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Rajpura Dariba |
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Rajasthan, India |
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Zn Pb
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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All papers now Open Access.
Available as Full Text for direct download or on request. |
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The Rajpura-Dariba zinc, lead and copper deposit is located some 86 km to the north-east of Udaipur in Rajasthan at approximately 24° 57'N, 74° 08'E. It is approximately 110 km to the south-west of the Rampura-Agucha Zn-Pb mine, and around 100 km to the north-east of the Zawar group of mines. The area lies within a plain, with isolated low ridges which protrude from the sandy cultivated land. The mineralised zone has a yellowish-brown gossan and forms a conspicuous physiographic unit.
Ancient workings, including 'large pits and deep shafts', are evident over a strike length of 4.8 km, from the village of Dariba in the south, to the village of Rajpura in the north, and again at Bethumi some 8 km to the NNE of Rajpura.
Published reserve and production figures include:
25 Mt @ 7% Zn, 1.9% Pb, 60 g/t Ag (Res., 1979),
19.82 Mt @ 7% Zn, 2% Pb (Prov. Res., @ 2% Zn+Pb cutoff, 1984, Jain & Rakesh, 1984),
55.5 Mt @ 0.15% Cu, 2.04% Zn, 2.79% Pb, 113.9 g/t Ag, 0.28 g/t Au (Geol. Resource,
Nat. Res. Canada website, 2006)
The ore contains around 6% barite, 570 ppm As, 200 ppm Sb, 2 ppm Hg and 680 ppm Cd.
Geology
The host rocks at Rajpura-Dariba are variously interpreted as belonging to the pre-Aravalli Bhilwara Supergroup and to the Aravalli Supergroup. The ore deposits are contained within a synformal inlier of meta-sediments enclosed by the much more strongly metamorphosed Banded Gneissic Complex (BGC) of the Bhilwara Supergroup. The BGC is represented by the Mangalwar Complex in the Rajpura-Dariba area. Some workers consider these meta-sediments to be relicts of the original Bhilwara sediments that have not been migmatised to form the BGC, while others interpret them to be structurally included inliers of Aravalli Supergroup sediments (Roonwal & Lowhim, 1986). Gupta et al., (1980) map several synformal inliers of upper Bhilwara Supergroup, post BGC sediments, the Rajpura-Dariba Group, in two main, and a number of subsidiary keels which have a cumulative length of 75 km. The northern of these two zones, which trends in a general north-south to NNE-SSW direction, is some 30 km long, and contains the Rajpura-Dariba and Bethumi mineralisation.
If Gupta et al., (1980) are correct, the Rajpura-Dariba Group would be pre-Aravalli Supergroup, but post Sandmata and Mangalwar Complex in age. As such then, the Rajpura-Dariba host rocks would be younger than those at Rampura-Agucha, but older than those at Zawar. The majority of the literature favours a pre-Aravalli age for the Rajpura-Dariba host.
Gupta et al., (1980) describe the Rajpura-Dariba Group as dolomitic marble, graphitic kyanite-staurolite-mica schist, bedded chert, quartzite, calc-biotite schist, actinolite schist and garnetiferous-biotite schist. According to Roonwal & Lowhim, (1986) these sediments have been metamorphosed to epidote-amphibolite to amphibolite facies grades, while Chauhan (1977) states that the sequence has apparently been metamorphosed to grades between a quartz-albite-biotite assemblage of the greenschist facies, and staurolite-almandine of the amphibolite facies.
Approximately 70 to 80% of the bedrock of the Rajpura-Dariba synform is covered by soil (Raja Rao et al., 1972). According to Jain & Rakesh, (1984) the Pre-cambrian sequence below the soil cover within the synform comprises the following, commencing from the western limb, to the core of the structure, a distance of around 12 km:
Dolomitic marble which is shown as lensing out to both the north and south;
Calc-schists and para-amphibolite. The amphibolite appears to represent tremolite rich calc-silicates found within the sequence;
Graphitic and sulphide-bearing mica-schist, with local sulphidic calc-silicate marble/chert lenses at the lower contact. This unit hosts the ore;
Quartzite/chert interbanded with graphitic mica schist in the core of the synform.
Roonwal & Lowhim, (1986) do not differentiate the two lower units on their map, namely the dolomitic marble and the calc-schists and para-amphibolite, but instead map the interval as calc-quartz-biotite schist, with large transgressive zones of ferruginous breccia.
According to Chauhan (1977), the chief lithologies within this broader sequence, that are exposed along the Dariba Main Lode, from the footwall to hangingwall are: i). Biotite-muscovite schist, which is occasionally garnetiferous and has thin bands of intercalated dolomite; ii). Siliceous dolomite and calc-silicates; iii). Ferruginous quartzite; iv). Graphitic mica-schist; v). Dolomitic marble and calc-silicates; vi). Quartzites, usually with intercalated chert bands; vii). Calc-silicates; viii). Graphitic mica-schist; and ix). Graphitic staurolite-kyanite schist. It appears that all are probably within the 'graphitic and sulphide-bearing mica-schist' unit detailed by both Jain & Rakesh, (1984) and Roonwal & Lowhim, (1986), as listed above.
Roonwal & Lowhim (1986) however, list the immediate host stratigraphic sequence, again apparently from the lower sections of the graphitic and sulphide-bearing mica-schist¹ unit, as comprising; i). Kotri Mica Schist; ii). Mokhanpura Biotite Schist; iii). Dariba Graphite Mica Schist; iv). Calcareous Biotite Schist; v). Malikhera Dolomite; and vi). Anjana Muscovite Schist. The mineralisation is apparently largely within the upper sections of the Calcareous Biotite Schist.
The mineralisation lies on the east dipping western limb of the main synformal structure, dipping at 65 to 80° (Raja Rao et al., 1972; Roonwal & Lowhim 1986).
The immediate host lithologies are mainly graphite-mica schist and calcareous biotite schist, with intercalations of quartzite, siliceous dolomite/calc-silicate and amphibolite (Jain & Rakesh, 1984). According to Chauhan (1977), the principal host rocks are:
Graphitic mica-schist and kyanite-staurolite schist - containing mostly pyrite with some sphalerite, although in zones, as described below, the ore is also within the graphitic mica-schist, above and/or below the mineralised siliceous zones (Chauhan 1977). According to Raja Rao et al., (1972), the graphitic-schists are composed of graphite, talc, sericite, kyanite and staurolite and occur as bands in the central part of the area from Dariba, north-wards to Bethumi. It appears that the mica-schists are calcareous adjacent to the dolomites, with which they have a narrow gradational contact (Roonwal & Lowhim, 1986).
Banded quartzite-cherts - which contain the major zinc mineralisation, present as sphalerite, usually associated with pyrite and occasionally galena (Chauhan 1977). According to Raja Rao et al., (1972), the ortho-quartzite is brown, well banded and slabby, and contains partings of graphite schist. It is generally well jointed, with the joints being filled by milky quartz in many places. One continuous band is traced from Dariba to Bethumi and beyond, attaining a 'considerable' thickness in some sections. Some sections of the quartzite are garnetiferous. Black meta-cherts are described from the main ore zone also (Deb & Bhattacharya, 1980). While some authors refer to the quartzites and cherts as being the main host to ore, others place the majority of the ore in recrystallised siliceous dolomite. It appears the two are the same, and the dolomitic marble/calc-silicates described below are less siliceous.
Dolomitic marble/calc-silicates - which mainly contain chalcopyrite and galena, and occasionally some zones of sphalerite (Chauhan 1977). According to Raja Rao et al., (1972), the dolomites are fine grained, compact and pale yellow to buff in colour, occurring as wide bands on either side of the quartzitic rocks. The calc-silicate bands within the dolomite comprise biotite, chlorite, tremolite and actinolite, and have been locally silicified. Thin bands of garnet-rich granulitic rocks run parallel to the fault zone between Dariba and Rajpura. It consists of 70% fractured brown garnet, accompanied by siderite, staurolite and minor graphite. A band of calc-silicate within the same fault is mineralised. In the major mineralised areas the lower carbonatic unit comprises a calcareous quartz-biotite-schist in the footwall to the ore. This schist carries numerous euhedral disseminated hematite grains, which are apparently not derived from the biotite of the schist as it shows no relics of alteration (Roonwal & Lowhim, 1986). Where the sulphides of the orebody occur, there is an upward gradational change of the calcareous-biotite schist to grey quartzite. Adjacent to the mineralised zone the biotite schist is impregnated with graphite, with an associated assemblage of pyrrhotite, pyrite and magnetite. The major mineralised zone shows the following mineral assemblage transition from the calcareous-biotite-schist in the footwall to the cherty-quartzite of the hangingwall: i). Diopside-tremolite-calcite; ii). Calcite-dolomite-barite-tremolite, with traces of celestite; iii). Tremolite-dolomite-calcite; and then iv). Grey quartzite (Roonwal & Lowhim, 1986).
The mineralised zone appears to be disposed along a fault structure. According to Raja Rao et al., (1972), the 'extensive old workings, mine dumps, slag heaps and diagnostic gossan are found along a well defined strike fault that runs through high grade Pre-Cambrian metamorphic rocks from Dariba to Rajpura over a distance of 4.8 km ...'. This fault may in part define the western contact between the calc-schists unit and the mineralised graphitic mica-schist.
Mineralisation
The Rajpura-Dariba ore deposit is reflected as a gossan zone that outcrops sporadically over a distance of more than 5 km. Prominent zones of chert are seen on either side of the gossan in places, forming a string of low disconnected ridges that rise up to 80 m above the surrounding plains. The limonite gossans form depressions between the siliceous ridges. The width of the general gossan zone varies from a 2 or 3, up to 60 m. In other areas a broad, 500 m wide strip of graphitic schist contains numerous thin limonite bands, but no prominent gossan zone (Raja Rao et al., 1972).
The gossan shows dark brown, bright reddish-yellow, vermilion-red, brick-red, purple, yellow, pale-green, bluish-green, azure-blue, black, grey and white colours, indicating the presence of limonite, goethite, jarosite, jasper, malachite, azurite, turquoise, hematite, manganese oxides, clay minerals, etc., while siderite, covellite, chalcocite, chrysocolla, cerussite and smithsonite are also recorded. Cubic, cellular, honey-comb boxworks, and other voids are evident, as is dense limonite. Relict galena, pyrite and chalcopyrite are seen within the gossan. Early grid soil and gossan sampling returned values of up to 4000 ppm Pb and 3000 to 4000 ppm Zn in the Dariba area (Raja Rao, et al., 1972). Near surface gossan to the north grades up to 3% Pb, 0.1% Zn and 200 g/t Ag, with traces of Cu, Sb, As, Au, Hg and Mo.
Two principal lode systems have been identified, the Dariba Main Lode and the Dariba East Lode. Mineralisation extends over a strike length of 2.2 km in the former, and corresponds to the gossan and ancient workings at the surface. There are both barren sections and ore shoots within this strike interval of the Main Lode. From the south the first economic shoot is known as the South Lode. It has a strike length of 550 m and thickness of around 18 m. This is followed by a 300 m sub-economic zone, then the North Lode which is another 900 m long ore shoot that averages 14 m in thickness. These shoots make up the Rajpura A-block. The South and North Lodes had been proved to depths of 500 and 400 m respectively to 1984, and were open at depth (Jain & Rakesh, 1984). The South Lode shoot plunges steeply, at around 60, to the NNE (Chauhan, 1977; Jain & Rakesh, 1984). Other authors however, state that the South Lode bottoms at a depth of 300 m, both by narrowing and by a decrease in grade.
Further north again there is another development known as the Rajpura B-block. This latter zone has been deeply weathered with the gossan extending to depths of 300 to 400 m below the surface (Jain & Rakesh, 1984). In this area where the base of oxidation increases dramatically from around 40 m below surface, to about 300 m, some 34 mt of low grade gossan have been indicated. The overall Dariba Main Lode, constitutes a series of blocks, including the Rajpura A and B blocks, and has a strike extent of 3700 m (Deb & Bhattacharya, 1980).
The East Lode, which has a strike length of 600 m, is developed 200 m to the east of the South Lode, the southern shoot of the Dariba Main Lode. It is generally 19 m thick and in 1984 was drilled to a depth of 300 m, but had not been closed (Jain & Rakesh, 1984).
Each of these lodes pinches and swells along its length. The Dariba Main Lode varies between 1 and 47 m in width, while the East lode ranges from 2 to 35 m in thickness. The ores are generally parallel to both the lithological banding and the schistose foliation of the host rocks (Deb & Bhattacharya, 1980).
As described above the economic mineralisation is mainly hosted by the siliceous dolomite/calc-silicate rocks and the cherty quartzites, as well as locally, from 1 to 20 m of adjacent graphite-mica-schist. The main north and south lodes are essentially developed at the contact between dolomite/calc-silicate and graphitic/carbonaceous schist. The North and East Lodes are essentially Zn-Pb lodes, while appreciably higher Cu levels are known towards the footwall of the South Lode, making it essentially a Zn-Pb-Cu lode (Jain & Rakesh, 1984).
This Cu-rich footwall zone of the South Lode is represented by a 15 m wide zone of siliceous rock with splashy chalcopyrite, some tennantite, tetrahedrite, galena, sphalerite and a little fluorite.
The footwall of the South Lode is occupied by an Fe zone characterised by biotite schist which is impregnated with graphite, and has an associated assemblage of pyrrhotite, pyrite and magnetite. Schists lower in the sequence contain disseminated hematite (Roonwal & Lowhim, 1986).
Within the Cu rich section of the South Lode, the host is a recrystallised siliceous dolomite, with stringers, irregular massive patches and rarely concordant bands chalcopyrite, all with subsidiary galena and pyrite. The western contact of this zone is marked by a laminated¹ cherty quartzite. Towards the hangingwall the Cu-zone has a diffuse assay contact with a Pb-Zn zone within recrystallised siliceous dolomite characterised by galena, sphalerite and pyrite, with little or no chalcopyrite. Possible altered stromatolites are recognised locally within a band of finely laminated¹ calcareous rocks which are intimately associated with sphalerite-pyrite ores of the Zn-Pb zone. These stromatolitic bands are composed of a matrix of recrystallised diopside and dolomite with the interpreted stromatolite columns being composed of carbonaceous chert with fine carbonate laminae. Massive tetrahedrite-tennantite occurs sporadically within the zone as irregular patches. Towards the hangingwall of this zone, delicate bands of sphalerite and/or pyrite are intercalated with black carbonaceous cherts over widths of up to 20 m in places. Individual bands vary from 5 to 100 mm (Deb & Bhattacharya, 1980). Where sphalerite occurs within banded quartzite-chert it is present interstitially between the siliceous grains of the host, not as individual sphalerite bands. Galena within concordant bands occurs as small elongated grains or as aggregates of elongated grains, often with smooth outlines (Chauhan, 1977).
Towards the southern margin of the South Lode there is an elliptical patch of coarse diopside-rich rock, up to 75 m long, which is characterised by high Ag and As values in minerals such as geocronite and polybasite, together with massive tennantite-tetrahedrite and discordant veinlets of galena. There is a sharp contact between the Pb-Zn body in the hangingwall, with the Fe rich zone which occurs as pyrite and/or pyrrhotite interbanded with graphite-mica schist (Deb & Bhattacharya, 1980). Where banded pyrite occurs within graphite-mica-schist, the pyrite bands are generally 0.1 to 4 mm thick, rarely up to 1 cm. In the zinc zone, sphalerite bands of 2 to 10 mm alternate with layers of calc-silicates (Nair & Agarwal, 1976).
According to Roonwal & Lowhim, (1986), accumulations of barite as pods and lenses, are found in the hangingwall of the deposit. The details above suggest that the Zn-Pb orebodies have a halo on both the hangingwall and footwall of Fe as pyrite, pyrrhotite and/or magnetite, with a Cu rich zone in the footwall of the main Zn-Pb ore in one part of the overall lode zone.
Large discordant veins of galena and chalcopyrite are recorded in the footwall dolomitic marbles and calc-silicates. In addition, in the hangingwall calc-silicates there are accumulations of rare sulphosalts such as geocronite, boulangerite, tetrahedrite, tennantite, which only occur in minor amounts elsewhere in the orebody. These minerals occur in the inter-crystalline spaces of metamorphic minerals such as diopside, and represent an 'abnormal' accumulation of metals such as Sb, As, Ag and Hg (Chauhan, 1977).
The predominant sulphide minerals in the ores are sphalerite, galena, chalcopyrite, pyrite, pyrrhotite and tetrahedrite-tennantite. Minor minerals include arsenopyrite, cubanite, mackinawite, polybasite, owyheite, geocronite, argentopyrite, pentlandite and enargite(?), as well as magnetite, ilmenite and rutile. The major minerals tend to be concentrated in defined zones, while the minor constituents are scattered sporadically throughout (Deb & Bhattacharya, 1980). Sphalerite is the most abundant sulphide, followed by pyrite. Pyrite is found associated with all of the other minerals in the deposit (Chauhan, 1977). According to Roonwal & Lowhim, (1986), there are three common mineral associations at Rajpura-Dariba, namely: i). pyrite-pyrrhotite-magnetite; ii). chalcopyrite-galena-pyrrhotite; and iii). sphalerite-pyrite-galena.
Deformational fabrics are evident in the ore on all scales. These include: i). alignment of sulphides, mainly sphalerite, pyrite and pyrrhotite along strong penetrative schistosity, S1; ii). transposition of S1, along with the sulphides, into a well developed crenulation cleavage; iii). stretching of sulphide layers to form boudins; iv). meta-blastic growth of pyrite and arsenopyrite, with associated pressure shadows, and rounded porphyroblasts that are rotated within the enclosing schistosity; v). recrystallised granoblastic mosaics of softer sulphides enclosing clasts of more competent wall rocks and sulphides to form a 'durchbewegung' texture; and vi). the local mobilisation of the sulphides to form massive galena and chalcopyrite-rich discordant veins, healed breccias and fractures (Deb & Bhattacharya, 1980). In addition to these textures, some authors claim to recognise syn-sedimentary sulphide textures (Nair & Agarwal, 1976; Chauhan, 1977; and others).
Mesoscopic folds ranging from those with an amplitude of 1.5 m down to crenulations are observed in the sulphidic units, particularly the graphite-schist bearing pyrite bands. These folds are interpreted to represent flexural slip, flexural flow, shear and quasi-flexural folds. Within the chert hosts the sulphides have accommodated the strain. Cataclastic deformation is dominant in the layered sphalerite-quartzite-chert, where healed breccias are common. In these structures sphalerite and galena appear to be mobilised and recrystallised in spaces between clasts (Chauhan, 1977).
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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Mishra B, Upadhyay D and Bernhardt H-J 2006 - Metamorphism of the host and associated rocks at the RajpuraDariba massive sulfide deposit, Northwestern India : in J. of Asian Earth Sciences v26 pp 21-37
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Nair N G K and Agarwal N K, 1976 - Primary and secondary structures in the polymetallic ores of Rajpura-Dariba, Rajasthan, India: in Mineralium Deposita v.11 pp. 352-356
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