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.
REFERENCES
Clemons, R. E., 1982, Geology of Florida Gap quadrangle, Luna
County, New Mexico: New Mexico Bureau of Geology and Mineral Resources,
Geologic Map 52, scale 1:24,000.
Clemons, R. E., 1984, Geology of Capitol Dome quadrangle, Luna
County, New Mexico: New Mexico Bureau of Geology and Mineral Resources,
Geologic Map 56, scale 1:24,000.
Clemons, R. E., 1998, Geology of the Florida Mountains, southwestern
New Mexico: New Mexico Bureau of Geology and Mineral Resources,
Memoir 43, 112 p.
Clemons, R. E., and Brown, G. A., 1983, Geology of Gym Peak quadrangle,
Luna County, New Mexico: New Mexico Bureau of Geology and Mineral Resources, Geologic Map 58, scale 1:24,000.
Dunbar, N. W., Jacobs, G. K., and Naney, M. T., 1995, Crystallization
processes in an artificial magma: Variations in crystal shape,
growth rate and composition with melt cooling history: Contributions
to Mineralogy and Petrology, v. 120, p. 412-425.
Jacobs, G. K., Dunbar, N. W., Naney, M. T., and Williams, R. T.,
1992, In Situ Vitrification: Observations of petrological processes
in a man-made magmatic system: EOS, Transactions of the American
Geophysical Union, v. 73, p. 401-411.
Julyan, R., 1996, The place names of New Mexico: University of
New Mexico Press, Albuquerque, 385 p.
McLemore, V. T., 1996, Copper in New Mexico: New Mexico Geology,
v. 18, p. 25-36.
Northrop, S. A. and LaBruzza, F. A., 1996, Minerals of New Mexico:
University of New Mexico Press, Albuquerque, New Mexico, 356 p.
Weber, R. H., 1980, Rock Hound: New Mexico Geology, v. 2, p. 59-60.