Biotite Pegmatite Research Articles

SIMS U-Pb Dating of Uraninite from the Guangshigou Uranium Deposit: Constraints on the Paleozoic Pegmatite-Type Uranium Mineralization in North Qinling Orogen, China

Pegmatite-type uranium mineralization occurs in the Shangdan domain of the North Qinling Orogenic Belt, representing a significant uraniferous province. The Guangshigou deposit is the largest U deposit of the district. Within the North Qinling area, a series of Caledonian granitic igneous rocks intruded the Proterozoic metamorphic rocks of the Qinling Group in two magmatic stages: (i) the Early Silurian Huichizi granite that was derived from a low degree of partial melting of thickened lower basaltic crust combined with mantle-derived materials following the subduction of the Shangdan Ocean; and (ii) the Late Silurian–Early Devonian Damaogou granite and associated pegmatites derived from the same source but emplaced in a late tectonic post-collisional extension environment. In the Guangshigou deposit, the U mineralization mainly occurs as uraninite disseminated in U-rich granitic biotite pegmatites, which formed by assimilation-fractional crystallization magmatic processes. Petrographic observations showed evidence for coeval crystallization of uraninite and other rock-forming minerals of the host pegmatite including quartz, feldspar, biotite, zircon, monazite, apatite, and xenotime. In addition, the low U/Th ratios (~19) and Th, REE, and Y enrichments characterized a magmatic origin for uraninite, which was likely derived from fractionated high-K calc-alkaline pegmatitic magma that experienced various degrees of crustal material contamination. In situ U-Pb isotopic dating performed by Secondary-Ion Mass Spectrometry (SIMS) on uraninite from the Guangshigou deposit yielded a crystallization age of 412 ± 3 Ma, which is concomitant (within errors) with the emplacement age of the host pegmatite (415 ± 2 Ma) and constrained the U ore genesis to the Early Devonian, which corresponds to the late Caledonian post-collisional extension in the North Qinling area. Uraninite then experienced various degrees of metamictization and/or post-Caledonian hydrothermal alteration characterized by an alteration rim associated with coffinite, chlorite and limonite. Finally, the characteristics of the pegmatite-related Guangshigou deposit exhibiting Th-rich uraninite which was the product of assimilation-fractional crystallization of pegmatitic magma defined a model significantly different than the one established for the world-class Rössing deposit characterized by Th-poor uraninite hosted in alaskite dykes formed by low degree of partial melting of U-rich metasediments.

Origin and evolution of uraniferous pegmatite: A case study from the Xiaohuacha granite–pegmatite system and related country rocks in the Shangdan uranium mineralization district of North Qinling Orogenic Belt, China

ABSTRACT The Shangdan district in the North Qinling Orogenic Belt hosts the largest pegmatite-type uraniferous province in China. The Xiaohuacha U mineralization in this district occurs along the contact zone between biotite pegmatite and its host rock of biotite gneiss, adjacent to the composite Huichizi batholith, which is composed mainly of biotite monzogranite and syenogranite. Zircon U Pb dating of these granitoids reveals two-stage magmatism, namely, the early Silurian for biotite monzogranite (438.5 ± 4.4 Ma) and the early Devonian for syenogranite (417.8 ± 3.2 Ma). The biotite pegmatite shows an age of 415.8 ± 3.8 Ma for the mineralized marginal zone, and similar ages of 414.6 ± 4.3 Ma and 413.2 ± 3.5 Ma are obtained for the barren marginal and intermediate zones, respectively. The biotite monzogranite exhibits a restricted range of positive zircon eHf(t) values of +5.3 to +7.8, with TDM2 ages of 0.93–1.08 Ga, suggesting magma derivation from mainly pre-existing basaltic rocks in the orogenic root. Magmatic oxygen fugacity (fO2) was estimated by Ti-in-zircon thermometry and Ce incorporation within zircons. The biotite monzogranite was emplaced at a temperature of 736 ± 24 °C with log fO2 values of −22.28 to −14.19. In contrast, the syenogranite zircons have eHf(t) values of −1.4 to +2.4 and TDM2 ages of 1.25–1.49 Ga, indicating a derivation from the Mesoproterozoic crust with some mantle input. Pristine zircon domains from different zones of the biotite pegmatite display eHf(t) values of −2.8 to +1.8 and TDM2 ages of 1.29–1.58 Ga, similar to those of the syenogranite, indicating they share a common magma source. Combination of petrographical, geochronological, isotopic, and mineral chemical data supports a petrogenetic relationship between the high-T (842 ± 14 °C) syenogranite and biotite pegmatite (756 ± 21 °C and 709 ± 26 °C for the uncontaminated marginal and intermediate zones respectively) through magmatic differentiation from the same magma source during a period of tectonic extension. Processes of segregation and emplacement of the pegmatitic magma are consistent with open-system assimilation and fractional crystallization (AFC) leading to decreasing log fO2 values from the syenogranite (−11.23 to −7.02) to biotite pegmatite through a fractionation trajectory. However, log fO2 values increase from the uncontaminated pegmatite marginal (−15.88 to −10.9) to intermediate (−17.86 to −6.14) zones with closed-system pegmatite internal evolution. The interaction (hybridization) between magma and gneiss induced decrease of oxidation state and (K + Na)/Al, as well as modifications to zircon compositions in the mineralized pegmatite marginal zone. We propose a model with decreasing of fO2 and (K + Na)/Al during AFC being the main controls on the formation of the U deposit.

Genesis of the Guangshigou pegmatite-type uranium deposit in the North Qinling Orogenic Belt, China

The Guangshigou uranium deposit is an important pegmatite-type uranium deposit located in the North Qinling Orogenic Belt (NQB), China. The characteristics pertaining to its chronology, petrogenesis, and geodynamic setting remain unclear till date. In this study, we present systematic zircon U–Pb ages, Lu–Hf isotopic data, and mineral chemistry of uranium-rich biotite pegmatite dikes. In situ zircon U–Pb dating of the biotite pegmatite dikes yielded early Caledonian ages (422–417 Ma). The initial 176Hf/177Hf ratios of the zircons from the biotite pegmatites showed a narrow range (0.282378–0.282590), with εHf values ranging from −5.1 to 2.8 and Paleoproterozoic to Mesoproterozoic two-stage Hf model ages between 1723 and 1227 Ma. The bulk and mineral chemistries of the pegmatites also indicate that the pegmatite dikes were derived from low-degree partial melting of Qinling Group rocks with monazite as the residual mineral. Differences between the normal biotite pegmatite and the mineralized biotite pegmatite dikes with respect to mineralogy, texture, structure, and mineral chemistry indicate that the assimilation between the pegmatite dikes and the wall rock, may be the primary mechanism for the mineralization of the uranium, with the fractional crystallization process being another important mechanism. Amphibolite facies metamorphism, which was most likely caused by the exhumation of the subducted slab in the NQB, may have been a key factor in the genesis of the uranium-rich pegmatitic magma.

Geochronology and Geochemistry of Uraninite and Coffinite: Insights into Ore-Forming Process in the Pegmatite-Hosted Uraniferous Province, North Qinling, Central China

The biotite pegmatites in the Shangdan domain of the North Qinling orogenic belt contain economic concentrations of U, constituting a low-grade, large-tonnage pegmatite-hosted uraniferous province. Uraninite is predominant and ubiquitous ore mineral and coffinite is common alteration mineral after initial deposit formation. A comprehensive survey of the uraninite and coffinite assemblage of the Chenjiazhuang, Xiaohuacha, and Guangshigou biotite pegmatites in this uraniferous province reveal the primary magmatic U mineralization and its response during subsequent hydrothermal events. Integrating the ID-TIMS (Isotope Dilution Thermal Ionization Mass Spectrometry) 206Pb/238U ages and U-Th-Pb chemical ages for the uraninites with those reported from previous studies suggests that the timing of U mineralization in the uraniferous province was constrained at 407–415 Ma, confirming an Early Devonian magmatic ore-forming event. Based on microtextural relationships and compositional variation, three generations of uranium minerals can be identified: uaninite-A (high Th-low U-variable Y group), uranite-B (low Th-high U, Y group), and coffinite (high Si, Ca-low U, Pb group). Petrographic and microanalytical observations support a three-stage evolution model of uranium minerals from primary to subsequent generations as follows: (1) during the Early Devonian (stage 1), U derived from the hydrous silicate melt was mainly concentrated in primary magmatic uaninite-A by high-T (450–607 °C) precipitation; (2) during the Late Devonian (stage 2), U was mobilized and dissolved from pre-existing uraninite-A by U-bearing fluids and in situ reprecipitated as uraninite-B under reduced conditions. The in situ transformation of primary uraninite-A to second uraninite-B represent a local medium-T (250–450 °C) hydrothermal U-event; and (3) during the later low-T (100–140 °C) hydrothermal alteration (stage 3), U was remobilized and derived from the dissolution of pre-existing uraninite by CO2- and SiO2-rich fluids and interacted with reducing agent (e.g., pyrite) leading to reprecipitation of coffinite. This process represents a regional and extensive low-T hydrothermal U-event. In view of this, U minerals evolved from magmatic uraninite-A though fluid-induced recrystallized uraninite-B to coffinite, revealing three episodes of U circulation in the magmatic-hydrothermal system.

The Guangshigou uranium deposit, northern Qinling Orogen, China: A product of assimilation-fractional crystallization of pegmatitic magma

Uranium mineralization associated with biotite pegmatites occurs in the Shangdan triangular domain of the northern Qinling Orogen, constituting a significant uraniferous province. The Guangshigou deposit is the most productive uranium deposit in this area. Several Caledonian intrusions were emplaced within the ore district including the Huichizi granite (HG, 441.1 ± 1.3 Ma), Damaogou granite (DG, 420.9 ± 1.3 Ma), biotite pegmatite (BP, 415.6 ± 1.5 Ma), and muscovite-biotite pegmatite (MBP, 412.1 ± 1.4 Ma). Based on uranium concentration, each BP displays two different zones, i.e., an uranium-bearing zone (UBP hosted at the contact zone) and a non-mineralized zone (NBP hosted in the non-assimilation zone). Two magmatic pulses are distinguished. The Early Silurian HG was derived from a low degree of partial melting of thickened lower basaltic crust interfused mantle-derived materials following the subduction of the Shangdan Ocean. The Late Silurian–Early Devonian intrusions exhibited the same source region but formed during the weakening of convergent action. Fractional crystallization of high-K granitic magma (DG) produced a set of fractionated pegmatites, whereas UBP deviates from a normal magma evolution trend due to country rock contamination in the hybrid zone.Most of the U in UBP is hosted in primary uraninite. The biotite gneiss (BG) is the most favorable country rock for uranium mineralization. Bulk assimilation of country rock is driven by reactions during xenolith melting and pegmatitic magma crystallization. The mineralization occurs at the assimilation/hybrid zone, e.g., the contact zone between BP and BG, as well as the partially assimilated xenoliths.The enrichment of contact-hosted peritectic biotite and increase of FeOT, MgO, TiO2, MnO, LILE (i.e., Rb, Ba, K), HFSE (i.e., Nb, Ta) contents and initial 87Sr/86Sr ratios as well as decrease of εNd(t) values in BP arose from chemical interaction (assimilation/hybridization) with the country rock. The reactive bulk assimilation modified the normal crystallization process of pegmatitic magma. The uranium ores are the products of the assimilation-fractional crystallization of the pegmatitic magma. U-F complexes were destabilized by reducing fluorine activities during the formation of peritectic biotite in the assimilation/hybrid zone, triggering decrease of U solubility and saturation.

Zircon trace element and isotopic (Sr, Nd, Hf, Pb) effects of assimilation-fractional crystallization of pegmatite magma: A case study of the Guangshigou biotite pegmatites from the North Qinling Orogen, central China

Evidence for open-system magmatic processes related to wallrock assimilation accompanied by fractional crystallization (AFC) is present in the Guangshigou biotite pegmatites, North Qinling Orogen. The biotite pegmatite-gneiss contacts generally coincide with the greatest enrichment of U and Th. Zircon UPb dating constrains the crystallization ages of the biotite pegmatite (rim zone-415±2.6Ma; internal zone-413.5±2.5Ma), in line with a pyrite PbPb isochron age (413±22Ma). Metamict areas in zircon show generally elevated concentrations of trace elements and expulsion of radioactive Pb. Internal zone samples, representing uncontaminated magma, have negative to positive zircon (~413Ma) εHf(t) (−1.53−+3.24), low εNd(t) values (−2.4), and old Hf and Nd model ages (tDM2=1.5–1.19Ga, T2DM=1.35Ga, respectively), indicating a dominantly recycled Mesoproterozoic lower crustal material with involvement of some juvenile materials in the source region. The magmatic oxygen fugacity (fO2) and crystallization temperatures ranges from −24.81 to −13.34 of log fO2 and 570°C to 793°C, respectively. Compared to the internal zone, pegmatite rim samples display a variable and lower εNd(t) values (−3.9 to −2.8) and T2DM (1.47–1.37Ga), but similar Hf isotopic compositions, favouring a three-component isotopic mixing model (recycled Mesoproterozoic lower crust materials, juvenile materials, and host gneiss). Pronounced variations of Ti, Y, U, Th, Hf, and REE concentrations in zircon from grain to grain in individual samples and from area to area within individual grains suggest a fluctuating crystallization environment in hybridized magma from which the rim-hosted zircons crystallized. Variable and high radiogenic Pb ratios of pyrites forming in the hybridized magma were inherited from the matrix. Zircons from both zones exhibit similar Hf isotope patterns, indicating the rim-hosted zircons crystallized during the early stage of hybridization of magma. However, the heterogeneous Nd isotopic composition in individual pegmatites imply an incorporation of wallrock-derived melt with less radiogenic Nd.

Alkali metal and rare earth element evolution of rock-forming minerals from the Gatumba area pegmatites (Rwanda): Quantitative assessment of crystal-melt fractionation in the regional zonation of pegmatite groups

This study presents a general model for the evaluation of Rayleigh fractional crystallisation as the principal differentiation mechanism in the formation of regionally zoned common and rare-element pegmatites. The magmatic evolution of these systems from a granitic source is reconstructed by means of alkali element and rare earth element (REE) analyses of rock-forming minerals (feldspars, micas and tourmaline), which represent a whole sequence of regional pegmatite zonation. The Gatumba pegmatite field (Rwanda, Central Africa) is chosen as case study area because of its well-developed regional zonation sequence. The pegmatites are spatially and temporally related to peraluminous G4-granites (986±10Ma). The regional zonation is developed around a G4-granite and the proximal pegmatites grade outwardly into biotite, two-mica and muscovite pegmatites. Rare-element (Nb–Ta–Sn) pegmatites occur most distal from the granite.Alkali metal fractionation trends in pegmatitic K-feldspar (Rb 350–6000ppm, Cs 2–160ppm) and muscovite (Rb 670–7000ppm, Cs 10–150ppm) define a single and continuous trend, which is modelled by Rayleigh fractional crystallisation, starting from a parental granite composition (G4-composition: K 3.1wt%, Rb 222ppm, Cs 11ppm). The fractionation model shows, moreover, that the pegmatites adjacent to the parental pluton are the least fractionated, and the distal pegmatites are the most fractionated. Biotite pegmatites form from 0% to 69% crystallisation, two-mica pegmatites from 69% to 92%, and muscovite pegmatites from 92% crystallisation onwards. The extreme Rb- and Cs-enrichment in rare-element pegmatites requires at least 98% fractionation of the initial G4-granite composition. Mathematical derivation of the K/Rb versus Cs relationship in K-feldspars confirms Rayleigh fractional crystallisation as the main differentiation process in the development of regional pegmatite zonation. Moreover (1) it demonstrates the continuity of the fractionation process from biotite pegmatites to rare-element pegmatite and indicates a genetic link among them; and (2) it allows a general evaluation of pegmatite fields in terms of system parameters, i.e. the initial element concentration in the granitic melt and partition coefficients.REE patterns of the rock-forming minerals (feldspars, muscovite, biotite and schorl) show a distinct transition along the regional zonation. They evolve from sloping fractionated to more flat patterns starting from the biotite up to the muscovite pegmatites. Minerals from the rare-element pegmatites (feldspars, muscovite and elbaite) show again more fractionated, heavy REE-depleted patterns. The observed evolution in the REE patterns corresponds to early crystallisation of light REE-enriched monazite and a late crystallisation of mainly cogenetic heavy REE-enriched phases such as apatite, columbite-group minerals and beryl. Modelling of Rayleigh fractionation, starting from an initial parental granite (G4-composition: LaN/YbN 10 and ∑REE 83), shows that this evolution in mineral REE pattern is the result of fractional crystallisation of the pegmatitic melt and precipitation of REE-incorporating minerals such as monazite and apatite.Consequently, trace element modelling indicates that Rayleigh fractional crystallisation governs the mineralogical and geochemical evolution from a granite source to common and eventually rare-element pegmatites. This mechanism shows that granitic pegmatites are extremely fractionated, residual melts which are genetically and directly connected to a main granite body.

MINERALOGY OF SPODUMENE PEGMATITES AND RELATED ROCKS IN THE TIN-SPODUMENE BELT OF NORTH CAROLINA AND SOUTH CAROLINA, USA

The Tin–Spodumene Belt (TSB) along the border of North and South Carolina, southeastern USA, is composed of hundreds of en echelon dikes intruded into amphibolite-facies mica schist and amphibolite. The spodumene pegmatites are unzoned and consist of medium- to very coarse-grained spodumene, plagioclase, K-feldspar, quartz, muscovite, and accessory amounts of beryl, manganoan fluorapatite, triphylite, and cassiterite. The pegmatitic fabric is replaced by a fine-grained saccharoidal albite– quartz fabric that also heals fractures in spodumene and feldspar. Plagioclase (Ab95–96) in the pegmatite is only slightly less sodic than that (Ab98–99) in the replacement albite–quartz assemblage. Rare, nonperthitic K-feldspar is slightly sodic (Or95–99). Spodumene typically contains about 1% iron oxide, giving the mineral a light green color. Muscovite contains 1–2 wt% FeO and a trace of sodium (0.2–0.6 wt% Na2O). A Mn-bearing fluorapatite (altered to low-F and low-Mn apatite) and triphylite (altered to fairfieldite) occur within the pegmatite fabric and appear to be primary phosphate minerals. Secondary phosphate minerals, including several species first described from the TSB, are associated with fracture fillings in sheared pegmatite. Pre-tectonic plagioclase – quartz – muscovite – garnet (Alm71Sps26Prp3) pegmatites are folded and sheared. Muscovite and Mn-bearing fluorapatite compositions are similar to equivalent phases in the spodumene pegmatites. The spodumene pegmatites are sheared, especially in proximity to regional shear-zones, but are not folded. Post-tectonic plagioclase – quartz – K-feldspar – muscovite – biotite pegmatites have Mn-poor fluorapatite, unlike the spodumene pegmatites. The pre-tectonic pegmatites may represent a precursor to the spodumene pegmatites.

In situ radiation measurement and estimation of U/Th ratio to reflect on the uranium bearing potential zone in metamorphic rocks of Mahendragarh district, Haryana

Gross gamma radiation survey was carried out using Nal(Tl) scintillator based portable gamma ray spectrometer (PGRS) around areas of Gaonri, Dholera, Pachnota and Meghot in Mahendragarh district, Haryana. Geologically the area forms part of north Delhi fold belt comprising calc-silicate, quartz biotite schist, impure marble, quartzite and pegmatite rocks. Equivalent uranium (eU3O8) concentration in ppm was estimated in situ on a regular grid pattern of 500 m (E-W) × 1000 m (N-S) and grab samples were collected at grid locations for analyzing in the laboratory for estimating the contents of eU3Os Raeq, ThO2 and %K. A comparison with the laboratory analysed grab samples for eU308 data and in situ radiation measurements shows a good match of the two sets of data. The in situ measurements indicate higher concentration of eU3O8 in Chapra Bibipur in northeastern most corners, Maghot area in central part, Gaonri in western part and Pachnota in southwestern part of the study area.

The chemistry of some granitic veins from the Lewisian of Coll and Tiree, Argyllshire, Scotland

The chemistry of two different types of granitic rocks from a polymetamorphic Precambrian basement complex is described in the light of their metamorphic and tectonic history. Thirty-two specimens were analysed for major oxides and S, Cr, Ni, Cu, Zn, Rb, Sr, Y, Zr, Nb, Ba, La, Ce, Pb and Th, and are subdivided on the basis of their chemistry and origin into five groups. High grade concordant granitic veins, with abnormally high K/Rb ratios, and low K/Ba and Rb/Sr ratios, had their pattern of element distribution imparted by granulite facies metamorphism. They are shown to have been enriched in K and Rb, giving them normal values of these ratios, during a later amphibolite facies retrogressive metamorphism and deformation. Late biotite pegmatites have an unusual chemistry, showing high K/Rb, Ba/Rb and Ba/Sr ratios opposed to their origin by fractionation of a granitic fluid, and high Ba, Zr, Th, La and Ce contents, and low Ce/La ratios as would be found in highly fractionated granites. It is suggested that they have had a dual formative process, being initiated by selective mobilisation of quartzo-feldspathic portions of the gneiss complex, and affected later by a highly fractionated fluid.

Contact and Endomorphic Phenomena Associated with a Part of the Idaho Batholith

The intrusion of the Idaho batholith in the Paradise Ridge area of north Idaho was accompanied by clearly defined contact and endomorphic phenomena. Hydro thermal solutions emanating from deeper parts of the consolidating magma changed a part of the early, chilled marginal hornblende-biotite diorite of the batholith to a less calcic biotite-quartz diorite and some of the quartzite above to biotite-quartz diorite and to a quartz-biotite-diorite gneiss. In a part of the area the emanations were enriched in potash and these, permeating through the gneissic rock, changed much of it to an augen gneiss with microcline porphyroblasts and the adjacent biotite-quartz diorite to granodiorite. Toward the end of the period of igneous activity the emanations were highly silicic and introduced exceptionally large amounts of quartz into all the rocks, even causing silicification of the invaded quartzite. Minerals added to the rocks by hydrothermal replacements include biotite, andesirie, microcline, quartz, sphene, zircon, apatite, magnetite, garnet, allanite, zoisite, epidote, muscovite, sericite, and calcite. As consolidation progressed downward and hydrothermal solutions were drawn off from deeper levels, the solutions moved upward along fractures in the already endomorphosed rock and by replacement changed the bordering rock to coarse but variable-grained bodies of biotite pegmatite. One of these bodies is extraordinarily rich in garnet. As the solutions became more silicic, fissures were filled with quartz, or the wall rock was replaced by quartz, and bodies of silexite were formed. This study shows the importance of igneous emanations in the formation of quartz-feldspathic gneisses, augen gneisses, and pegmatites, and the extent to which hydrothermal replacements enter into their origin. It is suggested that such phenomena may be widespread in the crystalline schists of extensive metamorphic terranes.