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Mineral Deposits in Relation to Plate Tectonic Settings

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Mineral Deposits in Relation to Plate Tectonic Settings MINERAL DEPOSITS IN RELATION TO PLATE TECTONIC SETTINGS. PART 2: SUBDUCTION RELATED SETTINGS, STRIKE=SLIP SETTINGS AND COLLISION-RELATED SETTINGS. 1. Details of Module Subject Name Geology Paper Name ECONOMIC GEOLOGY & MINERAL RESOURCES OF INDIA Module MINERAL DEPOSITS IN RELATION TO PLATE TECTONIC Name/Title SETTINGS PART 2 Module Id GEL-05-153 Pre-requisites Before learning this module, the users should be aware of Mineral deposits in various geological settings. Basics of Subduction Island arcs and continental margins Strike-slip features and collision tectonics. Objectives To understand the various aspects of subduction and mineralization. To understand the features of minerlization related to island arcs and continental margins. To understand the processes at strike-slip and collision settings. Keywords Plate tectonics and ore deposits, continental basins, domes, rifts, aulacogens, ocean basins, continental margins. 2. Structure of the Module-as Outline: Table of Contents only (topics covered with their sub-topics) 1. INTRODUCTION 2 SUBDUCTION-RELATED SETTINGS 2.1 MINERALIZATION IN ISLAND ARCS 2.1.1 Allochthonous deposits 2.1.2 Autochthonous deposits 2.2 MINERALIZATION IN CONTINENTAL MARGIN ACRS 3. STRIKE-SLIP SETTINGS 4. COLLISION-RELATED SETTINGS 3.0 Development Team: Role Name Affiliation National Co-ordinator Subject Co-ordinators Prof. M.S. Sethumadhav Centre for Advanced Studies Dept of Earth Science (e-mail: Prof. D. Nagaraju University of Mysore, [email protected]) Prof. B. Suresh 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. INTRODUCTION Details pertaining to the association of mineral deposits to intracratonic basins, intracontinental rifts and aulacogens, oceanic basins, mid ocean ridges and passive continental margins were provided in the Part 1 of the Lecture notes. Part2 deals with the mineral deposits related to subduction-related settings, strike-slip settings and collision- related settings. 2. SUBDUCTION-RELATED SETTINGS In these settings, oceanic lithosphere is subducted beneath an arc system on the overriding plate (Fig.1 Fig.). The subduction zone is usually marked by a deep sea trench and, between this and the active volcanic arc, is the arc-trench gap. The arc-trench gap in some arcs is composed of an outer arc and a fore-arc basin. The outer arc, which may rise above sea level, is usually built of oceanic floor or trench sediments scraped off the subducting plate above low angle thrusts and tectonically accreted to the overlying plate to form an accretionary prism or tectonic me’lange. The fore-arc basin contains flat lying, undeformed sediments that may reach upto 5 km in thickness and which reflect progressive shallowing, as the basin fills up with turbidites, shallow marine sediments and fluviatile- deltaic-shoreline complexes. The sediments are derived mainly from the volcanic arc or by erosion of uplifted basement. Arc systems may be either island arcs or continental margin arcs. Island arcs are of several kinds: (1) clearly intra-cratonic ( like Tonga, Scotia), (2) separated from sialic continent by small semi-ocean basins (Japan, Kurile), (3) pass laterally into continental margin fold belt (Aleutian) and (4) built against continental crust (Sumatra- Java). In continental margin arc, the volcanic arc is situated land ward of the oceanic crust- continental crust boundary, as in the Andes (Fig. 1). Behind volcanic arcs there is the back- arc area usually referred to when behind island arcs, as the back-arc basin or marginal basin or when behind continental margin arc as a foreland basin or molasses trough, e.g., the Amazon basin behind the Andes. 2.1 MINERALIZATION IN ISLAND ARCS It is convenient to divide the mineral deposits in these arcs according to whether they were formed outside the arc-trench environment and transported to it by plate motion (allochthonous deposits), or whether they originated within the arc complex (autochthonous deposits. 2.1.1 Allochthonous deposits Rocks and mineral deposits formed during the development of oceanic crust may, by various circumstances, arrive at the trench at the top of subduction zone (Fig.2). This material is largely subducted, when some of it may be recycled; some, however, is obducted to form accreted terrain. Examples of what appear to be Cyprus-type massive sulfide ore bodies thrust into a mélange, occur in north western California, where the Island Mountain deposit and some smaller occurrences are found in the Franciscan Me’lange. Further north in Klamath mountains of Oregon (USA), the obducted ophiolite contains the important Turner Albright Zn-Cu-Ag-Au-Co massive sulfide deposit. Cyprus-type massive sulfides are also likely to be present at the base of island arc sequences; whether these are the initial succession or a second or later one formed by the migration of the Benioff Zone is not clear, because in most cases, the arc basement will have originated at an oceanic ridge. Cleary, any other deposits formed in new oceanic crust may also eventually be mechanically incorporated into island arcs and the most likely victims will be the chromite deposits of alpine-type peridotites and gabbros. Podiform chromites are the most important magmatic deposits in the allochthonous peridotites. The chromitites are cumulates that originated in the upper mantle, often at mid-oceanic ridges. Economic deposits of podiform chromitite occur in present day arcs in cuba, where they are found in dunite pods surrounded by peridotite. Numerous occurrences are present in ancient island arc successions. 2.1.2 Autochthonous deposits In considering these deposits, it is convenient to divide the arc development into three stages: initial or tholeiitic stage, main calc-alkaline stage and waning calc-alkaline stage. Initial or tholeiitic stage: The probable hydrous nature of such magmas and their derivation by partial melting of mantle material implies that cogenetic massive sulfides may well be expected to form during this stage. However, no massive sulfide deposits have as yet been detected in the earliest tholeiitic sequences of island arcs, but such sequences must be considered as promising for their discovery. Main calc-alkaline stage: This is the main period of arc development. Considerable sedimentation and subaerial volcanicity may have occurred during the tholeiitic stage, but the main island arc building and plutonic igneous activity belong to this stage. The early stage of island arc volcanism is dominated by basalt and basaltic andesite, the more evolved arcs have andesite as the dominant volcanic rock. The early tholeiitic stage is succeeded by magmas having a calc-alkaline trend and which are probably ultimately derived from two sources- subducted oceanic crust, and partial melting of the mantle wedge overlying the Benioff Zone. This will result in the diapiric uprise of wet pyroxenite from just above the Benioff Zone. The pyroxenite will undergo partial melting as it rises and the magmas so produced will fractionate as they rise to produce a wide range of magmas possessing calc-alkaline characteristics. The exposure of much more rock to subaerial erosion will greatly increase the volume of sediment that reaches inter-island gaps, the arc-trench gap and the back-arc area. Reef limestones will then be common. Rapid erosion on the flanks of subaerial volcanoes, ignimbritic activity (related to the increase in silica content of magmas) giving rise to submarine lahars, submarine slumping and so on, would all contribute to produce a vast volume of material for local sedimentation and for transportation by turbidity currents to the arc-trench gap, along submarine canyons into the trench itself and in the reverse direction into the back-arc regions. Volcanic massive sulfide, stockwork, skarn and vein deposits are formed in this stage. Conformable sulfide orebodies of Beshi-type develop at this stage in back-arc basins, accompanying the andesitic to dacitic volcanism. They occur in complex structural settings characterized by thick greywacke sequences. The associated turbidities may be of arc or continental derivation as indicated in Fig. 3. The lack of ophiolitic components, the petrological association and pyroclastic nature of the volcanics indicates a different environment from that of the Cyprus-type deposits. Sillitoe (1972) suggested that the solutions which deposited some of these massive sulfide deposits might have come directly from subducted sulfide bodies (Fig. 2), with their metals being derived from the subducted bodies and layers 1 and 2 of the subducted ocean floor. Metal-bearing brines expelled from layers 1 and 2 would induce fusion in the wedge of mantle above the Benioff Zone, ascending with the resulting hydrous calc-alkaline magmas to form porphyry copper deposits when the volcanism is dominantly subaerial. Many porphyry copper are present in island arcs (Fig. 4) with both diorite and Lowell – Guilbert model types being present. There is no evidence that continental crust is required for their formation; therefore they can be sought for in any island arc with or without continental crust. The young age of the orebodies in some recent arcs indicates that they belong to the late stages
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