New Mexico Geological Society Annual Spring Meeting — Abstracts

New Mexico is located at the eastern edge of one of the world’s great metal-bearing provinces. There are nine known Laramide porphyry copper deposits in New Mexico: Santa Rita (58.3 Ma, ⁴⁰Ar/³⁹Ar), Tyrone (Burro Mountains district; 54.5 Ma, ⁴⁰Ar/³⁹Ar), Little Rock (Burro Mountains district), Copper Flat (Hillsboro district, 75 Ma, ⁴⁰Ar/³⁹Ar), Hanover-Hermosa Mountain (Fierro-Hanover district, 57.6 Ma, ⁴⁰Ar/³⁹Ar), Lone Mountain (51.5 Ma, K/Ar), Gold Lake (White Signal district), McGhee Peak (Peloncillo Mountains), and the newly discovered Lordsburg porphyry copper deposit (58.5 Ma, ⁴⁰Ar/³⁹Ar). Additional Laramide skarn and polymetalic vein deposits in New Mexico formed in Paleozoic limestones and dolomitic limestones adjacent to calc-alkaline plutonic rocks emplaced during the Laramide compressional event; most of these areas have potential for additional porphyry copper deposits.

Unlike the regional Tertiary plutons, which are related to caldera volcanism, the Laramide plutons appear to be the roots of andesitic volcanoes. The geology of the Hillsboro district is dominated by Cretaceous andesite flows (75.4 Ma, ⁴⁰Ar/³⁹Ar), breccias, and volcaniclastic rocks. The Copper Flat quartz monzonite porphyry stock intruded the vent of the volcano and hosts porphyry copper mineralization, whereas two additional stocks are unmineralized (Warm Springs quartz monzonite, 74.4Ma, ⁴⁰Ar/³⁹Ar). ⁴⁰Ar/³⁹Ar dating of the Lordsburg andesites (67.9 to 66.3 Ma) suggest that volcanism was older than the porphyry. At Santa Rita, ⁴⁰Ar/³⁹Ar ages of different intrusions suggests a long-lived igneous system between 59.3 to 56.4 Ma.

Compositions of the Laramide intrusive rocks vary regionally and with time from early mafic and silicapoor diorites and monzonites to more siliceous and felsic monzogranites, quartz diorites, and monzogranites . They are typically calc-alkaline to mildly alkaline. Laramide volcanic rocks in New Mexico have arc-like chemical characteristics. This suggests volcanism was the result of dehydration of the subducting Farallon plate beneath the North American plate. Mineral exploration is currently underway in the Lordsburg and Lone Mountain districts. The Santa Rita and Tyrone districts are currently being mined.