Earth Resources
Geology Today
Chapter 16
Barbara W. Murck
and
Brian J. Skinner
Petroleum, solar energy,
and biomass - California
N. Lindsley-Griffin, 1999
Mineral Deposits
Reserve - known resource that can be extracted profitably at
current market conditions and levels of technology
Source: U.S. Geological Survey
Mineral Deposits
Mineral deposit - a naturally occurring accumulation of
mineral material of potential economic value
Banded Iron Deposit, Lake Superior
Ore - the naturally occurring material from which a mineral
can be profitably extracted
N. Lindsley-Griffin, Dolgoff, 1998
Mineral Deposits
The plate tectonic setting controls
which mineral deposits form
Fig. 16.21, p. 493
j
N. Lindsley-Griffin, 1999
Mineral Deposits
Mineral deposits
form by natural
Earth processes:
At depth, from internal
heat and pressure
Near the surface,
from rock interactions
with atmosphere and
hydrosphere
N. Lindsley-Griffin, 1998
Mineral Deposits
Deep mineralizing
processes at:
Divergent margins
Convergent margins
Hot spots
Mt. Hood, Oregon
Types of deposits:
Magmatic
Hydrothermal
Metamorphic
Migmatite, Wyoming
N. Lindsley-Griffin, 1999
Mineral Deposits
Magmatic Deposits
Form as molten magmas
crystallize
Metallic minerals
settle to form layers
in the magma
chamber
Chromium
Platinum
N. Lindsley-Griffin, 1999
Layered gabbro, Smartville ophiolite, CA
Mineral Deposits
Model for magmatic
deposit formation
Fig. 16.24, p.497
Chromite and plagioclase layers,
Bushveld complex,South Africa
N. Lindsley-Griffin, 1999
Mineral Deposits
Hydrothermal
Deposits
Hot water and gases
circulate through
fractures in crust
Hot water and sulfide particles
issuing from a black smoker,
East Pacific Rise
Sulfide minerals
deposited here
Metal ions leached from
rock at depth are
concentrated and
redeposited
Gold, zinc, lead,
copper
Woods Hole Oceanographic Institution
N. Lindsley-Griffin, 1998
Mineral Deposits
Hydrothermal deposits in ophiolites
(on-land fragments of ocean lithosphere)
Veins are deposited along
fractures in basalts of
oceanic crust Divergent margins,
oceanic rift valleys
Ores are transported by
subduction and plate
movement, emplaced on
land by terrane accretion
in ophiolites Convergent margins,
active continental margins
Houghton Mifflin, Dolgoff, 1998; N. Lindsley-Griffin, 1999
Mineral Deposits
Hydrothermal deposits
associated with convergent
margins form beneath
stratovolcanoes.
Hydrothermal solutions
deposit copper-iron
sulfides in porphyritic
andesites - porphyry copper
deposits
Metallogenic province of rich
porphyry-copper deposits along
the western edge of the Americas
Skinner et al., 1999; N. Lindsley-Griffin, 1999
Mineral Deposits
Hydrothermal veins may
form at depth beneath
any volcano
Geologist inspects a hydrothermal gold vein being
mined at Cripple Creek, Colorado (Fig. 16.22, p. 496)
N. Lindsley-Griffin, 1999
Mineral Deposits
Hydrothermal ore deposits are
forming today in the Imperial
Valley of California - a graben
formed by rifting along the
northern end of the East Pacific
Rise which runs up Gulf of CA.
Metallic ions are leached from the
sediments under the graben by hot
fluids resulting from volcanism.
Hot brines deposit siliceous scale
containing 20% copper and 8% silver
on the insides of pipes in drilled wells.
Fig. B16.1, p. 494
N. Lindsley-Griffin, 1999
Mineral Deposits
Hydrothermal deposits forming
today in the Red Sea:
Hot, dense brines rise up along normal faults that bound the graben.
Heated by deep magmas along the oceanic rift, they precipitate chalcopyrite,
galena, and sphalerite as they cool.
Fig. B16.2, p. 495
N. Lindsley-Griffin, 1999
Mineral Deposits
Hydrothermal deposits forming
today in the Red Sea:
Brines remain pooled in the deep graben because they are denser than sea water.
This hydrothermal deposit is called a stratabound deposit, because the minerals
are precipitated as layers interbedded with sediments.
Fig. B16.2, p. 495
N. Lindsley-Griffin, 1999
Mineral Deposits
Stratabound ore of lead and zinc;
Kimberley, British Columbia.
Layers of pyrite (yellow), sphalerite (brown), and galena
(gray) are parallel to the layering of the sedimentary host
rock.
Skinner et al., 1999; N. Lindsley-Griffin, 1999
Mineral Deposits
Metamorphic deposits form by the heat, pressure, liquids
associated with metamorphism
Iron ores, marble, serpentine
N. Lindsley-Griffin, 1998
Mineral Deposits
Metamorphic deposits form by
two main processes:
1) recrystallization during regional metamorphism along
convergent margins
2) contact metamorphism by hot solutions (hydrothermal
solutions) near magma
Houghton-Mifflin, Dolgoff, 1998; N. Lindsley-Griffin, 1999
Mineral Deposits
Metamorphism - ores of
tungsten, zinc and iron
Scheelite
(CaWO4)
Pyrite
(FeS)
Calcite
(CaCO3)
Fluorite
(CaF)
Ore, Tem-Piute Mine, NV (Fig. 16.23, p. 496)
N. Lindsley-Griffin, 1999
Mineral Deposits
Shallow mineral deposits form by:
Surface water
Mechanical concentration
Evaporation
Groundwater
Leaching
Secondary enrichment
Biochemical reactions in
seawater
Types of deposits:
Sedimentary
Placer
Residual
N. Lindsley-Griffin, 1998
Mineral Deposits Sedimentary deposits form by
evaporation and precipitation
Anhydrite, gypsum, halite
Evaporite Deposits at Bonneville Salt Flats, Utah
N. Lindsley-Griffin, 1998
Mineral Deposits
Sedimentary deposits form by
biochemical reactions in seawater
Banded iron formations were precipitated by biochemical
reactions in a low-oxygen atmosphere during the Precambrian
Banded Iron Deposit, Lake Superior
N. Lindsley-Griffin, 1998
Manganese Nodules form by
direct precipitation from seawater
Concentric rings
enriched in:
Copper, Cobalt,
Nickel, Manganese
Metallic ions from mid-ocean
ridge hydrothermal vents
Cold water lowers solubility
Found in thin marine oozes (young crust or slow sedimentation)
Best commercial potential: central Pacific Ocean
N. Lindsley-Griffin, 1998; Dolgoff, 1998
Mineral Deposits
Mechanical
Concentration
Placer deposits:
Heavy grains sorted
by currents
Olivine beach placers, South Point, Hawaii
Deposited in rivers
or beaches
Previously weathered
from bedrock source
Gold, platinum, diamonds,
chromite, Zirconium and
Titanium minerals
N. Lindsley-Griffin, 1998
Mineral Deposits
Placers are deposited:
Behind rock bars
In rock holes
Below waterfalls
In point bars inside
meander loops
Downstream from a
tributary
Along beaches and
behind undulations on
the ocean floor.
N. Lindsley-Griffin, 1999
Mineral Deposits
Residual mineral deposits form
by chemical weathering
Soluble minerals are leached - dissolved by rain water and carried
downward by infiltration, leaving behind less soluble minerals.
Laterites are mined for iron and sometimes nickel.
Iron ore, Australia
N. Lindsley-Griffin, 1999
Mineral Deposits
Residual mineral deposits
Bauxite is the main
source of aluminum ore
- found in laterites
formed in tropical
climates.
Fig. 16.26, p. 499
Bauxite (aluminum ore)
Weipa, Australia
N. Lindsley-Griffin, 1999
Secondary Enrichment - metals leached from the
surface are precipitated below the water table
Upper zone: insoluble iron oxides left behind
Leaching
Precipitation
Enriched zone: soluble metal
sulfides of Zn, Pb, Cu, Au, Ag,
Hg, Fe
Phelps-Dodge-Morenci open pit copper mine, Clifton, Arizona
N. Lindsley-Griffin; Dolgoff, 1998
Mining
Mining can harm the environment if
not done properly - that’s one reason
why recycling is beneficial
Sound mining
practices
include:
Subsurface
mine shaft
Surface mine
Reclamation
of mined areas
Spoil Banks
Proper disposal
of tailings and
waste water
Houghton Mifflin, Dolgoff, 1998; N. Lindsley-Griffin, 1999
Mineral Deposits
REVIEW: plate tectonic setting
controls which mineral deposits form
Fig. 16.21, p. 493
j
N. Lindsley-Griffin, 1999