#OPH= Ore Petrology
Handbook, David Groves, Course Notes, Geology 330
# = not in the Library
Ore is a metalliferous mineral, or an aggregate of
metalliferous minerals, more or less mixed with gangue, which from the
standpoint of the miner can be won at a profit, or from the standpoint
of the metallurgist can be treated at a profit.
Definitions of ore all emphasize:
(a) that it is material from which we extract a metal, and
(b) that this operation must be a profit-making one.
Economically mineable aggregates or ore minerals are
termed orebodies, oreshoots, ore deposits or ore reserves.
.......ore is rock that may be, is hoped to be, will be, is or has
been mined; and from which something of value may be (or has been)
Industrial minerals (after Noetstaller, 1988)
...have been defined as any rock, mineral or natural occurring
substance of economic value, exclusive of metallic ore, mineral fuels
...is simply the unwanted material, mineral or rock, with which ore
minerals are usually intergrown.
Growth comparison for mineral products
Mineral Resource Classification
Reserves - material that has been identified and can be
extracted at a profit
Reserve base - reserves and identified material that might be
extractable in the future, depending on geological factors
Resources - reserve base plus undiscovered deposits
regardless of economic factors
Geological setting of mineral deposits
Earth consists of four global scale divisions:
biosphere, and lithosphere
Mineral deposits are part of the lithosphere
Some review of definitions:
Mineral: control the distribution of elements in earth,
naturally occurring solids with a characteristic crystal structure and
define chemical compositions.
Solid Earth is divided into the core, mantle, and crust
Core: iron and related elements Mantle: ultramafic
rocks, contain Cr, Co, Ni Crust: ocean crust: 5-10km
thick - mafic rocks (basalts) continental crust: 20 to 70km
thick felsic igneous, metamorphic rocks, sedimentary rocks -
Mineral deposits are the direct results of plate tectonics which
involves the movement of lithospheric plates about Earths surface
Lithosphere: rigid outer 100km of Earth, including the continental
and ocean crust and the uppermost mantle
Lithosphere plates move about on the underlying, more plastic
mantle, which is known as asthenosphere
Lithospheric plates have three types of margin:
1. divergent margins
2. mid-ocean ridges (ocean) and rifts (continent)
3. Convergent margins (subductiion zones and island arcs)
Continental collision - obduction zones
Kesler, 1994, Fig. 2.2
MT = Meteoric water systems MW = Magmatic water
systems MM = Metamorphic water systems MG =
Magma SW = Seawater systems CW = Basinal
Ore deposits and plate teconics - exploration perspective
Implications of plate tectonics to mineral deposits:
1. utilization of plate tectonics has never resulted in the
discovery of a metal deposits! 2. the use of plate tectonic
principles has enormous significance to the exploration for metal
metal deposits are found by the investigation of mineralized or
altered outcrops, and follow-up of geochemical and/or geophysical
anomalies. Main conceptual tools are that of lithology and structure.
main impact of plate tectonics is to broaden and deepen our
understanding of the tectonic environments in which many of these
lithologies and structures occur.
clearer idea of ore-generating environment.
metallogenesis now understood within the context of continental
evolution, this in turn permits the conception of exploration programs
aimed at specific types of deposits.
utilization of plate tectonics in exploration provides a meaningful
framework within which the geologic and geochemical processes that lead
to economic metal concentrations can be more fully understood.
more specific: increased understanding of paleogeography,
metallogenic zoning in arc systems, arc segmenation, intraplate
magmatism, and nature and products of rifting.
Geological processes that form mineral deposits
Most of the geologic processes that form mineral deposits involve
chemical changes in rocks and minerals where elements or compounds that
were dispersed through large volumes of rock are collected and
concentrated to form ore minerals or compounds.
The most effective agents for chemical changes of this type are
water and magmas both of which dissolve elements and crystallize
Because hot water is a better solvent than cold water, waters that
form many mineral deposits are hot and are known by the special name of
The formation of ore minerals and compounds by magmas and water can
take place at Earth’s surface or at depth in the crust or mantle.
In many cases, these chemical processes make a mineral deposit that
is rich enough to be an ore deposit. In other cases, further
concentration is needed by an additional chemical process or a physical
Geological Processes and Mineral Deposits
Surface and near-surface ore forming processes :
Weathering Physical sedimentation Chemical sedimentation
Subsurface ore forming processes:
Involving water (hydrothermal) Involving magmas
Ore-bearing fluids can be subdivided into six categories:
1. silicate-dominated magmas or derived oxide, carbonate, or
sulfide-rich magmatic liquids 2. water-dominated hydrothermal fluids
that separate from magmas 2a. meteoric waters 2b. seawater
2c. connate waters trapped in pore spaces in sediments 3. fluids
associated with metamorphic processes
If the fluid is heated and liquid the aqueous fluids can be
considered a hydrothermal solution If the fluid is a gas, it is
called pneumatolitic above the critical point, we are dealing with
Magma and magmatic fluids
Magma is “naturally occurring mobile rock material, generated within
the Earth and capable of intrusion and extrusion....” (AGI Glossary of
As a magma cools it crystallizes and separates into fractions by
complicated processes of fractional crystallization or igneous
differentiation. metallic elements can be concentrated by rock-forming
mechanisms in various portions of the resulting igneous assemblages.
Locally, specific minerals such as chromite may be so abundant that
the resulting igneous rock itself constitutes ore.
If a partly crystallized magma is subjected to differential external
stresses, the fluid fraction can be squeezed away from the crystalline
mush. This process is called filter pressing.
If the liquid is squeezed out into the surrounding rocks, the
process is known as magmatic injection (magmatic injection deposit).
Schematic sequence of magmatic events leading to ore magmas and their
Igneous rocks, differentiation, and hydrothermal fluid involvement
associated with fractional crystallization
Processes that deposit ore minerals from hydrothermal solutions
mobile elements LIL Large ion lithophile such as Li, Be, B, Rb, Cs
also significant quantities of alkalis, alkali earths and volatiles
Na, K, Ca, Cl, and CO2 metals such as Fe, Cu, Zn
Water is the principle mobile constituent in all magmas, increases
in amount with increasing differentiation and plays an important part in
the transportation of many ore components.
Estimates of water in magmas range form 1 to 15%.
Micas, clay minerals, zeolites, and amphiboles contain small amounts
of chemically bonded water
water that has passed through and equilibrated with the atmosphere
is called meteoric waters.
meteoric waters and convection cells - circulation
stable isotope signature - meteoric water line
seawater as an ore-forming fluid are best described in the contexts
of evaporates, phosphorites, submarine exhalalites
deposits related to seawaters are:
Alpine Peridotite chromite,
Mt Isa and Broken Hill???
Sedimentary iron deposits
water trapped in sediments at the time they were deposited is known
as connate water
connate waters are fossil waters
observed in oil field exploration
rich in sodium and chloride, also considerable amounts of calcium,
magnesium, and bi-carbonate, and many contain strontium, barium and
can also contain light hydrocarbons
stable isotope ratios near SMOW
Mississippi Valley type deposits
recrystallization of hydrous and anhydrous phases
volatile and mobile constituents are activated during metamorphism
and forced from the rock to migrate toward cooler
Ore fluids and geothermobarometry
Necessary to understand the nature of ore-component transporting
media: liquid or gases
Deeper ore-bearing fluids are studied by observations of:
studies of ores (ore petrology, thermodynamics) and gangue,
from laboratory studies of appropriate synthetic systems (synthetic
fluid inclusions, sulfide systems),