ЛИТОЛОГИЯ И ПОЛЕЗНЫЕ ИСКОПАЕМЫЕ, 2011, № 1, с. 36-46
УДК 551
Detrital type Manganese Ore Bodies in the Iron Ore Group of Rocks, Orissa, Eastern India
© 2011 B. K. Mohapatra, P. P. Mishra, and P. P. Singh*
Institute of Minerals and Materials Technology Bhubaneswar-751013;
E-mail: bkmohapatra@immt.res.in, p_geology@yahoo.co.in *P.G.Deptt. of Geology, Utkal University Bhubaneswar-751004;
E-mail: ppsgl_uu@rediffmail.com
Detrital type of manganese ore bodies in the Precambrian Iron Ore Group of rocks occur in Bonai-Keonjhar belt, Orissa besides stratiform (bedded type) and stratabound-replacement types of deposits. These ores appear in form of large boulders within lateritised aprons at various depths, often reaching beyond 30 m from the surface. Overprinting of primary structures, presence of mixed Fe-clasts and Mn-ooliths/pisoliths, mineral species of different generations and wide chemical variation amongst morphological varieties and from boulder to boulder are the characteristic hallmarks of such ore bodies. Features associated with ores occurring in different morphologies, namely: spongy, platy, recemented and massive varieties from a typical profile of Orahari Mn-ore body in Keonjhar district are described. Recemented variety may be further classified into sub-varieties such as conga, agglomerate and mangcrete. Common primary Fe-minerals are hematite, mar-tite with relict magnetite. The secondary Fe-Mn phases are goethite, specularite, cryptomelane, lithio-phorite, chalcophanite, manganite and pyrolusite.
These are ore bodies of allochthonous nature developed through a number of stages during terrain evolution and lateritisation. Secondary processes such as reworking of pre-existing crust through remobilisation & solution, precipitation & cementation and transport etc are responsible for the development of such detrital ore bodies in Bonai-Keonjhar belt of Eastern India.
Manganese ore bodies of Orissa, India are confined to three Precambrian stratigraphic horizons viz. Iron Ore Group, Gangpur Group and Khondalite Group. Over 25 MT of manganese ores are reported to occur in Bonai-Keonjhar belt of Iron Ore Group where active mining operations are going on in many areas like Joda, Roida, Kusumdih, Orahari, Kalimati, Mahul-suka, Dubna etc. Barring limited battery grade manganese ore, most of it is used in ferro-manganese production. Compared to the size of the belt, the literature available on the manganese ores of Bonai-Keonjhar belt seems to be limited. Broad geology and geo-economic aspects of this region have been dealt by E. Spencer [1948], B. Sen [1951], S. Ray [1954], Ray
[1955], B. Engineer [1956], G. Prasad Rao, Murty
[1956], D. Mookherjee [1966], N. Basu [1969], Mur-thy, B. Ghosh [1971] R. Mishra [1994] discussed the classification/mode of occurrence/genesis of some manganese ore deposits of this region. Mineralogical and geochemical studies though reported by S. Roy [1981], M. Mohapatra, T. Bagchi [1961], and M. Aj-mal [1990], are of local significance. Recently, P. Mishra et al. [2006] have classified manganese ore bodies of this region into three broad categories: viz. stratiform, stratabound-replacement and lateritoid types. This paper reports another category of ore bod-
ies from the area namely "Detritus manganese deposit (DMD)". Such ore bodies are similar to that of detrital iron ore deposit (DID) reported from Hamrsley basin of Western Australia [Ramanaidou et al., 2003; Butt et al., 2001].
As no precise report is available on the detritus manganese ores in Iron ore Group of rocks of Orissa, this paper describes the typical profile, ore petro-graphic (mineralogic-textural) and geochemical characteristics with a view to establishing the developmental history of DMD in Bonai-Keonjhar belt of Orissa, India.
GEOLOGICAL SETTING
The manganese-ore bodies in the Bonai-Keonjhar belt (Jamda-Koira valley) are confined to Shale Formation of Precambrian Iron Ore Group. Based on mode of occurrence, these ore bodies are classified into three categories: stratiform, stratabound (- replacement) and lateritoid types [Mishra et al., 2006]. The stratiform type shows distinct lamination or banding even on mesoscopic scale. The stratabound type is structure - and shear zone-controlled and is often silicified showing effects of replacement. The lateritoid type occurs as float. Generally each ore body over its effective area is more or less tabular and is capped by a lateritic hat of variable thickness. The manganese ores showing different texture and habit are chemically grouped under high, medium and low-grades. The ores are mostly associated with shale, mainly of kaolinitic composition. The ores, in general, show variable proportion of secondary oxy-hydroxide
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Fig. 1. Sketch map of horseshoe shaped iron ore belt (red colour) in part of Orissa, India, showing location of detritus type Mn-ore deposit.
Mn and Fe-phases, with minor clay minerals. A different kind of ore deposit, Detritus Manganese ore Deposit (DMD), is reported in this paper.
DETRITAL MANGANESE ORE BODIES
this paper. Such types of ore deposits are usually boul-dery in nature. Large drifted boulders are seen below 10 m depth (Fig. 2a). Small boulders are sometimes welded to form large one. Sometimes boulders appear sub-rounded to ovoidal (Fig. 2b).
Field characteristics. The manganese ore bodies of detritus type have limited depth persistency and mostly of low-grade types. Field, ore petrographic and other geochemical characteristics of a typical detrital deposit for example, Orahari near western limb of horseshoe belt, NW of Koira town (Fig. 1), is described in
Ore Morphology vs. Mineralogy. The ores from such type of Manganese deposits can be grouped broadly into four morphological varieties/types. The mineralogical and/ textural characteristics of these four types are detailed below:
Fig.2. A view of lateritoid type Mn-ore deposit:
a — The bouldery nature of ore body;
b — An enlarged view of an ovoidal boulder at 20 m depth.
1. Spongy Type. Strongly leached, it exhibits porous and cavernous structure. Locally honeycomb structure is seen due to growth of quartz crystals. Py-rolusite, lithiophorite and quartz are the major minerals present (Fig. 4) in spongy variety. The lithiophorite occurs as mosaic grains lining some vugs (Fig. 6e). Occasionally, chalcophanite needles encrusting vug are also noted.
2. Recemented Type. This is the major morphology shown by ores of DMD. This can be further classified into following 4 sub-types.
Recemented-I. This looks like canga. It is composed of clasts of primary iron ore (hard laminated), of size ranging from pebbles to cobbles that are cemented together by dense Mn-phases (Fig. 3a). The XRD pattern shown in Fig. 4 indicates the major mineral to be that of hematite.
Recemented-II. It is an agglomeration where elongated fragments of primary laminated iron ore and manganese pisoliths are cemented by secondary Mn-phases (Fig. 3c). The major minerals confirmed by XRD include hematite, goethite and cryptomelane (Fig. 4). This sub-variety is termed as agglomeratic variety in a non-genetic sense.
Recemented-III. A secondary conglomeratic mass, this may be termed as mangcrete. In such case primary Mn-ooliths, in varied shape and size, are cemented together to form mangcrete lithounit (Fig. 3c). The ma-
jor minerals recorded in XRD are pyrolusite and lithiophorite with minor illite (Fig. 4).
Recemented-IV. It is hard and compact showing pebble form. But closer observation clearly illustrates its recemented texture (Fig. 3d). Major minerals recorded are pyrolusite and cryptomelane (Fig. 4). The massive ore develops when primary Mn-ore minerals are replaced by dense secondary Mn-phases that overprints primary structure and texture.
Recemented ore-II and -III largely constitute primary ooliths and pisoliths. These oolitihs and pisoliths are flattened, round to oval shaped closely or partially filled by accretionary grains. Ooliths are relatively smaller grains (<2 mm) of single composition, either composed of cryptomelane (Fig. 5a) or goethite (Fig. 5b). Often Mn-ooliths ofvaried shape and sizes occur in clus-tures (Fig. 5c). The pisoliths are relatively larger grains (>2 mm) and composed of either layers of cryptom-elane / romanechite or hematite core with cryptom-elane encrustation (Fig. 5d). The pisoliths grains are concentrically banded and often show transverse / radial cracks (Fig. 5e). Two pisolith are sometimes found welded simulating an eye shaped structure and are further enclosed by cryptomelane of later generation (Fig. 5f). Occasionally, Mn-oolith is found to be traversed by manganese vein of younger generation (Fig. 5f).
Texturally, the recemented types under microscope look more or less alike. In recemented-I, laminated martite / hematite minerals are found enclosed within tetravalent manganese oxyhydroxides (Fig. 6a). The
Fig. 3. Megascopic view of different recemented ore types:
a — Cryptomelane fragments recemented together by secondary Mn-phase (cryptomelane); b — Recemented Ore-I (canga): Primary iron clasts, in different dimensions, enclosed by secondary manganese rich phase; c — Recemented Ore-II (agglomerate): Primary iron clasts and Mn-pisoliths, in different dimensions and shapes, enclosed by secondary manganese rich phase; d — Recemented Ore-III (mangcrete): Primary Mn-ooliths, in different dimensions and shapes, enclosed by secondary manganese rich phase.
primary magnetites are martitised to hematite with remnants of the former (Fig. 6b).
3. Platy: Cris-cross plates often develop due to strong leaching showing boxwork structure. Presence of cryptomelane, goethite and kaolinite (Fig. 4) is characteristic.
The primary iron ore invariably show lamination, occurring either as alternately laminated
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