What is alteration?
Hydrothermal alteration is a complex process involving mineralogical, chemical and textural changes, resulting from the interaction of hot aqueous fluids with the rocks through which they circulate, under evolving physicochemical conditions.
Hydrothermal fluids chemically attack the mineral constituents of the wall rocks , which tend to re- equilibrate by forming new mineral assemblages that are in equilibrium with the new conditions.
Importance of Hydrothermal Alteration and Mineral Exploration
- Feature of hydrothermal ore deposits
- Relates to type of deposit-environment
- Provides halo around target
- Vectors towards mineralization
Indication of size/intensity of system, may equate to potential The areal extent of the alteration can vary considerably, sometimes being limited to a few centimeters on either side of a vein, at other times forming a thick halo around an orebody
Controls of Alteration
- Nature of host rock or wall rock composition
- fluid composition
- Concentration, acivity, and chemical potential of the fluid components like H+, CO2, O2, K+, and SO2
- fluid/rock ratio (f/r) in the alteration process: the total mass of fluid that passes through the system, in the unit time, divided by the total mass of rock in the system
if f/r is small==== negligable alteration and ore formation
Alteration degree or Rank = Temperature
Temperature defined by mineralogy,e.g.
- Biotite = High T, > 400 C
- Chlorite = Intermediate T, 200-400 C
- Smectite = Low T, < 150 C
Style of alteration
Terms: Weak, moderate, strong pervasive, non-pervasive, These terms essentially refer to
- State of preservation of the original rock,
- How far the alteration process has advanced, both at the single mineral scale and at the regional scale,
- Overall geometry of the alteration halo.
refers to how much a rock has been affected by alteration.
• Weak, or low alteration intensity would mean that only a few of the original minerals have been replaced with little or no modification of the original textures.
• pervasive alteration is characterised by the replacement of most, or all, original rockforming minerals. – This results in the partial or total obliteration of the original textures.
Types of alteration
There are two main divisions of wall rock alteration based on the mode of formation:
a) Hypogene alteration
b) Supergene alteration
Hypogene alteration is caused by ascending relatively high T hydrothermal solutions.
Supergene alteration by relatively low T descending meteoric water reacting with previously mineralized ground.
Types of alterations
- Potassic alteration
- Phyllic (sericitic) alteration
- Propylitic alteration
- Argillic alteration
1. Potassic alteration or Potassium silicate alteration
Potassium silicate alteration is formed as a replacement of plagioclase and mafic silicate minerals, at temperatures in the region of 600–450 C. Potassic (or K-silicate) alteration is characterized by the formation of new K-feldspar and/or biotite (green colored and Fe-rich), usually together with minor sericite, chlorite, and quartz. Potassic alteration is especially common and important in porphyry and epithermal systems, where it occurs in the high temperature core zones.
Accessory amounts of magnetite/hematite and anhydrite may occur associated with the potassic alteration assemblage. Common sulfides are pyrite, molybdenite and chalcopyrite
Common sulfides are pyrite, molybdenite and chalcopyrite
Anhydrite is also a common mineral in potassic alteration in porphyry systems.
A variation of potassic alteration involving substantial addition of Na and Ca (called sodic and calcic alteration and characterized by abundant albite, epidote and actinolite)
The K-feldspars of the potassic zones are characteristically reddish in color due to minute hematite inclusions.
2. Propylitic alteration
Propylitic alteration is characterized by the addition of H2O and CO2, and locally S, with no appreciable H+ metasomatism.
It comprises mainly chlorite and epidote, together with lesser quantities of clinozoisite, calcite, zoisite, and albite
In places sericite, Fe-oxides, montomorillonite and zeolite may also be common.
It is a mild form of alteration representing low to intermediate temperatures (200–350 °C) and low fluid/rock ratios
It characterizes the margins of porphyry Cu deposits as well as epithermal precious metal ores.
Outward propylite alteration grades into unaltered rocks.
3. Phyllic (sericitic) alteration
Sericitic alteration is essentially due to the destabilisation of feldspars by hydrolysis (H ion metasomatism) in the presence of OH, K and S, to form quartz, sericite (fine-grained white mica), pyrite, chlorite, and some chalcopyrite (sulphide content can be up to 20% by volume).
In the process Na, Mg, Ti, Fe and also K are leached out.
Phyllic alteration is associated with porphyry Cu deposits, but also with mesothermal precious metal ores and volcanogenic massive sulfide deposits in felsic rocks.
This alteration grades into the potassic type by increasing amounts of (K, aK ) K-feldspar and/or biotite, and into the argillic type by increasing amounts of (H, aH)clay minerals.
4. Argillic alteration
This alteration style is commonly subdivided into:
- a) intermediate argillic
- b) advanced argillic categories
Depending on the intensity of host mineral breakdown or composition of clay minerals.
a. Intermediate argillic alteration
Intermediate argillic alteration affects mainly plagioclase feldspars and is characterized by the formation of clay minerals kaolinite and the smectite group (mainly montmorillonite).
It typically forms below about 250 °C by H+ metasomatism
It occurs on the fringes of porphyry systems.
b. Advanced argillic alteration
Advanced argillic alteration represents an extreme form of base leaching where rocks have been stripped-off alkali elements by very acidic fluids active in high fluid/rock ratio environments
It is characterized by kaolinite, pyrophyllite, or dickite (depending on the temperature) and alunite together with lesser quartz, topaz, and tourmaline.
It is commonly associated with epithermal precious metal deposits where alteration is associated with boiling fluids and condensation of volatilerich vapors to form extremely acidic solutions.
Silicification refers specifically to the formation of new quartz or amorphous silica minerals in a rock during alteration and is usually a by-product of isochemical hydrolysis reactions where Si is locally derived.
The majority of fractures through which hydrothermal fluids have passed are at least partially filled with quartz to form veins.
The Si in these settings is usually derived by leaching of the country rocks through which the fluids are circulating. Intense silicification, however, forms as a result of cation metasomatism.
This type of alteration is characteristic of the high level epithermal precious metal ore deposits.
• refers to the formation of carbonate minerals (calcite, dolomite, magnesite, siderite, etc.) during alteration of a rock
• promoted by fluids characterized by high partial pressures of carbon dioxide (PCO2 ) and neutral to alkaline pH.
Greisens represent an alteration assemblage comprising mainly quartz, muscovite, and topaz, with lesser tourmaline and fluorite, usually forming adjacent to quartz–cassiterite–wolframite veins.