Magmatic anhydrite and calcite in the ore-forming quartz-monzodiorite magma at Santa Rita,New Mexico (USA):genetic constraints on porphyry-Cu mineralization

摘要

A quartz-monzodioritic dike associated with the porphyry-Cu mineralized stock at Santa Rita,NM,has been studied to constrain physico-chemical factors (P,T,fo_2,and volatile content) responsible for mineralization.The dike contains a low-variance mineral assemblage of amphibole,plagioclase (An_(30-50)),quartz,biotite,sphene,magnetite,and apatite,plus anhydrite and calcite preserved as primary inclusions within the major phenocryst phases.Petrographic relationships demonstrate that anhydrite originally was abundant in the form of phenocrysts (1-2 vol.%),but later was replaced by either quartz or calcite.Hornblende-plagioclase thermobarometry suggests that several magmas were involved in the formation of the quartz-monzodiorite,with one magma having ascended directly from >=14 km depth.Rapid magma ascent is supported by the presence of intact calcite inclusions within quartz phenocrysts.The assemblage quartz + sphene + magnetite + Mg-rich amphibole in the quartz-monzodiorite constrains magmatic oxygen fugacity at logfo_2,>NNO+1,in agreement with the presence of magmatic anhydrite and a lack of magmatic sulfides.The same reasoning generally applies for rocks hosting porphyry-Cu deposits,seemingly speaking against a major role of magmatic sulfides in the formation of such mineralizations.There is increasing evidence,however,that magmatic sulfides play an important role in earlier stages of porphyry-Cu evolution,the record of which is often obliterated by later processes.