MANGANESE OXIDE MINERALOGY, PETROGRAPHY AND GENESIS, PILBARA-MANGANESE-GROUP, WESTERN-AUSTRALIA

Supergene manganese oxides, occurring in shales, breccias and dolomite s of Proterozoic Age, in the Western Australian Pilbara Manganese Grou p, have Mn/Fe ranging from 1.9 to 254 and Mn4+ to Mn (Total) of 0.49-0 .94. The manganese mineralogy is dominated by tetravalent manganese ox ides, especially by cryptomelane, with lesser amounts of pyrolusite, n sutite, manjiroite, romanechite and other manganese oxide minerals. Th e manganese minerals are commonly associated with iron oxides, chiefly goethite, indicating incomplete separation of Mn from Fe during Terti ary Age arid climate weathering of older, manganiferous formations. Th ese manganese oxides also contain variable amounts of braunite and ver y minor hausmannite and bixbyite. The braunite occurs in three generat ions: sedimentary-diagenetic, recrystallised sedimentary-diagenetic, a nd supergene. The mode of origin of the hausmannite and bixbyite is un certain but it is possible that they resulted from diagenesis and/or l ow-grade regional metamorphism. The supergene manganese deposits appea r to have been derived from manganiferous Lower Proterozoic banded iro n formations and dolomites of the Hamersley Basin and overlying Middle Proterozoic Bangemall Basin braunite-containing sediments.