Biotite Compositions Research Articles

Chemistry of biotite as petrogenetic discriminators of plutonic and volcanic granitoids from south-central Côte d’Ivoire, West Africa

Chemical compositions of biotite have been used to understand the petrogenesis of intrusive and extrusive magmatic rocks. Central southern Côte d’Ivoire is home to a wide variety of lithologies, including granodiorites, tonalites, and dacites. In this paper, we compare biotites from intrusive rocks with those from extrusive rocks. We used the composition of the mineral to describe the nature of the granitic magma and estimate the temperature at which the studied rocks were emplaced. Grano- diorite and tonalite are composed of biotite, hornblende, plagioclase, apatite, magnetite, and ilmenite. Dacite contains the minerals biotite, muscovite, quartz, and plagioclase. This work has shown contrasts in composition between biotites. The biotites in the intrusive rocks are muscovite-free. Some of these biotites coexist with amphiboles, while others are not accompanied by other ferromagnesian minerals. Some biotites in extrusive rocks are not accompanied by ferromagnesian minerals, while others coexist with muscovite. Extrusive rock biotites do not contain amphiboles. The major elements of the biotites show that they are located between the annite and siderophyllite poles. However, biotites in granodiorites and tonalites are rich in AlIV, giving them a siderophyllite tendency, while biotites in extrusive rocks, which are low in AlIV, converge towards the annite pole. The biotites in the intrusive rocks studied are rich in Mg (Mg-biotite) and Ti, while the biotites in the extrusive rocks studied appear to be rich in Fe (Fe-biotite) and Mn. Plutons could have formed in an orogenic setting from calc-alkaline magma in a subduction environment. Volcanic rocks are peraluminous. The intrusive rocks derive from I-type granites, while extrusive rocks appear to derive from S-type granites. Based on the incorporation of Ti into biotites, the rocks studied were emplaced in a temperature range estimated at 550-650°C at a pressure of 4-6 kbar.

The composition of metapelitic biotite, white mica, and chlorite: a review with implications for solid-solution models

The composition of metapelitic biotite, white mica, and chlorite: a review with implications for solid-solution models

Mineral chemistry, petrography and crystallization conditions of the Middle Eocene Kazıkbeli Pluton (Eastern Pontides, NE Türkiye)

The Eastern Pontides host a diverse suite of plutonic rocks spanning a wide range of ages and compositions. Among these, the Middle Eocene Kazıkbeli pluton, located in the Kürtün district of Gümüşhane, stands out due to its distinctive petrological characteristics. This study aims to unravel the petrological implications of petrographic and mineral chemical data to constrain the physicaydınçakırochemical conditions (temperature, pressure, oxygen fugacity) under which the Kazıkbeli magma crystallized and was emplaced. By integrating mineral chemical data, we seek to quantify emplacement pressure, crystallization temperature, and oxygen fugacity. A comprehensive understanding of the genetic relationships and physicochemical properties of the Kazıkbeli pluton rocks, as determined through geological, petrographic and mineral chemistry, is crucial for elucidating the geological evolution of the Eastern Pontides. The Kazıkbeli Pluton exhibits a predominant NE-SW orientation and encompasses an area of roughly 46 km². Modal mineralogical analysis reveals a compositional spectrum ranging from gabbroic diorite to monzogranite, with granodiorite and tonalite being the most prominent rock types. Textural variations encompass fine- to medium-grained, porphyritic, poikilitic, and occasionally graphic textures. The primary mineral assemblage of the pluton comprises plagioclase, orthoclase, quartz, amphibole, biotite, and Fe-Ti oxides. Accessory minerals include zircon, apatite, sphene, and allanite. Plagioclases are labradorite to oligoclase (An26 to An66) in composition. K-feldspar minerals exhibited an orthoclase composition (Or80 to Or97). All amphiboles belong to the calcic amphibole field and exhibit a magnesio-hornblende (Mg#=0.63-0.73) composition. Biotites crystallized as solidified melt products with compositions between annite and phlogopite endmembers, plotting close to the magnesium-rich (Mg#=0.52-0.58) end of the phlogopite solid solution series. Calculated crystallization temperatures derived from amphibole and biotite data range from 712°C to 824°C. Pressure estimations calculated using amphibole-plagioclase, amphibole and biotite suggest a range of 0.04 to 2.06 kbar. Oxygen fugacity (ƒO2) values calculated using amphibole and biotite fall between -12.5 and -16.1. Amphibole-based water content estimations indicate a range of 3.7% to 5.7% for the pluton. Biotite compositions within the studied Kazıkbeli pluton rocks exhibit characteristics suggestive of a potential mantle origin. Geobarometric calculations based on mineral chemistry data with geological and petrographic features indicate the emplacement of the Kazıkbeli Pluton at relatively shallow depths within the crust (~1 to 8 km).

The role of biotite in the deterioration of pyrrhotite within concrete aggregates: Trois-Rivières case study

In Trois-Rivières, Canada, numerous commercial and residential structures suffered severe concrete degradation due to internal sulfate attack (ISA). The deterioration was attributed to the use of sulfide-bearing aggregates obtained from nearby quarries. The objective of this study was to investigate the mechanisms involving pyrrhotite oxidation and the role of biotite in this process. Detailed analysis, including reflected light optical characterization, mineral chemistry, and EPMA chemical mapping, revealed significant oxidation of pyrrhotite within damaged concrete, resulting in the formation of iron oxyhydroxides as secondary phases. Furthermore, a textural association between biotite mica and pyrrhotite was observed in fresh and damaged aggregate samples. While the chemical composition of biotite remains unchanged, it accelerates the deleterious reactions when in direct contact with pyrrhotite. As oxidation proceeds, naturally occurring biotite crystals near pyrrhotite act as pathways, facilitating the ingress of oxygen and water while simultaneously channeling the release of iron and sulfur compounds.

Contrasting mineralized and barren porphyries in the Zhongdian Arc, insights from biotite and apatite compositions and halogen fugacity.

Copper mineralization in the Pulang (PL) porphyry deposit, Langdu (LD) porphyry-skarn deposit and Songnuo (SN) porphyry prospect in northwestern Yunnan, China, is closely related to the emplacement of quartz monzonite porphyries. The chemical compositions of biotite and apatite from those porphyries were analyzed to calculate the halogen fugacity and to constrain mineralized and barren porphyries. Our data show that biotites from the PL deposit have higher MgO, SiO2, TiO2 and F contents than those from the LD deposit or SN prospect. Compared to those in the LD deposit and SN prospect, the Mg (atoms per formula unit (apfu)) and AlVI (apfu) value in biotite is greater, and the F content is greater and the SO3 and Ce2O3/Y2O3 ratio in apatite are lower in the PL deposit. Ti-biotite thermometry and apatite-biotite geothermometry show that the crystallization temperature of biotite from the PL deposit is higher than that from the SN prospect or LD deposit. The results suggest that oxygen fugacity, magmatic sulfur, and H2O contents cannot be used to efficiently distinguish the PL deposit from the LD deposit and SN prospect. However, the halogen chemistry of biotite from the PL deposit is distinctly different from that of the LD deposit or SN prospect according to the lower IV (F), indicating that mineralized quartz monzonite porphyries in the PL deposit formed during the late magmatic stage, which is in contrast to those in the LD deposit and SN prospect. The mineralized porphyries display a remarkable negative linear relationship (r = -0.96) with the log (f HF/f HCl) and log (f H2O/f HF) ratio, which can be used to distinguish the mineralized and barren porphyries. Compared with other typical porphyry Cu systems, there is a remarkable positive linear relationship between IV (Cl) and log (f H2O/f HCl). In addition, the linear slope and intercept for log (f H2O/f HCl) ratios and the IV (Cl) of biotite from the potassic and phyllic alteration zones are significantly greater than those from other porphyries.

Andesite Quality based on Compressive Strength Tests in the Ulujadi area, Palu City and the Banawa Area, Donggala Regency

Andesite Quality based on Compressive Strength Tests in the Ulujadi area, Palu City and the Banawa Area, Donggala Regency

The petrogenesis of orbicular granites in the Diana’s Pool area, Zimbabwe

Abstract This paper assesses a poorly studied outcrop of orbicular granite from the Matopos granite batholith in the Diana’s Pool area, Zimbabwe. Historical samples from Diana’s Pool exhibit closely packed orbicules in a granitic matrix. They are 9 to 14 cm in diameter and are characterised by different types of orbicules containing coarse-grained felsic cores, fine-grained and alternating ferromagnesian and feldspathic shells, and a coarse-grained matrix. The orbicules are generally spherical to ellipsoidal in shape, however, some appear to be abraded and deformed. The compositions and grain sizes of cores and the matrix are comparable. Both the matrix and the cores are medium- to coarse-grained and dominated by plagioclase (the matrix probably in slightly lesser proportions), microcline (in variable proportions, and seemingly absent in some cores), quartz, biotite and accessory hornblende and magnetite. Contrary to the cores and matrix, shells are fine-grained and exhibit polygonal textures. In addition to this, the shells are dominated by biotite and magnetite; however, they do not contain hornblende. Plagioclase shows an almost complete overlap of An contents, x¯ = 26 ± 2.3 (core), x¯ = 24 ± 0.9 (shell) and x¯ = 25 ± 2.0 (matrix). Biotite composition in the shells is significantly less magnesian (x¯ = 16 ± 2.4) than in core (x¯ = 27 ± 2.2) and matrix (x¯ = 25 ± 2.2), whose compositions overlap. Average initial 87Sr/86Sr ratios from plagioclase in all the analysed shells (x¯ = 0.70226) are slightly more radiogenic than in the matrix (x¯ = 0.70193) and cores (x¯ = 0.70187). Cores are autoliths, which are plagioclase-rich, cumulate, or rim fragments reworked by new magma inputs or injections. Heterogeneous nucleation leading to the formation of orbicular shells around the cores is attributed to adiabatic decompression of magma pulses ascending in dykes, leading to superheating and resorption of early solids, and volatile exsolution, inducing undercooling, supersaturation, and shell crystallisation. The coarse-grained matrix crystallised later, after the orbicules formed, creating the groundmass and locking the orbicules in place. The deformation of shells and cores suggests that the orbicules continued to evolve in the presence of a melt (matrix material). As part of the Matopos Hills World Heritage Site, the Diana’s Pool orbicular granites present a unique and noteworthy petrogenesis, which should be preserved as part of the region’s important geoheritage.

In situ analysis of mica from the Daguanshan granites in Western Yunnan: Implication for Sn enrichment and mineralization

The potential of tin mineralization associated with the Triassic Lincang granite batholith in Western Yunnan, China, has been poorly understood, despite the batholith being considered the northern extension of the Southeast Asian Tin Belt. To address this issue and gain insights into the mechanism of tin enrichment, we investigate the major and trace element compositions of biotite and muscovite from four granite units in Daguanshan, including hornblende-biotite granite, biotite granite, two-mica granite, and muscovite granite in the Hongmaoling Sn deposit. From the hornblende-biotite granite to two-mica granite, the biotite composition varies from Mg-biotite to Fe-biotite with increasing MnO and decreasing K/Rb. The Fe3+-Fe2+-Mg2+ diagram and decreasing Mg/(Fe + Mg) ratios of the biotite indicate reducing conditions. From two-mica granite to muscovite granite, the muscovite Li, F, Rb and Cs concentrations generally increase with decreasing K/Rb, suggesting a magmatic fractionation trend. The variations in zoned muscovite composition reflect a high degree of magmatic fractionation and a lack of fluid interaction. Biotite tin concentrations increase from ∼ 18 ppm in the hornblende-biotite granite to ∼ 36 ppm in the biotite granite and further to ∼ 62 ppm in the two-mica granite. In addition, tin concentrations increase from ∼ 180 ppm in the core to ∼ 330 ppm in the rim of muscovite in the muscovite granite. These data indicate that magmatic fractionation is critical to Sn enrichment in the Hongmaoling deposit. The Nb/Ta ratio and tin concentration in mica from tin mineralized granites are generally higher than those from barren granites, indicating they could be used as reliable indicators for assessing tin metallogenic potential.

A Tall Tale of a Tiny Pluton: The L’Enfer Norite, Grenville Province, Québec

Abstract We present new field observations and mineral and whole-rock geochemical data for the L’Enfer norite, a small pluton in the central Grenville Province of Québec. Although tiny, its plagioclase-rich and noritic character suggested that it might hold clues (possible parental magma?) to the origin of closely associated andesine anorthosites in this part of Québec. The pluton is characterized by spectacular coarse-grained diabasic textures and varies from relatively massive to locally layered varieties. Both coarse- and medium-grained types occur and may be present together in some outcrops. Rare occurrences of snowflake-textured plagioclase are also present. The mafic mineralogy is dominated by orthopyroxene and Fe-Ti oxides (ilmenite and magnetite) with minor clinopyroxene, amphibole (Ti-pargasite), and Ti-rich biotite. Olivine occurs in one of the norites and in a single occurrence of an ultramafic rock that is also enriched in Fe-Ti oxides. Plagioclase compositions range from An32 to An48, but can be divided into two groups on the basis of texture: in one subset of samples, the plagioclase contains exsolution lamellae of orthoclase (i.e., antiperthitic), whereas plagioclase in the remaining samples lacks such lamellae and has not exsolved. Despite these textural contrasts, whole-rock compositions of the two groups are similar. Surprisingly, the compositions of pyroxene, biotite, and ilmenite reveal two non-overlapping compositional groups that correlate with those defined by plagioclase texture. Specifically, mafic minerals in the group with unexsolved plagioclase are consistently enriched in minor elements compared to those in the group with antiperthitic plagioclase. We interpret these contrasts to reflect differences in cooling rate for at least two injections of noritic magma. Moreover, the snowflake plagioclase grains are consistent with some degree of undercooling and supersaturation brought about by magma mixing. Although the norite shares some mineralogical and geochemical features with nearby anorthosites, it does not appear to represent a viable parent magma, primarily due to a pronounced difference in Fe-Ti oxide assemblage (hemo-ilmenite in the anorthosites versus ilmenite + magnetite in L’Enfer).

Regional variation of biotite composition of granitoid and its significance to deep crust component and mineralization: A case study on biotite from Early Mesozoic granitoid in the Qinling orogen

Regional variation of biotite composition of granitoid and its significance to deep crust component and mineralization: A case study on biotite from Early Mesozoic granitoid in the Qinling orogen

On the association of Magmatic Microgranular Enclaves (MMEs) and cordierite-bearing monzogranites at the Sierra Bermeja Pluton (SW Spain)

Magmatic Microgranular Enclaves (MMEs) found within the cordierite-bearing monzogranites of the Sierra Bermeja Pluton (southern Iberian Massif) were investigated to understand their potential petrogenetic relationship. Variations in mineralogy, such as the absence of cordierite (Crd) and potassium feldspar (Kfs) in the MMEs, as well as differences in the compositions of biotite (Bt) and plagioclase (Pl), along with the lack of correlations of certain elements in whole-rock geochemistry, suggest a distinct origin for the MMEs compared to the host monzogranites. The MMEs may represent minor globules of a more mafic magma injected into and mixed with the host monzogranitic magma, with no significant alterations in the geochemical characteristics of the latter.

Age and petrogenetic constraints of the post-collisional Parati Granite, southeast Brazil

The Ribeira orogen is one of the orogens formed in the Pan-African-Brasiliano collisional event. Post-collisional Cambrian-Ordovician magmatism occurred after the main agglutination event of this orogen, being recorded in its central segment by a series of granitic bodies exhibiting few or no deformation, which have been divided into three suites, Suruí, Nova Friburgo and Sana. This work brings a contribution to the understanding of the generation of these bodies through the geochemical - petrological study of the Parati Granite. In addition, unpublished data about crystallization temperature and pressure conditions during the emplacement, as well as a proposition on source and generation model for this granite is also presented. The Parati Granite has a batholithic size showing a predominant inequigranular porphyritic facies with feldspar phenocrysts, besides another inequigranular to equigranular facies also composed of feldspar, quartz, biotite, hornblende and titanite. Geochemically, it is a high potassium, Hi-BaSr metaluminous, magnesian to slightly ferroan calc alkaline granitoid. REE and multielemental diagrams along with biotite composition suggest that Parati Granite was generated from crustal melting with involvement of an enriched mantle, which is corroborated by isotopes of Sr, Nd and Hf. Geothermobarometry results point to crystallization temperature ranging from 760° to 810 °C and pressures varying from 3 to 5 kbars. Geochronological data indicate crystallization age interval between ca. 520 to 505 Ma. Due to the similarities of geochemical patterns and isotopic data, it is believed that the source of this granite could be rocks of the Rio Negro Arc. As a geodynamic model, slab breakoff followed by uplift can be considered as a mechanism responsible for the formation of Parati Granite, as it explains its mantle and crustal signatures.

Distinctive Features of the Major and Trace Element Composition of Biotite from Igneous Rocks Associated with Various Types of Mineralization on the Example of the Shakhtama Intrusive Complex (Eastern Transbaikalia)

This article presents data on the composition of biotite from igneous rocks of the Shakhtama complex, which are associated with various types of mineralization in Eastern Transbaikalia: Au-Cu-Fe-skarn, skarn-porphyry, Mo-porphyry, rare-metal–greisen, Au-polymetallic and Au-Bi. The major element and halogen contents were determined by EPMA. The content of trace elements was determined by means of LA-ICP-MS. As a result, the specific traits of the composition of the biotite of igneous rocks associated with specific types of mineralization of the Eastern Transbaikalia were determined. The biotites of rare-metal–greisen deposits are characterized by the maximum content of fluorine (>2 wt. %) and low chlorine content (<0.04 wt. %). In addition, such biotites are characterized by high XFe (>0.47). Within Eastern Transbaikalia, igneous rocks developed at the Bystrinsky deposit are potentially ore-bearing for the “classic” porphyry type of mineralization. They have the highest values of IV(F/Cl) (4.9–7.1) and IV(F) (2–2.8) and the lowest values of Log(XMg/XFe) (0.1–0.4). The trace element composition clearly distinguishes biotites from rare-metal–greisen deposits and is identified by the highest contents (ppm) Ga > 65, Li > 600, Sn > 20, Mn > 2000, Cs > 50, Zn > 600. Biotites of Au-polymetallic and Au-Bi deposits occupy an average position between rare-metal–greisen and Mo-porphyry ones. Biotites of Mo-porphyry deposits differ in minimum values (ppm) of Sn < 3, Zn < 160, and low values of Li (150–290), V (290–440), and Ga (39–48). In general, the chemical composition of biotites shows that the degree of igneous rock fractionation of deposits increases in the series: porphyry–skarn–polymetallic–rare-metal.

Early Paleozoic granitic magmatism and mineralization in the Debao Sn-Cu deposit, South China: Constraints from zircon and cassiterite U-Pb geochronology and whole-rock geochemistry

Numerous W-Sn deposits are developed in South China. While the majority of these deposits are related to the Yanshanian granites, a few of them are associated with the Early Paleozoic granites, and their geological significance is poorly understood. The Debao deposit is a typical skarn Sn-Cu polymetallic deposit that is spatially related to the Early Paleozoic Qinjia granites. Zircon U-Pb dating of three lithofacies (coarse-grained biotite granite, medium-grained porphyritic biotite granite and fine-grained porphyritic biotite granite) of the Qinjia granites yielded emplacement ages of 437.8 ± 3.0 Ma, 440.2 ± 2.0 Ma and 441.0 ± 3.0 Ma, respectively, consistent with cassiterite U-Pb ages of skarn ore (439.6 ± 2.7 Ma), indicating a close genetic relationship between the granitic magmatism and Sn mineralization. Biotite from the Qinjia granites belong to the ferro-biotite series with relatively high F contents (up to 0.70 wt. %). The Biotite compositions plot near the Ni-NiO (NNO) buffer on a Fe3+-Fe2+-Mg2+ diagram, suggesting formation under relatively low oxygen fugacity. The Qinjia granites are characterized by highly silicious (70.37–72.88 wt. %), high alkali (Na2O + K2O = 6.98–8.88 wt. %) and metaluminous to strongly peraluminous with the A/CNK ratios of 0.92–1.21, belonging to the I-S transform-type granites. They are enriched in U, Th, Zr, Nd and Hf, and depleted in Ba, Sr, Nb, Ta and Ti. They display right-declining chondrite-normalized REE patterns with significant negative Eu anomalies (EuN/Eu* = 0.33–0.66), and have low Nb/Ta and K/Rb ratios, indicating that the Qinjia granites experienced a high degree of fractional crystallization. The zircon εHf(t) values (−2.7 to +1.4), together with TDMC of 1.59 to 1.33 Ga, indicate that the granites formed via partial melting of Mesoproterozoic crustal with minor addition of mantle materials. The reduced (fO2 near NNO and Ce4+/Ce3+ ratios of zircon < 100), fractionated and volatile-rich magma facilitated the enrichment of Sn in the residual melt and hydrothermal fluids. Combined with previous studies on the Early Paleozoic magmatic rocks and tectonic activities in South China, we suggest that the Qinjia granites and associated Sn-Cu mineralization were likely formed in the post-collisional extensional environment during the Wuyi-Yunkai orogeny.

Alteration Mineral Mapping of the Shadan Porphyry Cu-Au Deposit (Iran) Using Airborne Imaging Spectroscopic Data: Implications for Exploration Drilling

Abstract Porphyry copper deposits are associated with large alteration footprints, and alteration mapping plays a key role in the exploration of these deposits. Imaging spectroscopy is commonly deployed for exploration targeting, yet it has rarely been used to map deposit-scale alteration patterns before initiating drilling. To close this gap, the Shadan porphyry Cu-Au deposit was thoroughly studied using the HyMap hyperspectral data (visible near-infrared–short-wave infrared) at 5-m resolution corroborated by rock geochemistry, magnetometry, and laboratory spectroscopy. Shadan is a well-exposed deposit with near-perfect zonation located in the volcanic belts of eastern Iran containing &amp;gt;135 Mt of ore at 0.3% Cu and 0.4 g/t Au. Thirteen minerals, including white mica, Al smectite, kaolinite, ferric/ferrous minerals, biotite, actinolite, epidote, chlorite, tourmaline, and jarosite, were mapped by applying the multifeature extraction methodology. The propylitic zone was partitioned into actinolite, epidote, and chlorite subfacies. The compositions of biotite and white mica were observed to become Fe and Al rich, respectively, toward the mineralized zones. The chemistry of actinolite was observed to change from Fe to Mg rich inward, providing a new vectoring tool for porphyry copper exploration. The study provided significant information about fluid-rock interactions and the chemistry of the circulating fluids including the oxidation-reduction states and acidity. By integrating the mineral maps with other data sets using the fuzzy logic method, the promising (ore) zones were identified and used to plan the next-stage drilling. This work demonstrated that imaging spectroscopy can be effectively used to better understand porphyry systems and provide deposit-scale vectors toward the mineralized centers, facilitating drilling.

Origin of Quartz Diorite and Mafic Enclaves in the Delong Gold-Copper Deposit and Evaluation of the Gold-Copper Mineralization Potential

The Triassic Paleo-Tethyan magmatic belt in the East Kunlun Orogen (EKO) hosts a small number of porphyry-skarn deposits. The controls of these deposits, especially those in the eastern EKO, are poorly understood. In this contribution, we report new petrological, zircon U-Th-Pb-Hf isotopic, whole-rock elemental with Sr-Nd isotopic, and mineral chemistry data of the Delong quartz diorite and mafic enclaves to constrain their petrogenesis and metal fertility. The quartz diorite and mafic enclaves are emplaced in the Late Triassic (ca. 234 Ma). They are medium-K, metaluminous, enriched in large-ion lithophile elements (e.g., Rb, Ba, Th) and light rare earth elements (e.g., La, Ce, Nd), and relatively depleted in high field strength elements (e.g., Nb, Ta, Ti, P) and heavy rare earth elements (e.g., Gd, Er, Tm, Yb). The quartz diorite show similar (87Sr/86Sr)i (0.712584~0.713172) and more depleted εNd(t) (−6.4~−5.7) and εHf(t) (−2.3~+2.6) to those of mafic enclaves ((87Sr/86Sr)i = 0.712463~0.713093; εNd(t) = −6.4~−6.0; εHf(t) = −9.4~−4.8). Geochemical compositions of zircon, amphibole, and biotite yield high water content (5.3 wt.%~6.9 wt.% and 6.1 wt.%~7.3 wt.% based on amphibole, respectively) and high redox state for both the quartz diorite and mafic enclaves. These data, together with petrography, indicate the Delong intrusion was formed by mingling of magmas from enriched mantle and lower continental crust with juvenile materials. The oxidized and water-rich features of these magmas denote they have potential for porphyry Cu (±Au ± Mo) deposits, as do some Triassic magmatic rocks in the eastern EKO that show similar geochemical and petrographic characteristics with the Delong intrusion.

Characterization of Pegmatites in Ogodo-Odobola Area of Idah Sheet 267NW, Central Nigeria

The pegmatites of Ogodo-Odobola area around Ajaokuta, Central Nigeria belong to the pegmatite belt of Central Nigeria. A detailed geological mapping of the pegmatites and host rocks was conducted with the aim of understanding their mode of occurrence and field relationship and to assess their mineralization potentials. Twelve (12) representative samples of rocks were selected for petrographic analysis. The results show that the area is underlain by migmatite-gneiss, schist, with intrusions of granite and pegmatite. The pegmatites occur in tabular form with varying widths (2 centimetres – 6 metres) and lengths (12 – 200 metres). Principal joint direction is NNE-SSW which is believed to have influenced the pegmatite emplacement. Petrography of the representative rock samples revealed an average mineralogical composition of biotite (23.90%), microcline (22.15%), hornblende (15.05%), quartz (10.65%), plagioclase (10.35%), muscovite (8.00%), myrmekite (0.20%), and opaque and accessory minerals (9.70%). The pegmatites were found to be dominated by microcline and plagioclase feldspars, and then muscovite, biotite, and accessory and opaque minerals. The pegmatite of the Ogodo-Odobola area is worth probing as the results have shown prospect for possible economic minerals.

Comparing the biotite and amphibole single-mineral thermobarometric models: A case study from the Cordillera de San Buenaventura volcanic rocks, Puna plateau of Central Andes (Argentina)

We applied a new thermobarometric model based solely on biotite composition to the trachyandesitic to rhyolitic volcanic rocks of the Cordillera de San Buenaventura, a long-lived magma plumbing system (spanning from the Late Miocene to the Holocene) in the Southern Puna plateau (Central Volcanic Zone of the Andean Cordillera). Within these rocks, biotite is a widespread mineral phase found in association with amphibole, as either glomerocrysts, mutual inclusions, or free phenocrysts. This allowed us to compare the biotite-only model with other thermobarometric estimates that rely on amphibole, mineral-melt, and mineral-mineral composition. Our goal was to determine the conditions of formation for biotite and its petrogenetic significance in the evolution of the Cordillera de San Buenaventura magma system. The results indicate that biotite crystallization occurred mainly at middle-to upper-crustal depths (2.1–5.1 kbar) with temperatures below 1000 °C (763–919 °C). The biotite-only T-P estimates for individual lithologies are consistent with those obtained using the amphibole-only model, and this is particularly true for the thermometer. Furthermore, our findings align with the existing petrogenetic models, which suggest extensive fractional crystallization, and with the analyses of mineral textures, that indicates events of magma replenishment. Therefore, our results support the validity of the biotite-only model and its ability to elucidate magmatic processes, including magma differentiation via fractional crystallization, magma ascent and cooling, and magma replenishment and/or mixing events.

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.).

Apatite and Biotite in Syenitic Intrusions, Archean Karari Gold Deposit: Evidence for an Oxidized Magma and Oxidized Subsolidus Potassic (Biotite) Alteration

Abstract Apatite and biotite from syenitic intrusions at the Karari gold deposit, Kurnalpi Terrane, Archean Yilgarn Craton, Western Australia, were targeted for microprobe and LA-ICP-MS analyses in order to determine some parameters (e.g., temperature, oxygen fugacity) during magmatic crystallization and potassic (biotite) hydrothermal alteration. The understanding of magmatic conditions is limited by the almost complete hydrothermal modification of magmatic minerals but made possible by the analysis of refractory magmatic apatite and rare relics of Ba-rich biotite. The SO3 contents of apatite cores (up to 1 wt.%) indicate that the syenitic magmas were strongly oxidized (fO2 &amp;gt; NNO +1) and contained approximately 1360 ppm S. The Cl contents of apatite are very low, suggesting crystallization from a low-Cl magma resulting from early separation of a volatile phase that sequestered Cl from the magma. Trends defined by wt.% SO3, at constant XF/XCl, are consistent with partitioning of oxidized S into the volatile phase during apatite crystallization. Early potassic alteration of the syenitic intrusions produced fine-grained seams of hydrothermal biotite and compositional modifications of igneous biotite phenocrysts. The magnesian composition of the hydrothermal biotite (mg# &amp;gt; 0.5) suggests that oxidized magmatic conditions persisted during potassic alteration. Provinciality of compositional data for different intrusion samples suggests that potassic alteration took place at low fluid:rock ratios. The potassic hydrothermal fluid modified the rims of igneous apatite crystals, depleting them in F, light rare earth elements, Mn, and S. The rare earth elements were rapidly redeposited, as monazite, in the apatite rims or surrounding groundmass. The depletion of S in apatite rims indicates that the potassic (biotite) alteration fluid was under-saturated in S. Application of the biotite-apatite geothermometer suggests potassic (biotite) alteration took place at temperatures of about 660 °C. The results of this study suggest the syenitic magmas at Karari were strongly oxidized, similar to Canadian examples of syenite-associated gold deposits, and therefore potentially fertile for hydrothermal gold.