GRABEN TYPE CALDERAS OF THE SIERRA MADRE OCCIDENTAL: POTENTIAL SITES FOR HYDROTHERMAL ORE DEPOSITS

The Sierra Madre Occidental (SMO) covers most of the NW portion of Mexico. It is probably the largest hydrothermal metallogenetic province of the world hosting several worldwide important precious metal mining districts, such as Tayoltita, Fresnillo, Zacatecas, Guanajuato, and Bolaños, just to mention some. We have found that several of these mine districts correspond to sites of graben-type calderas. A graben-caldera can be defined as an explosive-collapse, volcano-tectonic structure, from which ignimbrite-forming pyroclastic flows were erupted through the graben’s faults during collapse of the roof of a shallow magma chamber that was structurally controlled by the graben domain (Aguirre-Díaz 2008). Graben type calderas are common in the SMO and were developed by the overlapping in space and time of the Basin and Range extensional province and the SMO volcanic province (Aguirre-Díaz et al. 2008). Major ignimbrites of the SMO were derived from fissure eruptions rather than from classic semi-circular collapse calderas (Aguirre-Díaz and Labarthe-Hernández 2003). Both graben formation and mid-Tertiary silicic volcanic activity coincided in space and time, particularly for the 38-23 Ma period, known as the Ignimbrite Flare-up event.

Our field observations indicate that there is a clear link between the graben-controlled fissure vents with precious metal hydrothermal mineralization. Timings of hydrothermal processes agree with those of post-collapse caldera settings. Major veins occur along the master faults of the graben calderas or in faults between intra-caldera downdropped blocks within the graben caldera formed during the caldera collapse. Water needed for the hydrothermal system comes from intra-caldera lakes, many times represented by post-collapse fluvio-lacustrine sediments. In some cases, hydrothermal mineralization is also related to rhyolitic domes of the post-collapse caldera phase. Our generalized conceptual model for developing a hydrothermal ore deposit related to a graben type caldera is based upon three key factors, 1) the volcano-tectonic structure: a graben caldera and associated collapse faults, 2) the heat source: silicic volcanism and corresponding shallow subcaldera magma chamber, and 3) the transporting agent: infiltrated meteoric water.