Investigations of the geochemical controls on anomalous arsenic enrichment in the Santiago Peak Volcanics of Southern California: implications for arsenic distribution in volcanic arc systems

The Santiago Peak Volcanics (SPV) of Southern CA and Northern Baja CA, Mexico are remnants of a Cretaceous subaerial volcanic arc system that underwent greenschist facies metamorphism contemporaneous with volcanism. Observed SPV exposed at the surface of Black Mountain Open Space Park (San Diego, CA) exhibit anomalous arsenic (As) enrichment (100 - 480,000 ppm) up to five orders of magnitude greater than average for igneous rocks (1.5 ppm). We hypothesize that these rocks underwent localized syn-volcanic hydrothermal alteration along a highly fractured zone that today trends between N10°W and N20°W, leading to anomalous As enrichment on the spatial scale of tens of meters. We suspect that such As has been further mobilized by modern water-rock interactions. Using standard geochemical techniques (e.g. XRD, XRF, EDX) and mass balance analyses, we aim to (1) summarize the extent of As enrichment in altered SPV, and (2) present an integrated view of the interactions between ancient hydrothermal volcanic arc processes, surficial weathering, and observed As anomalies. Alteration textures of samples range from partially altered phenocrysts (i.e. minimally altered) to massive hydrothermal replacement, in which virtually all primary phases are altered to new hydrothermal minerals such as epidote, Fe-rich chlorite, and sericite (i.e. highly altered). Highly altered rocks contain average As concentrations (mean = 37,680 +/- 15,396 ppm, n = 23) >10,000 times that of minimally altered SPV (mean = 26 +/- 6 ppm As, n = 19). In some rocks, As-rich iron oxide and gypsum containing up to 900 ppm As are present as surficial rinds, suggesting modern day remobilization of As from hydrothermal host minerals, like arsenopyrite. These findings indicate that such As is highly soluble and, therefore, may be further mobilized by physical and chemical weathering. No other trace metals (e.g. Pb, Cu, Ag, Au) are consistently enriched above upper-crustal averages, and As does not always occur with sulfur. The results of this study could aid in pinpointing other regions at risk for anomalous As, particularly in ancient volcanic arc systems, in addition to informing future research on subsequent impacts to ecosystem and public health.