Magmatic Differentiation, Magmatic–Hydrothermal Evolution, and Hydrothermal Alteration in Felsic Dikes: Insights from Intrusions in Ultramafic Rocks, Ronda Massif (Spain)

During the emplacement of granitic dikes, the magmatic–hydrothermal transition and subsequent hydrothermal alterations have the potential to deeply modify the texture, mineralogy, and geochemistry of felsic magmas. In order to characterize these transformations, we investigated the Miocene granitic dikes intruding the Ronda ultramafic massif, Betic Chain, Andalusia, Spain. There, the significant chemical contrast between the dikes and the ultramafic host rocks allowed the distinction of the signature of external fluids from that of internally derived ones. The overprint of the fluid produced during the serpentinization of the peridotite was confined to the permeable margins of the dikes, forming a Co-, Cu-, and Ni-rich rodingite. The center of the dikes was left free from alterations caused by external fluids. Differentiation en route to the surface occurred concurrently with hydrothermal alteration, as evidenced by the mineralogical and geochemical footprints of the rocks. We propose a model of dike evolution where aqueous fluids were exsolved during both magma ascent and magma crystallization. Auto-metasomatism occurred in the overlying differentiating granitic magma. Channeling of the fluids occurred along the permeable outer dike margins, leaving the upper dike section poorly altered. Along the margins, the exsolved fluids were mixed with serpentinization-derived fluids. As a result of alteration, a finite vertical zonation formed in the dike, producing poorly altered, highly differentiated rocks in the upper part of the dikes and highly altered, poorly differentiated rocks below. Additionally, we found that the less altered the dike samples were, the more Sn-W-rich they were. The most altered ones were barren. This underscores the importance of implementing an adequate sampling strategy within such felsic dike swarms in order to accurately estimate their metallic potential, which might otherwise be underestimated if exclusively highly altered rocks are sampled. In addition, the utilization of geochemistry to constrain the tectonic context of partial melting and magma emplacement is hindered by the global fluid overprint observed in such dikes.