*5.1. Petrography and EPMA Study*

### 5.1.1. Petrography of Host Rocks

Detailed petrographic studies of gangue and ore minerals of the host rocks were carried out to understand the relationship between gangue and ore minerals, nature of gold mineralization, microstructures, mineral paragenesis and hydrothermal alteration pattern. Quartz-mica schist is composed of muscovite, chlorite, sericite and quartz with an accessory amount of apatite, feldspar, tourmaline, biotite, ilmenite, rutile, hornblende, orthoclase and sphene (Figure 6A–F). A few sections also observe the alteration of biotite and amphibole to chlorite. The rock shows well-developed schistosity, marked by the parallel alignment of chlorite, muscovite, and sericite. S-C fabric, mica fish and pressure shadow are also observed within the rock. Orientation of mica fish and S-C fabric shows a dextral sense of shearing (Figure 6J) and alteration of feldspar to sericite. (Figure 6K). The rock shows well-developed crenulation cleavage and microfolds associated with the second phase of deformation. In chlorite, Fe/Fe+Mg ratio varies from 0.42 to 0.61 and Mg/Fe + Mg ratio ranges from 0.39 to 0.58. Most chlorites are ripidolite with few pynochlorite and diabantite (Figure 8E). Feldspar is mostly albite (Ab98.6–91An8.2–0.4Or3–0.1) to oligoclase [(Ab84.8–70.9An27.1–14.6Or2–0.3), (Figure 8F) and tourmaline is dravite variety.

Albitite comprises albite (Ab98.7–90.8An9.1–0.7) and tourmaline as major minerals (Figure 6D) along with apatite, biotite, muscovite, actinolite, quartz, orthoclase, rutile and ilmenite as accessory minerals. Albite occurs as subhedral, polygonal grains and generally untwined in nature. In some sections, albite constitutes up to 90% volume of the rock. Tourmaline occurs as euhedral to subhedral grains and shows strong pleochroism in brown shades. The concentration of tourmaline ranges from 5 to 50% volume of the rocks. A higher concentration of tourmaline is observed in the mineralized zones. Albite and tourmaline grains in albitite are elongated and aligned, parallel to the shear foliation close to the shear zone (Figure 6G,H).

#### 5.1.2. Ore Petrography and Ore Chemistry

The sulfide mineral assemblages of the Jagpura deposit are represented by pyrrhotite, chalcopyrite, arsenopyrite, loellingite and pyrite in the order of abundance (Figure 7A–O). Moreover, native gold, maldonite (Au2Bi) and hedleyite (Bi7Te3) are present as gold and bismuth ore, respectively (Figure 8D). The sulfide minerals occur as semi-massive to massive type, vein and fracture fill type, foliation parallel type, patchy and stringers type. Magnetite is the dominant oxide phase and is closely associated with gold-sulfide mineralization (Figure 7G,J,M); it is euhedral to subhedral in shape and occurs as disseminations and veins (Figures 7E,I,M and 8A,B). Apatite is euhedral, hexagonal in shape and occurs as disseminations parallel to the foliation plane of host rocks (Figure 7L). Apatite is associated with magnetite and chalcopyrite at many places (Figures 7L and 8C). The ore minerals indicate replacement, remobilization and deformational textures. Major sulfide phases show replacement textures. Two types of arsenopyrite are observed in the ore zones: one is coarse grained, granular and intensely fractured with cataclastic texture (Figure 7A) and the other is idiomorphic, euhedral, rhombic in shape, fine to medium grained without deformation texture (Figure 7B). The healing of fractures within the arsenopyrite by chalcopyrite shows that the former is replaced by the later (Figure 7C). The convexity of chalcopyrite grain boundary within pyrrhotite shows that pyrrhotite has formed earlier (Figure 7D). Occurrence of sulfides along weak planes viz. fractures, veins and shear planes

(Figure 7H,K) and the hinge of F<sup>2</sup> folds represents ore remobilization (Figures 6E,F and 7F). In mineralized zones, sulfide veins cross-cutting the albite-tourmaline grains (Figure 6I), deformed albite twin lamellae and cataclastic fragments of sulfide minerals indicate ore deformational textures (Figure 7K). Sulfide minerals such as pyrite and arsenopyrite are associated with gold [62]. Arsenopyrite has a higher gold content than pyrite, and the gold content of arsenian pyrites increases with arsenic content [63]. *Minerals* **2022,** *12*, x FOR PEER REVIEW 12 of 36 tourmaline grains in albitite are elongated and aligned, parallel to the shear foliation close to the shear zone (Figure 6G,H).

**Figure 6.** Photomicrograph showing mineral assemblages related to alteration zones, D2 deformation and shear system. (**A**) Quartz-mica schist is mainly consists of muscovite, chlorite and quartz; (**B**) Alternate bandings of quartz and chlorite rich layers within quartz-mica schist; (**C**) Mica fish structure within quartz-mica schist indicating dextral nature of the shear zone; (**D**) Gold-sulfide mineralization is associated with extensive hydrothermal alterations viz. albitization and tourmalinization, also to note that mineralization is occurring parallel to the fractures of tourmaline grains; (**E**,**F**) F2 fold in quartz-mica schist with disposition of opaque minerals at the hinge; (**G**,**H**) Elongated albite and tourmaline grains in albitite close to shear zone; (**I**) Sulfide vein cross cutting the foliation of albite-tourmaline grains in albitite; (**J**) Alteration of biotite to chlorite in proximity to sulfide disseminations; (**K**) Sericitic alteration in albitite (PPL); (**L**) Angular grains of magnetite within iron oxide breccia. (Mag = magnetite, Ms = muscovite, Qz = quartz, Chl = chlorite, Ab = albite, Tur = tourmaline, PPL = plane polarized light). 5.1.2. Ore Petrography and Ore Chemistry **Figure 6.** Photomicrograph showing mineral assemblages related to alteration zones, D<sup>2</sup> deformation and shear system. (**A**) Quartz-mica schist is mainly consists of muscovite, chlorite and quartz; (**B**) Alternate bandings of quartz and chlorite rich layers within quartz-mica schist; (**C**) Mica fish structure within quartz-mica schist indicating dextral nature of the shear zone; (**D**) Gold-sulfide mineralization is associated with extensive hydrothermal alterations viz. albitization and tourmalinization, also to note that mineralization is occurring parallel to the fractures of tourmaline grains; (**E**,**F**) F<sup>2</sup> fold in quartz-mica schist with disposition of opaque minerals at the hinge; (**G**,**H**) Elongated albite and tourmaline grains in albitite close to shear zone; (**I**) Sulfide vein cross cutting the foliation of albitetourmaline grains in albitite; (**J**) Alteration of biotite to chlorite in proximity to sulfide disseminations; (**K**) Sericitic alteration in albitite (PPL); (**L**) Angular grains of magnetite within iron oxide breccia. (Mag = magnetite, Ms = muscovite, Qz = quartz, Chl = chlorite, Ab = albite, Tur = tourmaline, PPL = plane polarized light).

The sulfide mineral assemblages of the Jagpura deposit are represented by pyrrhotite, chalcopyrite, arsenopyrite, loellingite and pyrite in the order of abundance (Figure 7A–O). Moreover, native gold, maldonite (Au2Bi) and hedleyite (Bi7Te3) are present as gold and bismuth ore, respectively (Figure 8D). The sulfide minerals occur as semi-massive to massive type, vein and fracture fill type, foliation parallel type, patchy and stringers type. Magnetite is the dominant oxide phase and is closely associated with gold-sulfide mineralization (Figure 7G,J,M); it is euhedral to subhedral in shape and occurs as dissem-

occurs as disseminations parallel to the foliation plane of host rocks (Figure 7L). Apatite

and the gold content of arsenian pyrites increases with arsenic content [63].

is associated with magnetite and chalcopyrite at many places (Figures 7L and 8C). The ore minerals indicate replacement, remobilization and deformational textures. Major sulfide phases show replacement textures. Two types of arsenopyrite are observed in the ore zones: one is coarse grained, granular and intensely fractured with cataclastic texture (Figure 7A) and the other is idiomorphic, euhedral, rhombic in shape, fine to medium grained without deformation texture (Figure 7B). The healing of fractures within the arsenopyrite by chalcopyrite shows that the former is replaced by the later (Figure 7C). The convexity of chalcopyrite grain boundary within pyrrhotite shows that pyrrhotite has formed earlier (Figure 7D). Occurrence of sulfides along weak planes viz. fractures, veins and shear planes (Figure 7H,K) and the hinge of F2 folds represents ore remobilization (Figures 6E,F and 7F). In mineralized zones, sulfide veins cross-cutting the albite-tourmaline grains (Figure 6I), deformed albite twin lamellae and cataclastic fragments of sulfide minerals indicate ore deformational textures (Figure 7K). Sulfide minerals such as pyrite and arsenopyrite are associated with gold [62]. Arsenopyrite has a higher gold content than pyrite,

**Figure 7.** Photomicrograph showing various generations of mineral assemblages such as magnetiteapatite-sulfides-gold and their linkages to the shear zone characteristics and deformation patterns. (**A**) Catascatically deformed arsenopyrite of the first generation, where fractures are healed by pyrrhotite; (**B**) Undeformed isomorphic, euhedral, rhombic shaped second generation arsenopyrite; (**C**) Fractures of arsenopyrite are healed by later formed chalcopyrite; (**D**) Convexity of chalcopyrite **Figure 7.** Photomicrograph showing various generations of mineral assemblages such as magnetiteapatite-sulfides-gold and their linkages to the shear zone characteristics and deformation patterns. (**A**) Catascatically deformed arsenopyrite of the first generation, where fractures are healed by pyrrhotite; (**B**) Undeformed isomorphic, euhedral, rhombic shaped second generation arsenopyrite; (**C**) Fractures of arsenopyrite are healed by later formed chalcopyrite; (**D**) Convexity of chalcopyrite grain in pyrrhotite indicate that pyrrhotite has formed earlier; (**E**) Occurrence of abundant subhedral magnetite grains within quartz-mica schist; (**F**) Saddle reef structure having sulfide mineralization at the hinge of microfold indicating ore remobilization; (**G**) Disseminations of arsenopyrite, pyrrhotite, chalcopyrite and magnetite of early stage mineralization; (**H**) Nature of mineralized shear system showing shear planes cross cutting the albite grains; (**I**) Magnetite vein of second stage mineralization, occurring parallel to shear fracture; (**J**) Coarse grained magnetite with associated chalcopyrite; (**K**) Deformed albite twin lamellae in albitite close to mineralized vein; (**L**) Occurrence of subhedral apatite grains associated with magnetite in quartz-mica schist; (**M**) Disseminated magnetite and arsenopyrite of early stage mineralization; (**N**) Native gold grains in quartz-mica schist; (**O**) Skeletal gold speck in second generation quartz vein intrusive within quartz-mica schist. (Apa = apatite, Mag = magnetite, Apy = arsenopyrite, Po = pyrrhotite, Ccp = chalcopyrite, Au = gold, Tur = tourmaline, Ab = albite, Chl = chlorite).

Ore petrography revealed that gold is mainly present within the arsenopyrite and loellingite. Within the arsenopyrite, gold occurs in association with loellingite, maldonite and hedleyite. Gold occurs as specks, fine flakes and films varying in size from a fraction of a micron to 40 microns (Figure 7N). Gold also occurs along the fractures within the arsenopyrite and loellingite. Epigenetic quartz vein within the host rocks also contains native gold (Figure 7O). Arsenopyrite and loellingite are intimately associated and form the main gold bearing minerals in the study area. EPMA analysis of native gold indicates that Au concentration ranges from 89.25 to 94.72 wt% and it contains silver as a minor impurity (6.15 to 8.46 wt%). Gold fineness ranges from 914–937‰, with average value of 927‰ (n = 6).

EPMA analysis of the sulfides also indicates the presence of invisible gold. Gold content ranges from 0.02 to 2.58 wt% in arsenopyrite, 0.02 to 0.20 wt% in chalcopyrite, 0.08 to 0.11wt% in pyrrhotite and up to 0.17 wt% in loellingite. The EPMA study reveals that loellingite and arsenopyrite occur as coexisting phase within the ore zones. However, loellingite is the preferred residence for gold mineralization compared to arsenopyrite (Figure 8D). Arsenic content is higher in loellingite (72.20 to 72.36 wt%) than arsenopyrite (47.07 to 51.05 wt%). Loellingite and arsenopyrite are also rich in nickel and cobalt wherein, loellingite shows higher values compared to arsenopyrite [Table 1]. The nickel and cobalt values in loellingite range from 2.10 to 2.45 wt% and 1.08 to 1.44 wt%, respectively. The nickel and cobalt values in arsenopyrite are 0.39 to 2.22 wt% and 0.18 to 0.40 wt%, respectively.

**Table 1.** EPMA data of Au, Au-Te, Au-Bi and sulfide phases (in Wt%) from the Jagpura gold-copper deposit, western India.




**Table 1.** *Cont.*

*Minerals* **2022,** *12*, x FOR PEER REVIEW 18 of 36

**Figure8.** BSE images of magnetite-apatite-gold-sulfide mineral assemblages along with plots suggesting hydrothermal nature of pyrite and magnetite in the study area. (**A**) Magnetite megacryst in quartz-mica schist; (**B**) Disseminated magnetite within quartz mica-schist; (**C**) Subrounded to rounded apatite grains associated with chalcopyrite; (**D**) Association of gold, loellingite, arsenopyrite, maldonite and hedleyite; (**E**) Mineral composition classification diagram of chlorite, (Red = albitite, green = quartz-mica schist); (**F**) Or-Ab-An classification diagram of feldspar. (Red = albitite, green = quartz-mica schist); (**G**) Co vs. Ni plot of pyrite show hydrothermal signature, fields for pyrite composition from references [64]; (**H**) Ti vs. Al covariation plots of magnetite indicates **Figure 8.** BSE images of magnetite-apatite-gold-sulfide mineral assemblages along with plots suggesting hydrothermal nature of pyrite and magnetite in the study area. (**A**) Magnetite megacryst in quartz-mica schist; (**B**) Disseminated magnetite within quartz mica-schist; (**C**) Subrounded to rounded apatite grains associated with chalcopyrite; (**D**) Association of gold, loellingite, arsenopyrite, maldonite and hedleyite; (**E**) Mineral composition classification diagram of chlorite, (Red = albitite, green = quartz-mica schist); (**F**) Or-Ab-An classification diagram of feldspar. (Red = albitite, green = quartz-mica schist); (**G**) Co vs. Ni plot of pyrite show hydrothermal signature, fields for pyrite composition from references [64]; (**H**) Ti vs. Al covariation plots of magnetite indicates magnetite is hydrothermal in nature, fields from reference [65]. (Apa = apatite, Mag = magnetite, Ccp = chalcopyrite, Au = gold, Mdl = maldonite, Lo = loellingite, Hed = hedleyite).

The EPMA study of magnetite from the study area reveals that the concentrations of TiO<sup>2</sup> ranges from(0.01 to 0.11 wt%), Al2O<sup>3</sup> (0.06 to 0.28 wt%), MnO (up to 0.01 wt%), FeO (83.39 to 89.02 wt%), MgO (0.01 to 0.08 wt%), V2O<sup>3</sup> (0.03 to 0.26 wt%), Cr2O<sup>3</sup> (0.01 to 0.09 wt%), NiO (0.04 to 0.31 wt%), CoO (0.02 to 0.05 wt%) and CaO (0.01 to 0.08 wt%) [Table 2]. Magnetite shows low concentrations of TiO<sup>2</sup> (0.01 to 0.11 wt%) and Al2O<sup>3</sup> (0.06 to 0.28 wt%). The Co/Ni ratio of magnetite is <1 (0.04 to 0.67) and Ni/Cr ratio is ≥1 (1 to 4). Co/Ni ratio of pyrite from the study area is >1 (1 to 2.54, n = 22). The EPMA analysis of apatite reveals that apatite is of fluorapatite variety with F content >1 wt% and >1F/Cl ratio [Table 3]. Apatite has a higher concentration of F (4.23 to 5.97 wt%, n = 10) and a lower concentration of Cl (0.08 to 0.73 wt%), FeO (0.01 to 0.28 wt%), and MnO (0.01 to 0.07 wt%).


**Table 2.** EPMA data of magnetite from the Jagpura gold-copper deposit, western India.


**Table 2.** *Cont.*


**Table 3.** EPMA data of apatite (in Wt%) from the Jagpura gold-copper deposit, western India. Lithology: Quartz-mica schist, Sample No. JGB-11.
