Alteration is a term that appears in almost every junior mining company press release or project description, often repeated many times in a single paragraph. It is usually prefaced by a strange assortment of words ending in –ic, like potassic, advanced argillic or hematitic.
Entire journals, textbooks and theses explore in detail the formation of alteration minerals and the patterns they form around the world’s most important ore deposits. Understanding alteration is essential to understanding where the next big discovery will be found.
What is alteration?
Ore deposits form in many different ways and valuable minerals accumulate in igneous, metamorphic and sedimentary settings.
The most common ore forming processes involve the relocation and concentration of metals by fluids. Some fluids are released by a magma, others are hot, aqueous fluids circulating deep within the crust or cooler groundwaters percolating down from the surface. Many are a complex combination of these.
In most cases alteration is caused by these fluids moving through the tiny spaces between the mi
nerals in a rock and along structures like faults.
Alteration involves the modification and replacement of the original minerals in a rock with a new suite of minerals with different chemistry.
Minerals can be altered by sudden changes in temperature and pressure too.
New alteration minerals are deposited in cavities or fractures, change the chemistry of existing minerals or replace them with new minerals entirely. Replacement of one ore mineral by another, or by a mineral formed during weathering is common in many ore systems.
Ore formation by hydrothermal fluids
Most ore deposit on earth are formed or modified by hydrothermal fluids. These warm to very high temperature fluids are mostly composed of water and range in salt content and acidity. With the help of a heat source, they move through the tiny, empty pores and structures within rocks to mobilize and deposit valuable metals and other elements.
Alteration and exploration
When companies talk about alteration in their press releases, they are usually referring to the end result of the alteration process – the minerals and collections of minerals (known as assemblages) resulting from certain types of alteration.
Individual drill holes, exploration properties and even large mines are often mapped by geologists according to the alteration style. Alteration is a key aspect of core logging, together with colour, texture, and rock type.
Finding certain types of alteration in drill core or field mapping can indicate certain types of mineralization and direct explorers to ore zones. Alteration can enrich an orebody, or expand its footprint, making it easier to find.
Although alteration can be a huge arrow pointing the way to a new ore deposit, it can also be a very confusing patchwork of unusual minerals, seemingly designed to confuse geologists, young and old.
Introducing the -ics
Here are the –ic terms most often associated with the term alteration in a company media release, and the ore deposits they are associated with.
Potassic alteration involves the formation of new potassium feldspar minerals and possible some biotite. There may also be small amounts of sericite, chlorite and quartz. This type of alteration is typically found at the core of porphyry copper deposits, the result of alteration by very high temperature potassium-rich fluids.
Phyllic (AKA sericitic)
Phyllic alteration is associated with porphyry copper deposits too, but also epithermal and volcanic massive sulphide deposits. It forms over a wide range of temperatures and is a very common alteration style. The resulting minerals include mostly a fine-grained white mica known as sericite – it is sometimes referred to as sericitic alteration – and some quartz, chlorite and pyrite.
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 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.
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 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.
If you are getting the hang of these terms now you may have guessed that carbonatization 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.
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.
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.
Any ore geology textbook, I used Introduction to ore-forming processes by Lawrence Robb (Blackwell Science Ltd)