General

The trip east is over preCambrian Town Mountain Granite and Valley Springs Gneiss. The first part of the trip parallels the Llano River that is crossed by several pegmatite dikes . On our 1950 field trip, we found smoky quartz crystals in the pegmatites. An outcrop of granite about four miles east of Llano has jointing and fracture rocks with marked exfoliation sheets. From Buchanan Dam east, the road is on Valley Springs Gneiss until the edge of the uplift near Burnet where Cretaceous sand sediments overlain by limestones rest on the preCambrian erosion surface











Town Mountain Granite

The Town Mountain Granite suite (TMG) of the Proterozoic Llano Uplift of central Texas consists of numerous large intrusions with similar characteristics and ages. The age of these granites is 1.12 to 1.07 billion years ago. TMG is generally pink, very coarse-grained, porphyritic granite with accompanying pink coarse-grained non-porphyritic granite. Mineralogically, TMG consists primarily of plagioclase feldspar, potassium feldspar (microcline), and quartz with biotite and/or hornblende (The vast majority plot in the granite field of the IUGS classification). Accessory minerals commonly include titanite (sphene), zircon, magnetite and/or ilmenite, and apatite with fluorite, allanite, and/or pyrite being present in some rocks.

Texturally the granites are dominated by the large pink microcline feldspar crystals. Differences in the color, shape, or amount of these microcline feldspar crystals are responsible for much of the differing appearance of building stone varieties. Faint to well-developed alignment of these large crystals (magmatic foliation) occurs somewhere in most intrusions.

Regionally, the granites intrude deformed schists and gneisses of the Llano Uplift. Most intrusions of TMG have roughly circular to teardrop-shaped surface expressions. Most granites are undeformed but high-temperature solid-state deformation is present in at least three plutons .

Valley Springs Gneiss

The Valley Spring Gneiss is a major metamorphic unit in the Llano Uplift. It typically is a quartz-feldspar gneiss with minor amounts of schist, amphibolite, and marble. The main minerals are quartz, feldspar, and biotite that combine to produce a weak foliation. Early studies documented over 8,000 feet of this unit in the Honey Creek area, although the base is not exposed. The unit is thicker and more variable in composition in the western part of the uplift.

The isotopic age of this unit is a little over 1150 Mya which overlaps with ages for much of the Packsaddle Schist. Likely protoliths could be rhyolitic, granitic, or arkosic, but because there are layers of marble interspersed within the unit, at least some parts of the unit are thought to have sedimentary parentage.

Packsaddle Schist

The Packsaddle Schist is one of the two major Precambrian metamorphic units in this area. Isotopic ages for many of these rocks fall in the range of about 1150 Ma to 1170 Ma (mid to late Proterozoic) which is approximately equivalent to Grenville age rocks from the eastern U.S. and Canada. The metamorphic rocks throughout the Llano Uplift are folded into broad anticlines and synclines that generally plunge gently to the southeast. The foliation is generally parallel to lithologic layering and here it can be seen that the layers are dipping to the southeast. Crenulations on the foliation layers trend parallel to major fold axes and can be clearly noted in the roadcut.

As originally described, the Packsaddle Schist measured over 20,000 feet thick and consisted of various rock types formed by low pressure, medium grade metamorphism. There are many different rock types within this unit, including: graphite, mica, amphibole, and cordierite schist; marble; quartz-feldspar rocks (commonly referred to as "leptites"); various syntectonic meta-igneous gneisses; and serpentinite with metagabbro.

Lode Gold

So much of the history of the Llano Uplift is focused on gold mines - lost or abandoned, that I included this section. The road logs do not have any "gold" stops.

The best-known gold mine was the Heath, located 5 miles northeast of Llano, north of Road 2241, on privately owned land. Gold-bearing quartz veins and stringers cut dark graphitic schist near its contact with intrusive pink granite. Most of the quartz veins strike about N. 70º W. Pegmatite dikes, some containing rare minerals, also cut the schist. The Heath Mine was active between 1896 and 1916, during which time several test lots of ore were shipped. One lot of picked ore reportedly assayed at 1.1 ounce gold/ton. The quartz veins contain free gold, pyrite, chalcopyrite, and other sulfides. Some individual quartz stringers had spectacular gold values. The schist itself contains disseminated gold, and the overlying residual soils reportedly run as high as 0.1 ounce gold/ton.

In 1901, a shaft was sunk to 615 feet, and six other vertical and inclined shafts were sunk in an area of about 20 acres. There were many drifts and crosscuts. A number of shallow pits and trenches were also dug. Erosion and vegetation have obliterated most of the old workings.

Road Log

legend for the map

road milage points are marked with white circles on the map. All road logs start at the intersection of highways 29, 16 and 71; north of the bridge over the Llano River























mile 0.0 (30o 45.590; 98o 40.510) - Junction of 29, 16, 71

mile 0.4 (30o 45.562; 98o 40.055) - turn right to river for overview

mile 1.5 (30o 45.703; 98o 39.003) - highway 2241 to Tow

mile 3.6 (30o 45.772; 98o 36.825) - Town Mountain Granite outcrop on north side of highway

legend for the map



















mile 6.6 (30o 46.881; 98o 34.123) - Little Llano Creek RR bridge . Access to Llano River if you stay in river bed. Near this point on the river is a porphyritic granite dike with crystals of smoky quartz (sampled 1951)

mile 6.7 (30o 46.90; 98o 33.98) - highway 202 on left

mile 10.7 (30o 44.577; 98o 31.197) - Granite Hills Ranch (private) on left; outcrop of TMG can be seen

mile 14.2 (30o 43.992; 98o 27.966) - highway 1431 to Kingsland and Marble Falls (see log under Shorter Highway Excursions)

mile 15.9 (30o 44.418; 98o 25.251) - TMG outcrop on right

mile 17.3 (30o 44.609; 98o 24.973) - Lake Buchanan dam

mile 18.2 (30o 44.869; 98o 24.109) - bridge across Colorado River

legend for the map




















mile 19.7 (30o 45.467; 98o 22.839) - rest stop, Valley Spring Gneiss north of highway and pinnacle outcrops to the north

mile 20.2 (30o 45.416; 98o 22.391) - Park Road 4 on right (see log under Shorter Highway Excursions)

mile 21.7 (30o 45.850; 98o 21.005) - outcrop on left

mile 25.9 (30o 45.755; 98o 17.077) - highway 2341 on left (see log under Shorter Highway Excursions)

mile 26.6 (30o 45.645; 98o 16.329) - outcrop of Cretaceous Glen Rose limestone on left, at top of hill. Contact with the VSG must be near the base of the hill; but cannot be seen beside the highway.

mile 29.2 (30o 45.476; 98o 13.729) - intersection of 29 with 281 in Burnet.

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