MINE GEOLOGY - OVERVIEW
The profusion of minerals in the area is attributed to a series of volcanic eruptions 2,000 million years ago. The Phalaborwa area also boasts the only Antimony mine in the Southern Hemisphere at nearby Gravelotte. The cone of the eruption has vanished, but the pipe, an astonishing geological feature, remains. The pipe is 19 square kilometres in area and is filled to an unknown depth with minerals such as phosphates, copper zirconium, vermiculite, mica and gold. A history of mining through the ages at Phalaborwa is here.
The current open-pit copper mining scheme commenced at Phalaborwa in 1964. Current integrated copper metal refining capacity is 135,000 tonnes per year, and the mine is also a major source of vermiculite and baddeleyite (zirconium oxide). However, open-pit mining ceased in May 2002 and is being replaced by an underground operation with a further 20-year life. Development investment in the underground mine is some $410 million. The operation employs around 2,400 people.
The Phalaborwa mine contains magnetite, vermiculite, apatite, zirconium, titanium and uranium as well as copper. The deposit is hosted in an alkaline igneous complex comprising mainly pyroxenite with occurrences of pegmatites, foskorite and carbonatite. Three separate mineralised zones have been identified within the complex’s 20km² surface outcrop, of which the most northerly is phosphate-rich while the central (Loolekop) zone forms the basis for Phalaborwa’s copper production. Detailed geology of the mine is here.
At the end of 2001, proven reserves were 3.3 million tonnes of 0.85% grade copper in the open pit and 225 million tonnes of 0.7% copper in the underground section of the ore-body. Probable underground reserves were 16 million tonnes of 0.49% copper.
Throughout its 35-year life, Phalaborwa has often been at the forefront of surface mining technology developments. A key feature has been its use of a trolley-assist system for haul trucks coming out of the pit, and it was one of the early users of both in-pit crushing and computerised truck despatching. However, the pit's last blast took place on 25 April 2002.
Shaft Sinkers was contracted to install the main service shaft and 1,280m-deep production shaft, while RUC Mining Contracting has been carrying out the underground development. This included driving around 36km of tunnels plus the underground crusher stations, ore handling infrastructure and the undercut level for the first block cave, situated 500m below the final pit bottom. The crushing stations are being fitted with four ThyssenKrupp 900t/h double-toggle jaw crushers that feed a 1.32km conveyor linking to the production shaft.
Phalaborwa employs one of the most complex recovery circuits installed at any copper mine, producing eight metal, mineral and chemical products in around 20 different varieties and grades. The complex includes a concentrator, copper smelter and refinery, currently capable of producing 135,000t/y of copper plus by-products. Phosphate-rich tailings are delivered to Foskor, while Phalaborwa sells its own copper, precious metals, nickel, zirconium, magnetite and vermiculite on domestic and world markets.
GEOLOGY OF THE MINE
The Phalaborwa Complex consists of a central backbone of ultrabasic rocks surrounded by numerous plugs of syenite and is 2030 (+/-18) million years old. The pyroxenite body (which probably represents the remains of a volcanic vent) encloses three areas of ultrabasic pegmatite. Copper with the co-products of silver, gold, phosphate, iron ore, vermiculite, zirconia and uranium are extracted from the rocks.
The Complex is unique when compared to other African alkaline complexes, because its carbonatite components contain copper ore. Magnetite, uraninite-thorianite and baddeleyite are subsidiary product of the copper mining. The ultramafic rocks of the complex also contain economic deposits of apatite and vermiculite. Large numbers of younger dolerite dykes intrude and cut all of the rocks of the complex, and it is in these that zeolitic mineralisation occurs.
The Phalaborwa Complex covers an area of 1950 hectares and it consists mainly of a phlogopite- and apatite rich pyroxenite. This pyroxenite is intruded successively by a series of more differentiated rocks - foskorite, and olivine- magnetite- apatite- phlogophite rock and finally a central intrustion of sövite (transgressive carbonatite). The sövite intrusion shows an intimate relationship with foskorite. The sövite (50 hectares at the surface) is composed of calcite and magnetite with minor amounts of dolomite, apatite, chalcopyrite, bornite and various silicates. Furthermore uraninite-thorianite and baddeleyite are imortant accessory minerals. The sövite is being mined by large scale opencast methods mainly for copper with uranium, zirconium and minute amounts of platinum as by-products. The foskorite is mined for the extraction of phosphate. The resources of apatite from the foskorite and the pyroxenite are enormous.
Over 50 minerals (excluding trace minerals) are reported from the Phalabora Mine. The minerals can be divided into those that are associated with the the many cross-cutting dolerite (diabase) dykes and those that are associated with intrusive rocks of the Complex. In particular the lining cavities in the dykes have produced some of the most attractive zeolitic specimens yet reported from South Africa. The dykes vary in width form a few cm to 50m. The zeolite mineralisation occurs mainly in the fractures and joints as chalky coatings, but in the wider fractures, and in particular in the faulted areas crystals occur. In March 1982, in the Main dyke on bench 24 of the Phalaborwa open pit, a series of open cavities completely lined with well-crystallised mineral were discovered. Specimens from the upper portions of the Main dyke are particularly attractive, especialy those with silky white mesolite crytals projecting from clear fluorapophyllite crystsals set in a matrix of green prehnite. Later specimens from deeper down show a wider variety of zeolites.
The first locality consisted of a shear zone, some 1,8m wide, and composed of brecciated dyke material which was cemented with prehenite and fluorapophyllite. The northern side of the zone was bounded by an open cavity lined with crystallised fluorapophyllite and spiky mesolite, with larger amber pseudocubic calcite crytals up to 15cm on edge. The southern boundary consisted of a series of interconnected cavities lined with hundreds of snow-white mesolite crystals up to 2cm long. In the central brecciated zone small cavities lined with crystallised prehnite, transparent pseudocubic crystals of fluorapophyllite, and occasionally calcite crystals, were found.
In the Main Dyke, crystals of analcime to 4mm and heulandite to 1mm occurred in open joints. Species of particular interest included soft pectolite balls associated with natrolite in long thin prismatic crystals to 2cm. Thomsonite associated with fluorapophyllite occured on one boulder. Prehnite, mesolite, calcite, datolite, laumonite and chabazite have also been found.
In the Ramp dyke a different suite of zeolitic minerals were found. The minerals included pectolite (in both fibrous and free-standing crystals) and stilbite as small crystals often in association with scolecite and fluorapophyllite. The carboatite is generally of limited interest regarding specimens. An exception is a very localised area on the eastern boundary where small cavities have been found associated with foscorite xenoliths within the carbonatite.
The micaeous pyroxenite contains economic deposits of vermiculite and apatite. These are however of little interest to the collector. Phalaborwa probably represents one of the best localities worldwide for the rare species zirkelite. Large apple-green diopside crystals and books of phlogopite are also of interest. The diopside crystals reach over 1m in length, but cannot be removed intact from the enclosing matrix. Many crystals of euhedral baddeleyite (1-2cm), which were embedded in the carbonatite, have been recovered. The largest single specimen known measures 3x10cm. Other minerals such as bright orange-red euphedral chondrodite crystals up to 5mm, highly lustrous octahedral magnetite of 1-2cm on the edge, fluoborite and the rare mineral iowaite are also found. The iowaite occurs as micro crystals up to 5mm. The mineral is very dark-green to brown-green to black and has a micaceous appearance.
The original carbonate outcrop was a large hill known as Loolekop. The surface of the hill was once littered with primitive workings excavated by local tribesmen. Primitive smelting was carried out in the surrounding hills.
The Arabs and possibly even the Phoenicians before them, traded for slaves, gold, ivory and copper in Africa. The Portuguese were amongst the first Europeans to penetrate into the interior. The copper and iron deposits of Loolekop were also known to the Dutch East India Company.
Although a considerable amount of copper was produced over the centuries, very few artifacts have been discovered. The only copper articles known are about 20 strangely shaped ingots locally known as the "mirale" (singular: "lirale") and a few dozen armlets. The mirale consisted of a cylindrical bar 1,5cm thick and 45cm long with a flat cone-shaped end piece and it is believed that they were used as a primitive currency. In the town of Phalaborwa several interlinked armlets were unearthed in a house foundation about 25cm below the surface. These contained 97.8% copper and 1.65% iron.
During the stripping of the surface of Loolekop in 1964, early underground mines were exposed. Some of the ancient shafts were up to 20m deep yet only 38cm wide. The walls of these shafts where smoke-stained and charcoal fragments from the floors of two stopes indicated an age of 1,000-1,200 years.
In 1934 the first modern mining started with the extraction of apatite for use as a fertiliser. The remoteness of the area lead to the termination of the first mining ventures. Interest however remained and 1946 a well known South African geologist Dr. Hans Merensky started an intensive prospecting on Loolekop to establish the existence of economic deposits of apatite in the foskorite rock. In the early 1950s the area was discovered to be radioactive - the uranium content was however not economic for mining. In the process of this prospecting a very large low grade copper sulphide ore body was discovered which proved to be potentially exploitable.
A joint venture between Rio Tinto Zinc and Newmont Mining followed and resulted in the formation of the Phalaborwa Mining Company in 1956. During 1957 to 1962 the ore body was proved and a pilot plant operation was built. In 1966 production commenced in one of the largest open pit copper mines of the world. The mine is currently undergoing another major transition from wholly open-cast to an exclusively underground operation.
Re-established after the re-discovery of immense mineral deposits in the last century, Phalaborwa supports a vast mining industry, including one of the world's largest open-cast mines supplying vital minerals to the country. It is here that most of South Africa's copper is mined, in an unique fashion- the open pit is 450m deep, measuring nearly 2km in diameter and, as the world's deepest open-cast mine with the tallest headgear, has become a unique tourist attraction.
The mine museum is located in the first house built in Phalaborwa. There is also a viewpoint over the Big Hole, once a notable saddle-backed hill called Loolekop. Famous geologist Hans Merensky in 1938 found valuable minerals in this kop and it has been totally mined away. The mine is currently undergoing a unique transition from wholly open-cast to an exclusively underground operation.