MINERALIZATION ALONG DIVERGENT PLATE BOUNDARY ENVIRONMENTS AND TRANSFORM FAULTS

I. Mineralization along Divergent Plate Boundaries:

An example of a recent thermal plume generated rift with sedimentary mineralization is that of the Red Sea.  Although the rifting there started 50 m.y. ago, the complete rupture leading to true oceanic accretion took place 5-6 m.y. ago.  The movement still continues with the sea opening at a half rate of 1 cm/yr.  Irregularities along the axial valley of the Red Sea has given rise to a number of hot brine pools.

- Hydrothermal activity at the ridges gives rise to 

    a) Sulfide deposits and

    b) Metalliferous sediments on the flanks of ridges.

   Important metallic deposits formed are Fe, Zn, Cu, Pb, Au and Ag.

  •  In the Red Sea metalliferous sediments containing Fe, Zn and Cu are being deposited.  Such deposits are also reported from a number of places on the East Pacific Rise (13o N, 21o N and 20o S), Juan de Fuca Ridge and Galapagos Ridge.

  • The slow spreading Atlantic Ridge is characterized by the precipitation of Fe- and Mn-oxides, eg. the TAG Hydrothermal field on the Atlantic Ridge.

  • Ultramafic rocks in ophiolites are hosts to asbestos, chromite and nickel ores.  These are generally accessible in Phanerozoic orogenic belts to which sites they have been transported due to plate movement.

  • Podiform chromite deposits associated with serpentinized ultramafic rocks.

  • Cyprus Type massive sulfide deposits (Cu-Fe rich) are also associated with ophiolites and represent hydrothermal deposits formed at ocean ridges.

   Fig. 1 shows the schematic illustration of an oceanic spreading centre and associated metallogeny.

 . Mineralization along Transform Faults:

  •  True transform faults appear to have little importance in metallogenesis because, except where "leaky" they have no magmatism associated with them.

  • A possible genetic relationship has been noticed between the Cenozoic Chaman transform fault in Pakistan and stibnite vein deposits.

  • The Coast Range of California also host late Cenozoic mercury deposits.

  • The intersections of a spreading axis and transform fault appear to important in terms of localizing metalliferous brine pools in the Red Sea.

  • The McLaughlin hot-spring gold deposit in California Coast Ranges north of San Francisco is localized along a major thrust fault that has been reactivated by the San Andreas Fault.

  • The Salton Sea geothermal system of the Imperial Valley, California has a significant energy potential and sits astride short spreading segments that connect offset strands of the San Andreas transform fault in California.

 An ocean basin begins to form as rifting processes culminate in the development of a mid-oceanic ridge (MOR), where new lithospheric crust is produced. 

  BLACK SMOKERS & VMS DEPOSITS 

 It is at these sites of extensive basaltic volcanism that modern "black smokers" occur, their dark colour the result of suspended sulphide particles. The circulation of water throught the ridge system begins with the downward percolation of sea water through extensive fissures on either side of the rigde axis. The water percolates down to the base of the dyke complex which underlies the ridge where it is heated to 400-450 C, before rising to be expelled throught the hydrothermal vents of the black smokers. It is during this high temperature circulation that the heated water alters and corrodes the surrounding basaltic rocks, dissolving the metals it contains and precipitating them at the vent in contact with cold oceanic waters. It is with a concentration factor of around 400 that the copper from the original basaltic rocks is concentrated to form volcaniogenic massive sulphide (VMS) deposits. These modern sites of ore deposition are only likely to be preserved in oceanic depressions or traps that exclude oxygenated sea waters. Modern examples are occurring in the Lau and Manus Basins, in the Red Sea and along the Pacific, Atlantic and Indian oceanic ridges.

 HYDROTHERMAL METALLIFEROUS SEDIMENTS 

 Ancient examples of VMS deposits can be found in the geologic record in both Late Archean (< 2800 Ma) rock successions and Paleozoic rock successions. Ancient VMS deposits generally occur in clusters with one or more "giant" deposits in association with numerous smaller deposits. They occur at specific stratigraphic horizons, commonly boundaries between contrasting lithologies within volcanic successions. Within the Archean rock successions it is the Granitoid-greenstone belts that contain the economic mineralisation. 

 VMS deposits may form as a result of mid-oceanic ridge basaltic magmatism, placing them at sites of extensionaltectonic regimes, or as a result of oceanic arc and back-arc basin magmatism, placing them in subduction related tectonic environments. Metal concentrations have been found to be higher at subduction related VMS deposits (in back-arc or arc related basins) than at mid-oceanic ridges. As a result, fossil volcanogenic deposits demonstrate that back-arc and arc environments with highly differentiated lavas are the most favourable settings for the formation of large VMS deposits. The Kuroko VMS deposits of Japan are the best known ancient analogues of modern Black Smokers. 

 MANGANESE NODULES ON THE OCEAN FLOOR 

 Ferromanganese nodules and encrustations occur both overlying basalt lava at mid-oceanic ridges where they are considered to be hydrothermal deposits, and overlying sediments away from ridge crests where they are considered hydrogenous or authegenic in origin. They are of economic interest because of their high copper, nickel and cobalt content, these metal abundances being particularly high where nodules are widely dispersed on the sea floor. The absence of manganese nodules from deep sea cores indicates that they do not normally survive burial by younger sediments. 

 MINERAL DEPOSITS OF OPHIOLITE SEQUENCES 

 Ophiolites are thought to represent slices of oceanic crust that have been thrust or obducted onto a continental margin during collision. Ophiolites are characterised by a sequence of rock types, consisting of deep sea sediments overlying basaltic pillow lavas, sheeted dykes, gabbros and ultramafic peridotites. Most large ophiolite bodies are no older than Triassic and the scarcity of Proterozoic ophiolites suggests that they are normally destroyed by erosion within a period of several hundred million years. This presumably represents the erosion rate of high-level nappes, flanking the core of orogenic belts, in which the larger ophiolite bodies usually occur. 

STRATIFORM MASSIVE SULPHIDES 

 The Troodos ophiolite complex of Upper Cretaceous age in Cyprus has become the type-example of stratiform massive sulphides associated with basaltic pillow lavas and cherts. The sequence of mafic and ultramafic igneous rocks overlying pelagic sediments originally formed at a submarine spreading ridge during the late Cretaceous. The massive sulphides and related stockwork mineralisation all occur within the pillow lava sequence, with over 90 deposits known from within the Troodos Complex. The Semail ophiolite in Oman provides another example, with all mineralisation associated with the volcanic centres strung out along the uppermost part of the ophiolite on top of the pillow basalt unit. The Semail ophiolite was formed at the same time as the Troodos Complex, and emplaced during the same Tethyian collision event. 

 PODIFORM CHROMITE DEPOSITS IN ULTRABASIC ROCKS 

 Although 97 % of the world's chromite reserves occur in non-ophiolite, layered mafic and ultramafic intrusions, over half of current world production comes from ophiolite-hosted chromite deposits. Lenticular podiform chromite or chrome-spinel deposits are known from many Alpine-type ultrabasic bodies, interpreted as on-land oceanic lithospere. The chromite bodies occur only within the harzburgite lithologies (including dunite) and not within lherzolite units (both peridotite rock types). The harzburgite unit forms the basement to the cumulate rocks, is generally tectonised, lacks recognisable cumulus magmatic textures, and is considered to be a depleted mantle source. Upper Mesozoic and Cenozoic examples include the ophiolite sequences in Cyprus, Oman, Greece, Turkey, Cuba and the Philippines. Lower Paleozoic deposits are known from Newfoundland, Proterozoic deposits from the Eastern Desert of Egypt and Saudi Arabia. 

 BASE METAL DEPOSITS IN THE RED SEA 

 Recent studies of on-shore fracture patterns and faults presumed from bathymetry within the Red Sea area show that recent brine pools and metalliferous sediments are located at or very near to the intersection of transform faults with the Red Sea spreading ridge. Many of the Miocene base metal deposits in the adjacent coastal areas are also located on extensions of transform faults or presumed transform faults.

 METALLOGENY ALONG TRANSFORM FAULTS

 Transform faults form plate boundaries along which plate motion is predominantly strike-slip with plates sliding past each other with relatively minor convergence or divergence. They are not sites of major  magmatsim or orogeny. Transform faults related to subduction developed in or adjacent to continental margins or island arc systems and on former passive continental margins during and following continental collision show little associated mineralisation at the present erosion level. However, transform fault extensions related to the development of ridge-ridge transform movement do have associated mineral deposits.

 CU-NI-PT-AU-TI BASIC / ULTRABASIC INTRUSIONS 

 Layered basic/ultrabasic intrusions within fracture zones considered to be related to transform faulting have been described from the South-Eastern Desert of Egypt and Sierra Leone. The Freetown layered basic complex in Sierra Leone consists of a rhythmic sequence of basic/ultrbasic rocks including trocotolite which host the ubiquitous sulphides which occur as late stage hydrothermal veins and replacements. It is suggested that the 193 Ma old complex occurs at the intersection of the Guinea Fracture Zone and the Atlantic protorift, its position being related to both early stage rifting and to the continental extension of a transform fault. 

 BASE METAL DEPOSITS IN THE RED SEA 

 Recent studies of on-shore fracture patterns and faults presumed from bathymetry within the Red Sea area show that recent brine pools and metalliferous sediments are located at or very near to the intersection of transform faults with the Red Sea spreading ridge. Many of the Miocene base metal deposits in the adjacent coastal areas are also located on extensions of transform faults or presumed transform faults. 

 PEGMATITE MINERALISATION 

 A series of parallel tin-tourmaline pegmatites and tin-lepidolite-albite pegmatites intruded into the Phangnga Fault in the Phuket-Phangnga area of peninsular Thailand are worked as tin ores. Tin-lepidolite pegmatites are also associated with the Ranong Fault system to the north which has also been interpreted as the continental expression of a transform fault.  

 KUROKO TYPE SULPHIDE DEPOSITS 

 Within the Green Tuff magmatic arc of eastern Japan a concentration of Upper Cenozoic Kuroko type deposits have been noted at intersections of the volcanic front with transverse faults postulated to be the extensions of oceanic transform faults on the basis of seismic evidence. Although more sporadic in their distribution, the Miocene-Pliocene gold-silver quartz vein deposits are also located close to the transverse faults.

 

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