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Evolution of Brittle Structures in the Colorado Mineral Belt and Vicinity

Task Objectives

The evolution of the Precambrian rocks in the Colorado Rocky Mountain Front Range is intimately associated with pervasive brittle deformation, fluid flow, and hydrothermal alteration. Regional structural control of the major mining districts and epigenetic hydrothermal mineral deposits has long been recognized in the Colorado Mineral Belt, but the origin and timing of the development of these key structures relative to, for example, late Cretaceous and Eocene deformation is poorly known. The unique structural style and developmental history of major brittle fault zones, joint networks, and the lithologic controls on the properties of these structures are also poorly documented and understood.

The major objectives of this subtask were to provide detailed structural information at key sites to aid in understanding the:

1. structural history of the central Front Range
2. evolution and localization of the metal-bearing hydrothermal systems in the Colorado Mineral Belt
3. effects of brittle structures on ground water quality, quantity, and interactions with surface water systems.

Highlights & Key Findings

Evolution of Front Range Brittle Structures:

1. Preliminary quantitative mineralogy completed from several faults show extensive illite in clay-rich fault gouge rocks. This suggests the excellent possibility for Ar/Ar dating of the last motions on these faults.
2. Two environmental chambers for non-destructive disaggregation of minerals were donated to the Survey by ExxonMobil. These instruments were used to obtain pure clay and quartz separates for fault-related age and paleo fluid and mineralization studies.
3. Funding for a summer intern was awarded. The student completed extensive GIS databases and acted as a field partner for project members.
4. Initiation, write-up, submission, and acceptance of a co-convened topical session on faulting and fluid flow at the 2005 Annual Geological Society of America Meeting in Salt Lake City, Utah, called “Fault Zone Controls on Fluid Movement, Earth Resources and Processes”.

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