1 Introduction

The Neoproterozoic (827 ± 8.2 Ma, Anand et al. 2018) Degana Peraluminous Granite (DPG), emplaced along the western fringe of the Delhi–Aravalli belt in Rajasthan, host significant tungsten deposit, which is located in the Nagaur district, NW India (Lahiri and Krishnan 1966; Khan and Laul 1974; Gupta et al. 1980, 1997; Sood 1989; Jain 1990; Grover 1990; Chattopadhyay et al. 1994; Srivastava and Sinha 1997; Anand et al. 2018). Tungsten mineralization in DPG and other Neoproterozoic granitoids (table 1) are mainly distributed in the western part of Aravalli craton (Anand et al. 2018); in Degana, it occurs in three neighbouring but isolated hillocks, Rewat Hill, Tikli Hill and Phyllite Hill. The study area is covered with 3–15 m thick soil cover as well. The Neoproterozoic DPG is part of Malani Igneous Suite and intruded within Mesoproterozoic phyllite of Delhi Supergroup (Tobisch et al. 1994; Pandian 1999; Pandian and Verma 2001; Krylova et al. 2012; Singh 2016; Anand et al. 2018). Table 1 represents the available U–Pb and Rb–Sr radiogenic isotopic ages of Neoproterozoic granitoid of Rajasthan, NW India.

Table 1 Radiogenic isotopic ages of Neoproterozoic granitoid of Rajasthan, NW India.
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The fertile DPG hosts tungsten (W) and associated mineralization is one of the most significant findings of strategic importance in Rajasthan, NW India. The tungsten and associated mineralization are considered to be of vein-type deposits, which is randomly and erratically distributed (Lahiri and Krishnan 1966; Khan and Laul 1974; Sood 1989; Grover 1990; Jain 1990; Pandian 1999; Pandian and Verma 2001; Krylova et al. 2012; Singh 2016; Anand et al. 2018). Tungsten and associated mineralization in the exposed part of these three hills have already been established by drilling (Lahiri and Krishnan 1966) and major tungsten mineralized quartz vein and pegmatite vein form Rewat Hill and Tikli Hill and stock works from Phyllite Hill were mined out up to ground reduced level (RL) of 340 m. Most of the works on the Degana Tungsten Deposit focus on either geology or geochemistry and only a few on ore-forming fluid of ore deposit (Pandian 1999; Pandian and Verma 2001; Krylova et al. 2012; Singh 2016; Anand et al. 2018). Therefore, occurrence, mineral assemblage and mineral chemistry of fertile DPG under alluvium cover around Rewat Hill remain uncertain. In this paper, we studied fertile DPG having tungsten (W) content equal to or more than 400 ppm (0.05% WO3). The understanding of the fertility of DPG in soil-covered areas is key to advance research into the Degana Tungsten Deposit and to enlighten further prospects and exploration. To trace the fertile DPG under soil cover, sub-surface exploration has been performed.

We report subsurface data obtained from subsurface exploration and concentration of W, rare metals and REE analyzed by XRF and ICP-MS analysis (major oxide and trace elements respectively), on the fertile DPG intersected under soil cover around Rewat Hill. The results allow us to identify new tungsten prospect that has strategic importance and establish likely evidences of fertile DPG (Type-I and Type-III mineralization) extending beyond Rewat Hill below soil cover containing W concentration in the range of 402.98–5025.45 ppm.

2 Geological setting

The Rewat Hill is entirely made up of fertile DPG and the adjoining south-western hill (Tikli Hill) is of fertile DPG and phyllite in which the DPG is intrusive (Lahiri and Krishnan 1966; Pandian 1999; Pandian and Verma 2001; Krylova et al. 2012; Singh 2016; Anand et al. 2018). The Phyllite Hill is composed entirely of phyllite with inter-banded quartzite although domed up and altered to a considerable degree due to underlying intrusive DPG (figure 1). Tungsten mineralization in the Phyllite Hill is hosted in only stock works (figure 2F). Rest of the area is covered by barren lands, Quaternary sediments and alluvium. The DPG is traversed by a number of NW–SE, NNW–SSE trending W mineralized quartz veins, pegmatite veins, which are highly enriched in tungsten (figure 2A–C) and greisen (figure 2D). Semi-consolidated gravel bed around Rewat Hill (figure 2G) contains erratically distributed wolframite grains of variable size (up to 1 cm). The isolated fertile DPG intrusion has an outcrop area of about 1 km2 and occurs as a stock and dominated by greisen veins and aplite dykes (figure 2D and E). The DPG comprises three consecutive phases distinguished on the basis of intrusive relation and texture. Phase-I, DPG cuts through phyllite, is medium to coarse-grained and shows equi-granular to inequigranular texture. Phase-II granite is porphyritic and localized within the Phase-I granitic body and separated by the presence of a zone of intrusive breccia of variable thickness. Phase-III granites are intrusive phases within Phase-I and Phase-II granite and characterized as fine-grained porphyritic granite. All the three phases of granites are peraluminous and show similar mineralogy, however, have different texture and grain size (Krylova et al. 2012; Anand et al. 2018).

Figure 1
figure 1

Simplified geological map of Degana Tungsten deposit along with boreholes drilled in soil cover.

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Figure 2
figure 2

(A and B) Annotated photograph of the contact between the fertile DPG and the quartz–wolframite vein type (Type-I) mineralization seen on the sidewall of mine adits of Tikli Hill and Rewat Hill, respectively. (C) Annotated photograph of the contact between the granite and the quartz–muscovite–polymetallic sulphide vein-type (Type-II) mineralization seen on the upper wall of mine adits of Rewat Hill. (D) Greisen veins in DPG hosted wolframite (Type-III), Rewat Hill. (E) Aplite dykes intruded in DPG. (F) Stock work in phyllite hill (Type-IV) and (G) Type-V tungsten mineralization in gravel-bed around foothill of Rewat Hill. Wol: wolframite; Mus: muscovite; Qtz: quartz; and Py: pyrite.

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3 Subsurface exploration

A total of 30 vertical shallow boreholes of 35–60 m depth and six inclined boreholes of 50–72 m depth were drilled (figure 1) around Rewat Hill to know the extension of W mineralized quartz veins in soil covered area (Kumar 2019). The vertical boreholes were drilled in soil covered area at a regular interval of 200 m from Rewat Hill to know the extension of Type-III, Type-IV and Type-V mineralization, whereas inclined boreholes were drilled in soil cover at 100 m spacing from the exposed part of mineralized NW–SE trending quartz veins to know depth and strike continuity of Type-I and Type-II mineralization. The borehole spacing for inclined boreholes was kept 100 m because of pinching and swelling nature of mineralized quartz–pegmatite veins. A total of five boreholes, viz., BHV-1, BHV-2 BHV-9 (vertical boreholes) and BHI-31 and 32 (inclined boreholes) were reported positive. In vertical boreholes, the W-mineralization was hosted in granite (Type-III), whereas in inclined boreholes the mineralization was hosted in quartz vein as well as granite (Type-I, Type-II and Type-III). It is also confirmed by sub-surface exploration that fertile DPG is continuing beneath soil-covered area around Rewat Hill and hosts anomalous W and associated mineralization at variable depth (table 2). It was also confirmed that W mineralized quartz veins are restricted to Rewat Hill and pinching out towards soil cover in northwest as well as in southeast part of the hill but depth continuity of mineralized veins (Type-I and II) at deeper level cannot be ruled out. Recent sub-surface data reveals that fertile DPG with potential W mineralization is continuing in soil covered northern and north-western parts of Rewat Hill (table 4).

Table 2 Details of positive boreholes intersected fertile DPG with Type-I and Type-III mineralization beneath soil cover around Rewat Hill.
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4 Sampling and analytical methods

Core samples (n=113) from five positive boreholes (figure 1), drilled around Rewat Hill in soil covered area, were collected and analyzed. Sampling was done from suspected mineralized zone hosted in DPG at an interval of 0.5 m (table 5). Chemical analysis was performed for tungsten (W), rare earth elements (REE) and associated rare metals (RM) of fertile DPG with the help of ICP–MS (VARIAN 820-MS SYSTEM) at the facilities of the Chemical Division, GSI, Jaipur and PANalytical AXIOS X-Ray Fluorescence (XRF) Spectrometer was used to analyse major oxide and trace elements (table 4). The mineral chemistry of wolframite from two of the studied samples was determined by CAMECA SX-100 electron microprobe microanalysis (EPMA) with five WDS spectrometers, including LLIF and LPET crystals at EPMA lab, GSI, Faridabad (table 3).

Table 3 Representative EPMA analysis (wt.%) of wolframite from core samples of fertile DPG, Rewat hill, Degana.
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5 Petrography

The petrographic studies of fertile DPG from Rewat Hill reveal that they consist of quartz, K-feldspar, sodic plagioclase muscovite and biotite as dominant phases along with apatite, zircon, topaz, rutile, iron oxide as accessory phases and show hypidiomorphic texture. Mineralogical composition is almost similar in all the three phases. They are coarse-grained (phase-I and II), medium to fine-grained (phase-III) rock consisting of quartz (up to 45–40%), plagioclase (20–15%), K-feldspar (25–20%, muscovite, biotite (15–10%) and accessory minerals up to 5 mode% (figure 3A–C). K-feldspar occurs as phenocrysts and in quartzo-feldspathic groundmass and shows well-developed cross-hatched twinning. The plagioclase is albite in composition; micas include muscovite and zinnwaldite (Pandian and Verma 2001). Wolframite occurs as major and minor inclusions within quartz and near the grain boundary of mica and feldspar (figures 3D, 4A and B).

Figure 3
figure 3

(A) Photomicrograph of Phase-I, (B) Phase-II and (C) Phase-III DPG granite from Rewat Hill, Degana. (D) Wolframite of Type-II mineralization in reflected light. Wol: wolframite; Mus: muscovite; Qtz: quartz; Ser: sericite; K-felds and Akf= K-feldspar.

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Figure 4
figure 4

(A and B) BSE image showing wolframite grain with quartz, muscovite and alkali feldspar. Wol: wolframite; Mus: muscovite; Qtz: quartz; and Akf: K-feldspar.

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6 Geochemistry

6.1 Degana peraluminous granite

6.1.1 Major oxides

The whole rock geochemistry of 10 samples of Type-III DPG from Rewat Hill is presented in table 4. Samples show high loss-on-ignition (LOI) values (1.30–5.93 wt.%), which might be due to the introduction of mica on greisenization of these granites associated with tungsten mineralization. All samples show some geochemical variation between them and are comparable in terms of major oxides because of post-magmatic alteration. The concentration of SiO2 varies from 62.52 to 74.80 wt.%, Al2O3 (12.99–20.75 wt.%), Fe2O3 (0.52–4.67 wt.%), MnO (0.01–0.19 wt.%), MgO (0.10–2.33 wt.%), CaO (0.20–2.88 wt.%), Na2O (1.38–4.19 wt.%), K2O (4.35–6.07 wt.%), TiO2 (0.01–0.68 wt.%) and P2O5 (0.01–0.17 wt.%) and the samples are confined to peraluminous granite field (figure 4A). Notable incidences of K2O and Na2O imply their potassic and sodic character.

Table 4 Whole rock geochemistry of DPG, Rewat Hill, Degana.
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Table 5 Analytical results of fertile DPG beneath soil cover around Rewat Hill, Degana.
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6.1.2 Trace elements

Elevated values of Rb (up to 1080 ppm), Ba (up to 715 ppm) Zn (up to 1–21 ppm) and Zr (up to 324 ppm) are indicating highly evolved nature of felsic magma with the influence of fluid-assisted alteration and metasomatism. Primary differentiation index is determined by immobility of Zr (figure 5) and its behaviour to monitor the mobility of other major and trace elements during post-magmatic hydrothermal alteration (Pearce et al. 1992; Polat and Kerrich 2000). Tungsten concentration in fertile DPG under soil cover ranges between 402.98 and 5025.45 ppm with higher LREE (58.36–288.67 ppm) and relatively lower HREE (14.64–92.48 ppm). The elements Nb, Ta, Sc, Sn, Zn, Cr, V, Mo, Cu, Pb, Zn, Zr, U and Th are depleted in fertile DPG and show lower concentrations of Nb (8–76 ppm), Ta (1.43–56.21 ppm), Sc (3.5–27.0 ppm), Sn (8.20–105.33 ppm), Cr (16–685 ppm), V (20–237 ppm), Mo (5.78–100.46 ppm), Cu (1–472 ppm), Pb (2–56 ppm), Zn (35–528 ppm), Zr (35–321 ppm), U (2.44–31.86 ppm) and Th (10–168 ppm). Variation of different major oxides, large ion lithophile elements (LILEs) and HFSE are plotted against Zr to understand their relative mobility (figure 6). The variation plot indicates highly evolved nature of DPG. Phase-I, porphyritic granite shows relatively lower ratio of Zr with major oxides, trace elements (LILEs and HFSEs) indicating unaltered nature of phase-I DPG granite. Whereas, more evolved phase-II and phase-III DPG show higher ratio of Zr against them indicate more interaction of hydrothermal fluid and alteration within them. A/CNK–A/NK plot (Shand 1943) for DPG indicates its peraluminous nature (figure 5A). Chondrite-normalized REE patterns (figure 5B) of the DPG show an enormous negative Eu-anomaly indicating presence of plagioclase in the residual phase and crystal fractionation and enrichment of volatiles during hydrothermal alteration (Chattopadhyay and Chattopadhyay 1992; Chattopadhyay et al. 1994).

Figure 5
figure 5

(A) A/CNK–A/NK plot showing peraluminous nature of DPG (after Shand 1943). (B) Spider plot showing huge negative Eu anomaly of DPG (after Nakamura 1974).

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Figure 6
figure 6

Major oxides and Trace element variations vs. Zr.

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6.2 Wolframite

The mineral chemistry data of wolframite from studied samples are provided in table 3. Wolframite shows variation in its composition between Huebnerite (Hb) to Ferberite (Fb) and molecular proportion of these two end members are determined by the formula given below:

$$ {\text{Hb}} = {\text{MnO}}/\left( {{\text{FeO}} + {\text{MnO}}} \right) \times 100, $$
(1)
$$ {\text{Fb}} = {\text{FeO}}/\left( {{\text{FeO}} + {\text{MnO}}} \right) \times 100. $$
(2)

It is observed that molecular proportion of Huebnerite from the wolframite crystal of DPG varies between 29.2% and 57.8% and Ferberite between 44.5% and 70.8% indicating Ferberite as dominant phase. A slight enrichment of LREE together with Nb2O3 0.97–1.67 wt.%, Ta2O5 0.06–0.34 wt.%, BaO 0.08–0.13 wt.%, Y2O3 0.05–0.28 wt.% and F 0.06–0.31 wt.% was also reported from wolframite (figure 4A and B).

7 Discussion and conclusions

The peraluminous mineralized granites are mostly emplaced at shallow crustal level, enriched in volatile elements, viz., F, Li, B and having high Rb/Sr ratio (Neiva 1984; Zhao et al. 2001; Xie et al. 2009; Fogliata et al. 2012; Anand et al. 2018). Rb/Sr ratio of DPG is very high as it shows enriched Rb (352–2103 ppm) and depleted Sr (6–535 ppm). The high Rb/Sr ratio of DPG indicates post-magmatic alteration involving enriched K-phases and depleted Ca-phases (Imeokparia 1981; Ekwere 1985). This Rb/Sr ratio can also be used to determine the fertility of DPG (Nockolds and Allen 1953; Taylor and Heier 1960; Taylor 1965; Imeokparia 1981; Ekwere 1985). Higher Rb/Sr ratio indicates fertility of DPG for W and associated mineralization. The Rb/Sr ratio of DPG occurring beneath cover varies between 7.86 (BHI-31) and 91.02 (BHV-1). On the basis of field evidences, we classified tungsten mineralization in the study area into five types; Type I: quartz–wolframite vein (figure 2A and B), Type-II: quartz–muscovite–wolframite–polymetallic sulphide vein (figure 2C), Type-III: DPG hosted wolframite (figure 2D), Type-IV: stock-works hosted wolframite in phyllite (figure 2F) and Type-V: gravel bed hosted wolframite (figure 2G). Rewat Hill is entirely made up of fertile DPG (Type-I to III, Type-V at foothills) and Tikli hill is characterized as fertile DPG (Type-I to III, Type-V at foothills) with phyllite (Type-IV). The Phyllite Hill is entirely made up of phyllite with Type-IV tungsten mineralization. Wolframite occurrences of Type-I to Type-IV are genetically related to hydrothermal events associated with Neoproterozoic granite magmatism and Type-V is much younger weathering-related placer wolframite process.

Sub-surface data reveals the presence of fertile DPG with potential W-mineralization in soil covered northern and north-western parts of Rewat Hill with W concentration in the range of 402.98–5025.45 ppm with higher LREE (58.36–288.67 ppm) and relatively lower HREE (14.64–92.48 ppm). The composition of wolframite from DPG varies between Huebnerite to Ferberite, among them, Ferberite is reported as the dominant phase. The elements Rb and Ba have strong positive correlation with W and range between Rb (352–2103 ppm) and Ba (54–779 ppm). Rb/Sr ratio of DPG granite is very high with enriched Rb and depleted Sr, indicating post-magmatic alteration. This Rb/Sr ratio can also be used to determine fertility of DPG, higher the Rb/Sr ratio indicates more fertile DPG granite for W and associated mineralization (Li, Rb, Ba).