The thesis describes host rock petrology, mineralization structures & textures, and genesis of four fluorspar deposits / prospects of Rajasthan, viz., Mando-Ki-Pal, Chowkri-Chhapoli, Jhalara and Asind. In the Mando-Ki-Pal area the country rocks are migmatites with occasional bands of muscovite-kyanite-schist. Lower Proterozoic Aravalli quartzite surrounds the migmatites of Precambrian Banded Gneissic Complex (BGC). This study has shown that the host rocks of fluorspar are not cherts and cherty limestone, as hither-to-considered, but are silicified cataclastic rocks. They include proto- to ultramylonite, blastomylonite, and gangmylonite. Consequently, the genetic model for the deposit will be different and its resource evaluation will have to be modified. Green and purple fluorite, calcite ((mangeneferous at places, displaying red fluorescence) and quartz, are the main vein minerals, which display characteristic cavity: filling structures, viz., fissure filling, joint filling, fracture filling, symmetrical crustification, cockade structure, segmented veins, breccia filling and brecciated veins etc. Grains of pyrite, chalcopyrite, galena, and sphalerite are rarely present. Green fluorite and Mn-calcite display fluorescence. Epimorphs of calcite in gangmylonite is also common. Silicification and chloritization of the host rocks is observed. At Kaila persistent stratabound veins of brownish / grayish fluorite occur in quartzite. Jhalara is an extension of Mando-Ki-Pal area and the mineralization is very poor, occurring in migmatites, pegmatite and quartzite. This area was abandoned after prospecting. At Chowkri the fluorspar occurs in tonalite boss intruding the Middle Proterozoic Ajabgarh garnet-biotite schist. While at the adjoining Chhapoli the fluorspar is confined to Ajabgarh feldspathic quartzite. Green & purple fluorite in both these areas occur as large, color-zoned octahedral crystals. Calcite and quartz are associated with fluorite. Secondary neodigenite and hydrous iron oxide are found in few sections. Mineral, zoning is noticed at Chowkri area. Fluorite, calcite, quartz occur in the central part while apatite, quartz form the outer zone. The hydrothermal mineralization has brought about kaolinization and hematitization of the host rocks. Ilmenite and magnetite is associated with hornblendite reefs in the area. Near Asind the fluorspar veins occur in migmatites of Banded Gneissic Complex; amphibolite, pegmatite, aplite, epidosite, unakite are also present. Epidotization of vein wall in migmatite is characteristic features observed in the area. Associated minerals are quartz and rarely calcite. Here too the mineralization is poor and the area was discarded after prospecting. The fluorspar mineralization in these areas is diplogenetic formed by low temperature hydrothermal solution related to migmatization at Mando-Ki-Pal, Jhalara, Asind and tonalite-granite at Chowkri-Chhapoli. Fluorine bearing solutions leached calcium front the country rocks and deposited fluorspar in the available cavities (fissures, fractures, joint planes, pore spaces, etc.). Since cavity filling in non-calcareous rocks is predominant and replacement poor, therefore, the deposits are of poor economic significance.
The
rocks around Karara (Rajasthan) belong to the late Proterozoic
Malani suite of volcanic rocks and a range of pyroclastic
rocks represented by: volcanic breccias, lapilli tuffs,
and welded tuffs including ignimbrites and extrusive basalt,
andesite, trachyte, dacite and rhyolite. S-pole diagrams
of joint planes show a peripheral spread of the sub-maxima,
which is typical of central type of volcanic activity. Field
and remote sensing studies in this desert region conclusively
prove that the area represents a volcanic vent that had
witnessed repeated volcanic-activity of violent nature.
The
thesis embodies a detailed fluid inclusion and rare earth element
(REE) study of Chowkri -Chhapoli and Mando-Ki-Pal fluorspar
deposits of Rajasthan. At Mando-Ki-Pal the fluorspar mineralization can broadly be classified into two types. 1. One is that which occurs in mylonite and granitic gneisses of basement complex (Bhilwara Supergroup). The mineralization displays fracture filling, joint filling, crustification, cockade structure etc. minerals present are-Green fluorite, purple fluorite, calcite, quartz with rare presence of galena, sphalerite, pyrite, pyrolusite. The minerals contain two-phase aqueous inclusions having high degree-of-fill. They are of irregular shapes and have wider range of size. Microthermometry data indicates presence of salts other than NaCl in the fluids, salinity of which range from 14 to 22 Eq. Wt. % NaCl in fluorite; in quartz and calcite it ranges from 10 - 19 Eq. Wt. % NaCl. The temperatures of homogenization range from 102° - 168°C for fluorite; 93° to 134°C for quartz; and 112° to 135°C for calcite. All the minerals contain large number of pseudosecondary and secondary inclusions. 2.
Another type of fluorspar mineralization is the one that occurs
as persistent stratabound veins in quartzite in Kaila and Nawagaon
Blocks. Fluorite is milky white or light brown in color
and occurs as fine granular aggregate of fluorite and quartz.
Both these minerals contain CO2, CO2-H2O
and rarely CH4 inclusions. Cryometric data indicate
that CH4 density is about 0.24 g/cc. Density
for the CO2 inclusions ranges from 1.10 to 0.6 g/cc
although density of 0.8 to 0.7 g/cc is more common. The
aqueous FIs have salinity of 4 - 9 Eq. Wt. % NaCl.
Rarely, a halite daughter crystal has also been observed in
some of the fluid inclusions, indicating that the solutions
were saturated with respect to NaCl. The temperature of homogenization
for the H2O - CO2 type inclusions has
a range of 185° to 360°C with maximum values lying between 220°
to 270°C.
India’s largest resources of anthophyllite occur within the ultramafic rocks exposed to the West of Udaipur, Rajasthan. The ultramafic rocks constitute a linear belt designated as Jharol-West belt that extends for a strike length of about 25 km in NNE - SSW direction with maximum width of nearly 2 km. The ultramafic bodies occur as intrusives within tile rocks of Jharol Group of Aravalli Supergroup and exhibits concordant relationship with them. The original nature of the ultramafic rocks is difficult to ascertain since they have undergone high degree of steatitization and anthophyllitization. Quartz-mica schist, quartzite, few bands of impure marble along with isolated outcrops of amphibolite and pegmatite mainly represent the Aravalli metasediments of the area. The anthophyllite deposits occur as lensoidal bodies of varying dimensions. They show a characteristic set of lithological zoning where a zone of phlogopite separates the ore-bodies from enclosing quartz-mica schist. Anthophyllite occur as parallel, triangular or radiating aggregates of acicles/fibers. Minerals occurring in association with anthophyllite are talc + antigorite + chlorite + biotite + phlogopite + magnetite + ilmenite + olivine ± tremolite ± quartz ± calcite. Based on geochemistry and field relationship it is concluded that the ultramafic rocks of the area are product of peridotitic magma (Orogenic "root-zone"), Iherzolite sub-type, which was emplaced in subsolidus state though of fractures developed in the crust during the Aravalli orogeny where the Aravalli sediments were suffering, folding. Lensoidal shape of the ore-bodies was developed due to pinch-and-swell structure due to ductile deformation after their emplacement. XRD & chemical study proved that the asbestos of the area is anthophyllite that is Mg-rich variety of ortho-amphibole. Anthophyllite is product of metamorphism of the ultramafic rocks in amphibolite facies. Based on minerals association and fluid inclusion study carried out on some of the co-genetic transparent minerals, it has been concluded that the rocks of the area were subjected to pressure of about 4.50 ± 0.20 kb. and temperature of about 670 ± 20°C. It is also concluded that metamorphic fluids were H2O + CO2 type with variable CH4 content from margin to core of the ore-bodies. Retrograde metamorphism in the form of' steatitization of anthophyllite is observed specially in the northern part of the belt. Anthophyllite deposits of the area are being developed by small-scale, manual, opencast methods of' mining. The resource produced from tile area is being processed within the state for its industrial use. Anthophyllite of the area has much higher tensile strength (40,000 - 45,000 psi) compared to values reported for anthophyllite from other areas of world (< 4260 psi). It also has good values of cement absorption capacity, resistance to acids and alkalis, wet volume (buoyancy), surface area. Therefore, the resource is extensively utilized in a number of industries and hence, it has significant contribution in the development of economy of the nation.
The scheelite skarn prospect of Kararavav (SW Rajasthan) belongs to Middle Proterozoic Kumbhalgarh Group of Delhi Supergroup. The principal rocks of the area are granite, skarn, calc-silicate, amphibolite and impure limestone. The general strike direction of metasedimentary litho units is N20° W to N35° W and dips are moderately steep ranging from 60° to 75° towards NE. The granite-limestone contact is marked by the development of skarn rocks where zonal pattern from limestone to intrusive is represented successively by limestone, calc-silicate, skarn, and granite. Two types of granites are occurring in the area. One is grayish to pink hornblende granite and other is gray biotite granite. These scheelite skarn generating granitoids have been geo-chemically studied to decipher I, S, A or M-type characteristics. The most important litho units of the area are skarn and calc-silicate that have been differentiated texturally and by the fact that skarn displays characteristic metasomatic zoning in terms of dominant mineral assemblages due to addition of material from the magma. Based on field relation, petrography and mineral chemistry, it is inferred that impure limestone was subjected to high-grade regional metamorphism and subsequent metasomatism to generate calc-silicate and skarn rocks of the area under the influence of magmatic emplacement. The activity of skarn forming fluids and chemical controls on scheelite deposition in metasomatized facies conditions are broadly reviewed and it is concluded that scheelite was precipitated between the zone where in-flowing W-rich boiling solutions became fairly Ca-rich for the solubility product of scheelite to be exceeded and the zone where W is being extracted from the out-flowing fluids due to scheelite deposition. The concentration of Ca ion in the under-saturated solution was raised with the interaction of calcium rich rocks (e.g., limestone). An estimate of P, T, X C02 and F02 conditions for the scheelite bearing host rocks (skarn and calc-silicate) has been made from coexisting stable phase equilibrium of mineral assemblages In the multi-component systems and from the fluid inclusion study of various calcio-silicate minerals. Well-established mineral stability fields have been utilized to ascertain P-T conditions of skarn and calc-silicate rocks. Various calcio-silicate viz., scheelite, vesuvianite, clinozoisite, garnet quartz and calcite, contain mostly H2O + CO2 type and multiphase fluid inclusions. Microthermometric data indicates low XCO2 content of boiling fluids that had temperature fluid activity occurred in the rocks as indicated by low temperature secondary fluid inclusions.
The
Precambrian formations of south-central Rajasthan host a unique
lead-zinc deposit near Rampura-Agucha. The deposit owes
economic significance due to presence of 39.2 MT proved, 13.8
MT probable and 10.7 MT possible reserves of ore having an average
grade of 13.6% Zn and 1.9% Pb. This deposit has aggrandized
the national resources by 16.6%. Its geological importance rests
with the fact that the ore is hosted in highly metamorphosed
Precambrian rocks having significant silicate, sulfide and oxide
mineral assemblages. Based
on field relations, petrography and petrochemistry, it is inferred
that the Agucha Gneiss is the product of regional metamorphism
of marine sediments of pelitic to graywacke composition.
Petrographic study under transmitted and incident light has
proved that the silicates and sulfides have involved in high-grade
regional metamorphism; cataclastic textures superimposed on
the recrystallization textures are also prominently present.
The textural evidence is consistent with and reinforces interpretation
of ore as stratiform metamorphosed type of mineralization. |
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