Geology and mineral deposits of the Gold Hills, Hidalgo and Grant Counties, New Mexico
Robert D. Beard and Douglas G. Brookins
The Gold Hills, located approximately 20 km northeast of Lordsburg, New Mexico, consist principally of Precambrian supracrustal and intrusive rocks. Supracrustal rocks, which are most abundant in the northwestern Gold Hills, are primarily amphibolites and gneisses; intrusive rocks range from diorites to granites. Supracrustal rocks may represent part of the Bullard Peak Series of Hewitt (1959), and larger intrusions of the area are part of the Burro Mountain batholith of Hewitt (1959). Amphibolites and gneisses are moderately foliated, and diorite and granitic rocks are only locally foliated. Contacts between the supracrustal and granitic rocks are often migmatic, and migmatites are sometimes extensive. Minor intrusions are mainly pegmatites, diabase dikes, basaltic dikes and felsic dikes. Diabase dikes parallel the north-northwest-trending foliation in the supracrustal rocks, and basaltic dikes, which trend north by northeast, crosscut the diabase dikes. Niggli numbers and trace elements indicate that the amphibolites are orthoamphibolites, and may represent original ocean floor basalts. Petrography and Niggli values indicate that most of the gneisses are paragneisses, although one isolated occurrence may represent an original andesite. The lack of a coherent stratigraphy and the high grade of metamorphism make any tectonic interpretation extremely tenuous. Metamorphic conditions were approximately 700 ± 50°C and about 4.2-4.5 ± 1.6 kb, as determined by garnet-biotite geothermometry and quartz-plagioclase-aluminum silicate geobarometry. Abundant secondary chlorite, muscovite and epidote throughout all of the lithologies of the Gold Hills suggest that a pervasive low-grade event has affected the area. Mineral deposits are principally gold-bearing quartz veins of the Gold Hill mining district, located in the northwestern part of the Gold Hills, and minor deposits of fluorite, REE-enriched pegmatite, scheelite and base metals. The southeasternmost Gold Hills also contain the Bound Ranch (Langford Hills) district, which is a geologic and topographic extension of the Gold Hills.
- Beard, Robert D.; Brookins, Douglas G., 1988, Geology and mineral deposits of the Gold Hills, Hidalgo and Grant Counties, New Mexico, in: Cretaceous and Laramide tectonic evolution of southwestern New Mexico, Mack, G. H.; Lawton, T. F.; Lucas, S. G., New Mexico Geological Society, Guidebook, 39th Field Conference, pp. 203-210.