EVIDENCE FOR THE INVOLVEMENT EXTERNAL FLUIDS IN HYDROTHERMAL ALTERATION OF AN EVAPORITE-HOSTED IGNEOUS COMPLEX, LA SAL MOUNTAINS, EASTERN UTAH

The La Sal Mountains (LSM), located within the Paradox Basin of eastern Utah, host several Oligocene intrusions. These igneous rocks intruded through the Paleozoic and Mesozoic strata, including evaporites of the Pennsylvanian Paradox Formation, and formed several hydrothermally altered, alkaline laccoliths in the upper stratigraphic units of the basin. This unique setting presents a natural laboratory to study the role of external, saline fluids in the development of hydrothermal alteration.

Mapped intrusive rocks in order of most abundance are: 1) trachyte, the only unaltered igneous rock observed in the range; 2) syenite, observed at Miner’s Basin and South Mountain; and 3) diverse dikes and breccias only observed at Miner’s Basin. Volume estimates for intrusive rocks are ~30 km³. The Miner’s Basin area consists of a diverse suite of veins: qtz-cc(±fl ±Cu oxides) lodes; qtz-cc-pyx(-act±chl) veins; qtz-py//ser-py veins; mag veins. Proximal to Miner’s Basin are altered rocks with fsp bleaching ± blue amph. Distal alteration, covering ~12 km² through the LSM, consists of white fsp bleaching; mafic minerals are replaced by sd with veins of hm or ep. Whole rock geochemistry values for altered rocks (Na₂O/Al₂O₃: ~0.2-0.9; K₂O/Al₂O₃: ~0.0-0.7) exceed those of fresh rocks (Na₂O/Al₂O₃: 0.5-0.8; K₂O/Al₂O₃: 0.1-0.5) and suggest alkali exchange between fresh and altered rocks. Measured dD_VSMOW in silicates (-80 to -110‰) and d³⁴S_CDT in Fe-Cu sulfides (-13 to 1‰) have calculated fluid compositions, respectively, of -40 to -90‰ and 12 to 26‰.

Geochemical data are consistent with exchange of alkalis between fluids and rock along both warming and cooling fluid flow paths. Stable hydrogen isotopes show signatures within the magmatic and basinal range, while sulfur isotopes exceed the range of magmatic sulfur, allowing potential sedimentary sulfur. The estimated volume of altered rocks (12 km³ with a depth of 1 km) is difficult to account for with the estimated volume of magmatic fluids alone (1.5 km³ fluids from 30km³ of intrusion). Considering the proximity to nearby evaporites, isotopic evidence permitting basinal brines, documented alkali exchange, and insufficient volume of magmatic fluids, we assert the role of external fluids in the development of hydrothermal alteration in the LSM.