Summary:
Mineral Deposit Type
The Great Salt Lake is a terminal lake that hosts enriched brine containing dissolved minerals at concentrations sufficient for economic recovery of certain resources. The mineral resource of the Great Salt Lake currently supports economic recovery of sodium (as NaCl), potassium (as SOP), and magnesium (as MgCl2).
Geologic Description
The GSL Facility produces saleable minerals from brines sourced from the Great Salt Lake. These brines are upgraded through solar evaporation within large constructed ponds. The following describes the geologic relevance of the Great Salt Lake and lays out the man-made aquifers within the evaporation ponds which host brines with high lithium concentrations.
The GSL Facility is located on the shore of the Great Salt Lake in northern Utah. This location is within the geographic transition from the Rocky Mountains, to the Basin and Range Province to the west.
The Great Salt Lake is a remnant of Lake Bonneville, a large Late-Pleistocene pluvial lake that once covered much of western Utah. At its maximum extent, Lake Bonneville covered an area of approximately 20,000 square miles. Lake Bonneville has been in a state of contraction for the past 15,000 years and has resulted in the formation of remnant lakes that include the Great Salt Lake, Sevier Lake, and Utah Lake (Figure 6-1). Evaporation rates higher than input from precipitation and runoff have driven the lake contraction and has served to concentrate dissolved minerals in the lake water. The GSL is one of the most saline lakes in the world.
The Great Salt Lake is currently the largest saltwater lake in the western hemisphere, covering approximately 1,700 square miles. But due to fluctuation in evaporation rates and precipitation, that size has ranged from 950 square miles to 3,300 square miles over the past 60 years. On a geologic timeframe, the Great Salt Lake water level has varied by many hundreds of feet over the past 10,000 years (UGS, 1980).
Over the course of modern record keeping, the water level of the Great Salt Lake has not varied by more than 20 ft. This is controlled through the balance of recharge and discharge from the lake. Lake level data indicated that historical lows were seen in the 1960s, while historical highs were seen in the mid-1980s, which required discharge of the Great Salt Lake brine into the west desert by the Utah Division of Water Resources and Utah Department of Natural Resources in an effort to control the lake level.
Inflow contributions to the Great Salt Lake are from surface water (66%), rainwater (31%), and groundwater (3%), with seasonal variation impacting the annual contribution (UGS, 1980). Discharge from the Great Salt Lake is primarily through evaporation.
Salinity throughout GSL is governed by lake level, freshwater inflows, precipitation and re-solution of salt, mineral extraction, and circulation and constriction between bays of the lake. Distinct salinity conditions have developed in the four main areas of the lake as a result of 1) fragmentation of the lake resulting from causeways and dikes and 2) the fact that 95% of the freshwater inflow to the lake occurs on the eastern shore south of the causeway (Loving et al. 2000). From freshest to most saline, the largest bays in GSL today are Bear River Bay, Farmington Bay, Gilbert Bay (the main body of the lake also referred to as the south arm) and Gunnison Bay (i.e., the North Arm). Since 1982, the salinity in Bear River Bay and Farmington Bay ranges from 2% to 9% (Map 2.5), though it typically stays between 3% and 6%). Figure 6-2 illustrates the range of salinities within the four bays of the GSL.
In 1960, a railroad causeway was constructed in replacement of a 12-mile-long wooden trestle. The causeway is a permeable rockfill barrier with box concrete box culverts that permit limited brine transfer, but prevent full mixing of brine on either side of the causeway. The causeway has therefore effectively divided the Great Salt Lake into two bodies of water (the north arm and the south arm), which have each developed distinct physical and chemical attributes most readily identified through a noticeable color difference in the waters.
Due to the location of the causeway, all surface freshwater flow enters into the south arm of the lake as river inflow from the Jordan, Weber, and Bear Rivers. Conversely, the north arm of the lake receives only mixed brine via limited recharge through the causeway and minor contributions from precipitation and groundwater. Furthermore, due to topography and microclimate conditions, the south arm receives greater precipitation, while the north arm has more favorable evaporative conditions (UGS, 1980). Considering there are no freshwater inflows to the north arm and the intensity of evaporation in the summer months, the north arm acts as a hydrologic sink in the GSL terminal lake system, receiving all of the inflows from the south arm. These conditions have resulted in the preferential concentration of minerals within the north arm brine relative to the south arm brine.
Recent sampling for the Utah Geological Survey (UGS, 2021) data shows that overall potassium, magnesium, and sodium concentrations in the north arm are typically more than double those found in the south arm. These data reflect the impact of the causeway and environmental factors and allow for a review of potential resources to consider the north arm and south arm of the Great Salt Lake independently.
Compass Minerals’ GSL Facility extracts brine from the north arm of the Great Salt Lake into a series of evaporation ponds. The brine is concentrated in these ponds, moving from pond to pond as the dissolved mineral content in the brine increases.