DEPOSITS RELATED TO CHEMICAL SEDIMENTATION. PART 1: BANDED IRON FORMATION, PHANEROZOIC IRONSTONES AND BOG IRON ORES 1. Details of Module Subject Name Geology Paper Name ECONOMIC GEOLOGY & MINERAL RESOURCES OF INDIA Module Name/Title DEPOSITS RELATED TO CHEMICAL SEDIMENTATION. PART 1: BANDED IRON FORMATION, PHANEROZOIC IRONSTONES AND BOG IRON ORES Module Id GEL-05-148 Pre-requisites Before learning this module, the users should be aware of: Sedimentary environment Geology of Iron formations Palaeoclimatic aspects Objectives To understand: Iron ore formation worldwide. Facies and genetic types of iron ore deposits. Deposition of iron formation. Keywords Iron ores, facies of iron ores, genesis of iron ores,type of iron ores. 2. Structure of the Module-as Outline: Table of Contents only (topics covered with their sub-topics) 1. Introduction 2. Banded Iron Formation 2.1 Oxide facies 2.2 Carbonate facies 2.3 Silicate facies 2.4 Sulfide facies 2.5 Types of Iron formations: Algoma type, Superior type; Rapian type 2.6 Genetic aspects 2.7 Some examples 3. Phanerozoic ironstones 3.1 Clinton type 3.2 Minette type 4. Bog iron ores 3.0 Development Team: Role Name Affiliation National Co-ordinator Subject Co-ordinators Prof. M.S. Sethumadhav Centre for Advanced Studies (e-mail: Prof. D. Nagaraju Dept of Earth Science [email protected]) Prof. B. Suresh University of Mysore, Mysore-6 Paper Co-ordinator Prof. M.S. Sethumadhav Centre for Advanced Studies Dept of Earth Science University of Mysore, Mysore-6 Content Writer/Author(CW) Prof. M.S. Sethumadhav Centre for Advanced Studies Dept of Earth Science University of Mysore, Mysore-6 Content Reviewer (CR) Prof. A. Balasubramaian Centre for Advanced Studies Dept of Earth Science University of Mysore, Mysore-6 1. INTRODUCTION About 95% of the world’s “reserve base” of iron ores, estimated at tons, occur in sedimentary ores [chemical sedimentary (74%) and volcano-sedimentary (19.4%)]. A meagre ~5% belongs to all other categories combined, including magmatic, epigenetic- hydrothermal, metamorphogenic and residual concentration types and iron stones and bog iron ores (Veizer et al., 1989). Five super large Precambrian iron ore districts, viz., the Hamersley Basin (Australia), the Transvaal-Griquatown region (S. Africa), the Minas Gerias (Brazil), the Labrador Trough region (Canada), and the Krivoy-Rog-Kursk Magnetic Anomaly region (Ukraine) together make up ~90% of the world’s total sedimentary iron ore resources. Other important occurrences are the Damara Belt (Namibia), the Yilgran Block (Australia), the Northern Basin (W. Australia), the Iron Ore Group and Dharwar Supergroup (India), the Lake Superior Province (USA), the Belozyorsky-Komsky region (Ukraine), the Michipicoten (Canada), the Intaka Complex (Venezuela) and the Liberia-Sierra Leone- Guinea- Ivory Coast Belt. In India the “reserve base” stands at tons and it ranks sixth in the world’s iron ore resources. Virtually all Indian ore production (57 million tons in 1991) came from Precambrian deposits in Goa, Karnataka, Madhya Pradesh, Bihar and Orissa states. Iron ores can be grouped into (a) Banded Iron Formation (BIF), (b) non-cherty ironstones and (c) Bog iron ores. The BIF are known in different continents under terms Banded hematite quartzite, Banded magnetite quartzite, itabirite, jaspilite, taconite etc. BIF is classified into three types: Algoma type, Lake Superior type and Rapitan type Non-cherty iron stones are classified into two types: Clinton type and Minette type. The BIF ores are dominantly Precambrian. In fact, the great bulk of the banded iron formations of the world was laid down in a very short time interval of 2500 – 1900 Ma ago (James and Trendall 1982). The amount of iron laid down during this period, and still preserved, is enormous- at least tons and possibly tons. Among the BIF subgroups the submarine volcanic-associated “Algoma type” BIF deposits were dominantly Archaean, though younger Algoma type iron ores are also known. The carbonate-orthoquartzite associated shelf/marginal basin deposits of the Lake Superior type BIF were almost exclusively of lower Proterozoic age. Rapitan type continental margin BIF deposits in glaciogenic sequences are essentially of upper Proterozoic age. The non-cherty ironstones of both Clinton and Minette types are exclusively Phanerozoic. The bog iron ores were formed on land from carboniferous to Recent times. 2. BANDED IRON FORMATION (BIF) BIF is characterized by its fine layering. The layers are generally 0.5-3 cm thick and inturn they are commonly laminated on a scale of millimetres or fractions of a millimetre. The layering consists of silica layers (in the form of chert or better crystallized silica) alternating with layers of iron minerals. The simplest and commonest BIF consists of alternate hematite and chert layers. James (1954) identified four important facies of BIF. 2.1 Oxide facies: This is the most important facies and it can be divided into the hematite and magnetite subfacies according to the dominant iron oxide. There is a complete gradation between the two subfacies. Hematite in least altered BIF takes the form of fine-grained grey or bluish specularite. Oolitic texture is common in some examples, suggesting a shallow water origin, but in others the hematite may have the form of structureless granules. Carbonates (calcite, dolomite and ankerite rather than siderite) may be present. The “chert” varies from fine-grained cryptocrystalline material to mosaics of intergrown quartz grains. In the much less common magnetite subfacies, layers of magnetite alternate with iron silicate or carbonate and cherty layers. Oxide facies BIF typically averages 30 – 35% Fe and these rocks are commercially viable provided they are amenable to beneficiation by magnetic or gravity separation of the iron minerals. 2.2 Carbonate facies: This commonly consist of interbanded chert and siderite in roughly equal proportions. It may grade through magnetite-siderite-quartz rock into the oxide facies, or, by the addition of pyrite, may grade into the sulphide facies. The siderite lacks oolitic or granular texture and appears to have accumulated as a fine mud below the level of wave action. 2.3 Silicate facies: Iron silicate minerals are generally associated with magnetite, siderite and chert which form layers alternating with each other. This mineralogy suggests that the silicate facies formed in an environment common to parts of the oxide and carbonates facies. However, of all the facies of BIF, the depositional environment for silicates is least understood. This is principally because of the number and complexity of the minerals and the fact that primary iron silicates are difficult to distinguish from low ranking metamorphic iron silicates. Probable primary iron silicates include greenalite, chamosite and glauconite, some minnesotaite and probably stilpnomelane. Most of the iron in these minerals is in the ferrous rather than the ferric state, which, like the presence of siderite, suggests a reducing environment. PCO2 may be important, a high value leading to siderite deposition, a lower one leading to iron silicate formation (Gross 1970). Carbonate and silicate facies BIF typically contain 25 – 30% Fe, which is too low to be of economic interest. They also present beneficiation problems. 2.4 Sulfide facies: This consists of pyritic carbonaceous argillites and occur as thinly banded rocks with organic matter plus carbon making up to 7 – 8 %. The main sulfide is pyrite which can be so fine-grained that its presence may be overlooked in hand specimens unless the rock is polished. The normal pyrite content is around 37 %, and the banding results from the concentration of pyrite into certain layers. This facies clearly formed under anaerobic conditions. Its high sulfur content precludes its exploitation as an iron ore; however, its has been mined until recently for its sulfur content at Chvaletice in Czechoslovakia. 2.4 Types of Banded iron formations Two distinct types, viz., The Algoma and the Superior type BIF, came to be recognised (Gross 1970) and the identity of the third (the Rapitan type) group as a distinct variety was also established (Trendall 1973; Dorr 1973). 2.4.1 Algoma type: This type is encountered in Archaean high-grade terrains and widespread development of it is in Archaean greenstone belts. It also occurs in younger rocks including the Phanerozoic. It shows a greywacke-volcanic association suggesting a geosynclinal environment and the oxide, carbonate and sulfide facies are present, with iron silicates often appearing in the carbonate facies. Algoma-type BIF generally ranges from a few centimetres to a hundred or so meters in thickness and is rarely more than a few kilometres in strike length. Exceptions to this observation occur in Western Australia where Late Archaean deposits of economic importance extending to several kms are found. In the Algoma type BIF, oolitic and granular textures are absent or inconspicuous and the typical texture is a streaky lamination. A close relationship in time and space to volcanic rocks hints at a volcanic source of the iron and many investigators regard the deposits of this type as being exhalative in origin, e.g., Fralick et al. (1989). Goodwin (1973) in a study of the Algoma type BIF in the Canadian Shield showed that facies analysis was a powerful tool in elucidating the palaeogeography and could be used to outline a large number of Archaean basins. His section across the Michipicolin Basin is shown if Fig. 1. The Algoma