Important Types of Ore Deposits

Affect of Elements on Mineral Color

• The major factors responsible for the production of color in minerals fall into five categories:

1) The presence of an element essential to the mineral composition

2) The presence of a minor chemical impurity

3) Physical defects in the crystal structure 4) The mechanical mixture of very fine

impurities 5) The presence of finely-spaced structures

in the mineral

• Chromium, Cr, produces the color orange-red color of crocoite, (lead chromate). PbCrO4

• Copper, Cu, produces the azure blue color of azurite, (copper carbonate hydroxide). Cu3(CO3)2(OH)2

• Iron, Fe, produces the red color of limonite, (hydrated iron oxide hydroxide). Fe2O3.H2O

• Manganese, Mn, produces the pink color of rhodochrosite, (manganese carbonate). MnCO3

• Vanadium, V, produces the red-orange color of vanadinite, (lead vanadate chloride) Pb5(VO4)3Cl

Beryl Be3Al2(SiO3)6

• Beryl containing iron (Fe): • Aquamarine = Fe++, beryl is blue • Heliodor = Fe+++, yellow • Green beryl :  due to mixtures of Fe2+ and Fe3+ • Beryl containing Manganese(Mn): • Morganite : Mn++ is pink • Red beryl : Mn+++ is red • Beryl containing Chromium(Cr): • Emerald = emerald green : Cr+++

• Oxidation state (e.g., Fe2+ vs. Fe3+) also affects the color!

Skarn Deposits• Skarn deposits are

formed by high temperature contact metamorphic processes where a silicic magma (granite) intrudes carbonate rocks

• Common minerals include magnetite, molybdenite, sphalerite, galena, scheelite, hematite, garnet, epidote

• Important source of tungsten, iron, molybdenum, zinc, lead

Evans 1997

Papoose Flat Skarn, E. CA

Limestone near plutonLimestone

away from pluton

Skarn

Volcanogenic Massive Sulfide Deposits (VMS)

• Volcanogenic massive sulfide ore deposits (VMS) are a type of Cu-Zn ore deposit associated with hydrothermal events. They are predominantly stratiform accumulations of sulfide minerals that precipitate from hydrothermal fluids in a wide range of ancient and modern geological settings (especially the seafloor but also land-based volcanics).

• VMS represent a significant source of the world's Cu, Zn, Pb, Au, and Ag ores, with Co, Sn, Ba, S, Se, Mn, Cd, In, Bi, Te, Ga and Ge as co- or by-products.

• VMS deposits consists of over 90% iron sulfide!

• Pyrite, Chalcopyrite, Galena, Sphalerite, Barite

Or hot springs

Simplified MVS Deposit- Bent Hill

VMS

• Most VMS deposits show metal zonation, caused by the changing physical and chemical environments of the circulating hydrothermal fluid. Ideally, this forms a core of massive pyrite and chalcopyrite around the throat of the vent system, with a halo of chalcopyrite-sphalerite-pyrite grading into a distal sphalerite-galena and galena-manganese and finally a chert-managanese-hematite facies.

VMS Today• VMS deposits are forming today

on the seafloor around undersea volcanoes along many mid ocean ridges, and within back-arc basins and forearc rifts (regions of extension and crustal thinning). Black Smoker

White Island, New Zealand

Mississippi Valley Type Deposits (MVT)• MVTs are hydrothermal lead-zinc ore deposits that are characterized by

– (1) low-temperature formation (100°-150°C) – (2) epigenetic (forming after) emplacement within restricted

dolostone or limestone strata of sedimentary basins (i.e., stratigraphically controlled)

– (3) precipitation from highly saline brines – (4) the presence of barite and/or fluorite mineralization

• MVT deposits tend to occur in clusters at the margins of sedimentary basins, and they are intimately associated with the evolution of these basins.

• They are also typically

found far from, and lack a

genetic relationship to,

igneous activity or igneous

rocks.

MVTs• Although they are found

around the globe, the MVTs within the Mississippi River drainage basin are the largest and were the first to be studied in detail.

• The most abundant minerals in MVTs are sphalerite and galena, barite, fluorite, calcite, dolomite, quartz and pyrite

Sphalerite

Calcite onfluorite

Galena

Porphyry Copper Deposit

reynolds.asu.edu/sierra_cobre/p_formation.gif

Porphyry Copper Deposits• Characteristic alteration and mineralization

zones around the porphyry• Groundwater is very important for concentrating

copper minerals• Pyrite, chalcopyrite, molybdenite, bornite, native

copperBingham Copper Mine, Utah

Borax Deposits, US Borax Mine, Boron, CA

• 19 Ma, active lava flows spread across the Mojave Desert. Over thousands of years, water collected in the basin, forming a lake. Clay from the surrounding rock formed an impermeable layer at the bottom.

• Hot springs rich in boron flowed into the lake, quickly cooling to form borax crystals. Finally, another layer of clay washed in, sandwiching the borax deposit in between (stratiform deposit). Then the lake dried up.

• Boron and Searles Lake (in E. CA) produce 95% of the world’s borax!

Pegmatites

• Pegmatites are coarse-grained igneous rocks of granitic composition, typically forming dikes.

• Most common minerals are quartz, feldspars, and micas.

• The crystals formed in pegmatite dikes are large because the molten material is accompanied by an unusually large amount of water-rich gas. Without the gas, granite will form.

• Gem pockets form where the gas is trapped to form a large bubble. This allows crystals free growth. Rare elements are concentrated in the gas and account for the unusual compositions and colors.

• Lepidolite associated with pegmatites is the main source of the rare metal lithium

Bauxite Deposits

• Bauxite is an aluminum ore and is not actually a mineral. It consists largely of gibbsite Al(OH)3, diaspore AlOOH, together with hematite, the clay mineral kaolinite and small amounts of anatase TiO2.

• Bauxite occurs in many countries of the tropical belt. Largest producers are Australia, Jamaica and Guinea.

• It is formed by lateritization (intense chemical weathering in hot, wet, tropical areas) of various silicate rocks such as granite, gneiss, basalt, and shale.

Epithermal Gold Deposits• Epithermal gold deposits

(like CA’s Mother Lode) form when hydrothermal fluids infiltrate cracks and faults in the rock.

• The gold is commonly associated with quartz veins

• Rock permeability is highest immediately following earthquakes. This is when most epithermal mineralization occurs.

• Tectonic setting is very important for this type of gold deposit

Summary

• Ore deposits typically require:– Heat– Permeability (faults/fractures)– Mineralizing fluid– Specific rock types– Specific tectonic conditions

Created by Nicolas Barth2007

Geology 114AUniversity of California, Santa BarbaraSome images herein borrowed from

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