Many chemically formed ore shoots yield good ore deposits. During vein formation, hot silica (quartz-rich), hydrothermal acidic fluids react with rocks with favorable chemistry; particularly rocks with calcium carbonate (limestone, dolomite). Thus, following a vein that cuts through limestone could lead prospectors to find skarns, replacement deposits, etc for gold and other valuable minerals. Replacement deposits and skarns are often formed when such mineralizing fluids come in contact with limestone or even rocks such as limy shales, or limy sandstones. Replacement deposits can be very rich.
|Cupriferous gossan - note the tawny limonite (hydrated iron oxide) with fissures filled with azurite (copper carbonate) typically found in many mining districts in Arizona and Utah.|
|The tan matrix in this specimen invaded fractures in a quartz vein and partially replaced|