Skarn formation at the Mason Valley mine occurred at a depth of 2,000 m on the outer fringe of a contact metasomatic aureole related to a Jurassic granodiorite to quartz monzonite batholith. The skarn is located in Upper Triassic limestone at the contact with a stratigraphically lower tuff unit and is systematically zoned relative to this contact. The general zonal sequence toward marble is: garnet, garnet-pyroxene-sulfides, pyroxene-sulfides, tremolite-magnetite-calcite, talc-magnetite-calcite, and dolomite-calcite. The zones migrated outward with time.Electron microprobe data indicate that garnets and pyroxenes in the barren garnet footwall zone have compositions similar to the districtwide early metasomatic hornfelses and are represented by low-iron diopsides and intermediate grandites. Pure andradite, as overgrowths and in cross-cutting veins, becomes more abundant as the hanging-wall skarn is approached. Both garnets and pyroxenes shift abruptly to higher iron contents in the hanging-wall skarn which formed at the contact between the early garnet zone and dolomitized limestone. Garnets maintain a constant lower limit of 55 mole percent andradite and are zoned to pure andradite. Later garnets, contemporaneous with chalcopyrite deposition, consist of pure andradite. The iron content of pyroxene increases gradually and systematically toward the marble contact, from an average value of 36 mole percent hedenbergite in the inner garnet-pyroxene zone to 56 mole percent hedenbergite in pyroxene vein centers on the marble contact, and then drops abruptly to 15 mole percent hedenbergite in vein envelopes. Two generations of amphibole are represented by: (1) early tremolite (0 to 10 mole percent ferrotremolite) associated with magnetite-calcite in outermost vein envelopes in marble; and (2) actinolite which contains the same Fe/Mg ratio as associated pyroxene and is contemporaneous with chalcopyrite deposition in the pyroxene and garnet-pyroxene zones.The initial silication process, as represented by zoned veins at the marble contact, may be attributed to isothermal metasomatic diffusion of Ca, Mg, and Si, with X (sub CO 2 ) decreasing toward the vein centers. The abrupt appearance of new minerals coincides with the attainment of appropriate chemical potential values through metasomatism, rather than the crossing of isobaric univariant T-X (sub CO 2 ) equilibria. Bulk composition gradients are extreme and are reflected in the rapid increase in iron content of tremolite and diopside over a few centimeters from vein envelope to vein center.Within the main skarn zone, which formed at higher temperature and/or lower X (sub CO 2 ) , bulk composition gradients are less extreme, and phase-composition trends are opposite to those that would be predicted by a simple isothermal diffusion model. The gradual inward decrease in the iron content of salite within skarn zones of relatively constant bulk composition may have been controlled in part by continuous Fe-Mg reactions.Comparison with phase-composition data from similar zoned skarns indicates that variation of phase compositions within zones is a characteristic phenomenon, but that zonal composition trends in some cases are opposite to those established here.