Two Bureau of Geology scientists, in collaboration with scientists at the United State Geological Survey, have discovered similarities between ground water systems that formed ore deposits 10 million years ago and modern ground water in the Rio Grande Rift. They reported their work in an invited presentation at the 2000 Annual Meeting of the Geological Society of America.
Dr. Virgil Lueth, mineralogist/ economic geologist, and Lisa Peters, senior lab associate at the New Mexico Geochronological Research Lab, have been studying the mineral jarosite in ore deposits from Chihuahua, Mexico, to Albuquerque.
Using 40Ar/39Ar geochronology, the age of the mineral can be determined. The composition of the mineral is favorable for stable isotope analysis of oxygen, hydrogen and sulfur. "Using stable isotopes we can infer the origin of sulfur, the origin of the water(s) and the temperature of formation. In addition, we can infer some climate features at the time of mineral formation. Similarly, we can also look at the climate record during the time of weathering of the deposit." Lueth explains.
The mixing of two waters, one geothermal and the other ground water, appears to be responsible for the formation of the ore deposits over the last 10 million years. This mixing occurs periodically and correlates to wet climatic periods in the southwest. Interestingly, the isotopic compositions of the waters inferred to form the deposits are very similar to modern geothermal and ground water found in the rift today. "Mineralization is happening in some places now."
The mixing of the waters that formed the ore deposits is also responsible for water quality degradation in the rift today. Hot geothermal waters, carrying fluorine, sulfur and other elements (including arsenic) are mixing with near surface drinking water and lowering its quality. "By understanding the processes that operated in the past, we can anticipate water quality problems in this rapidly expanding population corridor," Lueth points out.
Finally, by studying the weathering history of the ore deposits, the scientists can calibrate the climate record for the last 10 million years. "Our results compare favorably with other studies of climate during the same time period," Lueth says. "It appears that there has been a general warming and drying of the climate over that time punctuated by wet periods." Research is continuing to refine the climate record of southern New Mexico using ore deposits as climatic "record keepers."