RECENT INSIGHTS INTO COLORADO CENTRAL FRONT RANGE (CCFR) TECTONICS AND METALLOGENY

Modern interpretation of the Precambrian of Colorado as an accretionary terrane began ca. late 1970s to early 1980s. Studies by Condie, Karlstrom, Houston, and others recognized the Cheyenne Belt as a suture between the Archean Wyoming craton and Proterozoic arc terranes to the south. Research efforts of the 1980s yielded interpretations of arc-related units, based on petrology, geochemistry, and comparison to modern analogs. Over the last 20+ years, subsequent studies attempted to define age, size, extent, number, and type of arc sequences. While disagreement exists among researchers as to details, the general model of volcanic arc accretion stands; yet many questions remain to be answered. How best to collect the myriad of studies and create an integrated model? One of the PC² goals is to bring researchers together, encourage and support new and collaborative research, and determine further issues to be addressed.

The current state of knowledge of the CCFR is illustrated by the Proterozoic rock units of the informally named “Idaho Springs Formation”, which comprises: 1) a basal juvenile volcanic arc (mafic and felsic gneiss), 2) a volcanic backarc with calcareous and silicic sediments (hornblende gneiss), and 3) a basin-fill of pelitic and quartzitic sediment (mica schist and quartzite). Initially locally studied at lower metamorphic grade, these are now recognized regionally to include areas of increasing grade and anatexis.

In addition to contributing to the understanding of the accretionary complex, Proterozoic minerals of the CCFR are also economically significant. Diverse commodities include quartzites (used for primitive tools), base metals (Cu, Zn, Pb), W, Be, REEs, gemstones, and industrial minerals (e.g., feldspar, fluorite, trap rock). Syn-depositional iron formations and base metal occurrences are present in the system and may be associated with volcanic, hydrothermal, or exhalative processes. Notably, zinc spinels, whose regional distribution in Colorado Precambrian terranes is second only to the world class Broken Hill region of Australia, are recognized as a potential massive sulfide pathfinder, as are other lithologies found within the CCFR units such as rutile- and topaz-bearing gneisses.

Proterozoic units and mineral occurrences discussed here will be examined on Field Trip 403.