VOLATILE LOSS FROM HIGH-LEVEL MAGMAS AT BAJO DE LA ALUMBRERA PORPHYRY CU-AU DEPOSIT, NW ARGENTINA

At Bajo de la Alumbrera, textural features such as interconnected miarolitic cavities and comb-quartz layers are consistent with volatiles streaming upwards through the mineralizing intrusions. Cathodoluminescence imaging of comb-quartz layered textures from a late, poorly mineralized, apparently barren intrusion reveals well-defined luminescent bands disrupted by irregular embayments and tubules. The earliest growth bands contain high salinity (45-47 wt.% NaCl) fluid inclusions that homogenize by halite dissolution (≤405°C). Younger trails of coexisting brine (410-550°C; ≤45 wt.% NaCl) and vapor inclusions occur in zones affected by quartz resorption and subsequent overgrowths. Observed phase transformations, combined with experimental data, imply an apparent drop in pressure from ≤70 to 30 MPa (assuming no post-entrapment modification). Younger growth bands contain low-salinity (~3.0 wt.% NaCl) liquid-rich inclusions that homogenize around 320°C, and high-density vapor-rich inclusions. Based on inclusion behaviour, these liquid-vapor inclusions record entrapment of low-salinity fluids at near-critical pressure and temperature conditions.

Comb-quartz layer textures such as those at Bajo de la Alumbrera, are interpreted to form from fluid pockets in the carapace of the magma body. High salinity inclusions preserve fluids trapped at high pressure beneath the carapace. Our LA-ICP-MS analyses show that these fluids contain ~0.3 wt.% Cu. Cracking of the carapace resulted in decompression and melt vesiculation and caused the melt to become quartz-undersaturated, which explains the dissolution textures and quartz with abundant two-phase inclusions. The low-salinity inclusions provide evidence for pressure quenching and late degassing of the melt. Our observations confirm accepted physical models whereby these textures develop during volatile accumulation and release. Magmatic fluids escape these intrusions as the carapace cracks; these fluids then hydrothermally alter the rocks they pass through, and if in sufficient quantities may be involved in ore deposition. Recognition of these quartz textures and associated metal-rich inclusions shows that apparently unmineralized intrusions in porphyry Cu deposits were not necessarily devoid of metals.