A New Chronostratigraphic Paradigm for the Age and Tectonic History of the Mesoproterozoic Bushmanland Ore District, South Africa

摘要

The four giant stratiform Cu-Pb-Zn-Ag base metal deposits of the Bushmanland ore district have long been regarded as Paleoproterozoic sedimentary-exhalative deposits formed in a continental basin at about 1650 Ma and overlying the Achab Gneiss, an approximately 2.0 Ga basement complex, with no known Archean crustal history. The supracrustal sequence was thought to include the Hoogoor "pink" metavolcanic gneiss at the base and the ore-bearing metaquartzites to be unconformably overlain by the 1650 Ma Koeris Formation. All the supracrustals were regarded as predating the 1200 to 1000 Ma Namaqua-Natal orogenic cycle. In this work we present ion probe and laser ablation zircon dating of xenocrystic, magmatic, detrital, and metamorphic zircon which leads to fundamental changes in our understanding of the age and tectonic history of the Bushmanland ore district. The Bushmanland base metal deposits are not older than 1650 Ma, as entrenched in the literature, but younger than the 1285 +- 14 Ma detrital zircons found within the ore horizon at Gamsberg. The ore is probably older than the 1198 +-10 and 1154 +- 18 Ma detrital zircons found in the unconformably overlying Koeris Formation, and most likely older than the 1130 +- 35 Ma Koeris Formation metabasalts. The existence of an Archean crustal component in the Bushmanland ore district has now been established by detrital zircon ages and by Sm-Nd model ages. However, no outcrops of Archean rocks have yet been found. The Bushmanland Group metasediments were derived largely from 1850 to 2100 Ma Paleoproterozoic rocks, which probably reflects their provenance in the Richtersveld subprovince to the north. However, they also contain Mesoproterozoic detrital zircon which constrains their minimum age. The Bushmanland ore district metasediments conform to the same chronostratigraphic and tectonostrati-graphic pattern as recently established at Bitterfontein to the southwest. An older group represented by the Bushmanland Group and including the sedimentary-exhalative Cu-Pb-Zn-Ag ores, formed before, but within 80 Ma of the main collision event at 1200 Ma. A younger group, represented by the Koeris Formation, formed after the main collision at about 1130 Ma. The older group should thus have experienced an early collision-related D_1 folding event and metamorphism, whereas the younger group should show only D_2 to D_3 and M_2 to M3 metamorphism, between 1130 and 1000 Ma. This prediction should be tested against more structural observations. The 1204 +- 11 Ma Aroams Gneiss is now recognized as the oldest known granitoid in the Bushmanland ore district, in agreement with other recently published ion probe data. However it probably does not represent basement to the older metasediments. The Achab Gneiss, previously thought to be about 2000 Ma old and the polydeformed basement, is dated by our published 1166 +- 13 and 1163 +- 11 Ma concordia ages for magmatic emplacement. The Aroams Gneiss is confirmed to belong to the 1200 Ma syncollisional Little Namaqualand Suite documented in the Okiep copper district to the west. The 30 Ma younger Achab and Hoogoor Gneisses may be grouped in the Aggeneys Suite. Microbeam zircon dating has established a new chronostratigraphic paradigm for the geologic history of the Bushmanland ore district, however many issues related to ore formation and the deformation history remain to be resolved.