Download Lab #6 Notes - University of California, Santa Barbara

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
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•
•
•
•
•
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
away from
pluton
Limestone
near 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 landbased 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 byproducts.
• VMS deposits consists of over 90%
iron sulfide!
• Pyrite, Chalcopyrite, Galena,
Sphalerite, Barite
Simplified MVS Deposit- Bent Hill
Or hot springs
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 galenamanganese 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 backarc 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
Galena
Calcite on
fluorite
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
copper
Bingham 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 Barth
2007
Geology 114A
University of California, Santa Barbara
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