Skarn, manto, and breccia pipe formation in sedimentary rocks of the Cananea mining district, Sonora, Mexico

Skarns, mantos, and breccia pipes occur at Cananea in a 2- by 4-km horst of Paleozoic carbonate rock and minor quartzite. Surface mapping and core logging reveal a sequence of events beginning with early metamorphism that converted impure carbonate lithologies to iron-poor garnet-pyroxene + or - idocrase hornfels. Subsequent metasomatism formed garnet-py-roxene skarn along the pre-Cretaceous Elisa fault contact between carbonate rock and Mesozoic volcanic rock. Skarn is zoned from an andradite-rich center, through a zone containing both andraditic garnet and salitic pyroxene, to a mineralogic sequence near the marble front which is largely a function of the sedimentary protolith; chert nodules are rimmed by wollastonite, dolomite is converted to massive phlogopite-magnetite skarn, calc-silicate hornfels is over-printed by veins of skarn garnet, and relatively pure marble is replaced by coarse blades of iron and manganese-rich pyroxene. Multiple generations of garnet and pyroxene can be distinguished by subtle variations in color and texture. Age classification on the basis of crosscutting vein and overgrowth relationships indicates that late pyroxenes are more iron and manganese rich than the early generations; garnets show a less regular iron enrichment with time. The spatial distribution of the different garnet and pyroxene generations is irregular; garnet and pyroxene in late veins have iron-rich compositions both near the skarn center and near the marble front. Pyrite and minor chalcopyrite are the only sulfides associated with this stage of metasomatism.Following the main stages of garnet and pyroxene formation, veins and orbicular patches of amphibole + or - quartz + or - calcite occur replacing pyroxene and in some cases, garnet. Most of the amphibole is actinolitic in composition and is associated with pyrite and minor amounts of chalcopyrite. About 20 percent of the amphibole is subcalcic and is associated with or replaced by massive calcite. The alteration of prograde skarn to amphibole + or - quartz + or - calcite can be best explained by a general temperature decline.The destruction of skarn by alteration related to brecciation and breccia pipe formation and the replacement of previously unaltered carbonate rocks by stratiform blankets (mantos) of iron oxides and sulfides resulted in some of the highest grade orebodies. Breccia pipe and manto formation appears to have been largely contemporaneous with emplacement and subsequent sericitic alteration of a series of quartz monzonite porphyry stocks. Where breccia pipes crosscut skarn, garnet, pyroxene, and amphibole are converted to mixtures of calcite, quartz, chlorite, hematite, siderite, and sulfides. Where breccia pipes or porphyries crosscut previously unaltered carbonate rock, mixtures of magnetite, sulfides, chlorite, siderite, calcite, quartz, and serpentine form massive mantos. Veinlets of chlorite and magnetite + or - pyrite + or - serpentine extend tens of meters beyond the zones of massive replacement. There is a rough vertical and lateral zonation of sulfide minerals with respect to the Democrata breccia pipe from chalcopyrite + or - bornite or pyrite in the core and at depth to sphalerite-pyrite-chalcopyrite to pyrite-sphalerite-galena distal to the center of mineralization. Mineral stability relations suggest that brecciation and mineralization took place at lower temperatures (275 degrees -25 degrees C) and possibly under lower X (sub CO 2 ) conditions than the earlier skarn formation.The high Zn/Cu ratios in skarn, the zonation of most hypogene mineralization relative to the breccia pipes which crosscut skarn, and the lack of skarn spatially associated with the quartz monzonite porphyry stocks which intrude carbonate rock all suggest that, unlike skarn in most porphyry copper districts, skarn at Cananea is not related to the quartz monzonite porphyry stocks that are mined elsewhere in the district for disseminated supergene-enriched sulfide mineralization. Rather, the skarn may be related to a deeper magmatic system which has not yet been encountered in subsurface exposures.