The North Pilbara basement terrane of 3600-2800 Ma age consists of a series of ovoid multiphase granitoid- gneiss domes bordered by sinuous synformal to ...
CONTROLS ON REGIONAL DISTRIBUTION OF RARE METAL PEGMATITES IN THE ARCHAEAN PILBARA CRATON, WESTERN AUSTRALIA. Marcus T. Sweetapple1, Robert J. Hickey2 and Peter L.F. Collins1 School of Applied Geology, Curtin University of Technology, Bentley, Perth, W.A., 6845 2 School of Spatial Sciences, Curtin University of Technology, Bentley, Perth, W.A., 6845
1
The North Pilbara basement terrane of 3600-2800 Ma age consists of a series of ovoid multiphase granitoidgneiss domes bordered by sinuous synformal to monoclinal greenstone belts, composed of mafic-volcanic dominated supracrustal sequences, producing a distinctive dome and basin pattern. Pegmatites bearing rare metal mineralization are distributed throughout the terrane, frequently associated with the greenstone belts. A number of the larger pegmatite bodies host economically significant tantalum, tin, beryl and feldspar mineralization. Rare metal pegmatite occurrences across the terrane have been classified into three geochemical divisions. These divisions have been made on the basis of characteristic mineral assemblages into LCT (Li-Cs-Ta) or NYF (Nb-Y(REE)>F) family end members and an intermediate type between the two end members. These pegmatite groupings have a broad relationship with major craton-scale tectonic features. In most cases, there is substantial field and geochronological evidence for their generation from the late Archaean ‘younger’ granite suite. The LCT pegmatite suite is contained mostly within one tectonostratigraphic domain, with all major tantalum deposits (Wodgina/Mt. Cassiterite, Tabba Tabba, Strelley) being associated with a north-east trending lineament within this domain. These pegmatites are hosted entirely within bimodal volcano-sedimentary greenstone belts. Pegmatites of this suite are characterized by the occurrence of large discrete dykes of at least five metres thick, or complexes thereof. Pegmatites of the NYF suite are mostly contained within the southern portions of the Yule and Shaw Batholiths They are generally hosted within either the older gneissic granitoid complex or within the ‘younger’ granite suite itself. Some of the pegmatites belonging to this suite may be intermediate members between the LCT and NYF suites. Pegmatites of this suite tend to occur as swarms of narrow dykes, often less than one metre thick. Most tungsten mineralization is hosted in silicic pegmatites or veins associated with the Bonney Downs Granite, in the south-eastern part of the craton. Tin mineralization is associated with both the NYF and LCT pegmatite suites. Minor beryl-bearing pegmatites are distributed throughout the craton, in association with all of batholith complexes. The major controls on the distribution of rare metal pegmatites appear to be related to: 1.
2.
3.
Secular changes in the minor element chemistry of the ‘younger’ granite suite across the Pilbara Craton with time. Field evidence indicates that the LCT suite has been sourced from strongly fractionated, high silica, peraluminous, two mica leucogranites, while the NYF suite is expected to have been sourced from high fluorine metaluminous granites with A-type characteristics. Both groups of source granites can be considered to have ‘tin granite’ characteristics. Most rare metal pegmatite bodies are no more than 5 km way from their apparent parent granite of the ‘younger’ suite, and all are within 10 km. These distances are in accordance with experimentally derived models. Most of the major occurrences of LCT pegmatites are hosted within mafic-ultramafic meta-volcanic lithologies. Generation of the LCT pegmatite suite appears to require interaction with cooler, volatilerich greenstone belts sinking into the apical portions of granite domes. Contributions of H 2O and CO2 from these greenstones may have enhanced the development of a suitable environment for extraction and partition of incompatible lithophile elements. Structures within the greenstone belts have provided favourable sites for the concentration of low viscosity hydrous pegmatite magma from the cooling granites.
