GeologyRocks

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simonmjowitt
Monash University

Introduction

Ophiolite-hosted VHMS (Volcanic-Hosted Massive Sulphide) deposits, also known as Cyprus-type VHMS deposits, are seen in at least 25 of the worlds more than 200 known ophiolite terranes. The term Cyprus-type should now be considered a bit of a misnomer, as the Troodos ophiolite - to which the Cyprus-type name refers - is not a typical Mid-ocean ridge ophiolite - indeed it is considered to be a supra-subduction zone ophiolite, similar to the present day Mariana-Bonin arc. In effect, the Ophiolite-hosted VHMS category should be split into ‘normal’ oceanic ridges and mature back-arc basin environments, and supra-subduction (Cyprus-type) zone spreading axes occurring in fore-arc and immature back-arc environments. This classifies Cyprus-type VHMS deposits as a subdivision of the wider Ophiolite-hosted VHMS deposits (Galley and Koski, 1999).

Ophiolite hosted VHMS deposits of both types form a significant worldwide base metal resource. These deposits are copper and copper-zinc rich, compared to other VHMS deposit types; individual deposits can reach up to 30Mt, although the average deposit size is around 5Mt (Galley and Koski, 1999). Gold is usually present in the oxidised sulphide caps of these deposits - particularly those associated with the Tethyan Ocean, as seen on Cyprus (Constantinou, 1980).

As with all other VHMS deposit types, ophiolite hosted deposits are formed as a consequence of hydrothermal circulation. The primary heat source that drives the hydrothermal circulation at the ocean ridge axis are layered ultramafic-mafic plutonic magma chambers that are found below sea-floor spreading axes. These chambers are formed of layered peridotite grading up into gabbros and plagiogranites, with the overlying sheeted dyke complex representing fossilized conduits for magma ascending to the ocean floor. Overlying the sheeted dyke complex is a volcanic section formed of massive and thinner lava flows, pillow lavas, lava breccias and hyaloclastites, with feeder sills and dykes, and interstitial sediments, all usually overlain by a thick marine sedimentary cover sequence (Galley and Koski, 1999).

Spreading rates are key to the formation of VHMS deposits associated with ophiolites. Ophiolite sea-floor spreading centres known to host larger VHMS deposits, for example, the Troodos ophiolite, Cyprus, the Josephine ophiolite, Oregon, and the Semail ophiolite, Oman, are known to have had alterations in spreading rate over time. Intense magmatic activity is required to initiate large-scale seawater convection and resultant high-temperature fluid/rock interactions, within reaction zones where metals are ‘stripped’ from the rocks by the hydrothermal fluids. Intercalated sediments within the upper volcanic sequence provide evidence for slower rates of spreading later in the life of the spreading ridge, during which significant hydrothermal activity would produce large volumes of massive sulphide and stockwork mineralisation. These large volumes of ore would not have accumulated if the fast spreading rate had been sustained, as the hydrothermal circulation system would have been transported too quickly away from the driving force of the underlying heat of subsurface magma chambers. Slower spreading rates would have enabled far more ore to be formed and deposited than would be the case in a faster spreading regime (Galley and Koski, 1999).

Massive sulphides in ophiolite-hosted VHMS deposits are commonly clustered in groups along the exposed strike length of the ophiolite. Within these clusters of deposits and mining districts, base metal mineralisation can occur at any stratigraphic level, from the top of the magma chamber to the overlying sedimentary cover sequences (Harper, 1999). However, massive sulphide deposits and associated stockwork mineralisation are usually only found throughout the volcanic strata above the sheeted dyke formation. Indeed, in many ophiolites, stratiform massive sulphide deposits are most commonly located along a major contact, between differing suites of lavas, or between lavas and the overlying sediments, implying a hiatus in magmatism, allowing a hydrothermal circulation system to initiate during fast spreading, and propagate throughout a period of slower spreading. Deposits themselves consist of a stratiform massive sulphide lens and a proximal sulphide vein stockwork zone. These may be seen together or one component may be missing, due to displacement by faulting or deformation, or simply due to lack of preservation. The stockwork mineralisation itself may be up to several hundred metres wide and may be traceable through several hundred metres of depth into the sheeted dyke complex. A clear example of this is provided by the Aarja VHMS deposit, Oman, where a diffuse stockwork zone can be traced vertically over 3km, and in places is up to 500m wide (Galley and Koski, 1999).

The massive sulphide deposits themselves are all pyrite-rich, commonly having a massive base, overlain by sandy-textured and brecciated ore, with the pyrite rich massive core being cemented by base metal sulphides, for example chalcopyrite and sphalerite. The underlying stockwork zone is generally formed of a mixture of quartz and pyrite, with minor amounts of base metal mineralisation.

Discussion - Cyprus-type VHMS Deposits

The typical, and more traditional, view of a Cypriot copper deposit is as the type example of a sulphide deposit formed at an oceanic ridge. All Cyprus copper deposits are VHMS deposits, with massive sulphides having formed at the seawater-lava interface. These massive sulphides are formed from metal-bearing exhalative fluids, typically from black smokers, as seen in Mid-Ocean Ridge settings today (e.g. the TAG mound, on the Mid-Atlantic Ridge). The seawater-rock interface was presumed, in most orebody cases, to be represented by the junction between the Upper and Lower Pillow lavas, representing a hiatus in magmatic activity that allowed the hydrothermal circulation system, needed for the formation of these orebodies, to develop (Constantinou, 1980). Below the massive sulphide bodies are lower grade stockwork bodies; these bodies are presumed to fossilised routes for the hot mineralising fluids as they flowed towards the surface after interacting with the lower sheeted dykes, stripping them of metals (Richardson, Cann et al. 1987). This overall view has, however, not been re-examined. Current work, both on Cyprus and in areas such as the Ural Mountains where Cyprus-type sulphide deposits have been the focus of a recent multinational geological investigation, has created a better understanding of how these deposits formed.

The tectonic setting of Cyprus, and in particular the Troodos ophiolite, is the first major point to examine. As explained elsewhere in this dissertation, modern thinking on the formation of the Troodos has indicated that the ophiolite most probably formed in a supra-subduction zone setting, much like the Mariana-Bonin arc in the present day Western Pacific. This, therefore, must also be taken as the setting for the formation of the massive sulphide and stockwork deposits seen on Cyprus, as magmatism and spreading had not ceased after the formation of the later series of Pillow lavas and other extrusive igneous rocks, seen to cover many orebodies on the island (Adamides, 1990).

Orebodies on Cyprus are generally limited to the Pillow lavas and Basal Group units, with rare mineralisation occurring in the Sheeted Dykes and Gabbros. Originally thought to form on the contact between the Upper and Lower Pillow lavas, recent geochemical work has identified three distinct units of Pillow lavas, rather than the two previously proposed. Couple this with the fact that some orebodies are known to have formed in the Basal Group, and the former Upper Pillow lavas, the theory that this contact is a primary control on mineralisation is somewhat flawed (Constantinou, 1980). However, the fact that mineralisation is only found in certain units, such as the Pillow Lavas and associated sediments, the Basal Group, and the Sheeted Dyke Complex, etc. means that lithology is still a controlling factor in mineralisation.

Mineralisation-hosting rocks are part of the Troodos ophiolite, which was originally formed during the Jurassic-Early Cretaceous. However, no Jurassic crust is preserved on Cyprus, with the ophiolite having formed between the start of the Triassic, and the Early Cretaceous, where subduction started due to the convergence of the African and Eurasian plates. After the subduction of any young Cretaceous crust, older and denser Triassic crust entered the trench, causing the ‘roll-back’ of the subduction zone, and the hinge of the subduction zone to migrate backwards - causing the mantle wedge above the young subduction zone to extend, allowing asthenospheric material to well up, forming the 200km wide spreading fabric of the Troodos ophiolite - which continued to spread and form new crust until Turonian times (92-90Ma). Ages of host rocks for the Copper deposits on Cyprus therefore range from the Triassic to the Turonian, ceasing at least by the Eocene, with the orebodies necessarily forming within this time period, as the heat generated by the spreading ridge ‘drove’ the hydrothermal system that formed the ore (Robertson, 1998).

Massive Sulphide deposits on Cyprus take the form of layered deposits containing generally more than 60% pyrite, with comparatively minor amounts of chalcopyrite and sphalerite. Deposited on, or close to, the seafloor, the orebodies can be found anywhere within the volcanic sequence, although mineralisation usually occurs upon a ‘surface’ showing a cessation in magmatism that is needed to generate large tonnage orebodies. Post-mineralisation magmatism is usually, but not always, seen, with orebodies often originally being covered by pillow lavas and many being cut by later dyke intrusions, possibly following the same lines of weakness that the mineralising fluids followed to form the orebody (Constantinou, 1980). Some deposits show an ochreous layer between the sulphides and the overlying pillow lavas, suggesting submarine oxidation of the orebody took place (Boyle, 1990).

The massive sulphide deposits are, except where displacement or deformation has taken place, underlain by a mineralised stockwork zone, formed of mineralised and hydrothermally altered volcanics extending to considerable depths. These stockworks have a core of pyritic mineralisation, with traces of cobalt, lead and molybdenum mineralisation, with an outer layer of remobilised copper and zinc minerals at the periphery of the pyritic mineralisation. Minerals found in the stockwork and massive sulphide orebodies include pyrite (the majority of the orebody), chalcopyrite and sphalerite, with less common minerals, such as copper secondary minerals for example bornite, as well as marcasite, galena, pyrrhotite, cubanite, stannite-besterite and hematite (Richards, Cann et al., 1989).

A complex history of hydrothermal alteration, crustal aging, uplift and emplacement is recorded in the Troodos dykes and lavas. General wall rock alteration around stockwork deposits is found to be generally smectitic or chlorite-smectite mixed facies in grade. However, alteration associated with stockwork mineralisation can be divided into three different types, namely P-, K- and M-type alteration ‘pipes’, so called due to the pipe-like nature of the stockwork mineralisation (Gillis and Robinson, 1990).

P-type Pipes, named after the Pitharokhoma deposit, have sulphide mineralisation associated with chlorite-illite, smectite and chlorite-smectite mixed layer alteration. Possible leached-facies alteration may also be present. K-type pipes, named after the Kokkinopezoula mine, have smectitic and chlorite-smectite mixed layer alteration, as seen in P-type alteration. However, no chlorite-albite alteration is present, and a continuous alteration sequence from smectitic through chlorite-smectite to chlorite-rectorite facies is present. K-type pipes may well have formed through partial retrogression of P-type pipes. M-type pipes, however, have an entirely different type of alteration. Named after the Mathiati orebody, the central stockwork zone host rocks are altered to a chlorite-quartz-pyrite-anatase assemblage not seen in other types of pipe alteration, with chlorites of a comparatively distinct Fe-rich composition. Intermediate stockwork zones include chlorite-rectorite and chlorite-smectite mixed facies, with chlorites in the intermediate zone being more Mg rich than in the central stockwork zone (Richards; Cann et al., 1989).

The source of the massive sulphide mineralisation is seen within the sheeted dyke complex and upper gabbros, below the extrusive sequences of the pillow lavas and volcanics. Within the Sheeted Dyke Complex, zones of epidote-quartz-magnetite rocks up to 900m wide are found, having sharp contrasts with the background ‘normal’ diabase dykes. These ‘epidosite’ zones are thought to form the hydrothermal reaction zones of the major hydrothermal systems that produced the massive sulphide and stockwork deposits of the Troodos ophiolite. These areas have been stripped of copper, zinc and manganese, as well as removing some sodium, and several are seen to lie along strike from major mining districts in Cyprus. These zones form the base of the recharge zone of the hydrothermal ore-forming systems, where metals were leached from the host rocks by the hydrothermal fluids, before flowing towards the surface along fissures and faults, eventually forming the ‘pipes’ of the stockwork deposit and the overlying exhalative massive sulphides (Richardson; Cann et al., 1987). Isotopic evidence, from Sr, H, S and O isotopes, has also suggested a basement sourcing for the metals found in the massive sulphide and stockwork deposits (Bickle; Teagle et al, 1998). This is further confirmed by Pb isotopic evidence, which also details the alteration history of the sheeted dykes, with hydrothermal alteration being overprinted by a later secondary alteration event in the sheeted dyke complex. This is probably due to the later stage low-grade zeolite facies metamorphism, widespread and clearly visible across the Sheeted Dyke Complex (Booij; Bettison-Varga et al., 2000).

References

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Booij, E., L. Bettison-Varga, et al. (2000). Pb-isotope systematics of a fossil hydrothermal system from the Troodos ophiolite, Cyprus: Evidence for a polyphased alteration history. Geochimica Et Cosmochimica Acta 64(20): 3559-3569.

Boyle, J. F. (1990). The composition and origin of oxide metalliferous sediments from the Troodos Ophiolite, Cyprus. Ophiolites; oceanic crustal analogues; proceedings of the symposium Troodos 1987. J. Malpas, E. M. Moores, A. Panayiotou and C. Xenophontos. Nicosia, Cyprus, Minist. Agric. and Nat. Resour.: 705-717.

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