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.

Naldrettite (Pd2Sb): A new find in Brazil and comparison with worldwide occurrences

ABSTRACT Naldrettite (Pd2Sb) is a PGM discovered in 2005 in Mesamax Northwest deposit, Ungava region, Quebec, Canada. Before and after its approval, PGM with the naldrettite type composition have been reported from a number of localities worldwide. Most frequently, naldrettite has been documented in magmatic Ni–Cu–PGE sulfide deposits, hydrothermal veins in porphyry coppers of the Cu–Au type, and PGE deposits of Alaskan-type zoned intrusions. Naldrettite has been occasionally found in metasomatic Sb–As sulfide ore, metamorphic Ni–oxide ore, and podiform chromitites, although these occurrences have not been fully constrained by solid chemical analyses or paragenetic reconstruction. In this paper we report the first discovery of naldrettite in Brazil. This new finding occurs in a chromitite sample collected in the Luanga Complex, a Neo-archaean layered intrusion in the Carajás Mineral Province. Paragenetic association with alteration assemblages (ferrianchromite, Fe-hydroxides, chlorite) suggests precipitation of naldrettite from metamorphic hydrothermal fluids. The average composition of the Luanga sample (Pd1.76Pt0.24)Σ2.00(Sb0.57As0.43)Σ1.00 shows major substitution of Pt and As. These elements were derived from the breakdown of primary sperrylite, and were incorporated in naldrettite deposited by percolating fluids, at temperature below 350 °C (maximum temperature registered by the crystallization of associated chlorite). An overview of documented occurrences indicates that naldrettite can form in a variety of igneous rocks (ultramafic, mafic, felsic), even involving minimal concentrations of Pd and Sb. Crystallization of naldrettite generally occurs in the post-magmatic stage due to the activity of hydrothermal fluids containing volatile species Sb, As, Bi, Te, and Pd due to its higher mobility compared with the other PGE. A major issue concerns the origin of fluids that can be: (1) “residual”, after the main crystallization of the host magma, (2) “metamorphic”, during regional metamorphism or serpentinization, and (3) “metasomatic”, emanating from an exotic magma intrusion. The combination of two or three of these factors is the most likely process observed in the naldrettite-bearing complexes.

Mineralogy of noble metals (Au, Ag, Pd, Pt) in Volkovskoe Cu-Fe-Ti-V deposit (Middle Urals, Russia)

Research subject. The mineral compositions of titanomagnetitic (apatite, titanomagnetite) and copper-titanomagnetitic (bornite, chalcopyrite, apatite, titanomagnetite) ores of the Volkovskoe Cu-Fe-Ti-V deposit (Middle Urals, Russia).Methods. The research was carried out using a Jeol JSM-6390LV scanning electron microscope and X-ray spectral microanalyzers JXA-5 (Jeol) at the Geoanalitik Collective Use Center of the IGG UB RAS. Results and conclusions.Native gold (with ≤ 0.3 wt % Pd, 0.2–0.4 wt % Cu; fneness 800–914 ‰), tellurides of Pd, Au and Ag (merenskyite, keithconnite, sylvanite, hessite) and Pt arsenide (sperrylite) were found in the copper-titanomagnetitic ores. For the frst time, two generations of native gold (fneness 1000 and 850–860 ‰) and palladium telluride (keithconnite Pd3-xTe) were detected in titanomagnetitic ores. The sequence of ore mineral formation and the features of their genesis were revealed. Native gold (fneness 1000‰) in the form of microinclusions in titanomagnetite was attributed to the magmatic stage. Noble metal minerals, intergrown with copper sulfdes (bornite, chalcopyrite, digenite) and associated with late hydroxyl-bearing minerals (amphibole, epidote, chlorite), are superimposed in relation to the magmatic minerals (pyroxene, plagioclase, hornblende, apatite, titanomagnetite, ilmenite, etc.) of these ores. Merenskyite, sperrylite, high fneness gold (800–914 ‰), as well as carrolite, cobaltite, copper-cobalt telluride and bismuth tellurium-selenide kawazulite Вi2Te2Se are syngenetic with copper sulfdes. The Au-Ag tellurides were deposited later than these minerals. It is shown that the high fugacity of tellurium, which binds Pd, Au, and Ag into tellurides, prevents the occurrence of native gold containing high concentrations of palladium and silver.

Partitioning of REE between calcite and carbonatitic melt containing P, S, Si at 900-650 °C and 100 MPa

Many carbonatites host deposits of REE and HFSE, and fractional crystallization might be a potentially powerful mechanism controlling magma enrichment by these metals to economically significant values. At present, information about the control of fractional crystallization by partition coefficients of ore-forming elements at magmatic stage is incomplete. Here we present an experimental study of REE partitioning between carbonatite melt and calcite in the system CaCO3-Na2CO3 with varying amounts of P2O5, F, Cl, SiO2, SO3 at 650–900°C and 100 MPa using cold-seal pressure vessels and LA-ICP-MS. The presence of phosphorus in the system generally increases the distribution coefficients but its effect decreases with increasing concentration. The influence of temperature is great: at 900 − 770°C DREE ≥1, while at lower temperatures the values are below unity. Silicon also promotes the fractionation of REE into calcite, while sulfur contributes to the retention of REE in the melt. Our results imply that calcite may impose significant control upon REE fractionation at the early stages of crystallization of carbonatite magma and can be a closest proxy for monitoring the REE content in initial melt.

Pre-Pegmatite Stage in Peralkaline Magmatic Process: Insights from Poikilitic Syenites from the Lovozero Massif, Kola Peninsula, Russia

The Lovozero peralkaline massif (Kola Peninsula, Russia) is widely known for its unique mineral diversity, and most of the rare metal minerals are found in pegmatites, which are spatially associated with poikilitic rocks (approximately 5% of the massif volume). In order to determine the reasons for this relationship, we have investigated petrography and the chemical composition of poikilitic rocks as well as the chemical composition of the rock-forming and accessory minerals in these rocks. The differentiation of magmatic melt during the formation of the rocks of the Lovozero massif followed the path: lujavrite → foyaite → urtite (magmatic stage) → pegmatite (hydrothermal stage). Yet, for peralkaline systems, the transition between magmatic melt and hydrothermal solution is gradual. In the case of the initially high content of volatiles in the melt, the differentiation path was probably as follows: lujavrite → foyaite (magmatic stage) → urtitization of foyaite → pegmatite (hydrothermal stage). Poikilitic rocks were formed at the stage of urtitization, and we called them pre-pegmatites. Indeed, the poikilitic rocks have a metasomatic texture and, in terms of chemical composition, correspond to magmatic urtite. The reason for the abundance of rare metal minerals in pegmatites associated with poikilitic rocks is that almost only one nepheline is deposited during urtitization, whereas during the magmatic crystallization of urtite, rare elements form accessory minerals in the rock and are less concentrated in the residual solution.

Origin of alkali-rich volcanic and alkali-poor intrusive carbonatites from a common parental magma

The discrepancy between Na-rich compositions of modern carbonatitic lavas (Oldoinyo Lengai volcano) and alkali-poor ancient carbonatites remains a topical problem in petrology. Although both are supposedly considered to originate via fractional crystallization of a “common parent” alkali-bearing Ca-carbonatitic magma, there is a significant compositional gap between the Oldoinyo Lengai carbonatites and all other natural compositions reported (including melt inclusions in carbonatitic minerals). In an attempt to resolve this, we investigate the petrogenesis of Ca-carbonatites from two occurrences (Guli, Northern Siberia and Tagna, Southern Siberia), focusing on mineral textures and alkali-rich multiphase primary inclusions hosted within apatite and magnetite. Apatite-hosted inclusions are interpreted as trapped melts at an early magmatic stage, whereas inclusions in magnetite represent proxies for the intercumulus environment. Melts obtained by heating and quenching the inclusions, show a progressive increase in alkali concentrations transitioning from moderately alkaline Ca-carbonatites through to the “calcite CaCO3 + melt = nyerereite (Na,K)2Ca2(CO3)3” peritectic, and finally towards Oldoinyo Lengai lava compositions. These results give novel empirical evidence supporting the view that Na-carbonatitic melts, similar to those of the Oldoinyo Lengai, may form via fractionation of a moderately alkaline Ca-carbonatitic melt, and therefore provide the “missing piece” in the puzzle of the Na-carbonatite’s origin. In addition, we conclude that the compositions of the Guli and Tagna carbonatites had alkali-rich primary magmatic compositions, but were subsequently altered by replacement of alkaline assemblages by calcite and dolomite.

Agpaitic Alkaline Rocks in Southern Brazilian Platform: A Review

General information is presented on ten agpaitic occurrences located in southern Brazil and at the border between Brazil and Paraguay. All the Brazilian agpaitic rocks are Late Cretaceous in age, whereas the Paraguayan ones are older than Early Triassic. The most significant occurrence is Poços de Caldas, the largest alkaline massif in South America. In general, these agpaitic rocks contain mineral assemblages that indicate presence of typical halogen-bearing Na–Ca–HFSE phases, eudialyte-, rinkite- and wöhlerite-group minerals being the most frequent ones. However, these associations are indeed more complex in terms of composition, with accessory phases in some cases consisting of various minerals, including U–Th oxides/silicates, Nb oxides, REE–Sr–Ba bearing carbonates–fluorocarbonates–phosphates–silicates and Zr–Na rich silicates. They usually form late magmatic stage to hydrothermal/deuteric assemblages linked with coarse and fine-grained, mainly silica-undersaturated evolved rocks. Data also indicate significant differences in type, amount and composition of agpaitic minerals in all investigated occurrences.

Geochemistry and geochronology of alkaline dykes from the Finero Phlogopite Peridotite (Ivrea-Verbano Zone): insights into the Triassic-Jurassic tectono-magmatic events of the Southern Alps

<p>The Ivrea-Verbano Zone (IVZ, westernmost sector of the Southern Alps) represents a unique opportunity to investigate the Paleozoic to Mesozoic geodynamic evolution of the Gondwana and Laurasia boundary from the perspective of the lower continental crust. Only recently, the petrochemical record of Triassic-Jurassic magmatism has been recognized. It mainly affected the northernmost tip, the Finero Complex, where the continental crust was tectonically thinned before opening of Alpine Tethys. However, the Mesozoic magmatism in the Finero Complex is still poorly-constrained. Firstly, its extent is largely unknown, because the mantle and crustal intrusives were already enriched by Paleozoic processes. Secondly, Mesozoic melts migration started when the Finero Complex was still placed at P-T conditions typical of a continental crust-mantle transition (1 GPa): this has promoted the reopening of the geochronological clocks in both Paleozoic and Mesozoic rocks, which usually provides wide time intervals. Lastly, the finding of Mesozoic magmatism as composite veins/pods and metasomatised layers has not allowed an exhaustive reconstruction of the primitive melts geochemistry. To place further constraints on such issue, a new dyke swarm cropping out in the Finero Phlogopite Peridotite mantle unit has been investigated. Dykes usually cut at high angle the mantle foliation and are up to 60 cm thick. They are composed by coarse-grained hornblendite to anorthosite, both phlogopite/biotite-bearing. Many dykes are composite, showing variable proportions of hornblendite and anorthosite. In places, the dyke swam was affected by volatiles overpressure as late magmatic stage, which produced plastic flow and development of a porphyroclastic structure by deformation of the early cumulates, with widespread segregation of a fine-grained mica matrix.</p><p>Dykes mainly consist of pargasite, phlogopite/biotite, albite (An 8-10), in association with apatite, monazite, ilmenite, zircon, Nb-rich oxides, carbonates. Enrichments in Fe (amphibole and biotite) and Na (plagioclase) suggest segregation from evolved melts, strongly enriched in H<sub>2</sub>O, P, C. The large LILE and LREE contents in amphiboles, sometimes associated to high Nb, Ta, Zr and Hf concentrations, as well as the mineral assemblage, support an alkaline affinity of the melts. The strongly positive εHf<sub>t </sub>(+10) of zircons and the isotopic Sr composition of amphiboles (0.7042) point to a derivation of the melts from mildly enriched sources, possibly located at the crust-mantle interface.</p><p>Zircons from anorthosite layers are mostly anhedral fragments. They show homogenous internal structure or sector zoning. Concordant <sup>206</sup>Pb/<sup>238</sup>U zircon ages vary from 221 ± 9 Ma to 192 ± 8 Ma. The results of this study confirm that mantle input to the Southern Alps magmatism was of alkaline affinity from Norian to Sinemurian. A widespread fluids circulation induced by such magmatism at high P-T conditions was likely the main cause of the diffuse geochronological reset towards Mesozoic ages of the northern IVZ.</p>

Ore-Bearing Magmatic Systems with Complex Sn–Au–Ag Mineralization in the North-Eastern Verkhoyansk–Kolyma Orogenic Belt, Russia

This paper reports the results of a study of magmatic rocks with Sn–W–Au–Ag mineralization from the Kuranakh, Elikchan, and Istekh ore fields in the Northern batholith belt of the north-eastern Verkhoyansk–Kolyma orogenic belt in Eastern Russia. Using petrographic, mineralogical, geochemical, and isotopic methods, we determined the mineral compositions, petrochemistry, and geochemistry of magmatic rocks, the P–T conditions of their generation and crystallization, and their geodynamic affinity. The studied magmatic rocks have common geochemical characteristics that likely reflect the influence of fluids supplied from a long-lived, deep-seated mantle source. The ore fields are characterized by Sn–W–Au–Ag–Pb polygenetic mineralization. The magmatic and metallogenic evolution comprised five stages for the formation of magmatic rocks and ores. During the first stage (Berriasian–Barremian), arc-related magmatic rocks formed in an active continental margin setting and were associated with Au–Ag mineralization. The second, third, and fourth stages (Aptian–Campanian) took place in a crustal extension and rift setting, and were accompanied by Au–Ag and Sn–W mineralization. During the fifth (post-magmatic) stage, Sn–Ag–Sb and Pb–Ag mineralization occurred.

Multi-scale impacts of Antuco basaltic stratovolcano (Southern Andes, Chile) ca. 6.2 ka sector collapse: avalanche deposition, eruptive behavior transformation and hydrologic reconfiguration

<p>Reconstructing the complex processes triggered by catastrophic destruction of volcanoes on both their own magmatic system and the surrounding landscape, is a fundamental task for evaluating long-timescale volcanic hazards and controls on the development of volcanoes. Antuco stratovolcano (37.4°S, 71.4°W; Chile), is a dominantly basaltic composite edifice which original ca. 3300 m altitude edifice experienced a ca. 5 km<sup>3</sup> Bandai-type sector collapse at ~6.2 ka BP. We carried out field studies of its debris avalanche deposit (DAD), which was distributed to the W and consist of chaotic breccias, with a longitudinal facies transformation from 2 large proximal toreva-block facies (4 & 9 km W from the scar) to megablocks, blocks and matrix facies in distal areas (up to 20 km from the scar). Basal facies are fine grained shredded rocks and contain substratum injections and clastic dykes. The surface of the avalanche is hummocky, and the size, internal architecture and lithology of hummocks vary with distance. At El Peñón and Manquel (10 to 20 km W from the scar) the DAD is overlaid by a sequence of dilute pyroclastic density currents (PDCs) containing juvenile ash and highly vesicular porphyritic basalt scoria fine to medium lapilli size. Further W, one of the latest dilute PDC gave ca. 3.4 ky BP in charcoal.  These PDCs are separated from two thick, far-reaching basaltic andesite overlying lava flows (post-collapse Antuco basal flows) by a paleosol, and they show compositional features consistent with mixing of a highly zoned or compartmentalised magma storage system at <5km depth. Subsequently, that event was followed by the initiation of a renewed basaltic magmatic stage and cone regeneration at Antuco during the Late Holocene to the present. These observations plus the detailed study of the composition and texture of post-collapse products suggests a long-lasting reconfiguration of the plumbing system in response to depressurization induced by the sector collapse. The DAD also blocked the natural output of Lake Laja, increasing its level ca. 200 m and then triggering catastrophic outburst floods by dam rupture, preserved as alluvial beds interpreted as debris and hyperconcetrated flow deposits. The ancestral Laja lake outburst, eroded and redeposited tens of meters of basaltic sediments and boulders as far as 120 km within the Central Depression, W from the volcano. Downstream, along the Itata and Biobío rivers (the latter fed by Laja River) at least two fluvial/alluvial terraces are formed by these volcaniclastic materials, 140-170 km WNW from Antuco volcano. These deposits develop laminar, cross bedded and flaser structures. In addition, fragments of pumice, charcoal and archaeological ceramics have been recognised in the sediments. Ceramics where likely produced at the Talcahuano-1 archaeological site (ca. 1.890 BP), in agreement with charcoal that provides a maximum age between 1.8 and 1.85 ky BP for the younger flooding events. The coupled investigation of the impacts produced by massive debris avalanches, especially at basaltic-arc stratovolcanoes, is important to understand their long-term system evolution and hazards.</p>