EPSc 116: Resources of the Earth

Lecture 22 on Ch. 8: Geochemically Scarce Metals

Themes

Carefully read Focal Points, pp. 264-265.

Scarce metals (individually <0.1 wt. % of earth's crust)

35 scarce metals in TOTAL < 1 wt. % crust

Scarce metals enhance the efficiency of industry:

Alloys; special properties

"Enzymes" of industry

Elements most likely to run out

CVS4, Fig. 8.1

Copper

Nickel

Cobalt

Lead

Zinc

Chromium

Tin

Mercury

Gold

Silver

Platinum

Rare-earth elements

Geochemically scarce

How do scarce elements occur:

Usually not as their "own" minerals, e.g., NOT like Fe2O3 [hematite]

Atomic substitution or solid solution: e.g., of cobalt (Co) for calcium (Ca)

CaCO3 vs. (Ca,Co)CO3 [CaCO3 = calcite]

CaCO3 CaCO3 CaCO3 CaCO3 CaCO3 CoCO3 CaCO3 CaCO3 CaCO3 CaCO3

So, (1) element is dispersed, (2) desired mineral is dispersed, NOT massive

Effect on exploration, mining, processing, residues

Harder targets: smaller deposits, fewer deposits

May recover only small quantities of desired element

By-products: silver in solid solution in galena (PbS); cadmium in sphalerite (ZnS)

**What concentration is too little to bother with?

Special Considerations for Scarce Elements/Metals

Silver Cadmium

IF we do not deliberately separate out scarce elements from ore, they may "travel"

Pure calcite, CaCO3

Cobaltian calcite, (Ca,Co)CO3

Solid Solution = Atomic Substitution

Atomic Substitution in Al2O3, Corundum

Ruby Sapphire

Cr3+ Al 3+

Fe2+ + Ti4+ 2Al3+

Structure of corundum

Images from Wikipedia

Gemstones

Natural Abundance Makes a Difference

Natural crustal abundance (top scale, in ppm) vs. concentration needed for mining (bottom, in wt.%)

The more vertical the connecting line , the lessenrichment needed over the “natural” abundance: Al, Fe.The more horizontal the line, the more enrichmentneeded: lead, gold.

The most abundant elements form “their own minerals”and occur in relatively large, rich deposits.The more scarce elements “hop a ride” in mineralsdominated by other elements; they also occur insmaller, less concentrated deposits.

Abundant Scarce

Grade (%)

Grade (%)

“Abundant” elements

“Scarce” elements

Currentmining

Currentmining

Enrichment

Crustalabundance

Minegrade

Logarithmic scale: 102 101 100 10-1

Geochemistry Meets Economics

CVS4, Fig. 7.21

CVS3, Fig. 8.2

CVS4, Table 8.2

Crustal abund.

Enrich-ment

Minable grade

ppm

(from ore)

Typical Types of Minerals that are Ores of Geochemically Scarce Metals

Dominated by sulfides, oxides, and native metals

CVS4, Table 8.3

Base-metal sulfides

Alloy- and specialty-metal oxides

Native-element precious metalsPlatinum-

group metals

Ore deposits tend to be small in tonnage cf. abundant elements:

require huge-scale mining

Copper mine in Bisbee, Arizona

Special large-scalemining equipment

Special Needs for Scarce-Metal Mines

Geologic process or combination of

processes causes local

concentration and precipitation

Primary oremineral oxidized &dissolved here

Water’s metal contentreprecipitated assecondary oreminerals here

Primary ore

Surface

Secondary processing:

Oxidation + Reprecipitation

Open-pitmining

CVS4, Fig. 8.16

Precipitation by Hot Waters

Groundwater heated by igneous intrusion. Local heat source creates convection currents in the water. Leaching and pptn.

As in porphyry copper deposit

CVS4, Fig. 8.20

Porphyry copper deposits are clustered along coasts, above subduction zones.

Porphyry Copper Deposits of the World

CVS4,

Fig. 8.21

http://www.pmel.noaa.gov/vents/nemo/education/curr_p1_11.html

More Hot Water

scienceblogs.com/highlyallochthonous/2007/07/precambrian_black_smokers.php

Black smoker on the sea floor near mid-ocean ridge and rift

Copper, iron, and zinc sulfides precipitated in smoker “chimneys”

Cooler Waters (around 100°C)

Waters compressed out of sediments in a basin can move, leach metals, and precipitate minerals elsewhere.

CVS4, Fig. 8.24b

Lead-Zinc Deposits from Cool Waters

About 2 hrs. south of St. Louis

CVS4, Fig. 8.24a

Even Colder Water:

Ambient Temperature

Density separation and concentration of minerals by the flow of (river) water

CVS4, Fig. 8.32Examples = gold, diamonds

South African Gold Deposits: Ancient Streams and Deltas

CVS4, Fig. 8.34

Discovered by two prospectors in 1886

Ferrous/ferro-alloy metals: lend specific properties, especially to steels (Cr, V, Ni, Mo)

Nonferrous/base metals: Cu, Pb, Zn, Hg, Sn. Not very high in price (which annoyed the alchemists!)

Precious metals: Au and Ag, Pt, Pd, Os, Ir, Rh. "Noble metals”, meaning non-reactive (as well as”fit for a king”)

Specialty metals: Ta (electronics), Be (nuclear technology), Bi, Ga,Ge, Zr. Industry uses.

Rare-earth metals: heavily used in high-tech magnets; great needfor these in alternative/green energy producers (hybrid engines, windmills)

Classify Scarce Elements by Usage

Rare-Earth Metals

Non-ferrous base metalsFerrous metals

www.chemicalelements.com/

Cr, chromium represents 0.0096 wt.% of crust = 9600 ppm

Uses: stainless steel (hard, non-corrosive)

Aircraft engines, weapons

Chemicals--pigments, etc.

Refractory – chromite linings in furnaces

Chromium: Example of a Ferro-Alloy Metal

Geologically

Forms in large igneous rock bodies that cooled and crystallized

Formed layers that are 10's to 100's of miles long.

Dense crystals settle out of melt.

Represents how chromium builds up in melt after precipitation of minerals without chromium.

Politically

South Africa has 70% of world's chromium reserves.

Large supply of chromium. Limitations are political and social.

Chromium as a Strategic Metal: Essential to Well-Being of a Country

© Cédrick Gineste

Melt Crystals come out Chromium builds up

Chromitecrystallizes

Mineral chromite

Copper: Example of a Base Metal

Cu or Au was first metal used by humans; both occur in elemental (native) state

Malleable, electrical conductor, corrosion resistant

Uses: coins, wires, pipes, alloy.

Several kinds of ore deposits; deposition frequently controlled by plate tectonics

Magmatic segregation: immiscible liquids (compare to lava light)

Hydrothermal veins near magma bodies; porphyry Cu from subduction and melting of crust

Sedimentary fluids in normal sedimentary basins transport & precipitate much copper

Gold as a by-product of some copper miningMain copper oremineral, chalcopyriteCuFeS2

CVS4, Fig. 8.15

Growth in the copper industry (and use of copper) was spurred by the spread of electrification (copper wire)

Copper’s Environmentally InducedChanges in Ore Processing

Smelting

Acid dissolution(leaching)

Microbially mediateddissolution

Need to get the copper out of the copper sulfide mineral, CuFeS2

Heap leachingon a pad

Heat copper sulfide mineral

Drive off sulfur gas (leaving metal behind)

Trap sulfur gas BEFORE it reaches atmosphere.

Learn from nature how microbes help to break down sulfide minerals and release metal

From De Re Metallica (1558). Ancient metallurgists attempting to “win” various metals from their ore minerals by heating them.

What 6 countries account for most of the metal reserves of the world? Why?

Geologic specificity of the location of individual metals (ore deposits): plate-tectonic localization of ores (broad scale),

structural geologic & geochemical control (at progressively finer scales)

Plate tectonic lectures and text Figs. 8.4, 8.13, 8.21, 8.28.

Relations (over time) between:production/use and reserves (text Fig. 8.23, page 291)minable ore grades (text Fig. 8.2 on copper, p. 268, Fig. 8.31 on gold, p. 300)

Recycling: text Fig. 7.14 (page 249)

Lessons to Learn from Geochemically Scarce Metals

CVS4, Fig. 8.23

CVS4, Fig. 8.2

Average grade (wt.% Cu) of copper ore mined in the U.S. over time.

CVS4, Fig. 8.31 Average grade of gold ore mined in the US over time

~ 1 ppm