Geology
of California Robert
Lopez
Study
Questions 13: Sierra Nevada Granitoids and Gold
Sierra Nevada Rocks:
1)
What
are pluton, magma chamber, and batholith?
2)
Approximately
at what depth and temperatures did the Sierra Nevada Batholith form?
3)
What
were the emplacement temperatures for the batholith?
4)
What
are granitoids? Name some granitiods
(Fig. 8-12, p. 168).
5)
What
are instrusive series?
6)
Where
are the mafic gneisses of the Sierra exposed?
7)
What
are geobarometers? Name a geobarometer
used to determine information for Sierran rocks.
8)
What
are mafic enclaves and where are they exposed?
9)
In
the origin of the batholith, what tectonic setting is involved?
10)
What
happens to the ocean lithosphere at about 150 km depth in the subduction
zone? How does this facilitate melting
in the mantle if peridotite melts at 1400°C under dry conditions?
11)
How
does oceanic lithosphere get hydrated to begin with?
12)
Most
geologists agree that the room problem to fit such a large batholith into the
continental crust is accommodated by crustal melting, uplift and erosion of
pre-existing rocks, or extension in a compressional orogen? Recall that the chemical signatures of the
granitoids indicate a crystal fractionation differentiation.
13)
Triassic
through Cretaceous plutons in the Sierra indicate an eastward migration. What does this mean in terms of subduction
angle and length of the arc-trench gap?
14)
What
are the three conspicuous features associated with Sierran granites?
15)
Define
exfoliation. Give a California example
of an Exfoliated Dome.
16)
The
bulk of the granitoids in the Sierran Batholith are Triassic, Jurassic, or
Cretaceous?
17)
What
are cooling joints and master joint sets?
How did Vernal and Nevada Falls form? What are rock steps?
Exfoliation Domes in
California that You Can Hike to the Top of:
Moro Rock in Sequoia National Park
Sentinal Dome on the road to Glacier
Point
Half Dome (usually a day and a half
trip from the Valley, but it could be done in a very long, strenuous day)
Lembert Dome in Tuolumne Meadows
Castle Dome in Castle Crags State Park,
Off I-5 in the Klamath Mts. (Actually, it may be very difficult to hike this,
may need to climb—mountaineer).
Sierran Gold:
1)
Define
the term auriferous.
2)
Be
able to list and discuss four physical properties of native gold that are
important for its identification and concentration in nature.
3)
List
three ways to distinguish fool’s gold (pyrite) from native gold.
4)
What
are Lode and Placer ore deposits?
5)
Why
are placer deposits commonly more profitable to mine than veins?
6)
What
are the Tertiary auriferous gravels?
7)
Where
is the Mother Lode Belt (Fig. 8-28)?
8)
What
was hydraulic mining, what type of deposit was it used for, what were the
environmental problems with the method, and when and why was it stopped?
9)
Dredging
is used for what type of deposit?
10)
Where
was most of the dredging done in California? (See Harden, Fig. 8-32 for
pictures of dredge tailings.)
11)
When
was the gold rush and why did it end?
A few points of interest along
Hwy. 49, the Mother Lode Highway.
Mariposa: State Mining and Mineral Museum and County
Historical Center.
Kennedy: Hard Rock Mines
Coloma
State Park: Site of the first gold
discovery
Malakoff
Diggings: large pits still exposed after over a century of hydraulic mining.
Empire
Mine, Grass Valley: Placer mine
Sierra
City: Two hard rock mines (go to Lakes Basin drive to Jamison Mine, Grey Eagle)
Other Points of Interest:
Gold
Run on I-80: Hydraulic mine pit
Folsom: Tailings from dredge mining