The phrase "host rock" refers to the surrounding rock or material in which minerals or valuable substances are found. It is like a host for these minerals, providing the environment in which they are formed or deposited. Full definition
However, the world's largest sea cave has formed in the heavily fractured Caversham sandstone (Barth, 2013) changing our understanding of which host rocks can form large sea caves. (en.wikipedia.org)
A secondary target host rock type is mafic volcanic rock of the Upper Plate rock sequence, similar to Newmont's Twin Creeks Mine. (paramountnevada.com)