Weathering Sequence and Alteration Products in the Genesis of the Graskop Manganese Residua, Republic of South Africa

Abstract

Abstract--Although numerous, small, manganese oxide deposits associated with dolomite in the Eastern Transvaal escarpment, Republic of South Africa, have been known for many years, their mineralogical make-up is somewhat controversial. Chemical, mineralogical, and morphological properties of the weathering products of dolomite and the coexisting manganese oxide material in the Graskop area were therefore determined. Mn and Fe occur only in minor accessory minerals in the original rock; however, in the weathering residue, these elements are concentrated and occur as separate mineral phases, chiefly birnessite, nsutite, and goethite. Thin veins of pure muscovite and quartz traverse the residua. Rare, pure calcite and maghemite nodules were noted throughout the residual manganese material. The properties of this weathering sequence suggest that the manganese wad deposits were formed in situ as a result of the congruent dissolution of dolomite, leaving a porous, sponge-like structure, highly enriched in Mn and Fe oxides.

Morphological, chemical and mineralogical properties of some manganese-rich oxisols derived from dolomite in Mpumalanga province, South Africa

“…Table 4 and mass concentration of nodules in whole soil). Hawker and Thompson (1988) identified birnessite, todorokite and nsutite but not lithiophorite in wad material from the Graskop region. They did not investigate overlying soils.…”

“…Such pockets of Mn-rich soils are probably related to variable composition of the dolomite because their distribution correlates poorly with topography. Hawker and Thompson (1988) presented a value of 1.72% MnO in unweathered dolomite. The wad which they studied was deep-seated and was considered to have undergone little pedogenic alteration apart from intense residual weathering.…”

“…The weathering front is often black due to a residual accumulation of Mn oxides. In places a substantial wad has developed which has been mined in the past (Schutz, 1990) and has received detailed mineralogical attention (Hawker and Thompson, 1988). Soils which have formed on moderate to gentle slopes are deeply developed and acidic, with a surface horizon that is typically humus-rich and a subsurface horizon that is typically dark red or yellow as a result of enrichment with iron oxides.…”

“…At Graskop the enrichment of Mn through weathering of dolomite has given rise to deposits of porous wad which presumably constitutes the parent material of the overlying black sola. Hawker and Thompson (1988) conducted a mineralogical and chemical investigation of the wad but their sampling was confined to deep residuum and did not include soil profiles.…”

“…A second objective was to establish whether the Mn minerals identified in the wad material by Hawker and Thompson (1988), have been altered by pedogenic processes as the weathering environment switches from alkaline, in the dolomite residuum, to acidic in the overlying soil mantle. A third objective was to find out whether certain trace metals have accumulated because of their affinity for Mn oxides (Becquer et al, 2006), despite the normal expectation (Burt et al, 2003) of low concentrations due to intense weathering and leaching.…”