Introduction:

Lead-Zinc deposits in India are localized mainly in the Precambrian formations of the Peninsular Shield and to a smaller extent in the lesser Himalayas. The important economically viable deposits are located in two main regions of India viz., Western Region and Southern Region with some scattered deposits in other parts such as North and Eastern Regions.

Western Deposits:

The Western Region comprises Rajasthan and Gujarat States. Copper-lead-zinc mineralization occurs as bi-and multi-metal deposits and 95% of National Ore Resource is confined to this region. There are two distinct metallogenic provinces; the north-eastern parts characterized by predominantly copper rich province, confined to the rocks of Delhi Super group, whereas in South Rajasthan and North Gujarat, the mineralization is chiefly lead-zinc ores with subordinate copper in the rocks of Pre- Aravalli, Aravalli and Delhi Super group (Table 1).

Bhilwara Super group:

Rajpura-Dariba-Bethumni Belt:

The Rajpura-Dariba-Bethumani belt is considered to form the western limb of Banera-Bhinder synform extending from Bhinder in the south to Banera in the north over a distance of about 130km.

The Dariba-Bethumani belt, extending from Bethumani in the north and Dariba in the south is composed of a group of folded met sedimentary rocks belonging to the Bhilwara Super group of pre-Aravalli age. Numerous old workings, gossans and ferruginous breccias have been recorded in the southern part of the belt in the Rajpura-Dariba area. The ore deposit is strata bound and is enclosed in a sequence comprising metamorphic equivalents of ortho-quartzite, carbonates and carbonaceous faces which are flanked by a thick monotonous sequence of meta-argillites. Rajpura-Dariba ore body owes its importance to its multi-metal sulphide-sulphosalt associations within the ores.

Zinc is dominant metal followed by lead and copper. The important trace metals are cadmium and silver. The sulphide ores at Dariba mine show conspicuous mineralogical zoning. Copper, lead-zinc and iron rich zones appear successively from the footwall to hanging wall. Two important lodes viz., Main and East lode, separated by a parting of 150 m, occur at Rajpura-Dariba. The main lode is further divided into South lode and North lode.

Rajpura-Dariba has been developed as a 3000 TPD underground mine with matching beneficiation facilities. The other less significant deposits are Mokhanpura North and South, Sindesar Kalan (East) and Malikhera block which are low in metal content and hosted by graphitic schist.

HZL is also exploring at Bamnia Kalan, which is 14 km north of Rajpura-Dariba mine, where the lead-zinc mineralization occurs in calc-silicate/dolomite and garnet bearing carbonaceous schist. Two distinct zones of mineralization viz., Main Zone and West Zone occur in this area. In the Main Zone, ore lenses are of variable dimensions, disposed in encephalon pattern with dextral overlap in sympathy with complicated fold pattern.

Rampura Agucha Lead-Zinc Deposit:

Introduction

The Rampura Agucha lead-zinc deposit is located 15 km southeast of Gulabpura in the Bhilwara district, Rajasthan. The village Rampura, originally located adjoining the western flank of the deposit was rehabilitated in 1990, prior to the commencement of open pit operation and Agucha village is about 1.5 km southwest of the deposit. The proximity of these two village led to the naming of it's as" Rampura-Agucha deposit". Since its discovery in 1977, the deposit has attained significance because of its large potential and geological setting not known to be conducive for mineralization. The deposits forms a part of pre-Aravalli Banded Gneissic Complex consisting of gneisses, schist and intrusive of acidic and basic igneous rocks that occupy, predominantly, the southeastern plains of Ajmer and Bhilwara.

Discovery and History of Exploration:

Garnet is found in almost all rock types around Rampura-Agucha deposit. In connection with an assessment of the potential of semi-precious and abrasive varieties of garnet of the area, T.C. Rampuria of Directorate of Mines and Geology, Government of Rajasthan (DMG), observed multicolored gossan bands, slag dumps and a linear shallow depression in the vicinity, in August, 1977. Preliminary geological, geophysical and geochemical surveys were made by DMG Rajasthan in 1978 and 1979, which resulted in indications of good potential and surface drilling in May, 1979.

Such indications attracted the attention of Hindustan Zinc Limited (HZL), which proposed detailed exploration through 15,500 m of surface drilling and 500 m of underground development, which was approved in February 1980, at estimated expenditure of Rs. 12.5 million.

Geology of the Deposit

Since a greater part of the area is capped with soil cover and fresh rock exposures are scanty, much of the information for detailed geology was gathered from the drill cores. The rock units show NE-SW strike with steep dips in hanging wall (75°-80°) and moderate dips in footwall (60°-65°) towards south-eats and plunges towards NNE. The sequence of rocks, from hanging wall to footwall, can be broadly grouped as under:

1. Garnet-biotite-sillimanite gneiss with intermittent bands of calc-granulites, amphibolites and aplites/ pegmatite's

2. Garnet-mica-sillimanite gneiss/ schist

3. Garnet-biotite-sillimanite gneiss with lenses of quartzo-felsphathic bands, amphibolites, pegmatite's and aplites

4. Granite gneiss, and

5. Mylonitic rocks.

Ore Zone

Economic mineralization is predominantly in graphite-mica-sillimanite gneiss / schist over a strike length of 1550m. The ore zone has a sharp contact with the hanging wall and footwall. The hanging wall side of the lode is the richest and also wide, followed by comparatively lean grade in the middle and a narrow, rich, footwall zone. Coarse-grained crystalline galena, associated with pyrite and pyrrohitite are seen in the hanging wall rocks. The mineralization's in hanging wall and footwall contacts is invariably fine to coarse-grained, and is made up of sphalerite and galena with numerous inclusions or rounded to sub-rounded discrete grains of feldspar, quartz, hornblende, sillimanite and dark green chlorite.

Attitude of the ore zone:

The general strike of the ore zone is parallel to the enclosing rocks, which is roughly NE-SW. The dip of the ore zone varies along the strike and depth. Near the surface, the dip of the hanging wall contact is steeper (75° to 80°SE) as compared to footwall contact (about 60° SE). In depth, dips of both the hanging and footwall contacts show a tendency to flatten. In general, the dip varies from 50° to 80°. The ore zone shows a variation in width, both along the strike and dip. The width of the ore zone gradually widens to about 95 to 100m between latitudes S-200 and S-400.

Mineralogy of the Ore Body

The graphite-mica-sillimanite schist hosts the economic mineralizations in Rampura-Agucha deposit. The economic minerals in the order of decreasing abundance along with their ranges of modal percentages are: sphalerite (15-20%), pyrite (15-18%), pyrrhotite (12-14%), galena (1-2%) and sulphosalts (0.1-0.2%). Graphite is ubiquitous in the ore body (7-10%). The gangue minerals that amount up to 45% to 50% of the ore zone constitute quartz, feldspar (orthoclase, plagioclase), various micaceous minerals (sericite, chlorite, and biotite), sillimanite with significant amount of garnets, amphibole, pyroxene, rutile, apatite etc.

Mining

The Rampura Agucha deposit at comparatively shallow depth is ideally suited for open pit mining using conventional equipment. The ore body is comparatively narrower and richer in grade in the northern part and wider in the southern. Thus open pit operation can go deeper only in the southern part.

Aravalli Super group

Zawar Lead-Zinc Belt

The Archaean basement comprising of gneiss, schist, amphibolite, quartzite and granite dating back to 3.2 to 2.5 by. showing unconformable relationship with the Aravalli cover rocks, is clearly marked in and around Udaipur. Stratigraphic succession, established by Roy et.al., (1984,b) for the Aravalli Super group of the type area around Udaipur and Zawar show two major groups separated by an unconformity. The Upper Aravalli Group consists of greywacke-slate-phyllite, quartzite, dolomite and silty arenite (host for sulphides of zinc and lead) while carbonaceous and pelitic phyllites, dolomite, quartzite, stromatolyte, phosphorite, chlorite schist, amphibolites, quartz arenite and local conglomerate belong to Lower Aravalli Group.

In general, Aravalli rocks in Udaipur-Zawar region show a low-grade metamorphism. The recrystallisation of the silicate minerals suggests the grade of metamorphism to be of greenschist facies. A few dyke-like bodies, intrusive in Aravalli rocks of Zawar, occur as totally unreformed.

The present structural disposition of Zawar area is the manifestation of two distinct major periods of tectonic cycles, each of which was characterized by intense folding and faulting. In Zawar area, the north and south limb of cross fold is represented by Mochia, Balaria and Bowa, whereas Baroi and Zawarmala represent the north-south trending and northerly plunging first generation fold system.

The mineralization occurs as sheated zones, veins stringers and disseminations, forming lenticular bodies arranged in overlapping enechelon pattern. The individual ore shoots persist along strike between 50 and 500 m, dip between 50° and vertical and plunge between 30° and 60° west or north. The ore body varies in width between 1 and 40 meters.

The mineralization is restricted solely within dolomitic horizon along with the structural control, but regional stratigraphic and litho logical control is also evident. The main sulphide minerals are sphalerite, pyrite and galena. Chalcopyrite, pyrrhotite and arsenopyrite are the associated minerals. Appreciable amount of silver and cadmium occur within the ore minerals.

Zawar belt has been the oldest centre of lead-zinc production in the world. The producing deposits are- Mochia, Balaria, Zawarmala and Baroi. Zawar group of mines has a capacity of 4000 TPD from underground mines with matching beneficiation facility and infrastructure.