MASS TRANSFER AND METASOMATISM ASSOCIATED WITH ADULARIA-SERICITE EPITHERMAL OREBODIES

Adularia-sericite epithermal deposits have Au-Ag veins surrounded by widespread zoned alteration with distinct geochemical signatures. Whole-rock geochemistry can provide useful vectors for exploration and can enhance efforts to characterise and quantify the mineralogy and intensity of hydrothermal alteration. Gold and Ag provide the most direct vectors to orebodies, and pathfinder elements such as As, Sb, Hg, Zn, Pb and Se are similarly zoned around some orebodies. However, the minerals that contain these elements occur almost exclusively in veins, and therefore they may not allow identification of geochemical vectors related to wallrock alteration. Precious metal and pathfinder element anomalies may be limited to areas close to veins (tens of m) in larger areas of hydrothermally altered wallrock (thousands of m). Distal and deep alteration zones contain quartz-illite-chlorite-carbonate-pyrite, whereas samples closer to veins contain quartz-adularia-illite-pyrite, with lesser and variable chlorite-carbonate. Therefore, adularia-sericite epithermal ore deposits are characterized by K-rich alteration that increases in intensity toward sites most likely to contain mineralization. Areas that have been most intensely altered are commonly characterised by highest K contents, but these results can be complicated by variations in the composition of precursor rocks. Potassium mass change calculations provide a robust and rigorous assessment of K-metasomatism, however, knowledge about the composition of fresh rocks and correct identification of fresh precursor compositions of altered rocks are required. Alternatively, there are three alteration indices that provide effective evaluation of K-metasomatism independent of knowledge about precursor rock compositions: the (1) K number, which is the ratio K/(2Ca+K+Na) (molar); (2) Barrie value [(K2O * 100) / Sr]; and (3) Rb/Sr ratio. In most places these provide similar vectors toward mineralization, although K numbers can be complicated by the appearance of hydrothermal calcite and/or albite. Plots of K/Al vs (2Ca+K+Na)/Al (molar) allow estimation of the intensity of K-metasomatism and its mineralogical products, thereby complementing traditional XRD and petrographic studies of alteration minerals.