_07_Gold_Glaciers

_07_Gold_Glaciers - GEOLOGY 20 - LECTURE 7 - Sierra Nevada...

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1 GEOLOGY 20 - LECTURE 7 - Sierra Nevada (Ch. 8) Gold in the Foothills of the Sierra Nevada What is the significance of the Western Metamorphic Belt? Who cares? Along with all of the oceanic rocks that were accreted to the edge of North America by colliding island arcs during the late Paleozoic – early Mesozoic, these terranes carried something much more valuable – gold . - the vast majority of gold mines in the Sierra Nevada occur in the Foothills, which mark the location of accreted terranes of the Western Metamorphic Belt The gold was probably already present within the terranes when they were accreted onto the continent. Hot fluids containing elemental gold (Au) flowed through fractures in the rock as it was forming near underwater volcanic vents while the island arcs were still growing somewhere out in the ocean, off the coast of ancient North America. - after the terranes accreted along western North America, the gold was later ‘remobilized’ by hot fluids generated by intrusion of the Sierran batholith. It’s no coincidence that the gold mines in California are located in the Western Metamorphic Belt near where the metamorphic rocks were intruded by later granitic magmas. - these fluids also contained abundant dissolved silica (SiO 2 ) squeezed out from the magma. When these fluids could travel no further along cracks or faults, the dissolved silica precipitated out as quartz veins . Associated with these quartz veins is pure, elemental gold . - quartz is composed of silicon and oxygen – SiO 2 - gold-bearing quartz veins may be 30 cm to 30 m thick - quartz veins may also contain pyrite (fool’s gold) Quartz veins in the Western Metamorphic Belt were preferentially emplaced along faults and fracture zones because these were the easiest routes for hot fluids to travel through as they were expelled from the magma chamber. Much of the gold-bearing, silica-rich fluids used the faults between terranes as conduits during their upward migration. The Mother Lode gold belt of the Sierra Nevada is a 1.5 km (~1 mile) wide belt of gold- bearing quartz veins and mineralized rock that runs for 195 km along the Western Metamorphic Belt (roughly paralleling Hwy 49) – located along a major fault zone separating terranes
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2 Three main types of gold deposits in the Sierran foothills: 1) lode deposits, exploited by hardrock mining, 2) placer deposits in modern streams and rivers, and 3) placer deposits in ancient river sediments 1) Lode deposits (such as the Mother Lode) were the original source of most of the gold mined from the Sierra - ‘ hardrock’ mining – requires picks, shovels, drills, dynamite and other heavy equipment to break out the quartz, then chemically separate it from any associated gold (requires mercury or arsenic for chemical separation) - mines were dug with picks and shovels that followed quartz veins for miles beneath the ground
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This note was uploaded on 03/10/2012 for the course GEL 20 taught by Professor Osleger during the Winter '08 term at UC Davis.

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_07_Gold_Glaciers - GEOLOGY 20 - LECTURE 7 - Sierra Nevada...

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