Ing. Geologo

©2007 Society of Economic Geologists, Inc. Economic Geology, v. 102, pp. 923–948

Whole-Rock Geochemical Techniques for Evaluating Hydrothermal Alteration, Mass Changes, and Compositional Gradients Associated with Epithermal Au-Ag Mineralization
IAN WARREN,†,* STUART F. SIMMONS, AND JEFFREY L. MAUK
School of Geography, Geology and Environmental Science, University of Auckland, Auckland MailCentre, Auckland 1142, New Zealand

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 zonationshows 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. Sincemolar 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 compositionstoward 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 Kmetasomatism 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 geochemicaltechniques 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.

IntroductionHYDROTHERMAL alteration records the effects of fluid-rock interaction, and these effects are expressed as compositional changes that can be recognized in the geochemistry of the rocks. Where compositional changes define gradients that can be related to processes that form ore deposits, they potentially provide vectors toward sites likely to contain precious metal mineralization. Zoned hydrothermalalteration surrounding ore deposits is the mineralogical expression of temperature and compositional gradients centered on those ore deposits. Whole-rock geochemical studies have been widely applied to volcanic-hosted massive sulfide (VHMS) deposits (Gemmell and Large, 1992; Whitford and Ashley, 1992; Callahan, 2001; Gemmell and Fulton, 2001; Large et al., 2001a-c) and also orogenic gold deposits...