Summary:
Mineral Deposit
Boron is believed to have been sourced from regional thermal waters which flowed from hot springs during times of active volcanism. These hot springs vented into the Hector Basin when it contained a large desert lake. Borates were precipitated as the thermal waters entered the lake and cooled or as the lake waters evaporated and became saturated with boron. Colemanite, being the least soluble mineral, would evaporate on the receding margins of the lake. The evaporite-rich sequence forms a consistent zone in which the borate-rich colemanite zone transgresses higher in the section relative to stratigraphic marker beds.
Based on drilling results, the deposit is elliptical in shape, with the long axis trending N40°W to N50°W. extending over an area of about 606-acres at an average depth of approximately 1,300 ft to 1,500 ft bgs. Beds within the colemanite deposit strike roughly N45°W and dip about 10° or less to the southwest. Using an isoline of 5% B2O3, mineralization has an approximate width of 2,800 ft and a length of 11,150 ft with thickness ranging from 70 to 262 ft exclusive of barren interbeds.
The western margin of mineralization appears to be roughly linear, paralleling the Pisgah Fault which lies approximately 1 mi to the west Duval geologists consider this boundary to be controlled by facies change from evaporite rich mudstones to carbonate-rich lake beds, because of syn-depositional faulting. The northeast and northwest boundaries of the deposit are controlled by facies changes to more clastic material, reducing both the overall evaporite content and the concentration of colemanite within the evaporites. The southeast end of the deposit is open-ended, and additional drilling is necessary to define the southeastern limits of borate deposition per Wilkinson & Krier, 1985.
Stratigraphic Column
Four major units have been identified:
• Unit 1: is characterized by a 490 to 655 ft thick sequence of red-brown mudstones with minor sandstone, zeolitized tuff, limestone, and rarely hectorite clay beds. Unit 1 is located immediately below the alluvium and surface basaltic lavas;
• Unit 2: is a green-grey mudstone that contains minor anhydrite, limestone, and zeolitized tuffs. Unit 2 has a thickness ranging from 330 to 490 ft and is interpreted as lacustrine beds;
• Unit 3: is a 245-to-490-foot thick evaporite section which consists of rhythmic laminations of anhydrite, clay, calcite, and gypsum. Unit 3 contains the colemanite mineralization. Thin beds of air fall tuff are found in the unit which provide time continuous markers for interpretation of the sedimentation history. These tuffs have variably been altered to zeolites or clays. Anhydrite is the dominant evaporite mineral, and the ore deposit itself is made up mostly of an intergrowth of anhydrite, colemanite, celestite, and calcite with minor amounts of gypsum and howlite;
• Unit 4: is characterized by clastic sediments made up of red and grey-green mudstones and siltstones, with locally abundant anhydrite and limestone. The unit is approximately 160 ft thick and rests directly on an irregular surface of andesitic lava flows. Where drilling has intersected this boundary, it has been noted that an intervening sandstone or conglomerate composed mostly of coarse volcanic debris is usually present.
Mineralization
Mineralization occurs in a sequence of lacustrine lakebed sediments ranging in depths from 1,300 ft to 1,500 ft bgs. The mineralization is hosted by a sequence of mudstones, evaporites and tuffs, consisting of variable amounts of colemanite, calcium borate 2CaO 3B2O3 SH2O, and lithium. Colemanite and lithium are the target minerals. Colemanite is a secondary alteration mineral formed from borax and ulexite. The mineral is semi-hard with a Mohs hardness of 4.5 and forms as discreet monoclinic, prismatic crystals or masses. Colemanite typically forms as a translucent colorless, white, or gray crystal with a vitreous luster. Colemanite is insoluble in water but soluble in HCl and sulfuric acid (H2SO4). Colemanite is associated with thinly laminated siltstone, clay and gypsum beds containing an average of 9% calcite, 35% anhydrite plus 10% celestite (SrSO4) per Wilkinson & Krier, 1985. In addition to colemanite and celestite, elevated levels of lithium have been found through chemical analyses of drill samples.X-ray diffraction analysis of core samples from the deposit indicates the presence of the evaporite minerals anhydrite, colemanite, celestite, and calcite. The mineralogy of the detrital sediments include quartz, illite, feldspars, clinoptilolite, and zeolite. The deposit underlies massive clay beds which appear to encapsulate the evaporite ore body on all sides as well as above and below the deposit. This enclosed setting makes the deposit an ideal candidate for in-situ mining technology affording excellent containment of the leachate solution.