New Mexico Geological Society Annual Spring Meeting — Abstracts

Gold Hill, 20 km northeast of Lordsburg, New Mexico, is composed of Precambrian granitic- and upper amphibolite-facies metamorphic rocks. Granite, the dominant lithology, consists of quartz and microcline with variable amounts of minor biotite. Low SiO₂ (47-52%) calc-alkalic diorites are present and locally injected with granitic material. Metamorphic rocks consist of upper amphibolite-facies felsic gneisses and amphibolites, which are generally intercalated. Metamorphic rocks generally have a foliation, usually obscured in migmatitic areas, while granitic rocks are only locally foliated. Foliations in both metamorphic and granitic rocks trend northwest with near vertical dips. Gneisses locally contain garnet-biotite-sillimanite assemblages, especially near contacts with granitic rocks. Garnet-biotite geothermometry gives temperatures 650°-700°C, which are consistent with the migmatitic nature of the contacts.

Amphibolites have nearly flat REE patterns and low total REE, while gneisses show extremely variable REE patterns, with some enriched or depleted in HREE. The chemical and petrographic heterogeneity of the gneisses indicates that they may be metasediments. The intercalated nature of the amphibolites with the gneisses and chemistry of the amphibolites suggests that amphibolite protoliths could be basaltic sills, but this does not rule out the possiblity of a sedimentary protolith.

Gold-bearing quartz veins occur throughout the area and generally occur along numerous mafic dikes which roughly parallel foliations. The veins generally consist of massive white quartz, with variable amounts of limonite, hematite, and Mn-oxides. Minor sulfides are present in unoxidized vein material. Gold values are variable, and silver contents are very low.