Virginia T. McLemore, Nelia W. Dunbar and William C. McIntosh, New Mexico Bureau of Geology and Mineral Resources, New Mexico Tech, 801 Leroy Pl., Socorro, NM 87801
Rockhound State Park lies in the Little Florida Mountains southeast of Deming, New Mexico. It was established in 1966 as the first park in the United States that allowed collecting of rocks and minerals for personal use. Each visitor is allowed to collect as much as 15 pounds of rocks and minerals from the 250-acre park; mineral dealers are not allowed to collect. Rockhound State Park actually consists of two separate sites, the main park in the Little Florida Mountains and Spring Canyon Recreation Area in the northern Florida Mountains. The main park offers covered shelters for camping and picnicking, a group shelter, restrooms with showers, RV dump station, playground, hiking trails, wildlife viewing areas, and rock and mineral collecting. Spring Canyon lies in the northern Florida Mountains, south of the main park, and is open for day use only. Facilities include picnic tables, restrooms, and hiking trails.
Prehistoric people most likely camped thousands of years ago beneath rocks that provided some shelter from the elements and predators. The Mimbres or Mimbreno Indians settled north of the area about 1200-1150 A.D. Arrowheads and pottery shards are still found today that reminds us of their former presence. Later, Apache Indians moved into the area. The rugged Florida Mountains were well known as Apache strongholds until the mid-1880s. Spanish conquistadors and settlers traveled through the area in the 1500s, but did not find the desert environment or the Apache Indians very hospitable. Spanish explorers named the larger range in the 1700s. The range was shown on Juan Nentwig's map of 1762 as Sierra Floridas (Julyan, 1996). In 1804, Col. Manuel Carrasco, a Spanish army officer, began mining the copper at Santa Rita (McLemore, 1996). Mule trains loaded with copper from the mine passed near what is now the state park, on their way to Chihuahua in 1804-1834 (Weber, 1980).
The Little Florida Mountains consist predominantly of interbedded mid-Tertiary andesite, dacite, ash-flow tuff, rhyolite, and fanglomerate intruded by rhyolite domes and dikes (Clemons, 1982, 1984, 1998; Clemons and Brown, 1983). Altered sanidine and biotite from an ash flow near the base of the stratigraphic section at Little Gap give 40Ar/39Ar ages of 33.5 ± 0.2 Ma and 32.9 ± 0.2 Ma, respectively. This ash flow may correlate with the 33.5 Ma Oak Creek Tuff, erupted from the Juniper caldera in the northern Animas Range in the Boot Heel volcanic field (McIntosh and Bryan, this volume). Additional rhyolite and andesite samples from the state park range in age from 24.4 ± 0.4 to 28.5 ± 0.3 Ma (groundmass, 40Ar/39Ar; this report). A rhyolite dike at the head of Spring Canyon has been dated as 25.4 ± 0.7 Ma (groundmass, 40Ar/39Ar; this report). Gray perlite, thunder eggs, jasper, onyx, geodes, agate, rhyolite, and quartz crystals are among the more common minerals and rocks found in the park. Thompsonite, a zeolite, is found in amygdules in quartz latite (Northrop and LaBruzza, 1996). Agate is present in a wide range of colors, and is one of the minerals that many visitors collect at Rockhound State Park. Some geodes found at Rockhound contain multicolored agate in addition to well-formed quartz crystals.
One of the geologically interesting features of Rockhound State Park is the presence of numerous spherulites within the rhyolite lava. Many of these spherulites have cores that contain agate or quartz crystals. The spherulites range in size from less than 1 mm to greater than 30 cm. When broken open, the spherulites exhibit distinct concentric banding, as well as numerous silica-filled fractures and open-space filling silica. In order to gain some insight into the process by which these spherulites form, samples were examined using back-scattered electron imaging and qualitative X-ray analysis and imaging using a Cameca SX-100 electron microprobe. The imaging and X-ray analysis show that the spherulites are formed of bands of pure quartz intermixed with bands of intergrown feldspar and quartz (Fig. 1). This banding produced the concentric appearance of the spherulites. Bands of quartz crosscutting the banded structure are also observed (Fig. 1).
The observed patterns in the spherulites suggest that they formed during the cooling of the rhyolite lava. Similar spherulitic features were observed in an artificial melt that was rapidly cooled (Jacobs et al., 1992; Dunbar et al., 1995). The temperature of the artificial melt was monitored during the cooling process, and exothermic crystallization was observed at high temperatures (1100oC). Spherulitic growth occurred during this crystallization process, and the internal structure of the spherulites was indistinguishable from the internal structure of Rockhound spherulites, although in the artificial melt the phenocryst phases were plagioclase and pyroxene rather that quartz and alkali feldspar. This similarity suggests that the Rockhound spherulites formed by a high-temperature magmatic crystallization, rather than by sub-solidus, processes. The quartz veins and agate associated with the spherulites formed later, as evidenced by the crosscutting relationships of the hydrothermal quartz with the igneous quartz and feldspar.
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