Petrographical and mineralogical investigation of the rocks ofthe Bushveld Igneous complex in the Tauteshoogte-Roossenekal area of the Eastern Transvaal

[摘要] This study comprises a petrographical and mineralogical investigation ofrocks from an area 850 sq. km in size, situated about 80km northeast of Middelburg.Roughly half of the area is occupied by rocks of the epicrustal phase ofthe Bushveld Complex, and consists largely of Rooiberg Felsite and granophyreas well as leptite, microgranite and granodiorite. Numerous veins of finegrainedgranite traverse the leptite which is considered to be highly metamorphosedfelsite. These veins of fine-grained granite probably owe their originto the melting of the leptite. The coalescence of these products of melting gaverise to the thick sheet of._granophyre between the leptite and the felsite.Rocks of the Layered Sequence occupy the eastern half of the area andconsist of the Main and Upper Zones which were subdivided into various subzoneson the basis of characteristic rock types and marker horizons. Mineralogicalinvestigations are restricted to the minerals from rocks of the LayeredSequence, namely orthopyroxene, plagioclase, apatite and the sulphides of theUpper Zone.In Subzone A of the Main Zone, the orthopyroxene is present as cumuluscrystals, but it changes in texture to ophitic in the lower half of Subzone B wheresmall discrete grains of inverted pigeonite are also developed. Inverted pigeoniteis present in the upper half of Subzone B and in rocks of the Upper Zone, whereasthe orthopyroxene-pigeonite relationships in Subzone C of the Main Zone area repetition of those observed in the underlying rocks. The phase-change fromorthopyroxene to pigeonite takes place over a transition zone in which bothphases crystallized from the magma. It is envisaged that the first pigeonite tohave crystallized from the magma at high temperatures had a lower Fe/Mgratio than the hypersthene precipitating at slightly lower temperatures, with theresult that the early formed pigeonite was unstable and reacted with the magmato form hypersthene. This caused the formation of groups of grains of hypersthenewhich are optically continuous over large areas and which may contain afew blebs of augite exsolved from the original pigeonite. A few pigeonite grainswere effectively trapped in other minerals, mostly augite, and consequentlyescaped reaction with the liquid. These inverted to hypersthene at the appropriatetemperature and contain numerous exsolution-lamellae of augite. As fractionalcrystallization of the magma continued, it moved further into the stability field of pigeonite and out of the stability field of hypersthene with the result thatthe formation of hypersthene by the reaction of pigeonite with magma was replacedby inversion of pigeonite to hypersthene. This inverted pigeonite is alsopresent as groups of grains optically continuous and contains pre-inversionexsolution-lamellae of augite orientated at random, and post-inversion exsolution-lamellae which are orientated parallel to the (100) plane of the orthopyroxenethroughout a unit. The inverted pigeonite is orientated in such a way that itscrystallographic c-axis lies close to or in the plane of layering. This is explainedas being due to the load pressure of the superincumbent crystal mass duringthe inversion.Textural features of the plagioclase revealed interesting information onthe postcumulus changes in the rock. Reversed zoning, interpenetration andbending of plagioclase crystals as well as the presence of myrmekite are described.These are considered to be due to increased load pressure prior toand during crystallization of the intercumulus liquid. It is considered that thevarious types of pegmatoids may have originated by an increase in pressure onthe intercumulus liquid which was concentrated to form pipe-like bodies bylateral secretion or filter pressing.Cumulus apatite is developed in the olivine diorites of Subzone D of theUpper Zone. From unit cell dimensions it seems as if it changes in compositionfrom a fluor-rich hydroxyapatite at the base of this subzone to a relatively purehydroxyapatite 70m below the roof. There seems to be a substantial increase inthe fluor content of the apatite in the topmost 70m of the intrusion.Rocks of the Upper Zone contain considerably more sulphides than those ofthe Main Zone. This is ascribed to an increase in the sulphur content of themagma owing to fractional crystallization. The magma reached the saturationpoint of sulphur when rocks of Subzone D of the Upper Zone started to crystallizewith the result that these rocks contain numerous small droplets of sulphidewhich constitute on an average about 0, 5 per cent by volume of the rocks. A concentrationof the sulphides in these rocks would not yield a deposit of economicinterest because of the unfavourable composition of the sulphide phase, whichconsits of more than 90 per cent pyrrhotite. Sulphides in the rocks below thissubzone are intercumulus and a concentration could be of economic importancebecause the sulphide phase contains appreciable amounts of chalcopyrite andpentlandite. Although no economic concentration of sulphides are known from the Upper Zone, this study has revealed the presence of a mineralized anorthositebelow Lower Magnetitite Seam 2 which contains in places up to 1 per cent Cu,0, 18 per cent Ni and 1, 6g/ton platinum metals.Continuous, slow convection and bottom crystallization probably gave riseto the homogeneous rocks of the Main Zone. Injection of a considerable amountof fresh magma took place at the level of the Pyroxenite Marker which resultedin a compositional break and gave rise to a repetition in Subzone C of the rocksof the Main Zone below this marker. The oxygen pressure during crystallizationof the magma was probably low, causing a gradual enrichment in iron in the magmaand gave rise to the appearance of magnetite at the base of the Upper Zone.Intermittent increase in the oxygen fugacity is considered to be important in theformation of magnetitite seams.As a result of fractional crystallization the volatile content of the remainingmagma gradually increased. This is seen firstly, by the appearance of biotitesecondly by the appearance of cumulus apatite and droplets of sulphide and lastlyby hornblende in the rocks of the Upper Zone. Some water-rich residual liquidsapparently also intruded the overlying leptite, causing additional melting of thelatter and the formation of irregularly shaped veins and pockets of granodiorite.A lateral change in facies of the rocks of the Layered Sequence in a southerly direction is described. This is considered to be due to crystallization of themagma at slightly lower temperatures because of the more effective heat losswhere the magma chamber was thinner.Two parameters of differentiation for layered intrusions are proposed,viz. a modified version of the differentiation index and a modified version of thecrystallization index. The former seems more applicable for intrusions such asthe Bushveld Complex, whereas the latter seems to be more applicable for intrusionsin which there is a considerable development of ultramafic rocks. Thesetwo parameters can also be used to indicate the differentiation trend if they areplotted against height in the intrusion.

[发布机构] University of Pretoria