Geologists map Cartwright country

’Big Bonanza’ and the Comstock Lode

Remember the burning Ponderosa map at the beginning of the long-running TV show “Bonanza”? It’s up in flames before you can read all the place names.
Now a geologist at Washington University in St. Louis has replaced that map with one of the famous ore site known as the Comstock Lode, a part of which is the “Big Bonanza.”

While it’s doubtful that Hoss, Adam and Little Joe – not to mention the sages, Pa and Hop Sing – could make heads nor tails of it, the map is a valuable contribution to geology because it gives an interpretation of the flow of hot waters interacting with rock some 14 million years ago that created the ore district. Between 1859 and 1882, the Comstock Lode produced gold and silver in such quantities that the bullion would be worth several billion dollars in today’s markets.

Robert Criss, Ph.D., professor of earth and planetary sciences in Arts & Sciences, and his former graduate student Michael J. Singleton, Ph.D., now at Lawrence Berkeley National Laboratory, analyzed 327 rock samples collected from a portion of the Comstock Lode as well as historical samples and ones from the Smithsonian Institute and “visualized” a kind of symmetrical flow. They were able to determine the flow thanks to a mathematical technique called kriging that allows computer contouring of oxygen isotope data gleaned from the rock samples.

When water and rock interact in ore deposition they exchange isotopes. Isotopes are different variations of the same element. There are three oxygen isotopes, oxygen-16, -17 and -18. All three behave chemically as oxygen, differing only in their mass. Most is oxygen-16, but about one oxygen atom in 500 is oxygen-18, and only one in about 2,500 is oxygen-17. Rocks are about 50 percent oxygen by weight, water 90 percent. The exchange of isotopes – the researchers measured O-16 and O-18– creates “patterns of disturbance” in the rock, which the researchers can map by combining a lot of field work with lab analysis and computing.

“We can map and interpret these patterns long after the disturbance happened -– 12 to 14 million years ago” said Criss. ” The rocks preserve a record of what happened.”

Criss said the hydrothermal flow geometry that created the ore district was a longitudinal roll pattern superimposed on a unicellular flow system. Think of the longitudinal rolls as two parallel tubes and the unicellular system as a flat roll. The map is the first evidence showing the longitudinal roll pattern occurring in nature. The system had been predicted by theory but never seen before in an ore district.

“We’ve shown that these modes of convection can occur on Earth under the right circumstances,” Criss said. “It’s the first description of such symmetry in an ore district. The ore body positions have an obvious relationship to these rolls. ”

The research was published in the April issue of the Journal of Geophysical Research. It was supported by funding from the National Science Foundation.

The finding is important for geologists to understand the creation of ore deposits. These events occur underground and must be analyzed remotely. And it could have economic implications.

“It’s possible, under perfect conditions, to understand currents of fluid that make ore bodies,” Criss said. “If this could become part of a predictive tool to locate currents that form ore bodies, that would be a valuable outcome because we don’t have very good theories on how ore bodies are formed. It’s a very peculiar process.”