COULD THE COMBINATION OF MAFIC AND SILICIC MAGMAS BE VITAL TO PORPHYRY SYSTEMS: A DISCUSSION

The sulfur and metal-rich character of mafic magmas make them attractive ultimate sources for at least a portion of the ore constituents in porphyry systems. In addition, recent work increasingly invokes and identifies the importance of such magmas in large systems. However, could the dynamic combination of mafic and silicic magmas contribute anything special to potential ore-forming processes? Several possible roles can be envisaged for such a combination including: 1) silicic magmas might impede the eruption of mafic magma during episodic extension and lead to magma mixing and/or crystallization of the underplated magma; or 2) silicic magmas may capture and store exsolved volatiles and metals from crystallized mafic magma as a discrete fluid phase; or 3) magma mixing may induce episodes of volatile exsolution over large volumes of silicic magma due to release of less soluble gases such as CO2 or SO2 from the mafic magma; or 4) the lower solubility of sulfur in Fe-poor silicic magmas (and capture of sulfur from mafic magma) may lead to formation of magmatic sulfides that may temporarily store Cu, Au, and other chalcophile metals prior to exsolution of an ore fluid and resorption of magmatic sulfides; or 5) lower density silicic magmas may more readily form high-level (shallow) magma chambers to focus multiple intrusions and long-lived hydrothermal systems conducive to formation of large deposits; or 6) large, shallow silicic magma chambers may contribute most or all of the metals and volatiles to the ore fluid while the mafic magma may only induce and promote convection leading to fluid separation in a cupola.

Alternatively, none of these factors may be critical to ore formation and porphyry systems may be formed strictly by crystallization of mafic or silicic magmas. No definitive answers to this complex question are yet possible. Audience participation will be solicited.