Manganiferous Gossans: Formation, Features, and Exploration Significance

Introduction:

Gossans are the weathered, oxidised caps that form above many sulphide ore deposits, often appearing as rusty, iron-rich outcrops. However, not all gossans are the typical reddish-brown "iron hats." Some are dark grey to black due to a high content of manganese oxides – these are known as manganiferous gossans. In this post, we'll explore what manganiferous gossans are, how they form through geochemical and mineralogical processes, and why they are important in mineral exploration. We will also highlight key manganese minerals and real-world examples like Broken Hill and Mount Isa, where black manganiferous gossans provided critical clues to rich ore bodies.

What Are Manganiferous Gossans and How Do They Differ From Typical Gossans?

A gossan (sometimes nicknamed an iron hat) is essentially the remains of a sulphide-rich mineral deposit that has been thoroughly oxidised and leached by weathering. In a classic gossan, iron from minerals like pyrite or chalcopyrite oxidises to form iron oxides (such as hematite, goethite, and limonite), which stain the rock in red, orange, and brown hues. These iron-rich gossans are porous and often exhibit a boxwork texture – a honeycomb-like pattern of iron oxide left behind after sulphide crystals have dissolved away.

Manganiferous gossans are a special variety of gossan that contain significant manganese oxides in addition to iron oxides. While a typical gossan is dominated by rusty iron colours, a manganiferous gossan may appear dark brown, grey, or jet black in patches or overall. The black colouration comes from manganese oxide minerals (commonly minerals like pyrolusite, psilomelane, and others) that form during the oxidation of the original ore. Essentially, if the original ore body had a notable amount of manganese-bearing minerals or if conditions during weathering favoured manganese accumulation, the resulting gossan can be “blackened” by manganese compounds. This contrasts with the more common ferruginous (iron-rich) gossans which have minimal manganese and retain a reddish palette.

In the field, a manganiferous gossan can often be recognised by its dark streaks or coatings and sometimes by a slightly higher density (some manganese oxides are quite heavy). Early prospectors might not have known the chemistry, but they noticed that certain dark, heavy gossans often indicated rich lead or silver ores beneath. One famous example is the original Broken Hill outcrop in New South Wales, which was a ridge capped by unusually dark, manganese-rich gossan rather than the usual red ironstone. This distinctive black “iron hat” stood out in the landscape and ultimately led to the discovery of one of the world’s great ore deposits.

How Manganese Behaves During the Oxidation of Sulphide Deposits

To understand how a gossan becomes rich in manganese, it's important to look at what happens when sulphide mineralisation is exposed to oxygen and water. Consider a buried sulphide ore deposit (for instance, containing minerals like pyrite (FeS₂), sphalerite (ZnS), galena (PbS), and perhaps some manganese-rich minerals such as rhodochrosite (MnCO₃) or rhodonite (MnSiO₃)). When this deposit is uplifted or eroded close to the surface, rainwater, oxygen, and groundwater begin to percolate through cracks. The sulphide minerals react with oxygen in a process known as oxidation.

A key reaction is that of pyrite (iron sulphide) with oxygen and water, which generates iron oxides (basically rust) and sulphuric acid. The simplified reaction is:

4 FeS₂ + 15 O₂ + 8 H₂O → 2 Fe₂O₃ (iron oxide) + 8 H₂SO₄ (sulphuric acid)

This reaction shows that when pyrite oxidises, it produces iron oxide (which colours the gossan) and acid. The sulphuric acid percolating through the rock can dissolve many other minerals in the ore, including base metals like zinc, copper, lead, silver, and any manganese present.

Iron vs Manganese in the weathering zone: Iron released from pyrite and other iron-bearing minerals doesn't travel far because as soon as it oxidises to Fe³⁺ in the presence of water, it hydrolyses and precipitates as insoluble iron oxides/hydroxides (like goethite and hematite). These iron compounds stay close to where the sulphides were, painting the rock red-brown and forming the robust, rust-coloured gossan.

Manganese, on the other hand, behaves a bit differently. When manganese minerals (or minerals containing Mn) break down, manganese typically enters the groundwater as Mn²⁺ (a soluble ion) under the acidic conditions created by the oxidising pyrite. Unlike iron, manganese does not immediately precipitate upon oxidation because converting Mn²⁺ to manganese’s insoluble form (Mn⁴⁺ in MnO₂) requires more oxidising conditions or a neutralisation of the acid. In fact, Mn²⁺ is relatively mobile in acidic, oxygenated water – it can travel some distance away from the original sulphide zone before it eventually oxidises to Mn⁴⁺ and precipitates.

As the acidic, metal-bearing waters move away from the immediate zone of intense oxidation, they gradually encounter conditions that cause metals to drop out of solution. This could be a result of mixing with less acidic groundwater (raising the pH), or simply travelling far enough from the reactive pyrite zone that the environment becomes more oxidising (higher Eh) due to abundant oxygen or catalytic action by microbes. Manganese often oxidises and precipitates at or beyond the fringes of the main iron-rich gossan. This is why one might observe a zoning in some weathered ore deposits: a core of red iron oxides where the sulphides were, and a halo or patches of dark manganese oxides a short distance away or slightly downhill from the core (as groundwater flow carries Mn outward).

It’s worth noting that the oxidation of Mn²⁺ to Mn⁴⁺ (forming MnO₂ minerals) can be sluggish without help. In many environments, specialised bacteria and microorganisms assist by oxidising manganese as part of their metabolic processes. These microbes can speed up manganese precipitation, creating black coatings or nodules of Mn-oxide. Thus, manganiferous gossan formation is often a biogeochemical process – life can help create these geological features!

Summary of manganese behaviour: In essence, manganese released from primary minerals during weathering stays in solution longer than iron does. It only precipitates out once conditions become right (less acidic, more oxidising), often leading to manganese oxide minerals forming a bit removed from the original source. If the original ore had substantial manganese (for example, carbonate or silicate manganese minerals, or if manganese was present in solid solution within other sulphides), a large portion of that Mn can end up as a significant accumulation of black oxides in the gossan or nearby soil. That is what gives manganiferous gossans their characteristic dark appearance and composition.

Key Secondary Manganese Minerals in Gossans

Manganese in oxidised zones forms a variety of secondary minerals, typically oxides and hydroxides. Many of these are black or very dark, which is why even a small percentage of manganese oxides can turn a gossan noticeably darker. Important manganese-bearing minerals (and groups of minerals) commonly found in manganiferous gossans include:

Pyrolusite (MnO₂): Pyrolusite is one of the most common manganese oxide minerals. It often appears as soft black powdery coatings or as needle-like crystals that form fibrous aggregates in weathered rock. It has a characteristic black or dark grey colour and will leave a black streak if rubbed on a porcelain plate. Pyrolusite is the primary ore of manganese in economic deposits, but in gossans it appears as a secondary mineral where manganese has re-precipitated from solution.

Psilomelane: This is an older field term historically used for hard black manganese oxides of uncertain or mixed composition. Modern mineralogy has shown that many "psilomelane" specimens are actually mixtures or specific minerals such as cryptomelane or romanechite. Psilomelane in the field typically forms shiny black, botryoidal masses (with smooth, bulbous surfaces) in gossan outcrops. It’s essentially a catch-all name for any massive, hydrated manganese oxide mineral that is hard and dark.

Cryptomelane (KMn₈O₁₆): Cryptomelane is a specific manganese oxide mineral often found in gossans, identifiable by containing potassium. It belongs to the hollandite group of manganese oxides, which have a tunnel-like crystal structure that can accommodate large cations like K⁺, Ba²⁺, or Pb²⁺. Cryptomelane usually occurs as black crusts or finely fibrous masses. Interestingly, because cryptomelane contains potassium, geologists can use K-Ar or Ar-Ar dating on this mineral to determine the age of a gossan’s formation. For example, cryptomelane from the Mount Isa gossan has been dated to the Miocene epoch (~15–20 million years ago), indicating when that supergene oxidation took place.

Wad: Wad is not a single mineral but a general term for earthy, powdery manganese oxide mixtures found in soils and gossans. Wad is typically a dull black or brown-black material that might dirty your fingers like soot. It usually consists of a mix of Mn-oxide minerals (like birnessite, lithiophorite, etc.) intermingled with clay, iron oxide, and other impurities. In old mining parlance, prospectors used “wad” to describe any black, manganese-rich gossan material that was too fine-grained or massive to identify. Despite its nondescript nature, wad can be very important in exploration because it often soaks up the geochemical signature of an ore deposit (it can host elevated levels of metals such as silver or lead that were carried by the groundwater).

Coronadite (PbMn₈O₁₆): Coronadite is a lead-bearing manganese oxide – essentially a manganese oxide mineral where lead has been incorporated into the structure. It typically forms in the oxidised zone of lead-rich deposits. Coronadite is heavy (due to the lead content) and black, often forming fibrous or botryoidal coatings. The Broken Hill gossan famously contained abundant coronadite, which contributed to the outcrop’s dark colour and was a carrier of lead at surface. Coronadite can be thought of as a natural combination of manganese and lead: it locked up some of the lead in the gossan as an insoluble oxide rather than letting it wash away. Finding coronadite at surface is a strong indicator of a lead-rich deposit below.

Other Mn Oxides: There are many other manganese oxide and hydroxide minerals that can appear in gossans depending on local chemistry. For example, romanechite (a barium-bearing manganese oxide very similar to psilomelane) can occur in Ba-rich systems, and chalcophanite (ZnMn₃O₇·3H₂O) is a zinc-bearing manganese oxide that might form in zinc-rich gossans. Manganite (MnO(OH)), a manganese hydroxide, can also form in some weathering environments. The key point is that manganese readily forms a variety of secondary minerals, so a manganiferous gossan might host a whole suite of these black minerals, sometimes requiring laboratory analysis to fully identify them.

Visual Characteristics and Geochemical Indicators

From a visual standpoint, manganiferous gossans are often quite striking. Instead of the usual rusty reds and ochres, you may see portions of the outcrop that are dark chocolate-brown, steel grey, or black. Often the manganese oxides appear as a coating or stain on rock surfaces, sometimes with a varnish-like sheen. In some cases, you might find a brecciated rock (broken fragments cemented together) where the cement or matrix is a black Mn-oxide material, giving the rock a speckled black-and-brown appearance. High-manganese gossans can also display a dendritic pattern on fracture surfaces – delicate branching black tree-like shapes where manganese oxides crystallised from solution along cracks.

One simple field indicator is the streak and hardness of the material. Iron oxide gossans (limonite/goethite-rich) usually have a yellow-brown streak and are relatively hard (they can scratch glass). Manganese oxides, by contrast, often have a black or very dark streak and can be quite soft (for instance, powdery pyrolusite or wad will easily rub off and stain your fingers). If you pick up a piece of gossan and it leaves a black smudge on your hand or streak plate, that’s a clue to manganese. Also, manganese oxide minerals can impart a higher specific gravity to the gossan – a chunk of gossan rich in something like coronadite will feel unexpectedly heavy for its size because of the lead and manganese content.

Geochemically, manganese oxides in gossans are often associated with enrichments in certain metals. Because Mn-oxides are excellent scavengers of metal ions, a geochemical assay of a manganiferous gossan might show elevated levels of elements like silver, lead, zinc, cobalt, nickel, or barium relative to a plain iron-rich gossan. Prospectors learned to pay special attention to black manganese-rich material not only because it was visually different, but because it often yielded higher assay values for valuable metals. For instance, a dark gossan could return anomalously high silver or lead in a surface sample, offering a clue that those metals were once concentrated in the underlying ore body. By contrast, a purely iron-rich gossan might be depleted in those metals since many of them are leached away during the oxidation process.

It’s also worth mentioning that manganese oxides have a strong capacity to absorb trace elements. They can lock up elements like arsenic, antimony, or molybdenum from the circulating fluids. Thus, a manganiferous gossan can act like a geochemical tape recorder, holding onto a wide range of elements that passed through during weathering. This makes them especially useful in geochemical exploration: sampling the manganese-rich portions of gossan or soil can reveal multi-element anomalies (a fingerprint of the buried deposit) that might be missed if one only sampled the iron-rich portions.

Manganese Oxides as Metal Traps: Influence on Silver, Lead, and Zinc

When it comes to metal mobility, manganese oxides don’t just sit passively in the gossan – they actively influence the dispersion and capture of other metals during weathering. Think of manganese oxides as natural sponges or traps for metals. As metal-rich acidic water percolates through the weathering zone, minerals like goethite (iron oxide) and especially manganese oxides will tend to adsorb and lock up certain metals that might otherwise escape. Here’s how this plays out for a few important metals:

Silver: In many oxidised silver-rich deposits, a lot of the silver near surface ends up associated with manganese oxides. Silver in oxidising, arid conditions can form compounds like silver chloride (AgCl, also known as cerargyrite or “horn silver”). Manganese dioxide is a strong oxidiser; it can help convert silver from sulphides or other compounds into silver chloride or even native silver. The newly liberated silver then often precipitates right where the reaction takes place, frequently alongside the manganese oxides. As a result, black manganese-rich gossans in a silver-bearing system might contain visible specks of native silver or cerargyrite, and they often show very high silver assays. In some historical mining districts, prospectors discovered that the black gossan itself could carry bonanza silver grades – essentially serving as high-grade silver ore that formed through supergene processes.

Lead: Lead is relatively less mobile in the supergene environment compared to metals like zinc. When sulphide deposits weather, lead tends to form fairly insoluble secondary minerals like cerussite (lead carbonate) or anglesite (lead sulphate) not far from the oxidation zone. If manganese is present, lead can also be fixed into manganese oxide minerals (like coronadite) or adsorbed onto Mn-oxide surfaces. Thus, manganiferous gossans often show strong lead anomalies at surface, as lead is held in place rather than completely leached. At Broken Hill, for example, the gossan was famously rich in lead (often over 10% Pb) because much of that lead became part of coronadite and other Mn-oxide phases instead of washing away. In effect, manganese helped “fix” lead in the gossan cap.

Zinc: Zinc is one of the more mobile base metals in acidic, oxidising waters. When sphalerite (zinc sulphide) oxidises, the Zn²⁺ typically travels far from the source and only redeposits where conditions become neutral (often as smithsonite – zinc carbonate, or hemimorphite – zinc silicate – deeper down or at the water table). This is why gossans are often depleted in zinc at the top. Manganese oxides do not capture zinc as strongly as they do lead or silver, but they can adsorb some zinc or even form zinc-bearing Mn minerals (like chalcophanite) under the right conditions. So, a manganese-rich gossan might show a slight zinc anomaly at surface where an iron-rich gossan would show almost none. In practical terms, if you detect a bit of zinc in a gossan that is also high in manganese, it hints that the original ore had significant zinc – even if most of that zinc has been leached away to lower levels.

Beyond silver, lead, and zinc, manganese oxides in gossans can trap other metals too. They are known to scavenge copper (often, black manganese wad is found alongside green malachite or blue azurite stains, indicating copper presence), as well as cobalt and nickel (in fact, marine manganese nodules concentrate Co and Ni by similar processes). Barium is another element often found with manganese oxides – either as residual barite (BaSO₄) or within minerals like hollandite (a Ba-Mn oxide). All these tendencies underscore that manganiferous gossans act as surface geochemical traps, soaking up a bouquet of metals and making a potentially complex ore signature more detectable right at the outcrop.

Exploration Significance and Examples of Manganiferous Gossans

The presence of a manganiferous gossan in the field is a big red (or rather, black) flag for geologists and prospectors. Such gossans have played a role in several significant mineral discoveries:

Broken Hill, NSW (Australia): Broken Hill is a textbook case of a manganiferous gossan leading to a world-class ore discovery. A long, dark ridge of manganese-rich gossan (standing about 50 metres above the plain) marked the lode. Instead of the usual rusty colour, this ridge was black and brown; prospectors described it as a "blackened outcrop" against the lighter host rocks. Assays of the cap returned very high lead (over 10% Pb) and silver, but low zinc – a sign that zinc had been leached away. Geologists found lead-bearing manganese minerals (such as coronadite) and lead carbonates like cerussite in the gossan, explaining the metal-rich surface. Drilling beneath that black cap confirmed one of the richest Pb-Ag-Zn ore bodies ever found, proving that an oddly coloured gossan can indeed indicate an extraordinary ore deposit below.

Mount Isa, QLD (Australia): This giant lead-zinc-silver deposit was discovered in the 1920s after a prospector (John Campbell Miles) tested some dark, heavy gossan rocks he found by a creek. The gossan outcrop at Mount Isa was a silicified ironstone ridge with traces of lead carbonate and patches of black manganese staining – not as dramatic looking as Broken Hill’s, but still significant. Geochemical analysis of the gossan showed anomalously high manganese and barium, hinting at what lay beneath (indeed, the ore bodies contained sphalerite, galena, and barite). Manganese oxide coatings on the gossan carried minor lead and zinc as well, illustrating how metals were held at the surface. In retrospect, even without a jet-black outcrop, the elevated manganese in the gossan’s chemistry was a critical clue that guided drilling to the massive ore system below.

Other Lead-Zinc-Silver Systems: Manganiferous gossans are found in many base metal districts worldwide. In the American Rocky Mountains, for example, 19th-century prospectors noted that some silver-lead lodes were marked by blackened gossans rich in manganese and lead oxides — these often yielded high-grade “dry” silver ore at surface due to the presence of silver chloride (horn silver) and manganese oxides. In parts of Africa and Asia, certain carbonate-hosted Pb-Zn deposits have weathered to produce black "wad" gossans (manganese-rich earthy caps). Even sedimentary exhalative (SedEx) deposits, which usually form deep in basins, can develop manganiferous gossan outcrops if brought near the surface by uplift; a modern example is the Abra lead-silver deposit in Western Australia, where manganese-rich gossan fragments with anomalous lead geochemistry helped guide exploration drilling.

Gold and Epithermal Deposits: It's worth noting that manganiferous gossans are not limited to lead-zinc systems. Some epithermal gold-silver deposits, especially those with abundant manganese carbonate (rhodochrosite) or manganiferous calcite in their veins, can weather to produce a black "sooty" gossan. For example, certain silver-rich veins in Mexico and Chile developed dark manganese oxide caps (historically called “black cap” gossans by miners). In such cases the manganese is indicating the original gangue mineralogy rather than lead, but the dark colour still serves as a valuable marker for precious metal mineralisation. Prospectors learned that even in gold and silver terrains, an unusually dark gossan could mean something interesting is hiding nearby.

Exploration relevance: Manganiferous gossans are highly valued in mineral exploration because they tend to concentrate the geochemical signals of buried ore. Iron-rich gossans, while useful, often lose many of the valuable metals during weathering (those metals get leached downward and dispersed). Manganese oxides, by contrast, help retain some of those metals at the surface where geologists can detect them. A black gossan, therefore, often yields stronger and more coherent geochemical anomalies – it’s essentially a natural collector of metal ions near the surface.

By sampling gossan outcrops or soils with high manganese content, exploration teams can pick up elements that might otherwise be too diluted to notice. For instance, an iron-rich soil above a deposit might show only a faint lead or silver anomaly, but if manganese oxides are present and have sequestered lead and silver into a smaller area, that anomaly will stand out sharply against background levels. In remote or vegetated terrain where outcrops are scarce, geochemists often keep an eye out for manganese-rich fragments in soil or stream sediments as a sign that a mineralised system could be shedding material upslope.

Another practical point is that not all manganese at surface is from a hidden ore body – sometimes you might just have manganese-rich rocks due to other geological processes. But when you find manganese oxides together with clear hints of other metals (like lead, zinc, copper, or silver) in a gossanous rock, it’s usually a compelling signal to investigate further. In essence, manganese acts like Mother Nature’s highlighter – it amplifies the subtle geochemical whispers of a buried ore deposit into a clearer shout at the surface.

Conclusion

Manganiferous gossans are fascinating features at the crossroads of geology and geochemistry. They start out as ordinary sulphide ore bodies deep underground and end up as eye-catching clues on the surface. The addition of manganese to the mix transforms the usual red iron cap into a darker cap, one that holds extra secrets about the minerals and metals once hidden below. By examining these blackened gossans – identifying their minerals, understanding their formation, and analysing their chemistry – geologists can decode a wealth of information about the unseen ore body.

In summary, a manganiferous gossan forms when nature’s weathering processes oxidise not just iron but also manganese from an ore deposit, leaving behind a residue rich in manganese oxide minerals like pyrolusite, cryptomelane, and coronadite. These minerals paint the rock black and act as sponges for metals like silver, lead, and zinc. Visually distinctive and loaded with geochemical clues, manganiferous gossans have guided prospectors to major discoveries (Broken Hill and Mount Isa are testament to that) and remain important in modern exploration. For geology students and rock enthusiasts, they are a vivid reminder that even a weathered lump of rock at the surface can be a storied piece of the Earth’s crust – recording chemical battles between water and ore, and pointing the way to treasure concealed below.