The Hugo Dummett porphyry Cu-Au deposit in the South Gobi desert, Mongolia, has an indicated resource of 582 Mt at 1.89% Cu and 0.41g/t Au, and an additional inferred resource of 1020 Mt at 1.07% Cu and 0.21g/t Au (at 0.60% Cu equiv. cutoff), in giving a total 22 Mt Cu and 14.9 Moz Au. The porphyry system is related to Late Devonian quartz monzodiorite stocks that intruded basaltic rocks belonging to the Gurvansayhan volcanic arc terrane. Advanced argillic alteration of dacitic ash flow tuff overlying basaltic host rocks is dominated by quartz-pyrophyllite, with ubiquitous kaolinite and late dickite on fractures, plus varying amounts of alunite, zunyite, diaspore and topaz. The quartz monzodiorite intrusions form the core of the deposit and are intensely altered to muscovite-illite, without pyrite. The isotopic composition of alunite (d¹⁸O _(SO4) 11.8 to 20.1 ‰, dD -43 to -55 ‰, d³⁴S 9.8-17.9 ‰) indicates that it is derived from condensation of magmatic vapor, without involvement of meteoric water. Field relationships suggest that alunite formed at the margin of early quartz veins associated with K-silicate alteration. Pyrophyllite (d¹⁸O 6.5 to 10.9 ‰, dD -101 to -106 ‰) formed from a magmatic fluid, with a possible minor component of external fluid of currently undetermined origin. Muscovite (d¹⁸O 3.0 to 9.0 ‰, dD -101 to -116 ‰) in the core of the deposit formed from a magmatic fluid similar to that which formed the pyrophyllite, but with a possible meteoric water component. By contrast paragenetically later, and post-ore alteration minerals such as dickite (d¹⁸O -4.1 to 3.3 ‰, dD -130 to -140 ‰), and Mg-chlorite after biotite (d¹⁸O 5.5 ‰, dD -126 ‰) show clear evidence for meteoric water mixing. Relatively low dD values for this meteoric component, about -140 ‰ indicate a high-elevation, possibly high-relief environment, during the final stages of cooling of the porphyry system.