COMBINED (U-TH)/NE AND (U-TH)/HE HEMATITE AGES AND 4HE/3HE THERMOCHRONOLOGY OF THE TRANSVAAL SUPERGROUP, SOUTH AFRICA

The Transvaal Supergroup located in the Kaapvaal Craton of South Africa is a sequence of sedimentary and volcanic rocks providing some of the best-preserved geologic records from the late Archean through early Proterozoic, documenting a period of significant environmental change, including the Great Oxidation Event. Two major hematite ore forming events are known to have occurred in this region: a 2 Ga supergene enrichment associated with a major regional unconformity (Evans et al., 2002) and 1.25 to 1.1 Ga hydrothermal enrichment associated with regional normal faulting (Cairncross et al., 1997). We have implemented the combined (U-Th)/He and (U-Th)/Ne chronometers to hematite from six different iron mines within the Transvaal Supergroup to better quantify the depositional and erosional history of this important sequence and provide the first direct age determinations for hematite enrichment in this region. The Sishen-type ore of the Khumani Mine yields bulk (U-Th)/Ne and (U-Th)/He ages of 2030 and 2080 Ma, respectively. This is in remarkable agreement with the previously known age constraints for unconformity-related supergene enrichment. Sishen-type ore from Beeshoek Mine give bulk Ne and He dates of 1740 and 1760 Ma, respectively. This is younger than the expected supergene enrichment associated with the same regional unconformity but agrees with the timing of hydrothermal alteration observed in paleomagnetic studies (Evans et al., 2001). Bulk Ne and He ages of 1640 and 1510 Ma obtained from the Wessels-type hematite of the Black Rock Mine suggest a link between the formation of this hematite and the regional burial and metamorphism associated with the adjacent Black Rock Thrust. Interestingly, the pisolitic hematite from Belgravia Farm produces highly discordant bulk Ne and He ages of 1260 and 751 Ma. This difference likely reflects preferential diffusive loss of helium. Modeling of ⁴He/³He data will allow us to produce thermal histories of these samples over billion-year time scales over a temperature range from about 225°C to earth surface conditions.