The granite textures 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 of these granite intrusions are undeformed but high-temperature solid-state deformation is present in at least three plutons.
The isotopic age of this unit is a little over 1.15 Bya which overlaps with ages for much of the Packsaddle Schist. The original rocks could have been 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. At casual glance, it is difficult to distinguish between some of these gneisses and the granite rocks.
Ages for these rocks are 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 plunge gently to the southeast. The foliation is generally parallel to lithologic layering and the layers are dipping to the southeast. Crenulations on the foliation layers trend parallel to major fold axes and can be clearly noted in roadcuts.
| 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 Heath gold mine, located 5 miles northeast of Llano, was the best-known. This location is on privately owned land north of Road 2241,. Gold-bearing quartz veins and stringers cut dark graphitic schist are near its contact with intrusive pink granite. Pegmatite dikes, some containing rare minerals, also cut the schist. The mine was active between 1896 and 1916, with several test lots of ore shipped. One lot of 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. Panning for gold in the Llano River is a popular pasttime, and sites describing gold panning are available on the web. |
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 - 29.2(30o 45.590; 98o 40.510) - Junction of 29, 16, 71
mile 0.4 - 28.8 (30o 45.562; 98o 40.055) - turn right to river for overview
mile 1.5 - 27.7 (30o 45.703; 98o 39.003) - highway 2241 to Tow
mile 3.6 - 25.6 (30o 45.772; 98o 36.825) - Town Mountain Granite outcrop on north side of highway
mile 6.6 - 22.6 (30o 46.881; 98o 34.123) - Little Llano Creek
RR bridge.
Near this point on the river is a porphyritic granite dike with crystals of smoky quartz (sampled 1951)
mile 6.7 - 22.5(30o 46.90; 98o 33.98) - highway 202 on left
mile 10.7 - 18.5 (30o 44.577; 98o 31.197) - Granite Hills Ranch (private) on left; outcrop of TMG can be seen
mile 14.2 - 15.0 (30o 43.992; 98o 27.966) - highway 1431 to Kingsland and Marble Falls (see log under Shorter Highway Excursions)
mile 15.9 - 13.3 (30o 44.418; 98o 25.251) - TMG outcrop on right
mile 17.3 - 11.9 (30o 44.609; 98o 24.973) - Lake Buchanan dam
mile 18.2 - 11.0 (30o 44.869; 98o 24.109) - bridge across Colorado River
mile 19.7 - 9.5 (30o 45.467; 98o 22.839) - rest stop,
Valley Spring Gneiss
north of highway and pinnacle outcrops to the
north
mile 20.2 - 9.0(30o 45.416; 98o 22.391) - Park Road 4 on right (see log under Shorter Highway Excursions)
mile 21.7 - 7.5(30o 45.850; 98o 21.005) - outcrop of VSG on left
mile 25.9 - 3.3 (30o 45.755; 98o 17.077) - highway 2341 on left (see log under Shorter Highway Excursions)
mile 26.6 - 2.6 (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 - 0.0 (30o 45.476; 98o 13.729) - intersection of 29 with 281 in Burnet.