The co-occurrence of green and blue gahnite in the Namaqualand metamorphic complex, South Africa

Two contrasting types of gahnite are closely associated in a vadety of quarzitic and garnet-phlogopite rocks at the contact between metapelitic schists and quartzites ar Oranjefontein, 50 km east of Springbok in the Namaqualand Metamorphic Complex, South Africa. Green gahnite (Ghn59-6Hcrr_zoSplzo_::) formed during prograde metamorphism of an unknown Fe-, Mn-, Mg- and Zn-rich precursor assemblage to granulite grades. The presence of minor galena and chalcopyrite suggests that sphalerite was the premetamorphic Zn-bearing mineral. Blue gahnite (Ghn66-97Hc2-11Spls-1) occurs in or is closely associated with the green gahnite-bearfuig rocks and in part forms overgrowths on green gahnite cores. The blue gahnite formed at relatively low temperatures by retrogade metamorphism, involving the breakdown ofgreen gahnite, phlogopite and garnet, yielding an assemblage of end-member zinc spinel, zinc-bearing chlorite, hematite and rutile.