Table 2 Summary of petrographic features for the reservoir core samples

Caldera group

353 m

Xáltipan ignimbrite (H26-1; Fig. 4a-b): beige to light gray vitroclastic lapilli tuff with white to blackish phenocrysts, pumice, marble and lava fragments with a size of up to 5 × 15 mm; unaltered feldspars, lithic clasts, biotite; elongated glass fibers indicate a low grade of welding, some oxides were observed in altered clasts that most likely originated from greater depth

weak

none–argillic

633–673 m

Ignimbrite/andesite breccia: H23-1: beige to gray vitrolithic lapilli tuff to tuffaceous breccia with gray to blackish lava and marble clasts as well as gray, vesicular, porphyritic andesitic lava; overall fresh appearance, H22-1 (Fig. 4c-d): vitric tuff clasts and reddish-gray lava with large phenocrysts; fractured, moderate alteration, abundant hematite, calcite, quartz

weak–moderate

argillic

Pre-caldera group

796–1400 m

Mafic, andesitic, trachyandesite, and rhyolitic lavas: highly variable appearance, texture, alteration intensity, and facies; predominantly argillic alteration, abundant calcite, quartz, chlorite, occasionally epidote; rhyolites (H19-1, 981 m; Fig. 4i-j): moderate alteration along fractures, xenomorphic to idiomorphic quartz, chlorite, and calcite are filling the fractures, while the remaining matrix consists of very fine-crystalline felsic assemblage of quartz + feldspars; trachyandesites (H24-1, 1008 m; H26-2, 1200 m; Fig. 4g-h): moderate alteration, groundmass preserved, calcite, quartz, phenocrysts partially altered

weak–strong

argillic–propylitic

1500–2495 m

Basaltic to andesitic lavas: gray, greenish, yellow color; epidote, chlorite, albite, K-feldspar, quartz, calcite, pyrite, occasionally magnetite, hematite, titanite, Ti-Fe-oxides and in some samples clinozoisite, groundmass: plagioclase and K-feldspar often pervasively replaced by chlorite and epidote; some samples contain altered lava fragments that most likely originated from greater depth; H38-4 (1950 m; Fig. 6d–g): andradite garnet, acicular wollastonite and blocky calcite in fractures, relicts of amphibole, mineral inclusions in garnet contain diopside-augite, calcite, pyrite, and wollastonite

strong

propylitic

 ~ 2000 m

Silicified lavas (H26-4, H28-2): white–gray color, fractured, brecciated; H26-4 (Fig. 5m–p): microcrystalline groundmass (quartz) with remains of phenocrysts, plagioclase, locally mica, chlorite, and radial biotite filling secondary pores; H28-2: mixture of clay minerals (illite–smectite) and quartz, some scattered apatite and relicts of feldspars rich in Na; partial "bleaching" also observed in H39-2 (1650 m; Fig. 6a-c) and H24-3 (2300 m)

strong

silicification

2844–2900 m

Skarn (H24-4, H18-4): white–gray color, H18-4 (Fig. 6h-i): calcite, quartz, apatite, possibly acicular wollastonite, very small garnet crystals; H24-4: calcite, quartz, clays and actinolite, in some parts protolith still identifiable

strong

skarn

Pre-volcanic basement

1970–2414 m

Marble (H13-3, H15-4, H7-x): light to medium gray, coarse crystalline calcite matrix; very brittle due to numerous calcite-filled fractures and veins (< 200 µm to a few cm wide) composed of coarse interlocking calcite crystals (up to 1 mm); thinner veins in H13-3 (Fig. 6j-k) and H15-4 contain wollastonite, grossular and subordinate diopside, apatite, and pyrite in the central parts and interstitially intergrown with coarse calcite, occasionally interstitial wollastonite and grossular within the calcite matrix; H15-4: andradite garnets, epidote, and polymetallic sulfides in metasomatic veins; foraminifera-rich limestone identifiable, no complete alteration to marble

strong

marble