Formation of Ores

Q. What is the most profitable mineral

commodity that is mined today?

A. Sand and Gravel

It doesn’t have to be flashy to make money …

http://eps.berkeley.edu/courses/eps50/documents/lecture31.mineralresources.pdf

What is an ore?

An ore is an aggregate of minerals

from which one or more minerals

can be extracted profitably.

Economic Geology

• Understanding of how metalliferous minerals

become concentrated key to ore deposits…

• Getting them out at a profit determines

where/when they come out

http://eps.berkeley.edu/courses/eps50/documents/lecture31.mineralresources.pdf

Ores

• Principally we discuss ores as sources of

metals

• However, there are many other resources

bound in minerals which we find useful

• How many can we think of?

Ore Deposits

• A deposit contains an unusually high

concentration of particular element(s)

• This means the element(s) have been

concentrated in a particular area due to

some process

• What sort of processes might concentrate

these elements in one place?

Ore minerals

• Minerals with economic value are ore

minerals

• Minerals often associated with ore minerals

but which do not have economic value are

gangue minerals

• Key to economic deposits are geochemical

traps metals are transported and

precipitated in a very concentrated fashion

– Gold is almost 1,000,000 times less abundant

than is iron

Gold Au• Distribution of Au in the crust = 3.1 ppb by

weight 3.1 units gold / 1,000,000,000 units

of total crust = 0.00000031% Au

• Concentration of Au needed to be

economically viable as a deposit = few g/t

3 g / 1000kg = 3g/ 1,000,000 g = 0.00031%

Au

• Need to concentrate Au at least 1000-fold to

be a viable deposit

• Rare mines can be up to a few percent gold

(extremely high grade)!

Less than 15 oxides occur in

quantities of > 0.5% in crustal rocks

An ore is a geochemical anomaly.

Some unusual process must:

1) remove specific elements, compounds

or minerals from ordinary rock,

2) transport these elements,

compounds, or minerals

3) concentrate the elements,

compounds, or minerals preferentially at

one spot or zone where the transport

stops.

Formation of ore

removalconcentrate

the primary mechanisms for

concentrating minerals into ores

involves either:

sorting by density

sorting by solubility.

• Sedimentary

– Placer – weathering of primary minerals and transport

by streams (Gold, diamonds, other)

– Banded Iron Formations – 90%+ of world’s iron tied

up in these

– Evaporite deposits – minerals like gypsum, halite

deposited this way

– Laterites – leaching of rock leaves residual materials

behind (Al, Ni, Fe)

– Supergene – reworking of primary ore deposits

remobilizes metals (often over short distances)

Ore deposit environments

Weathering and erosion as a

mechanism of separating and

concentrating chemical

constituents

Mechanical weathering and erosion

can concentrate minerals

We’ve already seen examples -

Wave action concentrates sand on a beach

Quartz sand typically mined from ancient beaches, sand bars, etc.

nuggets

lumps of metal

gold nugets: concentration by abrasion

brittle quartz removed by weathering

gold left behind

Placer deposits.

placers: deposits of heavy mineral

particles in stream bed.

Steps for making a placer

1. weathering removes mineral

particles from country rock.

2. kinetic energy of high velocity

stream transports mineral particles.

3. where kinetic energy drops

suddenly, high density particles stop,

lower density particles continue

density contrasts are substantial:

quartz - 2.65gm/cc.

gold - 19gm/cc.

Panning – same principle as a

placer, but in a pan

economically important placers:

gold nuggets

silver nuggets

platinum nuggets.

diamonds (carbon).

zircon (zirconium silicate).

uraninite (uranium oxide).

rutile (titanium oxide).

Chemical weathering and erosion

can concentrate minerals

Bauxite – aluminum ore

Progressive dissolution of silica from clays

in wet soils will eventually turn the

kaolinite clay Al2Si2O5(OH)4

Into

gibbsite Al(OH)3.

these soils become bauxite, a major ore of aluminum.

Bauxite – associated with

tropical climates

Ore deposit environments• Magmatic

– Cumulate deposits – fractional crystallization processes can

concentrate metals (Cr, Fe, Pt)

– Pegmatites – late staged crystallization forms pegmatites

and many residual elements are concentrated (Li, Ce, Be,

Sn, and U)

• Hydrothermal

– Magmatic fluid - directly associated with magma

– Porphyries - Hot water heated by pluton

– Skarn – hot water associated with contact metamorphisms

– Exhalatives – hot water flowing to surface

– Epigenetic – hot water not directly associated with pluton

Settling of crystals in a magma

chamber

can concentrate minerals

Spinel Group of Minerals

all relatively dense

• spinel: MgAl2O4

• Fe2+, Mn2+, Zn2+ substitute for Mg2+

• Fe3+, Cr3+, Mn3+ substitute for Al3+

• Ti4+ and V3+ can also substitute into the structure.

Basaltic magmas contain about

ten percent Fe

a few percent Ti

and trace amounts of Cr, Mn, and V

Under favorable circumtances these

become concentrated in the spinels.

Since the spinel crystals are dense, they can

sink to the bottom of a slowly cooling magma

body and make layers of ore

basaltic magma: 2500 kg/m3

Cr spinel: 4800 kg/m3

Liquid Immiscibility

Oil and water

don’t mix …

As magmas cool, they can split into two

liquids of different composition and

density.

– One of these liquids is the silica-rich melt. It

has the most volume

– The other, typically much smaller in volume,

can be rich in metal oxides, sulfides or

carbonates.

High T Low T

Desirable element preferentially concentrated into

low-volume melt

Types of Immiscible Melts

Oxide melts can be rich in Fe (Fe2O3, hematite)

and Ti (FeTiO3, ilmanite).

Sulfide melts can be rich in Ni, Cu, and the

platinum-group elements, in addition to iron

sulfur (FeS, pyrrhotite).

Carbonate melts can be rich in niobium, tantalum,

rare earths, copper, thorium, and phosphorous.

Dissolution by water can

concentrate chemicals

Geochemical Traps

• Similar to chemical sedimentary rocks – must leach

material into fluid, transport and deposit ions as

minerals…

• pH, redox, T changes and mixing of different fluids

results in ore mineralization

• Cause metals to go from soluble to insoluble

• Sulfides (reduced form of S) strongly binds metals

many important metal ore minerals are sulfides!

• Oxides – Oxidizing environments form

(hydroxy)oxide minerals, very insoluble metal

concentrations (especially Fe, Mn, Al)

Aqueous fluids in magma

�As magma cools, the volatiles (mostly water and carbon dioxide) that

they contain can form super-critical fluids.

supercritical fluids are on the verge of making the phase transition from

liquid to gas.

because of their extremely high temperature, many elements are

soluble.

These fluids can concentrate copper, molybdenum, gold, tin, tungsten

and lead.

The fluids from a large pluton can invade surrounding rocks, along

cracks called hydrothermal veins).

Hydrothermal Ore Deposits

• Thermal gradients induce convection of

water – leaching, redox, and cooling

create economic mineralization

Aqueous

fluids from

granitic

magma

have

invaded

surrounding

rock

porphery copper ore

These deposits are typically very low grade,

but can be huge.

Porphyry copper-molybdenum deposits are

one example

Skarns

acidic fluids from a granitic pluton invade and react with limestones

The limestone is dissolved and replaced by:

Silicate minerals,

sulfides of iron, copper, zinc, lead and silver,

oxides of iron, tin, and tungsten

gold

Geothermal systems

magma is the source of heat

but the water is just groundwater, and not

derived from the magma

High temperature geothermal systems occur where

ground water comes in contact with magma near

volcanoes.

Example: Yellowstone

Mechanisms that involve oxidation state of

the water

Ground water can carry dissolved materials.

These can precipitate out of solution if the

water becomes more or less oxidizing.

Example: Mississippi Valley Type lead-zinc deposits

ore minerals occur as veins in limestone that overlie a sandstone.

Ores include: lead as PbS, zinc as ZnS, copper as CuFeS2, and flourine as CaF2.

very saline brines containing sulfate ions (SO42-). These

brines transported the metals in solution through the sandstone.

The sulfate was reduced to S2-, perhaps by reaction with methane, and the minerals then precipitated out.

Example: uranium ore

soluble U6+ is produced during the weathering of igneous rocks.

U6+ was transported by groundwater until it encounters reducing conditions. It is reduced to U4+ and precipitates as uranium oxide.

Uranium oxide (yellow) replacing petrified wood

Buried wood makes ground

water more reducing, caused

uranium to drop out of solution

Large deposit in Canada formed

this way

Uranium deposits occur at or near an

unconformity between a sandstone (upper unit)

and a graphite (carbon) bearing gneiss (lower

unit). The sandstone provides the conduit for

hydrothermal fluids. The gneiss provides the

reducing conditions.

Marine hydrothermal fields

common on mid-ocean ridges

Massive sulfide deposits

• Hot, briny, water

leaches metals

from basaltic

ocean rocks

• Comes in contact

with cool ocean

water

• Sulfides

precipitate

1. Sea water percolates thought the hot (>300C) basaltic

crust, preferentially dissolving oxides and sulfides from the

rock

2. Water discharges into ocean, minerals precipitate as

soon as the water cools.

3. This process can concentrate copper, lead, zinc and

silver as volcanogenic massive sulfide deposits.

4. The minerals precipitate at different places in the

system, since they precipitate at different temperatures.

• �

Mechanism for producing the ore