Host rock volume change during alteration promotes self-sealing of hydrothermal systems

Altered host rocks that surround hydrothermal mineral deposits exchange elements with hydrothermal fluids to produce alteration minerals. The changed mineralogy affects the altered rock's density and volume and this in turn affects the potential of a hydrothermal reaction to proceed. An increase in rock volume closes fluid pathways and inhibits further reaction, whereas a volume decrease has the opposite effect. In adularia-sericite epithermal deposits, K-metasomatism dominates mass change, where illite and adularia are the principal K-bearing phases. Formation of illite generally decreases host rock volume, whereas proximal adularia formation increases the rock volume. Carbonation may further increase rock volume or offset illite-related volume loss. In orogenic deposits, proximal alteration involves carbonation, sulfidation and sericitization. In each case, proximal net volume effects in appear to be in the order of 5-10% expansion. Therefore, proximal host rock alteration provides a first-order control on hydrothermal fluid flow; host rock alteration leads to volume expansion that promotes sealing of the fault fracture networks that provide the main conduits for hydrothermal flow.