A MAGMATIC-HYDROTHERMAL BRECCIA FORMED BY HYDRAULIC FRACTURING UNDER A COMPRESSIVE STRESS REGIME AT THE QUESTA PORPHYRY MOLYDENUM DEPOSIT

Breccia pipes, by definition, are nearly vertical bodies. They are possibly formed either by magmatic-hydrothermal processes (e.g. Burnham, Econ. Geol., 1985) or by phreatomagmatic explosions (diatremes). At the Questa porphyry Mo deposit (NM) a >6.0x10⁶ m³ mineralized breccia body differs from the typical pipe shape, suggesting either different breccia-forming processes or a different lithologic and structural context.

The Goat Hill breccia body has an elongated lens shape, striking S75°W with a N20° dip. It is £ 100 m thick, 200 m wide and 650 m long and is located above and southward of an aplitic stock apex. Jigsaw puzzle breccia textures indicate that pre-mineral dikes and surrounding andesitic rocks were hydrofractured by ore-forming fluids evolved from crystallizing water-saturated magma. Fractures propagated from this magma chamber (i.e. stock apex) along a pre-existing volcanic or structural fabric dipping gently toward the north.

Oxygen isotope thermometry and previous fluid inclusion studies indicate the breccia matrix crystallized at 550°-600°C. The isotopic composition of this matrix isdD=-138 to –112 (phlogopite) and d¹⁸O=+6.8 to +8.6 for quartz and +3.3 to +5.7 for phlogopite. At 600°C, the calculated water composition isd¹⁸O=+5.5 to +8.1 and dD=-125 to –99. It is proposed that ore- and breccia-forming water had little-to-no meteoric component. An initial state of overpressure in the magmatic-hydrothermal system required a low permeability in the solidified magma carapace and country rock. Thus, the escape of magmatic-hydrothermal fluids from the breccia system into the surrounding meteoric-hydrothermal system was impeded, even after brecciation.

The overall shape of the breccia in long section is similar to a large tension crack. It is suggested that hydraulic fracturing in a compressive stress regime (horizontals1 and verticals3) formed this structural feature. Brecciation did not propagate to the surface as in diatreme systems, perhaps due to the 3-4 km paleo-depth of formation calculated from stratigraphic reconstruction. Rather, the Goat Hill breccia body formed by processes similar to those that operate within magmatic-hydrothermal-type breccia pipes, possibly under a different stress regime within highly anisotropic host rocks.