WHOLE ROCK GEOCHEMISTRY OF NI-CO LATERITE OCCURRENCES OF NORTHERN CALIFORNIA AND SOUTHERN OREGON

Critical mineral commodities are important to modern society and vulnerable to risk in their supply chains. Among these commodities are Ni and Co, important for stainless steel, superalloys, and batteries, as well as Sc, important for fuel cells, ceramics, and alloys. Pervasive chemical weathering of ultramafic rocks can form Ni-Co+/- Sc laterite deposits. Ultramafic rocks of the Klamath Mountains of Northern California and Southern Oregon experienced intense weathering in the Eocene, creating laterite occurrences. Subsequent tectonic activity and surface hillslope transport reduced the laterite concentrations to a mixture of limonite/saprolite and saprock layers in colluvium and slumps. Whole rock geochemistry reveals the systematic loss of Mg and Si and concentration of Fe, Mn, Ni, Co, Cr, and Sc through the weathering process. Mean values for the protolith peridotite are: 24.3 wt.% Mg; 20.7 wt.% Si; 5.9 wt.% Fe; 921 ppm Mn; 109 ppm Co; 2870 ppm Ni; 111 ppm Co; 2786 ppm Cr; and 10ppm Sc. Mean values for the saprolite are: 12.5 wt.% Mg; 15.4 wt.% Si; 13.3 wt.% Fe; 3310 ppm Mn; 5668 ppm Ni; 619 ppm Co; 12639 ppm Cr; and 34 ppm Sc. There is a positive correlation between Co, Sc, Ni, and Mn. SEM automated mineralogy maps of protolith and saprock show the alteration of olivine, clinopyroxene, and minor chromite to serpentine and iron oxides. The Ni-Co laterites of Northern California and Southern Oregon are generally lower grade than global analogues with mean grades of 1.26 wt.% Ni and 490 ppm cobalt. While the Eocene paleoclimate enabled a chemical weathering process identical to those that generate economically significant Ni-Co laterite deposits, the tectonic uplift in the Klamath Mountains terrane created unfavorable conditions for preservation and mature development of the laterite profile which lowers the tonnage and grade. Therefore, the potential for significant critical mineral resources in this area is low.