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CALIFORNIA

Lost Cement Mine

GEOLOGY OF THE AREA

The region comprising the headwaters of the Owens River consists of uplifted granitic rocks of the Sierra Nevada Batholith to the west and extensive volcanic tablelands and terrane to the east. The volcanic features of the area are world-famous and include Devil's Postpile National Monument. The volcanic nature of the Mono Lake region is well-documented. Volcanic cones occur 5 miles southeast of Mammoth Lakes and 5 miles north of the lakes. Many local features and landmarks attest to the volcanic nature of the area south of Mono Lake including Pumice Valley, Crater Mountain, and Glass Mountain. Based on the great abundance of rhyolite and pyroclastic debris, most of the volcanism in this area was quite violent.

Volcanic activity has occurred in the area throughout most of its history. The Mono Lake region of eastern California is the center of an ancient and sporadically active source of volcanism extending from Silurian times virtually to the present! Metavolcanic rocks of early Paleozoic age (Silurian and Devonian), late Paleozoic age (Carboniferous and Permian), and early Mesozoic age (Triassic and Jurassic) can be found in the area. Indeed, most of the Ritter Range consists of Triassic and Jurassic metavolcanic rocks. Younger volcanics of Tertiary, Quaternary, and even recent age occur in the area, with Quaternary silicic rocks being the most common.

As mentioned previously, the western half of the area of interest consists mostly of uplifted granites of the Sierra Nevada Batholith, but in truth the geology is a bit more complicated. While Mesozoic granites are the most abundant rocks west of the volcanic field, a large exposure (roughly 100 square miles) of Triassic-Jurassic metavolcanic rocks occurs in the central part of the Ritter Range. A smaller body occurs just to the southeast of Mammoth Lakes. A broken belt of uplifted Paleozoic marine sedimentary rocks occurs along the eastern flank of the Sierra Nevada granites. This belt forms the boundary between the uplifted granites to the west and the volcanic field to the east. The belt is discontinuous and trends northwest-southeast. In many places it is overlain by Quaternary volcanic rocks of the great volcanic field previously mentioned. The marine sedimentary rocks consist of older Ordovician to Silurian units and younger Pennsylvanian rocks. The older Ordovician-Silurian beds are part of an uplifted, fault-bounded wedge of rocks that occur on the ridge just southwest of Crowley Lake. Indeed, the entire eastern half of the area of interest is part of a northwest-southeast trending fault zone. The eastern volcanic field consists mostly of younger Quaternary rhyolites, andesites, basalts, and pyroclastics with minor Tertiary andesites and basalts. The most abundant volcanic rocks are rhyolites and rhyolitic pyroclastics.

PROSPECTING POTENTIAL

Virtually every account of the Lost Cement Mine places it at or near the headwaters of the Owens River. The ore is almost invariably described as a strange reddish cement filled with lumps of gold. This curious gold-bearing cement reminds one of the rusty red "calcrete" deposits of Western Australia. Some truly incredible gold nuggets have been recovered from these cemented, hematite-rich deposits. Could the Lost Cement ledge be a "calcrete-like" sedimentary deposit or is it indeed a strange and unique lode deposit? Prospectors may want to concentrate on the headwaters of the Owens River, especially up near the divide that

separates the Owens River watershed from the adjacent drainage basin. Prospectors may also want to focus on the lower stretches of the river, along the old emigrant's trail. In any case, one must keep in mind that the Cement ledge does not look like the usual ore body. It is probable that more than one prospector has passed by the ledge without even knowing it. It is possible that the lost ledge lies in some geologic setting that no knowledgeable and experienced prospector would ever dream of checking. After all, gold is where you find it.