EES 119/219
Lecture 6
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Hydrothermal processes
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Magmatic processes
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Base metals
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Porphyry copper Deposits
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Massive Sulfide Deposits
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Endogenic vs. Exogenic
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Endogenic processes derive their energy
from the sun: weathering; transport due to the hydrologic cycle and atmospheric
processes
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Exogenic processes are based on energy
coming from the earth itself: Mountain building; intrusion of magma;
hydrothermal processes
•
Hydrothermal activity: Water is heated and
convects through rock
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Questions:
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Heat source: in most cases cooling magma
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Extrusive processes: cooling magma at or
close to the surface
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Intrusive processes: cooling magma at
depth
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Permeability: Rocks have pores, sometimes
connected
à
water can move through rock, but rates are very slow, between centimeters and
meters per year
•
Origin of water: Water can come from the
surface (as groundwater – meteoric water) or from the magma (magmatic water)
Relation to
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Hot water transports minerals in solution
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Location: Plate boundaries
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Subduction zones: intrusive, andesitic
volcanism
à Porphyry Copper
Deposits
•
Mid-ocean ridges: extrusive, basaltic
volcanism
à
Massive Sulfide Deposits
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Temperatures are high:
–
150oC < T < 400oC
•
Pressures are elevated
–
10 bar < p < 2000 bar
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Solubility of minerals in water depends
strongly on p and T; in most cases solubility increases with increasing T and p
(Example: NaCl, Fig. 1)
à Increase in T (and/or p) causes elements
to go into solution
à
Decrease in T (and/or p) causes deposition of
ore minerals
à
Changes in T and p are dominant for formation
of ore minerals
Base
Metals
•
Cu, Pb and Zn are called base metals
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They form generally minerals with S: Sulfides
à
CuFeS2 chalcopyrite
à
PbS Galena
à
ZnS Sphalerite
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Two important types of hydrothermal deposits
à
Massive Sulfide Deposits
à
Porphyry Copper Deposits
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Sn is often also referred to as base
metal, but has different mineralogy
à Cassiterite
SnO2
Porphyry
Copper Deposit
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General Descriptions:
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Large, low-grade deposit
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Example:
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Predominantly mined for Cu, with traces of Au
and Ag
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Grade: 0.5 à 1.5 Cu, mostly as
CuFeS2
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Concentric zoning of mineralization
Model for Porphyry Copper
Deposits (Fig. 2)
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Associated with andesitic volcanism:
àFormed close to
subduction zones (converging boundaries)
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Found in young formations (< 75 Ma)
àFormed close to surface
in elevated altitudes (erosion limits age of deposits)
•
Concentric zoning of mineralization
à alteration due to
convecting fluids
•
Disseminated ore
àSlow deposition due to
cooling in the crust
•
Associated with shallow, porphyritic
intrusion
à formation close to,
but not at the surface
Massive
Sulfide Deposits
General Description
·
Lens-shaped bodies of massive sulfide ores
·
Relatively small dimensions: 200à1000
m diameter; 20à
50 m thickness
·
High grades of Cu (2à
5%); Zn (5à15%);
Pb (5-15%) with traces of Au and Ag
·
Associated with basaltic volcanism
·
Deposits found throughout geologic history
Cross section through a Massive
Sulfide Deposit (Fig. 3)
Explanations:
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Multi-ore deposits, sulfide ores
à derived from fresh
magma
•
Association with basaltic volcanism
à Mid-ocean Ridge
setting (diverging boundary)
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Massive ore mineralization
à rapid cooling
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Strong zonation of ore mineralization
à reflects different T
ranges of mineralization
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Covered by sediments
à Seafloor setting
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Age distribution: from Archean to Present
à shielded from erosion
by sediments
A modern example of these
formation processes is the hydrothermal activity at Mid-Ocean Ridges (Fig. 4; Fig.5)