Hindawi Publishing Corporation International Journal of Mineralogy Volume 2014, Article ID 978793, 8 pages http://dx.doi.org/10.1155/2014/978793 Research Article First Report of Florencite from the Singhbhum Shear Zone of the East Indian Craton Maitrayee Chakraborty,1 Sayan Biswas,1 Nandini Sengupta,2 and Pulak Sengupta1 1 Department of Geological Sciences, Jadavpur University, Raja SC Mullick Road, Kolkata 700032, India 2 Department of Geology, University of Calcutta, 35 Ballygunge Circular Road, Kolkata 700019, India Correspondence should be addressed to Pulak Sengupta; [email protected] Received 29 June 2013; Accepted 6 November 2013; Published 3 February 2014 Academic Editors: M. Arima, E. Belluso, M. M. JordanVidal,andL.N.Warr´ Copyright © 2014 Maitrayee Chakraborty et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Metamorphic florencite is being reported from kyanite-rich rocks from the eastern part of the Palaeo- to Mesoproterozoic Singhbhum shear zone. This is the first report of florencite from the Precambrian rocks of the Indian Shield. Host rock of florencite is a kyanite-rich rock (>80 vol%) with small and variable amounts of quartz, lazulite, augelite, and rutile. Florencite forms small (<20 microns) idioblastic-to-subhedral crystals that are included in large kyanite grains. Rarely, florencite replaces kyanite. The florencite has small proportion of crandallite (8.7–11.8 mol%) and goyazite< ( 2 mol%) components. Florencite of this study is dominated by Ce (∼49 mol%) with significant La (∼30 mol%) and Nd (∼21 mol%). Compared to other florencite occurrences of the world, florencite of the studied rock is impoverished in S, Sr, and Ba and rich in P. Stability of the assemblage florencite- kyanite-augelite-lazulite and the quantitative thermobarometry in the adjoining rocks suggest that florencite was formed during ∘ Palaeoproterozoic metamorphism that culminated at the - range of 490 ± 40 Cand6.3 ± 1 kbar. Integrating all the geological features it is postulated that florencite was formed due to metasomatism of some aluminous protolith by infiltration of acidic fluids −3 chargedwithPO4 and LREE. 1. Introduction Repina [5], and Janots et al. [8]). Once formed florencite is very difficult to be destroyed even in the weathering profile Florencite is a rare but important mineral in the alunite and hence controls the mobility of LREE over a wide range super group with the general formula of AB3(XO4)2(OH)6, of geological conditions (Gaboreau et al. [3], Izbrodin et al. whereA-sitefilledwithCe,La,andNd,B-sitewithAland [7], and Repina [5]). Florencite is associated with different X-site with P (Bayliss et al. [1]). The structure of florencite 2+ 2+ 2+ types of hydrothermal or deposits including unconformity also accommodates variable amounts of Ca ,Pb ,Hg , type uranium deposits and thus the presence of this mineral + 2+ 2+ + 3+ 2+ 2+ 2+ K ,Ba ,Sr ,Rb (in A-site), Fe ,Cu ,Zn ,Sn , can be used as a pathfinder mineral in exploration of these 3+ 3+ 3+ V ,Cr ,andGa (in B-site) (Bayliss et al. [1], Dill [2]). deposits (Gaboreau et al. [3]). Florencite is commonly associated in hydrothermally altered Singhbhum shear zone (SSZ) of the East Indian shield rock (Dill [2], Gaboreau et al. [3], Hikov et al. [4], and Repina separates the Palaeo- to Meso- to Neo Archaean Singhb- [5], among others) and less commonly in metamorphic hum craton from Palaeoproterozoic metamorphosed vol- rocks (Nagy et al. [6], Izbrodin et al. [7], and Janots et al. cano sedimentary pile, known as North Singhbhum Fold [8]). Because of its open structure that can accommodate a Belt (NSFB, Figure 1 after Dunn and Dey [10]andSaha large number of cations and anions including the REE, the [11]). It is generally agreed upon that Palaeoproterozoic composition of florencite provides a wealth of information tectonism led to thrusting of the NSFB over the Archaen about the source and composition of the metamorphic and Singhbhum craton along the SSZ (discussed in Sarkar and hydrothermal fluids (Visser et al.9 [ ], Nagy et al. [6], Dill Gupta [12]). Multitudes of rocks are intermingled, intensely [2], Gaboreau et al. [3], Hikov et al. [4], Izbrodin et al. [7], shearedandhydrothermallyalteredalongtheSSZ.Repeated 2 International Journal of Mineralogy ∘ ∘ ∘ ∘ ∘ ∘ 85 45 86 0 86 15 86 30 86 45 ∘ 23 23 ∘ ∘ 0 0 5 10 76 E 92 E 0 (scale in km) Dalma volcanics N ∘ ∘ 24 Delhi 24 N SSZ N NSFB Arkasani granophyre Soda graniteTatanagar ∘ ∘ ∘ ∘ 22 SSZ 8 8 22 Chakradharpur SSZ N ∘ ∘ N 45 45 granite gneiss 76 E 92 E IOG Jaduguda sediments Singhbhum granite Ongarbira SSZ Ghatshila metavolcanics Singhbhum group IOG of metapelites sediments S Soda granite ∘ ∘ 22 22 30 Kanyaluka∗ Kolhan Dhanjori 30 Group of volcanics SSZ volcanics Dhanjori IOG metavolcanics sediments MG ∘ ∘ 22 Mayurbhanj granite 22 15 ∘ ∘ ∘ ∘ ∘ 15 85 45 86 0 86 15 86 30 86 45 Figure 1: Geologic map showing the distribution of stratigraphic units in a part of eastern India. After Dunn and Dey [10]andSaha[11]. SSZ: Singhbhum shear zone, NSFB: North Singhbhum Fold Belt, IOG: Iron Ore Group. The rectangle around Kanyaluka (marked by red asterisk) is blown up in Figure 2. hydrothermal activities developed different types of Cu- 2. Petrography and Mineral Chemistry Fe-U-P deposits that are associated with tourmalinization, muscovitization, and ferruginization (discussed in Sengupta In domains of minimum strain, kyanite-rich rock develops et al. [13], and Sarkar and Gupta [12]). Infiltration-driven randomly oriented blades of kyanite that occupy more than metamorphism in the SSZ produced a number of exotic rocks 80 vol% of the rock (Figure 3(a), abbreviations after Kretz including per-aluminous kyanite-quartz rocks that fringe the [16]). Grains of quartz and rutile occupy the interstitial space of the mesh formed by the kyanite blades (Figures northern boundary of the SSZ (Figures 1 and 2; Figure 2: 3(a) and 3(b)). The kyanite-rich rock develops centimeter to partly after Mukhopadhyay and Deb [14]). In the eastern decimeter thick bands of intense shearing. In the shear bands, part, where the SSZ takes a bend towards south (Figure 1), kyanite blades are kinked, bent, and fractured (Figure 3(b)) kyanite-quartz rock is spatially associated with chloritoid- and the quartz grains show undulose extinction. Locally, bearing schist, pssammopelites, mica schist, and bands of deformed kyanite blades are extensively replaced by augelite tourmalinite (Figure 2). All these rocks share a common and lazulite (Figure 4(a)). Unlike kyanite, lazulite and history of deformation and metamorphism that culminated augelite do not show any deformation (static growth). In ± ∘ ± at 490 40 Cand6.3 1kbar (Sengupta [15]). In this the backscattered electron (BSE) images, florencite grains communication, we are describing the mode of occurrence of appear as numerous small bright spots in the dull background > florencite in the host of kyanite-rich rock ( 80 vol% kyanite) composed of lazulite, kyanite, and quartz (Figure 4(b)). Small that is exposed near the village of Kanyaluka (Figure 2). disseminated idioblastic, subhedral to anhedral crystals (<20 Integrating all the petrological data we demonstrate that microns) of florencite are included in kyanite, quartz, and florencite was developed due to infiltration of acidic fluid lazulite (Figures 4(c) and 4(d)). Rarely, florencite replaces charged with P and LREE into the per-aluminous kyanite- kyanite (Figure 4(c)). Textural features attest to the view that rich rock at the culmination of metamorphism and deforma- florencite crystals are left stranded within lazulite when the tion. Incidentally, this is the first report of florencite from any latter mineral replaced kyanite (Figure 4(d)). This feature Precambrian rocks of India. suggests equilibrium coexistence of florencite and lazulite. International Journal of Mineralogy 3 ∘ ∘ 22 30 86 30 Subarnarekha N ∘ 22 30 Sankh Nalah Kanyaluka ∗ Bhalki ∘ 86 30 Mica schist (NSFB) Chlorite quartz schist Banded ferruginous (granular rock) quartzite Quartzite Kyanite quartzite/ kyanite mica schist Metabasic rocks Chloritoid schist Banded psammopelites Soda granite/ Biotite muscovite schist Feldspathic schist Fault Quartz mica schist River Tourmalinite/ tourmaline schist Figure 2: Lithological map of the South Eastern sector of Singhbhum shear zone (SSZ) around Kanyaluka. (Partly after Mukhopadhyay and Deb [14]). The location from where the samples have been collected is marked with red asterisk. Ky Ky Ky Rt Qtz Rt Qtz 50 50 m m (a) (b) Figure 3: (a) Randomly oriented kyanite grains forming a mesh-like appearance. Some corroded rutile grains and quartz are also seen in the interstitial spaces and (b) Kyanite poor zone in the host rock showing the presence of deformed (kinked) kyanite. Mineral abbreviations used are after Kretz [16]. 4 International Journal of Mineralogy Ky Ky Qtz Aug Laz Laz Fl Ky (a) (b) Laz Laz Ky Fl Qtz Fl (c) (d) Figure 4: BSE images showing (a) Kyanite blades replaced by augelite and lazulite, (b) overall view of the lazulite rich zone: relicts of kyanite showing corroded boundary are sparsely distributed here. The small bright spots seen here are florencite. (c) florencite grain with protruding grain boundary inside kyanite, showing that florencite replaces kyanite, (d) florencite grain stranded within lazulite. Aug: augelite. Fl: florencite. All other mineral abbreviations are after Kretz16 [ ]. Electron microprobe analyses and WDS spectrum of the outweigh the concentrations of HREE. The concentrations florencite are presented in Table 1 and Figure 5,respectively. of HREE, S, and As are below the detection limit of electron Also included in Table 1 are the representative analyses microprobe.ConcentrationsofSr(0.002to0.008apfu),Ba of kyanite, lazulite, and rutile. For comparison, florencite (∼0.00003 apfu), Ca (0.040 to 0.062 apfu), and K (0.001 to analyses from some well-known localities are also included in 0.012 apfu) are low.