Whole-Rock Geochemical Techniques for Evaluating Hydrothermal Alteration, Mass Changes, and Compositional Gradients Associated with Epithermal Au-Ag Mineralization

Abstract

Volcanic rocks hosting quartz ± calcite ± adularia ± illite epithermal deposits undergo elemental mass changes associated with K metasomatism, K-H metasomatism, and H metasomatism that are developed progressively upward and outward from the site of mineralization as hydrothermal fluid ascends, boils, and cools. Resultant hydrothermal mineral zonation shows increasingly K-rich mineral assemblages with proximity to deposits, thus patterns of K enrichment provide a vector toward those deposits. We describe whole-rock geochemical techniques for identifying these patterns and for evaluating attendant hydrothermal mineral zonation. Whole-rock geochemical anomalies are evaluated by calculating mass changes associated with hydrothermal alteration, using a modified version of Gresens’ (1967) equation to compare the composition of altered rocks to fresh-rock equivalents. Hydrothermally altered rocks most affected by K metasomatism will be characterized by the largest K gains and, generally, Na and Ca losses. Mass changes associated with K metasomatism are also evaluated graphically using plots of molar (2Ca + Na + K)/Al versus molar K/Al. Since molar values are used to construct the plot, compositions of altered rocks can be compared to the compositions of primary and secondary K-, Na-, Ca-, and Al-bearing minerals that are located in the same compositional space, allowing identification of important hydrothermal minerals (e.g., adularia, illite, smectite) and alteration processes that are reflected in trends from fresh-rock compositions toward the compositions of hydrothermal minerals. The intensity of K metasomatism, encompassing both K gains and Na and Ca losses, can be represented by the slope of the line between an altered rock composition and the origin (i.e., the molar K/(2Ca + Na + K) value). Determinations of mass changes in altered rocks surrounding selected epithermal deposits demonstrate the predominance of K metasomatism proximal to, and commonly increasing in intensity toward, mineralized veins. Comparisons of K mass changes to trace element concentrations indicate that the area affected by K metasomatism is more extensive (100s to 1,000s of meters) than that containing anomalous concentrations of precious metals, base metals, and pathfinder elements (10s to 100s of meters); therefore, whole-rock geochemical techniques potentially extend the area over which geochemical targeting may be effective. Data from this study show that intensity of K metasomatism (molar K/(2Ca + Na + K) values) and concentrations of precious metals and pathfinder elements increase toward ore and are greatest proximal to ore, so that pathfinder element and whole-rock geochemical anomalies are complementary.