DECIPHERING TEXTURAL RELATIONSHIPS AMONG LITHIUM ALUMINOSILICATES IN DIBS PEGMATITE, WINNIPEG RIVER DISTRICT, SUPERIOR PROVINCE, SE MANITOBA, CANADA

Textures distinguish pegmatites from typical igneous rocks and may unveil processes and conditions of their formation. The spodumene-quartz intergrowth (SQUI) is thought to result from the isochemical breakdown of petalite (LiAlSi₄O₁₀) to spodumene (LiAlSi₂O₆) plus quartz during cooling of complex-type, petalite-subtype pegmatites such as the 2.6 Ga old Tanco pegmatite, SE Manitoba. Drill-core samples from the buried, internally-zoned Dibs pegmatite, located ~ 2 km from Tanco, were investigated to better understand the SQUI texture in relation to primary petalite; the compositional zoning of secondary spodumene; and the entrapment conditions of fluid inclusions in quartz from SQUI. Dibs is an internally zoned, gently dipping body, >500 m long, >100 m wide, and <65 m thick.

We have used 1) petrography, cathodoluminescence (CL), and SEM to characterize the mineral relationships, potential reactions, and compositional zoning; 2) BSE-image analysis to assess proportions within mineral intergrowths and distinguish between isochemical and open-system reactions; and 3) CL and LA-ICP-Mass Spectrometry to describe trace-element compositional zoning of spodumene. Spodumene occurs in coarser-grained SQUI (Type 1) that completely replaced petalite and finer-grained SQUI (Type 2), that only partially replaced petalite.

The minimum pressure of ~320 MPa for the petalite-breakdown reaction is constrained by our fluid-inclusion isochores from quartz in Type 1 SQUI and the stability field of spodumene (London, 1984). For this pressure the primary fluid inclusions in SQUI formed at 345±25°C. The highly variable proportions Sp:Qtz and the complex associations of SQUI with albite, muscovite, and K-feldspar suggest that Type 2 SQUI formed largely in an open system. Furthermore, the concentrations of transitional elements such as Mn, Fe, and Sn in Type 1 SQUI spodumene ranging tens to hundreds ppm also suggest an open system. The Mn/Fe ratios in zoned spodumene vary from 0.71 ± 0.3 to 5.7 ± 2.5, in their cores and rims, respectively. Trace element geochemistry of spodumene suggests that transitional metals introduced hydrothermally were involved in the formation spodumene at the expense of pure petalite.