Ilmenite Inclusions Research Articles

Fluid inclusions in magmatic ilmenite record degassing in basic magmas

Magmatic volatile phases play a major role in igneous systems, but indirect sampling of the magmatic fluid, especially for basic magmas, remains challenging to document. Here, we report compelling evidence of primary fluid inclusions trapped within magmatic ilmenite from two different basic intrusive settings: Armorican Massif (France) and Central Iberian Zone (Spain). Fluid inclusions have a solute chemistry dominated by sodium, calcium, chlorine, sulfur and iron, with detectable contents of metals and metalloids and thus likely record the onset of late-stage magmatic volatile saturation in these basic magmas. Hence, we argue the presence of fluid inclusions in ilmenite may be a good indicator for magma degassing in these settings and importantly records the magmatic-hydrothermal transition. Additionally, this study suggests that the trapping of the magmatic volatile phase during ilmenite (and other opaque minerals) crystallization may be more common but at present an underappreciated phenomenon in basic igneous magmatic systems.

Retrograde Assemblages in the Muscovite-Biotite Gneiss of Oluyole Southwestern Nigeria, an Indication of Shear-Zone Environment

Petrographic and whole-rock geochemical study of biotite-muscovite gneiss was determined in order to interpret the metamorphic evolution of the Basement Complex of Southwestern, Nigeria. The gneiss shows a millimetric banding, and in some cases the quartzo-feldspathic bands running up to 10 cm. The gneiss has mineral assemblage biotite + plagioclase + quartz + garnet + K-feldspar + muscovite + chlorite + ilmenite ±titanite. Chlorite occurs along cleavage planes of biotite, and in some cases forms reaction rims around porphyroblasts of garnet. K-feldspar crystals are surrounded by muscovite. Titanite crystals are sub-idioblastic to xenoblastic in form, and have inclusions of ilmenite. Titanite, where present, occurs in close association with biotite and opaque minerals (ilmenite). Also, titanite forms a reaction rim around apatite. Mylonitic texture, fine-grained matrix of mica and quartz ribbons were observed. In addition, there is stretching of the quartz crystals. The SiO2 content is greater than 60 wt %, while CaO ranges from 3.05-6.91 wt %. The M1 foliation comprise of mineral biotite some of which are included in the opaque mineral, M2 represents the metamorphism which gave rise to porphyroblasts of ilmenite, while the M3 gave rise to foliations that forms a wraparound structure on the porphyroblasts of ilmenite. The last metamorphism gave rise to retrograde minerals; chlorite, titanite, and muscovite. The study suggests that this area of the Basement Complex has been subjected to multiple deformations, as well as multiple episodes of metamorphism. The structures observed are similar to those associated with shear zone environment.

Late Paleozoic igneous rocks at Clarke Head, Nova Scotia: magmatism at an arc to back-arc transition

Abstract This study re-examines a reported mylonitic ‘metabasic granulite’ block in a megabreccia that is the most outboard igneous rock outcrop in the Avalon terrane, near the Meguma–Avalon terrane boundary in the northern Appalachians. The block of foliated gabbro is one of several igneous rock blocks in a largely dissolved salt wall and in style of deformation, mineralogy, lithogeochemistry and Sm/Nd isotopes resembles foliated and locally mylonitized late Devonian–early Carboniferous gabbro plutons along the Cobequid Shear Zone to the north. Garnet porphyroclasts in the foliated gabbro are exceptional, with a distinctive composition of Alm 55 Pyr 25 Grs 13 And 4 Sps 3 . Inclusions of pyroxene, andesine and ilmenite, lack of zoning and corroded rims suggest the garnets are antecrysts. Elsewhere in the world, garnets of similar composition in arc-related andesites are interpreted to be from disintegration of comagmatic cumulate material at 0.8–1.0 GPa under hydrous conditions. The Clarke Head foliated gabbro has two mafic components, one resembling arc-related hydrous magma and the other with tholeiitic back-arc character, similar to coeval rocks in the Cobequid Highlands. The gabbro is a product of complex mixing in crustal magma chambers and rapid rise of magma containing lower crustal antecrysts along strike-slip faults.

Almandine jewelry garnet from the Kitelya deposit (Karelia): composition and spectroscopic properties

Research subject. The chemical composition and spectroscopic properties of almandine jewelry garnets from the Kite-lya deposit in the Northern Ladoga region (Karelia). Materials and methods. The chemical composition, impurity elements, mineral inclusions and spectroscopic properties of almandine jewelry crystals were studied using IR and Mössbauer spectroscopy. Results. Garnet crystals were found to exhibit a weakly pronounced zonal composition, varying from Alm75Pir15Sps7Grs3 in the center to Alm80Pir14Sps4Grs2 at their edges. Therefore, the Ca and Mn contents decrease towards the grain edges. This zonality of garnets is characteristic of the processes of progressive metamorphism of their host rocks. The garnet crystals feature small inclusions of quartz, chlorite, mica FACI (biotite), ilmenite, rutile, monazite, zircon and pyrrhotite. The composition of chlorite, biotite and zircon was established. The parameter of the cube unit cell ao = 11.522 ± 0.003 Å was calculated. The IR absorption spectra of 995, 966, 901, 878, 638, 568, 528, 476 and 455 cm–1 are characteristic of the pyrope-almandine difference. Mössbauer spectroscopy revealed an insignificant admixture of trivalent iron (Fe3+) in the structure of Kitelya garnets (≈1 % of the amount of isomorphic iron). The obtained optical absorption spectra of garnet plates in the visible light spectrum indicate that Fe2+ ions in dodecahedral positions, to a lesser extent dodecahedral Mn2+ ions, as well as possibly octahedral Fe3+ ions are responsible for the bright red-crimson color of pyrop-almandine from the Kitelya deposit. Conclusions. A “portrait” of typomorphic features (composition and properties) of the pyrope-almandine jewelry garnet from the Kitelya deposit was obtained. This portrait can be used when analyzing the historical finds of faceted or cabochonized differences of almandine in jewelry, church utensils in both Russia and Europe (where this jewelry material was exported during the 17th century). The preservation of garnet jewelry differences in the host rock is due to the presence of thin amorphous kelefite shells or soft minerals (sericite, chlorite, kaolinite etc.).

Subsolidus breakdown of armalcolite: Constraints on thermal effects during shock lithification of lunar regolith

Abstract Shock lithification of regolith breccias is a ubiquitous process on the surfaces of airless planetary bodies and may induce thermal effects, including melting on regolith breccia minerals. However, potential thermal effects on lithic and mineral clasts in regolith breccias have seldom been quantitatively constrained. Here, we report two types of micro-textures of armalcolite [(Mg,Fe2+)Ti2O5] in an Mg-suite lithic clast from lunar regolith breccia meteorite Northwest Africa 8182. One type of armalcolite contains oriented fine-grained ilmenite grains; the other occurs as an aggregate of ilmenite, rutile, spinel, and loveringite. We propose that the two types of micro-textures formed through subsolidus breakdown of armalcolite by different processes. The formation of ilmenite inclusions in armalcolite is related to slow cooling after the solidification of its source rock, whereas the ilmenite-rutile-spinel-loveringite aggregates probably formed during the shock lithification event of NWA 8182. The results indicate that the temperature at the margin of lithic clasts could be raised up to at least 600 °C during strong shock lithification of lunar regolith and has profound thermal effects on the mineralogical and isotopic behaviors of lithic and mineral fragments in lunar regolith breccias.

Counter-clockwise <i>P-T</i> history deduced from kyanite-bearing pelitic gneiss in Tenmondai Rock, Lützow-Holm Complex, East Antarctica

This paper first reports the counter-clockwise pressure-temperature (P–T) path for the Lützow–Holm Complex in East Antarctica. The metamorphic textures of kyanite-bearing pelitic gneisses from Tenmondai Rock including earlier spinel and ilmenite inclusions, geothermobarometric data, and pseudosection modeling indicate that the pressure increases prior to the peak metamorphic conditions at around 9 ± 0.5 kbar and 770–820 °C, followed by cooling to kyanite-stability field. We conclude that these gneisses, underwent granulite-facies metamorphism with a counter-clockwise P–T path, but this contrasts with the widely recognized clockwise P–T path of the Lützow–Holm Complex basement rocks in general. One plausible hypothesis we proposed could be that this counter-clockwise P–T path originated from magmatism with late compression and the rocks of the different structural levels are juxtaposed, while acknowledging that this hypothesis conflicts with previous studies and that further work is needed to clarify these issues.

The capturing and decompression of melt inclusions in olivine and ilmenite megacrysts from the Monastery kimberlite, South Africa

Ilmenite and olivine megacrysts from the 89 Ma Monastery kimberlite (Kaapvaal Craton, South Africa) captured abundant and large melt inclusions containing quenched Si-Mg-rich melt, calcite, spinel, perovskite, phlogopite, and serpentine. Textural observations and 3D X-ray tomography of ilmenite and olivine megacrysts show melt inclusion shapes, sizes and distribution patterns indicative of melt capture during primary crystal growth near the base of the subcontinental lithospheric mantle (SCLM). Patterns supporting secondary melt injection along fractures or veins, such as planar arrays of melt inclusions, are absent. Melt inclusions in olivine, in some examples reaching the dimension of centimetres, likely were captured in skeletal voids forming in fast growing, up to decimetre-sized olivine megacrysts. These large melt inclusions commonly decrepitated, forming apophyses, radial fractures, and veins, along which residual volatile- and Si-Mg-rich melt was extracted. We attribute the decrepitation of melt inclusions in olivine to the rapidly increasing difference between the melt pressure in the inclusions, captured at mantle depth, and the decreasing confining stress to which the host olivines were exposed during magma ascent and after emplacement. In ilmenite, melt inclusions up to ∼ 6 mm in diameter remained commonly intact during the kimberlite ascent from its mantle source to the shallow crust. The quenched silicate melt in olivine- and ilmenite-hosted melt inclusions, in some places preserved as unaltered hydrous and CO3-bearing glass, shows systematic major element compositional variations that suggest that this melt formed by similar fractionation and depletion processes, irrespective of the hosting megacryst phase. Apparent modal variations in quenched silicate melt, calcite, and oxide contents suggest that the melt batches captured as inclusions in ilmenite and olivine either record different evolution stages in the megacryst magma, or document compositional heterogeneities in this magma at the time of megacryst growth.

Multiple magmatic-hydrothermal processes recorded by titanite from the Early Cretaceous Liangwan pluton, Tongbai Orogen

Titanite is a common accessory mineral in igneous rocks and hydrothermal deposits. Micro-texture and in situ chemical analyses on titanite are expected to reveal complex magmatic-hydrothermal processes. The Liangwan granitic pluton in the Tongbai Orogen is dominated by monzogranite with abundant mafic microgranular enclaves (MMEs). In this paper, an integrated study of LA-ICP-MS U Pb ages, textural, trace elemental and Nd isotopic compositions of titanites were carried out for the Liangwan pluton to decipher multiple magmatic-hydrothermal processes. U Pb dating results of zircon and titanite show that the Liangwan monzogranite and MMEs have similar crystallization ages of ca. 130 Ma. Combined with petrography and titanite geochemistry studies, MMEs are considered as magma mingling origin. According to the occurrence, texture, and chemical characteristics of titanite, four main types (Ttn-1 to Ttn-4) and several sub-types of titanite are identified in the Liangwan pluton. Among these types, Ttn-1, Ttn-2 g, and Ttn-4 g are from the monzogranites, while Ttn-2 m, Ttn-3, and Ttn-4 m mainly form in the MMEs. Ttn-1 grains are euhedral and show core-mantle texture. Their cores are characterized by high REEs, Nb, Ta, Th and Y contents, Th/U, Lu/Hf, and Y/Zr ratios, and low La/Ce, Nb/Ta rations, and relatively lower εNd (t) (−13.3 to −14.6) values, indicating that they are of magmatic origin and crystallized from the host monzogranites. However, Ttn-1's mantles and Ttn-3 grains show a reverse geochemical feature to those of Ttn-1's cores. Their relative higher εNd (t) (−12.1 to −13.9) values, suggest that they crystallized from a mafic magma. Both Ttn-2 g and Ttn-2 m contain abundant magnetite and ilmenite inclusions. They display geochemical characteristics between Ttn-1's cores and Ttn-3 but are closer to their host rocks, respectively. These features suggest that they are the product of the dissolution-reprecipitation of Ttn-1 in different magmas. Ttn-4 with few fluid inclusions, mostly existing as interstitial phases, is characterized by low REE, Y, HFSE, Th/U, LREEs( N )/HREEs( N ), and high Nb/Ta, indicating that they are hydrothermal origin that were derived from the residual melt. Textural and geochemical features of various types of titanites indicate that repeated recharges of mafic magmas happened at the syn - to late- fractional crystallization of felsic magmas. Nd isotopic data on titanite recorded various degrees of isotopic re-equilibration between the two end-member magmas during different pulses of mafic magma. Our study demonstrates that titanite is a reliable indicator to reveal complicated magmatic-hydrothermal processes. • Monzogranite and MMEs of the Liangwan pluton were both formed at ∼130 Ma. • MMEs in the Liangwan pluton were formed by magma mingling. • Titanite is a reliable indicator to reveal complicated magmatic-hydrothermal processes • Early Cretaceous granite could provide hydrothermal fluids for regional Au—Ag mineralization.

Titanite geochemistry and textures: Implications for magmatic and post-magmatic processes in the Notch Peak and Little Cottonwood granitic intrusions, Utah

Abstract Textural and compositional variations in titanite constrain the roles of magma mixing and hydrothermal alteration in two plutons in central Utah: the Jurassic Notch Peak and the Oligocene Little Cottonwood stocks. In the Notch Peak intrusion, magmatic titanite grains usually have oscillatory zones combined with BSE-bright sector zones, in some cases surrounding simple unzoned cores. These grains are frequently overprinted by hydrothermal titanite with low concentrations of high field strength elements (HFSE). Magmatic titanite has an average δ18O of 6.0‰ and post-magmatic titanite is 6.2‰, as analyzed by SIMS. Average Zr-in-titanite temperatures are also similar, with 718 °C for magmatic and 711 °C for hydrothermal titanite. These observations indicate simple magmatic growth, followed by hydrothermal alteration by magmatic fluids. Titanite in aplite dikes and sills has lower concentrations of all trace elements except F. Many titanite grains in the aplites have late overgrowths of high-Fe titanite. This high-Fe titanite has δ18O of 6‰ and an average Zr-in-titanite temperature of 718 °C and likely precipitated from a last flush of exsolved magmatic water enriched in Cl and Fe. Titanite in the Little Cottonwood stock typically has distinct patchy cores with rounded and embayed ilmenite inclusions. Mafic enclaves have abundant titanite that is similar in texture and δ18O (5.1‰) to titanite in the host (δ18O = 4.9‰), but it has a slightly higher average Zr-in-titanite temperature (731 vs. 717 °C). The patchy cores in the enclaves have the highest average Zr-in-titanite temperature (759 °C) and distinctive REE patterns. The textural and compositional data indicate that a hotter, more reduced, ilmenite-bearing mafic magma mixed into an oxidized felsic magma, destabilizing existing ilmenite and allowing crystallization of titanite. In the granodiorite and in the enclaves, hydrothermal growth of titanite is evidenced by distinct narrow rims as well as anhedral titanite that grew between sheets of chloritized biotite. Secondary hydrothermal titanite typically has lower concentrations of most HFSE, but is relatively enriched in F, Mg, Mo, and U, and it has higher Nb/Ta and lower Th/U ratios. Post-magmatic titanite also has strikingly different REE patterns than magmatic titanite, including the absence of pronounced Eu anomalies and lower REE abundances. These chemical features are controlled by element solubilities in aqueous fluids. In most cases, hydrothermal titanite has δ18O values similar to magmatic titanite, indicating alteration and recrystallization from exsolved magmatic fluids. The involvement of meteoric water with low δ18O is evident locally; individual spots have δ18O as low as 1.7‰ in the Little Cottonwood stock. Titanite compositions and textures provide important insights into the origins of granitic rocks and can be used to distinguish separate batches of magma, gauge the evolution of magmatic rocks, assess mixing processes, and infer compositions of mixing components. Because titanite also forms hydrothermally, it retains hints about the composition, temperature, and oxygen fugacity of the hydrothermal fluids and reveals details about titanite-forming reactions. However, the Al-in-titanite geobarometer does not yield realistic pressures of crystallization and the use of titanite as a geochronometer is compromised by the development of U-rich hydrothermal titanite.

Krystalograficky orientovaný srůst rutilu a ilmenitu a doprovodné minerály z Pasek nad Jizerou (krkonošsko-jizerské krystalinikum)

A detailed mineralogical study of a sample of quartz gangue containing macroscopically visible acicular rutile, which was collected in area of former Cu(-Ag) ore deposit NE from Paseky nad Jizerou, yielded a number of mineralogical findings. The mineralization is bound to strongly elongated quartz lenses, arranged conformably with schistosity of host phyllite of the Krkonoše-Jizera Crystalline Complex. Rutile is polysynthetically lamelled and compositionally zoned, with elevated contents of Fe (0.004 - 0.015 apfu), Nb (0.001 - 0.004 apfu), V (0.001 - 0.002 apfu) and W (0.000 - 0.006 apfu). It contains microscopic platy inclusions of ilmenite, which are regularly distributed in up to three crystallographic directions (probably cleavage planes) regardless of compositional zoning of host rutile. In addition, sporadic occurrence of aggregates composed of fluorapatite, phengitic muscovite and clinochlore was found too. The studied mineralization has metamorphic-secretory origin. The observed microstructural arrangement of inclusions of ilmenite in rutile host is very uncommon worldwide. With respect to evidently LP-LT origin of mineralization, we interpret its origin not in terms of exsolution but in terms of younger hypogene alteration of pre-existing rutile by fluids with relatively low Eh. The activity of such “suitable” fluids can be illustrated during crystallization of chlorite, which was formed at temperatures 137 - 160 °C and at log fO2 between -49.2 and -53.1 bar.

Neoarchean arc magmatism and Paleoproterozoic high-pressure granulite-facies metamorphism in the southern Motloutse Complex, eastern Botswana: Implications for the western extension of the Limpopo Complex

We provide the first record of the occurrence of high-pressure garnet-bearing amphibolite and mafic granulite hosted by amphibolites from the Maope area of the southern Motloutse Complex in eastern Botswana. The mineral assemblages of the rocks are garnet + edenite + plagioclase + ilmenite + rutile + quartz (garnet-bearing amphibolite), magnesio-hornblende + orthopyroxene + clinopyroxene + plagioclase + quartz (mafic granulite), and magnesio-hornblende + plagioclase + quartz + ilmenite + epidote + titanite (host amphibolite). The poikiloblastic garnet in the garnet-bearing amphibolite contains inclusions of edenite, ilmenite, quartz, and plagioclase as prograde minerals, while it is mantled by ferro-edenite + plagioclase coronae formed during retrograde metamorphism. The geochemical signatures of the mafic granulite and amphibolite, combined with the enrichment of large-ion lithophile elements and depletion of high-field strength elements (Nb and Ta), indicate subduction-related volcanic arc affinity, whereas the garnet-bearing amphibolite indicates subduction-related volcanic arc or within-plate basalt affinity. The peak metamorphic condition of the garnet-bearing amphibolite was constrained as 850–910 °C and 10.0–10.5 kbar based on phase equilibrium modeling in the Na2O-CaO-FeO-MgO-Al2O3-SiO2-H2O-TiO2-Fe2O3 (NCFMASHTO) system and geothermobarometry. Slightly lower conditions were obtained from the mafic granulite (745–825 °C) and amphibolite (610–750 °C). Zircon grains from the garnet-bearing amphibolite with lower Th/U ratios of 0.02–0.23 yielded a weighted-mean 207Pb/206Pb age of 2020.2 ± 4.9 Ma as the timing of high-grade metamorphism. In contrast, oscillatory-zoned zircons from the mafic granulite with higher Th/U ratios of 0.44–0.86 recorded a magmatic age of 2648.8 ± 9.8 Ma. As the age is consistent with previously reported age of a tonalite gneiss (2645 Ma) from the same region, we infer bimodal arc magmatism during Neoarchean. The results of this study indicate that the southern Motloutse Complex underwent high-pressure granulite-facies metamorphism with post-peak decompression along a single clockwise P–T evolution during Paleoproterozoic Era (∼2.02 Ga). Our integrated petrological and geochronological studies suggest that the southern Motloutse Complex is nearly equivalent to the Beit Bridge Complex as a possible western extension of a collisional suture (the Central Zone of the Limpopo Complex) between the Archean Kaapvaal and Zimbabwe Cratons. The occurrence of ca. 2.0 Ga high-pressure granulite-facies rocks from the southern Motloutse Complex might suggest multiple collisions of discrete crustal units during Neoarchean to Paleoproterozoic.

SAFIRA NA SERRA DA MANTIQUEIRA, SUL DO ESTADO DO RIO DE JANEIRO: ESTUDOS DE PROVENIÊNCIA COM USO DE ANÁLISE MINERALÓGICA E QUÍMICA MINERAL

O Brasil é detentor de reservas significativas de pedras preciosas, com destaque para aquelas oriundas das Províncias Pegmatíticas Oriental e da Borborema, como turmalina paraíba, esmeralda e topázio imperial. As explorações de coríndon (rubi e safira) ainda são reduzidas no país e estão localizadas principalmente nos estados de Mato Grosso, Goiás, Santa Catarina e Minas Gerais. Campanhas prospectivas realizadas em drenagens na porção sul do estado do Rio de Janeiro coletaram 25 grãos de coríndon, com dimensões menores que 2 mm e entre 2 e 5 mm. Análises de química mineral por microfluorescência de raios X permitiram o estudo de proveniência, com base nas razões Cr2O3/Ga2O3, indicando formação por processo metamórfico. As análises de MEV/EDS identificaram inclusões sólidas de biotita, apatita e ilmenita. A presença das associações minerais espinélio, cianita e cordierita nos metassedimentos neoproterozóicos reforçam a associação genética do coríndon com rochas metamorfizadas em fácies anfibolito superior a granulito.

Geochemical and tectonic constraints on the genesis of iron formation-hosted magnetite-hematite deposits at the Guanhães Block (Brazil) by contact metasomatism with pegmatite intrusions

The Horto-Baratinha (HBD) and Cuité (CTE) are two high-grade iron ore deposits (>60% wt. Fe) located in the Guanhães Tectonic Block (GB), at the eastern margin of the São Francisco Craton bordering the late Neoproterozoic–Cambrian Araçuaí-West Congo Orogenic belt. The orebodies are hosted by Orosirian to early Statherian metamorphic banded iron formation (IF) associated with quartzite and aluminous schists. The metasedimentary units are time-correlated with the Serra da Serpentina and Serra de São José Groups exposed in the Southern Espinhaço fold-thrust belt. They occur as tectonic slivers inserted in the pre-Rhyacian gneissic rocks and the granitic plutons of the Statherian Borrachudos Suite that comprise the GB. In the GB, the IF was intruded by anatectic pegmatites of Ediacaran to Cambrian crystallization age with the development of massive, granular magnetite-rich high-grade orebodies at the contact zone, contrasting with the hosting schistose quartz-hematite IF. The Fe-mineralization of the IF during the Brasiliano Orogeny developed in three distinct stages. (1) Syn- to late-collisional stage (ca. 580–560 Ma) with partial Fe enrichment along sheared zones, crystallization of oriented platy hematite crystals and development of discontinuous lens-shaped schistose high-grade orebodies. (2) Syn- to late-collisional stage (ca. 560–530 Ma) with the formation of massive magnetite bodies by contact metamorphic-metasomatism with the pegmatite. (3) Late- to post-tectonic stage with the oxidation of magnetite to kenomagnetite, martite and crystallization of granular hematite, probably by increased input of meteoric fluids during gravitational collapse of the Araçuaí-West Congo Orogen (ca. 530–490 Ma). In the HBD deposit, the first mineralization stage was unimportant for the enrichment of the IF that exhibit Fe contents of 32.45 to 43.50 wt%.Whole-rock data indicates that the massive hematite-magnetite high-grade iron ore pods exhibit Fe contents between 50.24 and 69.46 wt%, and increasing content of Al2O3, MgO and CaO and the trace elements Zr, Y, Zn, and Ni near the contact with pegmatite, associated with the presence of interstitial silicates and carbonates in their composition. Lamellar hematite exhibit very low contents of trace elements as in other specularitic IFs and orebodies from the eastern São Francisco Craton. A consistent increase in the content of trace elements, mainly Mg, V, Cr, Mn, Co, Ni, Zn, and Ga, is observed in kenomagnetite. Granular hematite grains are enriched an Al, Ti, Zr, Nb, Mo, Sn, Sb, Hf, Ta, and W. Notably, Mn and Ti may occur in relatively high concentrations in kenomagnetite and hematite with the crystallization of ilmenite inclusions, associated with the contact-metasomatic stages.

Gemstones-bearing sediments in the Mbiame floodplain, northwestern Cameroon

Sediment samples obtained from pits in the Mbiame floodplain gemstone prospect in the northwestern part of Cameroon were studied for their bulk geochemical properties together with the mineral composition of the gemstones found therein to decipher their provenance. Five main layers were isolated in each pit comprising an organic rich humus layer, muddy layer, unconsolidated and poorly sorted sandy layer, clay layer and gravel layer. The gravel layer is the most high-yielding in terms of gemstones content. The sediments have high rare earth element (REE) concentrations varying between 135 and 456 ppm with variable light/heavy rare earth element (LREE/HREE) ratio values (5.8–21.5). REE chondrite normalized patterns show similar high LREE enrichment and no Eu anomalies (Eu/Eu* = 0.9 − ~ 1) for all samples. The gemstones selected from the various horizons range in color from light gray, light blue/green, dark blue, pale brown and red. Some of the grains have zircon and ilmenite mineral inclusions while some of the zoned grains show simple core—rim zonation as well as oscillatory and sectorial zoning patterns. Bulk sediment geochemistry of sediments from the Mbiame gem prospect suggests an igneous provenance generally of felsic intermediate source input. The alluvial gemstones from the Mbiame gem prospect are the sapphire variety of corundum and the grains combined with zircon and ilmenite inclusions and megacrysts suggest a derivation from felsic to intermediate igneous rocks.

Determining the origin of inclusions in garnet: Challenges and new diagnostic criteria

ABSTRACT Permian metapegmatite garnets from the Koralpe region (Eastern Alps, Austria) contain abundant submicrometer- to micrometer-sized inclusions of rutile, corundum, Fe-Mn phosphate, ilmenite, xenotime, zircon, and apatite. Variations in inclusion abundance, phase assemblage, habit, and size define sector and concentric zones in the garnets, tracing low-indexed garnet facets. Zoning resulted from a process occurring at the garnet-melt interface, homogeneous along each facet, but sensitive to its crystallographic plane. Furthermore, inclusion and host lattices interacted, generating host-inclusion crystallographic orientation relationships (CORs). These phenomena exclude inclusion formation via overgrowth of pre-existing phases, infiltration of fluids/melts, or dissolution-reprecipitation. Magmatic garnet rims contain rutile needles up to 100 μm long, showing an interface-dependent shape-preferred orientation (SPO) that cannot be explained by exsolution models. Furthermore, the COR distribution for needles is unique, and implies large 3D lattice mismatches. These phenomena suggest that needles originated via oriented heterogeneous nucleation at the garnet interface and subsequent simultaneous growth of both phases. The origin of equant inclusions in core domains is less clear. With some assumptions, integrated compositions remain compatible with closed system exsolution or open system precipitation (OSP) involving divalent cation loss. Still, the oriented interface nucleation hypothesis seems to better explain the fact that the frequency of rutile-garnet CORs varies strongly not only between cores and rims but also between garnet core domains. Inclusion formation by oriented interface nucleation and simultaneous growth can explain many observations commonly attributed to exsolution, making distinguishing between these two mechanisms a challenge. We suggest interface-dependence of SPOs and COR frequencies as criteria for identifying inclusion formation via oriented nucleation at an interface and subsequent simultaneous growth.

The Mineralogical Findings in Coastal Outcrops of the Taimyr Peninsula

The unique mineralogical findings in the coastal outcrops of the Taimyr Peninsula have been investigated. The internal structure, mineral and chemical composition of the mineral formations have been considered. The drusoid formations were established to be glendonites, and the spherical mineral aggregates were found to compose of carbonate material mainly, the last one containing numerous inclusions of pyrite, ilmenite, plagioclase, chlorite, amphibole, apatite and barite. Due to the usage of physical and chemical modeling it is shown that the investigated mineralogical findings proved to be the result of chemical processes in the system “water–permafrost rocks”.

Multiple generations of titanites and their geochemical characteristics record the magmatic-hydrothermal processes and timing of the Dongguashan porphyry-skarn Cu-Au system, Tongling district, Eastern China

Dongguashan is one of the largest porphyry-skarn Cu-Au deposits in eastern China and titanite is found in the ore system. Five types of both magmatic and hydrothermal titanites are distinguished, based on their textural and chemical characteristics. Types 1 and 2 titanites are magmatic in origin whereas types 3, 4, and 5 are hydrothermal. Magmatic titanites are wedge-shaped and display concentric and sector zoning, whereas hydrothermal titanites show a wide range of textures that vary with different alteration stages. REE, Sn, Mo, and Cu are enriched in magmatic titanites (types 1 and 2). In contrast, vanadium and HFSE are enriched in hydrothermal titanites, especially in type 4 titanite from the propylitic alteration zone. Magmatic titanite (type 1) coexists with magnetite and K-feldspar and crystallized in oxidized and H2O-rich magma at 730–760 °C. Type 2 titanite contains ilmenite inclusions reflecting changes in the fO2 of the magma chamber, possibly caused by input of mafic magma. The mobility and enrichment of HFSE and association with fluorapatite reflect the F-rich composition of the magmatic-hydrothermal fluid at Dongguashan. The relationship between titanite textures and chemistry indicates that titanite can serve as a recorder of magmatic-hydrothermal processes in porphyry copper systems. U-Pb dating of type 4 titanite from the propylitic alteration zone and type 5 titanite from skarn yielded ages of 139.0 ± 2.6 Ma and 137.0 ± 2.0 Ma, respectively, indicating that it formed synchronously with the associated quartz monzodiorite.

Crystallographic orientation of ilmenite inclusions in amphibole – an electron backscatter diffraction study

AbstractOrientated ilmenite inclusions have been discovered in amphibole of hornblendite from the Zhujiapu area, Dabie ultra-high-pressure (UHP) metamorphic terrane, China. In order to characterize the crystallographic orientation relationships between ilmenite inclusions and amphibole host and reconstruct the mechanism of their formation, we present an electron backscatter diffraction (EBSD) analysis combined with energy dispersive spectroscopy (EDS) analysis and electron microprobe analysis (EPMA) for ilmenite inclusions and amphibole host. The inclusions can be subdivided into four groups: (1) 60.2% of ilmenites have the crystallographic orientation {0001}Ilm// {100}Amp, (101̅0)Ilm// {010}Amp, [112̅0]Ilm// <001> Ampand [112̅0]Ilm// <012 > Amp. (2) 16.5% of ilmenites have <0001> Ilm// <001> Amp, (101̅0)Ilm// {010}Amp, (112̅0)Ilm// {100}Ampand [3̅031]Ilm// <012> Amp. (3) 13.8% of ilmenites have <0001> Ilm// <012> Amp, (112̅0)Ilm// {100}Ampand [3̅031]Ilm// <001> Amp. (4) 9.5% of ilmenites have <0001> Ilm// [1̅12]Amp, (101̅0)Ilm// {201}Amp, [112̅0]Ilm// [1̅12]Ampand[1121¯]Ilm${[11\overline {21} ]_{Ilm}}$// <010> Amp. By comparing the lattice relationship between ilmenite inclusions and amphibole hosts, it is shown that the frequency of the ilmenite inclusions in different groups is related to the lattice coherency and oxygen packing. Group-1 of the ilmenite inclusions was most likely be formed via a solid-state exsolution process by cooling of the hornblendite after the intrusion was emplaced. The other three groups of ilmenite inclusions were probably formed via reduction reaction in an open system. The formation temperature of the ilmenite inclusions is estimated by using the TiO2solubility geothermeter in amphibole. The minimum formation temperature of the ilmenite inclusions is about 1025 °C, and the maximum formation temperature of the ilmenite inclusions is about 1126 °C.

Mineralogy, geochemistry and genesis of the post-Gondwana supergene manganese deposit of the Carletonville-Ventersdorp area, North West Province, South Africa

The post-Gondwana period is characterized by some of the unique geologic events which shaped the present geomorphological landscape in various parts of the world. One of the most important and unique features associated with this geologic period is the formation of high valence state (Mn4+) manganese oxides in weathering crusts of uplifted landscapes, underlain by manganiferous protores. The study of these manganese oxide minerals has proven not only to be industrially beneficial but they also provide insights into the evolution of the paleo surfaces, especially in tropical regions. Here we present new results on mineralogy, geochemistry and genetic model of the North West Manganese Deposit in the General Nice lease area. The General Nice ore deposit represents a near surface accumulation of supergene manganese wad and nodule deposit in a weathering crust underlain by manganiferous dolostones of the Transvaal Malmani Subgroup. The ore formation is attributed to in situ dissolution, leaching and surficial weathering of the underlying manganiferous dolostones of the Malmani Subgroup and carbonaceous protore of the Waterval Saprolite in a typically oxidic lacustrine setting. The combined processes led to the release and mobilization of Mn rich colloidal particles (sols) into the overlying water column of the depository, which later precipitated as manganese nodules upon interaction with the alluvial sediments which acted as substrates and centers of accretion. Mineralogical studies conducted on both scanning electron microscope (SEM –EDS) and X-ray diffraction (XRD) indicated the predominance of the manganese oxides; romanechite, cryptomelane and galaxite as major manganese oxide phases and pyrolusite, vernadite occurring in minute quantities. Diagenetic features indicate alteration of pyrolusite and its replacement by romanechite. This is evident from the romanechite veinlets cross-cutting the fractured groundmass of pyrolusite. Galaxite appears to represent the latest stage of authigenic mineral precipitation, and occurs mostly around the pore-spaces. Other mineral components include hematite/goethite, clays, muscovite, silica and inclusions of ilmenite and detrital zircon. These accessory minerals occur either in the form of nuclei grains or inclusions in manganese oxide cement.

Gold in ash and slag material of the Primorsk hydroelectric power

The paper shows the results of studies on the identification of noble-metal mineralization in various components of ash and slag formations: underburning, aluminosilicate microspheres, initial ash (fed to the magnetic separation of the technological section of Primorsk SDPP), magnetic and non-magnetic fractions – using analytical and mineralogical methods, electronic microscopy. An integrated approach made it possible to reliably establish significant concentrations of gold, to identify the factors of technological persistence of anthropogenic materials. All products contain free gold grains ranging in size from 50 to 230 micrometers and microscopic inclusions of 0.5-20 microns. The content of free grains of gold is from 0.4 to 1.37 mg / kg. The gold is pure or with an admixture of silver (up to 17% of the mass). Depending on the research materials, gold contains variable inclusions of magnetite, ilmenite, quartz, glass, amphiboles, pyroxenes, clay minerals, and carbonaceous formations. Gold grains have complex structure and morphology. Micronized inclusions, usually rounded, also contain an admixture of silver.