RE-OS ISOTOPIC DATING OF THE MESOPROTEROZOIC HAIB PORPHYRY COPPER DEPOSIT, SOUTHERN NAMIBIA

The Precambrian Haib porphyry copper district in southern Namibia is hosted by the metavolcanic Orange River Group and younger Haib Quartz Feldspar Porphyry (QFP) stock. Seven pyrite and chalcopyrite samples yield a Re-Os model 3 isochron age of 1.18 ± 0.18 Ga, with radiogenic initial ¹⁸⁷Os/¹⁸⁸Os ratios. These sulfide samples occurred in veins in the biotite, sericite, and chlorite alteration zones.

Our 1.18 Ga isochron date coincides with the 1.2- 1.04 Ga Namaqua Orogeny, a period of granitic magmatism and compressional tectonics in southwestern Africa. The Haib QFP differs, however, from Namaqua age granitic intrusions elsewhere in the region in that the Haib QFP lacks metamorphic fabrics or identifiable metamorphic minerals. This lack of physical evidence of Namaqua age metamorphism in the QFP suggests our late Mesoproterozoic Re-Os isochron represents the age of mineralization, which is significantly younger than the previously reported Paleoproterozoic 1.8 Ga age inferred from correlation with the Vioolsdrif Intrusive Complex (Reid, 1979 reported in Minnit, 1986). Alternatively, we cannot exclude the possibility of thermal resetting of Re-Os isotopes above the 400ºC Re-Os trapping temperature, albeit without deformation or metamorphic recrystallization. If the Namaqua thermal event reset Re-Os isotopes, then care must be taken in interpreting fluid inclusion and stable isotope data in the Haib district.

The initial ¹⁸⁷Os/¹⁸⁸Os ratio at Haib is similar to that of much younger porphyry deposits elsewhere in the world, suggesting magmas ascending through a continental crust with elevated geothermal gradient during the Proterozoic did not experience significantly greater assimilation of radiogenic ¹⁸⁷Os from continental crust than occurs in Phanerozoic systems.