POLYPHASE GOLD MINERALIZATION IN THE STRUCTURALLY CONDENSED GRANITOID-GREENSTONE CONTACT AT THE NEW CONSORT GOLD MINE, BARBERTON GREENSTONE BELT, SOUTH AFRICA

Gold mineralization of the New Consort Gold Mine in the Mesoarchaean Barberton Greenstone Belt in South Africa is hosted by discrete mylonitic units. These mylonites form part of an imbricate stack in which (ultra-)mafic schists of the ca. 3.26 Ga Onverwacht Group are juxtaposed against overlying metapelites of the ca. 3.26-3.22 Ga Fig Tree Group. The imbricate stack is situated in the immediate hangingwall of the basal granitoid-greenstone contact of the greenstone belt that is characterized by condensed metamorphic gradients increasing from upper greenschist facies conditions (510 – 530°C, >3kbar) in rocks of the Fig Tree Group, to upper amphibolite facies (650 – 700°C, 6 – 8kbar) in the basal Onverwacht Group. Combined structural and petrological data indicate two stages of mineralization. The earliest mineralization is characterized by high T ore parageneses (mainly loellingite + pyrrhotite + gold) associated with the development of a pervasive, southerly-dipping foliation (S1) in upper amphibolite facies rocks of the Onverwacht Group. The second phase of mineralization is associated with the subsequent D2 deformation, during which the supracrustals were folded into two main SE plunging synclines that are separated by an anastomosing network of NNW trending shear zones. The geometry of the main (D2-related) ore bodies is controlled by (1) the orientation of D2 fold axes, and (2) the intersection of D1 and D2 shear zones. The mineralization (arsenopyrite + pyrrhotite + pyrite + chalcopyrite + gold) hosted by rocks of the Fig Tree Group is syn-peak metamorphism, while the mineralization (arsenopyrite + loellingite + pyrrhotite + chalcopyrite + gold) in the underlying Onverwacht Group postdates the metamorphic peak. This retrograde mineralization is spatially closely associated with the intrusion of syn-kinematic pegmatite dykes that have previously been dated at ca. 3.1 Ga. PT estimates of ca. 550-600°C and 4-5 kbars from the structurally lowest Fig Tree Group rocks are in agreement with temperatures of ca. 550°C for the retrograde mineralization in the Onverwacht Group, suggesting that this mineralization coincides with the juxtaposition of the two units.