ASSOCIATIONS OF MN-BEARING MINERALS AS INDICATORS OF OXYGEN FUGACITY DURING THE METAMORPHISM OF METALLIFEROUS DEPOSITS

Abstract

The paper summarizes experimental and calculation data on the effect of oxygen fugacity on the origin of mineral assemblages in Mn-bearing rocks and demonstrates the possibility of application of these data to the reconstruction of conditions under which metalliferous deposits were metamorphosed. A new variant of the T-log fO2 f_{O_2 } diagram is proposed for the Mn-Si-O system, which differs from previous ones by the location of the lines for the formation (decomposition) of braunite and tephroite. These two minerals are the most universal indicators of oxygen fugacity during the metamorphism of Mn-bearing deposits, because these minerals are widespread in nature and can be formed in diverse environments: braunite at high fO2 f_{O_2 } values in the pore solution, and tephroite at low fO2 f_{O_2 } values. The occurrence of Mn oxides and rhodonite (pyroxmangite) in a rock makes it possible to constrain the oxygen fugacity range. An original T-log fO2 f_{O_2 } diagram is constructed for the Ca-Mn-Si-O system. As follows from this diagram, a Ca admixture expands the stability field of rhodonite toward higher oxygen fugacity values. Johannsenite can be formed in these rocks at even higher fO2 f_{O_2 } . The stability of both minerals is constrained in the region of low fCO2 f_{CO_2 } . The paper reports data on the Fe-Si-O and Mn-Fe-Si-O systems and discusses the possibility of applying the results of experiments in the Mn-Al-Si-O system to the estimation of conditions under which andalusite, spessartine, and galaxite can be formed in Mn-bearing rocks. Data on the mineralogy of numerous Mn deposits metamorphosed under various PTX parameters indicate that the origin of Mn-bearing mineral assemblages depends not so much on the temperature and pressure as on the oxygen fugacity, which is, in turn, controlled primarily by the composition of the pristine sediments (the presence or absence of organic matter in them) and host rocks and depends on the permeability of the rocks to oxygen, the P-T conditions, and the duration of the metamorphic processes.