Melt Inclusion Studies of Pb-Zn Ore Deposits of Rajpura-Dariba-Bethumni Belt in District Udaipur, (Rajasthan) India

The Proterozoic Aravalli-Delhi orogenic complex hosts a large number of economically important stratabound base metal sulphide deposits. In the present work, rock samples taken from Outcrop and Underground mines of Sindeskar Kalan, Vedanta Group, Rajpura Dariba-Bethumni Belt which were located at a distance of 76 Kms from Udaipur District (Rajasthan) have been studied. The chief litho units of the group were identified which contain sulfide-bearing calc-silicate and graphite mica schist, dolomite marble, calc-biotite schist and quartzite. Importance to the ore minerals like galena and sphalerite which have been reported in association with the buffer minerals like pyrite and pyrrhotite occurred in the host rocks of the study area. The study of melt inclusion (sulfide melt) of sphalerite showed the maximum temperature of melting which was 923 °C on the Linkam-TMS94 (the maximum temperature limit of the system is up to 1500 °C). The maximum temperature of melting of sphalerite reveals that the dolerite and/or pegmatite intrusion might have supplied sulfide rich melt during the study area in the geological past.

• Sphalerite
• Zn-Pb Ore
• Melt Inclusion
• Rajpura Dariba-Bethumni Belt
• Rajasthan

1. Introduction

Zinc-Lead deposits of various sizes and grades occur throughout the belt in calc-silicate bearing dolomite and graphite mica schist horizons [1] , the latter in general containing low-grade disseminated sulfides of large volumes. The ores from the various deposits in the belt have more or less similar mineral assemblage, differing mainly in their relative proportions from deposit to deposit. The stratiform ore bodies [2] mainly comprised sphalerite, galena, chalcopyrite, and pyrite-pyrrhotite. The stratiform ores of Rajpura-Dariba are characterized by the presence of different verities of laminated sphalerite such as lemon yellow, light brown, and dark brown, a feature that is absent in other deposit in the belt.

Silicate melt inclusions (MI) are small samples of melt that are trapped during crystal growth at magmatic pressures and temperatures. The MI represent a sample of the melt that was isolated from the magma during host crystal growth. Thus, MI provide a valuable tool for constraining the magmatic history of igneous systems because they provide an unambiguous method to directly determine compositions of melts from which the host crystal grew. The temperature and pressure at which the rocks are formed or the ores are deposited, ranges from about very high at a depth to atmospheric at the surface. Geothermometry is a measurement of estimation of temperatures at which the geologic process takes place, and the indicator which provides this information is known as geothermometry. There are practical as well as theoretical reasons for studying the temperature of ore deposition and the character of trapped melts, beside from the fact that genesis of the minerals cannot be understood until the conditions of depiction and the nature of cooling.

The Archaean basement comprising of gneiss, schist, amphibolite, quartzite and granite dating back to 3.2 to 2.5 b.y. showing unconformable relationship with the Aravalli cover rocks, is clearly marked in and around Udaipur. Stratigraphic succession, established by [3] for the Aravalli Supergroup of the type area around Udaipur and Zawar, shows two major groups separated by an unconformity. The Upper Aravalli Group consists of greywacke-slate-phyllite, quartzite, dolomite and siltyarenite (host for sulphides of zinc and lead) while carbonaceous and pelitic phyllites, dolomite, quartzite, stromatolyte, phosphorite, chlorite schist, amphibolite, quartz arenite and local conglomerate [4] belong to Lower Aravalli Group. In general, Aravalli rocks in Udaipur region show a low-grade metamorphism. The recrystallisation of the silicate minerals suggests the grade of metamorphism to be of greenschist facies [5] (Figure 1).

Figure 1

Regional geological map, Dariba Belt. Modified after [ 6 ]

2. Geology

The geology of Rajasthan has been studied by many workers [7] -[18] over decades and the different major geological units and prominent faults and lineaments of this region. The crust of the northwestern Indian Craton in Rajasthan comprises the Achaean Banded Gneissic Complex (BGC) forming the basement, overlain by the Proterozoic Delhi Aravalli Fold Belts of Delhi and Malani Igneous Suite, most of which are covered by the Tertiary and Quaternary sediments. Studies by [19] have provided constraints on the ages of the pre-Aravalli basement rocks. Detailed geological mapping by [20] suggests a wide variation in the spatial and temporal evolution of the region through different geodynamic processes.

The Aravalli mountain range in the northwest part of India extends over 700 km in length with a general NE-SW trend It consists of two main Proterozoic sedimentary and volcano sedimentary successions, the Aravalli Supergroup and the DelhiSupergroup, respectively, which are bounded by the Great Boundary Fault to the east and the Western Marginal Fault to the west. These Proterozoic successions rest unconformably on Archean granitoid basement [21] , commonly referred to as Banded Gneissic Complex/BGC [22] . The minimum age of the basement rocks is 2500 Ma [23] . The Aravalli Supergroup, a sedimentary succession with minor volcanic flows near the base, developed as a cover sequence on the granitoid basement [21] , the BGC of [22] . The existing geochronological data for the Aravalli Supergroup are insufficient to date precisely the opening and closure of the Aravalli Basin. However, the maximum age of the Aravalli Supergroup comes from the Sm-Nd systematics of the basal volcanic indicating 2326 ± 321 Ma [24] and the minimum age is considered to be 1900 ± 80 Ma from the Rb-Sr dating of Darwar Granite that was emplaced synkinematically with the earliest deformation of the Aravalli [25] .

Stratigraphy of the study area [26] .

3. Material and Methods

A total of 25 samples were collected from the different Levels of underground mine as well as outcrop samples from the selected mine (both of ores and rocks) of the study area, out of which fresher and unweathered samples were selected for melt inclusions studies polished blocks of ores were carried out under transmitted and reflected light respectively.

4. Melt Inclusions Studies

4.1. Introduction

Melt Inclusion in rocks are the vital source of information about the composition of parent magma. Melt inclusions preserve compositions that are different from those of erupted lavas. Melt inclusions are small parcels or “blobs” of melt(s) that are entrapped by crystals growing in the magmas and eventually forming igneous rocks. In many respects they are analogous to fluid inclusions. Melt inclusions are generally small―most are less than 60 micrometers.

Inclusions can act like “fossils” trapping and preserving these melts before they are modified by later processes. They are glassy or crystalline and are found within both extrusive and intrusive rocks because they can form at high pressure and are contained within relatively incompressible hosts, they may retain high concentrations of volatile elements that normally escape from magma during degassing. As such analysis of these inclusions provides direct information on the volatile contents of magmatic systems [27] . [28] was the first to document microscopic melt inclusions in crystals.

4.2. Biphase Inclusions (Figures 2-5)

Main type of Biphase inclusion is:

Figure 2

Photomicrographs showing A and B Biphase melt inclusion, Gas inside the bubble

Figure 3

Photomicrograph C showing Biphase melt inclusion having microtube, D showing Polyphase melt inclusion of globular shape having some rod shape inclusion material

Figure 4

Photomicrograph I and J Biphase melt inclusion showing negative crystal cavity

Figure 5

Photomicrograph K Biphase melt inclusion showing open cavity having daughter crystal, L showing Along trail and bottle shaped Polyphase melt inclusion

Ÿ Gas Phase (CO₂) in glass inclusions

Ÿ Silicate Phase in glasses

Ÿ Opaque Phases in glasses.

These Bi and Mono phase are connected with a microtube called hourglass inclusions.

4.3. Polyphase ( Figure 6 and Figure 7 )

In this type more than 3 phases occur like gas, silicate phase and other daughter mineral in glasses.

5. Geothermometry

5.1. Introduction

The Melt inclusion microthermometry study was carried out using a Linkam-TMS94 stage attached to a Leitz microscope having CCD camera (Figure 8). The stage was caliberated using synthetic CO₂ inclusion supplied by the M/S Linkam Ltd. Analysis was carried out for biphase and polyphase melt inclusion in wafer section of the sphalerite. The maximum temperature limit of the system is up to 1500˚C. At this high temperature oxidation

Figure 6

Photomicrograph E and F showing CO 2 rich Polyphase melt

Figure 7

Photomicrographs G showing Polyphase melt inclusion containing calsic material, H showing Microtube polyphase melt inclusion appearing like airphone

Figure 8

Photomicrograph showing different phases of Melt Inclusion

may take place. So to reduce the oxidation effect Argon (inert gas) is used. The surrounding environment of the sample should be inert. So, we make it vaccum. Thermo couple is used to know about the temperature of heating (Platinum Radium) thermocouple is used. The sample is placed above a sapphire glass. Water circulation is there to maintain the temperature of vaccum chamber at a constant rate.

5.2. Microthermometric Study

The main aim of microthermometric study is to know the temperature of homogenization of the entrapped inclusion. For the microthermometric study small fragments of already examined doubly polished wafer thin section of sphalerite (Figure 9) is selected. The heating study (Table 1 and Table 2) is carried out on inclusions of large size and primary origin. Heating started at room temperature and pressure.

Figure 9

Photomicrograph showing the changes observed during heating

Table 1

Showing the rate of temperature variation during heating

Rate of temperature increase per minute	Temperature limit in ˚C
30	200
20	500 (Heating)
10	900
5	1250 Maximum temperature
15	900
10	500 (Cooling)
25	200

Table 2

Observation during heating at different temperature pressure condition

Vacuum pressure (mbar)	Temperature ˚C	Behavior of melt inclusions description
6	30	Heating started
6.5	200	No change in melt inclusions
6	399	No change in melt inclusions
8	498	No change in melt inclusions
10	594	No change in the boundary of S1 and slight change in M1 and M2
7.5	690	Melt inclusions boundary start merging
9.3	799	Merging continues
8	880	Partial melting of M1, M2 and no change in S1
14.2	1020	Partial melting continues
6	1110	Only relict of melt Inclusion
8	1170	Complete melting of M1 and M2 but S1 not melt may be some opaque or solid inclusion
9	1250	Homogenization temperature of M1 and M2 but S1 still remain

6. Conclusion

Melt inclusion studies result shows that the temperature of complete melting of sphalerite is 1170˚C and maximum temperature of heating is 1250˚C, which is the temperature of homogenization. On the basis of melt inclusions petrography under the ore microscope reveals that the melt inclusions found only at 375 level and little bit at 350 meter level which probably suggest that the pegmatite intrusion occurred up to this level and the recognition of melt inclusion is a proof that a rock was partially melted at some time in its history.

Acknowledgements

The authors are thankful to Indian Institute Technology Bombay for field and laboratory support in carrying out

the investigation, to Chairman of Department of Geology, AMU Aligarh for providing facilities to carry out the study.

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