A Shallow to Deep View Inside the Hydrothermally Altered and Mineralized Silverton Caldera Complex: New Geologic Insights Gained From Modern Geophysical Interpretations

Science Center Objects

The Silverton caldera complex in southwest Colorado hosts base and precious metals that have been mined since the late 1800s. Extensive mine workings, excellent bedrock exposures, and deeply incised drainages make this area a natural laboratory ideally suited for furthering our understanding of the mineral systems in a volcanic environment. In addition, state-of-the-art geophysical data processing, combined with physical property measurements, allows us to model subsurface changes in magnetization and electrical resistivity, thereby providing a three dimensional model of the mineralization and alteration within Silverton caldera complex. The 3D geologic framework will provide insight into mineral system formation and associated environmental issues.

Silverton caldera location map

Simplified geologic map showing location of the Silverton caldera complex. From Yager and Bove, 2002.

(Public domain.)

Science Issue and Relevance

The Paleogene Silverton and San Juan caldera complex in the western San Juan Mountains near Silverton, Colorado provides an opportunity to investigate the shallow to deep geophysical expressions of a highly altered and mineralized epithermal system that formed after caldera formation. The area was the focus of numerous benchmark USGS studies on the processes related to caldera formation, mineralization, and the environmental effects of legacy mining, such as the 2015 release of toxic mine waters
from the Gold King mine. However, little information exists about the hydraulic conductivity through the colluvium, glacial moraine, and bedrock in the study area. This project is leveraging new and existing geophysical data to better understand the subsurface heterogeneities attributed to the origin and flow of acidic, metal-rich fluids. The results should provide a better understanding of shallow processes important for groundwater quality issues. In addition, the project hopes to better understand the "roots" of structures at depth that may have been conduits for hydrothermal fluids and whether they are groundwater flowpaths today or serve as aquitards. Thus, the resulting 3D models will provide insight into the porphyry epithermal system and associated environmental issues.

Methods to Address Issue

Red Mountain, Colorado

Intensely altered propylitic rocks (red and yellow) in the Red Mountain mining district, overprinting the regional, propylitically altered igneous rocks (grayish-green), peaks at top of image. View to west.

(Credit: Dr. Raymond Johnson, USGS. Public domain.)

To better understand the 3D geologic framework of the Silverton-San Juan caldera complex, the Project intends to: 

  • construct 3D models of geology and hydrothermal alteration based on existing maps and processed remote sensing data,
  • collect physical property measurements (magnetic susceptibility and electrical conductivity) on lithologies that have been hydrothermally altered,
  • use state-of-the-art geophysical data processing techniques, combined with physical property measurements, to develop subsurface models showing changes in magnetization and electrical conductivity,
  • collect and process magnetotelluric data to image deep structures within the caldera complex, and
  • integrate the modelling results into an interpreted geologic model of the caldera complex.