Mineral Resources

Prof. Gorring
GEOS 112 PHYSICAL GEOLOGY
Dec. 3, 1998 Mineral Resources

1. Concentration of Economic Minerals

Recoverable deposits that have valuable metals in high concentrations are called ores; the minerals that contain the metals are ore minerals (sulfides, oxides, and silicates).
Most elements are widely dispersed in Earth's crust. They must be concentrated by geologic processes in order to be economic. Concentration factor must increase as crustal abundance decreases in order to be economic (5x for Fe, 4000x for Au).

2. Ore-forming Geologic Processes
Three general conditions 1) accessible source of elements; 2) transport mechanism; 3) site and mechanism for deposition.

Hydrothermal Ore Deposits:

Ore minerals that crystallize from hot, aqueous fluids. Fluids derived directly from degassing magma or from circulating groundwater. Ore deposits of native metals (Ag, Au) and sulfide minerals containing Pb, Fe, Zn, Cu, Hg.

Three main types of deposits:

1. Veins - along discrete fractures or joints (usually w/quartz and calcite).

2. Disseminated (porphyry) - widely distributed in tiny network of cracks.

(1 and 2) commonly occur in above subduction zones

3. Hydrothermal Vents - mid-ocean ridge environment; hot, circulating seawater emanating from vents ('black smokers") on sea floor. Hot water leaches metals from basaltic oceanic crust; precipitate when hot water comes in contact with cold seawater.

Igneous Ore Deposits

Ore minerals that crystallize from directly from magma.

Layered Mafic Intrusions: minerals concentrated in magma chambers by gravity settling or flotation. Most of the world's Pt, Cr, and Ni. Fe- and Cu- sulfides also.
Pegmatites: coarse-grained granitic intrusions; very late-stage differentiated magma thus many odd elements are concentrated. Source of B, Li, F, Nb, U.
Kimberlites: H₂O- and CO₂-rich ultramafic magma that comes from deep in the mantle (~150- 250 km). Sole source of diamonds (C).

Sedimentary Ore Deposits

Ore minerals concentrated by sedimentary processes.

Non-metallic: limestone (cement, building stone, fertilizer); evaporites (gypsum for Ca, Na- and K-salts); phosphate deposits (P for fertilizer); clean quartz sand and sandstone (glass, fiber optics); clays (ceramics).

Banded Iron Formations (BIF's): Most of the Fe for steel. Exclusively Precambrian in age. Onset of oxygenated atmosphere caused previously soluble Fe²⁺ to be oxidized to Fe³⁺, massive quantities of Fe³⁺ precipitated in marine settings.