Magmatic history of the Little Hatchet Mountains, Hidalgo and Grant counties, southwestern New Mexico
— Ryan Channell, Nancy J. McMillan, Timothy F. Lawton, Matt Heizler, Richard P. Esser, and Virginia T. McLemore

Precambrian(?) to Tertiary rocks in the Little Hatchet Mountains provide information on numerous geologic events that occurred in southwestern New Mexico in the last 600 million years. The Precambrian(?) Hatchet Gap granite in the southern Little Hatchet Mountains has controversial U-Pb alpha dates of 640 and 605 Ma (Zeller, 1965), suggesting that it could be related to a suite of Cambrian–Ordovician alkalic and carbonatitic igneous intrusions in New Mexico rather than to older Proterozoic igneous rocks (~1.1 and 1.4 Ga) in the area. The Hatchet Gap granite is in fault contact with Paleozoic strata, which are overlain by the Jurassic Broken Jug  Formation. The Broken Jug Formation, correlative to the lower Bisbee Group, consists of marine and nonmarine sedimentary rocks as well as subaerial basalt flows formerly interpreted as mafic Cretaceous–Tertiary plutonic rocks. Numerous Cretaceous–Tertiary plutons of the Sylvanite intrusive complex intrude Bisbee Group rocks as young as the Lower Cretaceous U-Bar Formation. The Sylvanite intrusive complex is interpreted as the subvolcanic equivalent of the 71 Ma andesitic Hidalgo Formation in the northern Little Hatchet Mountains. Sylvanite plutonic rocks are mafic–intermediate in composition (50-65% Si0₂), and have high Al₂0₃ (17-19%) and La/Ta (28-39%), similar to samples from the Hidalgo Formation (Young et al., this volume), supporting this correlation. These geochemical data suggest that the Sylvanite intrusive complex was emplaced in a continental arc above the subducting Farallon plate during the Laramide orogeny. ⁴⁰Ar/³⁹Ar age determinations on biotite and K-feldspar of the Granite Pass granite in the southern part of the range yield a weighted mean age of 32.32 ± 0.17 Ma and confirm its Tertiary age. An ⁴⁰Ar/³⁹Ar age determination on the groundmass of a diorite in the Eureka intrusive complex in the northern part of the range suggests that the Eureka complex is no younger than 35.5 ±1.7 Ma. Thus, the Eureka complex could represent either a shallower and more quickly cooled phase of the Granite Pass granite or a separate, older intrusion.