FLUID EVOLUTION DURING CRYSTALLIZATION OF THE KING'S X PEGMATITE, FLORENCE COUNTY, WISCONSIN, USA

The King’s X-2 pegmatite (KX-2) is a 1-3 m thick, >600 m long dike in the Hoskin Lake pegmatite field associated to late to post-Penokean granites (<1,835 Ma) in Northeastern Wisconsin. KX-2 is a spodumene-amblygonite bearing, internally zoned dike intruding amphibolites of the Quinnesec formation, related to the previously described King’s X outcrop and in close proximity to the world-famous Animike Red Ace pegmatite (Falster, 2005; Sirbescu et al., 2008). Current models propose that zoned granitic pegmatites crystallize sequentially, from outer towards inner zones, at high degree of undercooling. This systematic petrographic and microthermometric fluid inclusion study aims to constrain the conditions of crystallization and fluid evolution during the solidification and cooling of the KX-2 intrusion.

We have identified two main inclusion types in quartz, tourmaline, and spodumene samples collected from fine-grained to comb-textured outer zones and coarser inner zones. Type 1 inclusions are primary or modified CO₂-H₂O-NaCl fluids of moderate salinity averaging 7.7 ± 2.8 wt% NaCl_eq (N=93). Type 2 inclusions are secondary H₂O-NaCl fluids of lower salinity averaging 5.0 ± 3.4 wt% NaCl_eq (N=64). CO₂-CH₄ and CO₂-CH₄-H₂O-NaCl inclusions also occur, but only in border zone samples.

Type 1 inclusions were further subdivided. Subtype 1a inclusions are of unambiguous primary character and preserved mostly in tourmaline, whereas 1b inclusions are secondary or suffered post-trapping changes and hosted mainly in quartz. Assuming a pressure of 300 MPa, type 1a inclusions suggest average crystallization temperatures ranging from 435 to 467°C of the outer zones and from 522 to 576°C of the inner zones, suggesting significant liquid undercooling. The degree of undercooling decreases towards the inner zones, which is consistent with rapid crystallization under a thermal gradient established as the sheet of magma intruded into the colder rocks. Types 1b and 2 inclusion assemblages have generally lower isochoric temperatures and cover a broader compositional spectrum. We interpret these inclusions as products of continuous fluid immiscibility, fluid-mineral interactions, and exchange with the country rocks during and/or after pegmatite solidification.