Alteration in the Madera limestone and Sandia formation from core hole VC‐1 Valles Caldera, New Mexico

Abstract

Core hole VC‐1 penetrated the southwestern ring fracture zone of the 1.1 Ma Valles caldera and at a depth of 333 m intersected the top of the Paleozoic section including the Abo Formation, Madera Limestone, and Sandia Formation, reaching a total depth of 856 m. The Paleozoic rocks, which consist of thin‐bedded limestone, siltstone, mudstone, sandstone, and local conglomerate, are overlain by volcanic rocks of the caldera moat that are less than 0.6 Ma. Diagenetic and at least three hydrothermal alteration stages were identified in the Madera Limestone and Sandia Formation. Diagenetic clay alteration was pervasive throughout the sedimentary rocks. Volcanic activity at 16.5 Ma and continuing through the formation of the Valles caldera resulted in high thermal gradients, which caused recrystallization of diagenetic clay minerals. Interstratified smectite‐illite is the most diagnostic clay mineral throughout the section; structurally, the illite component in the ordered interstratified illite‐smectite changes gradationally from 70% at the top of the Madera Limestone to 95% at the base of the section in the Sandia Formation. Pyrite that occurs as small clots and lenses as well as finely disseminated is interpreted as being of diagenetic origin, especially in organic‐rich beds. Low permeability of much of the Paleozoic section precluded the deposition of hydrothermal minerals except in fractures and intergranular space in some of the more permeable sandstone and brecciated horizons. Three stages of hydrothermal mineral deposition are defined. Stage I is widespread and includes mainly chlorite, calcite, pyrite, and interstratified smectite‐illite that was formed prior to caldera development at temperatures approximating 200°C. Stage II is characterized by quartz, sericite, and scarce sulfides deposited locally by fluids at approximately 275°C shortly before or at the time of early caldera‐related volcanism. The Stage III hydrothermal event, associated with formation of the Valles caldera, caused complex brecciation accompanied by deposition of phengite and molybdenite at temperatures locally approaching 300°C mostly in rocks underlying the Madera Limestone. During the present drilling program, thermal aquifers were encountered at 483 m (113°C), 533 m (125°C), and 845 m (184°C), very close to the Stage II and III mineralized zones; however, measured temperatures of the present hydrothermal system are much lower than during the main hydrothermal events. Good permeability has been maintained in these three levels for a long time while the rest of the Madera Limestone and Sandia Formation was sealed by hydrothermal mineral precipitation.