Alkaline Rocks and Carbonatites of the World

Setup during HiTech AlkCarb: an online database of alkaline rock and carbonatite occurrences

La Sal Mountains

stripes

Occurrence number: 
174-00-078
Country: 
United States
Region: 
Utah
Location: 
Longitude: -108.22, Latitude: 38.55
Carbonatite: 
No

The Central parts of the La Sal Mountains comprise three separate groups of clustering laccoliths. South Mountain consists predominantly of porphyritic diorites which contain a little aegirine-augite. Middle Mountain consists essentially of the same rock type but in the northwestern part is the Brumley Ridge intrusion, which is a nosean syenite consisting of orthoclase and perthite, about 12% of nosean and 3% of aegirine-augite. The complex of laccoliths, stocks and dykes which build North Mountain include peralkaline syenites, granites, rhyolite and feldspathoidal syenite. The outer parts of the complex consist of overlapping laccoliths of rocks described as diorite porphyry and monzonite porphyry by Hunt and Waters (Hunt, 1958), but they often contain a little aegirine-augite. The central part of the complex consists of a multiple intrusion, the North La Sal stock, covering about 5 km2 and composed of various porphyritic rocks including diorite, syenite, nosean syenite, aegirine granite and peralkaline rhyolite porphyries. There is also a dyke swarm complex and a number of breccia vents. The earliest rocks were dioritic but the plagioclase has been replaced by albite and perthite, and hornblende by aegirine-augite, biotite and another amphibole. These rocks are cut by an irregular intrusion of syenite consisting essentially of large microperthite phenocrysts, augite zoned to aegirine-augite and biotite, but Hunt and Waters (Hunt, 1958, p. 329) describe the syenite as being veined by an even more sodic syenite of alkali feldspar and aegirine-augite. Large rafts within the syenite of amphibolite, schist and gneiss are wholly or partially replaced by biotite and aegirine-augite. The dyke swarm complex, which in places comprises 85% dykes, consists chiefly of feldsparphyric syenites of alkali feldspar, aegirine-augite and some biotite. There are some dykes of sodalite syenite within the complex and extending into the laccoliths surrounding the central stock and these may be related to the nosean syenite at Brumley Ridge. Apart from euhedral sodalite (or ?nosean) these rocks consist of perthite phenocrysts up to 4 cm in diameter in a trachytic groundmass of K-feldspar and aegirine-augite. The breccia plugs consist of angular fragments of various types of syenite in a comminuted matrix locally rich in calcite. Sheets of aegirine granite occur within the breccia vents and one vent cuts an intrusion of aegirine granite. Aegirine rhyolite dykes represent the last phase of igneous activity. There is extensive deuteric and pneumatolytic alteration within and around the North Mountain complex which has produced concentrations of some metals which are of economic interest.

Economic: 
The area of North Mountain has been extensively explored and worked for Au, Cu, Pb, Zn and Se, the most extensive mineralization being in the hydrothermally altered rocks.
Age: 
K-Ar ages obtained on aegirine-augite from soda syenite porphyry and monzonite porphyry were 25.5 ± 2.5 and 22.5 ± 3.3 Ma and a hornblende from a diorite porphyry gave 54.8 ± 1.5 Ma (Stern et al., 1965, Table 3).
References: 

HUNT, C.B. 1958. Structural and igneous geology of the La Sal Mountains, Utah. Professional Paper, United States Geological Survey, 294-I: 305-64.
STERN, T.W., NEWELL, M.F., KISTLER, R.W. and SHAWE, D.R. 1965. Zircon uranium-lead and thorium-lead ages and mineral potassium-argon ages of La Sal Mountains rocks, Utah. Journal of Geophysical Research, 70: 1503-7

Map: 
Fig. 1_152 North La Sal stock, La Sal Mountains (after Hunt, 1958, Plate 40).
Scratchpads developed and conceived by (alphabetical): Ed Baker, Katherine Bouton Alice Heaton Dimitris Koureas, Laurence Livermore, Dave Roberts, Simon Rycroft, Ben Scott, Vince Smith