ORIGIN OF THE HOSKIN LAKE PEGMATITES, FLORENCE CO., WISCONSIN

The Hoskin Lake(HL) pegmatite field in Florence Co., Wisconsin, contains over 200 known pegmatites. Several of the larger ones reach up to 300 m. Geochemically, they are highly evolved LCT-type pegmatites. Several pegmatites in the Hoskin Lake pegmatite field have been studied in detail: the Animikie Red Ace (ARA) and the King’s X (KX) pegmatites. The surrounding country rock consists of metavolcanic and metasedimentary rocks. The ARA occurs in Quinnesec metasediments (QQMS), and the KX occurs in Quinnesec amphibolite (QA). The ARA is tourmaline rich and contains moderate amounts of Ta and Nb oxides minerals, small amounts of phosphates and very low Ti. The KX on the other hand, is tourmaline-poor, very rich in Ta, Nb, Sn, and Ti-bearing minerals and rich in phosphates. The surrounding amphibolite is apatite-rich and in high Ta-Nb-titanite. Titanite in the QA in the proximity of the pegmatite shows strong effects of mobilization.

The bulk chemistry of the HLG averages 13.8 wt % Al₂O₃ whereas the pegmatite wall zones as well as the QQMS average 20 wt % Al₂O₃. Spider diagrams of the metasediments and amphibolite more closely resemble that of the pegmatite wall zones than that of the HLG (Fig. 1). The HLG is higher in P₂O₅ and REE and is depleted in B and Li. The pegmatite wall zone of the ARA is low in P, elevated in Na, B, Li, and shows low K/Rb and K/Cs ratios. The K/Rb and K/Cs ratios are high for the HLG and intermediate to low for QQMS and QA.

Age determination data are only available for the granite (1.8 Ga) and the country rocks (2-2.2 Ga) and not for the pegmatites. The last regional thermal event was the Penokean orogeny which was essentially in its waning stages when the Hoskin Lake granite was intruded.

Considering the mineralogical and geochemical data available, it is unlikely that the pegmatites are fractionates of the Hoskin Lake granite. It is more likely that the pegmatitic melts formed from partial melting of the QQMS and/or the QA caused by either the thermal effects of the Penokean orogeny or as a result of heating resulting from intrusion of the HLG.