MICROBIAL AND INORGANIC “CAVE DEPOSITS” FROM AN ABANDONED GOLD MINE, SAN BERNARDINO MOUNTAINS, CA

An abandoned gold mine in Las Flores Canyon, San Bernadino Mountains, CA contains a variety of inorganic and microbially-mediated speleothems. The mine consists of a main horizontal shaft, approximately 200 m long, and a secondary horizontal shaft, approximately 75 m long that follow a contact between the Pelona Schist and Cretaceous granitoids. The percolation of groundwater within the mine has led to the precipitation of a variety of inorganic and microbially-mediated speleothems. Inorganic speleothems include millimeter-thick flowstone coatings along the walls of the mine, and small stalactites 2-5 mm in length, and soda straws, up to 2 cm in length, hanging from the roof of the mine. Cave pearls commonly occur on the mine floor, and range from a few millimeters to 2 cm in diameter. The cave pearls are typically comprised of mm-scale layers of aragonite crystals growing perpendicular to a central nucleus (typically a schist clast or an older agglomeration of cave pearls). The irregular lamination of the cave pearls, coupled with the presence of entrapped organic matter between laminae is indicative of a microbially-mediated origin (likely related to cyanobacterial mats and patches that are found along the walls and floor of the mine). Cave pearl laminae are probably the result of alternating wet and dry seasons that led to precipitation or dissolution of the aragonite. The cave pearls are often cemented together to form grapestone, and in places form a pavement along the floor of the mine. Overall, flowstone coatings are typically thicker in the deeper parts of the mine, although the distribution of flowstone is patchy. Standing pools of water are commonly free of speleothems, or the speleothems within the pools appear to be undergoing dissolution; speleothem precipitation is aided by agitation of the mine waters, which results in carbon dioxide degassing and calcite supersaturation. The source of the calcite within the mine is most likely marble that occurs within the Pelona Schist. Precipitation of calcite within the mine is occurring at a rather rapid rate: the mine was abandoned approximately 100 years ago, resulting in precipitation rates of 0.05mm/year, or even faster, as modern debris within the mine is strongly cemented to the cave floor.