Abstract The phenomenon of subsidence has plagued the mining industry for centuries. Although subsidence is directly caused by withdrawal of ore from underground workings, the particular combination of rock type, geologic structure, and method of mining at the site is responsible for the physical character of subsidence appearing at the surface. Continued mining of porphyry copper ores at the San Manuel Mine for ten years has resulted in the formation of three large subsidence pits in a wedge-shaped caprock of Tertiary conglomerates. The pits have a total surface area exceeding 160 acres. Peripheral growth of the pits is controlled by the geologic structure and engineering properties of the caprock and of the ore body, as well as by the relative location of caved blocks and rate of withdrawal of ore. Specific items of interest at the site are numerous subparallel faults, a definite pattern of advancing ground fractures, and well-defined units of mass wastage within the pits. The study has shown that there are definite relationships between geologic structure in the area and the growth and configuration of the subsidence pits.

Abstract This study exploits a cross-sectional view of the Laramide magmatic arc in the northern Tortilla Mountains, central Arizona, that was created by tilting during severe Tertiary extension of the Basin and Range province. Building upon earlier work, we combine the results of geologic mapping of rock types, structures, and hydrothermal alteration styles, with a palinspastic reconstruction, to provide a system-wide understanding of the evolution of the composite magmatic and hydrothermal Tea Cup porphyry system. Geologic mapping revealed the presence of at least three, and possibly four, mineralizing hydrothermal systems in the study area that are associated with widespread potassic, sericitic, greisen, sodic (-calcic), and propylitic alteration. The alteration envelops both porphyry copper and porphyry molybdenum (-copper) mineralization. Two areas flanking compositionally distinct units of the composite Tea Cup pluton are characterized by intense potassic and sericitic alteration. Intense alteration and mineralization akin to iron oxide-copper-gold systems was recognized in several areas. The U-Pb dating of zircons from porphyry dikes suggests that hydrothermal activity in the study area was short lived (~73–72 Ma). Subsequently, between ~25 and 15 Ma, the Tea Cup porphyry system was tilted ~90° to the east and extended by >200 percent due to movement on five superimposed sets of nearly planar normal faults. Each fault set was initiated with dips of ~60° to 70°, but modern dips range from 70° to 15° overturned from the youngest to the oldest set. Tertiary normal faulting resulted in the exposure of pieces of the porphyry system from paleodepths of >10 km. Palinspastic reconstruction of a ~30-km-long cross section reveals that the Tea Cup pluton formed by sequential intrusion of at least four compositionally distinct units. Each major unit generated its own hydrothermal system. The most intense alteration in each hydrothermal system formed above the cupolas of each major phase of the pluton. Potassic alteration dominates the core of each system, whereas feldspar-destructive acid alteration overlaps with the potassic alteration but also extends to higher levels within each system. Deep sodic (-calcic) alteration overlain by iron oxide-rich chlorite-sericite-pyrite alteration flanks these central systems and generally extends 2 to 4 km away from the center of the hydrothermal systems. Greisen-style alteration was recognized 1 to 2 km beneath the potassic alteration in one porphyry copper system but overlaps and extends above the exposed porphyry molybdenum (-copper) system. Propylitic alteration occurs in a distal position and surrounds the other alteration styles. The alteration mapping, combined with the palinspastic reconstruction, revealed two covered exploration targets centered on intense potassic alteration, demonstrating that palinspastic reconstruction represents a powerful exploration technique in a district with more than 100 years of exploration history.