K-RB RATIOS AS A SOURCE INDICATOR FOR HYDROTHERMAL FLUIDS AT THE SENECA VOLCANOGENIC MASSIVE SULFIDE DEPOSIT, BRITISH-COLUMBIA

The K/Rb ratio in hydrothermallya ltered rocks canb e highlyv ariablea nd is dependenotn the rock/water ratio, temperaturep, H, amounto f time available for reaction, and the relative K and Rb content of the fresh rocksa nd aqueousp hase. Wherea queoufsl uidss eparatefr om a crystallizing magma, the fluid is enrichedin Rb and has a lower K/Rb ratio relative to the original parent magma. Rocks altered by this fluid also acquire a low K/Rb ratio. Where meteoric water is heated in rocks of low porocity, the rock/water ratio is large enough to allow significant leaching of Rb resulting in a low K/Rb ratio in the aqueousp hase. Therefore,K /Rb ratiosc annotb e usedt o distinguishb etweend eposits formedb y magmaticv s heatedm eteoricw ater. Where the rock/water ratio is small, such as in submarine debris flows and explosion breccias, the high K/Rb ratio of sea water remains high even if substantiaRl b is leachedf rom the fragmentalr ocks. If sericite, or other secondary K minerals, forms from reaction of wall rocks and this aqueous fluid, the altered rocks will have a high K/Rb ratio. Abnormally high K/Rb ratios in altered rocks may indicatet hat seaw ater madeu p a significanpt art of the hydrothermal fluid. Positive Rb anomalies extend beyond the zone of K metasomatisma t porphyry copper depositsi n Chile and produce enlarged targets for mineral exploration. The Rb anomaly at the Seneca volcanogenic deposit is restricted to the zone of K enrichment, that is, to the zone where K alteration minerals were formed. However, the K and Rb anomalies may still be larger than the zone of mineralization, thusin creasitnhges izeo ft h[e xploratitoanrg et.