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Zawar District - Mochia, Balaria, Zawarmala, Baroi |
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Rajasthan, India |
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Zn Pb Ag
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Super Porphyry Cu and Au
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The Zawar Belt comprises a group of carbonate hosted deposits spread over a strike length of up to 20 km, within dolomites and phyllites of the Lower to Middle Proterozoic Aravalli Supergroup (2400 to 1700 Ma). The individual deposits, which include Zawar, Mochia and Balaria, are hosted by dolomites as disseminations, stringers and veinlets and together contain >40 Mt @ 4.5% Zn, 1.9% Pb (Res., 1992).
The three main Zawar lead-zinc deposits, Mochia, Balaria and Zawarmala are located around 24° 20' N, 73° 42' E, some 25 km due south of Udaipur in Rajasthan. As such they are approximately 100 km to the south-west of the Rajpura-Dariba Zn-Pb mine and 215 km south-west of the Rampura-Agucha Zn-Pb open pit.
The Zawar zinc-lead belt, as defined by known occurrences ancient workings and current mines, extends for a distance of around 20 km in a general north-south, to NNE-SSW and then around a fold nose to an ESE-WNW direction. Mochia and Balaria have been in production for some time, while Zawarmala was under development in the early 1990's. Another resource with quoted reserves, the Baroi deposit, is also within this. All are carbonate hosted deposits, that have been subjected to less intense metamorphism than either Rampura-Agucha or Rajpura-Dariba.
The mines are operated by Hindustan Zinc Limited, a division of the Vedanta Group. Hindustan Zinc was originally formed in 1966 by the nationalisation of the lead-zinc mines of the Zawar district. These mines were in financial difficulties at that stage, and nationalisation was seen as the only way to keep them operating (Jain and Rakesh, 1984).
Published reserve and production figures include:
Resource, Indian Bureau of Mines - 250 Mt @ 0.75 to 1.5% Pb, 1.5% Zn, (Krishnaswamy and Sinha, 1988).
Inferred District Resource, 1970 - 77.5 Mt @ 7% Pb+Zn,
Reserve, Mochia, 1978 - 11 Mt @ 4% Zn, 1.4% Pb, 35 to 40 g/t Ag,
2006 - 27 Mt @ 3.8% Zn, 1.7% Pb (National Resources Canada website),
Reserve, Balaria, 1978 - 19 Mt @ 5.4% Zn, 0.8% Pb, 35 to 40 g/t Ag,
Geological Reserve, Zawarmala - 1978 - 8 Mt @ 4% Zn, 2.05% Pb, 35 g/t Ag,
Indicated. Reserve, Baroi - 1978, 25 Mt @ 1.6% Zn, 3.3% Pb.
Geological Reserve, Mochia+Balaria+Zawarmala+Baroi - 63 Mt @ 3.5% Zn, 2% Pb, 35 g/t Ag,
Reserve, Zawar+Mochia+Balaria, 1991 - 36.9 Mt @ 4.5% Zn, 1.9% Pb (AME, 1992).
Reserve, Balaria, 1991 - 11.2 Mt @ 5.3% Zn, 1.0% Pb (AME, 1992).
Reserve, Mochia, 1991 - 4.5 Mt @ 4.6% Zn, 1.9% Pb (AME, 1992).
Reserve, Baroi Central, 1991 - 9.2 Mt @ 5.3% Zn, 1.0% Pb (AME, 1992).
Proven+Probable Reserve, Balaria - 1993, 5.6 Mt @ 5.5% Zn, 2.6% Pb (AME, 1995).
Proven+Probable Reserve, Mochia, 1993, - 4.5 Mt @ 4.6% Zn, 1.9% Pb (AME, 1995).
Proven+Probable Reserve, Zawarmala + Baroi, 1993 - 2.7 Mt @ 3.7% Zn, 1.7% Pb (AME, 1995).
Annual production from the Zawar group of mines in the early 1990's was of the order of 35 000 t of contained zinc and 9000 t of lead (AME, 1992). In 1992 the ore production and recovered grades from each of the mines was as follows: Balaria - 500 000 t @ 3.4% Zn, 0.7% Pb, 4 g/t Ag;
Mochia - 320 000 t @ 3.2% Zn, 1.6% Pb, 15 g/t Ag; Zawarmala - 300 000 t @ 3.3% Zn, 0.6% Pb (AME, 1995).
Geology
The Zawar string of deposits plot within the Udaipur Group, as mapped by Gupta (1980). However, according to Singh (1988), it lies within the upper Aravalli Supergroup of Roy et al., 1984).
According to Roy, et al., (1988), the Aravalli Supergroup in the Udaipur district is composed of the:
Lower Aravalli Supergroup - commencing with meta-volcanics which gradually pass on to rocks of dolomitic association, comprising dolomitic limestone, quartzite, graphitic-phyllite, argillitic-phyllite and schist. A thin but persistent stromatolitic phosphorite is found near the base of the dolomitic sequence.
Upper Aravalli Supergroup - started with the deposition of immature terriginous sediments to form a succession of alternating greywacke, slate and phyllite in the shelf sea, and conglomerate, arkose and quartzite in marginal subsiding rifted troughs. These were followed by another carbonate sequence, composed of dolomite, quartzite, argillite, graphitic phyllite and conglomerate. This latter carbonate sequence hosts the Zawar mineralisation, within the eastern shelf facies.
In this area there are preserved inliers of unconformably underlying basement exposed through the Aravalli Supergroup, particularly the large Sarada Inlier to the south. The domal form of these inliers, and the conglomeratic nature of the overlaying basal Aravalli Supergroup, imply that they may have been basement highs.
These two sub-divisions of Roy et al., (1990) appear to locally represent sections of the Debari and Udaipur Groups of Gupta, et al., (1980). The sequence in the area is interpreted to represent deposition in a shelf environment, punctuated by elongate rifted basins and some horst block basement highs. Metamorphism in the Zawar hosts in the Udaipur-Zawar region is of lower greenschist facies. Two main deformations are recorded, the first producing an early penetrative cleavage, the second a crenulation and a complex interference structural pattern. Minor pegmatites, but no major post-Aravalli granitoids are known in the area. Ultramafics to the south of Zawar have the same deformation history as the host sediments (Singh, 1988).
Deformation in the district includes both faulting and folding. The Zawarmala area is occupied by an isoclinal, north plunging anticline. Folding seems to have two preferred orientations, namely generally north-south, and WNW-ESE. These two directions are locally superimposed. In addition to the folding, there is a well developed set of generally east-west, to WNW-ESE faults that offset the quartzite ridges in particular. Flattening and a large amount of layer parallel shearing is also evident in the Zawarmala Antiform (Singh, 1988; Mukherjee and Sen, 1980).
East-west shears are a significant feature of the Mochia block. These shears are influenced by lithological boundaries between dolomite and phyllite, pure dolomite and siliceous dolomite and siliceous dolomite and phyllites. At times the shears along the dolomite-phyllite contacts deviate into the dolomite and form a breccia. Overall the shearing is developed in the lower half of the Main Dolomite, close to its boundary with the underlying phyllite unit. This shear may be traced from Mochia to Balaria.
The main rocks surrounding the Zawar mines are quartzites, dolomites, greywackes, phyllites, carbonaceous-phyllites and conglomerates. The general succession in the mineralised area, comprises, from below the ore, to the top (Singh, 1988; Mukherjee and Sen, 1980):
* Carbonaceous phyllite - which is a thick, widespread unit underlying the whole district. It appears to overlie a sequence of quartzite and conglomerates regionally.
* Greywacke and phyllite - that forms the immediate footwall to the host dolomitic units. In places it includes polymictic conglomerate. The phyllites in this unit contain chlorite, quartz, biotite, dolomite and pyrite cubes.
* Dolomite - composed of a varied sequence which includes dolomite, quartzite, dolomitic-quartzite, and pelites and graphitic schists. This unit is the host to all of the orebodies. The dolomitic rocks in places show cryptic and rhythmic layering and are composed of dolomite, calcite, quartz, plagioclase, microcline, biotite, muscovite and chlorite shreds, and tourmaline, set in a matrix of argillaceous and silty material. The silicates are generally rounded and are interpreted to be detrital. Recrystallisation of the carbonate minerals is common however, with crystalline rhombs being obvious under the microscope, sometimes replacing quartz and feldspar. Carbonate minerals are observed in thin section to penetrate cracks in larger detrital grains.
The main host to ore is a relatively pure dolomite, consisting of about 60% dolomite, 20 to 30% calcite and subordinate amounts of quartz and feldspar. As well as the pure dolomite, there is siliceous dolomite, sericitic dolomite, ferruginous dolomite and dolomitic arkose within the unit.
* Quartzite - which occur above the mineralised dolomitic units and form chains of high ridges.
* Psammo-pelites - that overlie the quartzites, but form low lying flats that are often obscured by superficial cover.
The ore is hosted by dolomite beds, which together with quartzites, some thin phyllites and arkoses, form a distinct unit about 1 km above the base of the Aravalli sequence. The ore horizon is underlain by a succession commencing with conglomerates which rest unconformably on the older basement gneisses. These are followed by shale and then greywackes, and in turn, the ore bearing dolomites. Above the host unit there is a strong quartzite, and then more shales. The sequence is tightly and complexly folded, though only poorly metamorphosed. Dips are generally between 50° and vertical.
Mineralisation
The Zawar orebodies consist of fracture fill lodes, stringers and veins with some disseminations, arranged in en echelon, lenticular bodies which make an acute angle with the bedding, both in plan and in section. The long axes of the ore lenses trend north-west at Mochia and NNW at Balaria. The dip of the ore lenses is to the south or SW, contrary to the dip of the host sediments. The ore lenses plunge at 30° to 60° W at Mochia where the ore zone is 600 m long and had been tested to a depth of approximately 400 m in 1979. The host dolomite has a maximum width of 150 m, while individual lenses vary from 2 to 40 m in width, and may be up to 200 m in length. The ore lenses are separated by barren gaps along strike that range from 200 to 1 km in length.
Mineralisation at Mochia is controlled by both shearing and lithology. On the 5th level for instance, it has been noted that the lode strikes east-west and dips steeply to the north. At this level it essentially fills the entire Main Shear and is around 2 m thick. Numerous oblique off-shoots are common, parallel to tension shears and openings. Subsidiary lodes are found in the Footwall Shear, while minor mineralisation occurs along the numerous cleavage slip planes. Mineralisation is confined to the lower half of the Main Dolomite, except where shears continue into the footwall phyllites. At the eastern end of the lode, mineralisation is largely massive galena and sphalerite, although eastward the shear passes from pure dolomite into siliceous dolomite and the galena decreases, sphalerite increase and the sulphides become less massive and more vein like. Pyrite is common in the carbonaceous dolomite, but is only a minor constituent in the massive galena-sphalerite ore. Accessory arsenopyrite, chalcopyrite and cubanite have been reported.
At Balaria the mineralisation is also largely controlled by stratigraphy and structure. The Main Dolomite is bounded by a hangingwall and a footwall phyllite, and is subdivided by two internal phyllites into three zones. Mineralisation is confined to the upper two dolomite sub-divisions over a total strike length of 1600 m. In the lower of the two mineralised sections there are 24 en echelon ore lenses of sphalerite and pyrite which trend obliquely to the foliation. They have sharp physical boundaries. Within these lenses mineralisation varies from massive to fracture filling stringers and disseminations. Banding of ore and dolomite is characteristic, reflecting close spaced shear planes. Pyrite is often replaced by sphalerite. The upper mineralised zone in the Main Dolomite is principally pyrite and galena in lenses sub-parallel to the regional strike.
The common ore minerals in decreasing order of abundance are, sphalerite, galena [both a Ag rich and a normal variety], pyrite, pyrrhotite, arsenopyrite and chalcopyrite. In addition to these minerals, native silver occurs in the form of thin films in dolomites, while some native copper has also been reported (Singh, 1988). The sphalerite is a marmatite with 2 to 8% Fe and is dominant along thin fractures, or in patches of breccia fill. Galena is found sporadically as slugs, while pyrite is rare at Mochia, but forms up to 5% of the ore at Balaria. The gangue is mainly dolomite with a little chlorite (Mackenzie, 1979).
Galena and sphalerite generally occur as coarse granular aggregates. Post-crystallisation deformation has been noted in sphalerite and galena in samples examined by Singh (1988). Pyrite is found as both porphyroblastic grains (Singh, 1988) and as three diagenetic forms (Chauhan, 1984). The diagenetic stages result in: 1) An early accumulation of very small [2 to 12 µm] pyrite grains which form in fine, dominantly pyrite laminae [mainly as cubes, then pyritohedra and octahedra], as well as in distinct framboids. These forms generally have no associated sphalerite or galena; 2) A second stage when aggregates of cubic pyrite begin to form, and fine pyrite cements cracks in framboids. Sphalerite first appears in minor amounts in this stage, as irregular, possibly interstitial grains, and as fine grains within or close to framboids; 3) As larger accumulations of coarser pyrite, and as over-growths joining adjacent framboids. This growth leads to thicker bands of pyrite being formed in the host. The majority of the sphalerite crystallises in this stage, while galena makes its first appearance, and crystallises late in the stage. Both galena and sphalerite are commonly concentrated in the pressure shadows of pyrite framboids. At the same time, galena and sphalerite which is dispersed in the intergranular spaces of the host silicate minerals, grew into irregular, elongate grains parallel to the overall layered structure of the metallic sulphides. Sphalerite and galena become abundant and dominate among the layered sulphides when pyrite assumes a coarsely crystalline nature (Chauhan, 1984).
It has been observed that massive pyrite framboids or groups of framboids, are in many places, separated by tectonic shattering and cemented by introduced sphalerite. The relict pyrite fragments are encased by skeletal frameworks of silver. Remobilisation of ore is interpreted in all of the Zawar orebodies, ranging from diagenetic, to solid-state, to fluid-state movement (Mukherjee and Sen, 1980). Pyrrhotite and arsenopyrite occur in association with pyrite, while chalcopyrite is occasionally found as small grains in sphalerite (Singh, 1988).
A large number of minor elements are recorded in galena and sphalerite of which Ag and Cd are recovered during smelting. The average content of these elements in the hinge zones at Zawarmala is 44 g/t Ag and 145 ppm Cd (Singh, 1988).
Within the central section of the Zawarmala orebody the ore zones have been classified into three categories, namely:
1) Hinge Zone Orebodies - where three 'major mineable' lenses have been outlined. All are confined to the dolomite and quartzose-dolomite beds and are bounded by the graphitic phyllite to the north and by interbedded quartzite and dolomite to the south. Veins are conformable with the S0/S1 cleavage and to S2. Many veins are forked. The S0/S1 veins commonly display pinch and swell structures. The thickest developments are along the intersection of the two sets. Individual veins, or zones of veining, in plan view, appear to have thicknesses of <5 up to >20 m (Singh, 1988). There is a variation in the mineralogy in the hinge, with pyrite increasing from east to west. Sphalerite and galena dominate in the eastern part of the hinge zone, and pyrite to the west.
2) East Limb Orebodies - which are composed predominantly of sphalerite and galena, with pyrite being rare. In the central section of Zawarmala the east limb has a strike length of 1000 m, and is made up of several shorter lenses orientated in a sinistral, en echelon pattern
3) West Limb Orebodies - which have the richest accumulations of pyrite, with galena and sphalerite being subordinate. These orebodies are generally parallel to the S0/S1 cleavage pattern, in contrast to the East Limb, with a dextral pattern of distribution of ore veins.
The distribution of mineralisation varies with cut-off grade. Mineralisation above 3% Pb+Zn forms long, irregular 'shredded' patterns, while at a 7% cutoff, smaller lensoid, drop like areas are defined in plan view (Singh, 1988).
In summary it seems that the ore sulphides at Zawarmala are strictly stratabound within a favourable lithology. There is evidence of diagenetic emplacement of sulphide in bands parallel to lithological trends. This mineralisation has increased in intensity with diagenesis and has subsequently been tectonically remobilised into veins and vein zones.
For detail consult the reference(s) listed.
The most recent source geological information used to prepare this decription was dated: 1996.
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
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Selected References:
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Talluri J K, Pandalai H S, Jadhav G N 2000 - Fluid chemistry and depositional mechanism of the epigenetic, discordant ores of the Proterozoic, Carbonate-hosted, Zawarmala Pb-Zn deposit, Udaipur district, India: in Econ. Geol. v95 pp 1505-1525
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Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge. It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published. While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
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