The Wallapai district is in the Cerbat Mountains, a range composed chiefly of crystalline rooks that are pre-Cambrian in age. The oldest rocks are quartzite, mica schist; hornblende-diopside schist, and amphibolite.Younger and larger in amount are granite gneiss and granite. Much of the gneiss is believed to be a product of granitization. Gabbroic and dioritic gneiss, pegmatite, aplite, diabase, and mylonite also occur as part of thebasement complex. The original rocks were disposed in an orogenic fold system, but igneous intrusion and granitization have obliterated all but a few remnants of the folds. The rock types and the pre-Cambrian history aresimilar to those of the Grand Canyon area.
The region contains no trace of Paleozoic rooks and probably none or Mesozoic age. Intrusions of granite porphyry in the Chloride and Mineral Park districts are believed to be Tertiary in age. Granite porphyry, pegmatite, aplite, and lamprophyre dikes are associated with these intrusions
Small areas of andesitic and rhyolitic extrusive rooks, presumably of Tertiary age, exist along the flanks of the Cerbat Range. Thin sheets of Quaternary basalt cap these rocks along the west side of the range. This basalt also lies on and interfingers with alluvium. Numerous rhyolite dikes and a few andesite and basalt dikes occur within the basement rooks.
Several periods of faulting and erosion are visible in the volcanic rocks. The latest faulting is of basin-range type and has outlined and caused the elevation of the present mountains. The greatest displacement has been on the west side, and the Cerbat Range is an eastward tilted fault block, modified in part into a horst. This mountain building seems to have occurred in late Tertiary and Quaternary time. Large quantities ofdetritus have accumulated in the adjoining basins and a pediment has been cut along the base of the range. Recent fault-block movement is suggested by the presence of terraces within the canyons and by the dissection of the pediment at the west base of the range.
Mineralization is believed to have occurred after the close of Tertiary volcanic activity. The first phase consisted of "porphyry copper" mineralization in a strongly shattered portion of the granite porphyry stock in the Mineral Park district. This was succeeded by the profuse formation of fissure veins, which carry the lead-zinc ores of the region. The veins are superimposed upon and grouped symmetrically around the "porphyry copper" mineralization. Tectonic action is postulated as the fundamental control for the emplacement of the granite porphyry stock and for the two types of sulfide mineralization that are areally associated with the stock. Turquoise deposits have developed by supergene processes in the capping of the "porphyry copper" deposit.
A late fault cuts the alluvium west of the Mineral Park district. This fault and the adjacent alluvial blanket have been mineralized by chrysocolla and form the Emerald Isle copper deposit. The chrysocolla is believed to be hypogene in this deposit.
