The gold-rich jasperoids of the Superior district, Arizona: Controls on their location, and their relationship to porphyry-related carbonate-hosted massive sulfide bodies

The gold-rich jasperoid bodies of the Lake Superior and Arizona (LS&A) and Queen Creek mines of the Superior district, Arizona, are composed of fine-grained quartz (60-90%), massive to earthy hematite, and minor goethite, and are hosted by a {dollar}sim{dollar}10 m-thick dolostone which also hosts a major proportion of the massive sulfide bodies at the Magma mine (located {dollar}sim{dollar}1 km to the east of the jasperoids). Field and petrographic evidence indicates the jasperoid bodies are the product of weathering and subsequent silicification of former massive pyrite-rich bodies. Mass balance and mass transfer calculations support a model in which silicification occurred from silica leached from overlying mid-Tertiary volcaniclastics, transported by groundwater, and deposited within the partially weathered massive sulfide bodies and/or replaced the dolostone host rock. Silica precipitation was favored by adsorption onto the iron oxides and by the higher permeability of the leached and weathered sulfide bodies. Because supergene silicification is a common process in many areas of the world, particularly in arid to semi-arid climates, silicification can significantly affect the overall grades and their spatial distribution in many types of deposits.; Results from detailed field, textural, and geochemical studies of the carbonate host rocks and mass transfer calculations of the replacement process show that: (1) the chemical and mineralogical composition of the host rocks did not play a significant role in the replacement process; and (2) selective replacement was controlled by contrasts in permeability (both fracture and intergranular) between the favorable and unfavorable beds. These results are consistent with observations at other high temperature carbonate-hosted districts (e.g., the northern Mexico lead-zinc deposits). Permeability control of selective replacement is consistent with an origin due to reaction front self-focusing. Self-focusing is favored by high reaction rates and high permeabilities, both of which are common in the "favorable" carbonate beds and explain why self-focusing occurs in this type of deposits. The similarity in the solubility and reaction rates of calcite and dolomite at the conditions of formation of this type of deposits explain why wall rock composition is not a major control on replacement.