The role of silica in the hydrous metamorphism of chromite

• Published in: Ore geology reviews 2017-11, Vol.90, p.274-286
• Main Authors: Colás, Vanessa, Padrón-Navarta, José Alberto, González-Jiménez, José María, Fanlo, Isabel, López Sánchez-Vizcaíno, Vicente, Gervilla, Fernado, Castroviejo, Ricardo
• Format: Article
• Language: English
• Subjects: Chromite; Earth Sciences; Metamorphism; Open system model; Petrography; Sciences of the Universe; SiO2-rich fluids; Tapo Ultramafic Massif
• ISSN: 0169-1368; 1872-7360
• DOI: 10.1016/j.oregeorev.2017.02.025

[Display omitted] •Alteration of chromitites involve dissolution-precipitation in a fluid-saturated open system.•μSiO2-μMgO diagrams quantify the role of SiO2 and MgO in the metamorphism of chromitites.•P-T-X diagrams model the alteration of chromite as an open system relative to SiO2 and MgO.•Reducing and SiO2-rich fluids are released from host ultramafic rocks during metamorphism. Retrograde hydrous metamorphism has produced three types of microstructures in chromite grains from chromitites and enclosing rocks of the Tapo Ultramafic Massif (Central Peruvian Andes). In semi-massive chromitites (60–80vol% chromite), (i) partly altered chromite with homogeneous cores surrounded by lower Al2O3 and MgO but higher Cr2O3 and FeO porous chromite with chlorite filling the pores. In serpentinites (ii) zoned chromite with homogeneous cores surrounded by extremely higher Fe2O3 non-porous chromite and magnetite rims, and (iii) non-porous chromite grains. The different patterns of zoning in chromite grains are the consequences of the infiltration of reducing and SiO2-rich fluids and the subsequent heterogeneous interaction with more oxidizing and Fe-bearing fluids. During the first stage of alteration under reduced conditions magmatic chromite is dissolved meanwhile new metamorphogenic porous chromite crystallizes in equilibrium with chlorite. This reaction that involves dissolution and precipitation of minerals is here modeled thermodynamically for the first time. µSiO2-µMgO pseudosection calculated for unaltered semi-massive chromitites at 2kbar and 300°C, the lowest P-T conditions inferred from the Tapo Ultramafic Massif and Marañón Complex, predicts that chromite+chlorite (i.e., partly altered chromite) is stable instead of chromite+chlorite+brucite at progressive higher µSiO2 but lower µMgO. Our observation is twofold as it reveals that the important role of SiO2 and MgO and the open-nature of this process. P-T-X diagrams computed using the different P-T pathways estimated for the enclosing Tapo Ultramafic Massif reproduce well the partial equilibrium sequence of mineral assemblages preserved in the chromitites. Nevertheless, it is restricted only to the P-T conditions of the metamorphic peak and that of the latest overprint. Our estimations reveal that a high fluid/rock ratio (1:40 ratio) is required to produce the microstructures and compositional changes observed in the chromitites from the Tapo Ultramafic Massif. The circulation of SiO2-rich fluids and the mo