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.