Summary of assemblages of alteration minerals, commonly used terminology, and the environment of formation. (Source : Atlas of Alteration; A Field and Petrographic guide to Hydrothermal Alteration minerals. Editors : A.J.B. Thompson& J.F.H. Thompson, MDD Series editor: K.P.E. Dunne, 1996)
|
Mineral Assemblage �(Key minerals are in bold) |
Standard Terminology |
Environment of Formation |
|
Intrusion-related |
||
| biotite (phogopite), k-feldspar (orthoclase), magnetite,quartz, apatite, anhydrite, sericite, albite-sodic, plagioclase, actinolite, rutile, 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 wallroks (without K-feldspar) that zones out to propylitic alteration. |
| K-feldspar (orthoclase or microline), 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 | Occur with minor mineralization in the deeper (peripheral in some cases) parts of some porphyry systems and is a host to minerlization in porphyry deposits associated with alkaline intrusions. |
| sericite (muscovite-illite), quartz, pyrite, cholrite, hematite, anhydrite | phyllic, sericitic | Commonly forms a peripheral halo around the core of pporphyry 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; percursor 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, diasposre, 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. |
| topaz, muscovite, quartz, tourmaline | greisen | Localized high-temperature alteration 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 hedernbergite are more common in reduced skarns (Au, W, and Sn) |
| fosterite-diopside or serpentinite-talc, calcite, magnetite, tremolite | magnesium skarn | Magnesium skarns are developed as metasomatic replacements of dolomitic limestone. HIgh-temperature magnesium skarn are characterized by fosterite and diopside and low-temperature magnesium skarn contain serpentinite and talc, both of which occur as retrograde minerals after fosterite 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. |