Timing and evolution of late Oligocene to early Miocene magmatism and epithermal mineralization in the Bolanos Graben, southern Sierra Madre Occidental, Mexico

Key questions in the study of hydrothermal deposits related to magmatism are when, where, and how do magmas acquire their metals and fluids; and how do magmas contribute these components to the hydrothermal system for further ore concentration. This link between the magmatic ore source and hydrothermal deposits is unilaterally constrained due to emphasis on studies of ore bodies and diminished attention on the associated igneous evolution. The mid-Tertiary Sierra Madre Occidental (SMO) of Mexico is a large silicic igneous province in close spatio-temporal association to a premier silver epithermal region, and represents a suitable location to study the magmatic-hydrothermal link. To study the evolution of metal sources, rather than the pathway and final ore accumulation; this research has applied a magmatic point of view: employing geological fieldwork and zircon chronochemistry, together with rock, crystal and melt inclusion chemistry. This thesis presents the relation found between magmas and mineralization at a local scale in the Bolanos Graben, southern SMO. Pre-Extensional (Late Oligocene) and Syn-Extensional (Early Miocene) magmatic stages were recognized to record the volcanic history of the graben, with mineralization occurring during the second one. A novel igneous texture identified is interpreted to record the metal enrichment of magmas during the Pre-Extensional stage. In Early Miocene times, crustal recycling via the remelting of the enriched igneous crust formed in the Oligocene, aided by the onset of extension, may have enhanced magmatic enrichment and produced fertile magmas. These magmas may have contributed with metals, volatiles and other components to mineralization; or the formation of an enriched crust via a prolonged magmatic history provided material for hydrothermal fluids to scavenge. In conclusion, the petrogenesis of silicic igneous rocks and their evolution can give insights in when, where and how magmas acquire their metals for hydrothermal deposits to form