The geology of the Florida Mountain area has been described in general by Lindgren (1900) and Piper and Laney (1926). More detailed studies were carried out by Earth Resources and NERCO as documented by Lindberg (1985), Porterfield and Moss (1988), and summarized by Mosser (1992). The oldest stratigraphic unit is the Late Cretaceous Silver City granite, which is unconformably overlain by the mid Miocene lower basalt to trachyandesite lavas. The granite and lower basalt are overlain by a sequence of andesitic volcanic-sedimentary and tuffaceous lacustrine rocks, which are in turn intruded and overlain successively by units of quartz latite, tuff breccia, and porphyritic rhyolite of the Silver City rhyolite (e.g., Lindberg, 1985). As at DeLamar, the tuff-breccia unit is interpreted as a near-vent pyroclastic unit erupted as a precursor to emplacement of the rhyolite flows and domes.
In contrast to the DeLamar area, the Silver City granite crops out on the flanks of Florida Mountain and was extensively penetrated by workings of the historical underground mines. It was designated granite (Figure 7.7) by the Integra geologists. Field relations demonstrate the lower basalt flows partially buried an erosional, paleotopographic high of Silver City granite. Surface exposures and maps of the underground workings, as well as early drilling at Florida Mountain, led Lindberg (1985) to infer the granite forms a northeast-trending ridge beneath a relatively thin capping of quartz latite, tuff breccia, and one or more flows of rhyolite lava.
The Earth Resources, NERCO and Integra geologists interpreted certain rocks at Florida Mountain to represent volcanic vents from which portions of the rhyolite flows and possibly tuffs were presumably erupted, and which later were important foci of hydrothermal activity, alteration, and mineralization (e.g., Porterfield and Moss, 1988; Mosser, 1992). However, exposures of rock units at Florida Mountain were generally poor prior to the start of mining by Kinross in 1994 as explained by Lindberg (1985), and the criteria used by the above authors to define the vent facies units and to delineate their geometries are not known to the authors. Moreover, most of the drilling at Florida Mountain was done by conventional rotary and RC methods, which can make outcrop-scale rock textural characteristics much more difficult, to impossible, to discern and correctly interpret.