Heavy Mineral Placer Sand Deposits of Kontiagarh Area,Ganjam District,Orissa, India

The Kontiagarh placer deposit in the Ganjam district, Orissa, India extends in northeast direction having a width of 700–1000 m. A total of 187 samples were collected meterwise from 55 bore holes in a grid pattern from beach, frontal, intermediate and back dunes covering an area of approximately 1 km². Light minerals decrease in size from the beach to the back dunes, whereas the size distribution of heavy minerals in the beach and dunes is more or less uniform. The average heavy mineral content in the beach and dunes vary from 9.38% to 24.20%. The heavy minerals are ilmenite, garnet, sillimanite, rutile, monazite, and zircon with trace amounts of magnetite, hornblende, diopside, sphene, tourmaline, and epidote. Heavy minerals are mostly less than 350 µm in size, with a peak distribution in the range between 180 and 125 µm. Ilmenite shows exsolution intergrowth with hematite. Mineral chemistry of ilmenite, hematite, leucoxene, magnetite, monazite and sillimanite are examined by EPMA. Leucoxene is lower in Fe and higher in Ti, Al, Cr and V than ilmenite. The litho‐units of the Precambrian Eastern Ghats Mobile Belt, comprising primarily khondalite, charnockite, calc‐silicate granulite and gneiss, are the source of heavy minerals for this deposit. The bulk sample has 7.30% ilmenite, 5.24% sillimanite, 9.16% garnet, 0.18% rutile, 0.14% monazite, 0.06% zircon and 0.52% other heavy minerals. The deposit has good potential for economic exploitation of ilmenite, rutile, sillimanite, monazite, zircon and garnet.     

Mineralogical Characteristics of the Separated Magnetic Rutile of the Egyptian Black Sands

The Egyptian black sands contain several economic minerals, such as ilmenite, magnetite, garnet, zircon, rutile and monazite. During the concentration and separation of a high-grade rutile concentrate a bulk magnetic fraction is obtained. This fraction is composed mainly of opaques, titanhematite, ilmenite–titanhematite exsolved intergrown grains, magnetic leucoxene in addition to chromite, and magnetic rutile. The magnetic rutile occupies 6 wt.% of the bulk magnetic fraction or approx. 4 wt.% of the original rutile content in the raw sands. Most of magnetic rutile crystals are contaminated with opaque inclusions, staining-coating and/or composite locked grains. This magnetic rutile has a magnetic range from strongly paramagnetic to very weak paramagnetic. Electron microprobe analysis for twenty-three magnetic rutile grains identified mineral components of rutile, titanhematite, pseudorutile, leached pseudorutile and ilmenite in decreasing order of abundance. Some other inclusions are also detected in the different magnetic rutile grains. They are most probably garnet, silica, amphibole, ilmenite, feldspar, mica and zircon. The presence of these inclusions reflect the derivation of magnetic rutile of various crystalline igneous and metamorphic rocks. The magnetic susceptibility of magnetic rutile depends on the associated mineral components and their relative volumes in comparison to the rutile mineral component. Magnetic susceptibility of magnetic rutile is also related to both type and size of the associated mineral inclusions. The average chemical composition of the magnetic rutile is 66.34 wt.% TiO₂, 21.71 wt.% Fe₂O₃, 6.39 wt.% SiO₂, 1.80 wt.% Al₂O₃, 1.19 wt.% CaO and 0.10 wt.% Cr₂O₃. Thus, the contamination of magnetic rutile in the non-magnetic rutile concentrate would decrease the market value of the rutile concentrate. Alternatively these magnetic rutile grains are recommended to be blended with magnetic leucoxene or some types of ilmenite concentrate to improve the overall marketable specifications especially for both of Ti, Fe and Cr contents.     

Genesis and evaluation of heavy minerals in black sands: A case study from the southern Eastern Desert of Egypt

Black sands in the southern Eastern Desert (SED) of Egypt contain substantial reserves of heavy minerals (up to 5 %), and are found mainly in three basins namely: Hodein, Ibib and Diit between Shalateen and Halayeeb cities. The heavy minerals in these black sands include ilmenite-leucoxene (31 %–44 %), magnetite (15–18 %), zircon (11–21 %), garnet and green silicates (11–15 %), rutile (6–12 %) and monazite (2–4 %). Cassiterite, thorite, uranothorite, gold, xenotime and chromian spinel are minor quantities (<1 %). Magnetite (FeO: 75–93.5 wt%) and ilmenite (TiO₂: 42.7–56.9 wt%), hosting high Mn, V, Zr, Zn, Cr, Nb and Co, were probably derived from gabbroic rocks. The detrital chromian spinel composition (Cr#, 0.51–0.61; Mg#, 0.5–0.63; TiO₂ < 1.0 wt%) and its morphology are similar to those of spinels in fore-arc peridotites from the SED of Egypt, suggesting dominance of fore-arc basins for peridotite emplacement. These basins were formed during arc-arc or arc-oceanic crust collision and encolsed ophiolites, gabbroic rocks and I-type granites as sources of the SED black sands. The studied garnets are mostly almandine in composition with few grossularite and spessartine; they might have been derived from I-type granites and gneisses sources. The rutile and monazite show enriched LREE relative to HREE, and display marked defeciency in Eu, suggesting highly fractionated granitic rocks as a main source. Two distinct types of zircon are recorded: radioactive (Hf: 1578–8770, Y: 319–1335, U: 36–114 and Th: 40–64 ppm) and non radioactive (Hf: 427, Y: 44, U: 2 and Th: 2 ppm); they were probably derived from different granitic sources. Compositions and P-T conditions (T: 655–970 °C, P: 1.18–9.53 kbar) of magmatic amphiboles are similar to those derived from I-type granitoids. Bulk analyses of the economic heavy mineral assemblages show significant concentrations of Fe (393 kg/ton)_, Zr (183 kg/ton) and Ti (129 kg/ton) with minor Cr (14 kg/ton), Ba (7 kg/ton), Hf (4.9 kg/ton), Th (up to 3.34 kg/ton) and U (0.29 kg/ton). The elevated contents of Th and U could be related to the occurrence of monazite and zircon with subordinate thorite, uranothorite and xenotime. The total REE contents of these bulk analyses range from 1 to 4 kg/ton, where LREEs form 80–90 % of total REEs. Monazite (ΣREEs: 443604 ppm on average), garnet, zicon (ΣREEs: 421 ppm) and rutile (ΣREEs: 309 ppm) are the main host of REEs in the investigated black sands. Tonnages of raw sands, to a depth of one meter, are estimated per 10 km² in each basin, giving 18 million tons for Ibib basin and 19 million tons for both Diit and Hodein basins. Economic heavy minerals constitute 6–26 % of the total heavy minerals and around 1.0 % of total raw sands. Calculated reserves of these economic minerals, per 10 km² of black sands, range from 0.1 million ton in Ibib and Hodein basins to 0.2 million ton in the Diit basin.     

江汉平原主要河流沉积物重矿物特征与物源区岩性的响应

通过对江汉平原主要河流沉积物的重矿物组合、特征矿物以及能够反映沉积物稳定状况、物源及成熟度的重矿物特征指数(ATi,GZi和ZTR)进行对比分析,发现在江汉平原范围内,长江和汉江及其长江主要支流的沉积物中重矿物特征具有显著的差异。长江的重矿物组合模型为：  锆石+绿帘石+辉石+绿泥石+金属矿物,特征矿物是锆石和辉石;   汉江的重矿物组合模型为：  绿帘石+角闪石+石榴石+绿泥石+金属矿物,特征矿物是角闪石、石榴石;   另外,清江、漳河、沮水和玛瑙河的重矿物组合及特征矿物也都完全不同。而且各水系的沉积物的重矿物特征与其源区的岩性分布显示出极好的相关性。研究表明在江汉平原利用重矿物特征及组合模型来进行物源示踪的方法开展水系演化研究是可行的。     

Estimation of excess lifetime cancer risk and radiation hazard indices in southern Iraq

Relatively high activity concentrations of some radionuclides (²²⁶Ra, ²³⁸U, ²³²Th, and ⁴⁰K) have been measured in surface and subsurface soils in areas (southern Iraq) where many warfare actions have taken place during the Iran–Iraq and Gulf wars. Such high activity concentrations might be related to the increase in cancerous injuries and birth defects recently reported. The study was aimed to estimate the activity concentrations of some nuclides and a comparison of results with international health hazard indices. In addition, the excess lifetime cancer risk (ELCR) was assessed. Soil samples were collected at three depths (0, 30, and 60 cm) in three locations in Abu Al Khasib and Ad Dayr in Basrah governorates. The average activity concentrations were estimated employing hyper-pure germanium HPGe gamma-ray detection technology. The values obtained in Abu Al Khasib were: 58.44 Bq/kg (²²⁶Ra), 43.56 Bq/kg (²³⁸U), 19.38 Bq/kg (²³²Th), and 321.76 Bq/kg (⁴⁰K) whereas in Ad Dayr 45.71 Bq/kg (²²⁶Ra), 35.53 Bq/kg (²³⁸U), 20.33 Bq/kg (²³²Th), and 337.02 Bq/kg (⁴⁰K). According to UNSCEAR (Sources and effects of ionizing radiations: a report on the effects of atomic radiation to the general assembly with scientific annexes, Annex B, United Nations, New York, 2000) report, higher levels of ²²⁶Ra, ²³⁸U concentrations than the world’s average values (35 Bq/kg) were observed in both locations. In addition, some radiation hazard indices were determined for both locations: average outdoor external dose (D _out), average indoor external dose (D _in), total average value of the external dose (D _tot), average annual outdoor effective dose (E _out), average annual indoor effective dose (E _in), and averaged total annual effective dose (E _tot). Values for D _tot and E _tot were higher than the worldwide median (143 nGy/h and 0.48 mSv/y, respectively) in both regions. The measured values of activity concentrations were also used to estimate the outdoor, indoor, and total ELCR. The ELCR values were higher than the worldwide averages. Further analyses and studies need to assess the real risks for human health and possible soil remediation.     

Natural radioactivity measurements in beach sand along southern coast of Orissa,eastern India

The beach placer deposits in the southern coastal Orissa, India may have significant levels of radiation due to the presence of Th and U bearing minerals such as monazite and zircon. In this study, Gopalpur and Rushikulya beach regions were selected to study the ambient radiation environment. The average activity concentrations of radioactive elements such as ²³²Th, ²³⁸U and ⁴⁰K of beach sand samples were measured by γ-ray spectrometry using a HPGe detector, and found to be much higher than the internationally accepted values. The cross plots of eTh/eU and eTh/K inferred that the sand samples of Gopalpur and Rushikulya beach placer deposit were deposited in leached uranium and an oxidising environment. The absorbed γ dose rate levels of the study areas are similar to other monazite sand-bearing HBRAs of southern and southwestern coastal regions of India and of world. Different radiation hazard indices were estimated for the present study area and were found to be much higher than the internationally accepted values. Hence, Gopalpur and Rushikulya beach placer region can be considered as a high background radiation area and a potential zone for radiogenic heavy mineral exploration.     

Mineral Chemistry of Albite-Enriched Granitoids at Um Ara,Southeastern Desert,Egypt

Mineral chemistry and typomorphic characteristics are used to monitor the physicochemical evolution of late-magmatic to postmagmatic alteration processes that resulted in the development of a radioactive and albite-enriched microgranite stock. The mineral paragenesis indicates that postmagmatic fluids were enriched in Nb, Zn, Mn, U., Th, Zr, and Y, in addition to Rb, Li, and F Manganocolumbite with extremely high Nb/(Nb+Ta) (0.99), Mn/(Mn+Fe) (0.82), and zircon with high Zr/(Zr+Hf) (0.97) indicate crystallization under alkaline, relatively high-temperature conditions (>425° C). The close association of manganocolumbite, Nb-Mn-Zn- rich ilmenite (with 1.2 to 14.5 wt% ZnO), spessartine garnet (with 68.2-89.4 mol% spessartine), zircon, xenotime, zinnwaldite mica (up to 5.98 wt% F), and fluorite indicates the strong affinity of the elements of Nb, Y., Zr, Mn, and Zn for stable complexing by K⁺, Na⁺, Li⁺, and F^? rich supercritical fluids during the course of extraction and transportation.

The enrichment of the interacting fluid in U and Th is depicted by the presence of up to 1.6% UO₂ in manganocolumbite and Hf-bearing zircon, and up to 10.5% ThO₂ in monazite, in addition to locally abundant thorite and uranophane. It is suggested that the uranium mineralization, mainly as fracture fillings, formed during the waning stage of hydrothermal activity.     

Ilmenite composition in the Tellnes Fe–Ti deposit,SW Norway: fractional crystallization,postcumulus evolution and ilmenite–zircon relation

Major and trace element XRF and in situ LA-ICP-MS analyses of ilmenite in the Tellnes ilmenite deposit, Rogaland Anorthosite Province, SW Norway, constrains a two stage fractional crystallization model of a ferrodioritic Fe-Ti-P rich melt. Stage 1 is characterized by ilmenite-plagioclase cumulates, partly stored in the lower part of the ore body (Lower Central Zone, LCZ), and stage 2 by ilmenite-plagioclase-orthopyroxene-olivine cumulates (Upper Central Zone, UCZ). The concentration of V and Cr in ilmenite, corrected for the trapped liquid effect, (1) defines the cotectic proportion of ilmenite to be 17.5 wt% during stage 1, and (2) implies an increase of D _V^Ilm during stage 2, most likely related to a shift in fO₂. The proportion of 17.5 wt% is lower than the modal proportion of ilmenite (ca. 50 wt%) in the ore body, implying accumulation of ilmenite and flotation of plagioclase. The fraction of residual liquid left after crystallization of Tellnes cumulates is estimated at 0.6 and the flotation of plagioclase at 26 wt% of the initial melt mass. The increasing content of intercumulus magnetite with stratigraphic height, from 0 to ca. 3 wt%, results from differentiation of the trapped liquid towards magnetite saturation. The MgO content of ilmenite (1.4–4.4 wt%) is much lower than the expected cumulus composition. It shows extensive postcumulus re-equilibration with trapped liquid and ferromagnesian silicates, correlated with distance to the host anorthosite. The Zr content of ilmenite, provided by in situ analyses, is low (<114 ppm) and uncorrelated with stratigraphy or Cr content. The data demonstrate that zircon coronas observed around ilmenite formed by subsolidus exsolution of ZrO₂ from ilmenite. The U-Pb zircon age of 920 ± 3 Ma probably records this exsolution process. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Mineralogy, Chemical Characteristics and Upgrading of Beach Ilmenite of the Top Meter of Black Sand Deposits of the Kafr Al-Sheikh Governorate, Northern Egypt

Egyptian beach ilmenite occurs in a relatively high content in the naturally highly concentrated superficial black sand deposits at specific beach zones in the northern parts of the Nile Delta at Rosetta. Microscopic study shows that the ilmenite occurs as fresh homogeneous black or heterogeneous multicoloured altered grains and exhibits three types (homogeneous, exsolved and altered) of ilmenite varieties. XRD data of ilmenite indicates their association with minor hematite and quartz, whereas leucoxene shows its association with Nb‐rutile, pseudorutile and hematite. Grain size distribution suggests a very fine sand size of >89% and 80% and a fine sand size of 10.5% and 18.3% for fresh and altered ilmenites, respectively. The density of fresh, altered ilmenite and leucoxene concentrates varies from 2.70, 2.50 to 2.40 ton/m³, suggesting a gradual decrease from high grade fresh to leucoxene and consistent with variation in magnetic susceptibility as a consequence of the leaching of iron. Mass magnetic susceptibility reveals 97.6% of ilmenite and 92% of the altered form are obtained at 0.20 and 0.48 ampere. Fresh ilmenite exhibits variable TiO₂ (47.18%) and Fe₂O₃^T (46.10%) with minor MnO, MgO and Cr₂O₃ (1.22, 1.10 and 0.51%). The altered ilmenite is higher in TiO₂ (76.16%) and SiO₂ (4.68%) and lower in Fe₂O₃^T (14.45%), MnO, MgO and Cr₂O₃ (0.39, 0.52 and 0.11%) compared with the fresh form. Three concentrates of ilmenites (G1, G2 and G3) were prepared from crude ore using a Reading cross belt magnetic separator under different conditions, revealing a gradual increase of TiO₂, SiO₂, Al₂O₃ and CaO accompanied by a decrease of Fe₂O₃^T, MgO and Cr₂O₃ with repetition of the separation processes. Several ore dressing techniques were carried out to upgrade the ilmenite concentrate.     

Distribution of heavy minerals in Nizampatnam-Lankavanidibba coastal sands,Andhra Pradesh,East Coast of India

Heavy mineral analysis was carried out for the beach and fore dune sediments along 60 transects of Nizampatnam-Lankavanidibba coastal area. The heavy mineral assemblage in this area with decreasing abundance of opaques (Ilmenite + magnetite, 47.67%), pyriboles (20.35%), garnets (3.66%), epidote (3.23%) and with less than 3.0% concentration of sillimanite, zircon, staurolite, kyanite, apatite, spinel, monazite, biotite, topaz, leucoxene and chlorite. The heavy mineral concentrations are high in the finer fractions i.e., +120 and +230 (ASTM) than the coarse fraction (+60) of sand. In the seven sectors, heavy mineral assemblage is same but their concentrations are different. The sectors nearer to the river mouth contain high concentration of high specific gravity heavy minerals (ilmenite and magnetite) than sectors away from the river mouth. The redistribution of heavy minerals is controlled by creek dynamics, longshore currents, size and specific gravity of the heavy minerals.     

Long-lived Nb-Ta mineralization in Mufushan,NE Hunan,South China: Geological,geochemical, and geochronological constraints

The Mufushan Complex (MFSC), located in northeastern Hunan, is a significant producer of Nb-Ta-Li-Be rare metals in South China. The present study examines the genetic relationship, material provenance, fluid evolution, and metallogeny of the co-developed ore-free pegmatite (OFP) and ore-bearing pegmatite (OBP) in granite-related pegmatite-type Nb-Ta rare-metal deposits in MFSC. Three minerals (columbite-tantalite (coltan), zircon, and monazite) were chosen for analysis. The coltan grains display both primary crystallization structures (crystal homogeneity, oscillatory zonings, and primary growth rims) resulting from equilibrium and disequilibrium reactions due to localized changes in the physicochemical conditions and environment, as well as later replacement structures (alteration rims, patches, irregular zonations, and complex zonations) from metasomatic replacement processes related to hydrothermal fluid activity. The coltan yielded two weighted mean ²⁰⁶Pb/²³⁸U ages of 138.1 ± 2.1 Ma and 125.3 ± 2.0 Ma corresponding to magmatic and hydrothermal Nb-Ta mineralization ages. For the OFP, zircons also yielded two weighted mean ²⁰⁶Pb/²³⁸U ages of 138.4 ± 0.8 Ma and 131.5 ± 0.7 Ma, whereas monazite gave a weighted mean U-Pb age 142.9 ± 1.2 Ma. The ages of 142–138 Ma and 131 Ma represent the early and late stages of OFP crystallization and barren pegmatites in the MFSC, respectively. Zircon Lu-Hf isotopic compositions link rare-metal metallogenesis to the Lengjiaxi Group, which was the source material to the Mufushan composite batholith. Calculated ε_Hf(t) values and T_DM2 ages from the OFP (?7.6 to ?3.6 and 1676–1418 Ma, respectively) and the OBP (?14.1 to +4.9 and 2976–1548 Ma, respectively) are akin to those of schists and metasandstones of the metasedimentary Lengjiaxi Group. We propose a long-lived (ca. 13-Myr) event involving two metallogenic episodes of Nb-Ta mineralization in the Mufushan region. This study demonstrates the potential of zircon, coltan, and monazite for fingerprinting minerals and classifying the mineralization potential of pegmatite veins.     

Chemical composition of rock-forming minerals in gold-related granitoid intrusions,southwestern New Brunswick,Canada: implications for crystallization conditions,volatile exsolution,and fluorine-chlorine activity

Chemical composition of rock-forming minerals in Appalachian Siluro-Devonian granitoid intrusions, southwestern New Brunswick, was systematically determined by electron microprobe. The mineral chemical data together with petrographic examination was used to test magmatic equilibration and to constrain crystallization conditions, volatile exsolution, and fluorine-chlorine activity of fluids associated with these intrusions. Mineralogical distinction between Late Silurian to Early Devonian granodioritic to monzogranitic series (GMS) and Late Devonian granitic series (GS) rocks is evident, although both are subsolvus I-type to evolved I-type granitoids. Oxidized to reduced GMS rocks consist of quartz, plagioclase (An>10), K-feldspar, biotite, apatite, titanite, zircon, monazite, ± hornblende, ± pyroxene, ± magnetite, ± ilmenite, and ± sulfide. GS rocks comprise quartz, K-feldspar, plagioclase (An<10), mica group minerals, zircon, monazite, apatite, sulfide, ± ilmente, ± magnetite, ± topaz, ± columbite, and ± xenotime. Inter-intrusion and intra-intrusion variations in mineral chemistry are interpreted to reflect petrogenetic processes (e.g., assimilation and fractional crystallization) during granitoid evolution. Although magmatic equilibration among rock-forming minerals are disturbed by subsolidus hydrothermal processes, GMS rocks appear to have higher magmatic temperatures, variable levels of emplacement, a range of (i.e., reduced intrusions 10^−16.7∼10^−13.4 and oxidized intrusions 10^−14.0∼10^−10.5 bars), and relatively low f _HF/f _HCl ratios (10^−3.0∼10^−1.0) in exsolved fluids, compared to GS rocks. Reduced GMS intrusions bear higher gold potential and thus may be prospective targets for intrusion-related gold systems. Electronic Supplementary Material Supplementary material is available for this article at     

The mineral chemistry of new titanates from the jagersfontein kimberlite,South Africa: Implications for metasomatism in the upper mantle

New members of the crichtonite mineral series are described in which K, Ba, Ca and REE are in significant concentrations (5 wt% oxides) filling the A formula position in AM₂₁O₃₈. These phases are chromium (16 wt% Cr₂O₃) titanates (58 wt% TiO₂) enriched in ZrO₂ (5 wt%) and constitute a mineral repository for refractory and large ion lithophile elements in the upper mantle. The mineral senes coexists with Mg-Cr-ilmenite, Nb-Cr-rutile, and Ca-Cr (NbZr) armalcolite that have equally unusual chemistries. Kimberlitic crichtonites are depleted in the intermediate lanthanides but highly enriched in LREE and HREE with chondrite normalized abundances of 10³ to 10⁵. Crichtonite, armalcolite, and Nb-Cr-rutile occupy a compositional range in TiO₂ contents bridging the gap between ilmenite and rutile, two minerals having a widespread distribution in kimberlites and mantle-derived nodule suites.In common with other associations, and based on similarities in mineral chemistry, it is concluded that these minerals formed at P = 20–30 kb, 900–1100°C by reaction of peridotite with metasomatizing fluids. Kimberlitic crichtonite may be expressed as spinel + Cr-ferropseudobrookite, and armalcolite is equivalent to Cr-geikielite + rutile in the system (FeMg)-TiO₂-Cr₂O₃. This system contains a number of Cr-Ti compounds not found as minerals but it is proposed that the ubiquitous occurrence of ilmenite intergrowths in kimberlitic rutile results from decomposition of high pressure αPbO₂-type crystallographic shear structures. The new minerals have exotic chemistries and the high K-affinities broaden the scope for the origin of alkalic rocks, the generation of highly potassic magmas in the upper mantle, and suggest that alkali metasomatism may be pervasive.     

Heavy mineral distribution and characterisation of ilmenite of Kayamkulam — thothapally Barrier Island, southwest coast of India

Mineral concentration and ilmenite characterization of the Thothapally — Kayamkulam Barrier Island of the southern Kerala has been studied. 96.86% concentrations of heavy minerals are recorded in the surficial and core samples (4 m) in the southern Kayamkulam and northern Thothapally areas. The total heavy mineral content decreases with depth. The primary heavy mineral suite of the surficial and core samples consists of ilmenite, sillimanite, zircon, garnets, rutile, monazite and magnetite. Longshore current and onshore-offshore movements of sediment during the southwest monsoon are primarily responsible in sorting of the heavy minerals. TiO₂ content in ilmenite is significantly higher in the Kayamkulam core sediments than the surface samples. XRD analysis supports intensive weathering and alteration leading to the higher TiO₂ concentration. Higher percentage of ferric iron than ferrous iron in the core samples reveals that considerable weathering occurred under burial condition. SEM examination of ilmenite grains reveal the presence of solution pit, chemical leaching, corrosion and replacement textures, supporting the intense epigenetic alteration and weathering under subaerial condition and post-depositional changes by water-table condition.     

Uranium-rich accessory minerals in the peraluminous and perphosphorous Belvís de Monroy pluton (Iberian Variscan belt)

The strongly peraluminous, perphosphorous (<0.85 wt% P₂O₅) and low-Ca granites from the Belvís de Monroy pluton contain the most U-rich monazite-(Ce) and xenotime known in igneous rocks. Along with these accessory minerals, P-rich zircon occurs, reaching uncommon compositions particularly in the more fractionated units of this zoned pluton. Monazite displays a wide compositional variation of UO₂ (<23.13 wt%) and ThO₂ (<19.58 wt%), positively correlated with Ca, Si, P, Y and REE. Xenotime shows a high UO₂ content (2.37–13.34 wt%) with parallel increases of LREE, Ca and Si. Zircon contains comparatively much lower UO₂ (<1.53 wt%) but high P₂O₅ (<14.91 wt%), Al₂O₃ (<6.96 wt%), FeO (<2.93 wt%) and CaO (<2.24 wt%) contents. The main mechanism of incorporating large U and Th amounts in studied monazite and U in xenotime is the cheralite-type [(Th,U)⁴⁺ + Ca²⁺ = 2(Y,REE)³⁺] substitution. Zircon requires several coupled mechanisms to charge balance the P substitution, resulting in non-stoichiometric compositions with low analytical totals. Compositional variations in the studied accessory phases indicate that the substitution mechanisms during crystal growth depend on the availability of non-formula elements. The strong P-rich character of the studied granites increases monazite crystallization, triggering a progressive impoverishment in Th and LREE in the residual melts, and consequently increasing extraordinarily the U content in monazite and xenotime. This is in marked contrast to other peraluminous (I-type or P-poor S-type) granite series. The P-rich and low-Ca peraluminous melt inhibits uraninite crystallization, so contributing to the U availability for monazite and xenotime.     

Crystallization of titaniferous chromite,magnesian ilmenite and armalcolite in tholeiitic suites in the Karoo Igneous Province

Titaniferous chromite (up to 8 wt% TiO₂) and magnesian ilmenite (up to 10 wt% MgO) coexist at the base of the differentiated tholeiitic Mount Ayliff Intrusion in the Karoo Province of southern Africa, suggesting that the original magma was TiO₂-rich. Picritic lavas with 3% TiO₂ from the Lebombo monocline of the Karoo Province also contain microphenocrysts of magnesian ilmenite (up to 6 wt% mgO) and armalcolite (up to 7 wt% MgO). These oxide mineral associations and compositions are atypical of tholeiitic magmas, in which chromite usually has less than 1 wt% TiO₂, ilmenite less than 3 wt% MgO and armalcolite is rarely a primary mineral. Experiments have been conducted at one atmosphere pressure on a range of compositions to determine the effect of TiO₂ on the crystallization and composition of chromite, ilmenite and armalcolite. The results indicate that increasing the TiO₂ content of picritic magmas increases the TiO₂ content of the spinel, mainly at the expense of Al₂O₃, whereas Cr₂O₃ is not affected. Spinel compositions in the Mount Ayliff Intrusion (with over 45 wt% Cr₂O₃, less than 10 wt% Al₂O₃ and 8 wt% TiO₂) were duplicated in experiments on a picrite at temperatures of about 1,200°C at the Ni/NiO buffer. Increasing fO₂ from fayalite-magnetite-quartz to Ni/NiO buffer is shown to increase the crystallization temperature of armalcolite and to decrease that of ilmenite. The total FeO content of the liquid has little influence on the crystallization temperature of these phases. The TiO₂ content of the liquid, when either ilmenite or armalcolite crystallizes, varies inversely with SiO₂ content. The MgO content of the liquid at which ilmenite or armalcolite crystallizes depends upon the TiO₂ content of the starting composition, with naturally occurring and experimetally determined saturation being demonstrated for liquids with 5 wt% MgO and 5.5 wt% TiO₂. The partition coefficent for MgO between armalcolite or ilmenite and liquid is about 1.5. Observed magnesian armalcolite and ilmenite compositions in picrite lavas (both minerals) and in the Mount Ayliff Intrusion (ilmenite only) are consistent with crystallization from a TiO₂-rich magma with approximately 5 wt% MgO. The Fe ₂ ³⁺ TiO₅ component of armalcolite in the picrite lavas matches those formed experimentally at temperatures of 1,150–1,110°C and fO₂ of the Ni/NiO to Ni/NiO+1 log unit. Similarities also exist between the compositions of chromite, ilmenite and armalcolite and liquid fraction-ation trends of some Hawaiian high-TiO₂ lavas and the experimental studies presented here.     

Residence of REE, Y, Th and U in Granites and Crustal Protoliths; Implications for the Chemistry of Crustal Melts

A systematic study with laser ablation—ICP-MS, scanningelectron microscopy and electron microprobe revealed that 70–95wt% of REE (except Eu), Y, Th and U in granite rocks and crustalprotoliths reside within REEYThU-rich accessories whose nature,composition and associations change with the rock aluminosity.The accessory assemblage of peraluminous granites, migmatitesand high-grade rocks is composed of monazite, xenotime (in low-Cavarieties), apatite, zircon, Thorthosilicate, uraninite andbetafite-pyrochlore. Metaluminous granites have allanite, sphene,apatite, zircon, monazite and Thorthosilicaie. Peralkaline graniteshave aeschinite, fergusonite, samarskite, bastnaesite, fluocerite,allanite, sphene, zircon, monazite, xenotime and Th-orthosilicate.Granulite-grade garnets are enriched in Nd and Sm by no lessthan one order of magnitude with respect to amphibolite-gradegarnets. Granulitegrade feldspars are also enriched in LREEwith respect to amphibolite-grade feldspars. Accessories causenon-Henrian behaviour of REE, Y, Th and U during melt—solidpartitioning. Because elevated fractions of monazite, xenotimeand zircon in common migmatites are included within major minerals,their behaviour during anatexis is controlled by that of theirhost. Settling curves calculated for a convecting magma showthat accessories are too small to settle appreciably, beingseparated from the melt as inclusions within larger minerals.Biotite has the greatest tendency to include accessories, therebyindirectly controlling the geochemistry of REE, Y, Th and U.We conclude that REE, Y, Th and U are unsuitable for petrogeneticalmodelling of granitoids through equilibrium-based trace-elementfractionation equations. KEY WORDS: accessory minerals; geochemical modelling; granitoids; REE, Y, Th, U     

Petrochemistry,Age and Petrogenesis of the Shexian Pluton in Southern Anhui

The Shexian gneissic granodiorite in southern Anhui trends NE 55° from Shexian in the west to Guitoujian in the east with a length of 22 km and an outcrop area of 32 km. It was considered formerly to be Caledonian on the basis of a biotite K-Ar age of 474 Ma (1982). However, new evidence indicates that it may be Early Jinning in age as shown by: (1) it is found intruding into the Mid-Proterozoic Shangxi Group and is unconformably overlain by the Sinian Xiuning Formation, and (2) a zircon U-Th-Pb age of 928 Ma is obtained for the pluton. The pluton is composed of plagioclase (An=27.37%), K-feldspar(14%), biotite(16%) and quartz(32%). Accessory minerals are ilmenite (150g/T), xenotime (15g/T). garnet(25g/T), monazite(10g/T), zircon (20g/T) and apatite (104g/T). Petrochemical characteristics of the intrusion are:(l) Al-enrichment (A/NKC=1.30); (2) H₂O enrichment (H₂O= 1.74%); and (3) low oxidation index (f ’=0.10). It belongs to the continental crust transformation type as evidenced by: (1) MF and Mg/Y values of biotite are 0.41 and 0.31 respectively; (2) (⁸⁷Sr/⁸⁶Sr)=0.71119; (3) δ Eu=0.52; and (4) A/NKC=130. The Shexian pluton is therefore considered as a product of melting of phyllite at depth in the light of similarities in trace element and REE contents with the phyllite of the Banxi Group. Calculations of REE batch partial melting indicate that it may have resulted from 75% melting of the Banxi phyllite.     

Mineralogy,geochemistry, and genesis of co-genetic granite-pegmatite-hosted rare metal and rare earth deposits of the Kawadgaon area,Bastar craton,Central India

Co-genetic pegmatites associated with the granite of the Kawadgaon area in the Bastar craton, Central India, contain a wide range of ore minerals of Nb, Ta, Be, Sn, Zr, Ti, and REE, including columbite-tantalite, ixiolite, pseudo-ixiolite, wodginite, tapiolite, microlite, fersmite, euxenite, aeschynite, beryl, cassiterite, monazite, xenotime, zircon, ilmenite, triplite, and magnetite. There is a distinct vertical zonation between the rare metal and tin pegmatites in apical parts of the host granite. Geochemically, these are LCT-S type, beryl-columbite-phosphate pegmatites that have notably high contents of SiO₂ (av. 73.80%), Rb (av. 381 ppm), and Nb (av. 132 ppm). The investigated granites probably were derived from the melting of older crustal rocks, as indicated by a high initial ⁸⁷Sr/⁸⁶Sr isotopic ratio, and the major-element geochemistry of the granites and pegmatites. Plots of mol. CaO/(MgO+FeO^t) vs. mol. Al₂O₃/(MgO+FeO^t) suggest that the source rock was pelitic metasediments. Based on the available data, it is postulated that the derivation of pegmatites from the parent granite occurred shortly after granite emplacement in the late Archaean-early Proterozoic (~2500 Ma). The K/Rb, Ba/Rb, and Rb/Sr ratios of the felsic bodies reveal that a substantial part of the granite formed from evolved melts, and further fractionation produced the co-genetic pegmatites and associated rare metal and rare earth deposits.     

Mineral distribution and provenance of heavy mineral sands (zircon,ilmenite, rutile) deposits from the NW Murray Basin,far western NSW,Australia

Abstract

There is significant economic interest in the Murray Basin of southeastern Australia as it is proving to be a major heavy mineral sands (HMS) province that will be one of Australia’s major source for production of rutile, zircon and ilmenite. The distribution and provenance of HMS resources in the Murray Basin is poorly understood because of its huge size, limited exploration and the complex depositional, structural and weathering mechanisms in their development. In this paper, we focus on the Copi North and Magic deposits some 130–180?km south of Broken Hill, NSW. The heavy mineral assemblages of the Copi North and Magic deposits are very similar, with the main economic minerals being ilmenite, leucoxene, pseudorutile, rutile and zircon. Intensive fracturing and brecciation are identified in many samples and are inferred to have been initially caused by multi-stage deformational processes associated with metamorphism and then further developed through alluvial and eolian transportation. Both deposits are classified as ‘medium sands, symmetrical, mesokurtic and moderately well-sorted’. The majority of economic minerals are of low to medium sphericity and subrounded, along with abundant polished eolian quartz grains. The Copi North deposit has coarser and more poorly sorted sediments with higher HMS grades and magnetics content than the Magic deposit, reflecting a higher energy depositional environment. The main source for the HMS for the Copi North and Magic deposits is largely ascribed to the Broken Hill Block. Previous studies have shown that the Broken Hill orebody underwent substantial sub-aerial weathering over hundreds of millions of years. In addition, the complex metamorphic events experienced by the Broken Hill Block were capable of forming the broad series of minerals identified within the Copi North and Magic deposits. The HMS were believed to have been transported through paleovalleys near the Mulculca Fault in a southeast direction representing a feeder system into the NW Murray Basin. Both deposits feature a relatively linear geometry (roughly parallel to the strike of the paleoshoreline), with high HMS grades, modest tonnages, and coarser sediments when compared to WIM-style offshore deposits. Compared to other strandline HMS deposits of the Murray Basin, they are smaller in size although have similar high grades of 3.7–6.9% THM and similar proportions of the HMS assemblage of ilmenite, leucoxene, rutile and zircon. Deposits of similar size and grade are likely to occur throughout the northern part of the Murray Basin.

1. KEY POINTS

2. Both the Copi North and Magic deposits contain similar mineral assemblages with the provenance of the heavy minerals ascribed to the Broken Hill Block.

3. A relatively high energy inshore environment is inferred for the Copi North deposit while a lower energy environment associated with either a foreshore or backshore environment is inferred for the Magic deposit.

4. Deposits of similar mineralogy, grades and size are likely to occur elsewhere throughout the northern Murray Basin.