The present is the key to the past. — Sir Charles Lyell (1797-1875) Geologist Steven Scott, a professor in the Department of Geology at the University of Toronto, has one of the most exciting research labs in the world. He spends much of his time on ships at sea and in a tiny submersible at the bottom of the earth’s oceans. He is one of a handful of geologists who have actually glimpsed hydrothermal vents form into sulphide deposits — something which, until now, was left to the imagination of geologists studying massive sulphide ores. These deposits, some of which are relatively rich in gold, are forming near rifts on the ocean floor. Scott and his colleagues are applying the knowledge they gain from such observations to the exploration of ancient ore-forming systems — systems with which we, on dry land, are more familiar. From these studies, two ore genesis models are emerging. At two sites in the eastern Pacific Ocean, the contrasting physiography of the active rift is having an influence on the deposit types being formed. Of the numerous sites explored, the Guaymas Basin in the Gulf of California is an example of a heavily sedimented ridge crest. The Southern Explorer Ridge (ser for short) is being used as an example of a sediment-starved ridge crest. The fundamental formation process is the same in both cases, but, because the rocks filling the axial region are different (predominantly sedimentary versus volcanic), quite different types of deposits are being formed. Both sites have analogues in the ancient geological record. The geology of the Guaymas Basin deposits has many similarities (especially stratigraphic and mineralogical features) to the Windy Craggy copper-cobalt sulphide deposits, in northwestern British Columbia; and the ser deposits display similarities to the sulphide deposits of the island of Cyprus in the Mediterranean Sea. Structural features in the massive sulphide deposits at Sturgeon Lake, in northwestern Ontario, are similar to features of modern hydrothermal sulphide deposits even though the compositions of the host lavas are quite different (primarily felsic versus mafic). Comparisons between ancient and modern-day deposits are coarse, but they do suggest where new deposits of both types might be found.
The formation of hydrothermal vents above subvolcanic sills at Guayma Basin and at Windy Craggy implies a similar environment of formation — a spreading rift system within continental crust. Other similarities include sediments lying on top of pillow basalts, presence of turbidites, the injection of sills into sediments, pyrrhotite instead of pyrite as the dominant iron sulphide, and the presence with the sulphides of calcite originating from calcium carbonate in the marine sediments.
Sea floor polymetallic sulphide deposits at the ser exhibit many morphological and mineralogical features in common with the Cyprus-type massive sulphide ores, including a stockwork beneath a massive sulphide lens, copper-to-zinc vertical zoning, predominance of iron sulphides associated with varying amounts of copper and zinc sulphides, and a cap of sediments (ochre) rich in iron oxide and silica. The environment that forms such deposits is a sediment-starved oceanic spreading ridge — a mid-ocean spreading ridge for ser deposits and probably in back-arc or inter-arc settings for the Cyprus-type deposits. However, regardless of the actual tectonic setting, it is the comparison of the local geology and process leading to ore formation that is attracting the attention of researchers.
Major structures in the Archean rocks at Sturgeon Lake, in northwest Ontario, are comparable to structures in present-day extensional environments on the ocean floor. In the Sturgeon Lake mining camp, the most extensive and persistent massive sulphide deposition was along a graben and, in particular, is associated with boundary faults and eruptive fissures. Wherever these structures are accompanied by areas of high-temperature hydrothermal alteration, an excellent exploration target has been identified on land.
Studying the modern-day volcanogenic massive sulphide deposits is an important step toward improving the understanding of, and exploration for, their ancient analogues now found and mined on land. Conversely, the land deposits provide important insights into what can be expected on the sea floor; and sea floor deposits, in turn, can also be re-examined to develop new techniques for land exploration. The exchange of ideas between geologists familiar with modern and ancient deposits results in new insight into both.
Massive polymetallic sulfide deposits, ancient and contemporary, are formed in extensional (rift) environments. The modern sea floor deposits demonstrate that the tectonic setting of the rift and the type of rock within it have a profound influence on the nature of the deposit produced, an influence that probably explains the diversity of ancient massive sulphide deposits as well.
The formation of a submarine volcanogenic massive sulphide deposit requires the following:
* a fluid capable of carrying several parts per million metals,
* abundant reduced sulphur as hydrogen sulphide,
* a buried heat source of sufficient size to cause large-scale circulation of this fluid through several kilometres of sea floor rocks,
* a fracture system that allows fluid circulation through these rocks and focuses the discharge on the sea floor as a hotspring, and
* a mechanism for precipitating the metallic sulphides from the fluid.
To preserve a deposit, a sufficient flux of new volcanic or sedimentary material must bury it. The geologic environment that satisfies all these criteria for the formation of volcanogenic massive sulphides is a submarine rift.
The Gulf of California is a new ocean basin formed by the rifting of Baja, Calif. from mainland Mexico. The spreading rate is moderately fast: about 6 cm per year. The ridge-crest is broken into short en echelon spreading centers, producing a series of deep basins only one of which, Guaymas, is hydrothermally active at the moment. The Guaymas Basin, which consists of two grabens that overlap at a non-transform offset, is a receptacle of sediment about 400 m thick. This sediment consists of roughly equal portions of pelitic and organic-rich pelagic material accumulating at the ex-tremely fast rate of 0.1 to 0.2 cm per year. The organic carbon content of the sediment reaches 4%. A consequence of the sedimentation is that magmatism at the ridge-crest forms small plugs and thin sills at shallow depth.
The hydrothermal circulation model for Guaymas Basin is shown in Figure 1. Reaction between basalt and sea water well below the sea floor generates a hydrothermal fluid (such as that at 21 degrees N on the East Pacific Rise) that subsequently passes through carbonate- and organic-rich sediment before venting onto the sea floor. The fluid reacts with the sediment, resulting in much higher carbon dioxide and calcium content, a high ph and a hydrogen sulphide content which is about the same as that of fluids from sediment-starved locations. Here too, there is lower total metals content compared with fluids from sediment-starved localities.
There are about 100 hydrothermal sulphide edifices within a kilometre- long segment of the spreading center, many of which are actively venting fluids at temperatures up to 360 degrees C. Most of the mounds lie above sills, but these sills are too small and too cool to be the source of the thermal energy necessary to drive the venting. Deep Sea Drilling Project (dsdp) cores and interstitial waters from below the sills show evidence of a deeper and much larger heat source. The role of the sills is structural.
Sills represent an impermeable cap to upwelling fluids, similar to that seen beneath some ancient massive sulphide deposits. By analogy with the ancient deposits, the Guaymas Basin sills are presumably transected by fractures that focus the flow to the surface and, indeed, this has been demonstrated in dsdp cores at one site. Away from the sills, thermal anomalies are weaker, upward fluid-
flow through the sediments is diffuse and no deposits are forming. The sulphide deposits have two end-member morphologies: mounds and chimneys (or spires). Hydrothermal mounds are convex, rounded or elliptical accumulations of mineral precipitates generally 5 m to 25 m high and 10 m to 50 m across. They are steep-sided and composed predominantly of barite, anhydrite, calcite and pyrrhotite. Little pelagic sediment covers the mounds, despite the high rate of sedimentation, suggesting they are all very young. Hydrothermal fluid actively circulating through the interiors of many mounds escapes through chimney structures and through small openings, although much is probably prevented from effusing by an impermeable outer crust. Chimneys and spires are columnar structures that vary greatly in height from a few centimetres to a towering 30 m or more above their substrate. They are situated directly on hydrothermal mounds or lithified basinal sediment close to a hydrothermal mound or isolated clusters removed from mounds. True chimneys have a central orifice up to several centimetres in diameter, whereas spires have no central orifice but, rather, contain numerous holes 0.1 cm to 1 cm in diameter or interconnecting pores through which hydrothermal fluid escapes.
The predominant, stable chimney sulphide is pyrrhotite, which is pseu- domorphically replaced by marcasite in mounds. High-isosphalerite and wurtzite are the next most abundant metallic sulphides; chalcopyrite, iso- cubanite and galena are generally present in small amounts. Calcite is abundant in chimneys and spires but, because it is soluble in cold sea water, is generally absent in mounds. Generally, chimneys and spires are richer in metallic sulphides and anhydrite than are surficial mound samples, whereas barite and amorphous silica are more abundant in mounds. However, the composition of the deep interior of the mounds is not known.
Abundant liquid hydrocarbon, with the smell and consistency of crude oil, saturates the hydrothermal mounds. The oil forms by rapid thermal maturation of organic matter in the sediments and is carried to the mounds in the hydrothermal fluid. In high-temperature vents, the oil is distilled, leaving residues of yellow and brown waxes and black tar lining the interiors of the chimneys.
The association of sulphide mineralization with sills intruded into sediments at Guaymas Basin is also a feature of the Late Triassic section of the Windy Craggy sulphide deposit. The Windy Craggy deposit may, therefore, have formed in a similar environment — that is, a sedimented, spreading rift system within continental crust.
The Windy Craggy stratiform copper/cobalt massive sulphide deposit is in the Alsek-Tatshenshini River area of the St. Elias Mountains in the extreme northwest corner of British Columbia. It is a concordant, tabular, steeply northeast-dipping massive sulphide body containing no less than 350 million tonnes of 1.5% copper and 2 lb cobalt per tonne, with a significant zinc-rich zone. A gold-rich zone is being investigated more closely.
Host rocks are folded and altered Late Triassic pillow basalts with limy and cherty sedimentary interbeds. The deposit contains gold-bearing copper-cobalt-zinc massive sulphides that occur in pillow lavas and sediments. The abundance of sills suggests that eruption of basalt was accompanied by shallow-level injection of magma into relatively unconsolidated basinal sediments.
The ser is an actively spreading (6 cm per year), sediment-starved ridge- crest located within Canadian territorial waters about 200 km west of Vancouver Island. It is a high-standing, northeast/southwest-trending ridge, 65 km long, 1,760 m to 2,600 m deep and 5 km to 8 km wide, characterized by two parallel valleys of similar dimension informally named East Valley and West Valley. Side-scanning sonar imagery suggests that ser is being built by effusive volcanism rather than just by tectonic processes. The ridge appears to be in a transition between a period of extension accompanied by little or no volcanism and a period of intense volcanism. In this sense, ser is more akin to a much-elongated seamount than it is to a conventional spreading ridge. It is thought that the East and West Valleys were originally a single 1,500-m-wide graben and that sheet flow volcanism from a central eruptive fissure is building the ridge which now separates the two valleys.
Normal faulting accompanying fissuring on ser is intense. The strike is primarily parallel to the ridge, as expected, but a number of faults at right angles to the ridge axis has produced horst and graben topography with box canyons. The orthogonal faulting is particularly prevalent in the area of overlap between ser and Seminole Ridge, a young southward-propagating segment located to the east.
More than 60 individual occurrences of copper, iron and zinc sulphides have been mapped along an 8-km segment of ser. Most of the deposits are at the northern end of the East Valley. The deposits consist of large, massive sulphide mounds that form by a combination of hydrothermal precipitation and the accumulation of sulphide-rich talus from mechanical and chemical breakdown of unstable chimneys. In places, several mounds coalesce to form continuous massive sulphide bodies up to 200 m in diameter.
The mineralogical composition of the mounds is similar to ancient massive sulphide deposits on land with high-temperature copper- and iron- rich sulphides beneath lower-temperature zinc- and iron-rich sulfides, barite and amorphous silica.
Samples collected from one mound at ser are composed of chalcopyrite and marcasite at the base, sphalerite and wurtzite near the top, and late barite and silica in the outermost zones. Samples of chalcopyrite and marcasite from the base of the mound and in large spires are coarse-grained and appear to have been extensively recrystallized; finer framboidal and colloform marcasite occur in amorphous silica on the outsides of spires and mounds. Accessory minerals were deposited along with the late, relatively low-temperature silica. These include galena and at least two phases of silver-bearing lead/arsenic/antimony sulphosalts. Associated with these sulphosalts are high gold values up to 1.5 parts per million (ppm) and averaging 0.8 ppm in 28 bulk samples. The controls on gold content in both modern and ancient massive sulphides are remarkably similar, and are primarily dependent on, the maturity and chemistry of the hydrothermal system. For many deposits, a high gold grade is associated with a sulphur- rich system, which is revealed in the ores by iron-poor sphalerite and other indicators.
Cyprus-type are copper-rich pyritic deposits associated with basaltic pillow lavas within ophiolite complexes (ancient ocean crust). The best examples are on the western Mediterranean island of Cyprus, but Newfoundland abounds with such deposits. The deposits are believed to have formed at a sediment-starved oceanic spreading ridge, although those on Cyprus were probably in a back-arc or inter- arc setting rather than at a mid-ocean spreading ridge. The ser sulphide deposits do have some remarkable physical similarities (and differences, to be sure) with the Cyprus ores and their surroundings. These features range in scale from kilometres to millimetres. Comparable features include lava morphologies, the importance of graben structures and their relations to sulphides, orthogonal faulting, the size and morphology of sulphide deposits, mineral textures and some ore types. Similarities do not hold however, for the compositions of the host basalts and of the sulphide deposits. A major difficulty in making meaningful comparisons is that the sea floor is viewed in plan with virtually 100% “outcrop” but with only a few tens of metres of vertical section exposed. Cyprus is seen primarily in section. Critical areas are covered by sedimentary rock and recent alluvium.
Massive polymetallic sulphide deposits, ancient and modern, are rift- related. The modern sea floor deposits demonstrate that the tectonic setting of the rift and the type of rock within it hav
e an influence on the nature of the deposit that is produced, an influence that also explains the diversity of ancient massive sulphide deposits. The synergy among researchers studying ancient and modern deposits is important in that it improves our understanding of, and exploration for, both types.
Research by Prof Steven Scott and his graduate students, funded primarily by the National Sciences and Engineering Research Council of Canada, is in co-operation with international researchers and the Canadian mining industry. REFERENCES Hannington, M.D., Peter, J.M. and Scott, S.D. (1986): Gold In Sea Floor Polymetallic Sulfide Deposits. Economic Geology, Vol. 81, pp. 1867-1883. Scott, S. D. (1987): Seafloor Polymetallic Sulfides: Scientific Curiosities or Mines of the Future? In P.G. Teleki, et al., editors of Marine Minerals, D. Riedel Publications Co., pp. 277-300. Scott, S. D. (1985): Seafloor Polymetallic Sulfide Deposits: Modern and Ancient. Marine Mining, Vol. 5, No. 2, pp. 191-212. Joyce Musial is a Toronto-based consulting geologist and freelance writer.
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