ELEMENTAL GEOCHEMISTRY OF MERCURY DEPOSITS AND HOT SPRINGS OF THE CALIFORNIA COAST RANGES BY HANDHELD X-RAY FLUORESCENCE (HHXRF)

The Geysers-Clear Lake area of the northern California Coast Ranges is a region that is host to numerous fossil and active Hg-rich hydrothermal systems. Mineralization in the region is related to increased heat flow and volcanism caused by the migration of the Mendocino Triple Junction through the region beginning at 2.3 Ma. There are two types of mercury deposits present in the area, hot-spring deposits that formed at shallow depths (<300 m) and silica-carbonate deposits that extend to depths of 1000 m. Active springs and geothermal areas continue to precipitate Hg and Au and are analogues to the fossil hydrothermal systems preserved as ore deposits.

Comparison of a HHXRF equipped with a silicon drift detector (SDD) and on-board processor with independent laboratory results show typical correlations (R²) > 0.90 and repeatability < 5 % relative standard deviation (RSD) for most major, minor and trace elements from Mg to U. Dried spring precipitates and rock chip samples from The Geysers-Clear lake area were analyzed for 30 seconds each on the low, main and high energy filters (for analyzing elements K to U), and 60 seconds on the light filter setting (Mg to S) for a total analysis time of 150 seconds.

Au-bearing springs deposited black, sulfur-rich mud enriched in Sb, As, Hg, and Ag. The presence of Si, S, Ca, Cr, Fe and Mg are consistent with observations of quartz, calcite, native sulfur, pyrite, chromite and chrysotile in the spring precipitates. Chromite is likely detrital in origin from the surrounding ultramafic country rock. Hot spring-type Au deposits have a similar trace element signature to active Au-bearing spring deposits. The alteration associated with the Au-bearing veins is rich in Si, Al, S and K. These major elements are hosted by quartz, alunite and adularia identified in thin section and by XRD. Silica-carbonate Hg deposits contain significant amounts of Sb, As, Ni, Zn and Cu associated with alteration and cinnabar mineralization. The alteration assemblage is characterized by Si, Mg, Ca, Fe and S hosted by quartz, chalcedony, magnesite and Fe-sulfides. Handheld XRF analysis provided fast geochemical results for mineralization and alteration assemblages in the field on scales ranging from cm to meters that can be used in geochemical exploration and field mapping.