The Middle Fork copper prospect is in the central Cascade Range of Washington State, near the southeastern margin of the Miocene, composite Snoqualmie batholith. The principal zones of chalcopyrite mineralization and related wall rock alteration occur in medium-grained, equigranular, main-phase Snoqualmie granodiorite. The mineralized zone is characterized by en echelon, northeasterly trending shearing. Significant sulfide concentrations appear to occur in linear breccia zones commonly near intersections of northeasterly trending shears with steeply dipping, north to north-westerly striking cross faults.Geologic mapping, petrographic data, and whole rock chemical analyses suggest the following sequence of alteration and mineralization:(1) A northeasterly trending zone of unmineralized, quartz-deficient biotite-plagioclase breccia and peripheral quartz-stable, biotitic granodiorite breccia formed from main-phase Snoqualmie granodiorite during a period of presumed deuteric alteration. The removal of quartz from the quartz-deficient zones resulted in a volume loss of approximately 30 percent.(2) Subsequent introduction of solutions rich in potassium, iron, sulfur, copper, and minor molybdenum was restricted primarily to the quartz-deficient biotite-plagioclase breccias. The alteration took place at constant volume and resulted in an assemblage of hydrothermal orthoclase and secondary biotite associated with relict igneous plagioclase and primary biotite. Subequal amounts of disseminated chalcopyrite (with pyrrhotite blebs) and pyrite occur within this alteration zone. Retrogressive alteration of biotite to chlorite and plagioclase to sericite and carbonate minerals commonly is superimposed on the earlier assemblage.(3) A late-stage assemblage of sericite, pyrrhotite, quartz, carbonate, and chalcopyrite is especially well developed along northwesterly striking discontinuous shear zones. The sericite-pyrrhotite alteration occurred at constant volume and probably represents deposition from the same solutions responsible for the orthoclase alteration, but at decreased K (super +) /H (super +) ratios and lower temperatures.(4) Northerly to northwesterly striking post-mineralization faults cut the mineralized zone into discrete blocks.The Middle Fork deposit has hydrothermal alteration assemblages similar to southwestern United States porphyry copper deposits, but differs from them in that it (1) occurs in medium-grained quartz dioritic-granodioritic intrusive rocks of batholithic dimensions, (2) has linear, structurally controlled zones of alteration and mineralization, (3) has potassium silicate alteration assemblages remarkably deficient in quartz, (4) lacks a peripheral propylitic zone, (5) contains pyrrhotite as a major sulfide mineral, and (6) has virtually no supergene enrichment. Despite these differences, the Middle Fork deposit appears to belong within the broad spectrum of porphyry copper deposits although diverging widely from the unifying model of Lowell and Guilbert (1970).