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).
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