Table 1: Summary of assemblages of alteration minerals, commonly used terminology, and

the environment of formation. Most of the key minerals in this table are dealt with
individually in the Atlas; some minerals have multiple entries because their characteristics
change in different
environments.
Mineral Assemblage (Key minerals are in bold)
Standard Terminology
Environment of Formation
Intrusion-related
biotite (phlogopite), K- feldspar (orthoclase), magnetite, quartz, anhydrite, albite-sodic
plagioclase, actinolite, rutile, apatite, sericite, chlorite, epidote
potassic (biotite- rich), K-silicate, biotitic
Generally found in the core of porphyry deposits, particularly those hosted by more mafic
intrusions (diorite, monzonite, granodiorite), or mafic to intermediate
volcanic/volcanoclastic wallrocks. May form a large peripheral alteration zone in wallrocks
(without K-feldspar) that zones out to propylitic alteration.
K-feldspar (orthoclase or microcline), quartz, albite, muscovite, anhydrite, epidote
potassic, K-silicate
Found in the core of porphyry systems, particularly hosted by felsic intrusions
(granodiorite - quartz monzonite, granite, syenite).
albite (sodic plagioclase), actinolite, clinopyroxene (diopside), quartz, magnetite, titanite,
chlorite, epidote, scapolite
sodic, sodic-calcic
Occurs with minor mineralization in the deeper (peripheral in some cases) parts of some
porphyry systems and is a host to mineralization in porphyry deposits associated with
alkaline intrusions.
sericite (muscovite-illite), quartz, pyrite, chlorite, hematite, anhydrite
phyllic, sericitic
Commonly forms a peripheral halo around the core of porphyry deposits: it may overprint
earlier potassic alteration and may host substantial mineralization.
sericite (illite-smectite), chlorite, kaolinite (dickite), montmorillonite, calcite, epidote,
pyrite
intermediate argillic, sericite-chlorite-clay (SCC), argillic
Generally forms a structurally controlled to widespread overprint on other types of
alteration (potassic) in many porphyry systems; precursor textures are usually preserved.
Argillic is often used for texturally destructive alteration that has a similar clay-rich
mineralogy, and which occurs in and around structures in the upper parts of porphyry
systems.
pyrophyllite, quartz, sericite, andalusite, diaspore, corundum, alunite, topaz, tourmaline,
dumortierite, pyrite, hematite
advanced argillic
Intense alteration, often in the upper part of porphyry systems, but also form envelopes
around pyrite-rich veins that cross-cut other alteration types.
Atlas of Alteration

Mineral Assemblage Standard Environment of Formation (Key minerals are in bold)

Terminology topaz, muscovite, quartz, grelsen Localized high-temperature alteration
tourmaline
associated with peraluminous granites and related mineralization.
garnet, clinopyroxene, wollastonite, actinolite- tremolite, vesuvianite, epidote
calcic skarn
Generally forms replacement zones in wallrocks (exoskarn - typically in limestone or
occasionally mafic to intermediate volcanic rocks), or within intrusions (endoskarn).
Andradite and diopside occur in oxidized assemblages related to porphyry Cu systems;
grossular and hedenbergite are more common in reduced skarns (Au, W, and Sn).
forsterite-diopside or serpentinite-talc, calcite, magnetite, tremolite
magnesium skarn
Magnesium skarns are developed as metasomatic replacements of dolomitic limestone.
High-temperature magnesium skarns are characterized by forsterite and diopside and low-
temperature magnesium skarns contain serpentinite and talc, both of which occur as
retrograde minerals after forsterite and clinopyroxene.
| calcite, chlorite, hematite, illite-smectite, montmorillonite-nontronite, pyrite
retrograde skarn
Commonly replaces earlier skarn alteration but may also affect adjacent wallrock -
limestone.
chlorite, epidote, albite, calcite, actinolite, sericite, clay, pyrite
propylitic
Commonly forms the outermost alteration zone at intermediate to deep levels in porphyry
systems. In some systems, propylitic alteration is mineralogically zoned from inner
actinolite-rich to outer epidote-rich alteration.
Intrusion-related — High-sulphidation Epith
ermal
quartz, rutile, alunite, native sulphur, barite, hematite, pyrite, jarosite
vuggy silica, vuggy quartz
Typically occurs in structural zones or as replacement bodies in permeable lithologies,
usually in the core of zones of advanced argillic alteration. This extreme form of leaching
can occur in the upper parts of porphyry systems (telescoped) but is more common at
higher (epithermal) levels.
Atlas of Alteration

barite, pyrite, hematite

Mineral Assemblage Standard Environment of Formation (Key minerals are in bold)
Terminology quartz, chalcedony, alunite, | silicic Represents the addition of silica to the
rock, resulting in replacement or, more commonly, the fill to vugs created during intense
leaching. Silicification is common in high-sulphidation systems at porphyry to epithermal
depths. It is sometimes confused with intense quartz stockwork veining at the top of some
porphyry deposits.
quartz, kaolinite/dickite,
alunite, diaspore,
pyrophyllite, rutile, zunyite,
alumino phosphate-
sulphates, native sulphur, rite, hematite
advanced argillic - acid sulphate
Forms widespread zones in the upper parts of some porphyry systems (lithocap): also as
more restricted alteration halos around high-sulphidation epithermal deposits.
kaolinite/dickite, montmorillonite, ¡llite- smectite, quartz, pyrite
argillic, intermediate argillic
May be present as a zone of alteration between advanced argillic and propylitic alteration,
particularly in the high- sulphidation epithermal setting.
calcite, chlorite, epidote, albite, sericite, clay, pyrite
propylitic
May occur as an outer regionally extensive alteration zone in systems at moderate depths
(>500m).
Low-sulphidation Epithermal — Geothermal
quartz, chalcedony, opal, pyrite, hematite
silicic
Pervasive replacement of the rock by silica minerals. Occurs in some epithermal and
geothermal systems as wallrock alteration around fractures and veins or within permeable
zones, usually at relatively shallow levels. Also forms blanket-like zones of replacement at
the water table below steam-heated advanced argillic alteration. Stratiform replacement
silicification may be mistaken for sinter.
orthoclase (“adularia”), quartz, sericite-illite, pyrite
“adularia”
Varies from wallrock alteration around veins, fractures and permeable zones to selective
replacement of plagioclase in alteration envelopes. Common at shallow to intermediate
depths in epithermal or geothermal systems; may be associated with boiling. Pervasive
replacement by “adularia” 1s difficult to distinguish from silicification.
Atlas of Alteration

Mineral Assemblage (Key minerals are in bold)

Standard Terminology
Environment of Formation
sericite (muscovite), illite- smectite, montmorillonite, kaolinite, quartz, calcite, dolomite,
pyrite
sericitic, argillic
Occurs as wallrock alteration around veins and replacement zones in permeable
lithologies. May exhibit progression from sericite to mixed layer clays with increasing
distance from mineralized (upflow) zones. Blanket-like carbonate- bearing alteration zones
in the upper part of some geothermal/epithermal systems may reflect the condensation of
gases (CO) from deeper boiling zones. Carbonate may also be important in some deeper
base metal-rich systems.
kaolinite, alunite, cristobalite (opal, chalcedony), native sulphur, jarosite, pyrite
advanced argillic - acid-sulphate
Forms extensive areas of alteration above the water (paleowater) table related to the
condensation and oxidation of gases (HS). Associated with mud pools, fumaroles and
deposits of native sulphur.
quartz, calcite
silica-carbonate
Replacement of ultramafic rocks in the shallow parts (low temperature) of geothermal
systems.
calcite, epidote, wairakite,
propylitic, zeolitic
Regionally extensive alteration around
pyrite, pyrrhotite
chlorite, albite, illite- alteration epithermal and geothermal systems.
smectite, montmorillonite, Mineralogical changes from zeolite-rich to
pyrite propylitic assemblages reflect increasing depth and temperature. The concentration
of CO, also influences the stability of zeolites and the relative importance of calcite versus
epidote.
Mesothermal
calcite, ankerite, dolomite, | carbonate Wallrock alteration in and around veins or
quartz, muscovite (Cr-/V- shear zones, and extensive replacement of
rich), chlorite, pyrite, ultramafic to mafic rocks. Carbonate-rich
pyrrhotite alteration may be regionally extensive and is not always related to
mineralization.
chlorite, muscovite, quartz, | chloritic Wallrock alteration in and around veins
actinolite, pyrite, pyrrhotite and shear zones, particularly in mafic volcanic and
volcaniclastic sedimentary rocks.
biotite, chlorite, quartz, biotitic Wallrock alteration in and around veins
and shear zones, particularly in sedimentary rocks.
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Atlas of Alteration
Mineral Assemblage Standard Environment of Formation Key minerals are in bold)
Terminology Sediment-hosted gold quartz, pyrite, hematite jasperoid Complete
replacement of limestone, and
occasionally other rock types, by fine- grained quartz; often associated with brecciation.
Jasperoids can form as regionally extensive zones, as small bodies related to sediment-
hosted Au deposits (Carlin-type”), and as the upper or outer alteration zones associated
with intrusion- related skarn/sulphide replacement bodies. Depth of formation is probably
moderate (52 km - *mesothermal”), although shallower zones of jasperoid may form; fluids
may be metamorphogenic (classic mesothermal), connate, or magmatic.
Volcanogenic massive sulphide
sericite, quartz, pyrite, chlorite, andalusite, chloritoid
sericitic
Pervasive replacement of rocks in the footwall below massive sulphide lenses;
concentrated in stockwork feeder zones but may be laterally extensive both deeper in the
footwall and extending into the hanging wall in some deposits. Most common in
intermediate to felsic volcanic rocks but may also replace the more mafic units in lower
temperature systems. Andalusite and chloritoid occur in metamorphosed alteration zones.
chlorite, quartz, sericite, pyrite, cordierite, biotite
chloritic
Pervasive replacement of rocks in the footwall below massive sulphide deposits. Fe-rich
chlorite occurs in the core of stockwork feeder zones in mafic footwall sequences (e.g.
Archean deposits) whereas Mg-rich chlorite has a more erratic distribution, generally
around the periphery or upper parts of stockwork zones. Cordierite +/- biotite is common
in metamorphosed Mg-Fe-rich alteration zones.
Atlas of Alteration
11

Mineral Assemblage Standard (Key minerals are in bold) Terminology

Environment of Formation
quartz, pyrite, sericite, K- silicic feldspar
Pervasive replacement of rocks in the footwall below massive sulphide deposits;
particularly common in permeable siliceous ash-rich beds, where the silicified rock may be
mistaken for cherts (chemical sediments). Also occurs as wallrock alteration in some
quartz vein stockwork zones.
dolomite, siderite, ankerite, | carbonate calcite, quartz, sericite, chlorite, pyrite
Usually occurs as disseminated alteration in footwall sequences, commonly over extensive
lateral and stratigraphic intervals. The composition of carbonates may change with
distance from ore zones.
Sediment-hosted massive sulphide
quartz, muscovite, siderite, | silicic dolomite, garnet, celsian, pyrrhotite, pyrite, barite
Pervasive replacement of footwall strata below massive sulphide; also of baritic facies of
the sulphide body and less commonly in the hanging wall. Well developed within
calcareous strata but more cryptic in siliciclastic strata. Can be mistaken for siliceous shale
or chert. Occurs as “garnet quartzite” in high grade metamorphic rocks.
tourmaline, muscovite, tourmalinite
quartz, pyrrhotite
Pervasive replacement of footwall strata below massive sulphide. Associated with
abundant disseminated sulphide in shallow footwall. Limited to feldspathic strata.
ankerite, siderite, calcite, carbonate quartz, muscovite,
Disseminated carbonate, often as euhedral crystals, in the shallow footwall below
pyrrhotite massive sulphide and baritic facies. Disseminated ankerite/siderite can be very
extensive within calcareous strata along major structures.
sericite, chlorite, quartz, sericitic Pervasive replacement of strata in broad
pyrrhotite, pyrite, albite
halo around massive sulphide deposit; forms discordant bodies in structurally controlled
vent areas. Occurs as potassium feldspar in high grade metamorphic rocks. Best developed
in feldspathic strata.
albite, chlorite, muscovite, | albitic biotite
Pervasive replacement of strata, and massive sulphide; more typically along structures and
around mafic intrusions. Limited to feldspathic strata.
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Atlas of Alteration