The Reilly Peak Tertiary(?) intrusive--A high-silica rhyolite
— Linda Lee Davis

Paleozoic sedimentary rocks at Reilly Peak are intruded by a high-silica intrusive rhyolite, felsic dikes, and mafic dikes. An extensive fracture system and skarn are developed in the sedimentary and igneous rocks. Breccia pipes are common. Textures in the intrusive rhyolite range from nearly 95% spherulitic to trachytic. Felty intergrowths of feldspar and cristobalite(?), granophyric and glomeroporphyritic quartz and feldspar, and micropoikilitic quartz commonly envelope the spherulites, and are also present where spherulites are not developed. Trachytic textures are common near the margins of the intrusive and in its northern extension, which is a sill or dike. Fibrous biotite, epidote, and titanite are interstitial to spherulites and appear to have grown after their development. Microphenocrysts of plagioclase (Ab _95-97 ) and lath-like biotite, and resorbed quartz are in the cores of the spherulites. This implies that they crystallized before or during spherulite growth. Alteration of the intrusive is represented by the kaolinite, sericite, and biotite. Microprobe analyses of biotites from several localities within the intrusive indicate varying degrees of hydrothermal alteration throughout the stock and sill. The biotites in each sample are significantly different from biotites in other samples. Two biotite species per sample can be distinguished on the basis of morphology, relative analytical totals, number of Si ions/unit cell, and sometimes by K₂O% and CaO%. H₂O and CaO have been added to, and K₂O and SiO₂ removed from, both biotites. The fluids that altered the intrusive rocks are probably the same fluids that formed the skarn at Reilly Peak and Iron Mountain.