earth resources

Earth ResourcesEarth Resources

Geology TodayChapter 16

Barbara W. Murck

and

Brian J. Skinner

N. Lindsley-Griffin, 1999

Petroleum, solar energy, and biomass - California

Reserve - known resource that can be extracted profitably at current market conditions and levels of technology

Source: U.S. Geological Survey

Mineral DepositsMineral Deposits

Mineral deposit - a naturally occurring accumulation of mineral material of potential economic value

Ore - the naturally occurring material from which a mineral can be profitably extracted

Banded Iron Deposit, Lake Superior

N. Lindsley-Griffin, Dolgoff, 1998


The plate tectonic setting controls which mineral deposits form

j



Fig. 16.21, p. 493

Mineral deposits form by natural Earth processes:

At depth, from internal heat and pressure

Near the surface,

from rock interactions with atmosphere and

hydrosphereN. Lindsley-Griffin, 1998


Deep mineralizing processes at:

Divergent marginsConvergent margins

Hot spots

Types of deposits:

Magmatic

Hydrothermal

Metamorphic

Mt. Hood, Oregon



Migmatite, Wyoming

Magmatic Deposits

Form as molten magmas crystallize

Metallic minerals settle to form layers

in the magma chamber

Chromium Platinum


Mineral DepositsMineral Deposits Layered gabbro, Smartville ophiolite, CA

Model for magmatic deposit formation

Fig. 16.24, p.497Chromite and plagioclase layers, Bushveld complex,South Africa



Hydrothermal Deposits

Hot water and gases circulate through fractures in crust

Metal ions leached from rock at depth are concentrated and

redeposited

Gold, zinc, lead, copper


Woods Hole Oceanographic Institution

Hot water and sulfide particles issuing from a black smoker, East Pacific Rise

Sulfide minerals deposited here


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

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

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)



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



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

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

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

Metamorphic deposits form by the heat, pressure, liquids associated with metamorphism

Iron ores, marble, serpentineN. Lindsley-Griffin, 1998


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

Metamorphism - ores of tungsten, zinc and iron

Ore, Tem-Piute Mine, NV (Fig. 16.23, p. 496)



Scheelite (CaWO4)

Calcite (CaCO3)

Pyrite (FeS)

Fluorite (CaF)

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


Sedimentary deposits form by evaporation and precipitation

Anhydrite, gypsum, halite

Evaporite Deposits at Bonneville Salt Flats, UtahN. Lindsley-Griffin, 1998


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

Manganese Nodules form by direct precipitation from seawater

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

Concentric rings enriched in:

Copper, Cobalt,

Nickel, Manganese

Mechanical ConcentrationPlacer deposits:

Heavy grains sorted

by currents

Deposited in rivers

or beaches

Previously weathered

from bedrock source

Gold, platinum, diamonds, chromite, Zirconium and Titanium minerals

Olivine beach placers, South Point, Hawaii



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.



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

Residual mineral deposits

Bauxite is the main source of aluminum ore

- found in laterites formed in tropical

climates.



Bauxite (aluminum ore)

Weipa, Australia

Fig. 16.26, p. 499

Secondary Enrichment - metals leached from the surface are precipitated below the water table

Phelps-Dodge-Morenci open pit copper mine, Clifton, Arizona

Upper zone: insoluble iron oxides left behind

N. Lindsley-Griffin; Dolgoff, 1998

Enriched zone: soluble metal sulfides of Zn, Pb, Cu, Au, Ag, Hg, Fe

Leaching

Precipitation

Sound miningpractices include:

Reclamationof mined areas

Proper disposalof tailings and waste water

Mining can harm the environment if not done properly - that’s one reason

why recycling is beneficial

Houghton Mifflin, Dolgoff, 1998; N. Lindsley-Griffin, 1999

MiningMining

Surface mine

Spoil Banks

Subsurfacemine shaft

Acid sp

ill in

stre

am

REVIEW: plate tectonic setting controls which mineral deposits form

j



Fig. 16.21, p. 493

earth resources

Documents

mineral depositsmigmatite

mineral depositsdeep

hydrothermal ore deposits

mineral depositslayered

hydrothermal veins

hydrothermal depositshot

hydrothermal gold vein

convergent margins form