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September 28, 1998
Sedex massive sulphide deposits, Part 1
by Derek Wilton
Sedimentary exhalative (sedex) is a type of massive sulphide deposit associated with sedimentary rocks. Sedex deposits are major producers of lead and zinc, and constitute most of the world's largest metal deposits, including: the Sullivan mine in British Columbia; Red Dog in Alaska; Mount Isa, Broken Hill and HYC in Australia; and Rammelsberg in Germany. It has been suggested that half of the world reserves of lead and zinc occur in deposits of this type.
Sedex deposits consist of layers of massive sulphide (a rock composed of at least 60% sulphide minerals) interbedded with layers of sedimentary rock. These intercalated sedimentary rocks include chemical sediments that form through the chemical precipitation of their constituent elements (chert, which is precipitated silica; barite, which is precipitated barium sulphate; and carbonate) and clastic sediments (shale, mudstone, argillite) that form through the accumulation of sediment on the seafloor. The term "sedimentary exhalative" reflects the current thinking that the massive sulphides precipitated from hydrothermal fluids exhaled or vented on to the seafloor. A generalized morphology for these deposits depicts them as consisting of a vent zone that cuts through underlying (footwall) sedimentary rocks and passes into the massive sulphide horizons above. Feeder vents are present as vein networks and/or wallrock replacements in the footwall rocks. These vent features can be difficult to detect and are not found in all sedex deposits. In some cases, the massive sulphides either moved as a package of sediment along a topographic feature, such as a mound or hill, away from the vent, or the sulphide precipitated along the seafloor at some distance from the vent. As the layered massive sulphide is part of the overall stratigraphy of the host rocks (vertical sedimentary layering), it is usually termed "syngenetic," meaning the ore formed at the same time as the host rocks (the opposite structure is "epigenetic," as found in Mississippi Valley-type, or MVT, deposits). Some suggest, however, that the sulphide mineralization forms when metal-rich hydrothermal fluids move through the host sediments, replacing pyrite that has formed in the early stages of diagenesis (the cementation process that turns an unconsolidated sediment into a rock). The massive sulphides are composed of alternating layers of iron sulphide (pyrite and/or pyrrhotite) with lesser amounts of sphalerite and galena. Interlayers of clastic and chemical sediments can be present between the massive sulphide layers. Individual massive sulphide layers range in thickness from millimetres to metres, and can extend laterally hundreds or even thousands of metres from the vent. Lead, zinc and silver grades decrease with distance from the vent.
The sedimentary basins in which sedex deposits form are most often bounded by faults. The ore-forming hydrothermal fluids, like those involved in the formation of MVT deposits, are thought to have been saline brines derived from sediments deeper in the basin. However, fluids involved in the formation of sedex deposits, at around 300C, were much hotter than those related to MVT deposits.
The brines circulated through the sedimentary pile, leaching metals out of the sediments, and then flowed to the seafloor along the basin-bounding faults.
On the seafloor, these fluids precipitated as the massive sulphide horizons. The veins and/or host rock alterations in the feeder/ vent zones represent areas where the rising hydrothermal fluids were concentrated. Magmatism may play a role in initiation of the crustal stretching, as well as provide some of the heat energy that drives the fluid movement. However, unlike so-called volcanogenic massive sulphide deposits, igneous rocks are not an integral component of sedex deposits.
-- The author is a professor of geology at Memorial University in St. John's, Nfld.
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October 5, 1998
Sedex massive sulphide deposits, Part 2
by Derek Wilton
Sedimentary exhalative deposits grade between 4% and 30% combined lead and zinc, with tonnages of up to 200 million tonnes. The giant deposits at Sullivan, in British Columbia, contained 170 million tonnes of ore with 5.5% zinc and 5.8% lead; Mt. Isa in Australia contains 125 million tonnes grading 6% zinc and 7% lead; Broken Hill in Australia contained 300 million tonnes grading 12% zinc and 13% lead; and Red Dog in Alaska has 77 million tonnes with 17.1% zinc and 5% lead.
Unlike volcanic-related massive sulphide deposits, sedex deposits contain no copper, though they do have significant amounts of lead, compared with most (but not all) Mississippi Valley-type deposits. Besides lead and zinc, sedex deposits also produce silver.
The targets of first-phase exploration are usually the large sedimentary basins in which these deposits tend to appear. The basins range in age from 300 million to 1.8 billion years. Sedex deposits generally occur in smaller, fault-bounded sub-basins within a larger basin. Follow-up targets include horizons that are the stratigraphic equivalents of known deposits, and mineralized veins and stockworks that may have acted as feeder zones. Sedimentary fill within prospective basins would include sulphur-rich shale-argillite clastic sedimentary rocks, which are interlayered with chemical sedimentary rocks, including chert, carbonate (calcite, siderite and ankerite) and barite.
Other prospective exploration targets in the search for sedex deposits are fault-bounded sub-basins, since hydrothermal exhalations were controlled by fluid movement along these faults. Synsedimentary faults can be identified by the presence of synsedimentary fault breccias, which are composed of sedimentary fragments cemented by more sedimentary material. Because the sulphide horizons are large and considerably more conductive and denser than the host sedimentary rocks, geophysics can often locate a deposit. Such geophysical exploration often includes airborne and ground surveys for magnetic, gravity and electromagnetic properties, as well as ground-based induced-polarization surveys.
Another initial exploration technique could be regional geochemical surveys for enhanced lead, zinc and barium in regions underlain by suitable sedimentary rocks.
Vent regions have geochemical halos in lead, zinc and silver, the values of which increase toward the vent. Therefore, if a regional geochemical survey detects enhanced concentrations of lead, zinc and silver, follow-up surveys for these metals could be used to track down a vent and, hence, possible massive sulphides.
-- The author is a professor of geology at Memorial University in St. John's, Nfld.
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October 26, 1998
MVT lead-zinc deposits, Part 1
By Derek Wilton
Mississippi Valley-type (MVT) deposits are named for the region of the U.S., along the Mississippi River, where these deposits were first discovered.
Deposits of this class are major producers of lead and zinc in North America. They have a relatively simple mineralogy consisting of pyrite, galena (lead sulphide), sphalerite (zinc sulphide) and, in some cases, marcasite, a low-temperature polymorph of pyrite (the same chemical composition but a different crystal structure). Secondary calcite and dolomite are also common. Organic matter, in forms such as petroleum, is associated with base metal sulphides in many deposits, and appears to have appeared late in the genesis of the mineralization. Fluorite and barite can also be found in some MVT deposits. These deposits are hosted by undeformed, predominantly calcium and magnesium carbonate rocks. MVT deposits are classic epigenetic types in that the sulphides formed after the host rock. In the case of MVT deposits, the sulphides precipitated from hydrothermal fluids that filled pre-existing holes and voids in the host rock. The sulphide mineralization also typically cements broken fragments of wallrock. Secondary calcite or dolomite may cover surfaces on wallrocks and fragments. In many cases, the mineralization is said to fill paleo-karst (caverns, caves or holes formed in carbonate rocks as a result of dissolution of the rock through the action of rain or ground water) in the host carbonates. For instance, sulphide bodies in the host carbonate at the former Pine Point mine in the Northwest Territories resemble plums in a plum pudding. At the Nanisivik mine, also in the Northwest Territories, the orebodies resemble filled caves. There is some debate about the role of the ore-forming hydrothermal fluid in this so-called ground preparation (the development of the open spaces in the carbonate). Some geologists suggest that ore fluids actually dissolved the host carbonate to create the open spaces, whereas others believe that those spaces formed before hydrothermal metal-bearing fluids flowed through them. Researchers are attempting to determine the age of the sulphide mineralization relative to the host rocks. MVT deposits occur in rocks of Lower Proterozoic age (2-3 billion years old), though most are in rocks of Paleozoic age (600-300 million years). The ore-forming hydrothermal fluids are of low temperature (between 80 and 200 C), low pressure (that is, formed shallow in the earth's surface) and highly saline (with up to 30% dissolved salts, known as brines). As a result of the physical characteristics of the fluid, MVT deposits formed without the influence of magmas and igneous rocks or metamorphism. Hence, they formed in sedimentary rocks without the influence of heat external to the sedimentary basin. The metals and the fluids in which they are transported are thought to have derived from basinal shales, with the fluids migrating through permeable sediments and porous rocks, such as paleo-karsted dolomites, to the site of deposition. In many ways, the formation of these deposits resembles that of petroleum, in which fluids are squeezed from sediments to the site of deposition. In some cases, MVT fluids and petroleum seem to have been derived from the sedimentary basin at different times. As such, the MVT fluids and petroleum are part of a continuum in the migration of fluids in sedimentary basins, with petroleum following the the MVT fluids that are expelled first. There are two main models for the driving force behind the fluid movement. One model posits that the fluids are gravity-driven, with saline fluids flowing with groundwater from zones of recharge at higher elevations to lower-lying regions where the fluids discharge. This view is a variant on classic hydrology models. The other model of fluid movement is based on the premise of fold-and-thrust tectonics, in which fluids in a sedimentary basin are squeezed by external, plate tectonic-related forces. For example, the Canadian Rockies exhibit classic fold-and-thrust tectonic features. -- The author is a professor of geology at Memorial University in St. John's, Nfld.
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November 9, 1998
MVT lead-zinc deposits, Part 2
by Derek Wilton
Mississippi Valley-type (MVT) deposits are important producers of zinc and lead, with zinc production being dominant. Some deposits, such as that of Newfoundland Zinc Mines, have contained negligible amounts of lead (galena). Typically, most MVT deposits have combined lead and zinc grades of less than 10%, more than half of which is usually zinc. Some MVT deposits are exclusively zinc producers.
Important past and present Canadian producers include: Newfoundland Zinc Mines' operation, with 7.2 million tonnes grading 8% zinc; Pine Point, in the Northwest Territories, with 75 million tonnes grading 6.5% zinc and 2.9% lead; Polaris, in the Territories, at 22 million tonnes grading 14% zinc and 4% lead; and Nanisivik, also in the Territories, with 10 million tonnes grading 10% zinc. In the MVT districts of the central U.S., many deposits -- up to 400 by some counts -- are estimated to have produced in excess of 1 billion tonnes of lead and zinc ore since the turn of the century. Silver content in MVT deposits is generally low, but appreciable silver is present in certain deposits. For example, the Nanisivik ores contain up to 60 grams silver per tonne. Cadmium, which is associated with sphalerite, has also been recovered at some MVT deposits. The orebodies represent discrete pods or lenses of massive sulphide (galena and sphalerite) in the host rocks. As such, they are amenable to both underground and open-pit operations, depending on how deep a deposit lies. The inherent porosity of the carbonate host rocks may present problems as it could allow ground water to flow into a mine. As MVT deposits are restricted to undeformed dolomitic carbonate rocks, exploration is directed towards areas underlain by such rocks. Significant deformation or metamorphism either will reduce the permeability of the rocks for the ore-bearing hydrothermal fluids or obliterate already present mineralization. A refinement to this technique is to examine carbonate successions with karst or paleo-karst features, sedimentary facies changes or erosional unconformities, all of which indicate secondary porosity. The deposits also occur in clusters or districts, and a carbonate sequence with known MVT deposits may host more. MVT deposits can be difficult to find using geophysical methods, owing to the relatively non-conductive and non-magnetic natures of both host rocks and ores. Airborne geophysical surveys would be of little use except to define areas underlain by carbonate rocks. However, owing to the difference in densities between sulphides and carbonate hosts, gravity surveys would prove useful. Electrical methods, such as induced-polarization or resistivity, can also be used in ground exploration. Regional geochemical surveys of lake sediments could locate areas of enhanced lead and zinc in a carbonate terrane, which could then be used to vector towards an MVT deposit. The Newfoundland Zinc Mines deposit was discovered in this manner. -- The author is a professor of geology at Memorial University in St. John's, Nfld.
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December 14, 1998
by Derek Wilton
Volcanic igneous rocks play a critical role in the formation of volcanogenic massive sulphide (VMS) deposits, which are important producers of copper, lead and zinc, as well as lesser amounts of gold and silver.
VMS deposits are found in every Canadian province and territory except Alberta and Prince Edward Island, and are significant producers in British Columbia, Manitoba, Ontario, Quebec, New Brunswick and Newfoundland. Like sedimentary exhalative deposits, they are a type of mineralization formed by the exhalation of hydrothermal fluids on to the sea floor. VMS occurrences are associated with submarine igneous rocks, though sedimentary rocks may also be in the vicinity. There are three main types of VMS deposits: copper-zinc, copper-zinc-lead, and Besshi. These deposits are well-defined, with a stockwork feeder zone grading into the massive sulphide. The stockwork zone is epigenic (that is, younger than the host rock) and represents the region through which hydrothermal fluids exhaled on to the sea floor. The stockwork zone appears in dormant exhalative systems as networks of predominantly quartz and sulphide veins with disseminated sulphides. These networks are central to altered country rock in that secondary mineralogy is produced when hydrothermal fluids pass beneath the massive sulphide on their way to exhalation. The massive sulphide layer forms when the dissolved components (sulphur and base metals) in the hydrothermal fluids precipitate directly on the sea floor. The massive sulphides are then bounded by enclosing volcanic and sedimentary rocks. This process is described as syngenetic, meaning that the massive sulphide formed at the same time as the host rocks. Chemical sedimentary rocks such as chert, barite, gypsum and carbonate are commonly associated with the massive sulphide horizons, and formed through the p recipitation of fluids from the system that also produced the sulphide. The massive sulphide layers are stratabound (sandwiched between other rock strata) and generally stratiform (layered in appearance). Igneous rocks play a key role in the formation of VMS deposits, and can be divided into two groups: the high-temperature basaltic-gabbroic group and the lower-temperature rhyolitic-granitic group. The difference between the two is that rhyolitic rocks contain free silica (quartz), whereas those of the basaltic group do not. This chemical difference also manifests itself in the abilities of associated magmas to flow. Since basalts are less viscous than rhyolites, magma chambers filled with rhyolite are more likely to "dome up," and ultimately explode, than those filled with basalt, as the basaltic magma will flow from the chamber. Basalts are dark in color and referred to as mafic rocks, whereas the lighter-colored rhyolitic rocks are called felsic rocks. Also, mafic igneous rocks have greater copper concentrations, while felsic igneous rocks have more lead. VMS mineralization has been forming throughout Earth's history. The oldest deposits are about 3.4 billion years old, whereas the youngest form even today on the sea floor. In fact, VMS occurrences are the only significant class of mineral deposits that we can observe in the process of formation. The generic VMS model posits that such deposits form when sea water circulates through permeable rocks on the ocean floor. The sea water intake occurs at some distance from a magmatically heated zone, wherein water is drawn downwards towards the heat, becoming heated itself in the process. The heated sea water progressively reacts with the rocks through which it is flowing, dissolving metals out of rock and concentrating them. (This fluid essentially changes its composition from that of sea water.) The flui ds then flow upwards along fractures in the oceanic rock to the sea floor where the now-hot fluids exhale. This cyclical convection can be likened to what happens to water boiling in a pot: cold water flows from the sides of the pot down to the base, where it is heated, before rising pwards through the centre.
Igneous rocks provide the metals, sea water provides the fluid, and the cooling magma chamber provides the heat to drive the ore-forming hydrothermal system. In some deposits, it has been demonstrated that the magma chamber itself may have contributed some metals or fluids to the ore-forming system. -- The author is a professor of geology at Memorial University in St. John's, Nfld.
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December 21, 1998
Zinc-copper VMS deposits, Part 1
By Derek Wilton
There are two main sub-types of copper-zinc volcanogenic massive sulphide (VMS) deposits: those that occur in Archean-Proterozoic greenstone belts (the Noranda-type) and those that formed in ocean environments less than 600 million years ago (the Cyprus- or ophiolite-type).
These deposits exhibit the typical VMS architecture, with a massive sulphide horizon overlying an alteration/stringer zone or pipe. In both types, the massive sulphide is predominantly composed of iron sulphide with less than 10% chalcopyrite (copper ore) and sphalerite (zinc ore). The iron sulphide is typically pyrite but can include pyrrhotite in metamorphosed occurrences or marcasite in lower-temperature deposits. In general, there is variation in the content of copper and zinc throughout the massive sulphide horizons, with occurrences of these metals being more frequent near the base. This zonation is thought to reflect the temperature at deposition, as copper would be carried in higher-temperature fluids and zinc in lower-temperature fluids. The sulphide mound can be envisaged as a thermal blanket. Higher temperatures at the base form copper-rich zones, whereas zinc-rich zones form higher up as the temperature gradually decreases. This may also reflect the evolution of the hydrothermal fluids, with earlier lower temperature types being relatively richer in zinc. Hydrothermal fluids that formed later and at a higher temperature contain copper. Silicate minerals intergrown with the sulphides are mainly quartz, chlorite and sericite. Chert and iron-oxide chemical sedimentary rocks overlie, and are typically associated with, these deposits, and presumably represent the final stages of exhalation from the circulating hydrothermal fluid sy stem. The stringer or alteration zone beneath copper-zinc VMS deposits can have a larger areal extent than the massive sulphide zone itself. There is a general zonation of alteration associated with the stringer zones or pipes. In pipes below spreading ocean ridge-type deposits, where alteration is most intense, silica (quartz) is added and iron-rich chlorite overgrows the host rock. This zone grades outwards into altered and unaltered country rock. (The altered rock is composed of secondary magnesium-rich chlori te with sericite.) In Noranda-type deposits, the most intense core alteration is sericite and silica surrounded by chlorite halos. However, both types of deposits contain disseminations and veins of pyrite and chalcopyrite. Magnetite (iron oxide) may also be present, as can small amounts of sphalerite. Alteration layers composed of epidote and quartz within the footwall rocks may also extend beneath the massive sulphides. Greenstone belts are deformed layers consisting of volcanic and sedimentary rock surrounded by gneissic-granitic terrains. These can be thought of as islands of volcanic and sedimentary rocks floating in a sea of granite-gneiss. VMS deposits in greenstone belts occur in both mafic and felsic igneous rocks, but the most common host is felsic footwall rocks. The massive sulphides typically flank small domes of massive rhyolite, which may be quite brecciated. The stockwork alteration zone overprints the rhyolite, as well as other mafic to intermediate units that may underlie the dome. In the Noranda region, several massive sulphide deposits are associated with spatially or temporally distinct rhyolite domes. The spreading ridge-type VMS deposit is associated with ophiolite, or oceanic igneous rock. The ophiolite sequence slices through oceanic crust from below. The base of ophiolite, which represents upper mantle rocks, is composed of ultramafic cumulate lithologies that pass up through gabbro into sheeted dykes, the feeders for magmatic rocks on the ocean floor. The dykes then form pillow basalts, which indicate a subaqueous genesis. At the top of the sequence, sediments drape over the basalts. Within the pillow basalt zones of the ophiolite sequence, the massive sulphides occur typically in small, fault-bounded basins. -- The author is a professor of geology at Memorial University in St. John's, Nfld.
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December 28, 1998
Zinc-copper VMS deposits, Part 2
By Derek Wilton
The largest greenstone-hosted volcanogenic massive sulphide (VMS) deposit in Canada is at Kidd Creek, where 115 million tonnes of ore contain an average of 2.2% copper and 7.25% zinc plus 145 grams silver per tonne. In the Noranda camp, the Millenbach deposit hosts 3.5 million tonnes grading 3.5% copper, 4.3% zinc and 56 grams silver. The Flin Flon deposit in Manitoba contains 62 million tonnes of 2.2% copper, 4.1% zinc and 43 grams silver.
The oceanic spreading ridge type are smaller deposits, typically containing less than 15 million tonnes and averaging 2-3 million tonnes. Canadian examples include Tilt Cove, in Newfoundland, with more than 8 million tonnes grading 6% copper and Gullbridge, also in Newfoundland, which contained more than 4 million tonnes of 1.02% copper.
Gold can be found in both types of deposits. In Canada, the greenstone deposit of the Horne mine (54 million tonnes grading 2.2% copper, 13 grams silver and 6.1 grams gold) in the Noranda district of Quebec contains gold, as does the spreading ridge type deposit at the Rambler mine in Newfoundland (400,000 tonnes grading 1.3% copper, 2.2% zinc, 23 grams silver and 5.1 grams gold).
There is considerable debate as to why some copper-zinc VMS deposits contain gold. One theory is that the original VMS mineralization was overprinted by a mesothermal lode gold system. The other posits that ore-forming fluids either had a unique composition or underwent low-pressure exhalation similar to an epithermal gold system.
The massive sulphide is usually the target of mining operations, but in some deposits the massive sulphides have been removed either by erosion or deformation. When this happens, the deposit being mined would be a stockwork zone. Mining operations are usually underground with mill complexes to crush ore and separate sulphides from host rock and each other by flotation techniques. Open-pit mines are developed where the sulphide deposits occur close to the surface.
Exploration for these different types of copper-zinc VMS deposits occurs in completely different geological and tectonic environments. Exploration for the greenstone-type deposit occurs in greenstone belts in Precambrian shield areas, particularly in volcanic mafic and felsic rock sequences and rhyolitic domes with definable breccia zones (also called mill rock). Exploration for oceanic spreading ridge deposits focuses on ancient orogenic belts, such as the Appalachian or Cordilleran, or modern seafloor vents, such as those on the Juan de Fuca Ridge near Vancouver Island. These deposits were originally thought to have formed at mid-ocean ridges, but high-precision geochemical analyses points to back arc spreading ridge systems as the sites of formation. This geochemical distinction offers a potential exploration tool in oceanic terranes, in that these specific geochemical signatures can be sought in prospective rocks.
Overall, massive sulphide layers in Noranda-type deposits have a bulbous form, while those associated with spreading ridges have a bowl-shaped appearance, reflecting the structure of the footwall rocks. Sulphides in the Noranda-type deposit are typically associated with rhyolitic domes, while the spreading ridge type occur in small, fault-bounded basins within underlying basaltic oceanic rocks.
Follow-up work often includes efforts to identify alteration zones or pipes in footwall rocks. These zones are typically larger than the massive sulphide itself, thus offering a better target. Aside from mapping mineralogical zonations within a potential alteration stockwork, documentation and mapping of subtle changes in geochemical signatures associated with alteration in bedrock may prove valuable in vectoring towards a massive sulphide horizon. Detailed mapping of chemical sedimentary rocks that overlie sulphide horizons could also be used as indicators. Owing to the conductive nature of their constituent metallic sulphides, these can be detected via ground and airborne electromagnetic and magnetic surveys. Airborne radiometric surveys may also prove useful, as can induced-polarization surveys.
The author is a professor of geology at Memorial University in St. John's, Nfld.
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