SEA FLOOR - VOLCANOGENIC MASSIVE SULPHIDES
"Cyprus" or "Besshi"-type VMS Deposits
Volcanogenic Massive Sulphide (VMS) deposits - General
Volcanic-related massive sulphide deposits, or volcanogenic massive sulphide (VMS) deposits, occur throughout the world and across the entire geological column. Extensive research over the last few decades has identified two broad types of VMS:
- "Back-Arc Rift" Type
- Perhaps the most common type, "Back-Arc Rift" VMS deposits are associated with areas of Continental Crust where attenuation (thinning) and rifting has occured.
- Typically, this type of VMS is spatially related to felsic volcanic material located beneath the deposit itself -- representing underlying Continental Crust
- These types of deposit are generally polymetallic, consisting of zinc, lead, copper, gold, silver and barite
- Sea Floor -- "Cyprus" or "Besshi" Type
- "Cyprus" or "Besshi" type VMS deposits form on the sea floor and are related to the new formation of Oceanic Crust and spreading zones where new crust is being created
- These VMS deposits are related to extrusive, often pillowed (representing formation under water), mafic basalts, usually underlain by ultramafic intrusive and cumulate rocks
- Their metal content is usually restricted to copper, gold and with less common zinc
The term "massive sulphide" relates to rocks which consist of >50-60% sulphide minerals. VMS deposits form as lens-like or tabular bodies usually parallel to the volcanic stratigraphy or bedding. They are often underlain by a stockwork zone of vein or stringer sulphides, which form either perpendicular to the volcanic stratigraphy or sometimes parallel to it. These stringer zones may be related to faults and fractures which were present at the time of VMS formation.
Host rocks surrounding VMS deposits are often altered with the existing rocks converted to epidote, chlorite, sericite, silica or clay minerals. It is the footwall rocks -- or rocks underneath the main tabular VMS deposit - that are most highly altered.
VMS deposit are now believed to be formed by the circulation of sea water (plus perhaps some volcanic fluids) underwater in a rifting or spreading setting. For the sea water to circulate, there must be an underlying heat source -- in a rifting setting this is usually generated by melted subducted Oceanic Crusted or at a spreading centre, fresh magmatic material from the Upper Mantle. As the hot sea water (hydrothermal solution) circulates, it leaches metals from the surrounding volcanic & sediments, which in time are deposited on or close to the sea floor as temperatures and chemistry changes. The circulating sea water generally flows along faults or fissures towards the sea bottom. These hot fluids alter the surrounding host rocks -- alteration which becomes more intense immediately beneath the VMS deposit itself.
Close to the sea floor, the fluids start to interact with colder sea water and undergo a pressure release and chemical transformation. The metal carrying capacity of the solution decreases sharply and metals are deposited either in veins beneath the sea floor or as mounds on the sea floor. The type of metal deposited depends mainly on the temperature of the fluid and its metal content. Hot solutions generally deposit pyrite and chalcopyrite (copper sulphide) -- cooler solutions tend to deposit sphalerite (zinc sulphide) and galena (lead sulphide).
"Cyprus" or "Besshi" Type VMS Deposits
At a spreading centre, where new Oceanic Crust is continually forming, the environment is hot and dynamic. Not only are lavas and basalts erupted on the sea floor but hydrothermal solutions and vents are common. Today these are known as "Black Smokers" -- vents and fissures where hot solutions escape on to the sea floor. They are called "Black" smokers with the black representing fine sulphide particles which form crusts, mounds or plumes of solids which "rain" on to the sea bottom. Black Smokers are thought to be the modern equivalent of volcanogenic massive sulphides in the geological record.
Generalised cross-section through a typical hydrothermal VMS system
(After: Hannington, Gallery, Herzig & Petersen; 1998)
In the geological record as many as 200 ophiolite sequences are preserved world-wide, and these contain at least 50 major producing VMS copper mines. One of the largest and best known ophiolitic copper mining districts is the Troodos Massif of Cyprus. During the hiatus of Cypriot mining between the 1950's and 1974 some 1.0-1.5 million tonnes of copper was produced from a number of different districts. In Cyprus, major districts correspond to the locations of fossil graben structures defining distinct clusters of deposits with an average of 30 VMS's per cluster containing up to 20 million tonnes of massive sulphide.
Mining in Cyprus, as in Oman, began in the Bronze Age some 4,000 BC. Early mining focussed on copper and gold but later sulphur and zinc were extracted. On Cyprus, economic deposits range from 0.05 to 16.00 million tonnes in size with grades between 1.0-5.0% copper and up to a gram gold. Most of the past production of copper in Cyprus was from small number of larger deposits with about 50% of known occurrences >1 million tonnes and around 15% at >10 million tonnes in size.
Ophiolite VMS deposits are generally formed of two distinct components -- the vertical to sub-vertical stringer zone of vein and disseminated sulphide beneath -- and the more massive sulphide as tabular, blankets lying parallel to the sea floor. Generally the stringer zones are lower grade -- in Oman ranging between 1.0-1.5% copper but with the massive sulphide portions attaining much higher grades averaging between 2.0-2.5% copper but up to 6.0% copper.
The mineralogy of Cyprus Type VMS's is relatively simple with deposits typically formed of >90% pyrite and chalcopyrite. This lends itself to simple and relatively cheap metallurgical beneficiation, with concentration to a 20-25% copper grade concentrate. Gold and zinc report to the copper concentrate to varying degrees.
Oman Semail Ophiolite VMS Deposits
To date more than 150 volcanogenic massive sulphide (VMS) occurrences have been recognised along the approximate 500 kilometres of the Semail Ophiolite Belt of northern Oman. Most of these deposits cluster in groups about 20-25 kilometres apart. Most of the larger deposits are associated with a change of volcanic type -- from large pillow lava basalts (Lower Basaltic Pillowed & Massive Lava) to a smaller pillowed mafic volcanic (Upper Basaltic Pillowed & Massive Lava) -- mappable as the V1-1 and V1-2 units by JICA (see below). Most of the known deposits are similar to the Cyprus VMS occurrences.
In detail, there are two types of VMS metallogenic associations -- the typical Cyprus Type with copper, cobalt, selenium, nickel and molybdenum (eg Bayda & Lasail) -- and deposits with copper plus more zinc, lead, silver and gold (eg Aarja). The latter type indicate that the Semail Ophiolite VMS deposits may not just be spreading margin type but also have some component of subduction related processes.
Detailed mapping by JICA indicated that may of the known VMS deposits in Oman were directly related to sea floor faults. These penecontemporaneous faults focussed the fluid flow localising alteration and metal distribution.
JICA Consolidation Report, 2000
