FACTS & FIGURES — New Mexico’s Mogollon district site of

The Mogollon mining district in southwestern New Mexico is one of those mining areas that always seems to be on the verge of a rebirth.

Between 1885 and 1944, the district produced 2.1 million tons of ore from two different associations in epithermal-style veins — gold and silver on the one hand, and gold, silver, copper and lead on the other — and data indicate there is additional mineral potential in the district. Based on geology and mineralization in the Mogollon mining district, where minerals have previously been located, scientists believe the region could contain additional mineral deposits.

It is speculated that the far northern area contains potential for additional copper, the east contains extensions of known veins, and the west may contain ore at depth.

The hypothesis that there may be additional mineral potential in the northern area of the Mogollon mining district results from the occurrence of two distinct assemblages and their apparent spatial separation. Copper and lead are more abundant in the northwestern and northern portions of the district, while silver and gold predominate elsewhere.

Certain events believed to have taken place in the Mogollon mining district may have created this distribution of minerals. The Silver Creek graben transects the central part of the district and may reflect the presence of an intrusion beneath the entire Mogollon mining

district.

One may speculate that the distribution pattern of copper-bearing ores reflects a separate geothermal event vis-a-vis the gold-silver ores, possibly one related to the formation of a porphyry copper deposit in the northern part of the district.

The belief that there may be additional ore-grade mineralization to the west of the Mogollon district stems from observations in underground workings. Ore-grade mineralization extends to and is incorporated in the post-mineralization range-front faults, which separate the Mogollon Mountains from the Alma Basin on the west. Part of the district must lie buried beneath the (late Cenozoic) basin fill, perhaps as shallow as 350 metres. There has been scant production from the portion of the district east of the Queen fault where the veins are predominantly calcite and/or extremely fine-grained silica. Resistivity profiles from audiomagnetotelluric soundings in the district and the Bursum caldera provide evidence that as much alteration occurs east of the Queen fault as occurs to the west, where most of the mining took place.

The character of the veins, the geophysical data and probable offset along the Queen fault of several hundred metres are seen as permissive evidence that the part of the district east of the Queen vein, although it has never had any mineral production, may contain an amount of ore equivalent to that produced west of the Queen vein.

The Mogollon district straddles the topographic margin and ring-fracture zone of the Bursum caldera near the northwestern edge of the Mogollon Mountains cauldron complex, which consists of three calderas: the Mogollon (33-34 million years), Gila Cliff Dwellings (29-30 million years) and the Bursum caldera (28-29 million years). Two episodes of volcanic and intrusive activity followed the cauldron complex development, an andesitic sequence (23-26 million years) and basalt-rhyolite sequence (18-21 million years). At 18-17 million years, the Mogollon veins are related temporally to the waning stages of the basalt-rhyolite igneous activity.

The geologic and mineralogic relations in the district support the belief that additional mineral potential exists in the northern, western and eastern parts of the district.

— From a recent “Mineral Resources” newsletter of the U.S. Geological Survey.