The oldest sequence exposed in the study area is a basal reef limestone, up to 880 m thick, locally known as the Aurora Formation (Lower Cretaceous). This unit is overlain by the Washita Group (Albian/Cenomanian), of around 130 m thick, that is subdivided into three formations: (1) Georgetown Formation (40 m of mudstone-wackestone limestone); (2) Del Rio Formation (10 m of thin-bedded shale); and (3) Buda Formation (impure limestone-fossiliferous wackestone-packstone). The Laramide Orogeny affects the entire sedimentary sequence, and developed two major folding styles: (1) asymmetrical and elongated anticline folds; and (2) domal anticlines with steep flanks.

La Encantada-Buenavista, Aurora, and Corea fluorite low temperature, stratabound, carbonate-hosted fluorite mantos are the main high tonnage deposits mined in the district (Fig. 1). The main fluorite anomaly trend spans 215 km² following the argillite/limestone contact of Del Rio/Buda-Aurora formations, with an average thickness of 2 m. There is a strong structural control on the distribution of fluorite bodies. They are found at the intersection of low permeability barriers with low angle faults. Fluorite appears as white to colorless, corrosion-free crystals indicative of a passive precipitation mechanism. Late cavities are filled by idiomorphic, cubic, zoned fluorite crystals with deep-purple outer zones, minor idiomorphic calcite scalenohedrons, and extremely rare barite crystals (Tritlla et al., 2007).

The Oligocene volcanism in the area is of effusive style, with abundant rhyolite necks, in contrast to the predominantly explosive style found at the Sierra Madre Occidental Volcanic Province. The Oligocene volcanics are represented by high-silica, peraluminous rhyolites, showing strong enrichment in fluorine and in some incompatible lithophile elements (Rb, La, Sm, Yb, Y, Th, U, Nb, Ta), and strong depletion in the feldspar-compatible elements Sr, Ba and Eu. This contrasts with the high-K rhyolitic rocks of the eastern Sierra Madre Occidental. Both volcanism styles and the particular chemistry are explained by a high rate of crustal extension that promoted crustal melting at high melting/segregation rates, and rapid ascent of low-viscosity fluorine-rich magmas, which inhibited melt stagnation in magma chambers (Orozco-Esquivel et al., 2002).

The emplacement of the felsic magmatic bodies, usually as volcanic dome-shaped structures and necks, are closely related to both the fault system and the fold axes. The El Pilote rhyolite is the only known fluorite-enriched volcanic neck along the entire rhyolitic intrusive trend (S. Baca, 2004, pers. com.). The El Pilote rhyolite intrudes the entire sedimentary sequence, including the fluorite mantos of the La Encantada district, and is also responsible for a subvertical metasomatic halo affecting the Aurora Formation and the Washita Group. The Tertiary magmatic felsic event produces a fluid inclusion decrepitation halo in the fluorite when the rhyolite bodies crosscut the stratabound fluorite mineralization (Tritlla et al., 2007).

The El Pilote rhyolite neck is characterized by a well-defined, concentric magmatic foliation, with aphanitic texture, corresponding to a hypo-abyssal body emplaced at near surface conditions. This magmatic body is composed by phenocrysts of K-feldspar, plagioclase and quartz, with a banded, whitish glassy matrix. Neither fluorite nor topaz was found as a free mineral within the matrix.

Whole rock and fluorite samples were analyzed for major and trace elements at the Service d’Analyse des Roches et des Matériaux of the CRPG-CNRS at Nancy (France). The rhyolitic composition has been confirmed by whole rock chemical analysis (SiO₂ = 76.8%; K₂O = 4.4%; and K₂O/Na₂O = 1.2). The F-saturation in the melt was determined for a temperature of 850 ± 15 °C (Scaillet and Macdonald, 2004), the latter calculated with the zircon saturation thermometer (Hanchar and Watson, 2003). Fluorine saturation is then estimated around 0.26%.

Chondrite-normalized REE composition of the El Pilote rhyolite shows a flat pattern with a (La/Yb)_N = 3.32, a pronounced depletion in Eu (Eu/Eu* = 0.06) and beryllium enrichment identical to anomalous (F-Li-Be-rich) rhyolites already described in Mexico (Orozco-Esquivel et al., 2002).

Zircons recovered from the El Pilote intrusion were carefully selected for dating. Analyses were carried out at the University of Arizona (V.A. Valencia, analyst) using a LA-ICPMS Micromass Isoprobe. Analyzed zircons were small (< 100 microns) and prismatic or needle-shaped. They show a regular magmatic zoning and have no inherited cores or inclusions. Nineteen analyses provided ²⁰⁶Pb/²³⁸U ages dispersed between 27.7 ± 0.8 Ma and 29.6 ± 1.1 Ma (Fig. 2; Table 1). The weighted mean crystallization age of zircons from the El Pilote rhyolitic intrusion is calculated according to Ludwig (2003) at 28.4 ± 0.4 Ma (n = 19; MSWD = 1.11).

The fluorite mineralization at El Pilote occurs in a skarn developed vertically around the margins of the rhyolite neck. The endoskarn is poorly represented (a few centimeters wide), whereas the exoskarn formed within the host limestones and reaches up to 200 m in thickness. Hydrothermal alteration is controlled by north-south trending faults, bedding planes, and reactive rocks of the Aurora and Buda formations.

This skarn is made up by a lateral succession of: (1) an inner rhyolite neck; (2) an endoskarn composed of zoned garnet (from Adr₁₀₀ to Adr₂₀Grs₇₀Py₁₀), wollastonite (Wo₁₀₀ to Wo₅₀En₅₀), calcite, and green fluorite; (3) an exoskarn, several hundred meters thick, composed mainly by recrystallized calcite, proximal green and distal white fluorite, pyroxenoid (Wo₅₀En₅₀ to Wo₅₀En₃₀Fs_20; Fig. 3), garnet (from Adr₁₀₀ to Adr₂₀Grs_80; Fig. 3), spurrite ((Ca₅ SiO₄)₂(CO₃)), rankinite (Ca₃Si₂O₇), albite, tremolite, Mn-calcite, and ilvaite (CaFe₃O Si₂O₇ (OH)); and (4) an outer marble aureole, consisting of a foliated, manganiferous marble (with braunite and hausmannite) in which the mineralogical zoning is roughly parallel to the rhyolite contact, progressively grading outwards to the well preserved reefal limestone. This marble contains irregular and small patches of garnet, possibly reflecting original compositional inhomogeneities within the limestone.

The skarn system (endoskarn and exoskarn) is crosscut by a retrograde assemblage following fractures, represented by a limited propylitic alteration, with tremolite, sericite, quartz, calcite and a swarm of fluorite veins (Fig. 3). Purple-colored, retrograde fluorite occurs mainly enclosed within the rhyolite neck (with very minor quantities in the exoskarn) while white fluorite is solely present as a retrograde phase within the exoskarn. Sulfides have been detected neither within the prograde metasomatic event nor in the retrograde alteration episode.

One remarkable characteristic of this skarn is that fluorite only appears as minute octahedral crystals, both in the prograde (green) and retrograde (purple and white) events, lacking more complex (dodecahedrons) or usual (cubic) crystal shapes. Fluorite octahedron faces at El Pilote always have very rough surfaces, indicative of very rapid, almost skeletal growth. Fluorite octahedrons (skeletal growth) are characteristically found in the early stages of formation in hydrothermal ore deposits, while dodecahedrons are relatively frequent intermediate shapes and cubes are the main, late crystal forms (Glikin, 2009).

A fluid inclusion study was carried out on both the prograde (green fluorite) and retrograde (white and purple fluorite) alteration events. Fluid inclusion microthermometric results are presented in Figs. 4 and 5 and Table 2. Twenty double-polished fluorite wafers were selected for microthermometric analysis (n = 174). These were performed using a Linkam THMSG-600 heating-freezing stage, calibrated with synthetic fluid inclusions, the triple point of pure water (0.0 °C), the triple point of CO₂ in pure CO₂-bearing fluid inclusions (−56.6 °C), and chemicals of known melting point from the Merck Corporation. Accuracy is estimated at about ±0.2 °C for cooling runs and ± 2 °C for heating runs.

The wafers used for the fluid inclusion work were obtained for every single fluorite generation found, both prograde and retrograde, including all the colors observed. Moreover, special care was taken in selecting defect-free fluorite crystals, with no visible evidences of deformation. In all the selected samples fluid inclusions assemblages are conformed by primary fluid inclusions, either isolated or along growth zones, as well as pseudosecondary and secondary fluid inclusions trapped along fracture planes.

6.1 Fluid inclusion types and distribution

6.2 Fluid inclusions in retrograde fluorite

The El Pilote rhyolite chemistry is similar to the topaz-bearing rhyolites previously described in Mexico (Orozco-Esquivel et al., 2002) and in the USA, (Bodnar, 1993; Christiansen et al., 1983; Price and Henry, 1984), although neither free topaz nor free fluorspar were observed as rock-forming minerals.

The U-Pb age determined for the El Pilote rhyolite appears to be in good agreement with both the crosscutting relationships seen in the field and with the previous reported ages of comparable rhyolite intrusions related to F-anomalies in Coahuila (Cantegrel and Robin, 1979) and in northern Mexico (San Miguelito and Chinchindaro, K-Ar on biotite (Cantegrel and Robin, 1979); El Realito, El Refugio, K-Ar, whole rock (Labarthe-Hernandez et al., 1982; Tuta et al., 1988); Zapote, Cantera, Cuatralba, K-Ar on sanidine, (Orozco-Esquivel et al., 2002)). The results of U-Pb dating on zircon constrain both the age of the emplacement of the Pilote rhyolitic neck (28.4 ± 0.4 Ma) and, indirectly, the hydrothermal alteration related system.

The octahedral crystal habit of fluorite is influenced both by its internal structure and the conditions prevailing during the growth processes, as temperature, pH value, Ca/CO₂ concentration of the solutions and fluorine super-saturation (Zidarova, 1978, 1989), as well as the growth velocity and nucleation (Kostov, 2002). The presence of octahedral crystals during all process (from prograde green fluorite to retrograde white fluorite) suggests a continuum of the physico/chemical conditions.

Fluid inclusions trapped in prograde fluorite records a complex fluid history. Primary high temperature type 4 and, possibly, type 2 fluid inclusions trapped in prograde fluorite are interpreted to represent aliquots of a rhyolite-equilibrated fluid, regardless of its origin (either orthomagmatic, meteoric or a mixture). The emplacement of the subvolcanic rhyolite neck furnished the heat needed for the hydrothermal system to operate. Boiling, recorded in prograde fluorite, was triggered by both the heat supplied and the decompression of the whole system during magma ascent, with subsequent fluid mixing with surficial waters. Flashing during boiling originated types 1 (vapor-rich) and types 3 (with trapped minerals) and 4 (NaCl and KCl-bearing, with trapped minerals) fluid inclusions to form, as fluorite precipitated trapping heterogeneous fluids. Boiling accounts for a super-saturation and rapid growth of skeletal fluorite crystals, prevailing the octahedral facies over more evolved crystals facies (dodecahedrons, cubes), as discussed above.

Fluid inclusions in late, retrograde octahedral purple and white fluorite present a path towards low temperature and (NaCl-dominated) salinity, suggesting the influx of meteoric water, still scavenging small quantities of fluor from the rhyolite zone towards the outer limestone, during the waning of the hydrothermal system.

The source of fluor remains problematic. Two possible fluor origins can be invoked: (1) a preconcentration of fluor as a volatile due to a very-low degree of melting of the parent rock, prior to its ascent and emplacement, giving rise to a volatile-rich, rhyolitic melt (Christiansen et al., 1983; Price, 2004; Price and Henry, 1984); (2) the remobilization of fluor from the regional rocks (and probably the close fluorite mantos neerby) during the melt ascent, until the PTX conditions of fluorite precipitation are reached (Richardson and Holland, 1979).

The El Pilote rhyolitic intrusion presents a very low F⁻ concentration (less than 0.3%) compared to the F⁻ rich volcanic or haplogranite system at similar temperature (from 0.8 to 3.6 wt% CaF₂ (Dolejš and Baker, 2006)). Hence, the F low concentration in the melt suggests that an external source must be invoked. Based on the knowledge of the local geology, the low F⁻ saturation of the rhyolite, the fluid temperature and chemistry of the paleofluids, the only plausible source for the F appears to be the locally available stratabound fluorite mantos. The predominance of octahedral shapes in skarn fluorite, suggests that fluorite formed very rapidly in a very short time. The geological evidences place the stratabound carbonate-hosted fluorite deposits almost at the same stratigraphical level than the rhyolite neck. Hence, we suggest that a remobilization between the rhyolite intrusion and the preexisting fluorite mantos was the main mechanism that originated such an unusual fluorite concentration.

El Pilote fluorite deposit was generated by the intrusion of a rhyolite body at 28.4 ± 0.4 Ma, that developed an infiltrative calcic skarn, with a reduced endoskarn, and a well developed exoskarn zone. The dating of the emplacement of the El Pilote rhyolite indirectly dates the hydrothermal system.

The well-developed exoskarn shows a metasomatic column with inner garnet and outer prograde (green) fluorite passing to a highly recrystallized carbonate (marble). A widespread, mineralized stockwork represents the retrograde event with purple-colored fluorite exclusively present within and around the rhyolitic neck; white fluorite appears as a retrograde distal phase within the stockwork affecting the carbonates. In all cases, minute octahedral fluorite is dominant, suggesting a very rapid (skeletal) crystal growth.

Fluid inclusion data gathered on prograde fluorite indicates the dominance of hot (170 to > 400 °C) and saline to hypersaline fluids in this stage (from 6.3 to 32.5 wt% eq. NaCl). After the fluid inclusions assemblages found, boiling is thought to be the main mechanism of fluid evolution and fluorite nucleation and precipitation. This is mineralogically supported by the sole presence of fluorite octahedra throughout the prograde stage. The fluids in retrograde fluorite evolved progressively towards lower temperatures (from 182 to 79 °C) and salinities (from 15.7 to 0.1 wt% eq. NaCl). All this fluid data depicts a coherent scenario where firstly magmatic-equilibrated fluids boiled off and progressively mixed up with low-temperature, low-salinity meteoric fluids, until the cessation of the hydrothermal fluid circulation and mineralization.