Abstract The Mesquite Mining District lies on the southern piedmont flanks of the Chocolate Mountains in southeastemmost California, some 60 km east of the Salton Sea and about 50 km northwest of Yuma, Arizona (Fig. 1). This range is part of the eastern margin of the Salton Trough, the locus of active faulting along the modem San Andreas fault system (Crowell, 1979) and the locale for this field trip (Fig. 1). Inactive strands of the Neogene San Andreas fault system cut the Chocolate Mountains (Dillon, 1976; Crowell, 1979). Production from the Mesquite Mining district began in 1985 and is now one of the largest producers of gold in California (B umett, 1990). Announced economic reserves (Gold Fields Operating Co., 1990) are 93.7 million short tons at an average grade of 0.033 troy ounces per short ton. The Oligocene gold orebodies that constitute the mining district formed in an epithermal environment and are hosted by gneiss and granite of Mesozoic age (see below). This part of the field trip examines the setting of epithermal gold mineralization in the mining district. The structural control on the orebodies is the primary focus of the field trip and we will examine some of the evidence supporting the interpretation of a dextral strike-slip control on the mineralization (Willis and others, 1987, 1989; Manske and Einaudi, 1989). Another structural model links the formation of the gold orebodies to Tertiary extensional deformation and related detachment faulting (Drobeck and)