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.

Особенности структур распада твердого раствора Cu-Ag cульфидов на месторождении Теплое (Северо-Восток России)

The authors consider features of the triple structure of Ag-Cu-S solid solution decomposition resulted from redistribution of matter in the ores of the epithermal Au-Ag Teploye deposit due to the spatial combination of volcanogenic Au-Ag mineralization, with relatively later copper porphyry. The discovery of the mckinstryite, copper-silver sulfide, permits to consider the occurrence of copper in acanthite a non-structural impurity element, while the term "cupreous acanthite" appears insufficiently substantiated. It has been suggested that the mckinstryite-jalpaite (+ acanthite) structure was formed not by the decomposition of a solid solution or the crystallization of a normal hydrothermal solution, but but in the course of solidification of the sluggish, viscous, colloid-like substance in a metastable environment under low-temperature conditions, and was accompanied by structural rearrangement of its mineral constituents.

Lanthanum Diffusion in Fluorapatite at 400 °C, 50 MPa and 4–16 wt %

Apatite is an important carrier of rare earth elements (REEs) and phosphorite is a potential REEs resource. However, the influence of hydrothermal fluids on the migration and enrichment of REE in apatite remains controversial. The experimental study of the interaction between REE-bearing fluid and apatite is one of the essential ways to understand the chemical behavior of rare earth elements in apatite. In this study, we conducted the fluid–mineral reaction experimental study (at 400 °C, 50 MPa and 4–16 wt %) between high lanthanum (La) content hydrothermal solution and low REE content to reveal the influence of different salinities on the diffusion of rare earth elements in fluorapatite. Based on in situ geochemical analyses of experimental products, we show that the diffusion coefficients of La in fluorapatite are between 3.24 × 10−15 and 5.88 × 10−15 m2/s. The salinity of the fluid has a great influence on the diffusion coefficient, with the increase of salinity, the diffusion coefficient increase.

Minerals of Bolshoi Semiachik geothermal fields (Central Kamchatka, Russia)

<p>The volcanic complex Bolshoi Semiachik is characterized by intensive hydrothermal activity which is expressed by presence of thermal fields with gas-steam jets (T up to ~ 140 ºC), boiling pots (T up to ~ 100 ºC), warm lakes (T up to ~ 90 ºC) and ground (T up to ~ 97 ºC) . The circulating hydrothermal solution is rich in ammonium, sulfate and locally in carbonate. To date, little is known about surface mineralogy that occurs at the geothermal fields of the volcanic complex Bolshoi Semiachik. The major geological expeditions were carried out there in the 1960`s, and there was also some additional research carried out in the 1980`s. The study of minerals occurring at the surface of geothermal fields is relevant for planetary science since similar minerals are suggested for Mars and Europa (Jupiter moon) and geochemistry since such environments of mineral formation are very specific.</p><p>In the summer 2020 the expedition of the Institute of volcanology and seismology has been organized in order to monitor thermal fields and to conduct mineral and water samples for study. Here we report the first data on mineral identification of processed samples (at about 50). At that moment, minerals have been identified by powder X-ray diffraction and electron-microprobe analyses.</p><p>The surface of Bolshoi Semiachik geothermal fields is covered by clay minerals with montmorillonite that is rich in disseminated pyrite being the most abundant. Among salt minerals the common phases are sulfates: halotrichite-, copiapite and voltaite-group minerals, alunogen, gypsum and native sulphur. The SiO<sub>2</sub> polymorphs: tridymite, cristobalite are also found at the geothermal field surface. In the zone called Central Crater chalcantite has been found in association with rhomboclase and tridymite. Some samples with zeolite-group mineral - laumontite were also found, which at the moment is identified less reliably. The central (high temperature) part of deposits around steam-gas jet is composed of dickite in association with sulphur and quartz covered by alunogen and halotrichite efflorescent. The rim (at about 1 meter from the center) is composed of smectites, marcasite and natroalunite. This zonation is likely caused by pH which is lower at the central part where the steam unloads and increases at the peripheral area around the steam-gas jet.</p><p>Acknowledgment. The study has been supported by RFBR project # 20-35-70008. We are grateful to Volcanoes of Kamchatka for letting us to conduct the field works at Bolshoi Semiachik thermal fields. Experimental works on mineral identification have been carried out using Analytical Centre of IViS and Research Park of SPbU.</p>

New View on the Genesis of the Bashuihe Pluton, Laoshan Granites, China: Indications from Fluid Inclusions and H–O Isotopes

Oval caves have recently been discovered in the Bashuihe granite pluton of Laoshan Mountain, China. Oval caves typically occur in alkaline granites. This study conducted microthermometry and stable isotope analysis of quartz inclusions from oval caves and host rocks from the Bashuihe pluton to reconstruct the diagenetic evolutionary history of the Laoshan area. The temperature measurement results indicated a homogenisation temperature range from 162.5 to 261.6°C (mean 203.9°C), a salinity range of 2.1–8.3 wt% (mean 5.07 wt%), and a density range of 0.8–0.98 g/cm3 (mean 0.90 g/cm3), indicating a low-temperature, low-salinity, and low-density fluid. The emplacement depth ranged from 2.73 km to 4.43 km, indicating medium-shallow granite. A hydrogen and oxygen isotope analysis ( δ D = − 83.58 – − 67.17 , δ 18 O H 2 O = 0.83 – 0.39 ) revealed that the diagenetic fluids of the Bashuihe pluton represented a mixed hydrothermal solution composed of meteoric water and magmatic water. The results of a whole rock, H–O isotopes, rare earth element, and high field strength element analysis on the Laoshan alkali granites suggest significant hydrothermal activity in the late stage of magmatism. Primary oval caves in the Bashuihe pluton most likely evolved in the following sequence: fluid was enriched in the late diagenetic stage, diagenetic minerals crystallised under low temperature and pressure conditions, the crystallisation rate accelerated, and the magma condensed rapidly. Moreover, the increase in magma fluid enabled the movement and convergence of fluid. The accumulated fluid and volatiles occupied more space, and rapid magma condensation trapped the accumulated fluid and volatiles in the pluton, forming the oval granite cave. This research provides a crucial theoretical reference for the development and utilisation of underground space and engineering buildings in granite regions.

Gemology, Mineralogy, and Spectroscopy of an Attractive Tremolitized Diopside Anorthosite Gem Material from the Philippines: A New Type of Material with Similarities to Dushan Jade

In recent years, a new type of material called Philippines “Dushan jade” has appeared in the gemstone market in China. This new type of material, very similar in appearance and physical properties to Dushan jade, an important ancient jade with a long history in China, is causing confusion in the market and poses identification difficulties. Microscopy, electron probe microanalysis, Fourier transform infrared (FTIR) spectroscopy, Raman microprobe spectroscopy, and ultraviolet-visible (UV-Vis) spectroscopy were used to study the gemology, mineralogy, and spectroscopy of rock samples from the Philippines in order to differentiate them from Dushan jade. The studies showed that Philippines rock is composed mainly of anorthite and minor amounts of diopside, tremolite, uvarovite, titanite, chromite, zoisite, prehnite, thomsonite-Ca, and chlorite, among which uvarovite, diopside, and tremolite are the main color causing minerals. The origin of the color is related to the electronic transitions involving Cr3+, Fe2+, Fe3+, and charge transfer between the ions. The paragenetic mineral formation sequence of Philippines rock can be divided into three stages: (1) the magmatic stage: anorthite phenocryst, diopside, chromite, and titanite are formed first in the magma; (2) the metamorphic stage: anorthite phenocryst undergo fracture and recrystallization; the early fluid intrusion transforms diopside into tremolite forming uvarovite-grossular-andradite solid-solution around the anorthite and chromite; and (3) the late hydrothermal stage: the late hydrothermal solution fills in fractures with prehnite, thomsonite-Ca, and zoisite being formed. From the comparison studies, it was established that Philippines rock and Dushan jade are two completely different type of material. Philippines rock should be called “tremolitized diopside anorthosite”.

Genesis of Gold Deposit in Um Trambishi Area, NE Atbara, Sudan.

Um Trambishi area consists of volcano-sedimentary sequence intruded by syntectonic pluton of granitoids composition with marginal fissures injected by aplite, had been subjected to dynamic metamorphism and obliterated by recrystallization forming different rock units varying in composition, forming greenschist terrains. These are an assemblage of recrystalline schists, meta-acid, cataclastic tonalite, granodiorites, meta-andesite, chlorite schists, quartzite, marble, metabanded chert, together with two generations of unmineralized deformed and undeformed white quartz, which are oriented mainly in E-W and N-S direction following faults trend. The area is subjected to four deformational phases of brittle-ductile transition. The metamorphic grade is Green Schist Facies. The alteration around the endo-contact zone is characterized by minerals: albite, chlorite, epidote, sericite, carbonate and kaolin with anomalous Au=0.91ppm, Ag=1.75ppm, As=44.6XRF, Co=15.70XRF, V=65XRF, Ti=4796XRF, Cr=23XRF, Sr=269XRF, and base metals Pb=1.51ppm, Zn=22.02ppm and Cu=13.59ppm. The first phase of hydrothermal solution carrying gold is related to the magmatic high temperature associated with granitoidal intrusions, deposited into the planar fabric S1 and has a characteristic signature of hypothermal processes .The second mode of occurrence is the regeneration of auriferous quartz stringers filling into the tectonic fissures trending 340⁰ and oblique to S1 as a result of D3 (Keraf Shear Zone) and has a characteristic signature of mesothermal processes.

Micro-Nanoscale Characteristics of Pyrite and Its Implications for Gold Mineralization: Two Cases of Gold Deposits in the Youjiang Basin and Southwestern Tianshan Mountains

The mineralogical and compositional characteristics of gold-bearing minerals and the occurrence of gold are not only of great significance to exploring the sources of ore-forming materials and their formation mechanisms but also helpful for designing reasonable beneficiations and smelting schemes and achieving remarkable economic benefits. This paper presents an integrated study on the crystal characteristics, elemental composition and distribution of pyrite (the main gold-bearing minerals), on the basis of electron probe microanalysis (EPMA), scanning electron microscopy (SEM), laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) and nano-secondary ion mass spectrometry (NanoSIMS). The occurrence of gold in the Shuiyindong gold deposit and Ashawayi gold deposit has been studied by means of microscopy, SEM, and EPMA images, elemental correlations, S–Fe–As ternary diagrams, logAs–logAu diagrams and Au/As ratios. The gold in pyrite of the Shuiyindong deposit is in the form of nano gold inclusions and lattice gold. The gold in pyrite of the Ashawayi deposit dominantly exists in the form of nano gold inclusions or is present as micro-nano gold particles in the cracks or edges of pyrite, some of which can exist as lattice gold. The ore-forming hydrothermal solution of the Shuiyindong gold deposit is mainly underground hot brine, but it may be reformed by a deep magmatic hydrothermal solution or volcanic-subvolcanic hydrothermal solution. The ore-forming hydrothermal solution of the Ashawayi gold deposit is mainly derived from the metamorphic hydrothermal solution formed during the orogenic process, and the ore-forming process or post-mineralization process may be reformed by the leaching of underground hot brine. Finally, the characteristics of ore-forming fluids and evolution of the two types of deposits are determined via pyrite element surface scanning. This paper shows that micro-nanoscale study of gold-bearing pyrite is of great significance to understanding the gold mineralization process and is worth further study.