The pore structure of hydrothermally altered granites: insights into the magmatic-hydrothermal system of the Laguna del Maule volcanic complex, Chile.

The porosity, geometry and permeability of magmatic geothermal systems is important for interpreting geophysical signals at potentially active volcanoes, understanding geothermal production and the concentration of critical metals in fluids. Our case study is the Laguna del Maule volcano (Chile) which is undergoing rapid ground surface uplift that could have a geothermal or magmatic origin. We study porous hydrothermally altered granitic lithic clasts, older than 150ka, from the 17ka rhyolite ignimbrite eruption. There are no outcropping granites, so these lithics provide information on some portion of the subsurface, which has also been extensively studied by multi-parameter geophysics. The lithics vary in composition, crystal sizes and alteration, and those we studied are holocrystalline, fine-grained granodiorites and coarser-grained monzogranites. The latter break more readily, however, all samples contain fractures and miarolitic cavities. Amphibole geothermobarometry indicates crystallization pressures of 0.5-2 (+/- 0.6) kbar (2-8 km depth) at 600-800 (+/- 30)˚C, overlapping with a Magnetotelluric anomaly of >1 S m−1 at 3-5 km depths. We apply diverse techniques to quantify the porosity and pore structure of the lithics. Helium pycnometry data show they have up to 7 vol% connected porosity. SEM analysis reveals millimetre-scale miarolitic cavities, however, image analysis shows that the pore volume is dominantly comprised of a connected network of 5-25 μm-wide pores along grain boundaries, especially, but not exclusively, around quartz grains. We suggest the density increase associated with the beta-to-alpha quartz transition is an important mechanism for generating connected porosity in granites as they cool below ~570˚C, and can generate conductive and permeable regions with relatively low porosity. Calculations reveal the MT anomaly could be generated with a low-intermediate salinity brine (5-25 wt.% NaCl) with a fluid fraction of 0.03-0.06. We propose that the erupted granites were brought to the surface rapidly from an active hydrothermal system beneath the Laguna del Maule volcanic complex.