MAGMATIC EVOLUTION OF THE RATTLESNAKE HILLS ALKALINE COMPLEX AND IMPLICATIONS FOR RARE EARTH ENRICHMENT

Alkaline complexes are commonly associated with economic REE deposits but their crystallization history and mechanisms for concentration of REEs are poorly understood. Important processes include igneous differentiation, carbonate/silicate immiscibility, and alteration processes.

The 48-44 Ma Rattlesnake Hills alkaline complex in central WY was emplaced as series of stocks, domes, dikes, diatremes, and tuffs. It contains subalkaline/alkaline silicate rocks and sparse carbonatite dikes and veins but economically viable REE deposits have not been recognized.

We use K2O+Na2O content of the rocks as an indicator of differentiation from tephrite to tephriphonolite and trachyandesite to phonolite. Two small and deeply weathered carbonatite dikes have been recognized, but deep Cenozoic weathering may have removed much of any original material.

	Tephrite	Tephri-phonolite	Trachy-andesite	Phonolite
Phl	x	x	x
Cpx	x	x	x	x
Amp		x	x
Hyn			x	x
Cal	x	x
Ttn	x	x	x	x

Primary igneous textures are overprinted by late-stage igneous or solid state reactions and post-crystallization retrograde reactions. Least-differentiated samples include textural and chemical evidence of discontinuous crystallization suggesting temporally abrupt changes in magma chemistry. Cpx phenocrysts have interior oscillatory zoning. An abrupt shift to strongly Mg-rich cpx grading to Fe-rich cpx extends to the outer rim of the crystals. Amp phenocrysts are variably resorbed and mantled by “moats” of nepheline and an outer rim of cpx, oxides, and phl. These textures are absent in phonolites, and amp is absent in peralkaline samples. These textures indicate tephrite melts mixed with tephriphonolitic melts. This event occurred after emplacement at the final shallow levels because the delicate resorption textures are undeformed. Late propylitic alteration by oxidizing hydrous fluids includes polycrystalline pseudomorphs of Na-Ca-Al zeo after euhedral nph crystals, vesicle/void fillings of Mg-Al-Si-O materials, intense alteration of afs phenocrysts to ser and locally to hyn, and formation of dark outer rims on amp crystals. These events complicate interpretation of whole-rock differentiation trends for these melts.

Absence of REE enrichment may be the result of early extraction of immiscible carbonatite melt or retention by halogen-enriched fluids.