Volcanogenic massive sulphide deposits

There are more than a thousand volcanogenic massive sulphide deposits in the world. Collectively, they are a major source of copper, lead and zinc, and are important producers of gold and silver. The deposits are usually zoned and comprise massive syngenetic sulphide ores overlying an epigenetic stringer zone. They are associated with acid to basic volcanics extruded in aqueous, usually submarine, conditions. There are many classes and subclasses: amongst these are modem sulphide deposits including the 'black smokers', their possible fossil equivalents in ophiolite sequences, plus Besshi and Kuroko deposits and many types of metamorphosed massive sulphides.

Modern polymetallic sulphides were first discovered along the East Pacific Rise (21°N) in 1978, followed soon after by active hydrothermal vents associated with sulphides. Although presently none are mined, they have some potential for copper, silver and zinc. Many such sites are known, most are associated with mid-axis ridges but some are found close to off-axis volcanism, at seamounts. The earliest and hence best known examples include those along the East Pacific Rise at 21°N and 13°N, along the Juan de Fuca and Explorer ridges and the Galapagos Rift sulphide mounds. They are important because their formation conditions can be measured directly and they show how little time is needed to produce appreciable tonnages of 'ore'. Similarly, their rapid oxidation and destruction show the transitory nature of these deposits. They confirm the validity of the synsedimentary volcanogenic model and, indeed, have become its paradigm. In this model, convecting hot water (mainly. sea water) leaches metals from the underlying volcanics and returns to the seabed via faults, rifts or volcanic conduits. There, it mixes with normal sea water to precipitate sulphides and sulphates to form sulphide mounds, sulphide-rich chimneys and plumes of 'black smoke' (very fine-grained pyrrhotite with minor sphalerite and pyrite) and 'white smoke' (silica, baryte and pyrite).

The styles of mineralization and dominant sulphide mineralogy of modem seafloor sulphides are not uniform. At 21°N, copper, iron and zinc sulphides form chimneys resting on sulphide mounds; at Juan de Fuca, zinc sulphides form mounds, whereas the massive sulphides found at the Galapagos Rift are dominated by pyrite. (Rona, 1984)

There are few descriptions of sulphides associated with off-axis seamounts, but their mineralogy comprises major pyrite and marcasite, plus chalcopyrite, pyrrhotite and covelline with minor zinc sulphide and cobaltian pyrite, together with baryte, quartz and opaline silica. Limonite and manganese oxyhydroxide oxidation products are common. Many seamounts have thin cobaltian manganese oxide crusts but little or no sulphide. (Lonsdale et al., 1982; Hekinian and Fouquet, 1985)

Essentially these are copper-bearing massive pyrite deposits that produce by-product zinc, silver and gold. Amongst the most discussed examples are those of the Troodos Mountains, Cyprus and this class of deposit is often called the 'Cyprus-type'. The deposits are found within, or close to, basic pillow lava sequences; all have massive sulphide ores overlying stringer zones and many have oxidized ochreous zones comprising goethite, quartz and traces of maghaemite lying above the massive sulphides. The mineralogy of the massive ores is dominated by pyrite together with variable amounts of chalcopyrite, sphalerite and pyrrhotite. Subordinate ore types include chalcopyrite-bornite-pyrite and sphalerite-pyrite. Pyrite has euhedral and collomorphic generations and shows extensive shattering and replacement by chalcopyrite; sphalerite also is replaced by chalcopyrite. The underlying stringer zone ores are essentially quartz-pyrite-chlorite veinlets. The deposits are widely believed to be the fossil equivalents of the East Pacific Rise black smoker-type of mineralization, although some authors have suggested, on mineralogical grounds, that the Galapagos sulphide mounds or modern off-axis seamount sulphides are closer analogies. (Constantinou and Govett, 1972, 1973; Constantinou, 1976, 1980; Oudin and Constantinou, 1984)

Sulphide deposits in the Oman ophiolite: Three massive sulphide deposits, Lasail, Bayda and Aarja, occur within pillow lava units of the Semail ophiolite. They comprise massive pyrite with subordinate copper and zinc-rich ores that overlie a quartz-pyrite-base metal sulphide-chlorite stringer zone in footwall basalts. Lasail and Bayda are mineralogically similar but have significant differences from Aarja. All three deposits have been interpreted as being ocean-floor exhalative deposits associated with seamounts lying away from spreading axis magmatism. (Alabaster and Pearce, 1985)

The type locality is the Besshi Mine, Japan, but they are also known as kieslager. Copper, zinc and by-product silver and gold are obtained from these deposits. They are stratabound conformable copper-rich pyrite deposits found in complex sedimentary and basic volcanic sequences. The ores comprise pyrite, chalcopyrite and locally pyrrhotite, sphalerite and magnetite, and are found as bedded or lenticular bodies that are often restricted to a single stratigraphical horizon. Minor amounts of bornite, haematite, sphene, ilmenite and rutile are present. In unmetamorphosed examples, graded bedding and collomorphic textures are retained in the sulphides. Many Besshi deposits, however, are metamorphosed, so that the ores show tectonic folding and schistose textures and the sulphides are accompanied by muscovite, chlorite, quartz, epidote, actinolite, albite and stilpnomelane. (Kanehira and Tatsumi 1970; Kase, 1977; Yui 1983; Fox, 1984)

These were first described from the Miocene orebodies of the Green Tuff region of Japan and are associated with felsic volcanism. Their tectonic setting, associated volcanism and ore types are different from spreading axes sulphides. Most deposits are unmetamorphosed. Characteristically, the deposits are zoned. The complete sequence from hangingwall to footwall is: quartz-haematite-baryte with minor sulphides (tetsusekiei ore) lying above baryte-sulphide ores. Beneath these ores lie the main massive sulphide zones.

The uppermost zone is black ore (kuroko) which is divided into two subzones: an upper, sphalerite, baryte, pyrite, galena and tetrahedrite subzone and a sphalerite, baryte, pyrite, chalcopyrite and quartz subzone. The middle zone is copper-rich (oko) and comprises chalcopyrite, pyrite and quartz ores above chalcopyrite-pyrite, and the lowest zone is pyrite-rich, comprising pyrite, chalcopyrite and sphalerite. Beneath the massive ores lies a stringer zone (keiko ore) of quartz-pyrite veins with chalcopyrite and minor sphalerite and galena, that passes downwards into barren footwall rhyolites or pyroclastics. (Lambert and Sato, 1974; Shimazaki, 1974; Ohmoto and Skinner (eds.) 1983). Recently the origin of Kuroko deposits has been reinterpreted using textural evidence from doubly polished thin sections. In the original interpretation, the pyrite, chalcopyrite and sphalerite-galena zones were believed to reflect the progressive cooling of ore fluids as they moved away from their source.

Although the ores were essentially co-temporal, the pyrite ore was interpreted as being the earliest and highest temperature. This model is now accepted as being too simple. The present model (Eldridge et al., 1983) suggests that the initial hydrothermal fluids mixed with sea water and were cooled to < 200°C to precipitate a 'primitive black ore' (fine-grained collomorphic sphalerite, galena, pyrite and baryte with only minor chalcopyrite) as a sulphide mound. Later hydrothermal fluids were protected from the full cooling effect of the sea water by this 'primitive black ore', and as the temperature of the sulphide mound rose, the primitive collomorphic ore recrystallized and then was replaced by chalcopyrite. This ore, in turn, was replaced by pyrite as the maximum ore fluid temperature of approximately 350°C was attained. In addition to the successive replacement of lower temperature ores by higher temperature ores, earlier ore types were remobilized to stratigraphically higher levels.

The end result is the commonly seen sequence: pyrite-rich ores overlain by chalcopyrite-rich ores, themselves overlain by galena-sphalerite-rich black ores. This interpretation is consistent with the re-interpretation of the presence of crystallographically controlled chalcopyrite blebs in sphalerite, a texture that is common in these ores and many others. Previously, this texture was explained as chalcopyrite exsolution and taken as an indication of moderate to high temperatures of formation. The concentration of chalcopyrite blebs along grain boundaries, fractures, twin planes and cleavage in sphalerite and the experimental data that show the limited solubility of chalcopyrite in sphalerite (below 500°C) has led to the realization that the chalcopyrite distribution (now called chalcopyrite disease) is due to epitaxial growth of chalcopyrite on sphalerite, or is a replacement phenomenon. (Barton, 1978; Eldridge et al., 1983)

Many massive sulphide deposits share the characteristics of Kuroko deposits, especially in terms of their tectonic setting, associated volcanism and ore types. Often, however, they have suffered the combined effects of tectonism and metamorphism, so making their classification speculative.