The Pliocene Ixtacamaxtitlán low sulfidation epithermal deposit (Puebla, Mexico): A case of fossil fungi consortia in a steam-heated environment

The Ixtacamaxtitlán area in northern Puebla (central Mexico) contains middle Miocene Cu-Mo-Au porphyry/skarn and Pliocene low-sulfidation Au-Ag epithermal deposits that are geologically associated with the evolution of the Trans-Mexican Volcanic Belt (TMVB). In this paper, a new 40Ar/39Ar age (2.87 ± 0.41 Ma) is provided for rhombohedral alunite from a kaolinite + alunite ± opal ± cristobalite ± smectite advanced argillic alteration assemblage. This age contributes to the definition of a metallogenic province that is confined to the TMVB, a relevant feature for regional exploration. A ~12 My gap is established between the formation of the Cu-Mo-Au porphyry/skarn and low-sulfidation Au-Ag epithermal deposits, which rules out the possibility that their overlapping was the result of telescoping. Advanced argillic alteration is conspicuous throughout the mineralized area. This alteration assemblage consists of a widespread kaolinite-rich blanket that underlies silica sinters, polymictic hydrothermal breccias, and an alunite-rich spongy layer that consists of vertical tubular structures that are interpreted as the result of gas venting in a subaerial environment. The above indicate a shallow hypogene origin for the advanced argillic alteration assemblage—that is, formation by the partial condensation within a phreatic paleoaquifer of acidic vapors that were boiled-off along fractures that host epithermal veins at depth. The formation of the spongy alunite layer and silica sinters is interpreted to have been synchronous. Within the alunite-rich spongy layer, tubular structures hosted microbial consortia dominated by fungi and possible prokaryote (Bacteria or Archaea) biofilms. Such consortia were developed on previously formed alunite and kaolinite and were preserved due to their replacement by opal, kaolinite, or alunite. This means that the proliferation of fungi and prokaryotes occurred during a lull in acidic gas venting during which other organisms (i.e., algae) might have also prospered. Periodic acidic gas venting is compatible with a multi-stage hydrothermal system with several boiling episodes, a feature typical of active geothermal systems and of low-sulfidation epithermal deposits. The microstructures, typical for fungi, are mycelia, hyphae with septa, anastomoses between branches, and cord-like groupings of hyphae. Possible evidence for skeletal remains of prokaryote biofilms is constituted by cobweb-like microstructures composed of <1 µm thick interwoven filaments in close association with hyphae (about 2.5 µm thick). Bioweathering of previously precipitated minerals is shown by penetrative biobrecciation due to extensive dissolution of kaolinite by mycelia and by dissolution grooves from hyphae on alunite surfaces. Such bioweathering was possibly predated by inorganically driven partial dissolution of alunite, which suggests a lull in acidic gas venting that allowed living organisms to thrive. This interpretation is sustained by the occurrence of geometrical dissolution pits in alunite covered by hyphae. Fungal bioweathering is particularly aggressive on kaolinite due to its relatively poor nutrient potential. Such delicate microstructures are not commonly preserved in the geological record. In addition, numerous chalcopyrite microcrystals or microaggregates are found within the alunite layer, which could be related to sulfate reduction due to bacterial activity from the sulfate previously released by fungal bioweathering of alunite. Hydrogeochemical modeling constrains pH to between ~3.2 and ~3.6 and temperature to between 53 and 75 °C during the stage in which fungi and other organisms thrived. These waters were cooler and more alkaline than in earlier and later stages, which were characterized dominantly by steam-heated waters. The most likely process to account for this interlude would be mixing with meteoric water or with upwelling mature water that did not undergo boiling.

An occurrence of bavenite in the Cape Granite Suite, southwestern Cape Province, South Africa, and its implication on the formation of the host pegmatite

Bavenite, (Ca4[(Al,Be)4(Si9(O,OH)26-n)](OH)2+n), is present in a pegmatite of the Paarl Pluton, a metaluminous I-type granite of Late Precambrian age. We are not aware of any other previous description of a beryllium mineral occurrence in the Cape Granite Suite. The pegmatite consists essentially of quartz and microcline microperthite together with albite, calcite and fluorite. A hydrothermal alteration assemblage of epidote, chlorite and bavenite occurs in vugs and veins within the pegmatite. Stilbite, which is stable below 170ºC, is also present, but not texturally related to the alteration assemblage. Microthermometric analyses and mineral chemistry of associated minerals elucidate the conditions of formation for the bavenite. According to primary fluid inclusions in the cores of euhedral quartz, the minimum temperature of crystallization of the pegmatite is 450ºC. Homogenization temperatures of later fluids indicate a minimum temperature of 210ºC for the main hydrothermal event. Chlorite geothermometry yields crystallization temperatures around 320ºC. The bavenite formed between 210ºC and 320ºC, at a pressure of less than 2 kbar.

Low-temperature thermal history of the Gilmore Dome area, Fairbanks Mining District, Alaska

Eight apatite and two zircon fission-track ages provide evidence of complex Tertiary thermal overprinting by hydrothermal fluids in the Gilmore Dome area. Five ages on apatite from the Fort Knox gold deposit average 41 Ma, one from the Stepovich prospect is 80 Ma, and two from Pedro Dome average 67 Ma. Elevations of these samples overlap but their ages do not, indicating that each area experienced a different thermal history.Ages of apatite from the Fort Knox gold deposit decrease with elevation from 42 to 36 Ma but have data trends indicative of complex cooling. Two ~51 Ma ages on zircon indicate that maximum temperatures approached or exceeded ~180 °C. An alteration assemblage of chalcedony + zeolite + calcite + clay in the deposit resulted from deposition by a paleo-hydrothermal system. The data suggest that the system followed a complex cooling path from > 180 to < 110 °C between 51 and 36 Ma, and that final cooling to below 60 °C occurred after ~25 Ma.The 80 Ma age from Stepovich prospect either resulted from cooling after intrusion of the underlying pluton (~90 Ma) or records postintrusion thermal overprinting sometime after ~50 Ma. The 67 Ma samples from Pedro Dome may also have experienced partial age reduction during later heating. The differences in the data from the different areas and the presence of a late alteration assemblage at Fort Knox suggest that the fluids responsible for heating were largely confined to the highly fractured and porous Fort Knox pluton.

A pseudo-tactite assemblage in the footwall of a massive sulfide occurrence, Savant Lake – Sturgeon Lake Greenstone Terrains, Ontario

A mineral assemblage consisting of calcite–tremolite–quartz–clinozoisite–diopside–muscovite has been found associated with a small, stratiform pod of galena–sphalerite–chalcopyrite–pyrite–pyrrhotite. This alteration assemblage resembles that of a skarn, although lacking, significantly, some typical minerals including garnet, wollastonite, and vesuvianite. Limits on possible condition of T, Pfluid, and [Formula: see text] can be estimated for the formation of the assemblage. The assemblage must have formed in a water-rich fluid, as [Formula: see text] is sharply restricted by the reaction 2 clinozoisite + CO2 = calcite + 3 anorthite + H2O. In Pfluid–T space, the assemblage is stable between the univariant curves for the assemblages tremolite + calcite + quartz + diopside at fixed [Formula: see text] and clinozoisite + anorthite + grossular + quartz + calcite. These curves bracket the kyanite–sillimanite boundary. As kyanite and sillimanite occur in the wall rocks, the minimum Pfluid–T are those of the aluminosilicate triple point. Measurement of sphalerite–sphalerite/galena dihedral angles in the thermally annealed area yielded a calculated temperature of 390 °C. Metamorphic conditions suggest that a sequence of progressive contact metamorphic reactions of an original talc and (or) chlorite–quartz–calcite assemblage produced the presently observed assemblage. Proximity to a later granitic batholith is believed to have brought about these changes by reactions in the original alteration mineral assemblage.