“…NW Caldera site; de Ronde, Humphris, Höfig, et al., 2019). By contrast, water‐rock interaction with acid‐sulfate fluids produce advanced argillic alteration assemblages, such as quartz, illite, pyrophyllite, diaspore, alunite, and anhydrite (de Ronde, Humphris, & Höfig, 2019; de Ronde, Humphris, Höfig, et al., 2019; Gena et al., 2001; Lackschewitz et al., 2004). Thus, rocks with advanced argillic mineral assemblages are highly depleted in MgO.…”