New Mexico Mineral Symposium — Abstracts

A eudialyte-bearing dike approximately 1 m thick by 100 m long has intruded the surrounding country rocks near the western edge of the Wind Mountain laccolith. The dike consists predominantly of albite, potassium feldspar, nepheline, and acmite. The main accessory mineral is eudialyte. The eudialyte makes up about 5% of the rock although it is irregularly distributed in the dike and locally makes up 20% of the rock. The dike shows interesting textures with margins consisting of large crystals of acmite up to 4 cm long arranged perpendicular to the walls. The spaces between these crystals and the center of the dike consist of smaller (1-2 mm) crystals of feldspars, nepheline, acmite, and eudialyte. Quartz is found locally very near the margins of the dike and has presumably formed by assimilation of silica from the country rock, which is a marly shale to impure silty limestone. Eudialyte is concentrated toward the center of the dike. In thin section many of the eudialyte crystals show color zoning with a pink to brown pleochroic rim and a colorless core.

Compositionally the acmites are close to pure NaAlSi₂O₆ with minor amounts of CaO, Al₂O₃, ZrO2, and TiO₂ the main other oxides present. CaO ranges from 0.7 to 4.6 wt %. The larger crystals near the margin of the dike show Ca-rich cores (up to 1.7 wt. % Ca0) and Ca-poor rims (0.7 to 0.8 wt. % CaO). The cores of smaller crystals appear to be richer in Ca with some as high as 4.6 wt. % CaO. The acmites also show uncommonly high contents of ZrO2 of from 0.8 to 3.4 wt %. The eudialytes tend to be quite uniform in composition with little core to rim variation. Apparently the variation that accounts for the color zoning is an increase in Mn0 (from 3-4 wt. % in the core to 5 wt. % at the rim) and a corresponding decrease in Fe0 (from 3.5-4 wt. % in the core to 2.7-3 wt. % at the rim). A typical analysis of the eudialyte yields the following results expressed as percentages: SiO₂, 47.07; ZrO₂, 13.39; TiO₂, 0.18; Al₂O₃, 0.01; La₂O₃, 1.22; Ce₂O₃, 2.05; Pr₂O₃, 0.30; Nd₂O₃, 0.37; Sm₂O₃, 0.05; Eu₂O₃, 0.74; Gd₂O₃, 0.46; CaO, 3.69; FeO, 2.71;-MnO, 5.12; MgO, 0.21; Na₂O, 13.22; K₂O, 0.40; F, 0.59; Cl, 2.71 (estimated); total 92.16.

The albites range from Ab₉₈ to Ab₉₉ and the potassium feldspars range from Or64 to Or94. Both feldspars contain less than 0.5% of the anorthite end member. The nephelines show considerable silica in solid solution and approach the maximum silica content that can occur in nepheline (Ne₈₅ Qz₁₅).

The dike can be traced into the main body of the Wind Mountain laccolith (an analcime-nepheline syenite) where it appears to grade into a zone of poorly defined dike-like bodies. The dike is interpreted to have formed from a late-stage Zr-rich pegmatitic magma that was injected into the surrounding country rock from the laccolith, possibly along a fracture formed during the doming of the overlying sediments. The dike began to crystallize under water-rich conditions that lead to the formation of the large acmite crystals. Before crystallization was complete, however, the system lost water pressure (presumably by further fracturing and venting to the surface), and the remaining magma was pressure quenched, producing the fine-grained center of the dike. The quenching was due to the shallow level of emplacement of the laccolith, which has been estimated to have been less than 1 km. The center of the dike is enriched in eudialyte because of further concentration of Zr in the remaining magma during crystallization of the acmite.