Thermal impact of dykes on ignimbrite and implications for fluid flow compartmentalisation in calderas

Ignimbrites within calderas host intrusions with hazardous and/or economically significant hydrothermal systems. The Hvítserkur ignimbrite at Breioavík caldera, north-eastern Iceland, is intruded by basaltic dykes. The country rock ignimbrite is generally soft and zeolite-rich, has low permeability, high porosity, and few macrofractures. However, the ignimbrite immediately adjacent to the dyke is hard, recrystallised quartz, plagioclase, and alkali feldspar, with a low permeability and porosity and frequent macrofractures. At 1-2 m from the dyke, the ignimbrite is hard, dominantly glassy with pervasive perlitic microfractures, has high permeability, but low porosity and frequent macrofractures. A narrow zone of pervasive unlithified clay exists 2 m from the dyke. The dyke intrusion promoted a narrow zone of welding, fracturing, and perlitisation in the ignimbrite resulting in high permeability and focussed alteration. Our study shows how intrusions and their thermal aureoles create vertical pathways for, and horizontal barriers to, geothermal fluid flow.