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 of island arc evolution and perhaps overlap into the waning stage. The ultimate source of the metals in porphyry deposits is to be sought in subduction zones. It can account for the regional differences seen in these deposits, relating them to differences in the content of copper, molybdenum and gold in the subducted oceanic crust.

Skarn deposits are common at igneous-carbonate country rock contacts in the older and more complex arcs, such as those in Indonesia, the Philippines and Japan. Many of the granites and granodiorites with which these deposits are associated also possess spatially related pegmatitic and vein mineralization, and the resulting metallic association can be quite complex, as in the Kitakami and Abukama Highlands of Japan. Skarn and vein deposits of tin are well known from parts of the Indonesian Arc. Less well known are the gold veins in the contact zones of diorites and granodiorites of the Solomon Islands. Other vein deposits are not common in the younger arcs, including those of mercury, which is surprising in view of their development in association with andesites in the Alpine orogenic belt, e.g., the eastern

Carpathians. Cinnabar veins occur in the more complex arcs of the Philippines, Japan and

New Zealand.

Waning calc-alkaline stage: The massive sulfide deposits of Kuroko-type probably belong to the later stages of development of island arcs. They are associated with the more felsic stages of calc-alkaline magmatism and are marked by distinctive features. Kuroko-type massive sulfide ores in ancient island arcs include the Palaeozoic deposits of Captain’s Flat,

New South Wales; Buchans, New foundland; Parys Mountain, Wales; and Avoca, Ireland.

Present Opinion is that these deposits too are wholly or mainly developed in back arc settings and Leat et al. (1986) pointed out that an extensional setting favours the development of long- lived, rhyolitic high level magma chambers, often with bimodal basalt-rhyolite volcanic associations like those of North Wales, with which Kuroko-type deposits are frequently associated. The Kuroko-type deposits of Japan have formed in a weakly extensional environment at the waning stage of the opening of the Sea of Japan about 15.3 Ma ago.

Manganese deposits of volcanic affiliation are very common in eugeosynclinal successions of all ages from Archaean to Recent and island arcs are no exception. Large numbers of subeconomic deposits occur in the Tertiary volcanic areas of Japan, Indonesia, and the West Indies and in the recent volcanic sequences of the larger South Western Pacific islands. These deposits have been formed insitu and are not tectonic slices of pelagic manganese deposits of the oceanic crust. They belong to the waning calc-alkalline stage and the preceding stage of island arc evolution. A well documented Mn deposit belonging to the above occurrences is that of Southern Urals (Russia). Here, the Devonian Kusimovo deposit is associated with basic and intermediate volcanics and the ore layers or lenses rest conformably on the underlying pyroclastics. Ore layers are composed of braunite and these layers alternate with cherty Jasperoid layers. Varentsov and Rakhmanov (1977) attribute a volcanic-exhalative origin for Mn ores of the Kusimovo deposit. Colley and Walsh (1987) have described typical deposits of this type in Fiji.

The nature of back-arc basins and their sedimentary and volcanic contents are variable. There is a scarcity of observed mineralization in modern back-arc basins, partly because of the thick blanket sediment and volcanic ash present in most of them. According to various workers, Besshi-type sulfide ores form in back-arc environment and a number of ophiolites with their associated sulfide and chromite deposits have been assigned to this

setting. The Taupo Volcanic Zone of North Island, New Zealand demonstrates that the volcanogene exhalative mineralizing processes can take place in marginal basins. A similar but older Welsh Basin, UK, has Kuroko-style and Cu-Pb-Zn vein mineralization in addition to sedimentary and volcanic manganese deposits.

2.2 MINERALIZATION IN CONTINENTAL MARGINS.

The principal features of continental margin arcs (also referred to in literature inappropriate as “Andean” or “Cordilleran” arcs) include a trench with turbidites, high heat flow and regularly arranged metamorphic zones, as well as monzonitic-granodioritic plutons and batholiths, foreland basins that are often molasses-filled, and widespread sulfide and oxide mineralization (including extensive development of porphyry Cu and Mo deposits). Active volcanoes are frequently present in mountain chains underlain by a crust of greater than normal thickness due to either crustal shortening or under plating. Although these are regions of plate convergence with its concomitant compression, many graben structures are developed running parallel or obliquely to the arcs. Some magmatic activity and mineralization are related to this rifting.

The Andean Arcs lie along the Western margin of South America where it abuts against the Nazca plate, which is moving down a Berioff Zone beneath the present mountain chain (Fig. 4). This chain is immensely rich in ore bodies of various types and metals, but generally shows a plutonic-epigenetic affiliation rather than a volcanic-syngenetic one. This is no doubt to some extent a function of erosion. The profusion of metals may be related to proximity to the East Pacific Rise and the abundance of metals that are being added to the oceanic crust at this spreading axis. The Andean Cordilleran deposits possessed a close relationship in time and space with calc-alkaline intrusives emplaced at relatively shallow levels in the upper crust.

The Andes are characterised by linear belts of mineralization that coincide with the morphotectonic belts which run down the mountain chain. The coastal belt, mainly consisting of Precambrian metamorphic rocks is important for its iron-apatite deposits of skarn and other types. The Western Cordillera is a copper province particularly important for porphyry- type deposits carrying Cu, Mo and Au (Fig. 5).

A belt with Palaeozoic sedimentary rocks extending along Eastern Cordillera contains tin (+W –Ag –Bi) mineralization (The Bolivian Tin Belt, Fig. 6). This tin-silver belt south of

Oruro, has produced vast amounts of silver from Spanish colonial times to the present day, and indeed the eastern Pacific continental margin arcs from British Columbia to Chile are well endowed with large silver deposits, but the Western Pacific arcs have virtually none. The development of silver deposits is controlled by the relative abundance of silver in the subducted oceanic crust. The Western Cordillera of the Andes consists of several gold deposits.

The overall metal zonation of the Andes has been related to the remobilization of metals, subducted in the oceanic crust at different depths down the Benioff Zone, to migration of Benioff Zones, and to changes in their inclination. The porphyry deposits are related to I – type granitoids probably generated along the Benioff Zone or in the mantle wedge above it, but the tin deposits are related S-type granitoids, perhaps representing anatectic melts that derived their tin from continental crust. Similar situations occur in Alaska and china with tin mineralization related to deeper sections of the Benioff Zone than the porphyry Cu mineralization.

3. STRIKE-SLIP SETTINGS

Two different types of faults are included here: transform fault and the long known tear or wrench fault. Strike-slip faults vary in size from plate boundary faults, such as the San

Andreas Fault of California and the Alpine Fault of New Zealand, through microplate

boundaries and intraplate faults, such as those of Asia north of the Himalayas, down to small scale faults with only a few metres offset. Even if these structures occur as single faults or as fault zones, they may be important in locating economic mineral deposits.

Well recognized transform faults in continental crust have little or no associated mineralization. Transform faults are, however, loci of higher heat flow and could well be channel-ways for hydrothermal solutions, as certainly appears to be the case in the Red Sea.

Here, the brine pools and metal-rich muds appear to be located above transform faults.

Donnybrook-Bridgetown Shear Zone of western Australia, which is considered to be analogous to the San Andreas Fault System, hosts the Greenbushes pegmatite group (Sn-Ta-

Li producer) which is potentially the largest rare metal resources in the world.

Modern strike-slip basins occur in oceanic, continental and continental margin settings. They may also occur in back-arc basins. The classic onshore strike-slip basin is the

Dead Sea with its important salt production, and the best known offshore basins are those of the Californian Continental Borderland, some of which host commercial oil pools. Ancient strike-slip basins are difficult to identify. However, two examples of economic importance are the Tertiary Bovey Basin of England with its economic deposits of sedimentary kaolin and minor lignite, and the late Mesozoic, coal-bearing, lacustrine basins of north-eastern china. A similar structural relationship was noted by Groves and Foster (1991) in the Yilgarn

Block (Western Australia) where major Archaean gold deposits are commonly adjacent to transcraton shear zones and on the district scale are usually present in geometrically related, shorter strike-length, smaller scale structures.

Henley and Adams (1979) studied giant placer gold deposits and their tectonic settings. They showed that the majority of giant gold placer deposits are situated on the

Pacific margins, where they formed during the Tertiary in similar tectonic and sedimentary environments. The plate boundaries involved are both convergent and regional strike-slip

faults, such as the Alpine Fault in New Zealand and the San Andreas Fault of California (Fig.

7). Two factors of regional evolution were important in the development of each giant placer.

First there was the rejuvenation of an older orogenic belt containing significant quantities of bedrock gold to provide a source, and second, continual high rates of uplift of this belt resulting in multiple episodes of sediment reworking and the development of large concentrations of placer gold.

Fundamental faults and lineaments often have orebodies and even ore fields spatially related to them. Heyl (1983) drew attention to the 38th parallel lineament of the central USA and its relationship to a number of ore fields in the Mississippi Valley and elsewhere (Fig. 8).

This lineament is a zone marked by wrench faults, lines of alkalic, gabbroic, ultramafic and kimberlitic intrusions, suggesting connections into the mantle, and changes in stratigraphy across it. Important ore fields occur at the intersections of this lineament and structures, for example the central Kentucky Orefield occurs where the West Hickman Fault Zone and the

Cincinatti Arch cross it and the very important South-east Missouri Field where it is crossed by the Saint Genevieve Fault system and the Ozark Dome.

Gold deposits in Archaean Abitibi Greenstone belt of Ontario-Quebec (Canada) exhibit close connection with regional fault zones. Antimony-gold mines of the Archaean

Murchison Schist Belt, Kaapvaal Craton (S. Africa) lie along the 35-40 km Antimony Line, which is considered to be a zone of shearing hosting most of the mineralization. The largest deposits of the Stuart Shelf-Adelaide Geosyncline region of South Australia lie on two alignments (Fig.9). The giant among these deposits, Olympic Dam, is close to the intersection of two lineaments and a sinistral flexure. (Fig. 10).

4. COLLISION-RELATED SETTINGS

Continental collision results from the closing of an oceanic or marginal basin whose previous existence is revealed by the presence of lines of obducted ophiolites. In some orogens

interpreted as collision belts, such as the Damarides of Southern Africa, this evidence is missing. Collision can occur between two active arc systems, between an arc and an oceanic island chain or between an arc and a microcontinent, but the most extreme tectonic effects are produced when a continent on the subducting plate meets either a continental margin or island arc on the overriding plate. Subduction can then lead to melting in the heated continental slab and the production of S-type granites (Fig. 11). In many continental collision belts, highly differentiated granites of this suite are accompanied by tin and tungsten deposits of greisen and vein type Hercynides of Europe (e.g., Cornwall, UK Erzegebirge, Germany).

With continental collision of the type illustrated in Fig. 11, belts of older I-type granitoids possibly with associated porphyry copper and molybdenum deposits, are related to the subduction of oceanic lithosphere, and belts of younger S-type granitoids, which may have associated tin (and tungsten) mineralization formed during continental collision. In south eastern Asia (Fig. 12), there are three different granite provinces that run parallel to each other. The Eastern Granite Province contains hornblende-biotite and biotite granites of

Permean age (265-230 Ma). They are I-type granites, which are, relative to the Main Range

Granites of the Central Province, poor in tin, and the ratio of historic tin output of central

Province to Eastern Province granites in Malaysia is 19:1. Further north in this Province in

Thailand, there is a well developed Permean volcanic-plutonic arc with a porphyry copper deposit at Loei. The central Province hosts the Main Range Granites of Malaysia. These are post-tectonic, peraluminous, S-type, biotite granites whose ages lie in the range 220-200 Ma.

The famous Malaysia tin fields of the Kinta Valley and Kuala Lumpur occur in this Province, but the same province in central and northern Thailand is much less well endowed with tin.

The western Province is restricted to western Thailand and Burma and carries granite intrusions of Cretaceous to Tertiary age (130-50 Ma). The older granites are largely I-type and the younger granites are S-type. In the latter, tin has been worked for over a century or so

at Kao Daen, Phuket and Samoeng. The geological, geochronological and geochemical data of the tin-bearing granites can be explained by postulating the successive collisions of three microcontinental plates (Fig.13 a,b,c). In this reconstruction it is postulated that in the

Permian an ocean or, more probably, a marginal basin occupied the area of the present central

Thailand and eastward subduction of oceanic crust gave rise to the development of the

Permian volcanic-plutonic Eastern Province with its Porphyry copper mineralization (Fig.

13a). This marginal basin closed in the Triassic with a continental collision that resulted in the genesis of the S-type tin granites of the central Province (Fig. 13b). Following this collision, the marginal basin to the west was gradually closed, and again by eastward subduction of oceanic crust gave rise to I-type granite emplacement in western Thailand at about 100 Ma and produced a phase of S-type granite formation; but in this event considerable overlap of I- and S-type granite resulted, as in Phuket Island, perhaps as a result of more pronounced subduction of the leading edge of the Burma Plate (Fig.13c). Finally it may be remarked that belts of I-type granites and S-type tin granites similar to those in

Malaysia appear to be present in the Hercynides of Europe.

Uranium deposits, particularly vein type, may be associated with S-type granites as in the Hercynides (Fig. 14 ) and Damarides (Rossing uranium deposit, Namibia). Unfortunately, not all S-type granites have associated mineralization, for example those found in a belt stretching from Idaho to Baja, California. Saukins (1984) has suggested that the reason that some have associated tin mineralization, some uranium and others neither, presumably reflects the geochemistry of the protolith from which these granites were derived, and their subsequent magmatic history.

Summary

This lecture notes deals with the topic “Mineral deposits in relation to three different plate tectonic settings viz., (a) subduction-related settings, (b) strike-slip settings, and (c) collision related settings.

1  In the subduction –related settings, oceanic lithosphere is subducted beneath an arc system on the overriding plate. 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 comprised of an outer arc and fore-arc basin. The outer arc is usually built of oceanic floor or trench sediments. The fore arc basins contain undeformed sediments, Arc systems may be either island arcs or continental margin arcs.

2  Mineralization in island arcs can be divided into two types: allochthonous deposits (those formed outside the arc-trench environment and transported to it by the plate motion) and autochthonous deposits (those originated within the arc complex).

3  Podiform chromitites confined to obducted Alpine-type peridotite-gabbro complex (ophiolites) constitute the major allochthonous deposits of island arcs environment.

Few massive sulfide deposits are also encountered in obducted oceanic crust (e.g., Turner

Albright Zn-Cu-Ag-Au-Co massive sulfide deposits in obducted ophiolite in Josephine

County, Oregon, USA).

4  Atochthonous deposits developed during main calc-alkaline stage of island arc evolution include porphyry-type Cu and Mo deposits, Bessi-type volcanic massive sulfide deposits, skarn and vein deposits. Kuroko-type massive sulfide deposits are associated with more felsic calc-alkaline magmatism related to waning stage of island arc evolulion.

5  The principal features of continental margin arcs include a trench with turbidites, monzonitic-granodioritic plutons and batholiths, foreland basins and widespread sulfide and oxide mineralization, including porphyry Cu and Mo deposits. The Andean arcs running along the western margin of South America are characterized by linear belts of mineralization

consisting of iron – apatite scarn deposits, porphyry-type Cu, Mo, Au deposits, vein and replacement type Cu-Pb-Zn-Ag deposits (tin-silver and gold deposits). The North America

Cordillera consists of numerous porphyry Cu and Mo deposits and base and precious metal vein deposits.

6  Strike-slip faults vary in size from plate boundary faults (e.g., San Andreas Fault of California and Alpine Fault of New Zealand) through microplate boundaries and intraplate faults, (faults of Asia north of Himalayas) down to small scale fractures with only a few meters offset. Many of the diamond-bearing Kimberlites of West Africa lie along transform faults onshore base metal deposits of the Red sea region also show a similar relationship with transform fault.

7  Modern strike-slip basins occur in oceanic, continental and continental margin settings. Dead Sea is a classic onshore strike-slip basin. Californian continental Borderland with commercial oil pools is an offshore basin. Two examples of ancient strike-slip basins are: Sedimentary Kaolin-bearing Tertiary Bovey Basin of England and Mesozoic coal- bearing lacustrine basin of north eastern china.

8  Fundamental faults and lineaments often have ore bodies and even orefields related to them. In the Mississippy Valley a number of orefields are aligned to 38th parallel lineament (deep rooted wrench faults). Along the lineament are found intrusives of gabbro, ultramafic and Kimberlitic intrusives suggesting connections into the upper mantle.

9  Major Archaean gold deposits of Yilgran Block (Western Australia) are found adjacent to transcraton shear zones. Like wise, gold deposits in the Archaean Abitibi

Greenstone Belt of Ontario-Quebec (Canada) are closely connected to regional fault zones.

Olimpic Dam copper deposit of the Stuart Shelf-Adelaide Geosyncline region of South

Australia is close to the intersection of two lineaments.

10  Collision – related settings can be encountered between two active arc systems, between an arc and an oceanic island chain or between an arc and a microcontinent.

Continental collision results from the closing of a oceanic or marginal basin. In many continental collision belts highly differentiated granites are accompanied by greisen and vein type tin and tugsten deposits (e.g., Cornwall, UK and Ergebirge, Germany).

11  In south-east Asia (Thailand-Malaysia-Burma) three linear granitic belts were evolved as a consequence of successive collision of three microcontinental plates during 280-

230 Ma, 230-190 Ma and 130-100 Ma. The Permean volcanic- plutonic Eastern Province consists of porphyry Cu deposits while the S-type granites of the Central Province and S- and

I-type granites of the western granites consist of rich tin mineralization.

Did you know ?

1  Mineralization in island arc settings include podiform chromite deposits confined to obducted Alpine-type peridotite-gabbro complex, Bessi- and Kuroko-type massive sulfide deposits, porphyry Cu-Mo deposits, skarn and vein deposits.

2  Continental margin arcs are bestowed with widespread sulfide and oxide mineralization.

3  The Andean arcs of South America are characterised by linear belts of mineralization: iron-apatite scarn deposits, porphyry-type Cu, Mo and Au deposits, vein and replacement type Cu-Pb-Zn-Ag deposits, tin-silver and gold deposits

4  In west Africa many of the diamond-bearing Kimberlites lie along transform faults.

5  World’s largest rare metal pegmatite deposits are confined to Donnybrook-

Bridgetown Shear Zone and the latter is considered to be analogous to the San Andreas Fault system of California.

6  Major Archaean gold deposits of Yilgran Block (Western Australia) and Abitibi greenstone belt of Ontario-Quebec are found adjacent regional fault zones.

7  Highly differentiated granites in continental collision belts are accompanied by greisen and vein type tin and tungsten deposits.

8  In south-east Asia (Thailand, Malaysia, Burma) tin-bearing I- and S- type granite provinces were generated as consequence of three microcontinental plates during Permean,

Triassic and Eocene.

DIAGRAMS

MINERAL DEPOSITS IN RELATION TO PLATE TECTONIC SETTINGS. PART 2: SUBDUCTION-RELATED SETTINGS, STRIKE-SLIP RELATED SETTINGS AND COLLISION-RELATED SETTINGS