The island of Cyprus is located in the eastern Mediterranean Sea along the southern margin of the Anatolian Tectonic Plate. The area of interest for mineral exploration is a geological feature known as the Troodos Ophiolite, which is a large fragment of ocean floor and associated underlying crust (collectively referred to as oceanic crust) that has been physically obducted to become emergent as the island of Cyprus.
Tectonic framework of the eastern Mediterranean. Closed arrowheads indicate areas of crustal contraction. Open arrowheads indicate subduction zones. Large arrows indicate the relative plate motions of Africa.
The Troodos Ophiolite is a fragment of seafloor which erupted in a marginal intra-arc basin above a north-dipping subduction zone in the Tethys Ocean about 92 million years ago. Volcanism stopped as the Troodos seafloor collided with the Anatolian Plate and the entire Troodos domain was rotated about 90° counter-clockwise. These rocks were subsequently overlain by various calcareous marine sediments. During the Middle Miocene ( 14 million years ago) the Troodos Mountains started to rise due to serpentinite diapirism, with uplift accelerating over the last million years.
Today, the Troodos Ophiolite forms an anticlinal dome such that the deepest formed intrusive units (basal oceanic crust) are now the highest central hills, and the seafloor volcanic rocks and overlying sedimentary rocks are exposed around the flanks. There is evidence that the Troodos Ophiolite is still rising, with recent and historic earthquakes showing the area to be tectonically active.
Cross-sectional evolution of Cyprus geology
Cover Sequence Sedimentary Rocks:
Two discrete sedimentary sequences:
- Recent (<3 Ma) coarse-grained alluvial sediments; and
- Cretaceous to Miocene (<100 Ma) sedimentary sequence (<2 km thick) composed mainly of limestone, chalk and marl. This sequence conformably overlies the volcanic-intrusive ophiolite sequence.
Extrusive Sequence Rocks (Volcanic)
Two discrete sequences of basaltic pillow lavas are identified:
- Upper Pillow Lavas (UPL): 200-400 m thick, contains abundant olivine crystals and rare dykes. The top of the sequence is commonly marked by a thin (<20 m thick), Mn-rich chemical sediment known locally as “umber”; and
- Lower Pillow Lavas (LPL): up to 500 m thick, lacks olivine and contains abundant dykes.
The UPL and LPL are also differentiated by their very distinct geochemical compositions, and both units contain thin, discontinuous sedimentary units within and between the volcanic units.
The Sheeted Dyke Complex (Intrusive)
Ca. 2 km thick, and chiefly composed (50 – 100%) of steeply dipping mafic dykes (each dyke ca. 0.5 to 1.0 m thick) which intrude either gabbro (lower part of unit) or basalt lava flows (upper part)
The Plutonic Complex
Comprising a lower, ultramafic (Harzburgite) unit and an upper mafic (Gabbro) unit separated by an interlayered mafic/ultramafic unit. The lower unit represents the uppermost mantle, and the middle and upper units represent lower oceanic crust components. The thickness of the overall unit is estimated to be at least 5 km. Minor, more evolved (e.g. plagiogranite) intrusive rocks are also recognised as part of this package.
Cyprus VHMS deposits are composed of pyrite with varying contents of chalcopyrite and sphalerite, with rare galena (Bear, 1963). Marcasite, pyrrhotite, rutile, gold and silver are also present, with silver strongly associated with chalcopyrite. Clay and silica form primary alteration haloes around the deposits. Many of the deposits have been weathered with copper oxides, chalcocite, covellite, bornite, digenite, vallerite, tenorite, as well as jarosite, magnetite and hematite as the main secondary minerals (Pantazis, 1979).
Constantinou and Govett (1973) define three different types of sulphide ore within Cyprus VHMS orebodies. Zones A, B and C are defined according to the style of pyrite mineralisation and amount of contained sulphur. Most ore bodies comprise Zone A mineralisation with either Zone B or Zone C making up the remainder of the ore-body. Rarely are all three zones present.
Zone A forms in the upper part of the VHMS deposits and is massive with >40% S. It commonly comprises two sub-types: conglomeratic ore and underlying compact ore. The conglomeratic ore occurs in fragmented zones with pillow shaped or spheroidal blocks of pyrite in a sugary, friable matrix dominated by pyrite. The size and proportion of sulphide blocks increases downward, and the underlying compact ore is much less porous than the overlying conglomeratic ore, containing large blocks of pyrite often coated with chalcopyrite, with covellite forming along fractures.
Zone B underlies Zone A and is a pyrite-quartz zone grading from 40% S at the top, to 30% S at the base. Cu values are typically 1-2%.
Zone C is the stockwork zone, and underlies Zones A, and B if present. The stockwork zone contains <30% S, and contains vein hosted as well as disseminated pyrite. The mineralogy and chemistry of the massive sulphide bodies varies between localities. Some contain up to 4.5% Cu, and were previously mined for Cu. Others were mined historically for S and Fe, or occasionally their gossans for precious metals. All, however, possess some or all of the common features described above (Parvaz, 2014).
VHMS deposits and occurrences in Cyprus are likely to be associated with gossans and other useful exploration vectors such as gossans, exhalites, limonite and ochre.
Gossan is a red-brown Fe-oxide-rich zone (essentially rust) containing residual materials from the weathering or supergene alteration of sulphide material, remnants of which may or may not be present at depth. Some features of the original sulphide may be preserved in the gossan but more commonly the process is highly destructive. The most common minerals in gossans are goethite and hematite, along with jarosite, limonite and silica. Some gossans may also contain magnetite as a supergene mineral (Blain and Andrew, 1977). During gossan formation, large amounts of iron are fixed above the water table as goethite or hematite. Other metals can be leached by groundwaters and may be precipitated nearby, variably as oxides, carbonates and sulphates (Parvaz, 2014). Gossans may be in- situ or transported.
Exhalite formation commonly represents the distal facies equivalent of a massive sulphide orebody. The most intimately associated exhalites with VHMS deposits are umbers which are Fe-Mn-rich sediments which precipitate from hydrothermal plumes above black smoker vent fields (Boyle, 1990) or through precipitation from off-axis thermal springs (Robertson and Fleet, 1976). They are commonly brown to black, very fine grained, microscopically porous and composed of iron and manganese oxides (Boyle and Robertson, 1984). They vary from massive to finely laminated.
Limonite is a general term for mixtures of amorphous iron oxides, finely crystalline goethite with minor silica, hematite, jarosite, lepidocrocite, or manganese oxides in various proportions (Blain et al., 1977).
Ochre is defined by Parvaz (2014) as a bright red, Fe-rich, finely bedded sediment intimately associated with massive sulphide deposits. Ochre mineralogy is dominated by goethite, jarosite, quartz, along with amorphous Fe oxides and traces of hematite and gypsum (Herzig et al., 1991). The presence and concentration of secondary copper sulphides are also indicative of the proximity of primary VHMS mineralisation, albeit their distribution will depend on local topography, the nature and juxtaposition of different lithologies and structures, and the hydrological regime (Parvaz, 2014).
Modern seafloor volcanic centres are also areas of intense hydrothermal activity, which can deposit massive sulphide accumulations, commonly referred to as Volcanic-Hosted Massive Sulphide (VHMS) deposits. Ancient forms of these deposits are the exploration targets in Cyprus. The basic ore-forming processes of VHMS deposits are well understood – broadly coincident with magmatism, seawater is drawn down into the oceanic crust where it becomes progressively hotter and richer in metals and sulphur. This now ‘pregnant’ hydrothermal fluid then rises back towards the seafloor with the metal-sulphur either depositing along the way or erupting onto the seafloor itself as “black smokers”. The richest deposits are those that erupt onto the seafloor. The size and grade of a VHMS deposit is primarily controlled by the size and persistence of the hydrothermal system. The location of VHMS deposits is controlled by the available pathways for the hydrothermal fluid, i.e. faults and fractures, with particular relevance given to the main faults which control volcanic rifting. Given the dynamic nature of seafloor volcanic domains the VHMS deposits are typically buried by subsequent volcanic eruptions.
VHMS deposits are stratabound concentrations of sulphide minerals precipitated from hydrothermal fluids in extensional seafloor environments. The term volcanogenic implies a genetic link between mineralization and volcanic activity, but siliciclastic rocks dominate the stratigraphic assemblage in some settings. The principal tectonic settings for VHMS deposits include mid-oceanic ridges, volcanic arcs (intra-oceanic and continental margin), back-arc basins, rifted continental margins, and pull-apart basins.
The composition of volcanic rocks hosting individual sulphide deposits range from felsic to mafic, but bimodal mixtures are not uncommon. The volcanic strata consist of massive and pillow lavas, sheet flows, hyaloclastites, lava breccias, pyroclastic deposits, and volcaniclastic sediment.
Deposits range in age from Early Archean (3.55 Ga) to Holocene; deposits are currently forming at numerous localities in modern oceanic settings. Deposits are characterized by abundant Fe sulphides (pyrite or pyrrhotite) and variable but subordinate amounts of chalcopyrite and sphalerite; bornite, tetrahedrite, galena, barite, and other mineral phases are concentrated in some deposits. Massive sulphide bodies typically have lensoidal or sheet-like forms. Many, but not all, deposits overlie discordant sulphide-bearing vein systems (stringer or stockwork zones) that represent fluid flow conduits below the seafloor. Pervasive alteration zones characterized by secondary quartz and phyllosilicate minerals also reflect hydrothermal circulation through footwall volcanic rocks. A zonation of metals within the massive sulphide body from Fe+Cu at the base to Zn+Fe±Pb±Ba at the top and margins characterizes many deposits. Other features spatially associated with VHMS deposits are exhalative (chemical) sedimentary rocks, subvolcanic intrusions, and semi-conformable alteration zones.
The basic ore-forming processes of these hydrothermal systems are well understood, and research indicates that sulphides can also be deposited in the intrusive units beneath the volcanic rocks, associated with sheeted dike and ultramafic complexes.
Modern seafloor volcanic centres are also areas of intense hydrothermal activity, which can deposit massive sulphide accumulations, commonly referred to as Volcanic-Hosted Massive Sulphide (VHMS) deposits. Ancient forms of these deposits are the exploration targets in Cyprus. The basic ore-forming processes of VHMS deposits are well understood – broadly coincident with magmatism, seawater is drawn down into the oceanic crust where it becomes progressively hotter and richer in metals and sulphur. This now ‘pregnant’ hydrothermal fluid then rises back towards the seafloor with the metal-sulphur either depositing along the way or erupting onto the seafloor itself as “black smokers”. The richest deposits are those that erupt onto the seafloor.
The size and grade of a VHMS deposit is primarily controlled by the size and persistence of the hydrothermal system. The location of VHMS deposits is controlled by the available pathways for the hydrothermal fluid, i.e. faults and fractures, with particular relevance given to the main faults which control volcanic rifting. Given the dynamic nature of seafloor volcanic domains the VHMS deposits are typically buried by subsequent volcanic eruptions.
Cyprus Type VHMS deposit
Koski et al. (2012) define 5 categories of VHMS deposits based on inferred tectonic setting and lithological association. The mafic-ultramafic type is deemed to be analogous ‘Cyprus Type’ with the currently producing Skouriotissa deposit being the archetypal example.
A “black smoker” venting highly mineralised hydrothermal fluid on the ocean floor. The diagrams above show a schematic cross-section through a typical ophiolite sequence, and presents the distribution of various mineralisation styles expected within that lithostratigraphic sequence. Mineralisation may also develop during uplift and deformation of oceanic crust to form the ophiolite fragment.
Globally, mineralisation with VHMS affinities host significant base metal deposits. A sub-set of the VHMS classified deposits are the “Cyprus-Style” deposits (which are associated with seafloor spreading and ophiolite/ocean-floor stratigraphy and structure). Academic literature indicates that the Troodos Ophiolite hosts base metal deposits of a similar size to those identified in VHMS settings globally (Constantinou, 1980). Specific to “Cyprus-style” VHMS systems is a structural control along one or both margins (Figure 54), as well as a mantling of the deposit by a broader zone of characteristic alteration that decreases away from the deposit (Franklin et al., 1981). The alteration is dominated by chlorite, quartz, pyrite and epidote, and silicification is commonly sufficiently intense in the stockwork zone to be detected as a resistivity anomaly in geophysical data.
Major worldwide VHMS deposits.
(Adapted from Barrie and Hannington)
1) Iberian Pyrite Belt;
5) Big Stubby, Mons Cupri and Whim Creek;
6) Scuddles-Golden Grove;
8) Mount Read;
11) Northern Cordillera.