents new data on the compositions of microinclusions in fibrous diamonds from the Snap Lake area in the eastern part of the Slave Craton (Canada). The compositional trends of diamond microinclusions are consistent with those of diamond-forming media ranging continuously between a highly carbonatitic endmember and a highly silicic endmember. The microinclusions exhibit general enrichment of most incompatible elements, which is probably indicative of their crystallization during partial melting of mantle peridotites and eclogites. Our results also suggest that the diamond analyzed in this study may have formed as a result of interaction between carbonate-silicate melts and peridotitic wall-rocks at the base of a thick lithospheric mantle at depths below 300 km. The trace element distributions in the studied diamond microinclusions show a general similarity to those previously found in the parental kimberlites and carbonatites. These data suggest that diamonds may have crystallized either directly from a kimberlitic/carbonatitic melt or from a proto-kimberlitic fluid/melt, which was derived from a source also common to kimberlites. This is supported by differences in the major element compositions of diamond-forming fluids/melts and kimberlites.

Fibrous diamonds from the placers of the northeastern Siberian Platform: carbonate and silicate crystallization media

The first data are presented on the compositions of microinclusions in fibrous diamonds from the Ebelyakh placers, northeastern Siberian Platform. Their fluid/melt microinclusions are of silicate or carbonate compositions. In general, the trace element patterns for the microinclusions correspond to kimberlites and carbonatites. The major-element composition differs significantly; for example, the microinclusions are considerably enriched in K and Na. In two of the studied diamonds, the microinclusion compositions differ considerably in the cores and rims. In one of them, the composition of the medium changes from chloride–carbonate to predominantly carbonate (sample HI-90); in the other one, from carbonate to silicate (sample HI-98). Similar carbon isotope characteristics of diamonds with microinclusions of two contrasting media might suggest their crystallization from a mantle reservoir with the same carbon isotope characteristics. The geochemical features of the microinclusions in the placer diamonds revealed their relationship with protokimberlitic carbonate–silicate fluids. Such fluids might result from the metasomatic interaction of volatiles and/or the low-degree partial melting of peridotite and eclogite substrates.

High-Mg carbonatitic microinclusions in some Yakutian diamonds—a new type of diamond-forming fluid

The composition of microinclusions in 26 fibrous cubic and coated diamonds from the Daldyn-Alakit kimberlite field, Yakutia were studied using EPMA and FTIR and the carbon isotopic composition of 4 of the diamonds was studied using SIMS. Fifteen diamonds carry carbonatitic high-density fluids (HDFs) with high MgO content (17–28 wt.%) whereas the MgO content of fluids in other diamonds does not exceed 14 wt.%. We propose that the two groups are distinct and evolved separately. The low-Mg suite is similar to previously reported carbonatitic to silicic HDFs from Africa and Brazil. The high-Mg suite is carbonatitic, but its composition is distinct from previously defined end-members. As the MgO content decreases from 28 to 17 wt.%, CaO, Na 2O, K 2O and Cl also decrease while the silica, Al 2O 3, TiO 2 and P 2O 5 contents remain constant. Additionally, the water band in the FTIR spectra of high-Mg HDFs is wider relative to the water band in the spectra of low-Mg carbonatitic HDFs. By combining EPMA and FTIR data, we have constrained the major element composition of the high-Mg carbonatitic end-member, which comprises 78 wt.% carbonates, 9% silicates, 6% water, 5% apatite and 2% halides. The composition of the high-Mg suite is closer to that of near-solidus melts of fertile carbonate peridotites and harzburgites. Thus, it should be possible to produce the Mg-rich HDF either by incipient melting, or by cooling and crystallization of a proto-kimberlitic melt at depth. However, the peridotitic system alone cannot explain the high alkali and Cl content of the fluids. We suggest that the elevated alkali and Cl content of the high-Mg carbonatites are related to interaction of carbonate peridotite with saline fluids, or of peridotite with chloride–carbonate melts. The major element composition of the high-Mg carbonatitic HDF is dominated by the low-fraction melts of the peridotite while the content of potassium and other incompatible elements is influenced by the contribution of the saline or carbonatitic fluids.

PLATINUM-GROUP MINERALS FROM FIVE PLACER DEPOSITS IN BRITISH COLUMBIA, CANADA

Platinum-group minerals (PGM) from a selected suite of gold- and platinum-group-element (PGE) placer deposits in British Columbia have been analyzed using the electron-microprobe (EMP). The PGM placer grains ( n = 70, 0.1–1.5 mm in size) principally comprise various Pt–Fe–(Cu) alloys: “Pt 3 Fe”-type alloys [Fe-rich platinum (formerly, ferroan platinum: Bayliss et al. 2005) or isoferroplatinum], Fe-rich Pt with an atomic ∑PGE:(Fe + Cu + Ni) ratio of 3.6–5.6, “(Pt,Ir) 2 Fe”-type alloy, members of the tulameenite–tetraferroplatinum solid-solution series extending from Pt 2 Fe(Cu,Ni) toward PGE 1+ x (Fe,Cu,Ni) 1− x , less common Ir-dominant Ir–Os–(Ru–Pt) alloys, subordinate Os-dominant alloys, and minor Ru-rich alloys and rutheniridosmine, the latter with an atomic Ir:Os:Ru proportion close to 1:1:1. Trace amounts of PGE sulfides and sulfarsenides: cooperite PtS, Ni-rich cuproiridsite (Cu,Ni,Fe)(Ir,Rh,Pt) 2 S 4 , unusual sperrylite-type [(Pt,Rh,Fe)(As,S) 2− x ] and platarsite-type [PtAs 1− x S 1+ x , or unnamed Pt(S,As) 2− x ] phases, and unnamed (Ir,Rh,Pt)S (?) crystallized at a late stage in low-S environments. Some PGM grains contain micro-inclusions of diopside, augite, ferro-edenite, a potassian sodic-calcic amphibole (richterite?), talc, clinochlore and euhedral quartz. High values of mg# [100Mg/(Mg + Fe)] of the ferromagnesian minerals in these inclusions suggest highly magnesian source-rocks. Textural and compositional data, in particular the zoned intergrowths of Pt–Fe–(Cu) alloy grains, which broadly resemble the zoned Pt–Pd–Cu stannides from the Noril’sk complex, indicate the following sequence of crystallization: (Pt,Ir,Rh) 3 Fe → (Pt,Ir,Rh) 1+ x (Fe,Cu) 1− x or Pt 2 Fe(Cu,Ni). The zoned Pt–Fe–Cu alloys likely formed by fractional crystallization of primary solid-solutions under closed-system conditions as a result of increase in the activity of Cu in the residual liquid after an early-stage (magmatic) crystallization of the Cu-poor core. The Cu-rich Pt–Fe alloys formed around these core zones and at their peripheries during a significant drop in temperature at a late stage of crystallization of the composite alloy grains. The compositions of micro-inclusions and exsolution lamellae of Os- and Ir-dominant alloys in Pt–Fe alloys imply uniform temperatures of equilibration within the range 750–800°C. The compositions of Cu-rich Pt–Fe alloys, PGE sulfides and sulfarsenides, and exsolution lamellae of Os- and Ir-dominant alloys, observed in intimate association with Pt–Fe alloys, are likely related to the crystallization of the coexisting Pt–Fe alloys. Narrow zones of Pt–Fe alloys richer in Pt and poor in Fe, observed at the boundary of placer Pt–Fe alloy grains, provide evidence for a removal of Fe and corresponding addition of Pt as a result of interaction with a low-temperature fluid. The terrane affinities, compositions and associations of the placer PGM examined appear consistent with two types of potential source-rocks, associated with Alaskan-type intrusions and the Atlin ophiolite complex. The preservation of faceted morphologies on many of the placer PGM grains implies a relatively short distance of transport from their source.

Relationship of strontium isotope ratio and chemical composition of micro inclusions in fibrous diamonds: Implication for isotopically different components of the upper mantle.

Relationship of strontium isotope ratio and chemical composition of micro inclusions in fibrous diamonds: Implication for isotopically different components of the upper mantle.