SYLVANITE INTRUSIVE COMPLEX, LITTLE HATCHET MOUNTAINS, HIDALGO COUNTY, SOUTHWESTERN NEW MEXICO

The Sylvanite Intrusive Complex (SIC), located in the Little Hatchet mountains of extreme southwestern New Mexico, is interpreted to be the subvolcanic pluton for a Laramide (ca. 73 Ma) arc composite cone. The extrusive equivalent of the SIC, a suite of andesitic lava flows, lahars, and breccias of the Hidalgo Formation, is preserved in the northern Little Hatchets on the hanging wall of a Tertiary normal fault. The entire southern section has been rotated approximately 50^o to the southwest, providing the opportunity to view the subvolcanic plumbing system in cross-section. Mapping, petrography, and geochemical studies are being done of the SIC in order to link the intrusions to the Hidalgo Formation and to interpret the intrusion in light of studies of modern subvolcanic magma chambers.

The SIC consists mainly of two bulbous intrusions with overlapping compositions. The southern, shallower, bulb is slightly older and in general more mafic with slightly lower incompatible trace element concentrations (SiO₂=54-61%, MgO=2-4%, Nb=8–19 ppm, Zr=217–248 ppm, Ba=664–1544 ppm, Sr=405–1235 ppm, Rb=40–151 ppm, Na₂O + K₂O=6–9.75%) than the northern, deeper, younger bulb (SiO₂=59-63%, MgO=1.75–2.25%, Nb=13–22 ppm, Zr=190–252 ppm, Ba=491–1361 ppm, Sr=525–1179 ppm, Rb=60–121 ppm, Na₂O + K₂O=7–11.25%). Higher incompatible trace element concentrations at comparable SiO₂ in the northern bulb suggests that the major differentiation processes were occurring at greater depth, prior to intrusion at the subvolcanic level.

Modern arc volcanoes have roughly cylindrical, magma chambers that range in size from 7 to 10 km tall and 0.75 to 5 km wide, with the upper surface at depths between 7 and 10 km. By restoring post-emplacement deformation and considering stratigraphic thicknesses, we estimate that the minimum dimensions for the SIC were 1.6 km diameter, 7.2 km tall, with the upper surface at 3 km depth. These data suggest that the SIC was similar to a modern arc subvolcanic magma body.