ORIGIN OF FIBROUS AMPHIBOLES IN THE IRON HILL CARBONATITE COMPLEX, GUNNISON COUNTY, COLORADO

The habit and chemistry of amphiboles from mixed carbonate-silicate rocks, pyroxenite, and fenites within the Iron Hill carbonatite complex have been determined using petrographic, EPMA, XRF, LA-ICPMS, and fluid inclusion analyses. Fibrous richterite and magnesioarfvedsonite occur in the mixed carbonate-silicate rock zone and the southern fenite. Nonfibrous richterite, magnesioarfvedsonite, and magnesiokataphorite are found in the southern fenite, nonfibrous magnesiohastingsite and hastingsite are found in the northern fenite, and magnesiohastingsite and pargasite are found in the pyroxenite (nomenclature of Leake et al., 1997). All fibrous varieties contained 8 of 8 Si atoms in the IV-site while the non-fibrous varieties contained 6 to 8 Si atoms.

Trace element signatures of whole rocks, amphiboles, pyroxenes, biotites, apatites, and carbonates, suggest that two separate fluids, related to the emplacement of the carbonatite, formed amphiboles in the complex. Fluid inclusion freezing experiments suggest that amphiboles in the southern fenite and mixed rock zone formed from an aqueous brine (7-21 wt.% NaCl eq). Decrepitate analysis of fluid inclusions in apatite and calcite of the same rocks reveal the fluid to be composed of Na, Mg, Si, K, and Ca, rarely Al and Fe, and possibly C. Amphiboles in the pyroxenite and northern fenite formed from a second fluid inferred to have higher CO2:H2O and Al:Si than the first fluid based on mineral associations and chemistries. Equilibration temperatures for amphibole-pyroxene and amphibole-biotite pairs using Mg-Fe partitioning are 430-600ºC in southern fenite, ~560ºC in mixed rocks, 510-540ºC in northern fenite, and 530-540ºC in pyroxenite.

Factors controlling the habit of amphibole growth include amphibole chemistry, fluid composition, temperature, and bulk rock composition. Factors promoting fibrous growth are bulk rock composition high in silica leading to complete tetrahedral site occupancies by Si, deposition from a water-rich fluid with high alkali content, and formation within a zone of high cooling rates.