Metals

Ore Forming processes

Ore minerals vs. gangue minerals

Internal (primary or hypogene) vs external (includes sedimentary and secondary/supergene) ore forming processes

INTERNAL PROCESSES

Magmatic differentiation

Magma chamber; crystallization of minerals not simultaneous.

Chromite (chromium), magnetite (iron) metallic

platinum all crystallize early

Dense minerals sink in magma chamber - crystal settling

Hydrothermal mineralization

Magmatic water

Final stages of crystllization water concentrated. So are many economic metal atoms. As fluids escape along fractures they carry metals with them, then cool, precipitating minerals on the walls of the fractures

Also Meteoric water -convective circulation driven by heat of pluton

Two types of fluid may mix; distinguished by isotopic composition of oxygen - principally 18O

"Hydrothermal" may also include metals dissolved in other fluids - sulfides (sulfur at high temperature is a liquid/gas

Hydrothermal Ore Deposition Mechanisms

cooling; boiling; mixing; reaction with rocks (esp limestones)

contact metasomatism eg. skarn deposits

Forms of hydrothermal deposits

Veins - includes pegmatites deposits

Stockwork - fine scale veining

Disseminated - porphyry copper, molybdenum; gold&silver

Zonation of hydrothermal deposits

tin copper lead/zinc iron

Surface processes

Sedimentary

Placer deposits

clastic (detrital)

Chemical deposits

eg pre-C ironstones degree of oxidation/reduction important.

Manganese deposits

Residual and secondary enrichment (supergene) deposits

Enrichment of existing ores by leaching and redeposition.

Metallic Mineral Exploration

Even the biggest mineral deposits are not big enough and are quickly depleted. Therefore are always looking for new resources

Odds of finding a rich deposit not very good (have found obvious deposits in past and this means less to find in the future)

Skewed distribution -number of large deposits small in proportion to small deposits, therefore more likely to find small deposit than large one. Small deposits cannot be extracted as profitably as large ones (see later).

Thus most discoveries are "failures". Failure the norm! Anything that can be done to enhance this is welcome:- hence involvement of geoscientists such as geologists, geophysicists and geochemists

Prospect - potential resource

Exploration Methods

Geology

Mineral deposits tend to form in specific geological environments. Key is to study deposits already discovered, try to understand them; then apply this to newly discovered prospects

eg. mineral zonation a clue in hydrothermal deposits

field investigations - maps etc; geological sections

drill holes (boreholes) - vertical control

Geochemistry

Especially with hydrothermal deposits, rocks/soil tainted with elements

Stream surveys

Soil surveys

Pathfinder elements

Geophysics

Gravity

Magnetic

Electrical

Seismic

Deposit Evaluation

Drilling

Reserve estimations: volume and grade

Environmental Impact