<p>That is a great question and one that might not be simple to answer in a short response. For more detailed information you can look to the work of Drs. Marco Einaudi, John Dilles, Mark Barton (particularly his contributions to the Reviews in Mineralogy and Geochemistry text Contact Metamorphism), David John, and others. <a href="https://www.semanticscholar.org/paper/Porphyry-and-Epithermal-Mineral-Deposits-Dilles-John/a648e855f704a4157040811af83f50432a14597d" rel="nofollow noreferrer">This paper</a> is great for understanding magmatic-hydrothermal alteration.</p>
<p>In short: understanding alteration requires an understanding of mineralogy and geochemistry. When you're looking at a rock and want to know what caused the alteration you have identified, you should start by asking &quot;under what conditions does this mineral form&quot;. For example, when observing jarosite encrusting voids and kaolinite replacing plagioclase in a granodiorite, it is important to know that jarosite forms under low pH conditions and kaolinite under similar conditions by the chemical weathering of feldspars and the nearly complete removal of cations at modest temperatures. Geologists classify this as argillic to advanced argillic weathering (argillic i.e. argillaceous referring to the formation of clays) to help sort and distinguish it from other alteration environments and conditions. Looking at a phase stability diagram, such as one you will find in the paper linked above, will tell you that the pH required for argillic weathering is too low for it to be entirely meteoric and the acid species present implicate magmatic fluid contributions.</p>
<p>As an alternative example, you're looking at a plutonic rock that has had disseminated pyrite, magnetite, and some hematite with minor calcite in fractures; plagioclase altered to albite + epidote and mafic minerals to chlorite. Epidotization is a process wherein anorthite, being less stable than albite, unmixes from plagioclase and dissolves providing calcium for the simultaneous growth of calcic epidote at around 250C+. Carbonation reactions have formed calcite and oxidation hematite. The location of this alteration zone when mapped might be peripheral and/or above the main ore zone. This leads one towards the conclusion that alteration was driven by the interaction of non-magmatic fluids that might be interpreted as convecting due to the influence of a plutonic heat anomaly.</p>
<p>Hope this helps.</p>

CC BY-SA 4.0
2024-04-07T05:10:19.953Z
2024-04-07T05:10:19.953Z
Josiah
<p>When you see an altered (silicification-argilic-FeO / trachyandesitic) rock outcrop, how do you understand the cause of the alteration. Is it occured by hot or cold alteration? Was it occured through magmatic water vs meteoric water ?</p>

<minerals><magmatism><alteration>
How do you understand the occurence of an alteration zone?
Muharrem Yavuz
# How do you understand the occurence of an alteration zone?

(Asked by: username_0 on 2024-04-04)

When you see an altered (silicification-argilic-FeO / trachyandesitic) rock outcrop, how do you understand the cause of the alteration. Is it occured by hot or cold alteration? Was it occured through magmatic water vs meteoric water ?

**Tags:** minerals, magmatism, alteration


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## 1 answers

### Answer by username_1 on 2024-04-07. Score: 2 | Accepted answer

That is a great question and one that might not be simple to answer in a short response. For more detailed information you can look to the work of Drs. Marco Einaudi, John Dilles, Mark Barton (particularly his contributions to the Reviews in Mineralogy and Geochemistry text Contact Metamorphism), David John, and others. [This paper](https://www.semanticscholar.org/paper/Porphyry-and-Epithermal-Mineral-Deposits-Dilles-John/a648e855f704a4157040811af83f50432a14597d) is great for understanding magmatic-hydrothermal alteration.

In short: understanding alteration requires an understanding of mineralogy and geochemistry. When you're looking at a rock and want to know what caused the alteration you have identified, you should start by asking "under what conditions does this mineral form". For example, when observing jarosite encrusting voids and kaolinite replacing plagioclase in a granodiorite, it is important to know that jarosite forms under low pH conditions and kaolinite under similar conditions by the chemical weathering of feldspars and the nearly complete removal of cations at modest temperatures. Geologists classify this as argillic to advanced argillic weathering (argillic i.e. argillaceous referring to the formation of clays) to help sort and distinguish it from other alteration environments and conditions. Looking at a phase stability diagram, such as one you will find in the paper linked above, will tell you that the pH required for argillic weathering is too low for it to be entirely meteoric and the acid species present implicate magmatic fluid contributions.

As an alternative example, you're looking at a plutonic rock that has had disseminated pyrite, magnetite, and some hematite with minor calcite in fractures; plagioclase altered to albite + epidote and mafic minerals to chlorite. Epidotization is a process wherein anorthite, being less stable than albite, unmixes from plagioclase and dissolves providing calcium for the simultaneous growth of calcic epidote at around 250C+. Carbonation reactions have formed calcite and oxidation hematite. The location of this alteration zone when mapped might be peripheral and/or above the main ore zone. This leads one towards the conclusion that alteration was driven by the interaction of non-magmatic fluids that might be interpreted as convecting due to the influence of a plutonic heat anomaly.

Hope this helps.


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