Rare earth element geochemistry and fluid characteristics of scheelite in the Hutti gold deposit, Hutti-Maski schist belt, Raichur district, Karnataka, India

Journal Pre-proofs Rare earth element geochemistry and fluid characteristics of scheelite in the Hutti gold deposit, Hutti-Maski schist belt, Raichur district, Karnataka, India Nevin Cheruvathery Gopi, Hari Shankar Pandalai PII: DOI: Reference:

S1367-9120(19)30513-9 https://doi.org/10.1016/j.jseaes.2019.104161 JAES 104161

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Journal of Asian Earth Sciences

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4 June 2019 19 November 2019 21 November 2019

Please cite this article as: Cheruvathery Gopi, N., Shankar Pandalai, H., Rare earth element geochemistry and fluid characteristics of scheelite in the Hutti gold deposit, Hutti-Maski schist belt, Raichur district, Karnataka, India, Journal of Asian Earth Sciences (2019), doi: https://doi.org/10.1016/j.jseaes.2019.104161

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Rare earth element geochemistry and fluid characteristics of scheelite in the Hutti gold deposit, Hutti-Maski schist belt, Raichur district, Karnataka, India

Nevin Cheruvathery Gopi1, Hari Shankar Pandalai1 1Department

of Earth Sciences, Indian Institute of Technology Bombay, Powai, Mumbai-

400076, India Email address: [email protected]

Abstract Scheelite occurs as thin smears, isolated grains and vein-lets in auriferous quartz reefs and sulfidized and auriferous biotite-chlorite schist at Hutti deposit. Major host rocks at Hutti are mafic and sill-like felsic volcanic rocks and quartz reefs located at the shear contact of these rocks. Laser-Raman and fluid inclusion studies on scheelite show that H2O+CH4±CO2+NaCl fluids, similar to gold mineralizing fluid are responsible for scheelite mineralization at Hutti. Rare Earth Element (REE) geochemistry of scheelite shows two types of chondrite normalized REE patterns which are classified as Type I (hump-shaped pattern), characteristic of REE incorporation by coupled substitution involving Na or K, and Type II (flat pattern) involving REE substitution with creation of a vacancy. The type I pattern is seen in scheelite within biotite schists whereas Type II pattern is associated with scheelite in felsic volcanic rocks and quartz mylonites. The Type II pattern is subdivided into Type IIa and Type IIb pattern based on Eu/Eu* values with Type IIa showing larger Eu/Eu* value than Type IIb. This lower ratio in Type IIb scheelite which occurs in altered felsic volcanic rocks is attributed to the overall enhancement of MREE, due to some degree of coupled substitution involving Na relative to Type IIa. The slope of the REE distribution pattern of scheelite is distinctly different from that of felsic volcanic rocks but similar to that of mafic volcanic rocks. It is

inferred that REE fractionation between mineralizing fluids and greenschist–facies metamorphosed mafic rocks of the schist belt is not significant.

Key works: Scheelite, REE, Fluid Inclusions, Greenstone belt, Orogenic gold, Hutti deposit

1. Introduction Greenstone belts in Archean shield areas are the hosts of several orogenic gold deposits (Gwilym, 1987; Groves et al., 2003). Scheelite (CaWO4) is often associated with gold mineralization as a related hydrothermal product in many orogenic gold deposits such as the greenstone belts of Abitibi, Canada; Harare-Shamva, Zimbabwe; Ivisaartoq, West Greenland and Western Australia. The Kolar gold fields, the Hutti gold deposit and the Ramagiri schist belts of Karnataka, are examples from the Indian craton where scheelite mineralization occurs within greenstone-hosted gold deposits and scheelite is considered as a pathfinder mineral for gold mineralization in these terrains. The geochemistry of scheelite in greenstone-hosted gold deposits has received much attention in the literature (Ghaderi et al., 1999; Brugger et al., 2000a, 2000b; Wood and Samson, 2000; Dostal et al., 2009). Limited studies of fluid inclusions in scheelite to characterize the nature of fluids responsible for their deposition are also available in the literature (Schenk et al., 1990; Mair et al., 2006). Even though the occurrence of scheelite at Hutti is well known, the origin and geochemistry of scheelite mineralization in the Hutti gold deposit received attention of scientists only in the recent past (Nevin, 2012; Raju et al., 2016; Hazarika et al., 2016). The REE patterns of scheelite from the Hutti deposit were studied by Hazarika et al. (2016) using the LA-ICP-MS technique and were interpreted in terms of two stages of scheelite precipitation wherein variation in abundance of scheelite precipitation governed the evolution of distinct REE patterns in the two stages of its formation.

In the present work, fluid inclusion studies and geochemistry of scheelite from the Hutti deposit were carried out, with the aim of providing new data on the geochemistry of scheelite and on reporting the nature of fluids entrapped in scheelite for interpreting fluid sources and processes.

2. Geological setting The Hutti gold deposit is a major gold-producing mine and is located in the Hutti-Maski schist belt in the Eastern Dharwar Craton. The Hutti gold mine is located about 140 km west of Raichur district headquarters of Karnataka state in Southern India. The Hutti-Maski schist belt is a Archean greenstone belt which has a hook-shaped outline with an area of about 750 km2 (Srikantia, 1995) and consists of a heterogeneous assemblage of volcano-sedimentary units metamorphosed to various degrees ranging from greenschist facies to lower amphibolite facies. The schist belt is surrounded by the Peninsular gneissic complex in the west and two different granitoids, namely Yelagatti and Kavital granitoids, on the north, and south-eastern sides of the belt respectively. The volcanic rocks are of mafic and felsic types. Mafic volcanic rocks are mainly pillow metabasalts, massive metabasalts, fine- to coarse- grained amphibolite and amphibole-chlorite schists. The felsic rocks are generally of rhyolitic to dacitic composition and quartz-porphyries, but geochemically they have compositions that are akin to adakite (Manikyamba et al., 2009; Nevin, 2012). The main sedimentary units are BIF, limestone, carbonaceous phyllite and metapelites of amphibolitic composition. The schist belt contains many discrete shear zones which localize gold mineralization of economic significance. The geological map of Hutti-Maski schist belt after Sundaram et al. (1995) is given in Fig.1a. The metavolcanic and metasedimentary rocks of Hutti-Maski schist belt are grouped under the Hutti Group (Srikantia, 1995) which is divided into four formations, namely,

the oldest Hussainpur Formation, the 530-Hill Formation, the Bullapur Formation and the youngest Buddinne Formation in stratigraphic order.

Geochemical studies show that the mafic volcanic rocks are mainly tholeiitic in composition (Ananta Iyer and Vasudev, 1979; Giritharan and Rajamani, 1998; Rogers et al., 2007; Manikyamba et al., 2009; Ram Mohan and Sarma, 2010) with some komatiitic rocks reported in the Bullarpur formation (Srikantia, 1995). There are varied opinions in the literature on the tectonic setting of the Hutti schist belt. Hutti mafic volcanic rocks are reported to be similar to mid-ocean ridge basalts but similarities with abyssal tholeiitic basalt related to back-arc spreading centers or marginal basin basalts have also been reported (Ananta Iyer and Vasudev, 1979; Ananta Iyer et al., 1980). Others suggest that volcanic rocks formed in an island arc setting (Satyanarayana and Reddy, 1996). Major and trace element modeling by Giritharan and Rajamani (1998) suggest that a melt-enriched mantle source resulting from partial melting was the tholeiitic protolith for the volcanic rocks of the Hutti belt. Deep crustal multi-source fluids enriched in LREE that were generated in these settings is suggested to be the source of gold mineralization (Giritharan and Rajamani, 2001). The collision between the eastern and western cratonic blocks of Karnataka and the craton-wide magmatism is suggested as a main cause of evolution of the greenstone belts (Jayananda et al., 2013; Chadwick et al., 2000; Rogers et al., 2007). Manikyamba et al. (2009) suggest that the Hutti-Maski schist belt represents a paired back-arc-arc setting or an intra-oceanic arc sequence obducted onto an older cratonic lithosphere. They suggest that during intra-oceanic subduction, arc basalts are generated by slab dehydration followed by melting of the mantle wedge above the slab whereas melting of the mantle wedge hybridized by melts resulting from slab-melting generate adakites and Mg-andesites. Ram Mohan and Sarma (2010) relate the volcanism to subduction in an island arc setting. They correlate younger plutonism with gold mineralization

in the eastern Dharwar Craton.

Regionally, three phases of deformation (D1, D2 and D3) are recognized in the HuttiMaski schist belt (Roy 1979, 1991; Riazulla et al., 1996; Vasudev and Chadwick, 2008; Nevin et al., 2010). Of these, the first two phases are pronounced while the third phase of deformation is relatively minor. According to Roy (1979, 1991) gold mineralization in the Hutti-Maski schist belt is mainly controlled by tectonics and structures that developed during the first phase of deformation and is principally attributable to the stage of ductile deformation that led to shearing. Based on the structures within the Hutti gold mine, Kolb et al. (2004a,b, 2005a,b) proposed five stages of deformation and two-stages of mineralization at Hutti deposit. Previous studies (Badhe and Pandalai 2018, Badhe et al., 2019; Hazarika et al., 2015, 2016, 2017; Rogers et al., 2013; Mishra and Pal, 2008; Kreinitz et al., 2008; Kolb et al., 2005; Saha and Venkatesh,2002; Mishra and Pal, 2002; Naganna, 1987; Roy, 1991; Raju,1978; Raju and Sharma, 1991) also suggest that a second stage of gold mineralization may have led to concentration of gold. The rocks of Hutti-Maski schist belt vary in metamorphic grade from greenschist to lower amphibolite facies (Roy, 1979; Biswas, 1990; Raju and Sharma, 1991; Giritharan and Rajamani, 1998; Pal and Mishra, 2002; Mishra and Pal, 2008).

Pandalai et al. (2003) have attributed gold mineralization to the entire period of progressive deformation by ductile shearing. They suggest that during shearing and ductile deformation, quartz vein-formation has taken place repeatedly with earlier quartz veins being progressively deformed during successive stages of ductile deformation leading to a complex array of quartz veins deformed to varying degrees. Nevin (2012) has shown that all these veins have been deposited from nearly similar fluids with mineralization that varied marginally over the entire duration of the ductile deformation event.

Pal and Mishra (2002) identify mineralization in the altered wall-rock as well as in “laminated” quartz veins and attribute mineralization to precipitation from a H2O-CO2-(±CH4) fluid of salinity in the range of 3.9 to 13.5 wt % NaCl equiv. at temperatures between 280 and 3200C and pressures of 1.0 to 1.7 k bars. Mishra and Pal (2008) follow the broad chronological order of tectonic events suggested by Kolb et al. (2005a), but differ from them in that they suggest that the “D2 quartz veins” located in the proximal biotite alteration zone as well as the “D3 laminated quartz veins” in the mylonitic shear zones at Hutti are mineralized. The fluids giving rise to both the D2 veins and D3 veins defined by them are similar and were entrapment after phase-separation between the carbonic and aqueous phases. Mishra and Pal (2008) differ from Pal and Mishra (2002) in that they attribute gold deposition to lowering of total dissolved sulfur ( S ) in the aqueous phase due to phase separation in a previously homogeneous aqueous-carbonic fluid as also suggested by Pandalai et al. (2003) whereas Mishra and Pal (2002) stated that lowering of total dissolved sulfur by sulfide precipitation was the important mechanism that triggered gold deposition in the wall-rock as well as in the quartz veins. Mishra and Pal (2008) also suggest that although two stages of alteration accompanied by gold mineralization took place, “continuous fluid flow over a long period of time seems more likely”. This implies a continuous phase of progressive deformation, quartzvein formation and mineralization akin to the model of Pandalai et al. (2003).

Calcite and barren quartz veins trending N50oW to E-W are often seen in the Hutti mine. Nevin and Pandalai (2010) and Badhe and Pandalai (2015) report a late-stage of “post-ore” hydrothermal activity that produced cross-cutting veins of barite. Fluid inclusion and sulfur isotope studies indicate that these veins were formed at temperatures as low as 150oC. The barite is often closely inter-grown with calcite and the mesoscopic crystal forms developed

indicate the open-space growth of these minerals.

Vasudev et al. (2000) reported a SHIRMP weighted mean

207Pb/206Pb

zircon age of

2576±12 Ma from a clast of granodiorite in a volcano-sedimentary mixite called the “Palkanmardi conglomerate” by Roy and Biswas, (1982) that occurs in an intermediate position within the stratigraphic column of the Hutti-Maski schist belt. This zircon age of the granodiorite clast is often reported as the maximum age of basin-development including that of volcanism and sedimentation in the Hutti schist belt. However, Anand and Balakrishnan (2010) give Rb-Sr, Pb-Pb and Sm-Nd dates of metabasalt as 2706 ± 130 Ma, 2637 ± 150 Ma and 2662 ± 81 Ma respectively and these authors believe that the 2662 Ma year age represents the time of formation of the basalts at Hutti-Maski schist belt. Rogers et al. (2007) dated zircon from felsic volcanics of the belt by the SHRIMP U-Pb method and gave an eruption age of 2586±59 Ma for these rocks. Sarma et al. (2008) using the same technique on zircons from felsic volcanics gave an age of 2587 ± 7 Ma.

The combined dating of U-Pb from zircon and titanite from northern pink granodiorites (from Golapalle and Yelagatti) show ages of 2569±17 Ma while the western gray granodiorites yield an U-Pb titanite and zircon ages of 2557 ± 7 Ma and 2559 ± 19 Ma (Anand et al., 2014). On the basis of SHRIMP U-Pb dating of zircon, the intrusive age of the Kavital granitoid is given as 2543 ± 9 Ma by Rogers et al. (2007) and as 2545 ±7 Ma by Sarma et al. (2008). Anand et al. (2014) give U-Pb titanite ages of 2554 ± 6 and 2554 ± 24 Ma for this rock.

Sarma et al. (2008 a, b) reported the age of what they termed “hydrothermal” monazite that occur as inclusions in arsenopyrite present in the felsic volcanic rocks of the Hutti mine

as 2547±10 Ma (U-Pb method) and is correlated with age of mineralization. Rogers et al. (2007) report SHRIMP Pb-Pb zircon ages from Yelagatti granites that record ages of 2221 Ma, suggesting a later event. Badhe et al. (2018) reported 40Ar-39Ar ages of biotite samples from the biotite-chlorite alteration zone around mineralized shear zone and inferred this to be ages of re-setting of alteration biotite by the last phase of hydrothermal activity in this area at ~2330 Ma. Schmidt et al. (2003, 2004) gave Rb/Sr ages of 2298±23 Ma and 2373±23 Ma for two biotite samples from the alteration zone and earlier Safonov et al. (1980) determined ages of 2140±70 Ma and 2240±70 Ma for the same alteration biotite. Such late events are also reported in Nevin (2012) on the basis of Sm-Nd dating of scheelite.

3. Hutti deposit and scheelite mineralization The Hutti gold deposit is the major producer of gold in India. Exploration and mining started in Hutti during the first decade of the nineteenth century. More details of the history of this mine are available in Curtis and Radhakrishna (1995). At present the Hutti Gold Mines Ltd (HGML) is running the underground mining operations at Hutti. Ore reserves at Hutti are estimated at 12.21 million tonnes at 4.79 g/t Au up to a depth of 870 m

(HGML, 2018).

Estimates of WO3 by HGML in Strike and Middle reefs (the two reefs adjacent to felsic volcanic rocks) gave mean wt% of 0.08 to 0.1% WO3 over a strike length of 1000 m with a width of 2.9m within the reef and immediate wall-rock. Geology and mineralization in the Hutti gold deposit is given in Naganna (1987), Biswas (1990a), Raju and Sharma (1991), Nevin et al. (2010) and Nevin (2012). The Hutti deposit consists of nine parallel reefs running NNW-SSE that dip steeply (>70º) towards west. The nine reefs are Main Reef, Prospect Reef, Oakley`s Reef, Zone I Reef, Village Reef, Middle Reef, Strike Reef (Hanging wall and footwall) and New East Reef. Recently some of these reefs have been developed to depths more than 900 m. The main shafts are the Main Shaft, Mallappa Shaft, Village Shaft, Central Shaft,

North Shaft, Grey Shaft and Prospect Shaft. The reefs are 0.5 to 3 m wide and are highly sheared and mylontized. The quartz reefs have a strike length of 480-1360m and auriferous zones are preferentially developed in these shear zone-controlled reefs. In the Strike Reef, Middle Reef and New East Reef sheet-like felsic volcanic bodies are exposed. The geological map of the Hutti mine area is shown in Fig.1b. At Hutti, structural features control the localization of gold mineralization (Mahanti and Raju, 1974).

Fluid inclusion studies show that low salinity aqueous-carbonic fluids are responsible for the gold mineralization at Hutti (Pandalai et al., 2001; Pal and Mishra, 2002; Pandalai et al., 2003; Mishra and Pal, 2008; Nevin et al., 2010, Pandalai and Nevin, 2010). A model for deposition of gold from these fluids is presented in Pandalai et al. (2001, 2003), Nevin et al. (2010) and Nevin (2012). Pandalai et al. (2003) proposed a fault-valve mechanism as suggested by Sibson et al. (1988) and Sibson (1990) for deposition of the quartz reef and destabilization of gold complexes leading to gold mineralization at Hutti. A brief study of low-salinity aqueous-carbonic fluids in scheelite of the Hutti deposit is available in Nevin and Pandalai (2008).

Major rock types present in the Hutti mine are proximal biotite-chlorite schist, quartz reefs and felsic volcanic rocks. The contact of biotite schist with quartz reefs and with felsic volcanic rocks can clearly seen inside the Hutti mine (Fig. 2). In the Hutti gold deposit, scheelite occurs in the Middle and Strike Reef where felsic volcanic rocks are present on the foot-wall of shear zones. Scheelite occurs as streaks parallel to the sheets of mylonitized felsic volcanic rock, and in the fractures of the mylontic quartz that occurs along the Strike- and Middle- reef shear zones (Fig.3a). Scheelite occurs as dirty white to slightly yellowish grains. The scheelite grains are invariably stretched parallel to the mylonite foliation and sometimes

tightly folded. Under short wavelength UV light, scheelite shows bluish fluorescence (Fig. 3b). Scheelite occurs as thin strings and laminae parallel to the shear planes. Occasionally, at places tight folds are also observed on the roof of mine openings. Such scheelite laminae are co-folded with the mylonitic foliation planes along which they occur (Fig. 3.c, d). The axis of these folds are discernible at places and are co-related with the fold axes of F2 folds. Scheelite is also observed occasionally in the biotite-chlorite schist (and very rarely with metabasalt) that occur alongside felsic volcanic rocks. The total width of the zone enriched in scheelite is usually about 2-3m. Scheelite is most abundant within the quartz reefs and observation within the mine openings under UV illumination shows that grains of scheelite are clearly aligned parallel to the mylonitic shear planes in quartz.

Under polarized reflected light, scheelite grains are greyish in color and have a low reflectance. Bireflectance is not observed. Anisotropism is distinct, but is masked by shades of white internal reflections. In thin sections, scheelite is easily identified by its very high RI, strong relief, fractures and high birefringence. Scheelite occurs most commonly as porphyroclasts in mylonitized quartz veins and felsic volcanic rocks. Small veinlets of scheelite parallel to Sm schistosity in altered biotite schist are also present. The veinlets of scheelites have sulfide grains within them. The sulfides include pyrrhotite and arsenopyrite which occurs as worm-like veinlets within scheelite or interstitially between scheelite grains. In oriented thin sections, scheelite is seen to occur as porphyroclasts in sheared quartz and felsic volcanic rock. The porphyroclasts commonly show mantling with the mantled grains drawn into tails parallel to the C-planes of mylonites (Fig. 3e). The porphyroclasts in oriented sections of quartz and felsic volcanic rocks show a dextral sense of shear inferred from the σtype structure developed by the rotated scheelite porphyroclasts and is consistent with the shear sense inferred from other shear-sense indicators such as mantled feldspars and C’planes

observed in mylonitic felsic volcanic rocks. The mantled scheelite grains sometimes also wrap around plagioclase porphyroclasts (Fig. 3f). Observations of thin sections and hand specimens of quartz veins and felsic volcanic rocks shows that thin veinlets of scheelite are co-folded with quartz to form tight folds with axis parallel to the C-planes of mylonite. Sometimes broader folds in quartz veins with scheelite laminae and mylonitic planes co-folded together are observed. Fine microfolds with scheelite laminae are also observable under UV illuminator.

4. Sample description and analytical procedures Fluid inclusion studies were carried out on the scheelite samples collected from the Middle Reef and Strike Reef of the Hutti gold mines. The scheelite samples are principally associated with felsic volcanic rocks and mylonitized quartz veins, but they also occur as thin stringers, often with quartz veinlets in the altered biotite schist close to the reef quartz. The details of scheelite samples collected for fluid inclusion and geochemistry are given in Table 1.

Even though scheelite contains abundant fluid inclusions in many samples from Hutti, it was difficult to observe phase changes during microthermometry. Fluid inclusion studies on scheelite were difficult because of the following reasons: i) the difficulty of preparing doubly polished wafer sections due to the brittle nature of scheelite, ii) the high RI of scheelite that made the observation of phase changes difficult during microthermometry, iii) the small size (5 to 10 μm) of most fluid inclusions, and iv) the decrepitated nature of a significant proportion of fluid inclusions (perhaps caused during wafer preparation or deformation). Due to this, Microthermometric and Laser Raman studies were carried out only on sample HM15 of scheelite associated with felsic volcanic rocks and HM28 of scheelite with quartz from the

biotite schist zone.

For fluid inclusion studies, doubly polished wafer sections of scheelite grains of ~0.06 mm size were utilized. The microthermometric studies were performed in the Department of Earth Sciences, IIT Bombay, using a Linkam THMSG 600 stage attached to Leitz Ortholux II PolBK microscope with a 50X objective. The heating-freezing stage was calibrated using triple point of CO2 in synthetic fluid inclusions and the triple point of pure water to give an accuracy of ± 0.2°C during cooling. Melting point of potassium dichromate was used to achieve an accuracy of ± 1.0°C during heating. Laser Raman studies were carried out on a Thermo-Nicolet Almega XR dispersive Raman Spectrometer with 780 nm and 532 nm lasers. Laser light was focused with a microscope fitted with objectives (upto 100x) and confocal microscopy to provide good microprobe analysis of areas up to 1 to 2 µm in diameter and up to depths of 510 µm within the sample. The software Omnic-7 was used for data processing and spectral analysis.

Scheelite samples were collected from different mining levels of the Strike Reef and Middle Reef of the Hutti deposit (Table 1). Scheelite samples were powdered using a brass mortar and sieved using a standard set of sieves; grains of 60 and 80 mesh size were collected and scheelite grains were separated using the Franz isodynamic separator and heavy liquid separation. First the Frantz isodynamic separator a current of 0.7Amps and with a slide slope of 100 and a forward slope of 80 was used for separation of scheelite from magnetic minerals. Bromoform (sp.gr. 2.26) and methylene iodide (sp.gr. 3.32) was then used to separate scheelite from other non-magnetic minerals. Scheelites were easily separated by this method due to their very high specific gravity (6 to 6.9) relative to other non-magnetic minerals. From the cleaned and separated scheelite grains, required amount of pure scheelite grains were

separated by hand-picking under a stereozoom microscope. The homogeneity of the grains was checked under ultraviolet lamp. The finally selected samples were then slightly powdered (to -200 mesh size) using a cleaned agate mortar. Accurately weighed scheelite samples were utilized for geochemical analysis of scheelite. Maximum care was taken to avoid any kind of contamination during the sample preparation.

For the geochemical studies wet chemical method was adopted and solutions were prepared by digesting the samples using the lithium metaborate fusion method. For REE analysis, 0.25 g of the sample was mixed with 0.75 g of lithium metaborate and 0.50 g of lithium tetraborate. The platinum crucibles were covered with a lid and kept inside a muffle furnace, at 800°C. The temperature was increased to 1050°C. After ten minutes samples were taken out from the furnace and suddenly quenched by external cooling. After quenching by external cooling, the crucibles were immersed in 80 ml of 1N HCl contained in a 150 ml glass beaker. The solution was made up to a volume of 100 ml using a standard volumetric flask. The 100 ml sample solution was used for REE separation by the REE pre-concentration method suggested by Walsh et al. (1981).

The REE pre-concentration was carried out using chromatographic techniques. The chromatographic columns used were glass columns of 25cm length and 2cm internal diameter, fitted with sintered glass discs and PTFE (poly-tetrafluoroethylene) stopcocks. The columns were charged with 30 g of 200-400 mesh AG50W-X8 Bio-Rad® cation exchange resins with working volume of ~30 cm (equivalent to a settled height of ~10 cm in distilled water). The columns were cleaned and equilibrated the resin with 100 ml 1N HCl (with a flow rate of ~2.5 ml/min) and then loaded with digested samples. The unwanted elements were eluted by passing 250 ml of 1.75N HCl through the columns (flow rate of ~2.5 ml/min). The bulk REE

along with Y, Sc and Hf was collected by passing 200 ml of 6N HCl at a flow rate of 1.5 ml/min. The collected REE rich elute was kept on a hot plate at 150°C till complete dryness and picked up in 2N HNO3 and made up to 10 ml in standard volumetric flask. The used columns were then cleaned with 10 ml of 8N HCl followed by 1 N HCl and agitated with Milli Q water and equilibrated with 1 N HCl and made ready to use for the next set of samples. REE studies were carried out on the ICP-AES (JY Ultima 2) in the Department of Earth Sciences, IIT Bombay. The instrument was calibrated using REE multi-element solutions of various concentrations. The detection limit (in ppb) for elements analyzed are as follows: La – 60, Ce – 30, Pr – 800, Nd – 230, Sm – 80, Eu – 80, Gd – 150, Tb – 120, Dy – 80, Ho – 135, Er - 30 Yb – 55, Lu – 60. The calibration was checked using two rock standards GSP-2 and BCR-2 (Fig. 4). All REE showed less than 10 % error except for Pr which showed 25 to 30 % error. Analysis of major and trace elements in scheelite was also done on the JY Ultima 2, ICP AES and the instrument was calibrated with rock standards (GSP-2, JG-1a, BIR, JG-b2, JB1b and JSY). Major elements show 0.65 to 3% error and trace elements show errors varying from 1 to 10 %. Detection limits were less than 0.1 wt % and100 ppb for major and trace elements respectively. EPMA analysis and BSE imaging on scheelite were carried out on CAMECA SX100 instrument at the EPMA Laboratory, GSI, Kolkata. Line-scan and point-scan analysis was carried out at an acceleration voltage of 15KV and 12nA of current and the calibration is done using mineral standards provided by BRGM and Cameca, France. ZAF correction was applied to the data. The average detection limits (in ppm) of elements analyzed is as follows: Na – 801, K – 866, Ca – 1066, Cr – 1357, Mn – 1444, Fe – 1468, Mg – 473, Co – 2891, W – 9454.

5. Fluid inclusion studies on scheelite

Fluid inclusion petrography on scheelite reveals presence of monophase and biphase fluid inclusions. The different types of fluid inclusion assemblages observed in scheelite from Hutti are given in Fig. 5. Three major types of fluid inclusions present in each of the two samples studied and these are Type I: Isolated monophase carbonic fluid inclusions, Type IIA: Isolated biphase aqueous fluid inclusions, and,Type IIB: Isolated biphase carbonic fluid inclusions. The monophase fluid inclusions are dark in appearance and occur in irregular, oval and elongated shapes. The irregular-shaped monophase fluid inclusions are larger in size and usually range up to 20 µm in size. The oval and elongated monophase fluid inclusion vary in size from 8 to 12 µm. Biphase fluid inclusions seen in the scheelite are of two varieties; they are distinguished on the basis of the “degree of fill”. In the first type, the liquid to vapour ratio ranges from 90:10 to 80:20. These fluid inclusions are slightly pinkish in appearance and their size varies from 10-12 µm. The fluid inclusions are usually oval-shaped but, elongated and irregular types are also seen. In the second type of fluid inclusions, the liquid to vapour ratio varies from 60:40 to 40:60. These fluid inclusions show a maximum size of 25 µm and most of them are 8-10 µm in size. These fluid inclusions are slightly dark with rectangular, oval and irregular shapes. In rare cases, some fluid inclusions of this type , show three phases (L1+L2+V) at room temperature. The polyphase fluid inclusions seen in the D1 quartz and also reported in the quartz veins of Hutti (Pandalai et al., 2003; Nevin, 2012) are not observed in scheelite.

The above-mentioned monophase and biphase fluid inclusions are seen to occur as isolated three dimensional clusters as well as on trails. The fluid inclusions in isolated clusters are slightly bigger in size compared to the other types of fluid inclusions. On account of the difficulty in making observations during microthermometry, only limited studies were done

on the fluid inclusions present on the trails and clusters. Isolated clusters of fluid inclusions with monophase and biphase fluid inclusions in scheelite grains are considered to be representative of fluids that caused the scheelite mineralization. Microthermometric observations were made on such fluid inclusions in scheelite samples HM15 and HM28 (Table 1).

Phase changes observed in microthermometric runs on isolated fluid inclusions in both scheelite samples were carried out and the microthermometric data details are summarized in Table 2. The results of microthermometric runs are discussed below for each major fluidinclusion type. Calculations of salinity of aqueous fluid inclusions were made using the equation of Bodnar and Vytik (1994 ) and that of carbonic fluid inclusions using Diamond (1992).

Type I fluid inclusions: Most of the Type-I fluid inclusions do not show much change while cooling. However, a few Type-I inclusions develop a new phase around -91°C which was difficult to identify. No clear freezing is observed in these inclusions up to -190°C. Upon heating, these fluid inclusions homogenize around -24°C. The fluid trapped in these is interpreted as being composed predominantly of CH4 with minor CO2. Laser-Raman studies were ineffective in detecting characteristic peaks of the gaseous phase in these inclusions due to fluorescence of scheelite.

Type IIA fluid inclusions: Type IIA fluid inclusions are isolated aqueous biphase fluid inclusions. On cooling, these biphase fluid inclusions show total freezing which is marked by a sudden deformation of the vapour bubble and its disappearance. Upon re-heating, first icemelting and final ice-melting are observed. Homogenization is usually observed from the

liquid-vapour phase to the liquid phase. Based on the microthermometric studies the Type IIA fluid inclusions are classified into two categories, viz.Type IIA-i and Type IIA-ii.

In the first category (Type IIA-i), the temperature of freezing is between -55.6°C and 35.5°C, first ice-melting is between -40.3°C and -23.7°C and Tmice occurs between -7.7°C and -1.0°C. Salinity varies from 11.3 to 1.7 wt% NaCl equivalent. The homogenization temperature (Th) varies from +130.2°C to +196.0°C (LV→L). These fluid inclusions clearly belong to the H2O-NaCl type.

In the second category (Type IIA-ii), freezing takes place between -88.6°C to -60.7°C. First ice-meting is observed between -53.4°C and -38.1°C and the Tmice is between -25°C and -11.6°C. These are high-salinity type fluid inclusions with salinity from 15 to 27.5 wt% NaCl equivalent. Th varies from +132.8°C to +162.6°C (LV→L). The predominantly H2O-NaCl type of fluids in these inclusions may contain very low “undetected” CO2 or CH4. A comparison of salinity and total homogenization temperature (Th tot) of Type IIA-i and Type IIA-ii fluid inclusions in scheelite is given in Fig.6.

Type IIB fluid inclusions: The Type IIB fluid inclusions are isolated biphase carbonic fluid inclusions. In most of the Type IIB fluid inclusions, final ice-melting temperature (Tmice) and final clathrate-melting temperature (TmCLA) are noticed. Tmice in these fluid inclusions ranges from -23.8°C to -1.4°C and TmCLA, from +6.3°C to +17.5°C. This set of fluid inclusions is composed of CO2-CH4-H2O-NaCl fluids with low CO2 content in which solid CO2-melting temperatures (TmCO2) ranges between -79.9°C and -57.1°C. The low value of TmCO2 indicates the presence of other components, inferred to be CH4. In many fluid inclusions temperatures of total freezing (Tf) and TmCO2 was very difficult to observe. Partial

homogenization of CO2 is also not clearly observed in most of the inclusions. A few of these inclusions show TmCO2 at -57.7°C, temperature of clathrate melting (TmCLA) at +9.9°C and temperature of partial homogenization of CO2 (ThCO2) at +24.6°C (LV→L). These inclusions are “high-CO2” carbonic-aqueous fluid inclusions. The temperature of total homogenization (Thtot) of the H2O-CO2-CH4-NaCl fluid inclusions ranges from +224.8°C to +495.8°C and all of them show homogenization to the liquid phase. Values of Thtot in excess of 400°C were observed in three fluid inclusions only. These higher values may be due to leakage. Histograms of physical parameters of Type IIB fluid inclusions are given in Fig. 7. These biphase fluid inclusions show the presence of CH4 and graphite on a fluorescence-corrected Laser Raman spectrograph (Fig. 8). The intensities of peaks observed are very low and due to the fluorescence of scheelite. Although the Laser Raman spectrographs were influenced by the fluorescence of scheelite, indication of the presence of CH4 and graphite is unambiguous.

6. Geochemistry of scheelite EPMA and wet chemical analysis were carried out on scheelite. The BSE images of sample HS29 and HM40 on which most of the analysis was done is given in Fig. 9. The analysis consisted on two line-scans of 25 spots each across scheelite grain in HM40 and 21 other spot analysis over HS29 and HS30 of which only spots No. 3, 4 and 5 are in HS30. Samples HS29 and HS30 are from the Strike Reef and are located within the quartz reef and the biotite schist. Sample HM40 is from the Middle Reef and is in the biotite-rich alteration zone. The BSE images show that there is variation of back-scatter across some grains indicating that there may be subgrains or some intra-grain compositional variation. The results of EPMA analysis are given in Tables 3. Due to strong fluorescence of scheelite, the analysis was not good and the total wt % of the analysis ranges between 94.19 and 102.02%. The analysis shows that the scheelite is essentially a CaWO4 with hardly any Na or Mo substitution

in the lattice. CaO values vary from 17.95 to 19.52 wt % and WO3 from 76 to 82.61 wt%. Wt % Na2O ranges between undetectable values to a maximum of 0.08% and that of MoO3 upto a maximum of 0.34%. Other elements such as Mg, K, Cr, Mn, Fe and Co are very low. Ni was not detected and elements such as Ba and Sr were not analyzed due to lack of standards for these elements. To supplement the EPMA analysis of major-element content of scheelite, wet chemical analysis was done on scheelite samples by ICP-AES method. The results (Table 4) show low values (<1%) for oxides of MgO, Na2O, K2O, MnO, Al2O3, P2O5 and SiO2. Traceelements such as Ba, Sr, Nb Pb and Y are present in ppm levels.

The results of the EPMA analysis shows that major and minor elements analysed along the line-scans and on spots across the three samples do not show large variation although small variations are observed. The scheelites samples are low in Mo. The concentrations and analytical precision of Na values on the basis of EPMA and ICP analysis are too low to make any conclusion on the relation between Na and Ca or any of the other cations.

REE analysis was carried out on scheelite samples in quartz, felsic volcanic rocks and biotite schists (Fig.10) close to the contact with felsic volcanic rocks of Middle and Strike Reef in which small veinlets of are scheelite are observed (Table 1) and the results of the analysis given in Table 5. Chondrite-normalized plots of REE concentration in the scheelite samples is given in Figs 11, 12, and 13. Duplicate analyses were performed on sample numbers HM28 (HM28 and HM28B) to validate the stability of the chemical procedure.

Based upon the REE patterns in scheelite that have been observed by other workers (Sylvester and Ghaderi, 1997; Ghaderi et al., 1999; Brugger et al., 2000a, b; Dostal et al., 2009), the REE patterns observed in the present study are classified into the following two

types. 1. Type I: Hump-shaped pattern with MREE-enrichment and a relatively small (1.9) positive Eu anomaly (i.e. Eu/Eu* with Eu* = (Sm x Gd)0.5) 2. Type II: Flat pattern with a larger (2.9 to 9.8) positive Eu anomaly Fig. 11, 12 and 13 show that samples HM28, HM28b and HM40 belong to the Type I category whereas the rest of the samples belong to the Type II category. Samples belonging to the Type I pattern are characterized by the following: 

(La/Lu)N between 0.5 and 0.8



(La/Yb)N between 0.5 and 0.6



(Gd/Yb)N between 2.7 and 3.2



(La/Sm)N between 0.2 and 0.3



(∑LREE/∑HREE)N between 0.1 and 0.3

(the subscript N indicates ratios of chondrite-normalized values). It is clear from the above that there is significant enrichment of MREE in these samples. The total REE (∑REE) content of the 3 samples (which includes the duplicate HM28 and HM28B samples) fall in the narrow range of 203 ppm to 226 ppm.

The samples belonging to the Type II category are characterized and further classified into Type IIa and Type IIb because Type IIa scheelites are hosted by the quartz reefs whereas Type IIb scheelites are hosted by sheared and altered felsic volcanic rocks and show distinctive features as given below. 

Type IIa: Eu/Eu* = 6.4 to 9.8, (La/Lu)N= 0.3 to 0.8, ∑REE= 37 to 107 ppm



Type IIb: Eu/Eu* = 2.9 to 4.2; (La/Lu)N = 0.7 to 5.0, ∑REE= 73 to 341 ppm.

The scheelites belonging to the Type IIa and Type IIb category however show some degree

of similarity in respect of the slopes of their respective patterns as seen from the ratios given below. 

(La/Yb)N between

0.6 and 1.0

0.8 and 3.8



(Gd/Yb)N between

0.8 and 1.1

0.8 and 2.1



(La/Sm)N between

1.0 and 1.4

1.0 and 3.7



(∑LREE/∑HREE)N between 0.4 and 0.7

0.6 and 2.6

The characteristic features of different types of REE pattern in scheelite is summarized in Table 6. It is clear from Table 6 and Fig. 11 to 13 that there is significant enrichment of MREE in Type I samples. Type IIa scheelite samples have a high positive Eu anomaly and lower total REE (∑REE) content compared to the Type IIb scheelite samples

On observing the lithological association of the scheelite samples (Table 1) and correlating them with their respective REE patterns, it is evident that scheelite veins in altered biotite-chlorite schist display Type I pattern. Samples of scheelite collected from altered felsic volcanic rocks, quartz veins and thin quartz-scheelite veinlets in metabasalts show Type II pattern and relatively lower values of total-REE than Type-I scheelite samples (except for one sample, HM44, with ∑REE =341 ppm)

Of the three samples that show Type IIa pattern (that is with very high Eu/Eu*) two are from scheelite in quartz veins and one is from thin scheelite-quartz veinlets in mafic volcanic rocks. The two samples from the quartz veins show the lowest value of ∑REE.

7. Discussion REE literature on Hutti scheelite

On the basis of LA-ICP-MS analysis of scheelite samples from the Hutti deposit, Raju et al. (2016) report total REE values up to 35 ppm and mean (LREE/HREE)N ratios of 1.8. The total-REE values reported by them is low and the REE distribution in their analysis shows a significant negative Eu anomaly. Raju et al. (2016) have reported the occurrence of scheelite in the proximal biotite-chlorite schists and in the quartz reefs of Hutti, but have not clearly specified the association of samples analyzed making it difficult to compare their results with that of the present study.

Hazarika et al. (2016) who also studied REE distribution in scheelite from the Hutti deposit based on LA-ICP-MS analysis of scheelite, clearly note the difference in the REE distribution pattern in scheelites from the proximal biotite-schists attributed to stage-I of mineralization by them and those collected from the quartz reef in association with chlorite, epidote and albite which they attribute to stage-II of mineralization. The observation by them of a hump-like pattern in scheelite from the biotite schists and a relatively flat pattern with positive Eu anomaly clearly matches the observations made in this study. The Type I humpshaped pattern observed by them shows a negative europium anomaly with (Eu/Eu*)N values of 0.15 to 0.71. Other scheelite from the proximal alteration zone and associated with a later mineralization stage shows their “Type II” behavior with slight HREE enrichment and (Eu/Eu*)N values between 0.94 and 1.59. The “Type II” of Hazarika et al. (2016) bears similarity with Type Ib of Ghaderi et al. (1999), with the peak REE values shifted towards Gd and Tb as discussed below for the Type I samples reported in this study. The Type III pattern reported by them and related by them to a later stage of mineralization has a flat REE pattern with positive Eu anomaly and shows closest similarity to the Type IIa pattern reported in this study from scheelites from the quartz veins and veinlets in metabasalts in terms of shape and ∑REE.

Hazarika et al. (2016) have also analysed the REE patterns of fluorapatite, calcite and tourmaline that coexist with scheelite in biotite schists as well as in the quartz reef and fluorapatite which coexists with scheelite in the quartz reef. The scheelite is shown by them to be slightly HREE enriched with respect to fluorapatite and calcite with a slight positive slope in the REE pattern relative to fluorapatite and calcite and relative MREE enrichment in Type I scheelite. The REE pattern of tourmaline is flat and the total REE content of tourmaline is low in comparison with that of scheelite, apatite and calcite. The Fe3+/Fe2+ ratio of tourmaline from the proximal alteration biotite schist and the quartz reefs was shown to be both low and it was inferred by them that there was no significant change in the oxidation state of fluids depositing hydrothermal minerals in the two environments.

Hazarika et al. (2016) suggest significant differences in the geochemistry of their “Type I” scheelite associated with biotite schist and their “Type III scheelite that is associated with quartz chlorite epidote-albite in the quartz reefs. This conclusion is derived principally from higher values of Mo, Nb, Ta, Zr (all of which substitute for W6+) and lower (La/Sm)N in their “Type I” scheelite compared to their “Type III” scheelite. The difference is attributed to the biotite-hosted scheelite having been deposited during an early stage of mineralization (Stage 1 correlated with their D2 stage of deformation) as against the quartz-vein-hosted scheelite that formed at a later stage (Stage 2 correlated with their D3 stage of deformation). Hazarika et al. (2016) however note that the major and trace elements in tourmaline grains from the same stage 1 and stage 2 assemblage show no difference suggesting similar nature of fluids responsible for the deposition of tourmaline in both assemblages. On the basis of the tourmaline chemistry it is concluded by them that the fluids responsible for ore formation had similar chemistry and were generated by metamorphic devolatilization at two different

times from two crustal levels. Type I pattern is claimed by them to be the normal pattern of REE distribution in scheelite whereas REE distribution in Type III scheelite is influenced by fluid constraints where a “limited REE budget” in the fluid and large-scale scheelite deposition led to progressive depletion in MREE and the development of a flat pattern in the Stage 2 scheelite.

REE substitution Scheelite is a common accessory mineral in hydrothermal gold deposits and is paragenetically related to the deposition of gold (Uspensky et al., 1998). Scheelite (CaWO4) crystallizes in the tetragonal system and elements such as Na, Sr, Y Pb and REE can substitute for Ca2+ in the scheelite structure (Cottrant, 1981; Brugger et al., 2000b). In addition, Mo6+ and scheelite.

Nb5+ can also substitute for W6+ in the tetrahedral WO42- sites of

REE pattern of scheelite has been studied for their use as geochemical tracers to

constrain the origin of mineralizing fluids in many Archean gold deposits (Anglin et al., 1987; Ghaderi et al., 1999; Brugger et al., 2000a, 2008; Hazarika et al., 2016). In particular, Sylvester and Ghaderi (1997) and Ghaderi et al. (1999) identified a hump-shaped (Type I) and a flat (Type II) REE pattern in scheelite and showed that these arise due to differing mechanisms of substitution of REE in scheelite. Type I pattern was shown by Ghaderi et al. (1999) as arising due to a coupled substitution mechanism involving Na+, whereas the flat Type II pattern arises due to substitution for Ca2+and creation of a vacancy for each pair of REE. Other mechanisms proposed for substitution include charge-balancing of REE3+ substitution of Ca2+ by the simultaneous substitution of W6+ by Nb5+ (Nassau and Loiacano, 1963). This substitution mechanism requires Nb concentration in scheelite to be similar to ∑REE concentration.

Using variations in cathodoluminescence of scheelite, Brugger et al. (2000 a,b) demonstrated that REE distribution patterns vary, often in oscillatory manner, within a scale of a few microns to hundreds of microns within single scheelite grains. Brugger et al. (2000b) described REE end-member patterns consisting of an MREE enriched, low Eu anomaly (Type-I pattern) and a second (Type-II) pattern with no MREE enrichment, but a relatively larger Eu anomaly. The oscillatory variation between Type-I and Type-II patterns was attributed by them to periodic MREE depletions in fluid that result during rapid deposition of Type I scheelite. Among several studies that use cathodoluminescence of scheelite for interpretation of the mineralizing process, Li et al. (2019) in a recent study of the Jaipei deposit of South China, show that scheelites show variations in cathodoluminescence that can be used to distinguish scheelites deposited during different episodes of hydrothermal deposition in the deposit as well as variations in depositional conditions during a single mineralizing phase.

In the present study, Type I pattern refers to the hump-shaped and Type II to the flat REE pattern as stated earlier. REE substitution involving Nb is not considered significant on account of the low concentration of Nb vis-à-vis ∑REE concentration. It is interesting to note that scheelite veins in sulfidized biotite-chlorite schist have a Type I pattern while samples of scheelite in altered felsic volcanic rocks and quartz from the same reef and at the same level of the mine (24th level) and within 5 m of each other along strike and within 0.5 m of each other across strike have such distinctly different REE pattern. The observed shapes of REE distribution in scheelite carried out by ICP-AES analysis on scheelite grainsseparates are broadly similar to REE analysis reported by Hazarika et al. (2016) using LAICP-MS on similar scheelite grains from this deposit. The ICP-AES analysis presented here is the average pattern on individual scheelite grains while the LA-ICP-MS results reveal the

REE pattern at the spot level. The difference between the spot analysis and average analysis is that in the case of Type-I scheelite (from the proximal biotite-schists) spot analysis show a slight negative Eu whereas the average analysis shows a positive Eu anomaly. Grains of biotite-schist hosted and quartz-reef hosted scheelite therefore are inferred to have dominant Type-I (MREE-enriched) and Type-II (flat) patterns respectively. The interpretations that follow are based on this dominant pattern at the grain-level.

Type I pattern in scheelite of Hutti resembles Type Ib of Ghaderi et al. (1999). In such pattern there is a distinct shift of the crest of the hump of the chondrite-normalized pattern towards Gd and Tb. Such shift occurs with substantial K+ substitution along with Na+ substitution in scheelite 2Ca2+ = REE3+ + Na+ or K+ (Gadheri et al. 1999). The larger ionic size of K+ results in preferential structural accommodation of REE with a slightly smaller ionic radius as compared to Eu. The occurrence of Type I scheelite with a chondritenormalized maximum at Gd in the biotite-chlorite schist indicates that high activity of K+ in the fluid could be correlated with development of this pattern. Mass-change calculations (Nevin, 2012) have clearly demonstrated that there is a net gain of K, Ba and Na in the biotite alteration zone.

The present study shows that the scheelite hosted by the altered felsic volcanic rocks and the quartz veins show Type II pattern with an almost flat REE pattern. EPMA

analysis

of two Type II scheelite samples collected from felsic volcanic rocks show very low Na (< 400 ppm). This is characteristic of scheelites that show flat REE patterns (Ghaderi et al., 1999). This REE pattern is attributed by them to the substitution mechanism where 3Ca2+ = 2REE3+ + □, where □, denotes a structural vacancy. It is argued by them that the REE-sites in the lattices of scheelite lie on either side of the vacancy and that this site-occupation

pattern places no constraint on size-preference of trivalent REE in the substitution process. Thus all trivalent REEs in the fluids are incorporated into scheelite in the same proportion as they exist in the fluid and therefore trivalent REE pattern of the scheelite reflects the trivalent REE distribution in the fluid that coexisted with the mineral growing in equilibrium with it (Ghaderi et al., 1999).

The ∑LREEN /∑HREEN of Type 1 samples ranges from 0.1 to 0.3 and is therefore slightly lower than the observed ratio in Type II a (0.4 to 0.7) and significantly lower than that of Type IIb scheelites (1.1 to 2.6, except one sample, HM45 with 0.6). The (Ce/Lu)N ratio (termed “slope” of the REE pattern by Ghaderi et al., 1999) in the Type I scheelite samples of the present study (0.9-1.4) is slightly higher than that of Type IIa scheelite (0.4 to 0.8), but is lower than that of Type IIb scheelite (1.5 – 4.6, leaving out the outlier sample no HM45). The slope of the REE pattern in scheelite is inherited from the parent fluid (Gadheri et al., 1999). From the present data it can be inferred that Type I scheelites were precipitated from fluid of nearly similar character as that of Type IIa scheelite.

Eu anomalies The Eu/Eu* ratio of scheelite is a function of i) the relative concentrations of Eu2+ and Eu3+ in the hydrothermal fluid, ii) the availability of sites and mechanism of REE substitution in the scheelite and iii) the Eu/Eu* ratio inherited from the hydrothermal fluid (Ghaderi et al., 1999). Enhanced substitution of Eu2+ directly into Ca2+ sites increases the total Eu incorporated into the scheelite lattice. The distribution coefficient of Eu2+ between fluid and crystal and the Eu2+/Eu3+ ratios in the fluid influence how much total Eu substitution occurs in scheelite. It is not easy to determine the Eu2+/Eu3+ ratio of the fluid accurately in most cases. Eu2+/Eu3+ ratios of scheelite are obtained directly only through

techniques such as µ-XANES (Brugger et al. 2008). Negative and positive Eu anomalies are observed with MREE-enriched (Type-I) and Type-II REE patterns and are commonly used to interpret the redox potential of mineralizing fluids (eg. Li et al., 2018; Guo et al., 2016, Li et al., 2019, Sun et al., 2019). Zhao et al. (2018), Yuan et al. (2019) and Sun et al. (2019) correlated the Mo content of scheelite with the Eu anomaly observed by relating substitution of Mo6+ in W6+ sites to more oxidized depositional environments and thereby to higher Eu/Eu*values. At Hutti the oxidation state of the fluids depositing Type I and Type II scheelites has been inferred to have been similar by Hazarika et al. (2016) on the basis of Fe3+/Fe2+ ratios in tourmalines coexisting with scheelite in these two environments. This suggests that differences in oxidation state of hydrothermal fluids could not be a significant factor in variations in Eu2+/Eu3+ of scheelite in the two environments. The very low values of Mo in the samples analyzed point to a reducing conditions and correlates with the positive Eu anomalies observed in the scheelites studied.

In Type II scheelite, the low concentration of Sm and Gd, the low Na content of scheelite and the large positive Eu anomaly indicates the possibility of substitution of Eu into scheelite as Eu2+. Type IIb and Type IIa have Eu/Eu* values of 2.9 to 4.2 and 6.4 to 9.8 respectively. As Sm and Gd values increase in Type IIb scheelite (as compared to Type IIa scheelite) there is reduction in the Eu/Eu* anomaly. (In Type I scheelite the ratio of Eu/Eu* (1.9) is even lower than that of Type IIb). This indicates that as content of trivalent REE increases, the ratio Eu/Eu* decreases. It is inferred that with incorporation of trivalent REE by coupled substitution involving Na, the ratio of Eu/Eu* decreases. This also shows that a substantial part of the Eu in samples with Type IIa pattern exist as Eu2+. Further, Type IIa samples of scheelite in quartz veins have the lowest total REE and the highest Eu/Eu* ratio. This indicates that coupled substitution involving Na is the least in these samples.

Fluid chemistry The increased Na activity in fluids coexisting with altered felsic volcanic rocks in comparison to quartz veins in dilatant jogs of the shear fractures possibly explains the difference between Type IIa (quartz-vein hosted) and Type IIb (felsic volcanic rock hosted) pattern observed in scheelite from the two host rocks. The lower Eu/Eu* ratio of Type IIb than Type IIa is interpreted as due to overall enhancement of all MREE by coupled substitution involving Na or K. Since coupled substitution with Na+or K+ is the least in Type IIa samples, the fluid that precipitated this scheelite must have had an REE pattern close to that of these samples. Also, since the “slope” of the chondrite-normalized REE distribution of Type I samples is similar to that of Type IIa samples the type of fluid that gave rise to Type-I samples may also be similar with an average La/Lu value of

~1.36. This inference

is similar to that of Giritharan and Rajamani (2001) who studied the REE-geochemistry of metabasalts, bulk sulfide ore and sulphide minerals separated from samples collected from the underground workings of the Hutti gold mines also concluded that the mineralizing fluids at Hutti were slightly LREE enriched. The present study on scheelite shows that the fluids that were responsible for scheelite deposition have an REE pattern similar to Type II with (La/Yb)N = 0.6 to 3.8, (La/Sm)N = 1.0 to 3.7, (Gd/Yb)N = 0.8 to 2.1 It is suggested that the ore-fluids that caused mineralization at Hutti had a flat REE pattern with a small positive Eu anomaly as observed in Type IIa scheelite samples. The total REE content (Table 7) and REE patterns (Fig.14) observed in the host altered felsic volcanic rocks from underground alteration zone and unaltered felsic volcanic rocks at surface away from alteration zone at Hutti is similar (Nevin 2012). The ∑LREEN /∑HREEN of the altered felsic volcanic rocks ranges from 18.62 to 27.09 and the ratio in

unaltered felsic volcanic rocks from the surface is 28.84. These values are, however, very distinct from that of the ∑LREEN /∑HREEN ratio of Type II a (0.4 to 0.7) and Type IIb (0.6 to 2.6) scheelite associated with felsic volcanic rock. The altered and unaltered felsic volcanic rocks do not, in general, show positive Eu anomalies. The Eu/Eu* values of the unaltered felsic rocks ranges between 0.9 and 1.0 and that of altered felsic rock ranges between 0.8 and 0.9. In contrast, scheelite hosted by the altered felsic volcanic rocks has Eu/Eu* values that range from 3.0 and 4.2.

Fractionation of REE during alteration can give rise to alteration fluids with REE geochemistry significantly different from the protolith with an altered “residual” rock that is significantly different from the parent. However from Fig. 14, it is observed that the unaltered felsic rocks of Hutti (located far away from the shear zones that focused fluid-flow during mineralization) have nearly similar REE patterns as the altered felsic rocks in the immediate viscinity of the quartz reefs. This indicates that significant fractionation of REE has not taken place during the hydrothermal alteration of the felsic volcanics. The alteration of felsic volcanics could thus not have significantly altered the chemistry of the large volumes of mineralizing fluids that passed through the shear system.

Scheelite grains in the proximal biotite alteration zone which may have formed in a local environment that facilitated MREE enrichment through coupled substitution involving Na and K explains the Type I (hump-shaped) nature of these scheelite grains. Although the scheelite grains in the quartz reef may have had a similar thermal and fluid history as that of scheelite grains in the biotite alteration zone, the REE distribution in them have been influenced by the immediate local Na- and K- poor geochemistry in the quartz reef and rapid rates of deposition in dilatant zones in the quartz reefs may have led to dominant Type IIa

patterns in these scheelites. The fact that Type I and Type IIa show similar “slopes”, however, indicates that the fluid that generated them were similar and that the Type I character is influenced by the local geochemical surroundings with which the fluids around the scheelite equilibrated/re-equilibrated.

Microthermometry and Laser-Raman studies on scheelite wafers irrespective of whether they are from the proximal biotite alteration zone or from the quartz reefs indicate that the major fluid that causes the deposition of scheelite is a fluid composed of H2O+CH4±CO2+NaCl. High- and low- salinity fluid inclusions that are observed in the scheelite are also observed in quartz from the main reefs at Hutti (Pandalai et al., 2003). This clearly indicates that fluids that were responsible for deposition of the auriferous quartz reefs were similar to those that led to deposition of scheelite and that the quartz and scheelite grains in the biotite zone as well as the quartz reefs may have undergone similar processes during and after their deposition.

One of the important models of fluid generation for gold minerlization in Archean greenstone belts is that of metamorphic devolatilization of the predominant metamorphosed mafic volcanics of the greenstone belts (Groves et al., 2003; Phillips and Powell, 2010; Tomkins, 2013; Goldfarb and Groves, 2015; Finch and Tomkins, 2017). In the Hutti schist belt, a similar metamorphic source for fluids has been suggested by Hazarika et al. (2016) on the basis of minor and trace element chemistry of tourmaline. The REE pattern of hot hydrothermal fluids that have equilibrated with tholeiitic mafic volcanics is governed by release of REE into fluid by breakdown of primary minerals, partitioning of REE into newlyformed altered minerals, nature of complexing ligands, T and pH of the aqueous medium and sorption processes (Bau,1991). In the case of the Hutti-Maski schist belt, the REE

chemistry of scheelites indicates that the hydrothermal mineralizing fluid had a nearly flat REE distribution. The hydrothermally altered metabasalts and the unaltered metabasalts of the schist belt also show a flat pattern with some rocks showing slight negative Eu anomaly and some with slight positive Eu anomaly (Fig.15). Similar chondrite normalized REE pattern was also reported earlier for the metabasalts of Hutti by Ananta Iyer et al. (1980). The concentration of REE in altered and unaltered mafic volcanic rocks from Hutti is given in Table 8. If metamorphic de-volatilization of the mafic rocks during greenschist to amphibolite facies metamorphism is the source of the hydrothermal fluids, then there is no significant REE fractionation during this process since the mineralizing fluid and the residual metabasalts have similar REE patterns.

8. Conclusions The following conclusions are drawn from the study: (i) Microtectonic studies reveal that scheelite mineralization in quartz and felsic volcanic rocks occurs along the mylonitic foliation and along the S-planes of sheared host-rock. These scheelites commonly show textures that indicate that they have undergone deformation along with the enclosing mylonitized. In biotitized alteration zones close to the sheared quartz reefs they occur as thin veinlets parallel to shear foliation and are less deformed. (ii) Fluid inclusions in all varieties of scheelite consist of CO2-CH4-H2O-NaCl fluid and are similar to fluids reported in previous studies on fluid inclusions in

auriferous

quartz. (iii) EPMA spot-analysis shows that scheelite grains studied are calcium tungstate with very low or undetectable Mo and low values of most other elements including Na. (iv) The Type-I and Type-II REE patterns reported here by ICP-AES analysis of

scheelite grains is similar to the Type I and Type III patterns

reported by Hazarika

et al. (2016) using the LA-ICP-MS analysis of similar scheelite grains from this deposit indicating dominant Type–I and Type-II patterns and significant differences in depositional environment of scheelites in the two different host environments. The hump-shaped (Type I) pattern is characteristic of REE incorporation by coupled substitution involving Na or K. Shift of the peak of the hump to Gd that is characteristic of involvement of K in the coupled substitution mechanism is observed in biotite-schist hosted scheelite. Type II (flat) patterns explained by substitution of trivalent REE for Ca2+ by creation of a vacancy are observed in scheelite within the quartz reef and felsic volcanic rocks. As concentration of trivalent REE increases, the Eu/Eu* ratio decreases indicating that the high Eu/Eu* ratio is on account of incorporation of divalent Eu2+ over and above trivalent Eu3+. The low Mo values in all scheelites analysed and the positive Eu anomalies observed in them indicate that fluids were in reduced state during deposition of Type I and Type II scheelites. (v) The “slope” of the Type I pattern is similar to the “slope” of the Type IIa pattern and indicates a similar source of fluid for scheelite with Type I and Type II patterns. The substitution mechanisms thus appear to be locally determined based on local geochemistry, oxidation state of fluids, availability of Na and K and relative availability of REEs during deposition. (vi) The “slope” of the REE pattern in scheelite is distinctly different from that of felsic volcanic rocks.

However the slope of the REE pattern of Type IIa

scheelite is

not very different from the slope of the REE pattern observed in mafic volcanic rocks. It is inferred that REE fractionation between mineralizing fluids and greenschist–facies metamorphosed mafic rocks of the schist belt is not significant.

Acknowledgments The authors thanks IIT Bombay for the financial support and infrastructure facilities provided during the PhD research work of the first author. The authors acknowledge the generous support of the Hutti Gold mines Ltd (HGML) and the geologists of the Exploration Division of HGML for carrying out underground sample collection and for logistic support. The authors also thank the Geological Survey of India, CPL, CHQ, Kolkata for EPMA analysis and Dr. V. N. Shilpa of the Department of Earth Sciences, IITB for help with ICPAES analysis. Meticulous review by two anonymous reviewers helped to improve the manuscript significantly.

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Fig. 2 Field photographs shows different rock types observed inside Hutti mines a) contact between proximal biotite - chlorite schist and mylonatised quartz reef and b) contact of felsic volcanic rocks with that of biotite - chlorite schist. Fig. 3 Features of scheelite mineralization: a) Scheelite in the C-planes of the quartz veins b) Bluish fluorescence of scheelite under the short wave UV light shows scheelites parallel to the mylonitic foliation in quartz c) Microfolded scheelite in quartz vein d) Schematic diagram of Hutti museum sample showing F2 folds developed on scheelite, quartz and mylonitic foliation plane in felsic volcanic rocks and chlorite-biotite schist e) Photomicrograph of oriented thin sections of felsic volcanic rocks with sigma type porphyroclasts of scheelite showing dextral sense of shearing f) Photomicrograph shows mantled scheelite grains parallel to the shear planes and at places wrapping around plagioclase porphyroclast. Fig. 4 Chondrite normalized REE pattern of rock standards analyzed compared with REE concentration of reference standards BCR-2 and GSP-2. Fig. 5 Photomicrographs of fluid inclusions in the scheelite samples: a) irregular monophase, b) and c) oval and elongated monophase, d, e, f) liquid-rich biphase, g, h) vapour-rich biphase, and i) coexisting liquid-rich and vapour-rich biphase, fluid inclusion. Fig. 6 Histogram shows a comparison between Type IIA-i and Type IIA-ii fluid inclusions in scheelite in the case of a) NaCl mass% and b) Th tot. Fig. 7 Histograms of physical parameters of Type IIB (isolated carbonic biphase) fluid inclusions of scheelite. Fig. 8 Type IIB isolated biphase fluid inclusions in scheelite show the presence of CH4 and graphite on a fluorescence-corrected Laser Raman spectrograph Fig. 9 BSE images of scheelite showing a) locations of EPMA line scans on HM 40 and b),c), d) spot EPMA analysis on HS 29.

Fig. 10 Photomicrographs shows scheelite associated with a) biotite schist, b) quartz reefs and

c) felsic volcanic rocks. Fig. 11 Type I (hump-shaped) REE pattern in scheelite associated with biotite schist. Sample numbers are shown in the box. Fig. 12 Type IIa (flat) REE pattern in scheelite associated with quartz veins within meta basalt and in quartz reefs. Sample numbers are shown in the box. Fig. 13 Type IIb REE pattern in scheelite associated with felsic volcanic rocks. Sample numbers are shown in the box. Fig. 14 Chondrite normalized REE pattern of the unaltered (H12, H3) felsic volcanic samples from the surface and altered (HM13, HM10) felsic volcanic rocks on 24th level of Hutti mine. Fig. 15 Chondrite-normalized REE values of unaltered (H-10 & H-11) and altered (OR-MR-2 & 19, MR-SR-1,3,6,11) mafic volcanic rock samples on a log-scale. Praesodymium (Pr) values are removed due to large analytical error and Thulium (Tm) was not detected during the analysis.

List of Tables Table 1 Details of scheelite samples used in fluid inclusion and geochemical studies. Table 2 The summary of the microthermometric data on scheelite Table 3 Spot EPMA analysis (wt% oxides) of scheelite along two linear scans (1/1 to 1/25 & 2/1 to 2/25) across HM40 scheelite sample (Fig.9a) and on different spot locations (Spot No. 3 to 23) of scheelite grains (mainly on HS30 and HS29) and the BSE images of grains shown in Fig.9 b-d) . Numbers in italics indicate values below detection limit. Table 4 Major and trace element ICP-AES analysis of scheelite (sample no. HM44). Numbers in italics indicate values below detection limit. Table 5 REE analysis of scheelites collected from Hutti mines. Samples Nos. HM28, HM28B and HM40 are scheelites from biotite schist and the remaining are from altered felsic volcanic

rocks, quartz and altered mafic volcanic rock adjacent to felsic volcanic rocks. Table 6 Characteristic features of different types of REE patterns in scheelite Table 7 REE analysis of felsic volcanic rocks collected from Hutti-Maski Schist belt. Samples Nos. HM13 and HM10 are altered felsic volcanic rocks collected from Hutti gold mine. Samples H12 and H3 are unaltered felsic volcanic rocks collected from surface exposures away from Hutti mines. Numbers in italics indicate values below detection limit.

Table 8 REE concentrations in the unaltered and altered mafic volcanic rocks from HuttiMaski schist belt. Samples H-10 and H-11 are unaltered mafic volcanic rocks collected from surface exposures away from Hutti mines. Samples Nos. OR-MR-2, OR-MR-19, MR-SR-1, MR-SR-3, MR-SR-6 and MR-SR-11 are altered mafic volcanic rocks collected from within the Hutti gold mine. Numbers in italics indicate values below detection limit.

Fig. 1 a. Geological map of Hutti-Maski schist belt modified after Sundaram et al. (1995) and b. Geological map of Hutti Gold Mines/ Reefs modified after Curtis and Radhakrishna (1995)

Fig. 2 Field photographs shows different rock types observed inside Hutti mines a) contact between proximal biotite - chlorite schist and mylonatised quartz reef and b) contact of felsic volcanic rocks with that of biotite - chlorite schist.

Fig. 3 Features of scheelite mineralization: a) Scheelite in the C-planes of the quartz veins b) Bluish fluorescence of scheelite under the short wave UV light shows scheelites parallel to the mylonitic foliation in quartz c) Microfolded scheelite in quartz vein d) Schematic diagram of Hutti museum sample showing F2 folds developed on scheelite, quartz and mylonitic foliation plane in felsic volcanic rocks and chlorite-biotite schist e) Photomicrograph of oriented thin sections of felsic volcanic rocks with sigma type porphyroclasts of scheelite showing dextral sense of shearing f) Photomicrograph shows mantled scheelite grains parallel to the shear planes and at places wrapping around plagioclase porphyroclast.

Fig. 4 Chondrite normalized REE pattern of rock standards analyzed compared with REE concentration of reference standards BCR-2 and GSP-2.

Fig. 5 Photomicrographs of fluid inclusions in the scheelite samples: a) irregular monophase, b) and c) oval and elongated monophase, d, e, f) liquid-rich biphase, g, h) vapour-rich biphase, and i) coexisting liquid-rich and vapour-rich biphase, fluid inclusion.

Fig. 6 Histogram shows a comparison between Type IIA-i and Type IIA-ii fluid inclusions in scheelite in the case of a) NaCl mass% and b) Th tot.

Fig. 7 Histograms of physical parameters of Type IIB (isolated carbonic biphase) fluid inclusions of scheelite.

Fig. 8 Type IIB isolated biphase fluid inclusions in scheelite show the presence of CH4 and graphite on a fluorescence-corrected Laser Raman spectrograph

Fig. 9 BSE images of scheelite showing a) locations of EPMA line scans on HM 40 and b),c), d) spot EPMA analysis on HS 29.

. Fig. 10 Photomicrographs shows scheelite associated with a) biotite schist, b) quartz reefs and c)

felsic

volcanic

rocks.

Fig. 11 Type I (hump-shaped) REE pattern in scheelite associated with biotite schist. Sample numbers are shown in the box.

Fig. 12 Type IIa (flat) REE pattern in scheelite associated with quartz veins within meta basalt and in quartz reefs. Sample numbers are shown in the box.

Fig. 13 Type IIb REE pattern in scheelite associated with felsic volcanic rocks. Sample numbers are shown in the box.

Fig. 14 Chondrite normalized REE pattern of the unaltered (H12, H3) felsic volcanic samples from the surface and altered (HM13,HM10) felsic volcanic rocks on 24th level of Hutti mine.

Fig. 15 Chondrite-normalized REE values of unaltered (H-10 & H-11) and altered (OR-MR-2 & 19, MR-SR-1,3,6,11) mafic volcanic rock samples on a log-scale. Praesodymium (Pr) values are removed due to large analytical error and Thulium (Tm) was not detected during the analysis.

Table 1 Details of scheelite samples used in fluid inclusion and geochemical studies. Sample Location No HS 29 Strike Reef 24th level HS 30 Strike Reef 24th level HS31 Strike Reef 24th level HS 33 Strike Reef 24th level HS 35 Strike Reef 24th level HS 36 HM 15 HM 28 HM 40 HM 42 HM 44 HM 45

Strike Reef 24th level Middle Reef 24th level Middle Reef 24th level Middle Reef 18th level Middle Reef 24th level Middle Reef 24th level Middle Reef 24th level

Occurrence Scheelite rich veinlets associated with quartz reefs Scheelite rich veinlets associated with biotite-rich schist. Veinlets parallel to the foliation planes in felsic volcanic rocks, sometimes with the quartz veins near by. Millimeter-wide veinlets in quartz reefs Lensoidal grains parallel to shear planes within more silicified felsic volcanic rocks. The scheelite is fine-grained and lensoidal grains are 1-2mm in size Veinlets in quartz veins within metabasalts Veinlets and grains in felsic volcanic rocks Scheelite-quartz vein, 1-2 cm wide, within biotite-rich schist. Scheelite rich veinlets associated with biotite-rich schist. Very dark-yellow coloured scheelite-quartz veinlets in metabasalts Veinlets and grains in felsic volcanic rocks Veinlets and grains in felsic volcanic rocks

Table 2. The summary of the microthermometric data on scheelite Type IA

Tf (°C) TmCO2(°C) Te(°C) Tmice(°C)

Changes not observed, may contain small amounts of CH4

Type IIA-i H2O-NaCl (low salinity) N= 16 -55.6 to -35.5

Type IIA-ii H2O-NaCl (high salinity) N=7 -83.9 to -60.7

-40.3 to -23.7 -7.7 to -1.0

-53.4 to -38.1 -25 to -11.6

TmCLA(°C) ThCO2(°C) Thtot(°C) Estimated Salinity (wt% NaCl equivalent)

+130 to +196 (L) n = 10

+133 to +163 (L) n=5

1.7 to 11.3

15 to 27.5

Type IIB CO2+CH4-H2O N= 15 -96 to -80 -79.9 to -57.1

6.3 to 17.5 -23.8 to -1.4 +225 to +406 (L) n=9

Table 3 Spot EPMA analysis (wt% oxides) of scheelite along two linear scans (1/1 to 1/25 & 2/1 to 2/25) across HM40 scheelite sample (Fig.9a) and on different spot locations (Spot No. 3 to 23) of scheelite grains (mainly on HS 30 and HS 29) and the BSE images of grains shown in Fig.9 b-d) . Numbers in italics indicate values below detection limit. Spot No 1/1 1/2 1/3 1/4 1/5 1/6 1/7 1/8 1/9 1 / 10 1 / 11 1 / 12 1 / 13 1 / 14 1 / 15 1 / 16 1 / 17 1 / 18 1 / 19 1 / 20 1 / 21 1 / 22 1 / 23 1 / 24 1 / 25 2/1 2/2 2/3 2/4 2/5 2/6 2/7 2/8 2/9 2 / 10 2 / 11 2 / 12 2 / 13 2 / 14 2 / 15 2 / 16 2 / 17 2 / 18 2 / 19 2 / 20 2 / 21 2 / 22 2 / 23 2 / 24

Na2O 0.03 0 0 0.06 0 0 0 0.05 0 0 0 0.03 0.01 0.03 0.02 0.04 0.01 0 0.01 0.03 0 0 0.04 0 0.02 0.01 0 0.06 0.01 0.06 0 0.07 0 0 0.01 0.04 0.02 0 0.01 0.02 0.05 0 0.07 0.07 0.02 0.02 0.03 0 0.08

MgO 0 0 0 0 0 0.02 0 0.01 0.03 0 0 0 0 0.02 0.01 0.05 0.01 0.03 0 0.03 0 0 0 0.06 0 0 0 0.03 0 0.02 0 0 0 0 0 0.02 0.04 0 0 0 0 0.01 0 0 0 0.01 0 0 0

K2O 0 0 0 0 0.04 0 0.01 0.01 0 0.02 0 0 0 0 0.01 0 0.02 0 0.02 0 0 0 0.03 0 0.04 0 0.01 0 0 0 0 0.02 0 0 0 0 0 0 0 0 0 0 0.01 0.02 0 0 0 0 0

CaO 18.61 18.99 18.98 18.62 19.07 19.15 19.41 19.33 19.01 19.02 19.21 19.05 19.20 19.04 18.71 19.52 19.11 19.68 19.10 18.97 19.27 19.52 19.43 19.10 19.38 18.16 19.05 19.10 18.88 18.59 19.01 18.74 19.02 18.95 19.15 18.93 18.84 19.35 19.07 18.68 19.28 19.27 19.36 19.38 19.32 18.92 18.66 18.88 19.01

Cr2O3 0 0 0 0 0 0 0.15 0.19 0 0.09 0 0 0 0 0 0.05 0.07 0.06 0.06 0 0 0.01 0 0 0.15 0.04 0.27 0.01 0.06 0.08 0.02 0 0 0 0 0 0.08 0 0.15 0.17 0 0.08 0 0 0.21 0.05 0 0.13 0.09

MnO 0 0 0 0 0 0.18 0.04 0 0 0 0 0 0.05 0 0.02 0 0 0.05 0.06 0 0 0.03 0.1 0.04 0.17 0 0 0.06 0 0 0.02 0.08 0 0.04 0 0 0 0 0 0.08 0.08 0.01 0.02 0 0.1 0 0.06 0 0

Fe2O3 0.11 0 0 0 0 0.14 0 0 0 0 0 0.09 0 0.08 0.12 0 0 0 0 0 0.04 0 0 0.05 0 0.05 0.08 0 0 0.13 0.05 0 0 0 0 0 0 0 0 0.04 0 0.08 0.07 0.16 0 0 0 0 0

CoO 0 0.17 0.01 0 0 0 0 0 0 0 0 0.17 0.05 0.08 0.05 0.11 0.1 0 0 0 0 0.12 0.13 0.18 0 0 0 0.22 0 0.11 0 0 0.12 0 0 0 0 0 0.12 0.02 0.12 0 0 0 0 0 0 0 0

MoO3 0 0 0.19 0 0 0.08 0 0 0.08 0 0 0 0 0.06 0.09 0.02 0 0.23 0 0 0 0 0.26 0 0 0 0 0.01 0 0 0.08 0 0 0 0.18 0 0 0.06 0 0 0 0 0.34 0.01 0.16 0 0 0.16 0.1

WO3 78.4 74.91 74.81 79.15 75.97 77.82 76.6 80.65 71.63 74.1 75.61 80.48 77.66 77.23 77.82 80.6 77.61 78.06 75.65 76.83 76.31 80.4 76.36 74.99 76.83 79.3 75.78 80.34 76.85 75.96 75.85 77.27 77.68 79 77.37 77.29 81.13 82.61 76.88 78.07 76.86 76.01 78.4 77.17 77.78 76.91 77.14 78.99 77.06

Total 97.15 94.06 93.98 97.83 95.08 97.40 96.21 100.24 90.75 93.23 94.81 99.82 96.97 96.55 96.86 100.39 96.93 98.11 94.89 95.86 95.63 100.07 96.35 94.44 96.59 97.56 95.19 99.83 95.81 94.94 95.03 96.17 96.82 98.00 96.70 96.29 100.1 102.02 96.23 97.07 96.39 95.46 98.26 96.81 97.59 95.90 95.90 98.15 96.35

2 / 25 Spot No 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

0.01 Na2O 0 0 0 0 0.02 0.01 0.02 0 0 0 0 0.07 0 0 0.15 0 0 0.04 0.13 0.52 0.03

0.04 MgO 0 0 0 0 0.05 0 0 0 0 0 0.02 0 0.03 0 0.01 0 0 0 0.01 0.12 0

0 K2O 0 0 0 0 0.01 0 0 0 0 0.01 0 0 0 0 0.02 0 0 0.02 0.04 0.16 0

19.19 CaO 18.88 19.26 19.01 19.13 19.31 18.94 18.75 19.22 19.34 19.41 18.75 12.79 19.37 19.45 17.95 19.48 19.34 18.98 19.1 19.4 19.08

0 Cr2O3 0.18 0 0 0 0.12 0 0 0.02 0 0.16 0 0 0 0 0 0 0 0 0 0.07 0

0.03 MnO 0.03 0.11 0 0 0.02 0 0.03 0.03 0.06 0 0 0 0 0 0.05 0.09 0 0 0 0.02 0.08

0 Fe2O3 0 0.03 0 0 0 0 0.03 0 0.01 0 0 0 0 0 0 0.03 0.05 0 0 0 0

0 CoO 0.12 0 0 0.12 0 0.27 0.18 0 0.15 0 0.09 0 0 0 0 0 0 0 0.23 0 0

0 MoO3 0.02 0 0.03 0 0.09 0 0 0.05 0 0 0 0 0 0.29 0 0 0.09 0.04 0 0 0

74.16 WO3 76.53 75.95 77.81 76.46 77.01 73.72 76.27 77.31 77.82 75.97 75.60 20.04 77.33 77.93 73.30 77.18 77.99 72.90 73.91 74.95 75.01

93.44 Total 95.77 95.35 96.85 95.71 96.62 92.94 95.28 96.63 97.39 95.55 94.45 32.90 96.74 97.67 91.47 96.78 97.47 91.98 93.41 95.25 94.19

Table 4 Major and trace element ICP-AES analysis of scheelite (sample no. HM-44). Numbers in italics indicate values below detection limit.

Major oxides In wt% SiO2 0.97 TiO2 0 Al2O3 0.08 Fe2O3 0.02 MnO 0.001 MgO 0.001 CaO 2.11 Na2O 0.04 K2O 0.01 P2O5 0.004 Trace elements In ppm Sr 34.2 Ba 11.0 Y 23.78 Nb 21.68 Pb 0.54

Table 5 REE analysis of scheelites collected from Hutti mines. Samples Nos. HM28, HM28B and HM40 are scheelites from biotite schist and the remaining are from altered felsic volcanic rocks, quartz and altered mafic volcanic rock adjacent to felsic volcanic rocks. Concentration of REE (ppm) in scheelite samples Type I Sample

Type II a

HM28 HM28B HM40 HM42

Type II b

HS36

HS33

HS35 HM45

HS31 HM44

La

4.8

6.2

7.9

9.0

2.5

4.8

15.1

10.0

13.5

46.4

Ce

23.3

29.4

31.9

23.0

7.5

12.6

40.0

29.2

24.8

110.6

Pr

7.1

7.7

6.7

2.2

1.3

1.6

3.8

2.9

1.7

9.6

Nd

38.9

42.3

36.5

16.2

5.1

9.6

24.0

19.2

9.7

51.6

Sm

18.1

21.3

18.4

4.1

1.6

3.0

9.0

6.3

2.3

17.7

Eu

14.6

16.4

14.3

17.9

4.6

7.7

10.3

8.8

3.8

19.8

Gd

29.3

32.1

29.2

7.6

2.9

4.6

12.9

12.1

3.3

22.9

Tb

4.7

5.0

4.8

1.0

0.7

0.6

2.3

2.3

0.5

4.0

Dy

32.9

34.2

35.1

9.5

3.8

5.6

15.7

18.5

4.5

29.3

Ho

6.3

6.5

6.6

2.1

0.8

1.3

3.1

3.8

1.1

5.6

Er

14.1

15.9

16.4

6.1

2.6

3.7

8.1

10.7

3.7

13.8

Tm

-

-

-

-

-

-

-

-

-

-

Yb

7.6

8.2

9.0

7.1

3.0

3.3

5.5

8.8

3.2

8.9

1.1 202.8

0.9 226.2

1.0 217.7

1.7 107.4

0.8 37.4

0.7 59.0

0.8 150.6

1.7 134.3

0.7 72.8

1.0 341.3

∑LREE

74.1

85.7

83.0

50.4

16.5

28.6

82.9

61.3

49.6

218.2

∑MREE

105.9

115.5

108.4

42.1

14.4

22.7

53.3

51.8

15.5

99.3

∑HREE

22.8

25.0

26.3

14.9

6.5

7.7

14.4

21.2

7.6

23.7

Eu/Eu*

1.9

1.9

1.9

9.8

6.6

6.4

2.9

3.1

4.2

3.0

Ce/Ce*

1.0

1.1

1.1

1.3

1.0

1.1

1.3

1.3

1.3

1.3

(Ce/Lu)N

0.9

1.4

1.3

0.6

0.4

0.8

2.2

0.7

1.5

4.6

(La/Lu)N

0.5

0.7

0.8

0.6

0.3

0.8

2.1

0.7

2.1

5.0

(La/Yb)N

0.5

0.5

0.6

0.9

0.6

1.0

2.0

0.8

3.0

3.8

(Gd/Yb)N

3.2

3.2

2.7

0.9

0.8

1.1

1.9

1.1

0.8

2.1

(La/Sm)N (∑LREEN /∑HREEN )

0.2

0.2

0.3

1.4

1.0

1.0

1.1

1.0

3.7

1.7

0.1

0.2

0.3

0.6

0.4

0.7

1.1

0.6

2.6

2.3

Lu ∑REE

Table 6. Characteristic features of different types of REE patterns in scheelite

Parameters (La/Lu)N (La/Sm)N (Gd/Yb)N (La/Yb)N Eu/Eu* (∑LREE/∑HREE)N ∑REE

Type I 0.5 to 0.8 0.2 to 0.3 2.7 to 3.2 0.5 to 0.6 1.9 0.1 to 0.3 203 to 226 ppm.

Type IIa 0.3 to 0.8 1.0 to 1.4 0.8 to 1.1 0.6 to 1.0 6.4 to 9.8 0.4 to 0.7 37 to 107 ppm

Type IIb 0.7 to 5.0 1.0 to 3.7 0.8 to 2.1 0.8 to 3.8 2.9 to 4.2 0.6 to 2.6 73 to 341 ppm.

Table 7 REE analysis of felsic volcanic rocks collected from Hutti-Maski Schist belt. Samples Nos. HM13 and HM10 are altered felsic volcanic rocks collected from Hutti gold mine. Samples H12 and H3 are unaltered felsic volcanic rocks collected from surface exposures away from Hutti mines. Numbers in italics indicate values below detection limit. Rare Earth Element (in ppm) analytical data of felsic volcanic rocks Rock Sample No. La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Eu/Eu* Ce/Ce*

Altered felsic volcanics

Unaltered felsic volcanics

HM13 24.3 42.1 3.1 12.4 2.2 0.5 1.4 0.2 0.9 0.2 0.4 0.4 0.03 0.9 1.2

HM10 17.8 34.3 2.3 10.1 2.0 0.4 1.1 0.3 0.9 0.2 0.4 0.4 0.1 0.8 1.3

H12 33.6 63.6 4.8 25.8 4.7 1.2 2.7 0.3 1.4 0.2 0.5 0.5 0.1 1.0 1.2

H3 30.3 67.0 5.3 26.0 5.9 1.4 3.6 0.5 3.0 0.6 1.7 1.5 0.2 0.9 1.3

(La/Sm)cn

7.1

5.8

4.7

3.3

(Gd/Yb)cn (La/Yb)cn La/Sm

3.1 48.2 11.1

2.4 32.5 9.0

4.7 51.7 7.2

1.9 14.1 5.2

Table 8 REE concentrations in the unaltered and altered mafic volcanic rocks from HuttiMaski schist belt. Samples H-10 and H-11 are unaltered mafic volcanic rocks collected from

surface exposures away from Hutti mines. Samples Nos. OR-MR-2, OR-MR-19, MR-SR-1, MR-SR-3, MR-SR-6 and MR-SR-11 are altered mafic volcanic rocks collected from within the Hutti gold mine. Numbers in italics indicate values below detection limit. Rare Earth Element (in ppm) analytical data of mafic volcanic rocks Rock Sample

Unaltered mafic volcanic rocks

Altered mafic volcanic rocks OR-MR-2 OR-MR-19 MR-SR-1 MR-SR-3 MR-SR-6 MR-SR-11

H-10

H-11

La

4.2

4.4

2.8

2.7

2.5

3.1

2.3

2.8

Ce

12.2

13.6

7.7

8.0

7.5

8.8

7.5

9.0

Pr

1.5

1.6

0.7

0.9

0.6

0.9

0.6

1.1

Nd

9.4

10.0

5.6

6.2

5.7

6.2

5.7

7.3

Sm

3.3

3.7

2.0

2.1

1.9

2.2

2.0

2.6

Eu

1.2

0.8

0.6

0.9

0.5

0.9

0.6

0.6

Gd

4.3

4.2

2.6

3.1

2.8

3.0

2.7

3.5

Tb

0.7

0.8

0.4

0.5

0.5

0.5

0.5

0.7

Dy

4.7

4.8

3.2

3.6

3.3

3.5

3.3

3.9

Ho

1.0

1.1

0.7

0.8

0.7

0.8

0.7

0.9

Er

2.5

3.3

2.0

2.3

2.1

2.2

2.3

2.6

Tm

-

-

-

-

-

-

-

-

Yb

2.3

3.0

2.0

2.3

2.0

2.2

2.1

2.3

Lu

0.3

0.5

0.3

0.3

0.3

0.3

0.3

0.3

Eu/Eu*

1.0

0.6

0.9

1.1

0.7

1.1

0.8

0.6

Ce/Ce*

1.2

1.2

1.4

1.3

1.5

1.3

1.6

1.2

(La/Sm)cn (Gd/Yb)cn (La/Yb)cn La/Sm

0.8 1.6 1.3 1.3

0.8 1.2 1.1 1.2

0.9 1.1 1.0 1.4

0.8 1.1 0.8 1.3

0.8 1.2 0.9 1.3

0.9 1.1 1.0 1.5

0.8 1.1 0.8 1.2

0.7 1.2 0.9 1.1

Highlights 

Scheelite associated with alteration biotite and quartz mylonites from Hutti show H2O+CH4±CO2+NaCl fluids similar to gold mineralizing fluid.



Hump-shaped (Type-I) and flat (Type II) REE patterns in scheelite indicate differences in mechanism of substitution of REE in scheelite controlled by the geochemistry of the host environment.



Mineralizing fluid is inferred to have a flat REE pattern that is similar to the flat pattern observed in host metabasalts of the schist belt.

CRediT author statement Nevin Cheruvathery Gopi: Conceptualization, Methodology, Investigation, Formal analysis,Writing - Original Draft, Writing - Review & Editing Hari Shankar Pandalai: Supervision, Conceptualization, Methodology, Writing - Review & Editing