Mineralization at Cananea, Sonora, and the paragenesis and zoning of breccia pipes in quartzofeldspathic rock

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Correlative early and late deposit types in the Cananea district correspond to similarly classified deposit types in other porphyry copper districts. Potassic alteration, massive silicates in the Colorada orebody, and prograde skarn were produced by early, probably predominantly magmatic-hydrothermal fluids from a limited number of porphyry stocks. Later stockwork-disseminated deposits associated with sericitic alteration were contemporaneous with breccia pipes, retrograde alteration of skarn, and limestone replacement deposits. Fluids associated with the relatively late deposits were probably predominantly meteoric-hydrothermal, but geologic relations suggest that the earliest fluids in some of these deposits may have included fluid exsolved from porphyry. Mineralization in both early and late deposits precipitated in the sequence: silicates followed by sulfides, except for early molybdenite in some pipes.Many pipes in the district appear to have formed by collapse into open space at depth. Consistent paragenetic sequences, structures, and textures shared by these pipes suggest that collapse occurred into space formed at the same time relative to emplacement of the responsible porphyry phases and evolution of the associated hydrothermal systems. A possible analogue of the space into which collapse took place is abreccia at the outer contact of a large porphyry stock. Small outlying structures containing sulfide mineralization and breccia or intensely fractured rock appear to be associated with the upper terminations of pipes.Late mineralization has high overall pyrite/Cu(Fe) sulfides ratios, greater than 1. Wherever the sulfide sequence could be established, it is (1) pyrite, (2) Cu(Fe) sulfides, and (3) sphalerite + galena + tetrahedrite, except for small amounts of second-stage sulfides. Zoning of these numbered sulfide stages in breccia pipes in quartzofeldspathic rock and also in limestone replacement deposits is inferred to be 1, 1 + 2, 1 + 2 + 3, and 1 + 3 from the proximal to the distal ends of the deposits. Pipes within disseminated deposits contain the lower portion of the pattern. Textures indicative of local replacement of wall rock and early gangue minerals by later phases (e.g., sulfides) are common. The common occurrence of late paragenetic stages in a pipe indicates that the pipe remained open during precipitation of the main stage of economic sulfides.Garnet in pipes in quartzofeldspathic rock is diagnostic of limestone at depth nearby. Early adularia and quartz-adularia alteration of wall-rock fragments in the Cananea-Duluth pipe persist to much greater distances above limestone than garnet does and might prove to be similarly diagnostic.

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