GEOCHEMCIAL IMAGING OF THE WESTERN EDGE OF THE NORTH AMERICAN CRATON IN NORTHERN NEVADA

Multielement regional-scale geochemical anomalies in northern Nevada appear to be related to structures along the western edge of the North American craton. A GIS was used to prepare maps and carry out spatial analyses using over 10,200 newly analyzed (post-1990) National Uranium Resources Exploration (NURE) stream-sediment and soil samples that cover large parts of northern Nevada and southeast Oregon. These data are low density, sampled on an irregular to semi-regular grid at a spacing of approximately 2-3 km, with an average of 1 sample per 17 km².

The craton edge in northern Nevada was established in the late Proterozoic (~ 0.8-0.5 Ga) following polyphase rifting of the Rodinian supercontinent, giving rise to a passive Atlantic-type margin along the present-day western edge of the new subcontinent of Laurentia, which later became the cratonic core of North America. The craton edge is commonly believed to correspond closely to the ⁸⁷Sr/⁸⁶Sr I_Sr=0.706 or 0.705 isopleth. These values are derived from Mesozoic and Tertiary granitoids and considered to represent the boundary between magmas related to oceanic or continental lithosphere. NURE geochemical anomalies of As, Cd, Ti, Al, Na, Sr, and other elements display regional-scale trend surfaces that reflect the general shape of the buried craton edge as defined by the ⁸⁷Sr/⁸⁶Sr I_Sr=0.706 and 0.705 isopleths. Residual anomalies of As, Sb, Se, Sr, Al, Ba, Na, and other elements, generated by subtracting regional- from local-scale element concentrations, display varying degrees of linear and continuous trends over large distances (> 100 km) that extend across and oblique to most mountain ranges and drainages. For the most part, they do not correspond to geologic units or lithotectonic terranes. These geochemical anomalies are coincident with, or parallel to, (1) geophysical discontinuities in gravity, magnetic, and magnetotelluric anomalies, (2) sharp boundaries and bends in Pb isotope isopleths, (3) a late Paleozoic-early Mesozoic regional paleothermal anomaly, and (4) major sedimentary rock-hosted Au mineral trends and alignments. Numerous researchers have proposed that these features delineate the edge, or are related to deep-seated crustal structures along the margin of the craton.