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.
Re-Os isotopic dating of the
Mesoproterozoic Haib porphyry copper deposit, southern Namibia