Mass Transfer During Hydrothermal Alteration at the Lahendong Geothermal System, North Sulawesi

Abstract

Petrographic observations show that mass transfer has taken place in in the Lahendong geothermal system. For example, plagioclase replaced by adularia suggests that Ca has been removed and K was gained by the rock; the replacement of augite by epidote suggests the addition of Al, but depletion of Fe, Mg, and Ca. However, the calculated gains and losses of constituents in the wall rocks are not always consistent with the petrography. One possible reason is that during alteration processes the elements have simply ìreshuffledî. On the microscopic scale, gains and losses do not always correspond with the replacement mineralogy. Elements lost during dissolution were therefore, possibly incorporated into a small amounts of hydrothermal minerals but most were removed in solution. On the other hand, significant gains occurred where minerals fill vugs or veinlets were inadvertently sampled for analyses. Multiple alteration events have taken place in Lahendong, but the calculated mass transfer reflects the end product of series of fluid-rock interaction processes