Abstract
Several studies have demonstrated the effect of a second phase on the distribution of fluid phase and dissolution of quartz grains. However, as most observations came from aggregates deformed under hydrostatic stress conditions and mica-bearing quartz rocks, 3-D distribution of pores on quartz-quartz (QQB) and quartz-hematite boundaries (QHB) has been studied. Several fracture surfaces oriented according to finite strain ellipsoid were analyzed. The pore distribution characterizes the porosity and grain shape as highly anisotropic, which results from the nature and orientation of boundaries. QHB have physical/chemical properties very different from QQB, once the hematite plates have strong effect on wetting behavior of fluid, likewise micas in quartzites. They are pore-free flat surfaces, normal to compression direction, suggesting that they were once wetted with a continuous fluid film acting as faster diffusion pathway. At QQB, the pores are faceted, isolated, close to its edges reflecting the crystallographic control and an interconnected network of fluid along grain junctions. The QQB facing the extension direction are sites of fluid concentration. As consequence, the anisotropic dissolution and grain growth were responsible for the formation of hematite plates and tabular quartz grains significantly contributing for the generation of the foliation observed in the studied rocks.
Highlights
Several studies based on observations (Mancktelow 1987, Hippertt 1993, 1994) and experiments (Hickman and Evans 1995, Farver and Yund 1999) have demonstrated that the presence of a second phase has a profound effect on the anisotropic distribution of fluid phase and the dissolution of quartz minerals
The different pore topographies observed along boundaries with different orientations and the anisotropic distribution of pores along these boundaries may suggest that the fluid distribution and grain boundary structures in these rocks were highly anisotropic
Microstructural observations of quartz-hematite tectonites indicate that anisotropic fluid distribution and grain growth have exerted an important role on the fabric evolution of banded iron formation from Quadrilátero Ferrífero (MG)
Summary
Several studies based on observations (Mancktelow 1987, Hippertt 1993, 1994) and experiments (Hickman and Evans 1995, Farver and Yund 1999) have demonstrated that the presence of a second phase has a profound effect on the anisotropic distribution of fluid phase and the dissolution of quartz minerals. Most experiments carried out in aggregates deformed under conditions of hydrostatic stress have demonstrated that fluid distributions are in textural equilibrium, showing equilibrated pore geometries (e.g. Watson and Brenan 1987). These studies have presented wetting and nonwetting fluid distributions (interconnected tubules along grain edges and isolated pores, respectively) as well as continuous fluid film in equilibrium with the solid. The studied banded iron formations occur in the southern part of the Brazilian Archean-Proterozoic shield, São Francisco Craton (Fig. 1) In this location there is an extensive iron ore deposit, in the region termed Quadrilátero Ferrífero – QF, which is part of a granite-greenstone terrain of Archaean-Proterozoic age (Alkmim and Marshak 1998)
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