Topaz Granite Research Articles

The Permian Cornubian granite batholith, SW England; Part 1: Field, structural, and petrological constraints

The Permian Cornubian granite batholith (295−275 Ma) in SW England includes seven major plutons and numerous smaller stocks extending for ∼250 km from the Isles of Scilly in the WSW to Dartmoor in the ENE. The granites are peraluminous and classified as crustal melt S-type, predominantly two-mica granites, and biotite or tourmaline monzo- and syenogranites, with subordinate minor topaz granite and lithium mica granite. The granites and their host rocks are pervasively mineralized with tin (cassiterite), tungsten (wolframite, ferberite), copper (chalcopyrite, chalcocite, bornite), arsenic (arsenopyrite), and zinc (sphalerite) mineralized lodes. Quartz-muscovite selvedges (greisen-bordered) also contain enrichment of lithophile elements such as boron (tourmaline), fluorine (fluorite), and lithium (lithium-micas such as lepidolite and zinnwaldite). They are derived from both muscovite and biotite dehydration melting of pelitic-psammitic rocks and intruded from a common source along the length of the batholith. Pressure estimates from andalusite and cordierite-bearing hornfels in the contact metamorphic aureole (150 ± 100 MPa) show that the granites intruded to 3 km depth. Cupolas around the Land’s End and Tregonning granites show aplite-pegmatite dikes and tourmaline + quartz + muscovite veins (greisen) that are frequently mineralized. Synchronous intrusions of lamprophyre dikes suggest an additional heat source for crustal melting may have been from underplating of alkaline magmas. The lack of significant erosion means that the source region is not exposed. In an accompanying paper (Part 2; Watts et al., 2024), gravity modeling reveals possible solutions for the shape and depth of the granite and the structure of the lower crust. We present a new model for the Land’s End, Tregonning, and Carnmenellis granites showing a mid-crustal source composed of amphibolite facies migmatites bounded by prominent seismic reflectors, with upward expanding dikes feeding inter-connected granite laccoliths that show inflated cupolas with shallow contact metamorphism. The Cornubian granites intruded >90 m.y. after obduction of the Lizard ophiolite complex, and after Upper Devonian−Carboniferous Variscan compressional, and later extensional, deformation of the surrounding Devonian country rocks. Comparisons are made between the Cornubian batholith and the Patagonian batholith in Chile, the Himalayan leucogranites, and the Baltoro granite batholith along the Karakoram range in northern Pakistan.

Nature of the mineralizing fluids in the Balda and Motiya W-prospects, western India: Constraints from chemical and B-isotope composition of tourmaline

In the Proterozoic tungsten belts of Balda and Motiya in western India, tungsten mineralization is hosted in tourmaline-bearing quartz veins intrusive into pelitic schists and granites. Tourmaline is a ubiquitous phase in all rock types, and in this study, we use its major, trace element and B-isotope composition to constrain the nature of the tungsten (W)-bearing hydrothermal fluid and the processes involved in the precipitation of wolframite. We have also reconstructed the compositions of the W-precipitating fluids from mineral-fluid trace element partition coefficients that were extrapolated using the Lattice Strain Model. The tourmalines from both belts are of schorl composition and have high alkali, low Ca content, and moderate X-site vacancies. High Li, Mn, Zn, and Sn in the fluid in Motiya is suggestive of relatively saline fluid possibly derived from a granitic source. The high V/Sc ratios of tourmalines in the mineralized veins and wall-rock tourmalinites indicate an important role of biotite dissolution and fluid-rock interaction that contributed Fe–Mn for the precipitation of tourmaline and wolframite. The tourmalines in the mineralized veins at Balda (δ11Btur = −10.9 ± 0.7‰, 2σ; n = 10) and those in the associated granites (δ11Btur = −11.5 ± 0.7‰, 2σ; n = 6) have similar B-isotope composition, while those of the associated topaz granites and pegmatites (−13.9 ± 0.7‰, 2σ; n = 19) are isotopically lighter than the granites. The B-isotopic variation in the granite-pegmatite-vein system can be explained by fluid exsolution with the mineralized vein forming from exsolved fluid and the topaz-bearing granites and the pegmatites crystallizing from the residual melts. The δ11B of the mineralizing fluid is estimated to be ca. −7.4‰ at Balda and ca. −7.6‰ at Motiya, and were possibly derived from granites, consistent with extensive tourmalinization and muscovitization of the adjacent wall rocks, and high concentration of elements such as F, Li, B, Sn, Mn in the tourmalines and the reconstructed fluid. The chemistry and B-isotope composition of tourmalines in both the belts support the hypothesis that W-bearing hydrothermal fluid was primarily derived from a fractionated granitic source, and that precipitation of wolframite and tourmaline involved interaction of the granitic fluid with surrounding pelitic rocks.

Beryl occurrences in the granitic complex of Guerioune, Laouni, Southeastern Algeria: electron microprobe, infrared spectroscopy, and fluid inclusions data

The Guerioune beryl is green or light green; the color is mainly due to the presence of Cr substituting Al in the beryl structure. The Cr2O3 and V2O3 contents in the Guerioune beryl are higher than in the other beryls of the Sahara region but lower than in the analyzed emeralds from Brazil (Capoierana, Nova Cruzeiro Ribeirao de Fogo). It remains unclear if the chromium in the beryl of Guerioune originated from the mafic rocks of the Laouni metamorphics or had a different origin. The Guerioune tardi-to post-cinematic massif intruded at shallow depth during the Pan-African orogeny (535–520 Ma) the Laouni terrane which is located 230km southwest of Tamanrasset, Algeria (Fig. 1). Guerioune granites are a peraluminous rare metal granites (RMG) poor in phosphorus but hosts beryl occurrences. The post-orogenic granites of Laouni Terrane form a set of small cupolas, aligned along with trending regional fault directed by NE-SW (Fig. 1b). The central part of the Guerioune massif is mainly composed of pink biotite granites that contains rounded enclaves of fine-grained granite. Stockscheider pegmatite type marks the contact to the albite–topaz granite. A muscovite granite and a porphyric biotite microgranite occurs as dike in the NE and NW of the massif. Beryl is found in three different associations: (i) pegmatites occur under quartz vein in muscovite granite (most of the beryl occurrences are associated with this type of pegmatite), (ii) in miarolitic cavities in the pink biotite–granite, and (iii) rarely in pegmatite veins. The micro-thermometric and Raman studies reveal a low salinity of the fluid inclusions ( 500° C. These results are in a total accordance with the hypothesis of magmatic or metamorphic fluids that are responsible of the crystallization of beryl.

High-resolution LA-ICP-MS trace-element mapping of magmatic biotite: A new approach for studying syn- to post-magmatic evolution

ABSTRACT Biotite grains from 22 felsic intrusions in New Brunswick were mapped in situ using a laser ablation-inductively coupled plasma-mass spectrometer (LA-ICP-MS). We investigated the extent to which biotite can retain its magmatic zoning patterns and, where zoning does exist, how it can be used to elucidate early to late stage, syn-magmatic to post-crystallization processes. Although the major element and halogen contents of the examined biotite phenocrysts are homogeneous, two-thirds of the grains display trace-element zoning for Ba, Rb, and Cs. The results also indicated that zoning is better retained in larger grains (i.e., > 500 × 500 μm) with minimal alteration and mineral inclusions. An exceptionally well-zoned Li-rich siderophyllite from the Pleasant Ridge topaz granite in southwestern New Brunswick shows Ti, Ta, Sn, W, Cs, Rb, and V (without Li or Ba) zoning. Cesium values increase from 200 to 1400 ppm from core to rim. Conversely, Sn and W values decrease toward the rim (50 to 10 and 100 to 10 ppm, respectively). Tantalum and Ti values show fewer variations but drop abruptly close to the rim of the grain (100 to 20 and 2000 to 500 ppm, respectively). These observations may indicate crystallization of mineral phases with high partition coefficients for these highly incompatible elements (except Ti) (e.g., cassiterite and rutile) followed by fractionation of a fluid phase at a later stage of magma crystallization. The preservation of zoning may indicate rapid cooling post-crystallization of the parent magma.

Inhandjara topaz leucogranite: A late rare metal-mineralized stock within the A-type Itu batholith, SE Brazil

The Inhandjara leucogranite is a highly differentiated Ediacaran stock with crystallization age of ca. 570 Ma (SHRIMP zircon U–Pb dating). It is a satellite intrusion of the Itu batholith, a large post-orogenic A-type granite in south-eastern Brazil. The Inhandjara stock consists of two leucogranitic facies: (1) an early inequigranular to porphyritic biotite-bearing monzogranite (BG), with potassium feldspar megacrysts in a medium to coarse-grained matrix; and (2) a more evolved equigranular medium to fine-grained topaz-bearing alkali feldspar granite (TAG) with subhedral albite, anhedral quartz and potassium feldspar, and accessory Li-bearing siderophyllite, topaz, Y-bearing fluorite, zircon, ilmenite, and columbite-group minerals. Both facies display signs of autometasomatism, mainly as pervasive albitization and fissure to pervasive greisenization. The Inhandjara leucogranite is reduced, metaluminous to weakly peraluminous and shows A-type affinity. There is a progressive increase of SiO2, Al2O3, Na2O, F, Cs, Rb, Nb, Ta and Y from the early biotite granite to the late topaz granite, parallel to a decrease in TiO2, Fe2O3, MgO, CaO, Sr, Ba and Zr. The seagull-wing-shaped REE pattern of the topaz granite, with a strong negative Eu anomaly (Eu/Eu* = 0.003), is unique in the Itu batholith. Both facies have very negative εNd(570Ma) values, ranging from −11.7 to −13.2, with the lowest values within the Itu batholith. Zircon crystals from biotite granite show oscillatory zoning and chemical evolution from core to rim; whereas in the topaz granite, zircon is unzoned and strongly enriched in Y, Hf, Nb, Th, and U, suggesting crystallization from a residual melt in the presence of a fluid phase. The Inhandjara stock crystallized from a highly evolved magma derived from a lower crustal source. The strong enrichment in incompatible lithophile elements mostly results from protracted magmatic fractionation, enhanced by the interaction with magmatic-hydrothermal fluids. These characteristics reveal that the leucogranite is the most differentiated occurrence within the Itu batholith and support its relation with regional rare metal mineralization (Nb–Sn–W).

Constraints on the Formation of Granite-Related Indium Deposits

Abstract The use of indium in modern technologies has grown in recent decades, creating a growth in indium demand; thus, there is a need to constrain the spatial and temporal distribution of indium-bearing, granite-related deposits. Toward this end, a conceptual model and exploration vectors for the formation of granite-related indium deposits have been developed. The magmatic-hydrothermal system is modeled by consideration of crystal-melt and vapor-melt equilibria. The model calculates the efficiency of removal of indium from a melt into a volatile phase, which may serve as a component of an ore-forming fluid. The results of the model suggest that as the proportion of ferromagnesian minerals increases in the associated granites, the probability of indium ore formation decreases. Further, for a given modal proportion of ferromagnesian minerals, as the modal proportion of amphibole increases, the probability of indium ore formation decreases. Lastly, for a given modal proportion of biotite, as the magnesium content of the biotite increases (as would result from increasing oxidation of the magmatic system), the probability of indium ore formation decreases. Granites with the highest probability of being associated with indium ore formation will typically be part of A- or S-type igneous systems and will likely be highly fractionated (e.g., A-type topaz granites). I-type granites will generally have a lower potential of being associated with indium-bearing deposits. However, some I-type granites may be associated with indium-bearing deposits if the deposits contain granites (sensu stricto) or other related rocks (e.g., alaskites) that lack amphibole or other ferromagnesian phases.

Nb–Ta–Ti Oxides in Topaz Granites of the Geyersberg Granite Stock (Erzgebirge Mts., Germany)

Oxide minerals (Nb–Ta-rich rutile, columbite-group minerals and W-bearing ixiolite) represent the most common host for Nb, Ta and Ti in high-F, high-P2O5 Li-mica granites and related rocks from the Geyersberg granite stock in the Krušné Hory/Erzgebirge Mts. batholith. This body forms a pipe like granite stock composed of fine- to middle-grained, porphyritic to equigranular high-F, high-P2O5 Li-mica granites, which contain up to 6 vol. % of topaz. Intrusive breccia’s on the NW margin of the granite stock are composed of mica schists and muscovite gneiss fragments enclosed in fine-grained aplitic and also topaz- and Li-mica-bearing granite. Columbite group minerals occur usually as euhedral to subhedral grains that display irregular or patched zoning. These minerals are represented by columbite-(Fe) with Mn/(Mn + Fe) ratio ranging from 0.07 to 0.15. The rare Fe-rich W-bearing ixiolite occurs as small needle-like crystals. The ixiolite is Fe-rich with relatively low Mn/(Mn + Fe) and Ta/(Ta + Nb) values (0.10–0.15 and 0.06–0.20, respectively). Owing to the high W content (19.8–34.9 wt. % WO3, 0.11–0.20 apfu), the sum of Nb + Ta in the ixiolite does not exceed 0.43 apfu. The Ti content is 1.7–5.7 wt. % TiO2 and Sn content is relatively low (0.3–4.1 wt. % SnO2).

Rare-metal granites as a potential source of critical metals: A geometallurgical case study

Because of their low grades in critical metals such as Light Rare Earth Elements (LREE) or Sn, rare-metal granites are not considered as economic for metal recovery but, when altered, they are often exploited for their industrial minerals. The St Austell rare-metal granite is well known for its world-class kaolin deposits which formed as a result of the extensive weathering and alteration of the underlying granite. The St Austell granite body is composed of several granite components, each having its own accessory minerals assemblage. As a result of the kaolinisation process, some metal-bearing accessory minerals of the granite, such as monazite (LREE) or cassiterite (Sn), are partially liberated from the gangue which allow their pre-concentration in the micaceous residue which is considered as a potential source for critical metals recovery. Similarities with other similar rare-metal granites suggest that topaz granite is the most prospective for disseminated magmatic Sn-Nb-Ta-REE mineralization. However, comparison of the potentiality of 3 granite types i.e., biotite, topaz and tourmaline granites suggest that biotite granites is actually the most prospective due to higher degree of kaolinisation of the biotite granite which favour pre-concentration of its accessory mineral in the micaceous residue. In order to develop a geometallurgical framework for extraction of kaolin and metals from the selected granite component, a field sampling campaign is performed. Core samples are processed in the laboratory using a characterisation program that mimics the full-scale kaolin refining route. Two main products are recovered through this program, viz. MR180 (−180 +53 µm) and P5 (−5 µm), which correspond to a fine micaceous residue and a fine kaolin product respectively. These products are both analysed routinely for major and minor trace elements by XRF and yields are recorded to indicate process performance. A selected number of MR180 samples are also being characterised in terms of particle size by laser light scattering, geochemistry by ICP-MS, and mineralogy by QEMSCAN®. Comparison of characterisation results of MR180 samples and corresponding industrial residue samples shows a good correlation, suggesting that sample analyses are representative for the in-situ deposit and the processing behaviour. Monazite is found to be either fully liberated or fully locked from one sample to the other. Next, pilot-scale gravity concentration tests are performed on micaceous residue samples. Characterisation of the processing products shows that monazite lost in the tailings is mostly locked within tourmaline or micas and is fine grained. Then, predictive regression models for spiral separation performance in terms of recovery, product grade and enrichment as a function of the feed grade are developed for MR180 LREE grade data. Finally, kaolin resources can be classified using quantitative indicators such as yield of the P5 product and the iron oxides content which provides insight into the kaolin quality in terms of whiteness. This geometallurgical classification can be used to delineate zones of interest within the deposit. Although kaolin quality and recovery primarily inform extraction planning, zones which are also of interest for metal recovery can be identified. The proposed model predicts whether the expected LREE grade and recovery satisfy the by-product requirements.

The transition from granite to banded aplite-pegmatite sheet complexes: An example from Megiliggar Rocks, Tregonning topaz granite, Cornwall

The genetic relationship between a granite pluton and adjacent complex of rare-metal pegmatite-aplite-banded sheets (Megiliggar Sheet Complex - MSC) has been studied at the border of the Tregonning topaz granite at Megiliggar Rocks, Cornwall, SW England. Similarities in whole-rock chemical and mineralogical compositions, together with a gradual change in textures away from the granite margin, provide strong evidence for a genetic link between the Tregonning Granite and MSC. The sheets are likely to represent apophyses of residual melt which escaped from the largely crystallized roof of the granite pluton. The escaping melt was peraluminous, had a composition near the F, B, Li slightly enriched granite minimum, and, in comparison with other Cornish granites, was enriched in F, Li, Rb, Cs, Sn, W, Nb, Ta, and U, and depleted in Fe, Mg, Ca, Sr, Th, Zr, and REE. With increasing distance from the Tregonning Granite, the silicate melt crystallized as homogeneous leucogranite sheets and banded complex sheets (i.e. combinations of bands with granitic, aplitic and pegmatitic textures), then layered aplite-pegmatites; this sequence becoming progressively more depleted in the fluxing and volatile elements F, Li, Rb, and Cs, but showing no change in Zr/Hf ratios. The fixed Zr/Hf ratio is interpreted as indicating a direct genetic link (parental melt) between all rock types, however the melt progressively lost fluxing and volatile elements with distance from the granite pluton, probably due to wall-rock reaction or fluid exsolution and migration via fractures.Differentiation of the primary melt into Na-Li-F-rich and separate K-B-rich domains was the dominant chemical process responsible for the textural and mineral diversity of the MSC. On a large (cliff-section) scale, the proximal Na-Li-F-rich leucogranite passes through complex sheets into K-B-rich aplite-pegmatites, whilst at a smaller (<1 m) scale, the K-B-rich bands are interspersed (largely overlain) by Na-Li-F-rich segregations. The grain size differences between the aplite and pegmatite could be related to pressure fluctuations and/or undercooling.

Composition of zircons from the Cornubian Batholith of SW England and comparison with zircons from other European Variscan rare-metal granites

AbstractZircon from 14 representative granite samples of the late-Variscan Cornubian Batholith in SW England was analysed for W, P, As, Nb, Ta, Si, Ti, Zr, Hf, Th, U, Y, La, Ce, Pr, Nd, Sm, Gd, Dy, Er, Yb, Al, Sc, Bi, Mn, Fe, Ca, Pb, Cu, S and F using electron probe microanalyses. Zircons from the biotite and tourmaline granites are poor in minor and trace elements, usually containing 1.0–1.5 wt.% HfO2, &lt;0.5 wt.% UO2 and P2O5, &lt;0.25 wt.% Y2O3, &lt;0.2 wt.% Sc2O3 and Bi2O3 and &lt;0.1 wt.% ThO2. Zircon from topaz granites from the St. Austell Pluton, Meldon Aplite and Megiliggar Rocks are slightly enriched in Hf (up to 4 wt.% HfO2), U (1– 3.5 wt.% UO2) and Sc (0.5–1 wt.% Sc2O3). Scarce metamictized zircon grains are somewhat enriched in Al, Ca, Fe and Mn. The decrease of the zircon Zr/Hf ratio, a reliable magma fractionation index, from 110–60 in the biotite granites to 30–10 in the most evolved topaz granites (Meldon Aplite and Megiliggar Rocks), supports a comagmatic origin of the biotite and topaz granites via long-lasting fractionation of common peraluminous crustal magma. In comparison with other European rare-metal provinces, the overall contents of trace elements in Cornubian zircons are low and the Zr/Hf and U/Th ratios show lower degrees of fractionation of the parental melt.

The petrogenesis of the Early Permian Variscan granites of the Cornubian Batholith: Lower plate post-collisional peraluminous magmatism in the Rhenohercynian Zone of SW England

The Early Permian Cornubian Batholith was generated during an extensional regime following Variscan convergence within the Rhenohercynian Zone of SW England. Its component granites can be classified, using mineralogical, textural and geochemical criteria, into five main types, all of which are peraluminous (A/CNK>1.1): G1 (two-mica), G2 (muscovite), G3 (biotite), G4 (tourmaline) and G5 (topaz). G1 granites formed through up to 20% muscovite and minor biotite dehydration melting of a metagreywacke source at moderate temperatures and pressures (731–806°C, >5kbar). Younger G3 granites formed through higher temperature, lower pressure (768–847°C, <4kbar) biotite-dominated melting of a similar source. Partial melting was strongly influenced by the progressive lower–mid crustal emplacement of mafic igneous rocks during post-Variscan extension and a minor (<5%–10%) mantle-derived component in the granites is possible. Two distinct fractionation series, G1–G2 and G3–G4, are defined using whole-rock geochemical and mineral chemical data. Variations in the major elements, Ba, Sr and Rb indicate that G1 and G3 granites underwent 15%–30% fractionation of an assemblage dominated by plagioclase, alkali feldspar and biotite to form more evolved G2 and G4 granites, respectively. Decreasing whole-rock abundances of Zr, Th and REE support the fractionation of zircon, monazite, apatite and allanite. Subsolidus alteration in G2 and G4 granites is indicated by non-primary muscovite and tourmaline and modification of major and trace element trends for G3–G4 granites, particularly for P2O5 and Rb. Topaz (G5) granites show low Zr, REE and extreme enrichment in Rb (up to 1530ppm) and Nb (79ppm) that cannot be related in a straightforward manner to continued differentiation of the G1–G2 or G3–G4 series. Instead, they are considered to represent partial melting, mediated by granulite facies fluids, of a biotite-rich restite following extraction of G1 and/or G3 magmas; they do not exhibit the typical geochemical characteristics of intraplate A-type granites.

A tin-mineralized topaz rhyolite dike with coeval topaz granite enclaves at Qiguling in the Qitianling tin district, southern China

The Qiguling topaz rhyolite is present as a dike within the Qitianling biotite granite batholith of the Nanling Range of southern China. Here, the rhyolitic dike, 4.5m wide and 500m long, contains enclaves of topaz granite. These rhyolites contain up to 72wt.% SiO2, have alumina saturation index (ASI) >1.1, and have groundmasses with estimated fluorine contents of approximately 1.5wt.%. Textural relationships provide evidence of a quenched silicate melt that contains quartz, K-feldspar, albite, and zinnwaldite phenocrysts in a groundmass containing abundant topaz. The rhyolites in the study area are also strongly enriched in tin (90–2700ppm), and generally have a close association between cassiterite and zinnwaldite, although cassiterite is also present as sponge-textured fills between rock-forming minerals. Granite enclaves and their hosted rhyolite have similar major geochemical compositions and mineralogies to each other. Zircon U–Pb dating indicates that the topaz rhyolite (147–150Ma) and topaz granite enclaves (154Ma) were formed contemporaneously, with ages that overlap within analytical uncertainty. In addition, the major and trace element compositions of the rhyolite and their granite enclaves are dissimilar to those of the hosting Qitianling biotite granite. This discovery of granite enclaves within rhyolite dikes suggests the presence of a topaz-bearing granite body at depth that may host tin mineralization. The expected hidden tin granite may be of great interest in the further exploration.

Geochemical correlation between dykes of topaz granites and elvans (Kokchetav Block, Northern Kazakhstan)

Geochemical correlation between dykes of topaz granites and elvans (Kokchetav Block, Northern Kazakhstan)

CRYSTALLIZATION OF THE KYMI TOPAZ GRANITE STOCK WITHIN THE WIBORG RAPAKIVI GRANITE BATHOLITH, FINLAND: EVIDENCE FROM MELT INCLUSIONS

The crystallization of the late-stage F-enriched Kymi topaz granite, in southeastern Finland, has been investigated by remelting and analyzing crystallized melt inclusions in quartz and topaz grains from porphyritic and equigranular topaz granites and marginal pegmatite. The conditions of rehomogenization were 300 MPa and 700°C for the granites, and 100 MPa and 700°C for pegmatite, as well as 1 atm and 900°C for all phases of the stock. After rehomogenization, quartz from the granites and pegmatite contains four types of melt inclusion: two coexisting types of melt inclusion varying in vapor content, extremely high-F melt inclusions and melt inclusions with very low F contents. The compositions of the melt inclusions show that the topaz granites and the stockscheider pegmatite of the Kymi stock crystallized from very F- and H2O-rich melts, and that Li and B were present in the granitic melt at high concentrations. They also confirm that the equigranular granite and pegmatite crystallized from a more evolved melt than the porphyritic granite. The melt inclusions in quartz and topaz grains from the granites and pegmatite contain indirect indications of melt separation and formation of a minor peralkaline melt fraction in addition to the prevailing peraluminous melt at the final stages of the crystallization. Combined petrological and field observations as well as melt-inclusion studies of the Kymi stock indicate that the zoned structure of the stock is a result of the migration of a highly evolved residual melt from the deeper parts of the magma chamber to the carapace of the chamber. This migration may be related to an opening of the contact between the surrounding rapakivi granite bedrock and residual crystal mush of the porphyritic granite, and to a migration of the residual melt into the carapace along contacts and fractures, or to a migration of highly evolved interstitial magma and fluid through the crystallizing porphyritic granite and along the walls of the magma chamber to the carapace.

Proterozoic granites of the Llano Uplift, Texas: A collision-related suite containing rapakivi and topaz granites

Proterozoic granites in the Llano Uplift, central Texas, intruded metamorphic rocks in a southern segment of the Grenville orogen. Three compositional groups of granite are present; two of these have not been previously recognized. The most widespread group occurs in three emplacement styles: (1) irregular plutons (synkinematic) were emplaced at 1119–1116 Ma during low-pressure Grenville metamorphism and deformation; (2) associated with these, there are volumetrically minor rhyolite dikes (1098–1093 Ma) and hypersolvus granite; and (3) large 1093–1072 Ma postkinematic circular to ovoid plutons are the most abundant of this compositional group, and many show rapakivi textures. A second, more felsic group has much higher light (L) to heavy (H) rare earth element (REE) ratios. The third group is highly fractionated, contains topaz locally, is enriched in Rb, Nb, and HREEs, and is depleted in Sr, Zr, and LREEs. No other igneous rocks were emplaced at the same time as the granites at the current level of exposure. Llano granites cannot be considered anorogenic or A-type on the basis of their tectonic setting, timing, or compositions. Sr, Nd, and Pb isotopic data indicate that these granites were derived from Proterozoic mantle and crust. The granites intruded thick continental crust at one margin of Laurentia and completed the consolidation of one segment of the Grenville orogen.

Topaz granites in northern Kazakhstan

The paper presents the first data obtained on topaz granites from dikes in columbite-bearing granites of the Kokchetav block, northern Kazakhstan. The columbite-bearing granites (398 ± 3 Ma, Rb-Sr method) compose large massifs and ubiquitously contain columbite as an accessory mineral in amounts from a few to a few dozen grams per ton. The topaz granites make up dikes up to 70–100 m long and 0.4–1.5 m thick, which have sharp contacts with the host granites. The composition of the topaz granites is as follows: albite ∼50%, potassic feldspar ∼24–26%, quartz ∼20%, topaz 3–5%, biotite and muscovite ∼1%. Topaz occurs in inner contact zones in the form of large (3–4 cm) crystals, which grew inward from dike contacts and is ubiquitous in the fine-grained groundmass as small (no larger than 0.05 mm) crystals. The ore minerals in the topaz granites (zircon, thorite, bastnaesite, Y-cerite, monazite, Y-fluorite, W-ixiolite, tantalite, columbite, and uraninite) developed as very small grains, which can be identified only under a microprobe and electron microscope. W-ixiolite was found only in topaz crystals. The topaz granites are rich in albite, contain Na > K and F concentrations from 0.2 to 0.97 wt %. Compared to the host columbite-bearing granites, the topaz granites are 10–15 times richer in Ta and Li, 3-8 times richer in Rb, Cs, Nb, Sn, F, and Be but 1.2–2 times poorer in U, Th, and REE. The columbite-bearing granites have Ta/Nb = 1: 20, while this ratio in the topaz granites is 1: 3, a fact highlighting the enrichment of the granites in Ta and their geochemical autonomy with respect to columbite-bearing granites.

Phase relations of a F-enriched peraluminous granite: an experimental study of the Kymi topaz granite stock, southern Finland

Phase relations of F-rich Kymi equigranular topaz granite have been investigated experimentally at 100–500 MPa as a function of water activity and F content. Fluorite and topaz can crystallize as liquidus phases in F-rich peraluminous systems, but the F content of the melt should exceed 2.5–3.0 wt% for the crystallization of topaz. In peraluminous F-bearing melts containing more than 1 wt% F, topaz and muscovite are expected to be the first F-bearing phases to crystallize at high pressure, whereas fluorite and topaz should crystallize first at low pressure. Following reaction models the saturation of fluorite and topaz: CaAl2Si2O8 (plagioclase) + 2[AlF3]melt = CaF2 (fluorite) + 2Al2SiO4F2 (topaz). The obtained partition coefficient for F between biotite and glass D(F)Bt/glass ranges from 1.89 to 0.80 (average value 1.29) and can be used as an empirical fluormeter to determine the F content of coexisting melts. Combined petrological and experimental studies of the Kymi equigranular topaz granite indicate that plagioclase was the liquidus phase at nearly water-saturated (fluid-saturated) conditions and that the F content of the melt was at least 2 wt%. The mean F content of natural biotite (3.92 wt%) suggests that the late-stage crystallization of biotite occurred in melts containing about 3 wt% F. The early crystallization of biotite and the presence of muscovite in our experiments at 200 MPa contrasts with the late-stage crystallization of biotite and the absence of muscovite in the natural assemblage, indicating that crystallization pressure may have been lower than 200 MPa for the granite.

Anomalously high Sc contents in ore minerals from Sn–W deposits: Possible economic significance and genetic implications

An investigation on Sn–W and W deposits and associated granites in the Erzgebirge region, central Kazakhstan, Mongolian Altai, and central Mongolia reveals anomalously high Sc contents up to several thousands of parts per million in wolframite and cassiterite from Sn–W deposits in the eastern Erzgebirge; even though whole-rock samples of granites from these localities have low Sc contents typical of high-silica granitic rocks. It is possible to trace the distribution of Sc in rocks and veins using data on accessory and ore minerals enriched in Sc (columbite, rutile, cassiterite, wolframite, zircon, scheelite, and mica). There are no indications for an enrichment of Sc during the magmatic evolution of the granite intrusions. Magmatic fractionation causes rather low Sc contents in evolved granites. On the other hand, Sc enrichment in zircon found in these rocks is clearly related to processes of secondary alteration leading to high U, Ca, and Fe besides Sc enrichment in the mineral. Enhanced Sc contents were also found in whole-rock samples from the greisen. The occurrence of Sc-rich wolframite and cassiterite is mainly restricted to endocontact orebodies (greisen and veins). Outside the rare metal-bearing granites, the Sc content in wolframite drops off if host rocks are formed by schist and gneiss. Correlation between Sc and Nb concentrations in wolframite found in endocontact orebodies also disappears in the exocontact veins except if the vein system provides a focused fluid flow. Sc enrichment in Sn–W mineralisation is accompanied by strong Y/Sc, Y/Yb, and Th/U fractionation in Sc-bearing minerals as indicated by trace-element contents in wolframite and scheelite. Y/Sc ratios in wolframite below 0.1 are typical of Sn deposits. It is suggested that certain conditions of complexation, pH, and Eh in F-rich fluids are necessary prerequisites for Sc enrichment and deposition. Our results hint for a Sc input into the ore-forming system by an external F-rich fluid carrying also U and heavy REE and possibly derived from a mantle-related source. Although whole-rock samples of rare metal-bearing topaz granite have low Sc contents, the related Sn–W ores may represent an interesting source for Sc extraction because of their enrichment in Sc.

Petrological and geochemical evolution of the Kymi stock, a topaz granite cupola within the Wiborg rapakivi batholith, Finland

The 6×3 km Kymi monzogranite stock represents the apical part of an epizonal late-stage pluton that was emplaced within the 1.65 to 1.63 Ga Wiborg rapakivi batholith. The stock has a well-developed zonal structure, from the rim to the center: stockscheider pegmatite, equigranular topaz granite, porphyritic topaz granite. The contact between the two granites is usually gradational within a few centimeters, but local inclusions of the porphyritic granite in the equigranular granite indicate that the latter solidified later. Hydrothermal greisen and quartz veins, some of which contain genthelvite, beryl, wolframite, cassiterite, and sulfides, cut the granites of the stock and the surrounding country rocks. The equigranular granite contains 1 to 4 vol.% topaz, and its biotite is lithian siderophyllite; the porphyritic granite has 0 to 3 vol.% topaz, and the mica is siderophyllite. The equigranular granite is geochemically highly evolved with elevated Li, Rb, Ga, Ta, and F, and very low Ba, Sr, Ti, and Zr. The REE patterns show deep negative Eu anomalies and tetrad effects indicating extreme magmatic fractionation and aqueous fluid–rock interaction. The zonal structure of the stock is interpreted as a result of differentiation within the magma chamber. Internal convection in the crystallizing magma chamber and upward flow of residual melt as a boundary layer along sloping contacts resulted in accumulation of a layer of highly evolved, volatile-rich magma in the apical part of the chamber. Crystallization of this apical magma produced the stockscheider pegmatite and the equigranular granite; the underlying crystal mush solidified as the porphyritic granite. Much of the crystallization took place from volatile-saturated melt, and episodic voluminous degassing expelled fluids into opened fractures where they or their derivatives reacted with country rocks and caused alteration and mineralization.

Petrogenesis of the Gross Spitzkoppe topaz granite, central western Namibia: a geochemical and Nd–Sr–Pb isotope study

The early Cretaceous Gross Spitzkoppe granite is one of several igneous complexes that formed in the Damaraland of Namibia during an episode of continental rifting and magmatism associated with opening of the South Atlantic. This is a classic setting of rift-related, anorogenic magmatism showing the typical association of mantle-derived mafic and alkaline rocks along with silicic units. The Gross Spitzkoppe granite is an important example of the latter because its peraluminous composition and crustal isotopic signature makes it well suited to characterize the crustal contribution to magmatism. The Gross Spitzkoppe stock (GSS) is composed of three texturally distinct topaz-bearing biotite monzogranites (average 76 wt.% SiO 2, ∼30 wt.% Rb/Sr and ∼0.5 wt.% F). Iron-rich biotite is the only mafic silicate. In proximity to the stock are numerous felsic dikes (rhyodacites to rhyolites). The elevated high-field-strength elements (HFSE) contents and high Ga/Al ratios of the granites and felsic dikes are typical of A-type granites. The granites and dikes show similar geochemical differentiation trends and the compositions of the more evolved dikes overlaps with the granites. This and similar initial isotope ratios suggest that the dikes and granites have a common origin. Major and trace element modeling confirms that the composition of the Gross Spitzkoppe granite can be produced by fractional crystallization from magma with the average felsic dike composition. Rb–Sr isotope data for nine whole-rock samples of the GSS yield an age for emplacement of the complex at 125±1 Ma. Initial 87Sr/ 86Sr ratios vary from 0.710 to 0.716. Neodymium isotope compositions of the granites and dikes yield initial εNd 125 values from −5.6 to −6.4. The Pb isotopic ratios (K-feldspar or age-corrected whole rock) fall in the range 206Pb/ 204Pb 18.70–18.82, 207Pb/ 204Pb 15.66–15.71, and 208Pb/ 204Pb 38.57–39.07. These geochemical and isotopic characteristics are consistent with derivation of the Gross Spitzkoppe granitic magma from a granulite-facies metasedimentary source. Batch melting calculations suggest that low (10–20%) degree partial melting of a felsic granulitic source is capable of producing melt compositions similar to those of the felsic dikes, with further fractional crystallization being required to produce the granites. The source rocks are thought to be Damara Belt metasediments that were melt-depleted in the early Paleozoic by extraction of the S-type Damara granites. The most likely scenario for crustal melting is by basaltic underplating of the source region in the lower crust. The range in initial Nd and Sr values is attributed to minor mixing of basaltic magma with the crustal melts, although Sr was also affected by minor contamination from the radiogenic Damara granites near the level of emplacement.