Gold formation processes.pdf

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Gold Formation, Extraction, and Refining in the Sierra Nevada Mountain Region

Isabella Niloofar Cross-Najafi

ABSTRACT

Gold formation processes include hydrothermal processes in which water becomes heated

underground when it is close to a geothermal site, dissolving many ores which have been trapped

in the surrounding rock and then through cooling the ore precipitates into a solid and purified

metal. Gold was formed in the Sierra Nevada inside the Smartville block while it was still

underwater and it was later brought to the surface through tectonic movement. This formation

process exposed many sites which were ideal for gold mining. The miners learned that placer

deposits could be excavated in many ways. They also learned that hard-rock deposits were the

original sources for the eroded ore that was washed into the placer deposits. Miners learned that

the presence of granite and quartz indicated an ideal location for gold formation. Placer deposits

were mined in many ways using panning, drift, and hydraulic mining methods. Panning was a

way to mine placer deposits and the only tool used was a gold pan. Drift mining was used to

extract gold from deep within the earth. Hydraulic mining is used to excavate ore-rich gravel

from deep within the earth by blasting an exposed ore-rich bank of gravel with highly

pressurized water. There are many different methods of gold mining specifically used on the

gold deposits of the Sierra Region. These include heap leaching, open-pit mining, and extraction

through the Cornish pump. Heap leaching was one of the first methods used in mining in which

the ore was crushed and then drenched in cyanide to dissolve the gold out of the ore in order to

refine it. Gold is extracted from the ore in mills where the ore is crushed and then dissolved in

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cyanide. The cyanide is exposed to an electric current and the gold reforms into a solid which is

then remelted into gold bars. These gold bars are the final stage of the gold and are exported all

over the world. Many environmentalists are concerned about the mercury content of the soil and

drinking water in and around some of the old mining towns.

INTRODUCTION

The relationship between man and gold goes back 6,000 years. Gold artifacts survive

from ancient civilizations and cultures such as the Egyptians and the Mesopotamians (Winter,

1993). Gold is valued so highly because of its many uses and its physical beauty. The chemical

symbol for gold is Au. It has an atomic number of 79 and an atomic weight of 197.2 (Winter,

1993). It has a bright yellow color, is very opaque and has a metallic luster. In nature gold can

occur in four habitats: crystalline masses, flat plates, nuggets, and in grains, some of which are so

fine they are invisible to the naked eye (Bonewitz, 2008). Gold can also be found in a compound

of several elements such as sulphides and tellurides and it is known as the most ductile and

malleable metal in the world. It can be stretched and manipulated into many different forms,

which is helpful when metalworking. However, gold is also very soft and usually has to be

alloyed with another metal for it to retain its shape (Bonewitz, 2008). Silver, platinum, nickel,

and zinc can be alloyed with gold to form white gold, while gold alloys with copper to form pink

gold, and with iron to form a bluish gold (Bonewitz, 2008). This precious metal can be used in

jewelry, ornamentation, and as currency. Gold also has industrial uses in electronics due to its

high electrical conductivity and its non-oxidizing nature (Bonewitz, 2008).

The demand for gold has always been high due to its scarcity. The first discovery of gold

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in California occurred when miners came across significant ore deposits near Sacramento in

1848 (Whaples, 2010). This discovery triggered a massive influx of people from all over the

world who moved to the west coast of the United States in hopes of making a fortune. Gold has

been an important part of the economy of the Sierra Nevada region ever since. In this paper I will

discuss how gold in this region is identified, how it originated, how it is extracted, and what

processes are used to refine it.

GOLD FORMATION IN THE SIERRA NEVADA

Gold formation in the Sierra Nevada began as far back as the late Jurassic period. 160

million years ago, sediment was deposited in a wedge-shaped basin along the western edge of the

North American tectonic plate (Hill, 2006). This basin is named the Smartville complex, or

Smartville block, after a small mining town that was later built there (Hill, 2006). Gold is

constantly formed within the magma beneath the earth’s crust. The gold found in the Smartville

complex was formed by a process known as a hydrothermal process, in which gold ore is

brought to the surface from deep within the earth through lava flows along mid-oceanic faults

(Hill, 2006). As the lava cools on the sea floor, cracks form on the surface allowing seawater to

penetrate the lava. The chlorine from the salt in the seawater becomes heated, facilitating a

chemical reaction which helps dissolve the metals within the rock. As the ore-rich hydrothermal

solution rises through cracks in the oceanic crust, it cools, resulting in the re-solidifying of

dissolved metals such as gold (Hill, 2006). The large concentrations of gold and other ores in the

oceanic crust are due to these hydrothermal processes. This is how gold initially became

concentrated in the Smartville complex.

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The Smartville Block

The Smartville block is the source for most of the gold mines in and near the Sierra

Nevada region. This gold-rich block, which lay between the Pacific and North American

continental plates, now lies below both the Sierra Nevada and the Central Valley and is about 25

miles wide (Hill, 2006). The block began its voyage inland when it rode over two ocean trenches

off the coast of California. It is unknown whether the block began to subduct into the trenches

and then sheared off, or just rode over them completely, but in either case the Smartville block

continued to move inland and eventually collided with California and helped to form the Sierra

Nevada mountain range. With the Smartville complex came the large hydrothermal gold deposits

that lay within its crust (Hill, 2006).

Formation of Hydrothermal Gold Deposits within the Smartville Block

Twenty million years later, multiple faults formed throughout the slowly growing

mountain range (Hill, 2006). Ground water near the tectonic activity became heated by magma

from an igneous intrusion close to the earth’s surface (Hamburger, Hereford, & Simmons,

2010). The high pressure combined with high mineral concentration in the water of minerals

such as chlorine, fluorine, and carbon dioxide enabled the water to absorb and dissolve metals,

such as gold, in the surrounding rock (Hamburger, Hereford, & Simmons, 2010). This

new ore-rich solution, or hydrothermal solution, continued to flow closer to the earth’s surface

by entering other thin fractures, or joints, in the overlying rock (Hamburger, Hereford, &

Simmons, 2010). As the water cooled the ore precipitated into a solid form again and formed

veins of ore within the thin fractures, which are also known as hydrothermal deposits

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(Hamburger, Hereford, & Simmons, 2010). Ore minerals are generally found precipitated

with quartz, calcite, and other minerals (Hamburger, Hereford, & Simmons, 2010). It is

these other precipitates that have helped people locate potential source rocks for gold during and

after the gold rush of the late 1840‘s.

CONDITIONS OF GOLD DEPOSIT FORMATION

Placer Deposits

In many cases, the discovery of an ore-rich source rock happened entirely by chance.

Many people found gold known as placer deposits in rivers, where the ore was washed out from

hard rock deposits due to wind and water erosion (See Fig. 1.) (“Placer deposit”, 2010). Stream

placers, also known as

alluvial placers, are the best-

known of all placer deposits

(“Placer deposit”, 2010). A

stream placer is created

when a powerful body of

water transports gold, along

Fig. 1. Placer deposits are formed by the weathering of hard‐rock

deposits (Placer deposit, 2010)

with other precious minerals

and stones, far from its source within the hard rock deposit. The ore is sorted from the silt and

sand by the velocity of the water. Heavier minerals are dropped before the lighter ones as the

velocity of the water decreases (“Placer deposit”, 2010). Placer deposits were most likely the

earliest mined deposits due to the accessibility of the gold (“Placer deposit”, 2010). Alluvial

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