The tephroite, spessartine, pyroxmangite and rhodochrosite-bearing mineral assemblages of the manganese ores in the Manomunda-Goriajhar area of the Gangpur Group, Orissa, India. Petrological and chemical investigations and their genetic implications
Mücke, Arno; Mohapatra, Birendra Kumar; Nayak, Bibhuranjan
published: Feb 26, 2001
ArtNo. ESP154017601002, Price: 29.00 €
The manganese ores of the Manomunda-Goriajhar area belong to the Precambrian Goriajhar Formation of the Gangpur Group in the west central part of the Sundergarh district, Orissa, India. These rocks are known to be of gonditic nature. Gondites are defined as being mainly composed of spessartine ± quartz. However, although spessartine is a common mineral and occurs in nearly all the mineral assemblages of the manganese ores, these are predominantly not identical to gondite. Among other mineral assemblages, mentioned but not discussed in this paper, the tephroite (= Mn-olivine)-spessartine- pyroxmangite-rhodochrosite assemblage was investigated in detail. This assemblage was metamorphosed to greenschist facies conditions. In the Manomunda manganese assemblage the minerals are inhomogeneously distributed and occur in three parageneses which are arranged parallel to the o/Sl-surface. These are the tephroite, pyroxmangite, and spessartine-carbonate parageneses. In Goriajhar the manganese minerals are relatively homogeneously distributed and are restricted to the tephroite paragenesis. Apart from jacobsite, which is a constituent of all the parageneses, the other minerals have only divalent Mn and Fe indicating reducing conditions and high Mn: Fe-ratios of 2.5 to 12.6. Other minerals are quartz, apatite and barite. Tephroite is often altered to either Mn-rich minnesotaite or to a sepiolite-related mineral that resembles serpentine in Mg-olivine rocks. The carbonate (nearly pure rhodochrosite) is variably (Manomunda) or completely (Goriajhar) replaced by manganite, manganomelane and pyrolusite. The sedimentary environment of the protolith is interpreted to have been a marginal basin formed during a regressional phase within the area. The sedimentary protolith consisted of a manganese-rich and carbonate-bearing precursor of volcanogenic-exhalative origin mixed with continent-derived material of pelitic to psammitic composition and Fe3+-hydroxide. The mineral assemblages of Manomunda and Goriajhar evolved through metamorphic decarbonatization reactions in a low oxygen fugacity environment. Tephroite was formed at low fco2 and pyroxmangite at slightly higher fco2. Layer dependent parageneses (in the cm-range) observed at Manomunda indicate closed chemical systems with minimal redistribution of elements, oxygen and CO2 diffusion during metamorphism, and are thus preserved from varying composed protolithic material.