The development and fracturing of plutonic apexes: implications for porphyry ore deposits

Porphyry ore deposits are generally located above plutonic apexes, described as finger-like extrusions from a large underlying silicic magma chamber. Fractures and faults that concentrate around these shallow structures allow mineral-enriched hydrothermal and magmatic fluids to circulate and exchange heat and mass with the host rock. Plutonic apexes, however, are not necessarily mineralized. The physical mechanisms invoked for their development and fracturing are focused on the role of volatile pressure, and we have no clear explanation on the associated thermo-mechanical processes. Here we present (a) a semi-quantitative scenario to explain how significant relief could form at the magma chamber roof to give apexes frozen within the shallow crust, and (b) the results of our numerical modeling of fracturing at plutonic apexes. We suggest that morphologic instabilities, expressed by two-directional corrugations (crests and troughs) at the crystallizing roof of the magma chamber, could arise at the top of large silicic batholiths as a result of thermo-mechanical interactions between the reservoir and its surroundings. The corrugated roofs could form with local apexes several kilometers high. Given that a local extensional tectonic regime would surround such systems, crystallization of the apexes would promote a concentration of fractures and faults in their vicinity. In modeling the thermo-mechanical regime around a plutonic apex to show how fractures and faults could develop, we tested different values for temperature contrasts, extension rates and magma viscosity. Two main regimes can be identified, depending on the rheological contrast between the magma and its host rock: the one, a single thick fault connecting the apex to the surface (analogous to a breccia pipe), and the other a network of fractures surrounding the apex (analogous to a stockwork). Where two apexes are close together, one will cluster the shear stresses, regardless of its vertical extension, and thus only a single fracture will develop. We thus infer that barren apexes can be located near mineralized apexes if the distance between them is no greater than the thickness of the brittle layer, which in turn is highly dependent on local thermal and mechanical conditions.