Metasomatism

“Metasomatism” is the replacement of one mineral by another by simultaneous solution and deposition.  Water is a key component in most of these situations, so many reactions involve the addition of hydroxyl (OH^-) ions to the original minerals (ie, “hydration”).  Another common reaction is called hydrolysis, which is the addition of the H⁺ ion.  Hydration reactions can also cause the release of H+ ions, which in turn causes changes in the pH of the hydrothermal fluid.

Alteration Types

There is no ideal classification system for alteration types due to the extreme variability of the original mineralogy present and the wide variety of conditions which can exist in the hydrothermal alteration environment.  Below is a summary of the most widely recognized alteration types:

Potassic Alteration:   Potassic alteration results in the enrichment of potassium in the rock.  The minerals commonly formed include potassium feldspar, biotite and sericite.  Three types of potassium feldspar are possible, depending on the crystal structure:  orthoclase, adularia and microcline.  The crystal structure depends on what the temperature, pressure, and fluid composition conditions are.

Sodic Alteration:  Sodic alteration results in the enrichment of sodium in the rock.  Sodic alteration is associated with a wide variety of gold deposits and forms in a wide variety of host rocks, including all compositions of plutonic rocks, schists, and shale or slate.

Silicification:  Silicification involves the addition of silica (SiO₂).  In most cases it is the addition of quartz.  Many different forms of quartz are possible, including normal coarse-grained varieties, microcrystalline varieties like chalcedony, or amorphous (non-crystalline) varieties like opal.

Silicification is an extremely common type of alteration associated with a wide variety of different mineral deposits.  This is because quartz is a stable mineral in so many different temperature, pressure and chemical environments.  In many ore deposit settings, silicification forms an inner most ( or “proximal”) envelope (or zone) around the ore body.  The high quartz zone is commonly transitional into a variety of other alteration types which are further away from the ore body. 

Propylitic Alteration:  Propylitic alteration generally involves the introduction of chlorite, which often causes rocks to take on a greenish appearance.   It occurs in a wide variety of geologic settings and is associated with a wide variety of different ore deposit types.  It typically forms at the fringe of other alteration zones and decreases in intensity outward into unaltered rocks.

Argillic Alteration:  Argillic alteration involves the introduction of clay minerals, the most common being the clay mineral kaolinite.  It forms most often at shallow depths.  It is associated with some gold and silver deposits, especially in volcanic rocks and sinters formed by hot springs.  It is commonly found in association with many base metal vein deposits.

Carbonate Alteration:  Carbonate alteration is alteration which forms carbonate minerals such as calcite, dolomite, ankerite, siderite or other carbonates.  Carbonate alteration is common in a wide variety of ore deposits.  It can show a general increase in intensity in the direction towards some vein deposits, or it can occur as a pervasive footwall alteration in some volcanic-related deposits. 

Sericitic Alteration:  Sericitic alteration (sometimes called “phyllic alteration”), as the name implies, involves the formation of the mineral sericite, although this alteration type includes the formation of several different species of fine-grained white micas.  Sericitic alteration is almost always accompanied by the formation of pyrite and quartz.

Alteration Zoning

Alteration zoning refers to the patterns of minerals (alteration types) next to a mineral deposit.  As ore-bearing fluids migrate outward from their source, they encounter changing conditions, which results in different minerals becoming stable.  The changing conditions include changes in temperature (ie, cooling), changes in pressure, changes in pH, and changes in fluid composition as it reacts with different types of rocks it may encounter as it migrates.  The same process gives rise to zoning, or patterns, in metals or other elements. 

Alteration zoning must be thought of in all three dimensions, ie, both aerial distribution and depth.  If the fluids migrate from the center outward from a spherical source through homogenous (same throughout) rock, they would tend to form concentric shells. If they are migrating outward from a fissure, they would tend to form parallel bands or layers.  Seldom are the conditions so perfect in the field, so most zoning patterns have a certain amount of irregularity.  However, by mapping zoning patterns on the surface, or by constructing their patterns from drilling data, it is sometimes possible to predict where ore deposits (or the fluid source) is most likely to occur.  Alteration zoning can be present on a larger scale where it forms around an entire ore deposit (Figure F5), or it can form on a smaller scale around a single vein or ore body which is part of a larger deposit (Figure F6).  Complications arise when a zoning pattern around one deposit or vein overlaps another, or changing conditions over time cause overprinting of one alteration type on another.  Most, but not all, ore deposits have some type of recognizable alteration zoning pattern.