Paleoplacer deposits (1)
Paleoplacer deposits such as those of the Witwatersrand consist of
stratiform layers (bankets) of auriferous quartz-pebble conglomerate,
pebbly quartz arenite and cross-bedded arenite, with gold locally
enriched in thin carbonaceous seams. The deposits occur in mature flu-
viatile to deltaic facies rocks in extensive cratonic sedimentary basins.
The most significant deposits of this type occur in Late Archean and
Early Proterozoic sedimentary basins, perhaps reflecting an important
control by an oxygen-poor atmosphere.
The ore and associated minerals consist of native gold and pyrite, in
most cases of detrital origin, and of the heavy minerals magnetite, ura-
ninite, ilmenite and locally hematite. The ores are typically gold-rich rel-
ative to silver (Au:Ag = 10:1). Hydrothermal alteration, mainly
sericitization and chloritization, overprints some deposits and may have
locally re-distributed gold. In addition, pyrite in some deposits is parage-
netically late and may result from sulphidation of detrital oxide grains.
Neverthless, the detailed distribution of gold is controlled by primary sed-
imentary facies variations.
Submarine gold-rich massive sulphide deposits (2)
These deposits, as exemplified by Horne and Boliden, consist of
banded and stratiform massive lenses and adjacent stockwork zones but
significant syntectonic sulphide veins are also present in deformed and
metamorphosed deposits. The deposits occur in mixed submarine vol-
canic, volcaniclastic and sedimentary sequences in greenstone belts of
all ages, typically metamorphosed to greenschist and lower amphibolite
facies. They are distinguished from other massive sulphide deposits in
that gold concentrations in parts per million (ppm) exceed the percent-
age of combined base metals. Mineralization is composed mainly of
pyrite and base metal sulphides, but commonly contains complex high-
sulphidation assemblages including minor phases such as bornite,
sulphosalts, arsenopyrite and tellurides. Ore contains considerable iron,
variable amounts of Cu, Pb, and Zn and has locally high concentrations
of As, Sb, Hg. Silver content generally exceeds that of gold (Au:Ag = 1:2
to 1:10).
The deposits occur in districts containing subvolcanic intrusions
and other massive sulphide deposits that contain proportionately less
gold. They are hosted mostly by felsic volcanic tuffs and derived schist,
near their interface with basalt or sedimentary strata. The host rocks are
typically sericitized and chloritized, and some deposits are enveloped by
zones of aluminous alteration resulting from extreme alkali depletion.
The latter may be another distinguishing feature of gold-rich massive
sulphides and is thought to result from boiling of ore-forming fluids
(Hannington, 1994; Sillitoe et al., 1996). This further suggests that shal-
low water sequences showing transition to subaerial conditions may be
more favourable than other massive sulphide environments (Sillitoe
et al., 1996).
Hot spring deposits (3)
Hot spring deposits like McLaughlin contain siliceous sinter and
geyserite formed at the paleosurface but also include funnel-shaped
hydrothermal and tectonic breccias and quartz stockworks narrowing at
depth into structurally controlled feeder zones. These deposits occur in
belts of subaerial mafic and felsic volcanic centres and intervening
clastic sedimentary rocks in subduction-related arc settings. They are
mainly recognized in young volcanic belts but, perhaps due to their poor
degree of preservation, are not known to exist widely in older, deformed
terranes. Nonetheless, some Paleozoic examples have been reported
(Cuneen and Sillitoe, 1989).
Mineralization is generally hosted by vent and hydrothermal brec-
cias in volcanic, volcaniclastic or sedimentary rocks, as well as by sub-
volcanic porphyritic intrusions. It consists of micron-scale gold and
electrum in zones of massive silicification, less commonly in sinters
which are nonetheless defining features, and in crustiform banded
quartz, chalcedony ± adularia and barite + carbonate veins and stock-
work zones. Mineralization typically contains up to 5% pyrite ± marca-
site, pyrrhotite, cinnabar, stibnite, realgar or arsenopyrite and tellurides,
with elevated concentrations Hg, As, Sb, Tl, Ba and locally Mo and W. It
displays a characteristic steep vertical metal zoning with near-surface
enrichments in Au, Hg, Sb, Tl, and As and with increasing Ag and Ba
content with depth (Au:Ag from 1:1 near surface to 1:30 at depth). Asso-
ciated alteration consists of massive silicification and adularization of
breccia zones, grading outward into advanced argillic and argillic alter-
ation zones, and downward into narrower zones of adularia along vein
margins and as replacements along hydrothermal conduits. These
deposits are thought to represent the paleosurface expressions of deeper
adularia-sericite deposits (Figure 1).
Adularia-sericite epithermal deposits (4)
These deposits, also referred to as low-sulphidation epithermal
deposits, consist of subvertical banded and breccia veins of quartz and
chalcedony with associated irregular stockwork and hydrothermal brec-
cia zones and less common disseminations. They occur in volcano-plu-
tonic continental and island arcs at convergent plate margins, in
association with subaerial intermediate to felsic calc-alkalic volcanic
centres and related subvolcanic porphyritic intrusions or, less com-
monly, with alkalic-shoshonitic igneous rocks and related sedimentary
rocks. The deposits are hosted by extensional or transcurrent structures
and are commonly associated with calderas as in the case of Creede.
They commonly occur immediately above basement to the host volca-
nics but also in basement lithologies, and relatively impermeable rock
types play an important role in fluid ponding in some deposits. Most
significant deposits of this type are Cenozoic in age.
Veins and hydrothermal breccias consist of crustiform chalcedonic
quartz, adularia and Mn-carbonate with pyrite, electrum, high-Fe
sphalerite, arsenopyrite, silver-sulphides and sulphosalts. The associ-
ated metals include Au, Ag, As, Sb, Hg, Pb, Zn, and Cu, and mineraliza-
tion is typically vertically zoned and grades downward over distances of
hundreds of metres into precious-metal-poor, base-metal-rich (Zn, Pb,
Cu) ores. Mineralization is either base-metal poor with Au:Ag = 10:1 to
1:10, or base-metal-rich with Au:Ag <1:25. Hydrothermal alteration
grades outward from silicification and sericite-illite/smectite assem-
blages ± fine-grained adularia near the veins to a broader external zone
of propylitic alteration.
Alunite-kaolinite epithermal deposits (5)
These deposits, also known as high-sulphidation or acid-sulphate
types, consist of disseminated and replacement mineralization in irregular
strata-bound to mushroom-shaped discordant vuggy silica replacementzones and
less common hydrothermal breccias, stockworks and veins.
They are associated with subaerial, calc-alkaline andesite to rhyodacite
volcanic centers and related subvolcanic porphyritic intrusions and sedi-
ments in volcano-plutonic arcs at convergent plate margins of all ages. The
deposits are hosted by volcanic dome-vent complexes, maar diatreme and
clastic sedimentary rocks above basement, or by underlying basement
lithologies, in association with regional normal and transcurrent faults or
with diatreme ring faults. Most deposits such as Goldfield are Cenozoic in
age, with a few Mesozoic to Precambrian examples.
Mineralization commonly consists of “high sulphidation” assem-
blages including phases such as pyrite, enargite-luzonite, chalcopyrite,
tennantite-tetrahedrite and gold in a gangue of massive or vuggy silica
or of quartz ± alunite in veins and breccias. This ore assemblage com-
monly occurs within, and overprints, the core of zones of massive silici-
fication, above or grading outward into advanced argillic, argillic and/or
propylitic alteration zones which are hallmarks of deposits of this type.
The Au:Ag ratios of the ore range from 1:2 to 1:10, and associated metals
include As, Cu, Sb and Bi and locally Hg, Pb, Te and Sn. The deposits
have limited vertical extent (<500 m) and lack significant vertical zon-
ing. They commonly occur above porphyry Cu or Cu-Au systems.
Porphyry gold deposits (6)
Porphyry Au and Au-Cu deposits are irregular to pipe-like zones of
quartz-sulphide stockwork and associated sulphide disseminations
confined to intrusions and their immediate wall rocks. They occur in
volcano-plutonic belts (including greenstone belts) in continental or
island arc settings, overlying a wide range of basement lithologies. The
deposits are associated with composite stocks of calc-alkaline (diorite,
granodiorite, quartz-monzonite) and alkaline (monzonite, quartz syen-
ite) compositions, with locally preserved remnants of coeval volcanic
rocks. The quartz stockworks are less well developed in deposits associ-
ated with alkalic intrusions.
Pyrite is the dominant sulphide mineral; its abundance ranges from
1–3 vol.% in the ore zone to 5–10 vol.% outside, and it is accompanied
by up to 20 vol.% hydrothermal magnetite ± hematite either in the stock-
work or as wallrock disseminations. Ore typically contains more silver
than gold (Au:Ag <1), and associated metals include Cu, Bi, Te, ± Mo.
Mineralization is coincident with K-silicate alteration, or albite and calc-
silicate alteration in alkalic systems, in most cases grading outward into
large zones of propylitic alteration. In some deposits, argillic or advanced
argillic alteration overprints part, or most, of K-silicate alteration.
Breccia pipe deposits (7)
Such deposits, as represented by Kidston, consist of mineralized
funnel-shaped, pipe-like, discordant breccia bodies and sheeted frac-
ture zones in mafic to felsic calc-alkaline volcanic environments in vol-
cano-plutonic arcs and greenstone belts. They are controlled by graben
faults and ring complexes related to cauldron development. Ore is
hosted by a variety of breccia types, including magmatic-hydrothermal,
phreatomagmatic, hydraulic and collapse varieties. Breccia cement con-
sists dominantly of quartz, carbonate (calcite, ankerite, siderite), with
specularite and tourmaline at some deposits. The ore contains pyrite,
chalcopyrite, sphalerite, galena, and pyrrhotite, with minor molybden-
ite, bismuthinite, telluro-bismuthite and tetrahedrite, which occur
either in the matrix or in rock fragments. Ore is silver-rich (Au:Ag =
1:10), with associated Pb, Zn, Cu ± Mo, Mn, Bi, Te, W), and a lateral
(concentric) metal zoning is present at some deposits. A sericite-quartz-
carbonate-pyrite alteration assemblage and variably developed silicifi-
cation is coincident with the ore zones grading outward into propylitic
alteration. An early stage K-silicate alteration is present at some depos-
its. Breccia pipe deposits are commonly associated with intrusion-
related hydrothermal systems.
Skarn gold deposits (8)
Skarn deposits, as exemplified by Hedley, consist of disseminated to
massive sulphide lenses and crosscutting veins in carbonate platform
sequences superimposed by volcanic and/or plutonic arcs. Mineraliza-
tion is associated with Al-rich garnet-pyroxene skarn assemblages
replacing limestone, calcareous siltstone and carbonatized volcanic
rocks adjacent to diorite or granodiorite stocks, dykes or sills. They
occur in some districts along with porphyry Cu-Mo mineralization and
tend to be associated with more mafic, hotter intrusions.
Ore bodies are composed of pyrite, pyrrhotite, arsenopyrite and
lesser telluride minerals. Ores contain locally high concentrations of As,
Bi, and Te, and show wide variations in their gold to silver ratios (Au:Ag
= 1:10 to 10:1). Retrogression of prograde skarn assemblages is common
and gold mineralization is considered to be related to such retrogression.
Carbonate replacement (manto) deposits (9)
Carbonate replacement deposits like those at Ruby Hill, Nevada con-
sist of discordant pipes or tabular concordant bodies of massive sul-
phides replacing limestone or dolostone beds, commonly interlayered
with calcareous quartzite, quartzite and phyllite. They occur in conti-
nental platform carbonate sedimentary sequences superimposed by
volcano-plutonic arcs. The deposits occur close to a “marble front”
related to nearby intrusions, represented in some cases only by dioritic
sills and dykes, but they are in many cases remote from intrusive rocks.
Fault intersections are important in localizing discordant mineralized
pipes.
Ore bodies are composed largely of pyrite, and may contain variable
amounts of pyrrhotite, galena, sphalerite, chalcopyrite, magnetite and
arsenopyrite. The ores are typically silver-rich (Au:Ag < 1), with ele-
vated concentrations of As, Bi, Hg, and they may contain several percent
of combined Pb, Zn and Cu. Associated hydrothermal alteration is gen-
erally restricted to the immediate vicinity of the ore bodies and consists
of silicification of carbonate rocks and sericitization of adjacent clastic
sedimentary rocks.
Sediment-hosted micron gold deposits (10)
These deposits, also referred to as “Carlin-type” deposits, are irreg-
ular discordant breccia bodies and concordant strata-bound dissemi-
nated zones confined to particular stratigraphic units. They occur incarbonate
and impure carbonate-argillite facies of continental platforms
and shelves that have been overprinted by regional thrusting,
extensional faulting and felsic plutonism. The deposits are hosted
mostly by impure carbonate rocks of Paleozoic age, but also by clastic
sedimentary rocks, greenstones and rarely granitoid stocks. They com-
monly occur near hornfels, skarn or calc-silicate rocks, but typically
outward from the edge of contact metamorphic aureoles. They coexist
regionally with Cu and/or Mo porphyry, Cu or W-Mo skarn and Ag-Pb-
Zn vein and manto deposits.
Mineralization consists of disseminated very fine-grained pyrite
overgrown by arsenian pyrite rims containing sub-micron grain-size
gold inclusions. Orpiment, realgar, cinnabar and stibnite are common
accessory minerals at the deposit scale. The Au:Ag ratios of the ores are
highly variable but typically less than one, and the ores contain locally
high concentrations of As, Sb and Hg. Decalcification and silicification
(jasperoid) of carbonate rocks are typically associated with ore, and may
be enveloped by zones of argillic and sericitic alteration.
Non-carbonate stockwork-disseminated gold deposits (11)
This poorly defined group of deposits, which includes Porgera,
Muruntau and perhaps Hemlo, consists of discordant to strata-bound
stockwork and disseminated sulphide zones along faults, permeable
units and lithologic contacts (including intrusive contacts) in miogeo-
clinal siliciclastic and volcaniclastic sequences in volcano-plutonic
arcs in oceanic and continental settings. The deposits are hosted
mostly by supracrustal rocks, but in cases where felsic sills, dykes and
stocks are present, the ore may also occur within and along the contacts
of intrusions.
Disseminated sulphides (1–20 vol.%) are mostly pyrite, with lesser
amounts of chalcopyrite and arsenopyrite, accompanied by hematite,
magnetite, molybdenite, tellurides and anhydrite in some deposits.
Ores have variable but generally gold-rich compositions (Au:Ag > 1)
and contain elevated concentrations of Cu, As, Bi, Te ± W, F, B and
locally, Mo, Sb and Ba. Associated alteration involves K-metasomatism
(sericite or roscoelite, biotite or K− feldspar) and / or Na metasoma-
tism (albite), accompanied by carbonatization and, in some deposits,
silicification.
Au-Cu sulphide-rich vein deposits (12)
These deposits consist of groups of sulphide-rich veins (>20 vol.%
sulphides), up to several hundred metres in strike lengths, in volcano-
plutonic arcs and greenstone belts of all ages. As in the case of Rossland,
they occur in faults and fractures hosted by a wide variety of volcanic
and intrusive rocks; individual veins commonly follow dykes of dioritic,
tonalitic or lamprophyric composition. In many cases, there is a marked
structural control by regional fault systems.
The veins consists of variable proportions of pyrite, pyrrhotite,
chalcopyrite and magnetite, with subordinate amounts of sphalerite and
galena, in a gangue of quartz and carbonate with lesser amounts of chlo-
rite and sericite. The veins typically contain more silver than gold
(Au:Ag = 1:2 to 1:5) and 0.5–3% Cu. The associated hydrothermal alter-
ation consists of chloritization and sericitization and is generally
restricted to the immediate vicinity of the veins.
Batholith-associated quartz vein deposits (13)
These deposits, including Chenoan and Linglong, consist of quartz
veins in brittle ± ductile faults and adjacent crushed altered wallrocks
and veinlet zones in tectonic uplifts containing metamorphic basement
and abundant granitoid rocks. Ore bodies are hosted both by granitoid
batholiths and adjacent medium- to high-grade schists and gneisses.
Deposits are controlled by regional fault systems and form extensive dis-
tricts that locally contain porphyry and epithermal styles of mineraliza-
tion as well. Veins consist of small amounts of pyrite and minor base
metal sulphides and stibnite in some cases, in a gangue of quartz and
minor calcite. The ores contain nearly equal abundances of gold and sil-
ver (Au:Ag = 1:5 to 5:1) and locally high concentrations of Cu, Pb, Zn.
Hydrothermal alteration consists of sericitization and chloritization of
wallrocks, generally within a few metres from the veins.
Greenstone-hosted quartz-carbonate vein deposits (14)
Deposits of this group, typified by the Mother Lode and Grass Valley
and including many important Precambrian examples, consist of
quartz-carbonate veins in moderately to steeply dipping brittle-ductile
shear zones and locally in related shallow-dipping extensional fractures.
They are commonly distributed along major fault zones in deformed
greenstone terranes of all ages. Veins have strike- and dip-lengths of 100
to 1000 m either singly or, more typically, in complex vein networks.
They are hosted by a wide variety of lithologies but there are district-
specific lithologic associations.
The veins are dominated by quartz and carbonate, with lesser
amounts of chlorite, scheelite, tourmaline and native gold; pyrite, chal-
copyrite and pyrrhotite comprise less than 10 vol.% of the veins. The
ores are gold-rich (Au:Ag = 5:1 to 10:1) and have elevated concentra-
tions of As, W, B, and Mo, with very low base metal concentrations.
Despite their significant vertical extent (commonly > 1 km), the depos-
its lack any clear vertical mineral zoning. Wallrock alteration haloes are
zoned and consist of carbonatization, sericitization and pyritization.
Halo dimensions vary with the composition of the host lithologies and
may envelope entire deposits in mafic and ultramafic rocks.
Turbidite-hosted quartz-carbonate vein deposits (15)
.These deposits consist of veins and vein arrays in folds (saddle reefs),
faults and brittle-ductile shear zones in turbidite sequences of all ages,
deformed and metamorphosed to lower to upper greenschist facies.
Graphitic schists in such sequences are particularly favourable hosts,
and intrusive rocks are generally lacking within and immediately
around the deposits. The deposits are commonly associated with anti-
clines and related limb-thrust faults as exemplified by Bendigo and
Ballarat. Veins consist of quartz and carbonate, with lesser amounts of
chlorite and sericite; arsenopyrite and pyrite typically comprise less
than 10 vol.% of the veins. The ores are gold-rich (Au:Ag > 5) and con-
tain elevated concentrations of As and W. Wallrock alteration, in the
form of sericitization and some silicification, is generally restricted to
the immediate vicinity of the vein.
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