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Gold Deposits: Where,
When and Why
John L. Walshe and James S. Cleverley*
1811-5209/09/0005-0288$2.50 DOI: 10.2113/gselements.5.5.288
DEPOSIT CLASSIFICATION AND GENESIS
Gold concentrations in mineable deposits range from ~1 to 100 parts
per million (ppm), or higher in bonanza deposits, in comparison to an
average crustal abundance of ~1.3 ppb. A diverse range of elements may
be associated with Au (La, Ce, U, V, Cr, Mo, W, Fe, Co, Ni, Pd, Pt, Cu, Ag,
Zn, Hg, B, Tl, C, Si, Pb, As, Sb, Bi, S, Se, Te). A common mineralogical
association is gold and quartz, but gold accumulations are also found
with carbonates, carbon, feldspars, Fe sulfides and oxides, base metal
sulfides, Fe ± Co ± Ni arsenides, and Fe ± Mg ± Ca ± V ± Cr silicates.
Deposits classified as epithermal gold–silver deposits, copper–gold
porphyry deposits, gold skarn deposits, iron oxide–copper–gold deposits
and intrusion-related Au deposits show some spatial ± temporal affinity
with intrusive magmatic activity, commonly in shallow crustal settings
at active plate margins. Such deposits are inferred or assumed to be
genetically linked to magmatic-hydrothermal activity. Gold-rich volcanichosted massive sulfides are related to sub-seafloor volcanic–hydrothermal
processes, whereas gold-rich sedimentary-exhalative deposits are
associated with the expulsion of basinal brines onto the seafloor in
intra-cratonic and epi-cratonic rift systems. Syn-deformational, mostly
gold-only deposits in metamorphic terrains and greenstone belts are
commonly classified as orogenic lode Au deposits. The economically
significant Carlin-type Au deposits, characterized by “invisible gold”
in pyrite, take their name from a geographically restricted area of
northern Nevada, which is the type and major locality of these deposits.
Similarly, conglomerate-hosted Au–U deposits are defined by the
deposits of the Late Archean Witwatersrand Basin of South Africa and
account for a staggering 45% of total gold production.
Many of the characteristics used to classify deposits may not be
particularly significant in terms of the processes that formed them.
Very different element and mineralogical associations may arise in
quite different geological settings from similar underlying processes.
Observations across a range of scales suggest that a limited number of
processes, some not fully understood, have controlled the distribution
of gold in the Earth’s crust.
A MINERAL SYSTEMS PERSPECTIVE
A mineral systems approach (Barnicoat 2008) can be used to help
understand where, when and why gold deposits form. This approach
asks five questions relating to the (1) geodynamic history, (2) architecture,
(3) fluid sources and reservoirs, (4) fluid pathways and driving forces,
and (5) metal transport and depositional mechanisms of a mineral
system. In order to establish the salient characteristics of gold systems,
it is necessary to consider the five answers in the broadest possible
context over different time and length scales of the system. Some
periods of Earth history produced more gold deposits than others. Most
important are the Late Archean (~2.7 to 2.6 Ga: greenstone-hosted
deposits), Paleoproterozoic (~2.0 to 1.6 Ga: iron oxide–copper–gold and
lode gold deposits) and, after a billion-year gap, the Phanerozoic
(~0.6 Ga to the present: porphyry and epithermal deposits). These
periods of gold production broadly correspond with phases of new
crustal growth although, paradoxically, the deposits do not necessarily
occur in the most juvenile crust. Rather, gold deposits and provinces
are commonly spatially associated with cratonic margins, cross-arc
structures, tears in slabs and trans-crustal structures. These architectural constraints, together with geodynamic links to plate reorientation,
slab rollback and slab foundering, suggest gold metallogenesis is in
* CSIRO Exploration & Mining, PO Box 1130 Bentley, Western Australia 6102, Australia
E-mail: [email protected]; [email protected]
E lements , V ol . 5, p. 288
A crust–mantle perspective on gold mineralizing systems. Geodynamics
and architecture constrain the release of mantle fluids into the crust
and gold mobility. Significant deposits have developed where thermo-chemical
gradients (redox, pH, water activity) were sustained by focused flux of anhydrous,
oxidized (CO2 ± SO2) and hydridic (H2, CH4, HCl) mantle/crustal volatiles, triggered
by plate-scale deformation events.
Figure 1
some way related to fluid fluxes from the mantle. If there is a common
unifying theme to the genesis of gold deposits, it could lie in the nature
of the fluid reservoirs in the mantle and the forces that drive fluid
release. Storage of anhydrous volatile-rich fluids in the mantle and their
subsequent release are conceivable without significant magma production.
The abundance of SO2 in volcanic gases implies that anhydrous fluids
in the outer few hundred kilometres of the mantle are highly oxidized
(CO2 ± SO2 fluids). However, thermodynamic studies and the mineralogy
of inclusions in diamondiferous kimberlites suggest hydridic (H2 -rich)
fluids may dominate at depths greater than 300–400 km.
Fluxes of reduced and oxidized volatiles, of either mantle or crustal
derivation, could sustain thermo-chemical gradients (e.g. redox and
acidity), promote metal transport and drive terrain-scale metasomatism.
Gold mineral systems may be viewed as thermo-chemical engines with
their roots deep in the mantle (Fig. 1). Such a model links metallogenesis
to secular changes in architecture and geochemistry of the planet over
its 4.5-billion-year history.
GOLD DEPOSITION MECHANISMS
At the smallest scale, we need to understand the processes that drive
metal transport and deposition. Specific element, mineral and isotope
associations, particularly with bonanza gold deposits, imply deposition
has occurred in chemical environments dominated by fluids far from
equilibrium with local host rocks and ambient fluids. In situ Sr isotope
analysis of apatite associated with gold in laminated quartz veins point
to an oxidized, CO2-rich (water-poor) fluid with a strong mantle affinity
at the time of gold deposition. An understanding of the nature and extent
of gold complexing in anhydrous fluids has not yet been achieved. It
is possible that very rapid growth of deposits may have occurred, in tens
of years, from relatively small volumes of fluid, assuming the presence
of gold (and other metals) at concentrations of 103 –10 4 ppm. This
mineral systems approach addresses fundamental mantle–crust interactions
and suggests the following research priorities:
•p
roperties of non-aqueous fluids and the capacity of such fluids to
dissolve and transport metals;
• l inks between mantle degassing and fluid-driven processes in the
Earth’s outer ~10 km;
•m
apping of fluid flow paths to resolve their sources and sinks and
enhance mineral exploration.
REFERENCE
Barnicoat AC (2008) The pmd*CRCs Mineral Systems approach. In: Korsch RJ,
Barnicoat AC (eds) New Perspective: The Foundations and Future of Australian
Exploration. Abstracts for the June 2008 pmd*CRC Conference, Perth,
Geoscience Australia, 2008/09, pp 7-16
288
O c tober 2009