The Colorado mineral belt (CMB) is a swath of metalliferous mineral veins and faults spanning 15 to 30 miles in width and running ~250 miles in length between Dolores and Jamestown, Colorado. This NE trending zone encloses most, but not all, of the significant occurrences of gold and silver deposits found to date in Colorado.

Significant finds like the Cripple Creek district have been found outside the CMB, but these are exceptions to the trend. The large gold/silver/tellurium lode in the Cripple Creek diatreme is the result of a volcanic past that stands somewhat apart from the vein deposition processes that produced the CMB lodes.

What is especially intriguing about the CMB is that it is coincident with a significant gravity anomaly. It turns out that a particularly deep negative gravity anomaly exists in the Colorado Rocky Mountains. A few papers on this effect can be found on the web. In particular, a paper (ref 1) by Mousumi Roy at the University of New Mexico offers some details on the  extent of the gravity anomaly and some possible reasons for the effect.

At first blush it might seem odd that a negative gravity anomaly should coincide with a region known for heavy metal deposits. After all, dense matter has greater mass per unit volume, and if there is a lot of volume, then one might expect the acceleration of gravity to be a tiny bit greater than some reference value.

While this line of reasoning has merit, it turns out that despite the presence of thin metalliferous veins in the region, the overall density of rock below the CMB formation is somewhat low. A density contrast exists in the CMB formation and the surrounding rock. A large, low density formation in the crust and/or upper mantle would cause the local acceleration of gravity to be slightly below that of the reference geoid value.  The structure of the density contrast is the subject of some scrutiny and has been addressed by Roy and others.

A large low density mass below the surface is expected to have some buoyancy. A buoyant mass is one that would exert an upward distortion on the crust. The Colorado Rocky Mountains are part of a region characterized by numerous past episodes of mountain building. Whether mountain building was the result of large scale tectonic interactions or more localized effects of density contrasts, the fact remains that a gravity anomaly exists coincident with the CMB.

The mechanical effect of the upthrust of the lower members of the crust to form the Colorado Rocky Mountains has been that a series of faults and fractures have formed. These void spaces have provided networks for the flow of mineral rich hydrothermal fluids over geological time.

High pressure, high temperature aqueous fluids are prone to cooling and depressuriation as they work their way upwards into cooler and less constricted formations. At some point these fluids throw down their solutes and suspensions in the form of solids that occupy the void network. Eventually the flows become self-sealing and circulation halts leaving veins filled with chemical species that were selectively extracted and transported from other formations. 

The earths hydrothermal fluid system is continuously extracting soluble components and transporting them to distant locations where solubility properties force their deposition. But this process does not always produce solid, compacted veins. Void spaces can be left behind at all scales, from microscopic size to large chambers. These spaces are called “vugs”. Rock with a large fraction of void spaces is referred to as “vuggy”. It is possible to walk up to a mine dump in the CMB and find hand samples of vuggy rock. It is not unusual to find crystals of pyrite or other minerals lining the internal spaces of the vugs.

1.  McCoy, A., Roy, M., L. Trevino and R. Keller, Gravity models of the Colorado Mineral Belt, in The Rocky Mountain Region – An Evolving Lithosphere: Tectonics, Geochemistry, and Geophysics: American Geophysical Union Geophysical Monograph 154 (eds. Karlstrom, K.E. and Keller, G.R.), 2005.

[Note to the reader: Th’ Gaussling is just a chemist, not a geophysicist. But like many others, I have the ability to read and learn. When I learn something new and interesting, I like to write about it. It reinforces the learning.]