U-PB ZIRCON GEOCHRONOLOGY OF ARCHEAN FELSIC UNITS IN THE MARBLE BAR REGION, PILBARA CRATON, WESTERN-AUSTRALIA

Archaean supracrustal rocks that are well exposed in the Marble Bar region, Pilbara Craton, have been assigned to the Warrawoona Group and younger sequences. The predominantly volcanic Warrawoona Group, previously dated at 3300 to 3500 Ma, is largely basaltic with locally intercalated thick felsic volcanic units. The supracrustal rocks have been folded and are distributed around the margins of large, roughly circular to ovoid, "granitic-gneissic" batholiths. A U-Pb zircon geochronology study was undertaken to obtain precise age constraints for some of the ore deposits in the area, especially the Big Stubby, North Pole (barite), and Miralga Creek (Zn-Pb-Cu-Au) deposits, in support of efforts to improve Archaean lead isotope models. The results also help significantly in interpreting stratigraphic relationships and crustal evolution in the area. The new conventional zircon U-Pb data indicate that most of the Warrawoona Group (from the Duffer Formation upwards) was deposited over a period of approximately 20 Ma, from approximately 3470 Ma to 3450 Ma. The age of the Duffer Formation is closely constrained by results of 3471+/-5 Ma and 3465+/-3 Ma, and the age of the upper Salgash Subgroup appears to be established at 3454+/-1 Ma. The Panorama Formation at the southern margin of the North Pole Dome is 3458+/-1.9 Ma; unless the Duffer Formation was deposited over a period of at least 10 Ma, this casts doubt on a previously suggested Panorama-Duffer correlation. The age of the lower part of the North Pole succession is greater than approximately 3458 Ma, consistent with the correlation of the basal greenstones at North Pole with the Talga Talga Subgroup. The North Pole chert-barite unit, which is well known for its preservation of Early Archean stromatolites, microfossils, and evaporite sediments. is clearly older than 3458 Ma. The 3325 Ma age now established for the Wyman Formation shows that this unit must be excluded from the Warrawoona Group. The age of the Talga Talga Subgroup remains uncertain; a felsic schist in the Mount Ada Basalt, which has been dated at 3449+/-3 Ma, is interpreted to be a sill, probably related to the 3450 Ma granitoid bodies. An age > 3724 Ma for a zircon xenocryst in the Panorama Formation is the oldest obtained so far for zircon from the Pilbara granite-greenstone terrane, and indicates the existence of pre-Pilbara Supergroup sialic basement. Ages of 3465+/-3 Ma and 3449+/-2 Ma applied to previous Pb isotope data for the Big Stubby and Miralga Creek deposits, respectively, in combination with comparable data for other syngenetic Archaean sulphide deposits, yield a general single-stage Pb evolution model with the parameters T0=4560 Ma, A0=9.0818, B0=9.9002 and C0=29.343. This gives model ages in accord with known constraints for many Archean deposits. A model lead age of 3490 Ma for the North Pole deposit, which is older than 3457 Ma, may be too great, but attempts to directly date this deposit have not yet succeeded. There are striking parallels between the Warrawoona Group ages reported here and those recently obtained for the Barberton region, Kaapvaal Craton, South Africa. The Talga Talga Subgroup and Lower Onverwacht (Tjakastad Subgroup) sequences have approximately equivalent constraints, and an age for the upper Salgash Subgroup agrees closely with those for the Hoogenoeg Formation of the upper Onverwacht Group (Geluk Subgroup).