massive sulphide deposits

massive sulphide deposits Massive sulphides are rocks made up primarily of sulphide minerals. There is no accepted definition of the term ‘massive sulphide’, but it is most commonly used in describing strata-bound (and often stratiform) deposits in sedimentary or volcanic settings formed from the flow of heated fluids (usually sea water) through sediments or igneous rocks, or both, the circulation being driven by associated volcanic activity. Massive sul-phides range in age from Archaean to those forming today. Although they form a minor part of the Earth's crust, massive sulphides have received considerable atten-tion throughout history. This interest has arisen mainly because they are a major source of many metals (in par-ticular lead, zinc, copper, and silver) that have been critical in human technological development and are vital to a modern industrial economy. Exploration continues for new ore bodies, and considerable effort has been expended to develop models for the formation of massive sulphides in the hope that these will aid in the search for new reserves. Massive sulphide deposits are also of interest for the record they preserve of the magmatic and hydrothermal processes that control the composition of the Earth's crust, oceans, and atmosphere.

Generalized genetic model

Although various types of massive sulphide deposits have been described, they share some basic similarities in their paragenesis. Cold aqueous fluid (commonly sea water) is drawn down through sediments or igneous rocks and its temperature is raised by an underlying heat source. This heat source is usually a relatively shallow magma chamber or a recent igneous intrusion. As the cold water is heated to become a hydrothermal fluid, any dissolved sulphate that it contains is reduced to sulphide or precipitated as anhydrite (calcium sulphate). The fluid also becomes depleted in magnesium, and this causes a drop in pH. The resultant hot, acidic fluid reacts with the solid phase through which it is flowing. Various elements are leached from the rock and dissolved as complexes (predominantly chloride complexes, with smaller amounts of sulphide complexes). This modified hydrothermal fluid rapidly reaches equilibrium with an assemblage of secondary minerals. The hot hydrothermal fluid becomes less dense and flows upwards. As it nears the sea floor it cools. This causes precipitation of minerals such as pyrite (FeS₂), chalcopyrite (FeCuS₂), sphalerite (ZnS), and galena (PbS) that form massive sulphide deposits, together with sulphates (anhydrite (CaSO₄) and barite (BaSO₄)) as the hydrothermal fluid mixes with sea water entrained into the system. The exact mineralogy depends on the physico-chemical nature of the hydrothermal fluids and the extent of sea-water mixing. If, as usually happens, the rising hydrothermal fluids exit to the sea floor, they may then rise hundreds of metres into the overlying water column. Here they can form precipitates that can be carried many kilometres by bottom currents. Alternatively, if they are sufficiently dense, the exiting hydrothermal fluids may form pools of metal-rich brines similar to those observed in the Red Sea today. The modern oceans are generally well oxygenated, and the particulate plumes are consequently dominated by iron and manganese oxides. In the Archaean oceans the plumes would have consisted largely of sulphide minerals.

Idealized volcanogenic and sediment-hosted massive sulphide deposits are illustrated in Fig. 1. The deposits consists of a zoned body, in which the central core of iron-rich sulphides (pyrite or pyrrhotite, or both and a copper-rich sulphide (chalcopyrite) is overlain by more zinc- and lead-rich sulphides (sphalerite and galena) and is underlain by an alteration zone containing veins (‘stringers’) of iron and copper sulphides in a matrix of hydrothermal silicate minerals. This zone of alteration becomes progressively less distinct from the surrounding (‘country’) rocks as one moves laterally away from the deposit. The sulphides may be overlain by, or partially intermingled with, a cap of sulphate minerals. The deposit is usually mantled by iron and manganese oxides (‘ochres and umbers’) and iron-rich cherts that become thinner away from the deposit; these may extend for several kilometres before grading into the background sediments. In deposits that have been uplifted and exposed on land, the upper sulphides and oxides are invariably oxidized and weathered to a variety of secondary minerals (‘gossan’) by circulating meteoric water. During this process some metals, such as gold, may become enriched in the weathered zones.

Volcanic-hosted, or volcanogenic, massive sulphide deposits can be defined as deposits formed from circulation of hydrothermal fluids through volcanic rocks without involvement of significant amounts of non-volcanic sediments. Hence, these deposits contain only species leached from igneous rocks (except for a few elements derived from the circulating sea water). Sediment-hosted massive sulphide deposits are also generally associated with igneous activity, which provides the heat source for hydrothermal circulation. They differ, however, from volcanogenic deposits in that the hydrothermal fluids have also flowed through sediments overlying the igneous rocks. Hence, the hydrothermal fluids and resultant massive sulphide deposits may contain species derived from the sediments, as well as from the igneous rocks.

Volcanic-hosted deposits

Massive sulphide deposits are currently forming from hydrothermal systems at a large number of sites on mid-ocean ridges. The largest of these yet discovered is the TAG (Transatlantic Geotraverse) site at 26°N on the Mid-Atlantic Ridge at a depth of 3500 m below sea level. Detailed sampling by submersibles, surface ships, and deep-ocean drilling has provided much information about this site. Black smokers, debouching fluids at 350 °C into the overlying sea water sit atop a mound of hydrothermal minerals, including sulphides, sulphates, oxides, and silicates. The mound is 200 m in diameter and 50 m high and contains approximately 5 million tonnes of hydrothermal minerals. Radiometric dating suggests that hydrothermal activity has been intermittent at the site for about 50 000 years. There are also older hydrothermal deposits in the TAG area that are heavily weathered, the massive sulphides being altered to oxides and eroded by bottom-water currents and tectonic activity. It is likely that most mass-ive sulphides formed on mid-ocean ridges are destroyed within a few million years after hydrothermal activity ceases.

The closest analogues in the geological record to modern hydrothermal systems are the massive sulphides found in ophiolite belts throughout the world (e.g. Cyprus, Oman, the Urals). These deposits are generally small (few are larger than 20 million tonnes and most are smaller than a million tonnes) and are most commonly mined for copper, although some contain significant amounts of gold in gossans. The commonly accepted interpretation of these deposits is that they are located in portions of uplifted oceanic crust that formed in the same manner as modern, mid-ocean ridge hydrothermal systems. Massive sulphide deposits in ophiolite settings are commonly overlain by later flows of basalt pillow lavas that prevent them from being destroyed.

Kuroko-type massive sulphide deposits are also found in environments with modern tectonic equivalents, thought to have formed in back-arc and interplate settings. They differ from ophiolite-hosted deposits in a number of ways. They are associated with felsic volcanism and they were emplaced in relatively shallow seas (shallow enough for steam explosions). They have a higher proportion of galena and sphalerite in the massive sulphides and smaller amounts of chalcopyrite and pyrite than is found in ophiolite settings, and their sulphate minerals include a higher proportion of barite relative to anhydrite. The deposits are generally overlain by mudstones or tuffs, or both. Kuroko deposits tend to be larger than ophiolite-hosted deposits, with orebodies up to 75 million tonnes. Kuroko deposits were first described in Japan (Kuroko means ‘black ore’ in Japanese), but similar types of deposits have been identified in Fiji, Turkey, and the Philippines.

The largest volcanogenic massive sulphide deposits are found in greenstone belts in Archaean cratons, such as those in South Africa and Canada. They are not the products of presently active tectonic processes, but the general form of the deposits is similar to other volcanogenic massive sulphides. The greenstone-hosted deposits formed in shallow water in association with basic and more silicic rocks; they have an underlying hydrothermally altered zone and contain abundant copper, zinc, and lead sulphides. The greenstone-hosted deposits differ in that they lack a cap of sulphate minerals and they are overlain by later volcanic rocks rather than clastic sediments. The major difference is, however, one of scale. For example, the Kidd Creek deposit (in the Atitibi greenstone belt of south-eastern Canada) contains over 250 million tonnes of ore, and there are several other deposits containing in excess of 100 million tonnes of ore. The size of the greenstone-hosted deposits may be related to the greater intensity and duration of igneous activity in the Archaean.

Sediment-hosted deposits

There are several places in the oceans (e.g. Guaymas Basin, Gulf of California) where spreading centres are buried beneath sediments shed from the nearby continents. Hydrothermal circulation in these environments differs from sediment-starved mid-ocean ridge sites. Instead of the fluids flowing directly into sea water from the oceanic crust, they must first pass through an overlying layer of sediments. When the fluids do reach the overlying sea water, their chemistry is substantially altered. They have lower dissolved metal and sulphide concentrations due to precipitation of massive sulphides in the sediments and the chemical signature of the fluids provides clear evidence of leaching of elements from the sediments by the hydrothermal fluids.

The heat source that powered hydrothermal circulation that formed sediment-hosted massive sulphides is not always exposed, but fluid inclusion and mineralogical studies indicate that these deposits were the products of high-temperature (around 350 °C) fluids. Sediment-hosted massive sulphides have a wider range in mineralogy than volcanic-hosted deposits, reflecting the variation in the composition of sediments (e.g. arenites, carbonates, evaporites, etc.). In terms of their economic geology, sediment-hosted massive sulphide deposits tend to have higher concentrations of lead, zinc, and silver and relatively smaller quantities of copper and iron than volcanic-hosted deposits. Two of the largest sediment-hosted massive sulphide deposits are the Sullivan (Canada) and the Broken Hill (Australia) deposits. The Sullivan orebody comprises 160 million tonnes of massive and bedded sulphides, consisting mainly of sphalerite, galena, and pyrrhotite. The deposit is located in a thick sequence of continentally derived turbidites which were intruded by gabbro sills shortly after the sediments were deposited. These sills may have provided the heat source for hydrothermal circulation. The massive sulphides formed at, or close to, the sediment–sea-water interface, possibly in a brine pool. The deposit is overlain by turbidites that were altered to an albite-rich assemblage in the waning stages of hydrothermal activity. The funnel-shaped alteration zone underlying the sulphides extends to a depth of least 1500 m and is unusual in containing very high concentrations of tourmaline. The Broken Hill district comprises 180 million tonnes of lead- and zinc-rich orebodies and has been a major source of these metals in the world. Unlike Sullivan, it has been extensively metamorphosed, so that many of the original features of the ore bodies are obscured. However, it also appears to be the product of hydrothermal activity within a thick sediment column. Both Sullivan and Broken Hill are thought to have formed in early continental rifting environments.

The factors required to generate a large orebody include: a source of metals, a means of transporting large amounts of metals in solution, and a means of trapping precipitates from these hydrothermal fluids before they are dispersed or destroyed. An intriguing feature of both Broken Hill and Sullivan is that there is evidence for the involvement of non-marine evaporites in their formation, and these evaporites may have been the key to the genesis of these two very large, rich ore bodies. Evaporites contain high concentrations of chloride and other anions that are readily dissolved by circulating hydrothermal fluids and greatly enhance the solubility of metals in solution through formation of dissolved complexes. A modern example is provided by the Salton Sea geothermal field in southern California, where deep hydrothermal fluids interact with non-marine evaporites to produce very high dissolved concentrations of ore-forming metals and actively deposit iron–zinc–lead–copper sulphide minerals. Non-marine evaporites are relatively common in early rifting environments. Their presence may be very important in the formation of giant sediment-hosted massive sulphide deposits.

M. R. Palmer

Bibliography

Guilbert, J. M. and and Park, C. F. (1986) The geology of ore deposits. W. H. Freeman, New York.