Summary:
The deposit type of the Sevier Playa is a terminal lakebed (playa) brine deposit. The brine deposit is sedimentary in origin and composed of the natural concentration of mineral salts in groundwater found in the Playa. The brine is contained within the unconsolidated Playa sediments, composed primarily of clay and marl, a lime or calcium-rich clay. While the sediments may play a role in the mineral occurrence, development efforts to date have focused primarily on the mineral content found in the brine. The extractability of the brine Mineral Resource and the interaction between the brine mineralization and the potential for recharge by water flowing through the Playa sediments is part of this investigation, and an integral part of the determination of total extractable Mineral Reserves of SOP and other potential mineral compounds.
Composition of the thin salt crust covering the Playa consists of evaporite minerals and is approximately 3 to 4 inches (7.6 to 10.2 centimeters) thick, ranging up to as much as 18 inches (45.7 centimeters) in areas, as determined from drilling and augering data. Evaporite minerals forming the crust tend to be zoned on the Playa surface with halite (NaCl) being the dominant mineral in the center of the Playa, followed by glauberite (Na2Ca(SO4)2), and then gypsum (CaSO4) near the Playa shore (Godbe, 1984; Rasmussen, 1997).
Soluble salts, in the sediment-hosted brine, are the target mineralization of current development work. The source of soluble salts is the erosion and leaching of Paleozoic-era bedrock in the Sevier and Bear River drainages. Observation and sampling of Playa sediments and brine have identified the following features that characterize the top 100 feet (30.5 meters) of the deposit:
- Salt crust approximately 3 to 4 inches (7.6 to 10.2 centimeters) thick.
- Lateral zonation in crust mineral chemistry;
- Vertical zonation in sediment mineral chemistry;
- Variation in brine saturation both laterally and with depth;
- Variation in sediment grain size distribution;
- Artesian brine flow in some areas;
- Elevated concentrations of sodium, potassium, magnesium, calcium, chloride, and sulfate in the brine.
These features influence, to varying degrees, the target brine extent (volume) and potential for production of potash, halite, and bitterns from the brine. The focus of the Mineral Resource and Reserve estimates presented in this report are three shallow (depths less than 100 feet [30.5 meters]) brine saturated horizons termed the FCZ, MCZ and SCZ, from top to bottom. Below these horizons is a dry, hard clay.
Fat Clay Zone
The FCZ derives its name from its physical properties, being described predominately as plastic (fat) clay with low hydraulic conductivity. This dense gray clay is capped by a thin salt crust that is typically several inches thick over most of the Playa, but can range up to 18 inches (0.5 meters) thick in certain areas, according to CPMC auger logs(Gwynn, 2006). The FCZ averages approximately 12.8 feet (3.8 meters) in thickness and is comprised of two sub-horizons. The upper part of the FCZ consists of 9.35 feet (2.9 meters) thick homogenous, dense, plastic clay. This clay zone is observed to contain gypsum crystals up to 6- inches (15.2- centimeters) in diameter. Underlying this homogenous clay is a plastic clay zone, 1.64 feet (0.5 meters) thick that contains abundant organic material, commonly appearing as grass mats and root structures, representing a dry period when the Playa surface was covered by grassy beds. This organic clay zone is an important marker bed that separates the FCZ from the underlying MCZ below.
Marl Clay Zone
The MCZ is described as a gray, bedded, granular clay averaging 16.67 feet (5.1 meters) in thickness. The granular texture arises from what is observed to be silt-size granules of smaller clay particles loosely bound by a soft calcareous or gypsiferous matrix. The clay zone is also observed to contain numerous gypsum crystals up to 6-inches (15.2-entimeters) in diameter. An unconsolidated sand and gravel bed frequently occurs near the top of the MCZ, but is not consistent throughout the Playa. Where present, this sandy or gravelly zone averages a thickness of 18 inches (45.7 centimeters).
A dense zone of stiff clay averaging approximately 3.3 feet (1 meter) thick occurs approximately 2.9 feet (0.9 meters) below the sand and gravel bed, where present. It has been identified, in those exploration holes where handheld penetrometer readings have been taken, at regular intervals, in the core samples and used as a rough guide to determine the overall hardness of the Playa sediments. Penetrometer readings for the stiff clay zone of the MCZ range from between 1.5 to 3.0 st/ft2 (1.5 to 3 kg/cm2). For comparison, the surrounding marl clay has penetrometer readings between 0 to 1.25 st/ft2 (0 to 1.25 kg/cm2) The overlying fat clay has penetrometer readings between 0 and 0.5 st/ft2 (0.5 kg/cm2), and underlying siliceous clay has penetrometer readings ranging from 0.75 to 1.25 st/ft2 (0.75 to 1.25 kg/cm2).
Below the stiff clay bed is a further 9.8 feet (3 meters) of marl clay that transitions rapidly into predominantly siliceous clay, the underlying SCZ. The contact between the marl clay and underlying siliceous clay is easily supported by the sediment mineralogy and carbonate content test results from XRD mineralogy analyses. The average carbonate contents derived from drill core samples are illustrated in the stratigraphic column.
Siliceous Clay Zone
The SCZ is identified as an olive gray, quartz- rich clay with a relatively low carbonate content, averaging approximately 30% carbonates noticeably lower than the overlying MCZ. Four sand and gravel beds have been identified within the SCZ from drillhole records, but are not consistent throughout the Playa. The thickness of these sand and gravel units decreases from the margins of the lakebed toward the center of the Playa, where these beds are often missing from the drillhole records. Average thicknesses of the sand and gravel beds, where present, vary from 1.6 to 2.9 feet (0.5 to 0.9 meters). The base of the siliceous clay unit is marked by the presence of a dull red, dry, hard clay with hand-held penetrometer readings exceeding 5 st/ft2 (5 kg/cm2).
Drilling to date is insufficient to accurately determine a brine Mineral Resource potential below these three shallow aquifers.