Mica Alteration Research Articles

Gold Mineralization, Hydrothermal Alteration, and Li Isotope Fractionation at Cochenour Orogenic Gold Deposit, Red Lake, Canada

Abstract Lithium isotopes have been used to study subduction zones, continental weathering, and magmatic–hydrothermal transitions, but little is known about their behavior in metasomatic footprints surrounding hydrothermal ore deposits. Although minerals such as biotite, chlorite, and white micas may sometimes be characteristic of Au mineralization, they are not truly diagnostic given that they also usually occur outside the ore zones. Here, whole rock δ7Li compositions are measured in samples from drill core cutting through metasomatized basalts from the Cochenour orogenic Au deposit, Red Lake, Canada, to verify whether Au ore-related biotite, chlorite, white micas, and quartz have specific δ7Li signatures. Laser ablation ICP-MS maps show 1000–1100 ppm Li in biotite, 200–600 ppm in chlorite, and 20–100 ppm in white mica. In the ‘arsenopyrite-bearing replacement-style’ alteration and mineralization, Au is associated with arsenopyrite, pyrrhotite, and chalcopyrite in the ore zone, but is more concentrated in white mica than in sulfides outside the ore zone. In the cross-cutting ‘quartz-actinolite vein-style’ alteration and mineralization, Au is distributed with chalcopyrite and arsenopyrite in the selvages of the veinlets, but is associated with pyrite, calcite, and biotite outside the ore zone. Mass balance calculations suggest that Au ore-related biotite, chlorite, white mica, and quartz have δ7Li values of ∼+4‰, ∼0 to +2‰, ∼−2‰, and ∼+6 to +12‰, respectively. This is explained by Li isotopic fractionation occurring during the retrograde sequence of auriferous biotite-chlorite-white mica alteration and the late auriferous quartz vein event, identified in the present samples as well as in the Red Lake-Campbell mine complex and the Red Lake greenstone belt. In the ‘arsenopyrite-bearing replacement-style’ alteration, the intercorrelation of δ7Li and Au and their correlation over distance show greater amounts of Au in biotite-enriched rocks, where biotite has δ7Li values of ∼+4‰. Such values indicate a mantle source for the early biotite-related auriferous fluid at Red Lake.

Novel insights about petroleum systems from source and reservoir rock characterization, Cretaceous Deposits, Babouri-Figuil Basin, Northern Cameroon

The Babouri-Figuil Basin (BFB) is a frontier basin for petroleum in Cameroon. It belongs to the series of Cretaceous rift basins of the West and Central Rift System (WCARS), the origin of which is related to the opening of the South Atlantic. Within the same rift system, commercial hydrocarbon accumulations have been discovered in Chad, Sudan, Niger and, more recently, in Nigeria (Gongola Basin). The study of the geology of the BFB just recently received considerable attention, mainly because of its presumed hydrocarbon potential. In the pursuit of researching possible petroleum systems in the BFB, the current study provides a first look into the characterization of source and reservoir rock and its integration into a 2D lithostratigraphic model. The study was solely based on outcrop samples. Black shale and massive claystone are good to excellent hydrocarbon source rocks [e.g., up to 38 wt% total organic carbon (TOC), up to 943 mg/g hydrogen index, up to 85 m thickness, up to 20–30 km lateral extension], with moderate to high values of extractable organic matter (e.g., >10,000 ppm). Calcareous claystone, on the other hand, are poor source rocks [e.g., <0.20 wt% TOC]. The samples are thermally immature, except for those located close to volcanic intrusion at Golombe that have reached the threshold for oil generation (Tmax >435 °C, production index >0.1). The petrographic analysis of sandstone revealed that they are fine-grained to coarse-grained, poorly to moderately sorted, texturally and compositionally immature to submature, subarkosic to arkosic arenites. The main diagenetic processes that affected sandstones are as follows: moderate to intense compaction characterized by the development of long, concavo-convex, and sutured contacts between grains; cementation through calcite, iron oxide, and quartz cements; alteration of mica and feldspar grains; partial to complete dissolution of feldspar, mica, amphibole grains, and calcite cement; and the replacement of feldspar and mica grains by clay minerals. Alteration and dissolution increase the porosity of sandstone through the creation of secondary pores. However, mechanical compaction through the development of a pseudomatrix and cementation as pore-filling materials have significantly reduced the quality of sandstone beds as conventional petroleum reservoirs. Hence, the best reservoir-quality sandstones in the basin are generally located in the upper portion of the basin in terms of its lithostratigraphic model. They are the cleanest sandstones with the smallest amount of cement and the lowest ductile grain content (pseudomatrix), with a thickness that varies from 3 m to 120 m and a lateral extension of 20 km. The lithostratigraphic model of the basin is characterized by an extensive lacustrine environment that provided a thick sequence of organic-rich formations; sand deposited as extensive reservoirs sandwiched between shale/claystone beds; the development of stratigraphic traps through lateral facies change; and the widespread deposition of lacustrine and floodplain claystone that provide regional seals. The similarities between the Babouri-Figuil Basin and proven petroleum systems in other WCARS rift basins suggest that the basin may host at least one petroleum system where actively generating source rocks are present.

Influence of magmatic and magmatic-hydrothermal processes on the lithium endowment of micas in the Cornubian Batholith (SW England)

The Cornubian Batholith (SW England) is an archetypal Variscan rare metal granite with potential for Li-mica mineralization. We present a petrographic, trace element and multivariate statistical study of micas from the Cornubian Batholith granite series and related hydrothermally altered units to assess the role of magmatic vs subsolidus processes and of fluxing elements (F and B) on the Li cycle during the evolution of the system. The mica types are as follows: (1) magmatic, which include Fe-biotite, protolithionite I and phengite-muscovite from the most primitive granites, and zinnwaldite I from more fractionated lithologies; (2) subsolidus, which encompass high-temperature autometasomatic Li-micas and low-temperature hydrothermal muscovite-phengite. Autometasomatic species include protolithionite II, zinnwaldite II and lepidolite, which were observed in the most fractionated and hydrothermally altered units, and occur as replacements of magmatic micas. Low-temperature hydrothermal Li-poor micas formed via alteration of magmatic and autometasomatic micas or as replacement of feldspars, and albeit occur in all studied lithologies they are best represented by the granite facies enriched in metasomatic tourmaline. The evolution of micas follows two major trends underlining a coupling and decoupling between the Li(F) and B fluxes. These include as follows: (1) a Li(F)-progressive trend explaining the formation of protolithionite I and zinnwaldite I, which fractionate Li along with Cs, Nb and Sn during the late-magmatic stages of crystallization, and of zinnwaldite II and lepidolite forming from the re-equilibration of primary micas with high-temperature Li-B-W-Tl-Cs-Mn-W-rich autometasomatic fluids; (2) a Li(F)-retrogressive trend explaining the low-temperature hydrothermal muscovitization, which represents the main Li depletion process. Trace element geochemistry and paragenesis of late muscovite-phengite support that muscovitization is a district-scale process that affected the upper parts of the granite cupolas through acidic and B(Fe-Sn)-saturated hydrothermal fluids associated with metasomatic tourmalinization, which were mixed with a low Eh meteoric component.

Magmatic-hydrothermal fluid evolution of the tin-polymetallic metallogenic systems from the Weilasituo ore district, Northeast China

The large Weilasituo Sn-polymetallic deposit is a recent exploration discovery in the southern Great Xing’an Range, northeast China. The ore cluster area shows horizontal mineralization zoning, from the inner granite body outward, consisting of high-T Sn–W–Li mineralization, middle-T Cu–Zn mineralization and peripheral low-T Pb–Zn–Ag mineralization. However, the intrinsic genetic relationship between Sn-W-Li mineralization and peripheral vein-type Pb–Zn–Ag–Cu mineralization, the formation mechanism and the deep geological background are still insufficiently understood. Here, we use fluid inclusions, trace elements concentrations in quartz and sphalerite, and H–O isotope studies to determine the genetic mechanism and establish a metallogenic model. Fluid inclusion microthermometry and Laser Raman spectroscopic analysis results demonstrates that the aqueous ore-forming fluids evolved from low-medium salinity, medium–high temperature to low salinity, low-medium temperature fluids. Laser Raman spectroscopic analysis shows that CH4 is ubiquitous in fluid inclusions of all ore stages. Early ore fluids have δ18OH2O (v–SMOW) values from + 5.5 to + 6.2‰ and δD values of approximately − 67‰, concordant with a magmatic origin. However, the late ore fluids shifted toward lower δ18OH2O (v–SMOW) (as low as 0.3‰) and δD values (~ − 136‰), suggesting mixing between external fluids derived from the wall rocks and a contribution from meteoric water. Ti-in-quartz thermometry indicates a magmatic crystallization temperature of around 700 °C at a pressure of 1.5 kbar for the magmatic ore stage. Cathodoluminescence (CL) imaging and trace element analysis of quartz from a hydrothermal vug highlight at least three growth episodes that relate to different fluid pulses; each episode begins with CL-bright, Al-Li-rich quartz, and ends with CL-dark quartz with low Al and Li contents. Quartz from Episode 1 formed from early Sn-(Zn)-rich fluids which were likely derived from the quartz porphyry. Quartz from episodes 2 and 3 formed from Zn-(Sn)-Cu-rich fluid. The early magmatic fluid is characterized by low fS2. The SO2 produced by magma degassing reacted with heated water to form SO42−, causing the shift from low fS2 to high fS2. The SO42− generated was converted to S2– by mixing with CH4-rich, Fe and Zn-bearing external fluid which led to late-stage alteration and dissolution of micas in vein walls, thus promoting crystallization of pyrrhotite, Fe-rich sphalerite and chalcopyrite and inhibiting the precipitation of anhydrite. This study shows that ore formation encompassed multiple episodes involving steadily evolved fluids, and that the addition of external fluids plays an important role in the formation of the later Cu–Zn and Ag–Pb–Zn mineralization in the Weilasituo ore district.

Alteration of trioctahedral micas in the presence of inorganic and organic acids

The alteration of two trioctahedral micas, biotite and phlogopite, was investigated at the meso, micro, and nanoscale using three complementary microscopy techniques to better understand mica surface reactivity. In situ and ex situ experiments were performed to monitor the mineral interface during dissolution in acidic solutions (nitric and oxalic acid, pH ∼ 1–2), over a temperature range of 25–100°C. The inorganic acid was used as a benchmark condition to elucidate the effect of the organic acid on the dissolution behavior. The observed topographical changes that arose during mineral alteration revealed the simultaneous occurrence of different processes that heterogeneously shaped the mica surface: 1) the retreat of pre-existing and newly formed steps (edge surface reactivity). In the case of biotite, layer curling and peeling-off occurred in the presence of nitric acid whereas dendritic-shaped step edges resulted from the effect of oxalic acid; 2) the nucleation of etch pits and the formation of dissolution channels on the (001) surface. Oxalic acid promoted the growth of the pits to such an extent that they were discernible at each scale and resolution investigated; and 3) precipitation of secondary phases. Overall, a multi-scale approach offers new insights into the dissolution behavior of biotite versus phlogopite and provides and enhances understanding of the effect that oxalic acid has on the surface reactivity of mica.

Paleoproterozoic gold deposits at Alta Floresta Mineral Province, Brazil: two overprinted mineralizing events?

Abstract Large gold provinces commonly show complicated mineralization histories, and the Paleoproterozoic Alta Floresta, one of Brazil's most exciting Au–Cu mineral provinces, is a good example. The current models defined four deposit types, all connected to a single (1.88–1.75 Ga) magmatic–hydrothermal event. However, long Province history, diverse geodynamic environment and older ages of Type-1mineralization weaken the single metallogenic event and enable the hypothesis of overprinted mineral events. By scale-integrated analyses, we revise the tectonic–geological context, structural–hydrothermal alterations and chlorite–white mica geothermobarometry and propose the Type-1 as an older, granitoid-hosted orogenic mineralization, with subsequent overprinting by the magmatic–hydrothermal event. The older orogenic gold event developed orogenic gold deposits on WNW-trending shear zones in the Peixoto de Azevedo domain granitic–gneiss rocks. Phengite, biotite and chlorite–carbonate phyllonites (3.3–6.1 kbar, 300–420°C) host fault-fill quartz veins (pyrite–chalcopyrite–magnetite–pyrrhotite–gold–Bi–Ag tellurides). Mg-rich chlorite–phengite is the main alteration footprint for this mineralization type. A younger magmatic–hydrothermal event in the Juruena magmatic-arc rocks produced Fe-rich chlorite–white mica alteration zones (0.6–4.6 kbar, 120–380°C) and disseminated and stockwork–breccia ore (pyrite–chalcopyrite–gold–molybdenite–Ti minerals–allanite) in porphyry–epithermal deposits. Where the younger mineralization overprints the older, phyllic alteration destroyed the phengite orogenic gold phyllonite S n +1 foliation. The ages of two pyrite populations (1979 and 1841 Ma) in the older fault-fill veins and molybdenite in late fractures (1805–1782 Ma) or disseminated in the c. 1.79 Ga syenogranite porphyry suggest more than two episodes of mineralization. These two events differ in their alteration styles, P–T conditions and structural, mineralogical and textural ore styles. The multiscale approach sheds light on the relationships between the various mineralization events, allowing a new explorational potential within the province. Supplementary material: Chlorite and white mica complete chemical composition data and other characteristics of the studied deposits are available at https://doi.org/10.6084/m9.figshare.c.6056324

Lithological controls on soil geochemistry and clay mineralogy across Spodosols in the coastal temperate rainforest of southeast Alaska

The interaction of similar soil forming factors with different parent materials determines soil chemical weathering, influencing soil processes and properties. In the coastal temperate rainforests of southeast Alaska, Spodosols is the dominant soil order in well-drained parent materials regardless of the lithology. Yet, the role of lithology on chemical weathering, base cation depletion, and mineral transformation during pedogenesis remains elusive. Here, we established a lithosequence comprised of soils derived from tonalite, phyllite, slate, and metavolcanic rocks to test the hypothesis that despite the soils in southeast Alaska presenting similar taxonomy and morphology, the influence of lithology in chemical weathering intensity and mineralogy can be detected. We evaluated physicochemical properties, clay mineralogy, Fe-Al oxides, chemical weathering, and elemental mass balance on eleven Spodosols sampled along the lithosequence. We also propose a new weathering index, the Weathering Index for Spodosols (WISP), suitable to evaluate the weathering degree of soils with Al mobility and base-rich parent materials. This index fills a gap in traditional weathering indices that uses Al as an immobile element and/or does not evaluate the leaching of main base cations. We found the pedons across the lithosequence expressed similar physicochemical properties, with predominantly thick profiles, presenting andic properties, relatively thick spodic horizons, and thin E horizons. Podzolization imposed similar mineral transformation trends, mainly dissolution of chlorite, mica alteration to interstratified mica-vermiculite, and formation of smectite in E horizons. Kaolinite was detected in the soils from all lithologies, but only in trace amounts in phyllite soils. Fe oxides depth distribution was similar but with a higher concentration in the slate (44.5 ± 4.2 g/kg), followed by metavolcanic (31.6 ± 4.0 g/kg), phyllite (27.9 ± 3.8 g/kg), and tonalite soils (19.9 ± 5.7 g/kg) and predominantly as organometallic complexes (extracted by Na-pyrophosphate). The stronger weathering of chlorite and mica in the E horizons was reflected in larger losses of Mg and K, while Ca and Na were more depleted in the spodic horizons, suggesting plagioclase weathering. Slate soils were the most depleted in base cations, followed by phyllite, tonalite, and metavolcanic soils. The more depleted status of slate soils was supported by the higher WISP values (WISP = 62), followed by phyllite (WISP = 55), tonalite (WISP = 53), and metavolcanic soils (WISP = 45). Our results demonstrated that lithology controlled elemental depletion intensities, Fe oxide concentrations, and partially the clay fraction mineralogy in addition to podzolization acting as a dominant pedogenic process across all sites. These findings advance our understanding of the role of lithology on soil mineralogy/geochemistry that impacts critical soil functions, such as the soil carbon cycle, elemental fluxes, organo-mineral interactions, and solid-solution reactions.

Rare-Metal Enrichment and Nb–Ta Fractionation during Magmatic–Hydrothermal Processes in Rare-Metal Granites: Evidence from Zoned Micas from the Yashan Pluton, South China

Abstract Rare-metal mineralization in aluminous granites (i.e. rare-metal granites, RMGs) is usually associated with high degree of magma fractionation, strong enrichment in fluxing elements (Li, B, P, and F), and the operation of both magmatic and hydrothermal processes. Experimental data have established the theoretical basis for melt- or fluid-driven rare-metal mineralization. To investigate these mechanisms in natural samples, this paper presents detailed textural and compositional variations for zoned micas and rare-metal minerals from rocks of the Late Jurassic Yashan rare-metal granitic pluton, South China. This pluton preserves an evolutionary sequence from ore-barren rocks to those containing ores formed by Ta–Nb–Li mineralization during magmatic–hydrothermal processes. Three main units of the Yashan pluton are exposed: from bottom to top, these comprise protolithionite–muscovite granite (Unit I), Li-mica granite (Unit II), and topaz–lepidolite granite (Unit III), representing crystallization from successive magma batches from a deep-level magma chamber. The gradual decrease in Nb/Ta and K/Rb in both whole rocks and micas from units I to II to III illustrates fractionation within the magma chamber, accompanied by successive enrichment of incompatible elements in the residual melt. Furthermore, the strong enrichment of fluxing elements such as Li, P, and F in Unit III likely led to lower magma viscosity, which may have helped to expulse interstitial residual melt from the magma chamber. Zoned micas from the Yashan pluton are composed of Li-phengite or lepidolite inner cores and muscovite rims. Cores show increasing Li, F, and rare-metal elements from units I and II to Unit III, consistent with the trend of magmatic fractionation. In contrast, rims that formed in the residual hydrosilicate melt show marked decrease in F and rare metals (i.e. Li, Cs, Rb, Nb, Ta, Sn, and W). Chemical variations of the zoned micas and rare-metal minerals were used together with Rayleigh fractionation modeling calculations to reconstruct the contribution of melt–fluid immiscibility to the crystallization of rare metals, which took place via the combination of melts enriched in high-field-strength elements (e.g. Nb and Ta) and fluids enriched in mobile elements (e.g. Mn, Fe, W, and Cs). Strong fractionation of Nb and Ta, along with extreme enrichment of Ta, in RMGs is caused mainly by the crystallization of micas and columbite-group minerals, and also depends on the degree of fractional crystallization. The subsolidus alteration of micas by acidic fluids may have leached some Nb rather than Ta during chloritization, which could have partly contributed to the enrichment of rare metals and fractionation of Nb–Ta. It is concluded that magmatic–hydrothermal processes, including magmatic evolution and fluid exsolution, are critical for rare-metal enrichment and Nb–Ta fractionation in RMGs.

Zn-clays in the Kihabe and Nxuu prospects (Aha Hills, Botswana): A XRD and TEM study

Abstract Zinc clays are commonly found in oxidized Zn deposits and, even though they rarely represent the main target of the ore exploitation, they can be used as a proxy to restore the genetic conditions during ore-forming processes. This work sheds light on the micro- to nano-mineralogy and on the genesis of Zn-clays in the Kihabe and Nxuu prospects (located in the Aha Hills district, Northern Botswana), through an integrated XRD and TEM study of the mineralized facies occurring in the ore system. The Kihabe and Nxuu ores are hosted in a Neoproterozoic metamorphozed quartzwacke unconformably covered by the recent sedimentary rocks, also containing calcretes, of the Kalahari Group. In the analyzed samples, four distinct mineralogical facies have been recognized: (1) vanadate-calcrete facies, poor of Zn-clays; (2) low Zn-clay facies, characterized mostly by clays showing low Zn concentrations; (3) Zn-clay facies, containing proper Zn clay minerals; and (4) sulfide facies, devoid of Zn-clays. In all the facies detrital dioctahedral mica (muscovite and illite) is interstratified with smectite in the form of random (R0) to short-range ordered (R1) I/S, which locally shows significant Zn concentrations. In the sulfide facies kaolinite overgrowing onto mica packets has been detected. The low Zn-clay facies is dominated by Zn-bearing beidellite, with minor kaolinite and fraipontite. The Zn-clay facies consists mostly of a random (R0) interstratified clay between a 7 Å phase corresponding to fraipontite, and a 2:1 swelling clay component identifiable with the dioctahedral smectite, with minor sauconite. The micro- to nano-scale paragenetic study performed by TEM indicates that the above-mentioned clays formed through a multistage process, eventually ending with the genesis of Zn-bearing phyllosilicates assemblages. The main steps were: (1) alteration of detrital mica and dissolution of feldspar clasts, which led to the formation of epitaxial kaolinite and replacive beidellite; (2) fertilization of barren clays and formation of replacive to epitaxial fraipontite/smectite and of Zn-bearing mica, through input of Zn2+ deriving from sphalerite or willemite dissolution by mixed meteoric-hydrothermal fluids; (3) formation of low tetrahedral charge sauconite, either in pores or as replacement of K-feldspars under surficial hypersaline conditions, possibly also linked to the establishment of the arid climate in region. These processes provide insights into genesis of Zn-Pb-V ore deposits in northwest Botswana. Furthermore, the identification of a Zn-smectite species having a stevensite-like stoichiometry is valuable for future studies dealing with the systematics of clay minerals.

Micro- and Nanotexture and Genesis of Ball Clays in the Lower Cretaceous (SE Iberian Range, NE Spain)

Ball clay deposits in the SE of the Iberian Range (NE Iberian Peninsula) consist of Albian clays and siltstones with greyish and blackish colors, interbedded with subbituminous coals. The ball clays are nowadays mined for the manufacture of white color ceramics. The mineralogy of these deposits consists mainly of kaolinite, illitic phases, and quartz. The euhedral to sub-euhedral morphology of the kaolinites suggests their in-situ origin. The anhedral morphology of the illites and the presence of frayed illites suggest a detrital origin. At the micro-scale, authigenic kaolinite booklets are observed filling pores and forming mica/kaolinite intergrowths, in which the kaolinite grows between the cleavage sheets of pre-existing detrital mica. At nanometer scale, illite/smectite (IS) phases are detected forming interlayers with mica and kaolinite, and evidence of the replacement of mica by kaolinite is observed. The matrix consists of defective illite and kaolinite, and random mixed layers of kaolinite-I/S (Kln-IS), illite-I/S (Ilt-IS), and I/S-smectite (IS-S). The textures of illite and the presence of different types of mixed layers suggest that the expandable phases and kaolinite are products of mica alteration. The effectivity of the alteration was probably a consequence of the low pH that occurred in the environment due to the presence of abundant organic- and acidic- rich fluids.

Influence of carbonate rocks on soil properties in the humid tropical climate of atlantic forest, Rio de Janeiro –Brazil

Soil carbonate accumulation plays an important role in C sequestration, accounting for approximately 40% of the C stored in soils worldwide. Under humid tropical climate these carbonates are rapidly dissolved, intensifying CO2 emissions. Despite this, little is known about the weathering of carbonates in areas with complex geology. This study aimed to evaluate the influence of the parent material on soil attributes and weathering processes under humid tropical climate conditions along toposequences formed by dolomitic marbles and calc-silicate rocks interspersed with garnet-biotite gneiss. Samples and soils profiles were collected on sites of both parental materials and at different points of a toposequence, namely: upper third, middle third and lower third of a slope. The profiles were analyzed for their morphological, physical, chemical and mineralogical attributes. The attributes of soils formed from metamorphic sedimentary rocks were more influenced by geological processes than pedological ones. It was found that the nature of the parent material was the main responsible for the attributes of the profiles, especially the strongly alkaline pH, being greater than 8.0 in most soil horizons, and the high contents of exchangeable Ca2+ and CaCO3 equivalent, with maximum values of 40.6 cmolc kg−1 and 763 g kg−1, respectively. As for mineralogical composition, it was verified the occurrence of minerals such as talc, vermiculite and Fe and Ti oxides, with metamorphic processes and the addition of hydrothermal fluids being the main responsible for the presence of these minerals. The supply of high contents of basic cations, with maximum values of 61.4 cmolc kg−1, high Si/Al ratio (reaching values close to 5.0) and the high contents of CaCO3 conditioning the strongly alkaline pH hinder the dissolution of silicates, even those with low resistance to weathering, despite the hot and humid climate conditions. The action of weathering processes in humid tropical climate is more marked in acidic materials, being responsible for the in situ formation of kaolinite from the alteration of mica and feldspar.

Mineralogy of the Svetloye epithermal district, Okhotsk-Chukotka volcanic belt, and its insights for exploration

The Svetloye epithermal district (SED) is located within the Okhotsk-Chukotka volcanic belt (OCVB), 220 km southwest of Okhotsk. The OCVB, which formed during the Cretaceous with ore potential, is a large marginal volcanic rock belt associated with continental subduction. Svetloye occupies two volcanic edifices of different ages – the northwestern volcanic center (including the Emmy deposit) consists of basaltic-andesite of the Khetanian suite (Coniacian-Santonian time (K2), and the southeastern volcanic center (hosts the Elena, Tamara, Lyudmila, and Larisa deposits) comprises the dacite-rhyolite-leucogranite formation of the Urak suite (Campanian-Maastrichtian time (K2). Distinct host rocks and different levels of erosion in the two edifices of Svetloye led to various altered rocks, mineral assemblages, and diverse types of gold mineralization. The alteration halo of the Emmy deposit contains vuggy residual quartz, with Au-Ag-telluride and Au-Ag mineralization plus other tellurium-bearing minerals (goldfieldite, kawazulite, coloradoite, melonite, altaite, tellurobismuthite, tellurantimony, and native tellurium). Altered rocks of the other deposits are characterized by variable halos, including vuggy residual quartz, alunite, dickite, quartz-alunite-dickite, illite-chlorite, illite, illite–smectite, smectite, quartz-carbonate, and quartz-white mica alteration, hypogene and supergene processes. The hydrothermal (hypogene) and supergene ore processes within the SED occurred in 3 stages: leaching and barren quartz-rutile-pyrite zone with halos of quartz-alunite-dickite-pyrite (or quartz-illite-chlorite-calcite-pyrite), then quartz-pyrite with polysulfides and gold, gold-silver telluride, and finally supergene with 'mustard' gold. The alteration halo, epithermal ore bodies as hydrothermal veins and breccias with vug infill, colloform, and crustiform textures, hypogene sulfide assemblage represented by galena, sphalerite, tennantite-tetraedrite, and chalcopyrite of SED are typical of epithermal styles with an high-intermediate sulfidation signature, possibly indicates a porphyry deposit at depth. The lack of high-sulfidation-state sulfides such as hypogene covellite, enargite, famatinite, luzonite in the residual quartz may be due to their erosion at shallower paleodepth, and/or to their supergene oxidation below the present day surface.

Separating provenance and palaeoclimatic signals from particle size and geochemistry of loess-palaeosol sequences using log-ratio transformation: Central European loess belt, Czech Republic

Loess-palaeosol sequences represent the most complete terrestrial archives of Quaternary climatic cyclicality. Particle size and geochemistry are widely used proxy data in palaeoclimatologic analysis of loess-palaeosol sequences. The palaeoclimatologic signals hidden in the texture and chemical composition of the Czech loess-palaeosol sequences, which are part of the European loess belt, are modulated by temporal changes in the interplay of oceanic and continental macroclimates and by the diverse bedrock geology of the Bohemian Massif.Innovative tools of compositional data analysis, including log-ratio transformation and scalar-on-function regression, can substantially enhance the information value of large granulometric and geochemical datasets, when compared to classical statistics of raw data. Particle size distribution and bulk-rock geochemistry of 389 and 542 samples, respectively, from four Czech loess-palaeosol sequence sections representing the last glacial-interglacial cycle were analysed in this study. Centered log-ratio transformation was applied to the key elemental proxies and their spatial (between sections) and stratigraphic (within section) distribution. Centered log-ratio transformed densities of key elements were then plotted against particle size distribution to assess the particle size control on element concentrations.Nearly all loess-palaeosol sequence samples exhibit a bimodal particle size distribution with medium/coarse silt and coarse clay fraction representing the main modes. The dominant silt fraction is completely allogenic. The clay component is partly allogenic, transported by high altitudes air stream, and partly authigenic (neoformed in situ by pedogenic processes). However there is only a minor variation in particle size between the sections, the centered log-ratio transformed density functions reveal that the particle size control on the distribution of major and trace elements is highly site-specific. The provenance signal is recorded especially in coarser-grained fractions transported for a short distance from the source area by near-surface wind. The proportion of the authigenic clay fraction, the alteration of feldspars, micas and low alkali contents indicated by high Rb/K ratio and high values of Rb/Sr and Sr/Ca ratios, which indicate the intensity of carbonate leaching, reflect the intensity of chemical weathering, which is highest in the podzols. Precipitation rates were likely the main microclimatic factor that controlled the compositional differences. The loess-palaeosol sequences in areas with higher present-day annual precipitation show higher contents of clay minerals, higher degree of loessification (cementation) and subsequent pedogenic changes.

Petrography, diagenesis and geochemistry of the Cambro-Ordovician Khabour sandstones, western Iraq: Implications for reservoir quality and the impact of the Hirnantian glaciation

Deeply buried (2.3–3.5 km) sandstones of the Cambro-Ordovician Khabour Formation are important hydrocarbon reservoirs in western Iraq. An integrated approach incorporating petrography, scanning electron microscope (SEM) and X-ray diffraction (XRD) was applied to investigate diagenesis and diagenetic minerals and their impacts on reservoir quality. Major and trace element distribution using X-Ray Fluorescence (XRF) analysis also was carried out to identify sandstones deposition during or after the Hirnantian glaciation. The sandstones of the Khabour Formation are dominated by sublitharenites and subarkoses. Several diagenetic processes have affected the sandstones of the Khabour Formation. These include early diagenetic porosity occluding events, such as common formation of illite and sericite coatings, along with rare quartz overgrowths, and calcite cementation. This was followed by secondary porosity forming events which include calcite dissolution, leaching of feldspar, alteration of micas and fracturing. The pore systems are dominated by secondary pores, vuggy and fractured porosity, intergranular and intragranular pores and poorly-connected macropores. A decrease in heavy minerals in sandstones from the upper part of the Khabour Formation and an associated decrease in both Zr and Hf concentration, which are typically associated with zircon, supported by a decrease in mica content and low API gamma ray unit values are considered to be a signature of sandstones affected by the Hirnantian glaciation.

Protracted Magmatism and Mineralized Hydrothermal Activity at the Gibraltar Porphyry Copper-Molybdenum Deposit, British Columbia

Abstract The Gibraltar Cu-Mo deposit, with a total tonnage of 3.2 million tons (Mt) Cu, is located in the Canadian Cordillera and hosted by the Late Triassic Granite Mountain batholith. The batholith formed through multiple intrusions of tonalitic rocks over a period of ~25 m.y. beginning at 229.2 ± 4.4 Ma in the Quesnel island arc before the accretion of the arc to the North American continent. Late in its evolution, Cu fertile magmas intruded in the center of the batholith, during at least three events from 218.9 ± 3.1 to 205.8 ± 2.1 Ma. The fertile magmas were hotter and more mafic than older barren magmas. They generated magmatic-hydrothermal activity, forming potassic alteration and white mica alteration, and produced Cu mineralization as chalcopyrite-quartz veinlets and disseminated chalcopyrite. Zircon in the Cu-bearing tonalite intrusion (218.9 ± 3.1 Ma) shows high Ce4+/Ce3+ (681 ± 286 [2σ], n =15) compared to those from older barren intrusions (129 ± 56 [2σ], n = 118). Oxidation conditions for parental magmas are calculated using the compositions of zircon and amphibole. The magmas for Cu-bearing intrusions have an average of fayalite-magnetite-quartz buffer (FMQ) +1.7 ± 0.7 (2σ, n = 73), whereas those for older barren intrusions have slightly lower fO2 (avg FMQ +1.3 ± 0.5 [2σ], n = 108), although the values are overlapping for the two. The bulk rocks of Cu-bearing tonalite intrusions in the Granite Mountain batholith have low Sr/Y ratios (&amp;lt;22) independent of the degrees of alteration. The low ratios are also reflected by low Sr/Y in zircon, suggesting that the low Sr/Y ratios of bulk rocks represent those of unaltered rocks. The values are low compared to those associated with many other porphyry Cu deposits globally. The data suggest that igneous rocks elsewhere with low Sr/Y in bulk rocks may have a potential to host economic Cu deposits. Ratios of Ce/Nd and Ce/Ce* (=Ce/((NdN)2SmN)) in zircon are positively correlated with the Ce4+/Ce3+ in zircon from the Granite Mountain batholith. Since the former two ratios can be obtained solely from zircon composition, these ratios from detrital zircon may be useful in evaluating the occurrences of oxidized intrusions in regional mineral exploration.

Mineralogical constraints on the magmatic–hydrothermal evolution of rare-elements deposits in the Bailongshan granitic pegmatites, Xinjiang, NW China

The Bailongshan pegmatite district in south Xinjiang Province, NW China, is a newly discovered area of rare-element pegmatites. It is a superlarge Li–Rb reserve that includes an estimated 3.45 million tons of Li2O and 176,000 tons of Rb2O. On the basis of a detailed characterization of textures and minerals in the Bailongshan 2# and 5# pegmatites, six zones are identified: a saccharoidal albite zone (zone I), a graphic pegmatite zone (zone II), a blocky microcline zone (zone III), a muscovite–quartz zone (zone IV), a cleavelandite–spodumene zone (zone V), and a quartz–spodumene zone (zone VI). The detailed mineralogical characteristic is performed by EPMA for illustrating both the existent state of rare-elements and the magmatic-hydrothermal evolution of the Bailongshan pegmatite. Lithium, Be, Rb, Nb, Ta, and Sn are major economic rare-elements, which are concentrated in zones V and VI. The rare-elements are hosted by minerals including spodumene, montebrasite, eucryptite, lepidolite, beryl, Rb-enriched K-feldspar, columbite-group minerals, cassiterite, and tourmaline, which formed from the fractional crystallization of pegmatitic magma. The Bailongshan pegmatites show lower degrees of fractional crystallization than many other Li-Cs-Ta (LCT-family) pegmatites worldwide, as evidenced by the relatively low Nb-Ta, Zr-Hf, and K-Rb fractional degree in the carrier minerals. In zones V and VI, the primary minerals record intense interaction with hydrothermal fluids, which caused the alteration of spodumene and mica, the development of Cs- or Na-enriched veinlets in beryl, and the formation of Mn- and Ta-enriched veinlets coupled with micropores within columbite-group minerals. The hydrothermal fluids were composed predominantly of Li, F, Cs, and Ta, which conformed to the evolved trend of pegmatitic melt. These late hydrothermal fluids exsolved from the magma as it differentiated, and may have the benefit with rare-elements metallogeny.

Fluid inclusion and stable isotope constraints on the heavy rare earth element mineralisation in the Browns Range Dome, Tanami Region, Western Australia

This study reports on fluid inclusion and oxygen isotope compositions of mineralised and barren hydrothermal quartz veins and hosting metasedimentary rocks associated with the heavy rare earth element (HREE) mineralisation in the Browns Range Dome of the Tanami Region, Western Australia. The HREE mineralisation consists of quartz and xenotime-bearing hydrothermal veins and breccias that occur along sub-vertical faults within Archean to Paleoproterozoic metasedimentary rocks. Based on analysis of nearly 550 quartz-hosted primary fluid inclusions, three fluid inclusion types were identified in the mineralised samples: type I low-salinity H2O-NaCl (largely <5 wt% NaCl; consistent with meteoric water), type II medium-salinity H2O-NaCl (12–18 wt% NaCl), and type III low- to high-salinity H2O-CaCl2-NaCl (1 to ca. 24 wt% NaCl + CaCl2). Homogenisation temperatures of all fluid inclusion types vary over a relatively wide range from 100 to 250 °C. Barren quartz veins contain only type I low-salinity H2O-NaCl fluid inclusions, with homogenisation temperatures extending from 170 to 350 °C. Raman analyses of all three fluid inclusion types confirmed their aqueous nature with no carbon-bearing fluid species identified. The three fluid inclusion types indicate mixing of three hydrothermal fluids: a low-salinity H2O-NaCl meteoric fluid (<5 wt% NaCl), a medium-salinity H2O-NaCl (12–18 wt% NaCl) fluid, and a high-salinity H2O-CaCl2-NaCl (ca. 24 wt% NaCl + CaCl2) fluid.Limited LA-ICP-MS analysis found detectable Y, Ce, U and Cl only in the type III fluid inclusions, which indicates that transport of ore metals was (at least partly) by Cl complexes in the type III fluid. The δ18Ofluid values calculated from quartz from mineralised samples are in the range defined by the Archean metasedimentary host rocks of the Browns Range Metamorphics (δ18Ofluid = +1.8 to +5.2‰) and the unconformably-overlying Paleoproterozoic Birrindudu Group sandstones (δ18Ofluid = +8‰). Collectively, our fluid inclusion and oxygen isotope data, together with other field, mineralogical and geochemical data, support an ore genesis model involving mixing of the three hydrothermal fluids in fault zones and along unconformity surfaces in, and around, the Browns Range Dome. The meteoric low-salinity H2O-NaCl fluid potentially carried P from the Birrindudu Group sandstones, and the high-salinity H2O-CaCl2-NaCl fluid leached HREE + Y from metasedimentary rocks of the Browns Range Metamorphics. Ore deposition occurred following mixing of the P-bearing and HREE + Y-bearing fluids, and was associated with a widespread white mica alteration. The temperature and pressure during the fluid-fluid mixing and mineralisation was between 100 and 250 °C, and 0.4 and 1.6 kbar, respectively.

Age, geological setting, and paragenesis of heavy rare earth element mineralization of the Tanami region, Western Australia

Metasedimentary rock-hosted heavy rare earth element (HREE) mineralization occurs as numerous orebodies distributed across a large district of the Tanami region of central Australia, close to a regional unconformity between Archean metasedimentary rocks of the Browns Range Metamorphics (BRM) and overlying Proterozoic Birrindudu Group sandstones. The orebodies consist predominantly of quartz, xenotime, and minor florencite and occur along steeply dipping structures within a stockwork of hydrothermal veins and breccias. Paragenetic stages of the mineralization include (1) a pre-ore stage of a greenschist-facies overprint of detrital minerals including quartz, alkali feldspar, plagioclase, and muscovite aligned in the premineralization foliation; (2) syn-ore quartz and white mica alteration associated with a multistage mineralization of the ore minerals, primarily in breccias and veins; and (3) a post-ore stage of veining and brecciation forming several generations of quartz, plus hematite, barite, anhydrite, and pyrite. In situ U–Pb dating of xenotime from several deposits/prospects yielded an age range for mineralization of 1.65 to 1.60 Ga; this timeframe lacks local magmatism or orogeny and is significantly younger than the ca. 1.72 Ga 40Ar/39Ar age of the pre-ore muscovite. Far-field stresses associated with the distal Isan and Liebig Orogenies are invoked as drivers of large-scale fluid flow and fault (re)activation in the region. We propose that ore formation was achieved via fluid leaching of REE from the BRM, followed by fluid mixing in fault zones, especially in the vicinity of the unconformity between the BRM and overlying Birrindudu Group sandstones. This mineralization style shares many features with unconformity-related U deposits, and there is significant potential for discovery of further REE orebodies of this style, especially in the vicinity of regional unconformities, in intercontinental sedimentary basins.

Geology, igneous geochemistry, mineralization, and fluid inclusion characteristics of the Kougarok tin-tantalum-lithium prospect, Seward Peninsula, Alaska, USA

The Kougarok prospect is situated in a Sn-W (+Ta, Nb, Li, Be) metallogenic belt formed in a post-collisional to within-plate tectonic environment. Crystal fractionation of granitic magma, combined with its mixing/mingling with mantle-derived mafic magma, is proposed as the major process causing the formation of the complex B-rich to Li-F-type granitic suite and associated Sn-rare metal (Ta, Nb, Li) mineralization. An intrusion of alkalic (potassic lamprophyric) mafic magma into a crystallizing biotite-tourmaline granite magma reservoir may have supplied a geochemically distinct assemblage of volatiles (F) and associated metals (Li, Ta, Nb) into the boron- and Sn-rich granitic system. An alternative model considers differentiation-driven unmixing of mafic and granitic silicate melts sequestering different volatile and mineralizing species that could also trigger magma-fluid and fluid-fluid unmixing, with separation of gaseous fluid coexisting with immiscible granitic and mafic magmas in the form of magmatic melt-fluid-crystal “suspension.” Biotite-tourmaline granite was accompanied by early quartz-tourmaline-cassiterite greisen at > 430–380 °C followed by quartz-tourmaline-chlorite-cassiterite stockwork at < 350 °C. The immiscibility of F- and B-rich fluids was followed by preferred ascent of F-rich fluids upward in the granitic magma reservoir, with its strong enrichment in F, Li, Ta, and Nb and subsequent crystallization of zinnwaldite granite at a shallower level, with separation of homogenous high-salinity magmatic fluid at ~ 600 °C. Crystallization and fluid exsolution continued at lower temperatures, followed by formation of topaz-quartz stockscheider at 550–500 °C. Ta-Nb mineralization in the uppermost part of the zinnwaldite granite appears to be associated with final episodes of magmatic crystallization, particularly W-Nb-rutile and columbite-tantalite with a higher Nb content. Some Ta-Nb minerals, such as columbite-tantalite with a higher Ta content, appear to be also stable during the post-magmatic stage, in quartz-tourmaline+topaz greisen and higher-temperature quartz-albite-Li mica alteration that replace zinnwaldite granite at ~ 500 to 400 °C. Nearly contemporaneous, lower-temperature quartz-albite-Li mica alteration and quartz-muscovite, quartz-topaz, and quartz-fluorite greisens formed from boiling fluids at 390–350 °C and caused removal of Ta-Nb minerals. Instead, fluid cooling and neutralization of boiling fluids affected Sn solubility and promoted massive cassiterite deposition in the late greisens. The latest phyllic quartz-sericite-carbonate alteration assemblage, comprising arsenopyrite, pyrrhotite, chalcopyrite and other sulfides, native Bi, and Bi tellurides, formed from boiling fluids at < 310 °C.

Use of TEM-EDX for structural formula identification of clay minerals: a case study of Di Linh bentonite, Vietnam

Transmission electron microscopy linked with energy-dispersive X-ray spectroscopy (TEM-EDX) was applied to characterize mineralogical signals of weathering processes in the Di Linh bentonite deposit (Vietnam) and to visualize the effects of Na activation on the smectitic phases. Modelling of X-ray diffraction patterns (oriented mount) was applied in order to refine the computed structural formula. X-ray diffraction, X-ray fluorescence and Fourier-transform infrared spectroscopy (FT-IR) methods were also applied to verify the TEM-EDX results. AnExcel-based routine has been developed in this research to allow fast computation of structural formulae and classification of the investigated clay particles. This routine supports the acquirement of 100–300 TEM-EDX analyses as a representative set of individual particles for each sample. TheExcel-based routine involves end members of different clay-mineral groups and interstratifications with two or three members (e.g.illite–smectite interstratifications – IS-ml; dioctahedral vermiculite–smectite interstratifications – diVS-ml; and kaolinite–montmorillonite–dioctahedral vermiculite interstratifications – KSV-ml). The routine is now freely available. According to the identification procedure, the &lt;2 µm fraction of the Di Linh bentonite (Vietnam) is composed mainly of K- and charge-deficient illite–smectite interstratifications (or diVS-ml): montmorillonite-rich randomly ordered (R0) type and illite-rich regularly ordered (R1) type. Additionally, Fe-poor KSV-ml was identified. Industrial Na activation of the Di Linh bentonite resulted in an increase of the R1 diVS-ml portion and dissolution of a large part of the smectite-rich phases. The TEM-EDX approach also gave analytical proof of a sedimentary process for Di Linh smectite. The parent muscovite was altered in two different environments: (i) K-leaching and layer-wise alteration into kaolinite (weathering), and (ii) further edge-controlled alteration of mica into lath-like montmorillonite particles associated with a dissolution of kaolinite layers from the former kaolinite–mica intergrowths by heat impact (basalt flow).