Peralkaline Sodic Metasomatism from the Wilson Ridge Pluton, Northwest Arizona

Abstract

Peralkaline sodic alteration is well known with peralkaline magmatic systems such as carbonatites and some syenites, yet is essentially unreported in other settings. Possibly caused by hydrothermal circulation of alkaline lake fluids, distinctive peralkaline sodic metasomatism, characterized by the presence of hydrothermal riebeckite and aegirine, impacted at least 20 km² of the weakly peraluminous Miocene Wilson Ridge pluton in northwest Arizona. Mapping on the western edge (Kingman Wash) and satellite imagery across >50 km2 show that alteration is concentrated in the western, possibly upper parts of the tilted Wilson Ridge pluton. Where studied, the pluton primarily comprises biotite-hornblende granodiorite with subordinate diorite, aplites, and late biotite-bearing rhyolitic dikes. Over 20 km2 contain by volume approximately 1/3 peralkaline metasomatism consisting of sodic amphibole veinlets and breccias enveloped in zones where albite and sodic amphibole replace igneous K-feldspar, plagioclase and biotite. Intense zones contain albite + aegirine-augite ± sodic amphibole; these are often accompanied by quartz dissolution. Alteration, brecciation, and veining postdate all but the late rhyolite which appears to overlap in time with brecciation and extensive albite + sodic amphibole flooding. Whole rock geochemistry shows that although Na, Ca and K, and to a lesser extent Fe, are mobile, whereas high-field-strength elements (Ti, Zr, REE, Nb) are little changed. Initial 87Sr/86Sr is elevated in the sodically altered rocks (0.7095—0.7129) compared to their unaltered equivalents (0.7092—0.7101).Despite the mineralogical similarities to alteration linked with peralkaline magmas, an alternative, distinctive origin is implied by the unusually large area of alteration, the absence of peralkaline igneous rocks, the lack of minor element enrichments characteristic of fenites, and the distinctive, non-magmatic Sr isotopic signature. In contrast to typical sodic(-calcic) alteration which is found in many geologic settings, and which is consistent with near-neutral NaCl-rich fluids, alteration at Wilson Ridge requires a distinctly alkaline fluid. Given that the evidence seems to negate a peralkaline magmatic source, an evaporitic Na-CO3-SO4-Cl source similar to modern and ancient alkaline lakes seems most likely, perhaps contributed from contemporaneous evaporitic sediments of the nearby Horse Spring formation which have similarly elevated Sr isotopic ratios.