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Propylitic alteration is found at the margins of porphyry copper deposits and epithermal gold (and other)
deposits. This mild form of alteration happens at lower temperatures and creates mainly chlorite and
epidote with some clinozoisite, calcite, zoisite and albite.
Argillic
Argillic alteration is often referred to as ‘advanced’ or ‘intermediate’ depending on how severely the
minerals in the original host rock have been changed. Intermediate argillic alteration replaces
plagioclase feldspars with the clay minerals kaolinite and montmorillonite. It also occurs at the edges
of porphyry systems.
Advanced argillic alteration is much nastier. Affected rocks are leached by boiling, extremely acidic
fluids. It is commonly associated with near-surface epithermal deposits and recognized by the minerals
kaolinite, pyrophyllite, dickite and alunite, with some quartz, topaz and tourmaline.
Silication
Easily confused with silicification (up next), silication converts carbonate minerals into silicate
minerals. It is an essential step in the formation of skarn deposits and involves an acidic, magmatic fluid
invading a relatively easily-dissolved carbonate rock.
Silicification
Silicification introduces new quartz or amorphous silica minerals. It can occur as a halo around a range
of ore deposit types and characterizes what is
known as the sinter zone in high level
epithermal deposits.

Carbonate-hosted mineral deposits have
complex alteration envelopes (MDRU)
Carbonization
If you are getting the hang of these terms now
you may have guessed that carbonization
refers to the formation of carbonate minerals
like calcite, dolomite, magnesite and siderite.
This alteration type is almost always found
around Archean greenstone gold deposits.
Greisenization
A greisen is a cap of altered rock over a granite containing tin and tungsten mineralization. They contain
mostly quartz, muscovite and topaz with some tourmaline and fluorite.
Hematization
Last but by no means least is hematization; an alteration style caused by oxidizing fluids. It results in the
formation of hematite and some potassium-feldspar, sericite, chlorite and epidote. The enormous
Olympic Dam IOCG and uranium deposit is characterized by this iron-rich alteration style, and also
sediment-hosted copper-cobalt deposits in central Africa.
Further reading
Any ore geology textbook, I used Introduction to ore-forming processes by Lawrence Robb (Blackwell
Science Ltd)

Classification of Mineral Deposits

Geologists, are known to have more opinions than economists, so it should come as no surprise that
the classification of mineral deposits, is an on-going hot topic. The details will be debated until the sun
cools, but the broad-brush classification of mineral deposits is generally understood.

Why Do Classification Systems Matter?

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