Las Guijas tungsten district is located 58 miles southwest of Tucson, Arizona. The area studied has six pre-Quaternary rock units: granite, granite dikes, volcanic breccia, andesite and rhyolite dikes, black biotite minette, and sedimentary rocks of Cretaceous age. The granite and volcanic breccia occupy most of the area and serve as host rocks. The area has a series of vertical faults consisting of a north-south system (post-mineralization) and an east-west system (pre-mineralization). The deposits are associated structurally and genetically with the Las Guijas granite stock, and all the veins encircle the stock.Economic deposits were formerly mined but active mining has now ceased. The mineralogy is simple. Wolframite, huebnerite, and scheelite are the principal ore minerals along with sulfides and oxide minerals. Quartz is the prevalent gangue and sericite---an alteration product. Spectorgraphic analyses indicate the elements present in the ore minerals and confirm mineral identification.On the basis of field observations and in the light of the recent experimental studies, the Las Guijas tungsten deposits have been classified as epigenetic epithermal deposits.