Yamuna Singh

Early Proterozoic 87Rb-86Sr Model Ages of Pegmatitic Muscovite from Rare Metal-bearing Granite-Pegamtite System of Kawadgaon, Bastar Craton, Central India

Abstract Seven muscovite mica separates from the rare metal pegmatites of Kawadgaon, Bastar Craton, Central India, give model 87Rb-86Sr ages ranging from 2330 to 1850 Ma. The oldest age of the muscovite almost overlaps within 20 error with the age (2497k152 Ma) of the parent fertile granites. The data suggest possible derivation of pegmatites shortly after the emplacement of Kawadgaon granites at ca. 2500 Ma. Most of the muscovite ages (n = 6) indicate tectonomagmatic ages after pegmatite injections. The 87Sr/86Sr ratio (0.7142) of granites suggests their derivation from crustal material.

Bastnaesite from Kanigiri granite, Prakasam district, Andhra Pradesh

For the first time we report bastnaesite and hydroxyl bastnaesite (lanthanum cerium fluoro-carbonate) from the Kanigiri granite. The host granitoids are of A-type and vary in composition from quartz syenites to peralkaline granites. Rare metal and rare earth-bearing minerals identified by X-ray diffraction (XRD) studies in Kanigiri granite are bastnaesite and hydroxyl bastnaesite, besides columbite-tantalite, monazite, fergusonite, thorite and euxenite. Petromineralogical studies have also revealed the presence of bastnaesite. The presence of bastnaesite in Kanigiri granite suggests that the host felsic rocks may also form a potential source for light rare earth mineral, bastnaesite, apart from the already known rare-metal minerals.

X-ray crystallography and mineral chemistry of bastnaesite from Kanigiri granite, Prakasam district, Andhra Pradesh, India

The authors report the results of X-ray diffraction (XRD) and geochemical studies on bastnaesites (lanthanum cerium fluoro-carbonate) hosted in alkali Kanigiri Granite of the Prakasam district in Andhra Pradesh, India. The XRD pattern of the investigated bastnaesite displays sharply-defined reflections. The observed d-spacings of the bastnaesite are in very close agreement with those published for bastnaesite standard in International Centre for Diffraction Data (ICDD) Card No. 11–340. The calculated unit cell parameters (ao; co) and unit cell volume (V) of the studied bastnaesite (ao 7.1301–7.1413 Å, co 9.7643–9.7902Å and V 429.8940–432.3875 Å3) are almost equal to values published for bastnaesite standard (co 7.1290 Å, co 9.7744 Å and V 430.19 Å3) in the relevant data card.Geochemical data of bastnaesite reveals high content of Ce (mean 27.22%) followed by La (mean 16.82%), Nd (mean 6.12%) and Pr (mean 1.91%). Compared to light REE (LREE) content (mean 437165 ppm), heavy REE (HREE) content (mean 5867 ppm) is drastically low, with unusually high LREE/HREE ratio (mean 80). The chondrite-normalised plot also exhibits drastic enrichment of LREE relative to HREE with pronounced negative Euanomaly (mean Eu/Eu* = 0.15). High (LREE)N / (HREE)N, (La/Lu)N, (La/Yb)N and (Ce/Yb)N ratios reveal higher fractionation of LREE relative to HREE. The rare earth element (REE) contents of the studied bastnaesite are very close to REE contents of bastnaesite hosted in alkali syenite from Madagascar. The presence of bastnaesite in Kanigiri Granite and soils derived from it enhances the scope of further exploration for bastnaesite in several bodies of alkaline rocks and alkali granitoids present along the eastern margins of the Cuddapah basin, Andhra Pradesh.

Geochemistry of Proterozoic Radioactive Arkoses in Khammam District, Andhra Pradesh, India: Evidence for K-rich Evolved Granitoid Upper Crust in the Late Archaean

Abstract Geochemical studies on radioactive arkoses (43–153 ppm U and 387–862 ppm Th) of the Proterozoic Pakhal Supergroup from Bangaruchilka, Khammam district, Andhra Pradesh, India, indicate that their gross major and trace element chemistry reflect their mineral composition. Chemically, arkoses are rich in silica (83% to 88% SiO 2 ) and potassium (3% to 5% K 2 O), with consistently high Al 2 O 3 /Na 2 O (36 to 50) and K 2 O/Na 2 O (18 to 25) ratios, which indicate that they are chemically mature sediments. The arkoses also show higher concentrations of Ti, V, Cr, Ni, Cu, Y, Zr, Nb, La and Pb. The values (60% to 68%) of chemical alteration index (CIA) of studied arkoses are moderate, and indicate that the source rocks have undergone lesser degree of chemical weathering. Tectonic setting discriminate plots of Fe 2 O 3 (total)/MgO vs. TiO 2 and Al 2 O 3 /SiO 2 , and K 2 O/Na 2 O vs. SiO 2 and SiO 2 /Al 2 O 3 indicate that the Bangaruchilka arkoses represent the sediments that were deposited in passive continental margin (PM), which is further supported by association of platformal type of sediments (quartzites and phyllites) with them. Unlike middle Archaean sedimentary rocks, the studied arkoses are depleted in Na 2 O, MgO and CaO, and distinctly enriched in SiO 2 and K 2 O. These geochemical features match with post-Archaean clastic sediments, which argues for the involvement of late Archaean granitic crust in supply of detritus of studied arkoses. Enrichment of potassium alongwith abundant microcline and elevated concentrations of Y, Zr, Nb, U, Th, La, etc. in arkoses indicate K-rich evolved granitoid upper crust in the provenance during the late Archaean-early Proterozoic period. Dominance of mechanical weathering over chemical weathering favoured arkose formation, and also transfer of radio-elements with clastic sediments into the Pakhal basin. After sedimentation, uranium seems to have been remobilised from the rocks of Archaean-Pakhal system, consequent to post-sedimentation structural disruptions, and concentrated along suitable structural loci. Therefore, it is likely to encounter significant concentration of uranium in close proximity of unconformable Archaean and Pakhal contacts and tectonic zones, thereby making Pakhal basin and its environs a suitable terrain to search for concealed uranium mineralisation.

A new occurrence of thorianite from syenitic pegmatite near Bhaluchuan, Odisha

We report a new occurrence of thorianite from syenitic pegmatite near Bhaluchuan, Sambalpur district, Odisha. The thorianite is brown to deep-brown with round grains of 2 to 10 mm size. The chemical analysis of the investigated thorianite reveals 64.8% ThO2, 25% U3O8, 3.81% PbO and 1.7% Fe2O3. Calculated structural formula of the thorianite is (Th+40.61U+40.14U+60.08ΣREE+30.017Pb+20.04Ca+20.01Mn+20.001Fe+30.05Al+30.003Sc+20.002K+10.005Na+10.008 Si+40.04Ti+40.02)O2.08. Chondrite-normalised rare-earth element (REE) plot of the thorianite reveals enrichment of light REE (LREE) over heavy REE (HREE) with pronounced negative Eu-anomaly (Eu/Eu* = 0.35). The (ΣLREE/ΣHREE)N ratio is perceptibly high (2.76). The (La/Lu)N (42.31), (La/Yb)N (27.49) and (Ce/Yb)N (21.58) ratios are also very high.X-ray diffraction (XRD) pattern of the investigated thorianite displays sharply-defined reflections. Corresponding interplanar spacings (d-spacings) of all the reflections are in very close agreement with those published for thorianite standard in International Centre for Diffraction Data (ICDD) Card No. 4-556. However, I/Io of two reflections (1.9694Å and 1.6787Å) are lower than those published for thorianite standard. The unit cell parameter (ao) of the investigated thorianite (ao 5.5750Å) is also less than ao of thorianite standard (ao 5.6000Å and V 175.62Å3), which is because of extensive substitution of Th by U.

Ianthinite: A rare hydrous uranium oxide mineral from Akkavaram, Andhra Pradesh, India

Ianthinite is the only known uranyl oxide hydrate mineral that contains both U6 +  and U4 + . For the first time, we report ianthinite from India (at Akkavaram, Andhra Pradesh), which is hosted in basement granitoids. The mineral occurs in the form of tiny grains, encrustations and coatings in intimate association with uraninite and uranophane. X-ray diffraction (XRD) data reveals that d-spacings of the investigated ianthinite are in close agreement with the corresponding values given for ianthinite standard in International Centre for Diffraction Data (ICDD) card no. 12-272. The crystallographic parameters of the studied ianthinite are: ao = 11.3 (1) Å, bo = 7.19 (3) Å and co = 30.46 (8) Å, with a unit cell volume of 2474 (27) Å3. The association of investigated ianthinite with uraninite suggests that the former has formed due to oxidation of the latter. Since a major part of the uraninite was exposed to oxidizing meteoric water, much of it has been transformed into hydrous uranium oxide (ianthinite) and very little part remained unaltered as uranium oxide (uraninite). Absence of schoepite in the investigated ianthinite suggests that after its formation it (ianthinite) was not exposed to oxygen/oxidizing meteoric water. As the oxidation was partial and short lived, some amount of primary uraninite is also preserved.

Dating of Columbite-tantalite and Monazite from Pegmatites of the Kawadgaon–Challanpara Area, Bastar Craton, Central India

The results of geochronological studies on columbite-tantalite and monazite from the rare metal pegmatites of the Kawadgaon–Challanpara area in Bastar craton, central India are presented. Columbite-tantalite yielded U-Pb concordia upper intercept age of 1978±16 Ma (MSWD = 0.18). Radiogenic 207Pb*/206Pb* (T7/6) ages on 4 out of 5 columbite-tantalite vary in a narrow range of 1903 to 2077 Ma and are similar to U-Pb age, whereas, one sample shows younger 207Pb*/206Pb*(T7/6) age of 1728 Ma. Younger Pb-Pb age of 1744 ± 250 Ma (MSWD = 150) has also been indicated by these columbite-tantalite samples. Four out of five monazite samples define Pb-Pb errorchron age of 2050±370 Ma (MSWD = 165) and radiogenic 207Pb*/206Pb* (T7/6) ages on 3 out of 5 monazites show a narrow range of 1983 to 2083 Ma. Other two samples show younger 207Pb*/206Pb*(T7/6) ages as 1254 Ma and 1592Ma. Both monazite and columbite-tantalite indicate disturbance in Pb and U isotopic systematics as revealed by high MSWD. However, selected samples from both monazite and columbite-tantalite indicate age of their formation as c. 2000 Ma. Younger ages, i.e., 1254 to 1744 Ma are indicative of later geological disturbances. Reported age of c. 2000 Ma is comparable to Rb-Sr date of pegmatitic muscovite (1850-2330 Ma) from this area and is younger to intrusive granites of c. 2500 Ma. By analogy, therefore, it may be inferred that the age of the rare element mineralization may be ~2000 Ma old, and linked with younger granitic activity that spanned over the period from 2300 to 2100 Ma in the Bastar craton.

First report of metamict alluaudite (A sodium iron manganese phosphate mineral) from Pisangan, Ajmer district, Rajasthan

For the first time we report alluaudite from India, which is metamict and is hosted in a zoned pegmatite, near Pisangan, Ajmer district, Rajasthan. Alluaudites known till date, all over the world, are non-metamict type. Therefore, the alluaudite reported from the area is the first locality in the world for the metamict-type of alluaudite. The most conspicuous features exhibited by the alluaudite are various types of cracks in different directions and shapes, including its isotropic nature. The calculated crystallographic parameters of the reported alluaudite are: a0 = 11.9874 Å, b0 = 12.5144 Å, and c0 = 6.4136 Å, β = 114.240 with unit-cell volume (V) = 877.31 Å3, which are in agreement with the values of alluaudite standard. Geochemical data indicates high content of P (32.30% P2O5), Fe (30.2% Fe2O3), Mn (10.62% MnO), besides Mg (7.71% MgO), Ca (5.60% CaO), Na (3.40% Na2O) and Si (2.30% SiO2), and appreciable amount of water of hydration (3.50% LOI). It is highly radioactive due to uranium (2.28% U3O8) and thorium (185 ppm Th). Calculated structural formula of the alluaudite is Na1+0.59Ca2+0.54Mn2+0.80Mg2+1.02Fe3+2.03Al3+0.037U4+0.043P5+2.45O12. The chondrite-normalised plot shows enrichment of HREE relative to LREE with pronounced negative Eu-anomaly (Eu/Eu* = 0.46). Such a high negative Eu-anomaly suggests extremely fractionated nature of the host pegmatite.

Application of X-ray diffractometry in exploration and research for atomic minerals

On the analogy of the diffraction of light wave by gratings, it was observed that crystalline substances having an orderly arrangement of atoms along particular directions in the crystal lattice act similar to grating whose particular plane at a fixed angle diffracts the X-ray radiation. This happens only when the wavelength is of the order of the repeat interval of atomic planes. Such reflection of X-ray radiation is called diffraction, and the angle at which the planes diffract the beam is known as Bragg’s angle. As almost every matter consists of selected groups of atoms bonded to each other in different fashion, the repeat distances and the planes for each would be different. It is this property of the crystalline matter that is used to investigate the nature of the atomic arrangement in a particular matter, compound, and metal. The mineral aggregates which defy characterisation by conventional methods due to various reasons are easily identified by X-ray diffractometry (XRD). The X-ray commonly used in diffraction work is produced by bombarding a target material with fast-moving electrons which are capable of penetrating deep into the orbitals of the atoms. One of the most valuable roles of X-ray analysis is the ability to distinguish between crystalline and amorphous materials. Every crystalline substance has a unique X-ray powder pattern because its position depends on the size of the unit cell, and the line intensity depends on the type of elements present and their arrangement in the crystal. Thus, as each individual could be identified by characteristic fingerprints, in a similar way, an unknown crystalline solid could be pinpointed by its characteristic diffraction pattern. A comprehensive understanding of ore mineralogy is essential in constraining ore genesis, exploration, and beneficiation. In this context, XRD finds diversified applications in exploration and research for atomic minerals (EARFAM) which are in great demand for the nuclear energy programmes of India.

Geochemical behaviour of vanadium, chromium, cobalt, nickel, and copper during progressive chloritization of biotites in granites

Evaluation of the vanadium, chromium, cobalt, nickel, and copper contents of five biotites, chloritized to varying degrees, from a magmatic granitic series of the 2700-million-year-old Giants Range Batholith of Northeastern Minnesota, USA, on the southern margin of the Canadian Shield, determined by wavelength-dispersive x-ray fluorescence spectrometry, shows that, during progressive chloritization of biotites in granites, the content of vanadium decreases and the contents of chromium, cobalt, nickel, and copper increase. The results emphasise the need for great caution when interpreting whole-rock trace-element data in petrological research.