GEOLOGICAL, GEOCHEMICAL, AND GEOPHYSICAL CHARACTERIZATION OF GOLD IN THE DAWSON PERALUMINOUS INTRUSIVE-RELATED SHEAR ZONE GOLD SYSTEM, CENTRAL COLORADO

Assessment of new and existing geologic mapping, geochemistry, and geophysics conducted during a 2016 to 2017 gold exploration program at Dawson, south of Canyon City in central Colorado, has led to a new model for middle Proterozoic age gahnite bearing base and precious metal mineralization in the Colorado Front Ranges. All of the known base and precious metal occurrences are associated with what can be interpreted as high temperature hydrothermalite fractionates of peraluminous granitioid intrusive complexes emplaced during flat subduction near the end of the Yavapai Orogenic epoch in Colorado between 1715 and 1690 Ma. At Dawson and nearby Green Mountain, two styles of metallization are present. An older copper-zinc (Ag) metal event associated with banded, weakly peraluminous biotite aplite occurs in the upper portions of an E-W trending, steeply south dipping shear zone named the Dawson shear. A second gold (Te-Se-Bi) locally bonanza grade precious metal event is associated with a later pink, medium to coarse grained, weakly foliated peraluminous granite. It generally occurs down dip from and overlaps with the base metal mineralization. The fractionation of what are inferred to be peraluminous hydrothermalites may have been driven by depressurization in tensile zones developed along the Dawson shear zone during NE-SW shortening strain.

Many of these middle Proterozoic metal occurrences, including those at Dawson, were formerly misinterpreted as metamorphosed volcanogene massive sulfides, but their consistent association with peraluminous intrusions suggests otherwise. A literature survey of other gahnite bearing metallogeny such as Broken Hill in Australia and Franklin, New Jersey also indicates the presence of a robust peraluminous granitioid and pegmatoid association. Like Dawson, much of the textures and mineralogies in these deposits can be viewed as magmato-hydrohermal, immiscible, high temperature hydrothermalites hydrously differentiated from peraluminous melts evolved within the amphibolite isograd. This new perspective for the gahnite bearing base and precious metal metallogeny may uncover significant new exploration opportunities not only in Colorado but elsewhere on this planet in high grade orogenic metamorphic belts where gahnite and peraluminous magmatism co-exists.