Hydrothermal origin of soft hematite ore in the Quadrilátero Ferrífero of Minas Gerais, Brazil: petrographic evidence from the Gongo Soco iron ore deposit.

The high-grade soft hematite ore from Gongo Soco, Minas Gerais, Brazil, consists of laminae of platy hematite that define a prominent foliation (S[sub 1]), generally attributed to the Neoproterozoic Brasiliano Orogeny. The laminae alternate with bands (1-2 mm thick) of a porous goethitic matrix with S[sub 1]-parallel crystals of platy hematite. Both the laminae and bands are overprinted by masses of fine-grained crystals (~2-20 μm in length) of platy hematite that pervade along the banding at low angle without obliterating the foliated aspect of the ore. The ore microfabrics are interpreted in terms of hydrothermal leaching of quartz and dolomite from precursor itabirite, with goethitic alteration and pervasive hematitisation. Pyrolusite and a Mn-Ba oxide, possibly romanèchite,occur as late open-space infilling.The ore has a constant Al[sub 2]O[sub 3]/TiO[sub 2] ratio; electron-microprobe analyses indicate that titanium is not associated with residual clay minerals, but substituting iron in platy hematite. [ABSTRACT FROM AUTHOR]