CRYSTALLIZATION CONDITIONS AND FLUID CHEMISTRY OF THE ANIMIKIE RED ACE PEGMATITE, FLORENCE COUNTY, WISCONSIN

The Animikie Red Ace (ARA) pegmatite is the largest of ~200 pegmatites exposed in the LCT-type Hoskin Lake pegmatite field of Florence County, WI, which is thought to be related to post-Penokean (~ 1.760 Ga) granitic magmatism. The ARA pegmatite is a zoned, lithium-rich pegmatite, remarkable for raspberry-red tourmaline-rich wall zones. It is one of four known occurrences of rhodizite-londonite in the world (K, Cs, Al, Be borate). The Hoskin Lake pegmatites are somewhat similar to the gem-bearing, miarolitic (pocket) pegmatites of San Diego County, CA, because of their distribution in thin, sub-parallel layered dikes, the abundance of colored tourmaline, and comparable pressures of emplacement (~2-4 kbars). However, to date, neither pockets, nor gem-quality crystals have been reported in the Hoskin Lake pegmatites.

This fluid inclusion study integrates microthermometry and crush-leach analysis to evaluate the magmatic fluid chemistry and crystallization conditions of the ARA pegmatite in comparison to results from other pocket and non-pocket lithium-rich pegmatites. Microthermometric data were collected from zoned tourmalines. The primary fluids are H₂O-CO₂ mixtures with an average X_CO2 of 0.09 ± 0.03, salinities of 6.5 ± 1.2 wt% NaCl_eq, and trapping temperatures of 407 ± 25 °C (at a pressure of 3 kbar). The primary magmatic character of H₂O-CO₂ inclusions in tourmaline was inferred based on their distribution relative to crystal growth zones. The low crystallization temperatures are comparable to values reported for other lithium-rich pegmatites, and may be due to high concentration of fluxes such as B, P, alkalis and water, or to high degrees of undercooling.

The leachate analyses indicate that atomic ratios F/Cl (0.02-0.08) and Li/Na (0.01-0.08) of the ARA fluids vary systematically from pegmatite’s wall zones to its core. These values overlap with fluid ratios measured from non-pocket pegmatites including Tin Mountain (Black Hills, SD) and Palermo (Grafton, NH), but are lower by an average factor of 7 and 4, respectively, than ratios measured in pocket, gem pegmatites (San Diego, CA). These preliminary comparative results suggest that 1) the ARA does not contain pocket gems and 2) the chemistry of major and minor ionic species of fluid inclusions is a potentially useful tool in gem-pegmatite exploration.