Virgil W. Lueth, Richard Chamberlin, and Lisa Peters
Abstract- Cryptomelane (KMn8O16) samples from the Nancy and MCA mines in the Luis Lopez manganese district were collected in order to determine the age of mineralization using the 40Ar/ 39Ar dating method. Cryptomelane precipitation occurred at the onset of calcite mineralization following banded manganese ore deposition. X-ray diffraction analysis confirmed sample composition and purity. 40Ar/ 39Ar dating of two cryptomelane samples from the MCA mine and one from the Nancy Mine yielded age spectra with generally increasing radiogenic yields and apparent ages that suggest Ar loss. The apparent ages of the oldest furnace analysis heating steps from the MCA-2 sample (6.69±0.04 Ma) and sample 9189 from the Nancy Mine (6.33±0.04 Ma) are assigned as minimum ages of mineralization. These ages are consistent with age estimates for mineralization (ca. 3-7 Ma) based on geologic relationships in the area as established by previous workers.
These age determinations indicate manganese mineralization closely followed regional potassic metasomatism. Published 40Ar/ 39Ar ages of metasomatic adularia from jasperized conglomerates of the lower Popotosa Formation, north of the Nancy mine, indicate metasomatism persisted until at least 7.4 Ma. Minor manganese veins near the Nancy mine cut jasperized Popotosa conglomerates and cut numerous jasper veinlets in the Oligocene tuffs affected by this earlier alteration. A published model for the Luis Lopez district previously interpreted the mineralization to be derived from a manganese-depleted red alteration zone superimposed on thick intracaldera tuff (31.9 Ma) near the center of the district. A dike dated at 11.05 ± 0.05 Ma is unaltered where it cuts across the southern arm of the red zone. This indicates that the red zone is at least 4.4 million years older than manganese mineralization at the MCA and Nancy mines and genetically unrelated. A composite upper-crustal silicic pluton is inferred to have fed episodic eruptions of dacitic and rhyolitic lava domes (at 9.5, 8.7, 7.8, 7.5 and 7.0 Ma) in the Socorro Peak area, 8-9 km northeast of the Nancy mine. This plutonic complex represents the most likely heat source for the hydrothermal system of the Luis Lopez manganese district. Miocene playa claystones formed an impermeable cap to the hydrothermal system, thereby forcing significant lateral flow (i.e. 9 km) of the mineralizing waters.