ALBERT

Description: Meteorite impacts cause conversion of kinetic energy into thermal energy. Part of this thermal energy is used to form a melt sheet, part is dissipated to heat the target rocks and these together with the hot rocks that elastically rebound from the depth of several kilometres (central uplift) activate hydrothermal circulation. Impact-generated hydrothermal systems have been documented from several impact structures world-wide. Three Australian examples—Shoemaker, Woodleigh and Yarrabubba—provide evidence of hydrothermal fluid flow both within and around the structures. Field observations, and petrographic and geochemical data suggest a common evolutionary trend of post-impact hydrothermal activity from early high-temperature alkali metasomatism to a later lower temperature H+ metasomatism, resulting in the overprinting by hydrous mineral assemblages. Hydrothermal systems activated by meteorite-impact events are important because they may also form economic mineral deposits, as is documented for several impact structures in the world. A working model of hydrothermal circulation in terrestrial impact structures posits two main stages: (i) initial high-temperature fluids percolate downward causing widespread alkali metasomatism of the shattered target rocks below the melt sheet, resulting in their modification to rocks of syenitic affinity; and (ii) inflow of meteoric water and progressive cooling of the melt sheet leads to a lower temperature stage, in which hydrothermal fluid flow tends to move upward, resulting in mineral assemblages and alteration patterns that resemble those of epithermal systems. In addition, these fluids can discharge at the surface as hot springs.