Abstract
Metamorphosed manganese deposits at Koduru and Garbham in Andhra Pradesh, India, consist of bixbyite, braunite, hausmannite, hematite, hollandite, jacobsite, pyrophanite, and vredenburgite as primary oxides; pyrolusite, cryptomelane, psilomelane, ramsdellite, lithiophorite, and goethite as secondary oxides; rhodochrosite as primary carbonate; rhodonite, pyroxmangite, tephroite, manganophyllite, spessartite, and spandite as primary silicates; and apatite, barytes, K-Ba feldspar, gorceixite, and quartz as primary gangue minerals.Bixbyite, braunite, hausmannite, hollandite, jacobsite, and vredenburgite formed during peak metamorphism by transformation from syngenetic oxides which existed in the original sediment. Decarbonation of rhodochrosite and association with pyroxmangite produced tephroite. Rhodonite resulted by polymorphic inversion from pyroxmangite. After the peak metamorphism, with cooling, exsolution of hausmannite (second generation) from vredenburgite occurred. Pyrophanite exsolved from jacobsite. Jacobsite was partly martitized to hematite. With the restoration of normal atmospheric conditions, primary oxides and silicates altered to secondary oxides by supergene processes.Compositions of primary oxides indicate temperatures of peak metamorphism of at least 700 degrees C. Minimum pressure of metamorphism was 6 kb, and oxygen fugacities range broadly from 10 (super -2) to 10 (super -12) atm. Changes in oxygen fugacities in different areas produced local variations in mineralogy and mineral compositions--mainly in jacobsites and garnets.The Koduru and Garbham deposits have remarkable mineralogical similarities with gonditic deposits in northern India and other stratified metasedimentary manganese deposits, such as the Otjosondu (S. W. Africa) and Franklin (U. S. A.) deposits.
