The Chocolate Mountains of southeast California are a northwest-southeast trending range of uplifted igneous and metamorphic rocks. The range is heavily weathered and eroded and is a classic example of the block-faulting that is so characteristic of the Basin and Range Province of North America. The Chocolate Mountains consist of an uplifted core of Precambrian igneous and metamorphic rocks intruded by younger Mesozoic granites and younger still Tertiary volcanics and intrusives. The Tertiary volcanics include basalts, rhyolites, andesites and associated pyroclastic
rocks. Some large exposures of Pre-Cretaceous metasedimentary rocks occur along the length of the range.
Highway 78 cuts through an eroded gap in the Chocolate Mountains which has been a source of placer gold in times past. Many of these placer deposits occur within the Pleistocene gravel deposits that drape the flanks of the Chocolates in this area. Several mines and prospect pits occur in the area including the Rainbow Mine and Gold Basin Rand Mine.
The southern Chocolates are best known for their gold deposits. Placer gold occurs at the southern end of the range, along the western flank of the range, and in the dry washes of the Picacho Basin. Silver deposits, on the other hand, are uncommon in the Chocolates. Nevertheless, it is entirely possible that a silver-bearing vein lies hidden