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January 11-17, 1999
Zinc-lead-copper VMS deposits, Part 1
By Derek Wilton By Derek Wilton

Zinc-lead-copper volcanogenic massive sulphide deposits have also been called Kuroko-type, after the deposits in the Green Tuff Belt in Japan. Aside from these base metals, such deposits also produce precious metals.

As with copper-zinc VMS deposits, zinc-lead-copper VMS deposits consist of a stratabound, generally stratiform, massive sulphide body underlain by a stockwork feeder zone. Although the footwall rocks in the Kuroko VMS deposits are white rhyolite domes, they are typically felsic to intermediate breccia or ashflow volcanic rocks. This composition contrasts with more refractory volcanic rocks of oceanic spreading copper-zinc VMS deposits.

Zinc-lead-copper VMS deposits form as a result of circulation of seawater through the underlying volcanic layer. The felsic volcanic nature of the zinc-lead-copper footwall indicates greater involvement of felsic igneous rocks and, hence, a source for the formation of lead and zinc. The fluids that formed copper and zinc did not have access to rocks with such lead-rich compositions.

However, it has been suggested that the absence of lead in greenstone-type copper-zinc VMS deposits reflects the lead-poor nature of the Earth's crust as it was forming.

The sulphides in zinc-lead-copper VMS deposits exhibit a strong zonation, and the typical model for these deposits defines seven different mineralogical zones. However, the presence of all of these zones is rare, owing to erosion or poor development of individual layers.

These zones include: silicious ore -- the lowermost zone, consisting of stockwork pyrite, chalcopyrite and quartz; pyrite ore -- overlies siliceous ore and is stratiform massive pyrite with some veining and disseminations; oko or yellow ore -- composed of pyrite or chalcopyrite but can also include sphalerite, barite or quartz; black ore -- overlies oko ore, and consists of sphalerite, galena, chalcopyrite, pyrite and barite; barite ore -- a chemical sedimentary rock composed of massive barite (calcite, dolomite and siderite); chert-hematite -- constitutes the top of the sequence; and gypsum ore -- contains the chemical sedimentary rocks gypsum and anhydrite, and can occur on the edges of the sulphide mound laterally, away from the core.

The ores for copper, lead and zinc are chalcopyrite, galena and sphalerite, respectively.

As in the copper-zinc VMS deposits, the various zonations seem to reflect temperature differences and the outward migration of metals from the site of influx. It appears that anhydrite forms as the first phase of an exhalation system on the seafloor, and is then replaced by sphalerite and galena, followed by chalcopyrite and pyrite. The gypsum, barite and chert-hematite zones can extend laterally some distance away from the sulphide mound. Some divide the siliceous stockwork material into zones consisting of a core of siliceous pyrite that grades into siliceous yellow ore followed by siliceous black ore at the edges. A typical stockwork zone in a zinc-lead-copper VMS deposit has a quartz-sericite core (frequently with chlorite) that grades into a middle zone with sericite, clay minerals, chlorite and sometimes feldspar, and, finally, into an outer zone with zeolite and clay minerals.

These alteration systems -- particularly those consisting of sericite and chlorite, or zeolite and clay -- have been known to surround the sulphide mound and envelop the hangingwall for up to 300 metres above the sulphide body, and laterally for up to 1.5 km.

The layers containing massive sulphide mounds can exhibit soft sediment deformation features, indicating that the layers were plastic on the seafloor prior to cementation into solid rock. In some deposits, these sulphide layers moved en masse downslope from the underlying rhyolitic dome, becoming detached from their stockwork feeders.

Such ore horizons are called "transported ores." "Proximal ores" are in contact with their stockwork zones, whereas the massive sulphide in "distal ores" are not connected to the stockwork.

-- The author is a professor of geology at Memorial University in St. John's, Nfld.

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January 18-24, 1999
Zinc-lead-copper VMS deposits, Part 2
By Derek Wilton

Zinc-lead-copper volcanogenic massive sulphide deposits typically contain up to 18 million tonnes of ore, though deposits containing 100 million tonnes have been discovered.

In the Canadian portions of the Appalachian Orogenic Belt, two important zinc-lead-copper VMS districts occur. One is at Buchans, Nfld.; the other, at Bathurst, N.B. Buchans produced 16.2 million tonnes of ore grading 1.33% copper, 7.56% lead, 14.51% zinc, 126 grams silver and 1.37 grams gold per tonne from 12 orebodies that are among the richest of this sort of VMS. At Bathurst, more than 30 deposits contain 250 million tonnes of ore: the Brunswick No. 12 orebody contains 134 million tonnes of ore with 0.3% copper, 3.6% lead and 8.87% zinc, plus 100 grams silver per tonne; the Heath Steele deposit contains more than 33

million tonnes of 0.7% copper, 2.5% lead, 6.3% zinc, 60 grams silver and 0.62 gram gold; on Buttle Lake, on Vancouver Island in the Cordilleran Orogenic Belt, the H-W deposit contains 13 million tonnes grading 2.2 % copper, 0.3% lead, 5.3% zinc, 38 grams silver and 2.4 grams gold.

These massive sulphides are mined by standard underground or (where possible) open-pit methods. Sometimes a combination of the two is used. These operations usually include a crushing mill. The complex polymetallic nature of the ore requires elaborate flotation and separation techniques, both to process the sulphides and remove dense barite and other waste material.

Exploration for these deposits should be undertaken in the oceanic volcanic portions of orogenic belts -- in particular, those portions that have a strong felsic-to-intermediate igneous component with a calcalkaline geochemical signature.

Regional lithogeochemical surveys of volcanic rocks in the correct tectonic setting provide a first-order exploration tool. As the deposits often occur in clusters, exploration in regions with known deposits may prove successful. Exploration in prospective

volcanic belts can lead to the identification of alteration halos around these deposits. The haloes, as well as the stockwork zones, may be larger than the mineralized horizons themselves. Explorers can vector toward a massive sulphide by evaluating mineralogical changes, and associated geochemical changes, in a potential alteration stockwork.

Such a technique would not work in the case of transported ore, as no stringer zone occurs in these sorts of deposits. Larger, hangingwall alteration systems would also be good mineralogical or geochemical targets. Another potential exploration target, one also larger than an actual deposit, would be a mineralized horizon of chemical sedimentary rocks, particularly barite. The evaluation of barite distributions in regional lake sediment or stream sediment surveys can be used to define the location of this horizon

Since the deposits are electrically conductive, they can be detected via airborne electromagnetic (EM) and magnetic (MAG) surveys. Airborne radiometric surveys, which are capable of detecting the potassic content of felsic volcanic rocks or the alteration associated with the stockwork zones, can also prove useful in exploration. A more recent geophysical exploration innovation is a combined EM-MAG-radiometric airborne survey. These have been used in a recent Geological Survey of Canada mapping program in the Bathurst camp of New Brunswick. In the Buchans district of Newfoundland, Billiton Exploration recently

completed a combined EM-MAG airborne survey. Ground-based follow-up geophysical programs can include gravity, induced-polarization/spontaneous-polarization, MAG-EM and very-low-frequency surveys, any of which can be carried out in co-ordination with diamond drilling.

-- The author is a professor of geology at Memorial University in St. John's, Nfld.

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May 31-June 6, 1999
Porphyry deposits, Part. 1
By Derek Wilton By Derek Wilton

Porphyry deposits are the quintessential low-grade, large-tonnage mineral deposit. These formations are called porphyries because they are commonly, but not solely, associated with intrusive igneous rocks with large, well-formed mineral crystals (typically feldspars) set in a groundmass of finer-grained crystals.

The intrusive rocks that usually host the deposits are generally felsic to intermediate, ranging from granite to quartz monzonite to granodiorite. The deposits are rare in mafic intrusions, such as gabbros.

The porphyritic texture indicates that the magmas intruded and crystallized near the surface. Because of their near-surface nature, these intrusions are termed epizonal, but can also be moderately coarse-grained with uniform-sized crystals or mesozonal.

Porphyry deposits can be subdivided into different types based on their metal content. These types include copper, copper-gold, copper-molybdenum and molybdenum. In general, copper- and gold-rich porphyries are associated with intrusions derived from mafic magmas in settings such as island arcs. Molybdenum-rich deposits are associated with felsic intrusions derived from magmas with a substantial component of remelted continental crust.

Porphyry deposits are related both genetically and spatially to igneous intrusions. There are usually several bodies of intrusive rock, emplaced in multiple events, and porphyry copper deposits are often associated with dyke swarms and breccias. The country rock intruded by the porphyry can be of any lithological type.

Both the intrusion and the country rock typically exhibit strong and pervasive fracturing. The only geological requirement for porphyry mineralization is that the host rock be rigid or brittle.

Mineralization and alteration can develop in both the intrusive and country rock. The core of the mineralizing system demonstrates the most intense alteration -- called potassic alteration because potassium is added to the affected rocks. In the potassic zone the minerals biotite, potassium feldspar and quartz develop. The potassic zone grades outwards into the phyllic zone, which contains quartz and muscovite, usually in its fine-grained variety, called sericite. The phyllic zone then passes into the argillic zone, where

quartz and clay minerals develop. The propylitic zone, containing chlorite, epidote and carbonate, develops next, grading outwards into unaltered country rock. These zones do not all show up in every deposit: any one can be missing. The argillic zone, typically the smallest, is often entirely absent.

Usually, mineralization has a low-grade core containing disseminated pyrite that grades out into the ore zone. In the ore zone, pyrite with lesser chalcopyrite (copper ore) and molybdenite (molybdenum ore) are present in veins and disseminations. Sometimes an outermost zone containing only pyrite develops, and then passes into unmineralized country rock.

Formation of these deposits seems to involve two processes.

One, the orthomagmatic process, involves a mechanism called "second boiling," whereby water saturates the magma as a result of crystallization. With progressive crystallization of the magma, the volume of water dissolved in it increases at a relative rate since water will not seep into silicates. For example, suppose a magma contains 2% dissolved water: once 50% of the magma has crystallized into silicate minerals, the remaining magma would contain a dissolved water content of 4%.

Because water boils at 100�C and the magma has temperatures exceeding 600-700�C, excess water will essentially boil off (hence the term second boiling) if released near the earth's surface. When this happens, sulphur, copper, molybdenum and gold can be concentrated in solution in this water. When the aqueous part of the magma boils off, the pressure can cause the intrusive and country rocks to brecciate and fracture, providing pathways for the solution to travel through the rock and be deposited. This type of brecciation and fracturing is sometimes called "ground preparation."

The second means of formation, known as the "convective process," starts when continued cooling of the intrusive magma causes groundwaters to circulate through the surrounding country rocks, much as water convects to seafloor volcanic vents and forms volcanogenic massive sulphide deposits. These late-circulating hydrothermal fluids can add more metals to the ore-forming system, or redistribute metals that had been previously deposited in the orthomagmatic stage so as to upgrade the concentration of the sulphides.

Porphyry deposits occur in a similar geological setting to epithermal-style gold deposits, and share many of the same characteristics and processes of formation. Some epithermal deposits are part of a larger porphyry-deposit system.

-- The author is a professor of geology at Memorial University in St. John's, Nfld.

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June 7-13, 1999
Porphyry deposits, Part. 2
By Derek Wilton

Porphyry deposits can be found in orogenic areas such as the Canadian Cordillera, the Andes Mountains of Chile and Peru, and in the southwestern Pacific regions of the Philippines, Indonesia and Papua New Guinea.

These deposits are the most important source of molybdenum, and of rhenium, a platinum group element associated with molybdenite crystal lattice. They are also among the most important sources of copper -- reportedly contributing up to half of the metal mined worldwide -- and gold. Silver and a number of other metals, including tungsten, tin, lead and zinc, are also recovered from porphyry operations.

These deposits contain hundreds of millions to billions of tonnes of ore grading from 0.2% to more than 1% copper, 0.005% to 0.03% moly, and 0.4 to 2 grams gold per tonne.

As an example, the porphyry copper mine at Bingham, Utah, contains an average of 0.6% copper in more than 2 billion tonnes of ore. Since operations began in 1904, the mine has produced more than 16 million tonnes of copper. Other regions with porphyry deposits include: Butte, Mont., with more than 2 billion tonnes grading 0.85% copper; Chuquicamata, Chile, with more than 10 billion tonnes grading 0.56% copper; and Ok Tedi in Papua New Guinea, with more than 375 million tonnes grading 0.7% copper and 0.66 gram gold.

In Canada, all moly production and roughly half of all copper production are derived from porphyry deposits. With the exception of Quebec's Gaspe Copper, which is mining a deposit estimated at 150 million tonnes grading 0.37% copper, Canadian porphyry production is limited to British Columbia and the Yukon.

Current and past-producing mines in those areas include: Valley Copper in British Columbia, with 690 million tonnes grading 0.41% copper; Island Copper in British Columbia, with 345 million tonnes grading 0.42% copper and 0.017% moly; Brenda in British Columbia, with 360 million tonnes grading 0.16% copper and 0.039% moly; Mount Polley in British Columbia, with 230 million tonnes grading 0.25% copper and 0.34 gram gold; and Casino in the Yukon, with 162 million tonnes grading 0.37% copper, 0.039% moly and 0.48 gram gold.

Because of their low grades, porphyry mines must be low-cost. To keep costs down, these are mined as open-pit operations, which are less costly to run than underground mines. The size of many of these deposits renders such operations huge. For example, at 800 metres deep and 4 km wide, the pit in Bingham, Utah, is the largest man-made excavation in the world.

Exploration for these deposits focuses on regions with felsic-to-intermediate intrusive rocks, particularly those with a history of multiple intrusions and brecciation or fracturing in the contact zone with country rock. More detailed exploration would zero in on defining alteration halos that grade laterally from the core of the mineralizing system.

A vertical zonation in copper mineralization might also develop in hot, arid regions where surface waters tend to redistribute copper from an exposed porphyry system, concentrating it elsewhere. Such enrichments are called "supergene" and contain higher-grade copper minerals, such as chalcocite and bornite, than found in chalcopyrite. The oxidized surface waters dissolve copper from the original porphyry ore, called protore, and transport it in the water table until such time as the waters encounter a reduced zone and precipitate the copper. The presence of a supergene enrichment indicates the presence of a larger hypogene, or original, porphyry system.

Regional geochemical surveys for both metals and alteration, such as potassium, are useful exploration techniques. Regional airborne geophysical surveys, such as gamma-ray spectrometry, may prove useful in locating and defining alteration halos.

Some porphyry systems in the Andes were first detected through satellite imagery of alteration halos. Ground geophysical surveys useful in exploration include induced-polarization for disseminated and vein sulphides, and magnetic for secondary magnetite content.

-- The author is a professor of geology at Memorial University in St. John's, Nfld.

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August 9-15, 1999
Redbed copper deposits, Part 1
By Derek Wilton

As their name suggests, red-bed copper deposits form in red host rocks. The red colouration is actually rust, which is oxidation formed after the rock's exposure to the atmosphere.

The two types of redbed mineralization are volcanic and sedimentary. The volcanic-hosted types occur in sub-aerial (land-based) lava flows and associated fragmental rocks, such as agglomerates and tuffs.

In volcanic host rocks, breccias and layers of volcaniclastic sedimentary rocks can act as permeable zones. So can flows, if they contain vesicles (holes from which gas escapes). The vesicles are typically filled with low-grade metamorphic minerals. Such vesicles are called amygdules, and the host rocks are termed amygdaloidal flows. In many cases, the copper-bearing permeable horizons are crossed by faults or fractures.

Copper sulphide minerals -- including chalcocite, digenite, djurleite, covellite, bornite and chalcopyrite -- can form cross-cutting veins, or can be disseminated through the host rock, or can fill vesicles in volcanic rocks.

The second type of redbed deposit, sedimentary-hosted, forms in such environments as fluvial (river) systems. The red continental sediments in these oxidizing environments differ from the green-to-black, reduced sediments deposited in oxygen-poor (or anoxic) submarine environments. Typically, mineralization consists of disseminated chalcocite, with lesser bornite and chalcopyrite in permeable layers of the host rock.

The copper appears to have precipitated after encountering reduced material in the form of organic debris and pyrite. Oxic copper-bearing fluids were reduced through reaction with the iron sulphide pyrite, causing precipitation of copper sulphides.

Sometimes, copper precipitation is caused by the mixing of oxic copper-bearing fluids with hydrocarbon-rich fluids in a permeable horizon of a fluvial sequence. There is little or no alteration of the host rocks by the copper fluids, and no deformation. This combination suggests that the ore fluids were low in temperature and almost in equilibrium with the host rocks, except for oxidation potential. It is generally assumed that these fluids were diagenetic -- that is, that they were produced by dewatering of material elsewhere in the sedimentary pile.

A mineralogical zonation develops in many redbed copper occurrences, particularly in the volcanic-hosted type. Zonation is not fully developed in all occurrences and is barely present in many sedimentary-hosted types. Where fully developed, the zonation contains, at its core, native copper, which grades outwards through zones of chalcocite, copper- and iron-rich bornite, chalcopyrite and finally pyrite. The zonation is known as a "fluid front," and forms when oxidized copper-bearing fluids gradually replace reduced layers. The zone in which the reaction between reduced rock and oxidized fluid occurs is known as the "redox (reduction-oxidation) boundary."

In low-temperature, sandstone-hosted deposits, the fluid front is called a "roll front," owing to its concave shape. Copper precipitates when it reaches the redox boundary, and the continued influx of fresh ore fluid pushes the redox boundary through the permeable horizon and, at the same time, increases the copper content of the minerals behind the boundary.

-- The author is a professor of geology at Memorial University in St. John's, Nfld.

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August 16-22, 1999
Redbed copper deposits, Part 2
By Derek Wilton

Sedimentary redbed deposits are relatively small, and few are ever brought into production. There are no such mines in Canada, though deposits do occur in Nova Scotia and New Brunswick. Around the world, the Dzhezkazgan deposits of central Kazakstan and the Paoli deposit of Oklahoma are producers.

Conversely, volcanic-type redbed deposits are important producers.

Such mines in the Keweenawan district of Michigan have produced more than 5 million tonnes of copper since the mid-19th century from such deposits as the Calumet (70 million tonnes grading 2.64% copper) and Kearsarge (90 million tonnes of 1.05% copper). In addition to copper, redbed deposits also produce silver.

Canada's only volcanic redbed operation was the Mamainse Point mine, on the southeastern shore of Ontario's Lake Superior. Itproduced 850,000 tonnes grading 1.15% copper and 8 grams silver per tonne.

The Sustut deposit of north-central British Columbia, which contains 43.5 million tonnes grading 0.81% copper, and the 47 zone of the Northwest Territories' Coppermine district, reported to contain 3.2 million tonnes grading 3.4% copper, are undeveloped occurrences.

More than 250 volcanic- and sedimentary-hosted redbed deposits occur in the Seal Lake area of central Labrador.

The sedimentary-hosted deposits form when oxic (oxygen-rich) diagenetic fluids rise through permeable zones. The volcanic-hosted deposits form when oxic copper-bearing fluids flow along faults or fractures and encountered permeable horizons in which to precipitate copper minerals. Some models suggest that oxic fluids in volcanic-hosted deposits derive from the metamorphism of volcanic rocks from deeper stratigraphic levels. Fluids driven off by dehydration will concentrate copper in the rocks.

Kupferschiefer-type deposits are similar to the redbed type, and are found in the Central African copper belt, which straddles Zambia and the Democratic Republic of Congo, and its namesake district of Eastern Europe.

These large, regionally extensive deposits form in continental shelf sedimen-tary environments in which continental redbeds are transgressively overlain by reduced marine sedimentary rocks (involving the gradual submergence of land by a shallow sea). These deposits formed when oxic copper-rich fluids from the underlying beds were forced up to sedimentary marine rock, creating a redox zone in which copper minerals precipitated after the reaction of the fluid with the host rock.

Exploration for sedimentary-style redbed copper deposits occurs in areas with thick, undeformed packages of fluvial sedimentary rocks, whereas exploration for volcanic-hosted redbed copper deposits focuses on sub-aerial volcanic flows and associated volcaniclastic sedimentary rocks.

Exploration crews investigate fault zones and permeable horizons in oxic sequences that also contain some reduced zones or material. Delineation of mineral zonation often points the way to enriched copper horizons. Also in an explorer's arsenal are regional geochemical surveys for anomalous copper or silver and electrical-based ground geophysical surveys.

-- The author is a professor of geology at Memorial University in St. John's, Nfld.

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