Polyphase speleogenesis in Lick Creek Cave, Little Belt Mountains, Montana, USA

Abstract

Lick Creek Cave in northern Montana (USA) is hosted in limestones of the Lower Carboniferous Madison Group near Tiger Butte, an Eocene quartz–syenite porphyry intrusive dome. The cave is located within the zone of contact metamorphism of the dome, which crops out ∼ 300 m from the cave entrance. The cave consists of two genetically distinct cave systems separated by a fracture zone: (1) a ∼ 80 × 50 m dome-shaped cavern in breccias of a Carboniferous paleocave, and (2) anastomosing conduits ∼ 2–10 m across, parallel to the bedding of the Madison Group and extending 100 m up dip to the present cave entrance. The conduits are further subdivided into a tectonised and a maze zone and are variably decorated in several combinations by phreatic isopachous calcite spar cements, with crystals up to several cm long, and with vadose speleothems, including stalactite–stalagmite pairs, flowstone, corallite (cave popcorn), and moonmilk. Our database is comprised of field survey, thin section, XRD, and SEM observations along with 118 δ 18O/ δ 13C analyses and 27 87Sr/ 86Sr measurements from samples of county rock and speleothems. The limestone matrix samples with the heaviest δ 18O/ δ 13C ratios are interpreted as the least recrystallised proxy to Tournaisian seawater. Stable isotope data from other Carboniferous limestones, including paleocave breccias, follow a regional meteoric pathway established elsewhere in the Madison for the Late Carboniferous transition from greenhouse to icehouse conditions. Isopachous calcite spar cements from the conduit zone are interpreted as the result of late-stage, Eocene hydrothermal fluid circulation. Stalactite–stalagmite pairs, flowstone, corallite, and moonmilk carry a signature similar to modern or Quaternary high-alpine meteoric water. Previous workers have determined separate hydrothermal and meteoric δ 18O/ δ 13C stable isotope fields for speleothems in caves in Carboniferous limestones from the Black Hills, South Dakota. We re-define the stable isotope ranges for meteoric and magmatic–hydrothermal calcites based on a comparison of stable isotope data from the Little Belt Mountains with those from the Black Hills. We further propose that the hydrothermal calcite end-member δ 18O composition is around −20‰ PDB, represented by the lowest oxygen isotope values from all data sets, with a corresponding δ 13C of about −7‰ PDB. Sr-isotope data from speleothems, Carboniferous limestone wall rocks, and from the igneous intrusion itself support the interpretation of an Eocene hydrothermal speleogenic event. The integration of petrographic and geochemical data shows that Lick Creek Cave is the result of polyphase speleogenesis in three major episodes: (1) Middle to Late Carboniferous, (2) Eocene, and (3) (sub-)Recent to Recent. The Carboniferous and (sub-)Recent to Recent speleogenesis appear epigenic, i.e., driven by surface-derived waters, whereas the Eocene event was hypogenic, i.e., driven by ascending hydrothermal waters. Each of the three major speleogenic events probably consisted of two or more distinct “phases”, but our database does not permit these phases to be resolved with certainty.