The alteration of Miraflores Basalt (Panama): Mineralogical and textural evolution

Samples of The Miraflores Basalt sub-volcanic rocks (Miocene Late Basalt Formation) collected from different quarries on the Isthmus of Panama and several samples collected from the excavation of the third set of locks in the New Panama Channel were characterised in this study. All rocks studied had similar petrological and mineralogical characteristics; however, the samples had substantially different degrees of alteration, which varied in the NW–SE direction. The most altered areas were located at the SE (Sosa Hill Quarry), next to the Pacific Ocean, while the Cerro Escobar rocks, located further inland, showed only slight alteration. The rocks from the excavation and from the Cocolí and Aguadulce Hills area, located in the middle of the study region, exhibit features consistent with intermediate alteration between the two aforementioned extreme cases. Images and data from different alteration stages were obtained using optical microscopy, electron microprobe (EMP), and scanning electron microscopy (SEM). The earliest stage of alteration was characterised by the presence of iddingsite, which was almost the only product of alteration. As the alteration progressed, the generalisation of smectites throughout the rock was characteristic. The glass disappeared and transformed to palagonite, and the plagioclase and pyroxene crystals became altered both at the edges and inside the crystals. The crystals also showed numerous nanofractures, which were mainly perpendicular to the longer faces and that were filled by smectite. In the most advanced alteration state, all crystals and glass were deeply affected. Crystals were deeply transformed into smectite, and the porosity of the rock increased in a process that conserved the volume. Smectite appeared as the end-alteration product both from the major minerals (olivine, plagioclase, pyroxenes), as well as from the glass.