pubs.usgs.gov/pp/0415/report.pdfThe Answer is YES!
94 GEOLOGY AND MINERAL DEPOSITS, THOMAS AND DUGWAY RANGES, UTAH
Lithology.- The rhyolite varies from place to place in color, texture, abundance of phenocrysts, degree of layering,
and presence or absence of spherulites. The variations are gradational, and there may be as many differences between parts of a single flow as between two separate flows; it is rarely possible to determine, by inspection or even by microscopic study, what flow a given specimen of rhyolite came from. Two rhyolite flows in the northwestern part of the Thomas Range, however, can be distinguished in places by the presence of copper-colored biotite flakes in the older flow.
The rhyolite is divisible into three facies: obsidian, red spherulitic rhyolite, and gray rhyolite. A thin layer of obsidian, formed by rapid cooling, is generally formed at the base of a thick flow of rhyolite
(fig. 28).
Obsidian has also been found within or beneath layers of volcanic breccia in the northwestern part of the Thomas Range and about a mile northwest of the north end of Antelope Ridge. Discontinuous layers of obsidian occur, also, within other bodies of rhyolitic lava south of Dugway Pass, where they probably mark chilled surfaces of flows. The red spherulitic rhyolite is almost always found just above the obsidian near the base of a flow, but in a few places, notably in the northeastern part of the Thomas Range, it occurs considerably above the base. Whether in these places the red layers mark pauses in the outpouring of the lava is not known. Red spherulitic rhyolite resting on obsidian forms extremely irregular layers, which may vary laterally in thickness from less than an inch to 200 feet within a distance of 600 or 700 feet. The spherulitic rhyolite was found in all the subgroups except the lowest, in which only the top of the flow is exposed. The red rhyolite usually grades into gray rhyolite, through rock consisting of red spherulites in a gray matrix, but in ·some places the contact between the two facies is fairly sharp. The gray rhyolite makes up about 95 percent of all the rhyolite in the younger volcanic group.
Because the obsidian facies has sharp boundaries and is useful in distinguishing main subgroups from one another where pyroclastics are absent, it was mapped separately, but the gray and red rhyolites are represented by a single color symbol.
Chemical composition.-A chemical analysis was
made of black obsidian col1ected on the west side of
the Thomas Range, 1.8 miles north of Wildhorse
Spring (table 15, no. 8). This rock was very similar
in composition to analyzed specimens of other facies
FIGURE 34 .-Specimen from the base of an obsidian layer showing lenticular black obsid1a.n fragments in a brown glass matrix from west of the Autunite No. 8 claim . White bar is 1 inch long.
of the rhyolite, especially to the red spherulitic rhyolite (table 15, no. 9) that commonly overlies the obsidian layer from which the sample came, and to a gray rhyolite from the uppermost flow (table 15, no. 11).
The greatest difference in composition between the obsidian and the rhyolites is that the obsidian contains about 4 times as much H2O as any of the rhyolites. This is not surprising because glass is known for its high water content. A compilation of analyses published in U.S.G.S. Professional Paper 99 (Washington, 1917, p. 56-233) shows that 35 samples of obsidians, presumably rhyolitic, averaged 1.44 percent water. · The norm of the obsidian (tahle 15, no. 8) is almost identical with the norm of the gray rhyolite No. 10. Both rocks would be classified by Rittmann (1952, p. 95) as alkali-rhyolite. George (1924, p . 353-372) has shown that the indices of refraction and the specific gravities of natural glass are closely related to its chemical composition· he made graphs indicating roughly the amount
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of Si0 2 H 2 0, K 2 0, MgO, and CaO, and total Iron oxides corresponding to various specific gravities and indices of refraction. Ross and Smith (1955, p . 1071-1089) have also shown that the indices of refraction of rhyolitic glass is related to and varies with the amount of H20 present. In order to have a comparison of the chemical composition of other obsidians with that of the analyzed specimen, the index of refraction and the specific gravity of the analyzed specimen and of three other specimens were determined (table 9) . To determine how much difference in composition there may be between the spherulitic and nonspherulitic varieties of obsidian, the analyzed specimen (SC-14-54) was divided into two parts . Specimens used were selected fragments of fresh unaltered glass free of phenocrysts.
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Red spherulltlc facies Lithology .- The red spherulitic facies of the rhyolite is made up of numerous small spherulites set in an aphanitic or glassy groundmass (fig. 35). Its color is mostly hematitic red but varies to gray, purplish gray, pale red, and reddish brown. Most of the spherulites are easily visible in hand specimen, although in a few places none were noted except under the microscope.
The spherulites range from less than Y2 mm to about 10 mm in diameter. The larger ones, which generally have hollow centers, are found only in certain small areas, one of which is near the south end of Antelope Ridge and another on the south side of a small ridge 11/2 miles northwest of the Autunite No . 8 prospect. Abundant spherulites commonly occur in layers separated by layers that contain relatively few.
The spherulites consist mainly of radiating fibers of potassium feldspar and quartz, together with some glass. The radiating fibers may show a black extinction cross .vhen viewed between crossed nicols. The fibers do not generally form complete spherulites; they are commonly in sheaves or bundles that, in cross section, fill only a half or a third of a circle. Spherulites appear to have formed late during the crystallization, for they commonly surround or partly surround phenocrysts. They are also common in the matrix, where they are superimposed in some places upon a preexisting flow structure (fig. 38). Flow structure is common· in the red spherulitic rhyolite.
The individual laminae have an average thickness of about 0.25 mm. The flow lines bend around the phenocrysts, and in some places, particularly just above the obsidian layer, they are highly contorted.
Most of the red spherulitic rhyolite contains few phenocrysts that are visible in hand specimens, and even in some thin sections there are none. In some specimens, however, they make up as much as 30 percent of the rock. On the average they make up about 10 percent of the rock-slightly less than in the gray f acies. They are euhedral to subhedral crystals that range in length from 0.10 to 3.25 mm. They consist mainly of sanidine and quartz; these minerals vary widely in relative abundance, but sanidine generally predominates. Plagioclase, when present, is third in abundance and may constitute as much as 3 percent of the rock. It occurs in small subhedral to euhedral crystals wl{ose average composition ranges from An1s to An35·
The chief dark minerals are biotite and magnetite, but the amount of each nowhere exceeds 1 percent. A very little hornblende is present in some specimens, and a few minute grains of zircon were identified.
I MM
FIGURE 35 .-Came ra-lucld a drawing of spherulltic rhyolite showing trldymlte rings In spherulites. Radially fibrous areas are brown partially devitrified glass containing crystallites. Stippled Meas a re trldymite (t) and quartz (q) . Outer rings of spherulites and area between spherulites are devitrified glass. Southwest side of Antelope Ridge
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The groundmass of most specimens is made up of
(1) spherulites enclosed in a matrix that consists of glass containing some microlites and crystallites and (2) mosaics of · interlocking crystals with low birefringence. The crystal mosaics generally form eye-shaped pockets that in places form as much as 20 percent of the rock interlayered with spherulitic rock.
The mosaics consist chiefly of quartz and sanidine, but most of them also contain tridymite, cristobalite, or both. Other minerals noted in the groundmass include topaz in a specimen from the southern tip of Antelope Ridge and fluorite in a specimen from a small hill just north of Spor Mountain.
Chemical C0mposition.-A chemical analysis was made of a specimen of red spherulitic rhyolite from the east side of the Thomas Range, 2.5 miles south-southeast of Dugway Pass (table 15, no. 9). This analysis reveals no significant differences in composition between the red rhyolite and the analyzed specimens of the gray rhyolite (table 15, nos. 10-13).
Gray facies
Lithology.-The gray facies of the rhyolite is generally a dense aphanitic rock containing some spherulites and a few visible phenocrysts of quartz and sanidine in a glassy matrix. Its color in most places is light gray, but in some places it is pinkish gray, medium yellowish brown, and light brownish gray.
The exposed surfaces of this rhyolite have a characteristic honeycomb appearance, which is due to the weathering out of pockets along certain layers, especially those that contain lithophysae (fig. 36) . In some areas this rhyolite is broken into crude steps whose risers are vertical joints.
Flow layers are common in some places in the gray rhyolite, although not everywhere; they are commonly folded into anticlines or synclines. Some of these folds are several hundred feet across, but they tend to become smaller and more contorted near the tops or bottoms of flows.
The tops of these rhyolite flows commonly consist of
olive-green to brown glass containing numerous small cavities elongated parallel to the flow structure. In some places normal gray aphanitic rhyolite may be traced upward through partly glassy rock to nearly all glass at the top of the flow (fig. 29).
Some of the gray rhyolite contains no spherulites, but about half of it contains at least a few; on the whole, spherulites are not nearly as common in this rock as in the red facies.
The spherulites are mostly too small to be readily seen in hand specimens. Lithophysae, from about 'lti to 8 inches in diameter, are locally abundant, particularly in Topaz Valley. The
A
B
FIGURE 36.-Weathering of rhyolite, A., Rhyolite of the uppermost subgroup, north -central Thomas Range, showing vertical joints and "honeycomb" weathering. Field of view Is several hurudred feet across. B, Details of "honeycomb" weathering in rhyolite. Lineation of holes results from orientation of lithophysae parallel to fiow lines. Field of view Is about 15 feet across.
lithophysae are spherical and consist of a series of thin concentric shells of rhyolite separated by hollow spaces. The shells are mostly incomplete and tend to coalesce at one end. Some of the lithophysae have hollow centers. Small crystals of topaz, quartz, or sanidine commonly protrude into the hollow spaces at the center and between the shells. Some authors (Tyrrell, 1948,, p . 98; Grout, 1932, p. 41) consider lithophysae to be large spherulites, but this view is open to question because spherulites are generally made up of radiating fibers and lithophysae of concentric shells:
Locally, and especially in Topaz Valley, the rhyolite contains small cavities or vugs as much as 11/z
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98 GEOLOGY AND MINERAL DEPOSITS, THOMAS AN:D DUGWAY RANGES, UTAH
inches in diameter, which contain well-developed crys-
tals of topaz, specularite, quartz, pink beryl, bixbyite,
garnet, and pseudobrookite that have aroused the in-
terest of mineralogists and mineral collectors (p. 102
to 108).
In most places the gray facies contains only a few
small phenocrysts that are visible in the hand speci-
men. In a few places, however, as in an area at the
north end of the Thomas Range , 1 to 2 miles west of
Dugway Pass, and in another area on the east side
of the Thomas Range, 11/2 miles northeast of Colored
Pass, phenocrysts are about as abundant in this rock
as in the porphyritic rhyolite of the older volcanic
group. In thin sections the phenocrysts, mostly of
microscopic size, are seen to make up as much as 35
percent of the rock in some places but to be almost
absent in others; the average is about 14 percent.
The phenocrysts are chiefly of sanidine and quartz.
Sanidine occurs in subhedral to euhedral crystals from
0.05 to 3.5 mm in diameter, and quartz (which is
somewhat less abundant) in anhedral to subhedral
crystals from 0.10 to 3.5 mm in diameter. Plagio-
clase is much less abundant than quartz and is not
everywhere present, but in places it makes up as much
as 3 percent of the rock. It occurs in subhedral to
euhedral crystals, which range from 0.15 mm to 2.5
mm in , length. Because of its scarcity, a maximum
extinction angle measurement was difficult to obtain
in many specimens, but the extinction angles meas-
ured indicate that it ranges from about An 20 to An 53 •
Biotite and magnetite are the principal dark min-
erals, but neither is present in all thin sections, and
neither forms more than 1 percent of any section.
Biotite occurs in small euhedral crystals, which in
some places are partly altered; its usual color is cop-
per brown. Magnetite occurs in small rounded grains.
In addition to the above minerals, from a trace to
less than 1 percent of the following minerals have
been noted in from one to six specimens : zircon,
hematite, hornblende, sphene, augite, garnet, topaz,
and dark-purple fluorite. Of these minerals, topaz
and fluorite have a special interest because of their
apparent lack of relation to the total fluorine content
of the rhyolite. Topaz was noted in only 3 of the
76 thin sections of this rock, in which it forms small
anhedral grains or rosettes in the groundmass. A
chemical analysis of one of the specimens thus found
to contain topaz (table 15, no. lO) shows a fluorine
content of 0.14 percent. If the fluorine were all in
topaz, the topaz content of this rock would be roughly
0.9 percent, considerably more than the few small
grains found would indicate. Some of this excess
fluorine cou ld be in fluorite, but fluorite was found
in only one specimen from the north end of Spor
Mountain, where it was very scarce (fig. 37). In
thin sections from other analyzed specimens of this
rhyolite, in all of ·which there was considerable fluo-
rine, neither topaz nor fluorite was found.
The glass which generally makes up most of the
groundmass appears colorless to light or medium
brown in thin section. Some of it contains microlites
and crystallites, and perlitic cracks have been noted
in specimens from the northeastern part of the Thomas
Range. In some thin sections most of the groundmass
consists of devitrified glass containing only very
minute crystals; in others it is a mosaic of interlock-
ing larger crystals, chiefly of quartz and feldspar,
with minor amounts of biotite, magnetite, tridymite,
and cristobalite.
There are also small pockets or eyes of clear mate-
rial, made up chiefly of quartz and sanidine crystals,
commonly associated with tridymite or, locally, with
cristobalite. The tridymite is mostly in small rec-
tangular crystals, but in a few places it forms wedge-
shaped twins or is anhedral. Most of the cristobalite
shows no crystal form, although in places it forms
small rosettes. Where both of these minerals are
anhedral they are almost indistinguishable.
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