CHEMICAL AND TEXTURAL VARIATIONS OF APATITE, ANIMIKIE RED ACE PEGMATITE, FLORENCE COUNTY, WISCONSIN: IMPLICATIONS FOR CRYSTALLIZATION PROCESSES

The Animikie Red Ace pegmatite (ARA) is a highly fractionated, lithium-cesium-tantalum type pegmatite. It is a ~ 600 m long and up to 3 m wide dike, the largest in the Florence County pegmatite field. It intrudes the 1.8 b.y. Quinnesec Formation. It is mineralogically and texturally zoned, including a fine-grain border zone, wall zone, three intermediate zones, core margin, core, and aplite cross-cutting units. The pegmatite zones have distinct major minerals including blue and pink tourmaline, quartz, spodumene, lepidolite, albite and orthoclase. The question we are addressing is whether the ARA pegmatite formed 1) slowly, at equilibrium, through a fractional crystallization process or 2) rapidly, at disequilibrium, through rapid crystal growth, generating chemically distinct boundary layers.

Minerals and textures of various ARA zones were explored for clues on the pegmatite's cooling and crystallization history. We focused on the accessory mineral apatite. We investigated apatite's habit, color, location, crystal zoning, and Mn contents. We used Scanning Electron Microscopy Energy Dispersive X-Ray Spectrometry to characterize Mn contents on apatite collected systematically from the border to core zones. We conducted cathodoluminescence (CL) to investigate apatite zoning. Type 1 (aspect ratio ~1-4) gray apatites, in the border zone within 0-3 mm from the country rock, are CL zoned and Mn-poor. Type 2 (aspect ratio 3-10) blue apatites are in hanging wall zones, not CL zoned, and Mn-rich. Type 3 (aspect ratio ~1.25-2.0) gray apatites are abundant in aplite units, not CL zoned, and Mn-rich. Apatite frequency fluctuates from abundant in the border, wall, and middle intermediate zones and aplite units, to no apatite in all other zones.

Textural and chemical evidence suggests that the ARA pegmatite magma crystallized at disequilibrium, because of rapid cooling caused by intrusion into colder country rock. Textural evidence includes broad range of grain sizes across the ARA, skeletal morphologies, and varying apatite habits. Chemical evidence includes oscillatory zoning of border zone apatites, fluctuating abundance, and chemical composition of apatite in the pegmatite zones.