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(Sedimentary Exhalative) Processes 3. Summary 1 1. Details of Module Subject Name Geology Paper Name ECONOMIC GEOLOGY & MINERAL RESOURCES OF INDIA Module Name/Title SOME MAJOR THEORIES OF ORE GENESIS PART 2: ORIGIN DUE TO SURFACE (EXOGENIC) PROCESSES Module Id GEL-05-143 Pre-requisites Before learning this module, the users should be aware of Origin of orthomagmatic deposits. Genesis of igneous iron ore. Origin of pegmatites. Objectives To understand: the mode of occurrence or orthomagmatic iron deposits associated with intermediate to felsic volcanic rocks. the mode of occurrence of pegmatites, their mineral wealth, depth- wise classification of pegmatites and diverse views on their origin. Keywords Felsic magmatism, orthomagmatic deposits, pegmatites. 2. Structure of the Module-as Outline: Table of Contents only (topics covered with their sub-topics) 1. Introduction 2. Volcanic-exhalative (sedimentary exhalative) processes 3. Summary 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. B. Krishna Rao Former Professor, Department of studies in Geology, 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 2 1. INTRODUCTION Exogenic (surface) processes involved in the development of mineral deposits include: i. Mechanical accumulation: This process involves concentration of heavy, durable minerals into placer deposits. Placer deposits are classified as (a) residual placers, (b) eluvial placers (concentration in a moving solid medium, generally formed upon hill slopes from mineral released from a nearby source rock), (c) stream or alluvial placers, beach placers and offshore placers (all the three are the concentration in a moving water medium), and (d) Aeolian placers (concentration by air). Typical examples of Placer deposits are: Rutile and zircon sands of New South Wales, Australia and Trail Ridge, USA; Tin placers of Malaysia; Gold placers of the Yukon, Canada; Precambrian palaeoplacer deposits of Au, Au-U and U, including the large Witwatersrand Au-U placer deposit. ii. Chemogenic precipitation leading to the formation of sedimentary deposits. This processes takes place through precipitation of particular elements in suitable sedimentary environments, with or without the intervention of biological organisms. Typical examples of sedimentary deposits include Banded Iron Formations of Precambrian shields, manganese deposits of Groote Eylandt, Australia; manganese deposits in South Ukrainian Oligocene Basin (contains about 70% of the World’s reserves of manganese ore); Zechstein evaporite deposits of Europe; Phosphate deposits of Florida, USA. iii. Residual processes: Residual deposits are generated through the process of leaching from rocks of soluble elements leaving insoluble elements in the remaining material. This category of deposits include high level or upland and low level peneplain type lateritic bauxites, auriferous bauxites and laterites, residual deposits of nickel, chromium, REE and titanium. Typical examples of residual deposits include nickel laterites of New Caledonia; high level leateritic bauxites deposits of India; Kaolin deposits in Nigeria; Residual deposit of 2 3 REE over carbonatites at Mount weld near Laverton, western Australia; and Anatase deposits in the overburden of alkalic rocks in Parana Basin, Brazil. iv. Supergene enrichment of sulfide and oxide deposits: This process involves leaching of valuable elements from the upper parts of mineral deposits and thir precipitation at depth to produce higher concentrations. This process in fact generated economic-grade ores from sub economic ones in many porphyry-type base metal deposits. v. Volcanic exhalative (= sedimentary exhalative) processes: This process involves exhalations of hydrothermal fluids at the surface, usually under submarine conditions and generally producing stratiform ore bodies. Deposits resulted from this process is known in literature as volcanic-associated massive sulfide (VMS) deposits. Typical examples of VMS deposits are: Base metal deposits of Meggan, Germany; Sullivan, Canada; Mount Isa, Australia, Rio Tinto, Spain; Kuroko deposits of Japan: Black smoke deposits of modern oceans. Detailed account on (a) placer deposits (b) sedimentary deposits, (c) residual deposits and (d) supergene enrichment of sulfide and oxide deposits are provided in four lessons among the series of lessons on Economic geology and mineral resources of India (GEL-5). This lesson provides details on volcanogen exhalative process and massive sulfide deposits derived from this process. 2. VOLCANIC – EXHALATIVE (SEDIMENTARY EXHALATIVE) PROCESSES Volcanic – associated massive sulfide deposits frequently show a close spatial relationship to volcanic rocks, but this is not the case with all the deposits, e.g. Sullivan, Canada (Fig. 1)(Fig.2.14. Page 34 Evans) which is sediment-hosted and this and similar examples are referred to commonly as sedex (sedimentary-exhalative) deposits. In the volcanic-associated types the principal constituent is usually pyrite with varying amounts of copper, lead, zinc and baryte; precious metals together with other minerals may be present. 3 4 For many decades they were considered to be epigenetic hydrothermal replacement orebodies (Bateman 1950). In the 1950s, however, they were recognized as being syngenetic, submarine-exhalative, sedimentary orebodies, and deposits of this type have been observed in the process of formation from hydrothermal vents (black smokers) at a large number of places along sea-floor spreading centres (Rona 1988). These deposits are now referred to as volcanic-associated (or volcanogenic) massive sulfide deposits. The ores with a volcanic affiliation show a progression of types. Associated with basic volcanics, usually in the form of ophiolites and presumably formed at oceanic or back- arc spreading ridges, we find the Cyprus types massive sulfide deposits. These are essentially cupriferous pyrite bodies. Thcy are exemplified by the deposits of the Troodos Massif in Cyprus and the Ordovician Bay of Islands Complex in Newfoundland (Canada). Associated with the early part of the main calc-alkaline stage of island arc formation are the Besshi-type massive sulfide deposits. These occur in successions of mafic volcanics in complex structural settings characterized by thick greywacke sequences. They commonly carry zinc as well as copper and are exemplified by the Palaeozoic Sanbagwa deposits in Japan, and the Ordovician deposits of Folldal in Norway. The more felsic volcanics, developed at a later stage in island arc evolution, have a more varied metal association. They are copper-zinc-lead ores often carrying gold and silver. Large amounts of baryte, quartz and gypsum may be associated with them. They are called Kuroko-type deposits after the Miocene ores of that name in Japan, but similar deposits in the Precambrian are known as Primitive-type. All these different type massive sulfide deposits are normally underlain in part by a stockwork up which the generating hydrothermal solutions appear to have passed (Fig. 2) (Fig.2.20 Page, 39 Evans). There is today wide agreement that these deposits arc submarine-hydrothermal in origin, but there is a divergence of opinion as to whether the solutions responsible for their 4 5 formation are magmatic in origin or whether they represent Circulating sea water. To understand this let us look at the evidence from hydrothermal activity on the ocean floors. Hydrothermal mineralization at sea-floor spreading centres was first discovered in the Red Sea in the mid 1960s, but the resultant deposits do not appear to be true modern analogues of ancient volcanic and sediment-hosted massive sulfide deposits. Since then various forms of hydrothermal mineralization have been found at many sites along spreading centres with the black smoker type producing obvious analogues of ancient massive sulfide deposits (Rona 1988). Black smokers were discovered in the late 1970s during ocean floor investigations using a submersible. They are plumes of hot, black, sometimes white, hydrothermal fluid issuing from chimney-like vents that connect with fractures in the sea floor. The black smoke is so coloured by a high content of fine-grained metallic sulfide particles and the white by calcium and barium sulfates. The chimneys are generally less than 6 m high and are about 2 m across. They stand on mounds of massive ore-grade sulfides (Fig.3)(Fig. 4.11, page 72 Evans) that occur within the grabens and on the flanks of oceanic ridges. Ten of the largest mounds in the eastern axial valley of the southern Explorer Ridge (about 350 km west of Vancouver Island) average 150 m across and 5 m thick and are estimated to contain a total of 3-5 million tons of sulfides. The largest mound-chimney deposit so far found is the TTG mound on the Mid-Atlantic Ridge at 26°N, which is estimated to contain 4.5 million tons of sulfides. The mineralogy of the mounds is similar to that of massive sulfide deposits on land with high temperature copper-iron sulfides beneath lower temperature zinc- and iron-rich sulfides, baryte and amorphous silica. Silver-bearing sulfosalts with minor galena occur
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