Trace element (TE) partitioning in the system “mineral-hydrothermal solution” is studied by the method of thermo-gradient crystal growth coupled with internal sampling of a fluid phase. The analytical procedure used enables evaluating of structurally bound and superficially bound modes of TE in crystals and determining corresponding dual partition coefficients. The case of precious metals (PM—Au, Pt, Pd) at 450 and 500 °C and 100 MPa pressure is considered. The minerals are pyrite, As-pyrite, magnetite, Mn-magnetite and hematite and fluids are ammonium chloride-based hydrothermal solutions. The partition coefficients for structural and surficial modes, $D_p^{str}$ and $D_p^{sur}$, are found to be unexpectedly high (except for Au in pyrite). High concentrations of PM are attributed to superficial nonautonomous phases (NAPs), which can be considered as primary concentrators of PM. We also have studied the co-crystallization (exchange) coefficients (D_e) of REE (Ce, Eu, Er, Yb) and Fe in magnetite and hematite at 450 °C and 100 MPa. $D_e^{sur}$ is elevated to two orders of magnitude as compared to $D_e^{str}$. It is shown that not only physicochemical parameters affect REE distribution in hydrothermal systems, but also NAP presence and its composition. The crystal growth mechanism specified by the agency of NAP is suggested. The study of PM distribution in natural pyrite of gold-ore deposits supported the importance of differentiating between structurally and superficially bound TE modes for correct use of experimental D values to determining element concentrations in ore-forming fluids.

The study focused on the forms of occurrence and distribution of hidden (“invisible”) noble metals (NMs = Au, Ag, Pt, Pd, Ru) in the coexisting pyrites and arsenopyrites of four samples of mineral associations from three Au deposits in the north-east of Russia. The unique nature of our approach was the combination of methods of local analysis and statistics of the compositions of individual single crystals of different sizes. This allowed us to take into account the contribution of the surface component to the total NM content and to distinguish the structurally bound form of the elements. The following estimates of the distribution coefficients of the structural (str) and surficial (sur) forms of elements were obtained: ${\overline{D}}_{Py/Asp}^{str}$ = 2.7 (Au), 2.5 (Pd), 1.6 (Pt), 1.7 (Ru) and ${\overline{D}}_{Py/Asp}^{sur}$ = 1.6 (Au), 1.1 (Pd), 1.5 (Pt and Ru). The data on Ag in most cases indicated its fractionation into pyrite ( ${\overline{D}}_{Py/Asp}^{str}$ = 17). Surface enrichment was considered as a universal factor in “invisible” NM distribution. A number of elements (i.e., Pt, Ru, Ag) tended to increase their content with a decrease in the crystallite size in pyrite and arsenopyrite. This may be due to both the phase size effect and the intracrystalline adsorption of these elements at the interblock boundaries of a dislocation nature. The excess of metal (or the presence of S vacancies) in pyrite increased Ag and Pt content in its structure and, to a lesser extent, the content of Ru, Pd and Au. Arsenopyrite exhibited a clear tendency to increase the content of Pt, Ru and Pd in samples with excess As over S. Sulphur deficiency was a favourable factor for the incorporation of Ag and platinoids into the structures of the mineral associations studied. Perhaps this was due to the lower sulphur fugacity. Pyrite with excess Fe was associated with higher contents of some NMs. The presence of other impurity elements was not an independent factor in NM concentration.

Partitioning experiments were done by hydrothermal synthesis of crystals containing trace elements (TEs) by internal sampling of fluid at the temperature of 450 °C and pressure of 1 kbar. The crystal phases obtained were magnetite, hematite, and Ni-spinel, which were studied using X-ray diffraction (XRD), X-ray electron probe microanalysis (EPMA), laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), atomic absorption spectrometry (AAS), and atomic force microscopy (AFM). The solutions from the sampler’s fluid probes were analysed by AAS for TEs included elements of the iron group plus aluminium. The highest co-crystallisation coefficients of TE and Fe between mineral and fluid (D_TE/Fe) in magnetite were measured for V, Al, Ni and Cr (in decreasing order of n units in value), a lower value was observed for Co (2 × 10⁻¹), and still lower values for Ti, Zn, and Mn (n × 10⁻²–10⁻³). In hematite, D_TE/Fe values were highest for Al and V (order of n units in value), while lower values characterised Ti, Cr, and Co (n × 10⁻¹–10⁻³), and the lowest values were exhibited by Cu, Mn, and Zn (n × 10⁻⁵). Copper was confirmed to be the most incompatible with all minerals studied; however, Cu had a high content on crystal surfaces. This surficial segregation contributes to the average TE concentration even when a thin layer of nonautonomous phase (NAP) is enriched in the element of interest. The accumulation of TEs on the surface of crystals increased bulk content 1–2 orders of magnitude above the content of structurally-bound elements even in coarse crystals. The inverse problem—evaluation of TE/Fe ratios in fluids involved in the formation of magnetite-containing deposits—revealed that the most abundant metals in fluids were Fe followed by Mn, Zn, and Cu, which comprised 10 to 30% of the total iron content.

The peculiarities of the distribution and binding forms of platinum group elements (Pt, Pd, Ru, Rh, Os and Ir) in the arsenopyrites and pyrites of the Natalkinskoe gold ore deposit (Northeastern Russia) were examined using atomic absorption spectrometry with analytical data selections for single crystals (AAS-ADSSC), a “phase” chemical analysis (PCA) based on AAS of different size-fractions of minerals, scanning electron microscopy with energy dispersive X-ray spectrometry (SEM-EDX) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). The arsenopyrites and pyrites of the Natalkinskoe gold deposit were found to concentrate not only Au but also platinum group elements (PGEs) such as Pt, Pd, Ru and Rh. The PCA showed that the highest contents (in ppm) were found in the monofractions of arsenopyrite—Pt up to 128, Pd up to 20, Ru up to 86 and Rh up to 21—and comparably lower in monofractions of pyrite—Pt to 29, Pd to 15, Ru to 58 and Rh to 5.9. The AAS-ADSSC method revealed two forms of uniformly distributed Pt, Pd and Ru corresponding to the chemically bound element in the structure of the mineral and in the superficial non-autonomous phase (NAP). The superficially bound form dominates over the structural form and presumably exists in a very thin surface layer of the crystal (~100–500 nm). The maximum contents of these PGE, chemically bound in the structure of arsenopyrite, reached values of (in ppm) 48, 5.9 and 48; and in pyrite structure, 68, 5.2 and 34 for Pt, Pd and Ru respectively. The contents of Pt, Pd and Ru related to NAP on the surface of the crystal were significantly higher and amounted (in ppm) for arsenopyrite to 714, 114 and 1083; and for pyrite 890, 62 and 690 for Pt, Pd and Ru, respectively. Preliminary results for the Rh form in arsenopyrite crystals suggest that the surface-related form (154–678 ppm) is more abundant than the structural form (17–45 ppm). Data from studying the surfaces of sulphide minerals by SEM-EDX and LA-ICP-MS confirmed the presence of Pt, Pd, Ru and Rh on the surface of arsenopyrite and pyrite crystals. These methods generated primary data on the content of Os and Ir in arsenopyrite and pyrite in the surface layer. The maximum content of Os and Ir found in arsenopyrites was up to 0.7 wt%. PGE-enriched fluids (up to ~3 ppm Pt) may exist in the gold ore deposit. It is assumed that there is a common mechanism of impurities uptake associated with the active role of the crystal surface and surface defects for gold-bearing arsenopyrites and pyrites. The surface enrichment is due to peculiarities in the crystal growth mechanism through the medium of NAP and the dualism of the element distribution coefficient in the system of mineral–hydrothermal solution, which is higher for NAP, compared to the volume of the crystal. Although mineral forms of Pt, Pd, Ru, Rh, Os and Ir have not been found at the Natalkinskoe gold deposit, their existence in the form of nano-scale particles is not excluded. This follows from the evolutionary model of surficial NAPs, assuming their partial transformation and aggregation with the formation of nano- and micro-sized autonomous phases of trace elements. The presence of PGE in the ores and the possibility of their extraction significantly increase the quality and value of the extracted raw gold materials at the Natalkinskoe deposit, and adds to the list of known platiniferous ore formations.

The dual distribution coefficients (D) that are related to structurally and superficially bound trace element (TE) in pyrite (Py) and pyrrhotite (Po) associations, crystallized hydrothermally at 400 °C and 1 kbar pressure, were determined. Three independent methods were used to estimate the structural and surficial TE contents (C^str and C^sur) and the corresponding D Py/Po values (D^str and D^sur), which were found, on average, to be 12.4, 0.8, 0.9, and 0.06 (D^str) and 2.6, 0.7, 2.0, and 0.07 (D^sur) for Ag, Pd, Cd, and Mn, respectively. The coincidence of a dual D for several elements was a result of coupled changes in C^sur and C^str. The selectivity (S) of the surficial nonautonomous phases (NAPs) that were responsible for TE accumulation (which is the ratio of TE concentrations in surficial and structural modes) was determined. It was shown that the interpretation of TE uptake by surficial phases was adequate and that this phenomenon is common in nature, independently of the system where it occurs—i.e., in experimental autoclaves or in hydrothermal ore deposits. Studies of NAPs selectivity can help in evaluating the total element compatibility in minerals and the maximum possible contents of structurally bound admixtures of the element (solubility) in minerals under given conditions. A significant surficial impurity accumulation effect is most important and well-pronounced for incompatible micro-elements with concentrations of less than ~0.1 wt%. The surficial mode may be a source of Pd and other platinum group elements and more abundant and easily refined than the structurally bound mode.

—First results of the experimental study of a hydrothermal multisystem including lanthanides (Ln) and Fe oxides (magnetite and hematite) are presented. Ln concentrations in solutions and crystals were determined by ICP-MS and LA-ICP-MS, respectively. The Ln partition and cocrystallization coefficients obtained are interpreted as the maximum estimates of “true” values corresponding to structurally bound admixture. It is shown that Ln (except for Eu) are the compatible elements in hydrothermal magnetite; heavy Ln (beginning with Tb) are compatible in hematite. The pronounced tendency of the increase of both coefficients with increasing Ln atomic number beginning from Gd–Tb was established. This is of great importance when light to heavy Ln ratio is used as a typochemical guide for localization of ore source. The Ln-rich phases in association with magnetite and hematite were obtained. These phases have oxychloride (without Fe) and oxyhydroxide (with Fe) composition and exemplify the simultaneous occurrence of light and heavy Ln within a single hydrothermal system due to the co-crystallization of phases selectively accumulating light and heavy lanthanides.