Precipitation by depressurization and boiling in veins hosted in an extensional strike-slip duplex: microstructural and microthermometric evidence.

Abstract

The interplay between fluid transport and crustal deformation is fairly well known in theory; however, it has seldom been described in actual field examples. The structural, microstructural and microthermometric analysis of fault-vein networks spatially associated with the Caleta Coloso extensional strike-slip duplex (125-118 Ma) in the Atacama Fault System, in the north of Chile, provides key information regarding the thermodynamics (pressure, temperature and composition) of the fluids responsible for mineral precipitation, giving insights into the triggering mechanisms. Microstructural analysis suggests that quartz, epidote and calcite crystals grew under subhydrostatic pressures in fluid-filled, open spaces. Primary fluid inclusions in quartz and calcite yield homogenization temperatures between 102 and 253°C, with a wide salinity range (3.5%-23.7% of NaCl equivalent weight). Calculated pressures range between 0.4 and 7.4 MPa, which are extremely low values considering the geologic setting of the veins suggesting depths of at least 3 km. These pressure values document a significant, local fluid pressure drop that may have triggered boiling and mineral precipitation in upper crustal dilational sites of the Caleta Coloso Duplex.