Abstract

Cumobabi, a Cu-Mo-mineralized breccia-hosted deposit, is associated with 56-m.y.-old in- trusive rocks of the Mexican porphyry belt. About 35 breccias that crosscut volcanic flows and small intrusive bodies crop out in the Cumobabi area, but only five central breccias are mineralized significantly. These mineralized breccias occur as fingers of strong potassic alteration which crosscut surrounding zones ofpropylitic alteration. Distal, unmineralized breccias exhibit strong sericitization and tourmalinization. Stage I mineralization, consisting of abundant molybdenite-pyrite-quartz, occurred subsequent to both brecciation and the onset of potassic alteration. Later, stage II mineralization replaced pyrite with chalcopyrite and tetrahedrite in genetically related, but crosscutting, zones of strong sericitic alteration.Three types of well-developed primary inclusions (vapor, moderate-salinity liquid, and high-salinity liquid phases) are abundant in stage I quartz. The distribution of salinities of the inclusions suggest that episodic boiling of a probable magmatic fluid at 440 degrees to 350 degrees C accompanied and may have been responsible for mineral deposition. Episodic boiling is supported by (1) separation of inclusion salinities, (2) absence of intermediate salinity inclusions in quartz, (3) zoning of quartz crystals, with each zone representing a renewed boiling event, and (4) variation of pressure between hydrostatic and lithostatic load. A comparison of data from Cumobabi to experimental data on the NaCl-H 2 O system suggests that boiling of a 6 to 9 wt percent solution at depths of 1.0 to 1.5 km may have formed two residual phases: a lowsalinity vapor phase and a 30 to 50 wt percent salinity liquid rich in metal cations. The resulting concentration produced by boiling gave rise to a varied daughter mineral suite. Many of the daughter minerals from type III inclusions were identified under the SEM. Hematite, chalcopyrite, and delafossite, along with halite and sylvite, were common. Other unidentified daughter minerals were shown to contain Cu, Fe, Mn, and Zn, with anions presumed to be oxide, hydroxide, or carbonate. These results suggest that solutions are capable of carrying sufficient metals to form an ore deposit but may be deficient in reduced sulfur. In addition, the close association of molybdenite deposition with boiling suggests that such boiling is an important ore-forming process.Fluid inclusion-rich quartz samples from the mineralized breccia pipes were analyzed for gas and solute components. Gases were analyzed after vacuum line extraction by mass spectrometry, whereas solute components were extracted by leaching techniques and the washings analyzed by DC-plasma atomic emission spectrometry. Gas and fluid analyses along with appropriate thermodynamic data permitted computation of temperature dependent ionic equilibria, including pH and f (sub O 2 ) . These data provide requisite information to constrain fluid chemistry responsible for hydrothermal alteration and metallogenesis at Cumobabi.Computed reaction paths involving interaction of an NaCl brine with a hypothetical quartz monzonite rock illustrate the role of water/rock ratio on hydrothermal alteration at Cumobabi and the effect of temperature on metal mobility. Reaction path calculations demonstrate, for example, that high water/rock ratios stabilize sericite and propylitic assemblages at a constant temperature. Silicate reactions control pH, whereas f (sub O 2 ) controls sulfide deposition. Under the conditions modeled here Mo never exceeds concentrations of 3 ppm in solution, whereas Cu can be maintained in solution at 3,000 ppm. Temperature reduction results in albitization of feldspar and precipitation of sulfides, drastically reducing metal contents in solution.

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