Original paper

Genesis and evolution of the caves in the Naica Mine (Chihuahua, Mexico)

Forti, Paolo

Abstract

The Naica caves and their large gypsum crystals were discovered almost a century ago, but only recently four new caves were unveiled, the largest of which (Cueva de los Cristales) hosts by far the largest gypsum crystals of the world. Until now studies were focused on the mechanisms allowing for the development of the giant crystals while the genesis and the evolutionary steps of the Naica caves were completely ignored. The aim of the present paper is to close this gap. The thermal fluids responsible, since 25 Myr ago, of the evolution of the Naica ore bodies were always characterized by net deposition, therefore the permeability of the hosting formation was reduced and no karst voids had the possibility to develop. 1-2 Myr ago tectonic stresses partially displaced the ore bodies giving rise to open joints and fractures closely related to three main faults, which still now control the uplift of the thermal fluids. Since then the thermal waters became aggressive with respect to carbonate formations, thus small cavities had the possibility to develop at different levels inside the aquifer. This process lasted for a relatively short time interval as testified by the clear tectonic shape maintained by all these cavities. Scarcity of corrosional features did not allow for a detailed reconstruction of the cave evolution, which was in turn achieved by a multidisciplinary analysis of the thick chemical deposits hosted inside each cave. The evolutionary steps were several and complex, related to different speleogenetic mechanisms. Although they were always controlled by the presence of the thermal aquifer, the resulting evolution was somewhat different from cave to cave, reflecting alternately deep-seated phreatic, epi-phreatic and vadose environments. From this point of view the most interesting cave is Cueva de las Espadas, the evolution of which was characterized by several changes between these three environments through time, while the deeper caves suddenly changed from deep phreatic to vadose when the mine dewatering lowered the groundwater below the -290 m-level some 20 years ago. The anthropogenic processes induced by the mine depression cone had relevant consequences on the cave development giving rise to the evolution of several new diagenetic minerals (over 35 minerals - mainly oxides-hydroxides, but also sulphates and silicates - are actually forming, some of them new for the cavern environment) but also greatly enhancing the condensation corrosion and dissolution processes which, in a few years, will be responsible not only for the damage of the giant gypsum crystals but also for their complete destruction. For this reason one of the main tasks for the scientists involved in the study of the Naica caves is to search for the best way to preserve, at least partially, this incredible underground world.

Keywords

speleogenesishypogene cavescave mineralsmexico