FLUCTUATION IN POROSITY AND FLUID PRESSURE DURING HYDROTHERMAL EVENTS - TEXTURAL EVIDENCE IN THE EMUFORD DISTRICT, AUSTRALIA

Upper Paleozoic granites of the Emu Suite, north Queensland, show varying degrees of alteration. The petrographic and microstructural survey of albite-rich zones reveals the spatial overprint of hydrothermal events which are more correlated with changes in physical conditions (permeability, fluid pressure and flow regime) rather than with chemical variations of the percolating fluids. The sequence of alteration events chronologically comprises: (1) in situ substitution of primary microline to chessboard albite; (2) dissolution of magmatic quartz resulting in the formation of interconnected vugs; (3) formation of concentric cracks around some vugs; (4) infilling of vugs; and (5) development of a dense system of closely spaced, sheeted veinlets by a crack-seal process. The first two stages indicate an increase of the global permeability of the albite zones (chemically induced porosity), whereas the vug- and vein-filling stages indicate the reverse, resulting in an increase of fluid pressure from nearly hydrostatic to nearly lithostatic values (mechanically induced porosity). This contrast is discussed from micro- and mesostructural arguments and from results of a fluid inclusion investigation; it is explained by the progressive focusing of the circulating fluids via grain boundaries, then vugs, and through tapping by late-stage microfracturing.