The Upper Cretaceous Guaynopa IOCG and Guaynopita porphyry copper deposits, Chihuahua, Mexico

Abstract

The Guaynopa and Guaynopita mineralized areas in central-western Chihuahua are conterminous sets of ore deposits that formed in association with gabbroic and granitic intrusions that correspond to the Lower Volcanic Complex of the Sierra Madre Occidental silicic large igneous province. The Guaynopa IOCG deposit consists of (1) early iron oxide–copper–gold mantos accompanied by potassic (fuchsite, biotite and potassium feldspar) and/or calcic–sodic alteration (tremolite–actinolite) and hosted by marmorized limestones near the contact with intrusive granites, (2) later copper-rich stockworks and gold disseminations, and (3) late gold- and copper-rich quartz-calcite veins. Mantos contain most of the copper and gold ores in this deposit, and their hypogene mineralogy consists of magnetite, fuchsite, chalcopyrite, Ag-rich gold, cuprite, and late hematite. The Guaynopita porphyry copper deposit consists mainly of stockworks within potassic alteration zones, and includes ancillary sulfide copper- and lead-rich skarn deposits. Microthermometric studies of fluid inclusions were carried out on most mineral associations of the Guaynopa IOCG and in the Guaynopita porphyry copper deposits. In IOCG mantos, temperatures of homogenization (Th) in calcite and quartz vary between 152° and 310°C, and apparent salinities between 10.7 and 24.2wt.% NaCl equiv. In IOCG veins, Th in quartz vary between 310° and 400°C, and apparent salinities between 11.1 and 21.0wt.% NaCl equiv. In IOCG stockworks, Th in tremolite vary between 330° and 410°C, and the apparent salinity is 16.0wt.% NaCl equiv. In porphyry-copper stockworks, Th in quartz vary between 205° and 448°C, and apparent salinities between 8.1 and 21.3wt.% NaCl equiv. δ34S values for sulfides (mostly chalcopyrite) in IOCG associations range between −15.1 and 7.0‰ in mantos, between −3.7 and −0.2‰ in veins, and between −1.6 and 0.2‰ in stockworks. δ13CVPDB values from calcite in IOCG mantos range between −5.22 and 3.45‰, and δ18OVSMOW values between 9.61 and 17.23‰, which correspond to the interaction of magmatic fluids with host limestones at a broad range of temperatures and water to rock volume ratios. Likely ore-formation processes are isothermal mixing, conductive cooling, and boiling. In the case of the Guaynopita porphyry copper deposit, similar possible magmatic-dominated fluids underwent progressive dilution and cooling with time, with the occurrence of boiling at some extent.New geochronological data from hydrothermal minerals and host rocks for these deposits suggest a minimum ~14m.yr. span of conjoined magmatic and hydrothermal activity: biotite and chromian muscovite (fuchsite) from mantos at the Guaynopa IOCG deposits yielded 40Ar/39Ar plateau ages at 98.12±0.37 and 95.42±0.71Ma (Cenomanian), respectively, whereas U–Pb dating in zircons from diorite and granodiorite host intrusives, and a potassic alteration assemblage at Guaynopita yielded ages at 92.4±0.5Ma, 89.1±0.7Ma (Turonian to Coniacian), and 84.4±1.0Ma (Santonian), respectively. These ages place the Guaynopa–Guaynopita ensemble within the Mesozoic metallogenic provinces and epochs in northwestern Mexico. Unlike the majority of Mesozoic generic magmatic-hydrothermal iron oxide deposits in Mexico, Guaynopa in Chihuahua and Cerro del Oro in Sonora formed at a distal position from the paleo-Pacific margin (~500km inland) and therefore it cannot be ascribed to a general intra-arc “Coastal Andes Cordillera-type” tectonomagmatic setting. Therefore, this region may constitute a part of a previously unnoticed metallogenetic region, and petrogenetic studies should follow in order to properly evaluate the hypothetical possibility for a back-arc setting there during the Late Cretaceous.