New Mexico Mineral Symposium — Abstracts

The Hanover—Fierro (H-F) mining district extends from the community of Hanover north to mines in Shingle Canyon, northeast of the Continental mine. The host-rock stratigraphy consists of Paleozoic limestones, dolomites, and shales, overlain by Cretaceous siliceous sedimentary rocks, capped by middle Tertiary volcanic rock. The most important ore hosts are the Pennsylvanian Oswaldo Formation and the Mississippian Lake Valley Formation. Limestones in the Ordovician El Paso Formation also host chalcopyrite skarn. Late Cretaceous quartz-diorite sills and Tertiary dikes intrude the stratigraphic section, with little or no associated mineralization.

The major structural features of the district are the contacts of the stock and the Barringer fault zone, located near the northern end of the H—F stock near the community of Fierro. The Barringer fault, which pre-dates the stock and mineralization, is a major feature that forms the northwestern edge of the Santa Rita horst. The fault strikes north 75°-80° east, dipping steeply northwest on the west side of the stock, changing to a strike of about north 30°-40° east, dip of 60°-75° northwest to the northeast of the stock. Displacement to the west is about 1,600 ft, and 1,400 ft on the northeast. In the area of Hanover Mountain, the fault juxtaposes the Cretaceous Colorado Formation with the Paleozoic Lake Valley and Percha Formations, indicating about 1,400 ft of displacement. The fault zone acted as an important conduit for mineralizing solutions in the district. The important stock contacts run north south on the east and west sides of the stock where the Paleozoic rocks have been domed by the force of the intrusion. In the Fierro area, the lower Paleozoic rocks on the footwall of the Barringer fault and on the east side of the stock, dip steeply (+80°) to the east. Upper Paleozoic rocks on the hanging wall of the Barringer and on the west side of the stock, dip more gently (20°-25°).

Copper mineralization in the Continental mines (open pit and underground) near Fierro is in skarns associated with the 57 Ma H—F granodiorite stock. A large quartz-sericite-pyrite (QSP) hydrothermal alteration event involving large amounts of meteoric water did not occur at Hanover—Fierro as was the case at Santa Rita and Tyrone. As such, the district has little supergene enrichment. Most primary copper at H-F is hosted in skarns within the Paleozoic limestones and, to a lesser extent, dolomites around the north end of the stock. The skarns did not undergo a strong retrograde alteration event. The differences between Santa Rita and H—F suggest a deeper level of emplacement for the Hanover—Fierro stock. Weak to moderate QSP alteration is present in the Colorado Formation at Hanover Mountain, which has resulted in the formation of a relatively small chalcocite enrichment blanket on the hanging wall of the Barringer fault northeast of the Continental pit. The fault itself is also host of oxidized copper mineralization, most notably azurite roses from the Hanover #2 mine and a limited amount of turquoise.

Iron skarns are common in dolomitic units of the lower Paleozoic section. These units are in contact with the H—F stock in the middle stretches of the intrusive between the zinc deposits to the south and copper to the north. The Fusselman and Montoya Formations are the most common hosts. The iron occurs as magnetite, with much less specular hematite. Copper as chalcopyrite has been produced from some of the iron mines from the El Paso Formation underlying the Fusselman and Montoya Formations.