earth science

Supergene (geology)

Supergene (geology) is a earth science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Supergene (geology) rather than just read about it. In short: In ore deposit geology, supergene processes or enrichment are those that occur relatively near the surface as opposed to deep hypogene processes. Supergene processes include the predominance of meteoric water circulation (i.e. water derived from precipitation) with concomitant oxidation and chemical weathering.

Key takeaways

Supergene (geology) belongs to earth science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Supergene (geology) to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Supergene (geology) from memory before moving on to harder problems.

Reference excerpt

In ore deposit geology, supergene processes or enrichment are those that occur relatively near the surface as opposed to deep hypogene processes. Supergene processes include the predominance of meteoric water circulation (i.e. water derived from precipitation) with concomitant oxidation and chemical weathering. The descending meteoric waters oxidize the primary (hypogene) sulfide ore minerals and redistribute the metallic ore elements. Supergene enrichment occurs at the base of the oxidized portion of an ore deposit. Metals that have been leached from the oxidized ore are carried downward by percolating groundwater, and react with hypogene sulfides at the supergene-hypogene boundary. The reaction produces secondary sulfides with metal contents higher than those of the primary ore. This is particularly noted in copper ore deposits where the copper sulfide minerals chalcocite (Cu2S), covellite (CuS), digenite (Cu18S10), and djurleite (Cu31S16) are deposited by the descending surface waters. All such processes take place at essentially atmospheric conditions, around room temperature (25 °C) and standard atmospheric pressure (1 atm).

Zones

Distinct zones of supergene processes can be identified at various depths. From the surface down they are the gossan cap, leached zone, oxidized zone, water table, enriched zone (supergene enriched zone) and primary zone (hypogene zone).

Gossan cap Pyrite (FeS2) has oxidised to form goethite (FeO(OH)) and limonite (FeO(OH)·nH2O), which form a porous covering over the oxidized zone known as a gossan cap or iron hat. Prospectors use gossan as an indication of ore reserves.

Leached zone Groundwater contains dissolved oxygen and carbon dioxide. As it travels downwards it oxidizes primary sulfide minerals, concomitant with forming sulfuric acid and solutions of oxidized metals. For example, groundwater commonly interacts with pyrite (FeS2) to form an oxidized iron (FeO(OH)) and sulfuric acid (H2SO4), portrayed in the idealized chemical reaction below (intermediate steps omitted):

4 FeS2 + 12 H2O + 15 O2 → 4 FeO(OH) + 8 H2SO4 An intermediate in this process is ferric sulfate (Fe2(SO4)3), which oxidizes pyrite and other sulfide minerals.

Oxidized zone Above the water table the environment is oxidizing, and below it is reducing. Solutions traveling downward from the leached zone react with other primary minerals in the oxidised zone to form secondary minerals such as sulfates and carbonates, and limonite, which is a characteristic product in all oxidised zones. In the formation of secondary carbonates, primary sulfide minerals generally are first converted to sulfates, which in turn react with primary carbonates such as calcite (CaCO3), dolomite (CaMg(CO3)2) or aragonite (also CaCO3, polymorphic with calcite) to produce secondary carbonates. Soluble salts continue on down, but insoluble salts are left behind in the oxidised zone where they form. Examples of insoluble salts that are commonly found in the oxidized zone include lead precipitates like anglesite (PbSO4) and pyromorphite (Pb5(PO4)3Cl); copper precipitates like malachite (Cu2(CO3(OH)2), azurite (Cu3(CO3)2(OH)2), and cuprite (Cu2O); and smithsonite (ZnCO3).

Water table At the water table the environment changes from an oxidizing environment to a reducing one.

Enriched zone Copper ions that move down into this reducing environment form a zone of supergene sulfide enrichment. Covellite (CuS), chalcocite (Cu2S) and native copper (Cu) are stable in these conditions and they are characteristic of the enriched zone. The net effect of these supergene processes is to move metal ions from the leached zone to the enriched zone, increasing the concentration there to levels higher than in the unmodified primary zone below, possibly producing a deposit worth mining.

Primary zone The primary zone contains unaltered primary minerals.

Mineral alterations Chalcopyrite CuFeS2 (primary) readily alters to the secondary minerals bornite Cu5FeS4, covellite CuS and brochantite Cu4SO4(OH)6. Galena PbS (primary) alters to secondary anglesite PbSO4 and cerussite PbCO3. Sphalerite ZnS (primary) alters to secondary hemimorphite Zn4Si2O7(OH)2.H2O, smithsonite ZnCO3 and manganese-bearing willemite Zn2SiO4. Pyrite FeS2 (primary) alters to secondary melanterite FeSO4.7H2O. If the original deposits contain arsenic and phosphorus bearing minerals, secondary arsenates and phosphates will be formed.

Etymology The word supergene is derived from the Latin root super meaning 'above' and the Greek root -gene (-γενής) meaning 'born' or 'produced'. The terms supergene and hypogene refer to the depth at which they occur.

Illustrations

Worked examples

Example 1 — a first encounter with Supergene (geology)

Start with the simplest possible case. Write down what Supergene (geology) claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In earth science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Supergene (geology) before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Supergene (geology) ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Supergene (geology)

In research: Supergene (geology) appears in earth science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Supergene (geology) in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Supergene (geology) is common in secondary-school and first-year university syllabi. It links to neighbouring topics Economic geology, so understanding it makes those chapters shorter.
In everyday life: Look for Supergene (geology) outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Supergene (geology) in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Supergene (geology) means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Supergene (geology) out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Supergene (geology) in simple terms?

Why does Supergene (geology) matter?

Because it connects several earth science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Supergene (geology)?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Supergene (geology).