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The Rocks at this Site
This site consists of small, irregularly shaped silicified rhyolite and layers of interbedded latite, hornfels, and dacite. Parts of the volcanic section have been completely altered to quartz and hornfels. Volcanic textures are fairly common at this site.
Rhyolite is a volcanic rock, of silica-rich composition.
Latite is a volcanic rock.
Hornfels is the group designation for a series of metamorphic rocks that have been baked by the heat of volcanic masses and have been rendered massive, hard, splintery, and in some cases exceedingly tough and durable. Hornfels rocks were referred to by miners in northern England as whetstones.
Dacite is a volcanic rock. It is intermediate in composition between andesite and rhyolite. The process by which dacite forms has been used to explain the generation of continental crust during the Archean eon. At that time, the fabrication of dacitic magma was more ubiquitous due to the availability of young hot oceanic crust. Today, the colder oceanic crust that flows under most plates is not capable of melting before the dehydration reactions therefore inhibiting the process
Area Geology
The Southern Otay Mountain and Western Otay Mountain Wilderness Areas are underlain by metavolcanic rocks that are assigned to the Upper Jurassic Santiago Peak Volcanics. The Santiago Peak Volcanics are part of a discontinuous, northwest-trending, 10-mi-wide belt of volcanic and sedimentary rocks that lies within a few miles of the Pacific coast and extends for at least 400 mi from southern California, U.S.A., down the peninsula of Baja California, Mexico. These rocks were intruded on the east by the predominantly granitic Peninsular Ranges.
The Santiago Peak Volcanics are considered to have formed in and near an ancient volcanic archipelago. The volcanic arc was bordered by deep marine basins that received alternating volcanic eruptions and sediment fans derived from emerging volcanic islands. Soon after eruption and deposition, these accumulations were buried, folded, and thrust faulted, probably during an episode of regional deformation that accompanied the earliest intrusions of the Peninsular Ranges batholith. As the result of burial, heating, and local intrusion, the volcanic and sedimentary rocks underwent incipient to low-grade metamorphism. This metamorphism is marked by the development of one or more metamorphic minerals that include chlorite, illite, epidote, albite, calcite, muscovite/sericite, actinolite, serpentine, and stilpnomelane.
History of Mining on and around Otay Mountain
The first discovery of gold in the region was in 1828 when a small placer deposit was reported at San Ysidro about 10 mi west of the site. The earliest confirmed mining activity took place in 1877 when gold was found in the Mine Canyon area. This area, adjacent to the east boundary of the Southern Otay Mountain Wilderness Area is in the Dulzura (Oneida) mining district. Claims located here included the Golden Artery and the Chief of the Hills lode claims. A patent survey of these claims in 1894 reported a 570-ft-long adit, a 100-ft-deep shaft, and several open cuts. The ore averaged $8 per ton at $20 per-oz price. Little additional work was done on the property after it was patented in 1896.
Credit: Mineral Resources of the Southern Otay Mountain and Western Otay Mountain Wilderness Areas, San Diego County, California (USGS) Full report and credit acknowledgements can be found here:
Getting Credit
Flow banding is a geological term to describe bands or layers that can sometimes be seen in rock that formed from the substance molten rock or magma. Flow banding is caused by friction of the viscous magma which is in contact with a solid rock interface, usually the wall rock to an intrusive chamber or if the magma is erupted, the surface of the earth across which a lava is flowing.
Do you see any flow-banding on these rocks? Email me and tell me your answer and tell me what (if anything) you might deduce about the flow of magma when these rocks were being formed or transformed.