New Mexico Mineral Symposium — Abstracts

Introduction
Gypsum crystals of extraordinary size were discovered a decade ago in cave chambers intersected by mining activity at the Naica mine (Industries Peñoles) in Chihuahua, Mexico (Garcia-Ruiz et al. 2007). The cave chambers are genetically associated with a hydrothermal base metal deposit that is the source of the ore. The system is hot, ranging from — 40-60°C (104-140°F) in the accessible areas. Large volumes of water are continually pumped out of the mine to facilitate zinc, lead, silver, and minor copper extraction.

Inclusions are numerous within the giant selenite crystals. Most contain solid materials, fluids, and sometimes gas, but microbial microfossils are visible in scanning electron micrographs of some of the inclusions. In addition, highly colored orange and black deposits on the chamber walls exhibit microbial biofabrics. Copper seams in some of the mine passages also exhibit mineral deposits reminiscent of biofabrics that we have observed in copper-bearing study sites around the world.

During February 2008 and December 2009, we collected fluid and solid samples from a wide variety of mineralogical settings within the caves and elsewhere in the mine. These samples have been analyzed with direct microscopic inspection (optical and electron), bulk chemical analysis, live microbiological culturing, and molecular DNA analysis.

Microscopy
Scanning electron microscopy was employed to examine the contents of inclusions, iron- and manganese-rich wall materials, unusual blue copper stalactites, and other deposits from the mine. Both live and fossilized materials were present in inclusions within the selenite crystals, and microbial filaments and cell bodies were observed in many of the other materials.

Molecular analysis
Samples were taken from a variety of sites at Naica for molecular analysis. These samples yielded a number of mostly uncultivated and novel microorganisms. While many of the organisms are unknown to science, we can infer clues about their metabolic capabilities and adaptations by comparing them to known genetically close relatives. Many of the closest relatives to the Naica clones are high-temperature organisms, yet others are soil organisms. Several come from high CO₂ environments and some from volcanic environments. Interestingly, several strains come from other caves in far distant parts of the world. Although these are the closest known relatives to our strains, the genetic distance is still great, so the environmental characteristics of the relatives may or may not be shared by our strains.

Live cultures
Approximately 30 cultures of live organisms were isolated from the environments studied in Naica. Characterization of these slow-growing organisms is currently underway. Mineral precipitation, catalyzed by organism cell surfaces, has been observed. Work by other colleagues on Naica samples collected at the same time as ours, have shown large viral loads (Suttle et al., pers. comm.). Viruses live only as infective agents on other organisms, so their presence is another corroboration that there is a well-developed microbial subsurface ecosystem in the Naica caves.

Crystal ages
The age of the crystals provides a constraint on the time elapsed since the microorganisms were trapped in the crystal fluid inclusions. The age of the crystals in Cueva de los Cristales, the cavern with the largest crystals, were dated by Sanna et al. (2010) at 106-260 k.y. However, the crystals may be much older than the U/Th dates indicate, due to uncertainties in the location of broken crystal sampled for that analysis. Indeed, crystal growth experiments in the mine produced calculated ages of - 400 k.y. (Sanna et al. 2010). The deepest fluid inclusion that we sampled was at a crystal depth of - 3-4 cm, which yields an approximate age of 50 k.y. since entombment. This is based on a growth rate of 1.45 mm per 1 k.y. (Lauritzen et al. 2008).

Discussion and conclusions
Live organisms and analyzable DNA have been recovered from all of the sites sampled. Preliminary microscopic examination shows a variety of microbial forms growing in close association with minerals at the wall surfaces. Fossilized biomineral-encrusted microbial forms are also present within some inclusions in the giant selenite crystals. Live cultures from the fluid inclusions suggest that the organisms have survived significant periods of time in a crystalline time-capsule and remain viable and have extractable DNA. Live organisms still growing in the wall material resemble some of the preserved inclusion organisms and presumably constitute the microbial community that was trapped during crystal growth. Trapped pollen and soil microorganisms indicate at least intermittent connectivity with the surface. Thus, organisms may serve as tracers to understand the hydrologic circulation and mixing of meteoric and deep geologic waters. Such apparent long-term persistence of microorganisms in geological materials can be useful in understanding the history and connectivity of hydrogeological systems, the potential for long duration survival of organisms, and the potential for minerals and rocks to act as repositories of biodiversity.

References:

1. Garcia-Ruiz, J. M., Villasuso, R., Ayora, C., Canals, A., and Otalora, F., 2007, Formation of natural gypsum megacrystals in Naica, Mexico: Geology, v. 35, no. 4, pp. 327-330.
2. Garofalo, P. S., Fricker, M. B., Gunther, D., Forti, P., Mercuri, A.-M., Loret, M., and Capaccioni, B., 2010, Climatic control on the growth of gigantic gypsum crystals within hypogenic caves (Naica mine, Mexico)?: Earth and Planetary Science Letters, v. 289, pp. 560-569.
3. Lauritzen, S.-E., Constantin, S., and Forti, P., 2008, Chronology and growth rate of the Naica gypsum crystals: International Congress of Geology Proceedings, Oslo, Norway.
4. Sauna, L., Saez, F., Simonsen, S., Constantin, S., Calaforra, J. M., Forti, P., and Lauritzen, S.-E., 2010, Uranium-series dating of gypsum speleothems: methodology and examples: International Journal of Speleology, v. 39, no. 1, pp. 35-46.