Introduction
Gossan is intensely oxidized, weathered or decomposed rock, usually the upper and exposed part of an ore deposit or mineral vein. In the 19th and 20th centuries, gossans were important guides to buried ore deposits used by prospectors in their quest for metal ores. An experienced prospector could read the clues in the structure of the gossans to determine the type of mineralization likely to be found below the iron cap (Ozdemir & Sahinoglu, 2018). Easy to identify, gossans are probably a marker of mineralization, possibly base metals. The most prevalent sulphide in deposits connected to volcanic rocks is typically pyrite. Recognize the difference between gossan and the more worn, orange surface found on some rocks that are rich in other minerals that include iron. It is a positive indication that you are nearing your gold if you see these structures in your claim.
What are gossans?
Gossans are the signposts that point to what lies beneath the surface. Gossans are exceedingly ferruginous rock, which is the product of oxidation by weathering and leaching of sulfides mineralization (Haldar, 2018). They are distinguished by their many honeycomb patterns, which are referred to as boxwork structure, texture, and colors. Banded, diamond mesh, triangular, cellular, contour, sponge, and colloform textures are among the different types of boxwork textures. A gossan’s color can vary greatly, ranging from red to yellow to brown to black. The iron hydroxide and oxide mineral phases are what give this color. Gossan can reach depths of several hundred meters.
Historical significance of gossans in mining
Throughout history, gossans have helped to discover numerous deposits. For example, the Broken Hill deposit in Australia was discovered due to the study of gossans. Most of the outcrop of the main lode at Broken Hill is an unusual gossan: a dense black rock, consisting of intergrown aggregates of botryoidal plumbic coronadite, altered rock fragments, and some quartz and goethite. It is more siliceous towards the top, full of cracks and vughs filled with oxide botryoids and stalactites, but towards the base it increasingly exhibits cracks and scattered masses of cerussite and pyromorphite. With depth, the gossan gradually becomes less dense, containing less plumbic coronadite, which eventually occurs only in the form of veins and patches. Silver halides may also occur (van Moort & Swensson, 1984). Another example is the High Lake deposit in Canada. Gossan samples collected during a reconnaissance expedition to High Lake in Nunavut, Canada, were analysed to determine their mineral components and to define parameters for the geochemical environment in which they formed. The gossan represents a natural acid drainage site in an arctic environment that serves as an analogue to the conditions under which sulfate, and Fe-oxide possibly formed on Mars. Rock and soil samples were taken from three different outcrops and analysed using XRD, SEM/EDS and Mössbauer. Two main mineral assemblages were observed (West et al., 2009). It is important to note that early prospecting relied heavily on visual identification of gossans.
Gossans as pathfinders in modern exploration
Gossans are used to identify anomalies in metal concentrations. For example, higher copper and zinc values in gossan samples can point to significant mineralization zones. The spatial distribution of gossans helps geologists target areas for further exploration. In Turkey’s Kastamonu-Sinop region, gossans were critical in identifying VMS-type deposits associated with specific geological formations. In terms of remote sensing, modern satellite imaging methods allow for the detection of gossans from space. Hyperspectral imaging can distinguish gossans from false targets by analysing their unique reflectance signatures across the electromagnetic spectrum. Gossan zones can affect Induced Polarization (IP)/resistivity and Electromagnetic (EM) surveys. Gossans, which are surface-weathered zones of metal mineralization, often contain minerals like sulfides or oxides that influence geophysical properties such as conductivity and chargeability. These characteristics can create anomalies in IP/resistivity and EM surveys, making gossans detectable and significant in mineral exploration.
Types of mineralization associated with Gossans
Gossans can help discover several types of mineralisation including base metals (Copper, Lead, Zinc), precious metals (particularly in shear-hosted and VMS settings) and massive sulphide deposits (VMS, SEDEX) and relationships with gossans. The table 1 shows nature of gossan developed by different ore minerals.
Table 1: Nature of gossan developed by different ore minerals.
| Sulphide Deposits | Gossan colours | Boxwork Textures/Structures |
| Chalcopyrite | Deep brown/brick red, yellowish | Coarse, cellular with blebs, masses with angular walls |
| Bornite | Chocolate | Triangular, crusted, curved |
| Bornite-chalcopyrite | Yellow, brick red | Fine, cellular, thin, small, friable walls, blebs |
| Chalcocite | Deep maroon | Porous/ Pitchlike limonite, no cells |
| Sphalerite | Tan to brown | Cellular, siliceous, thin, rigid angular walls |
| Galena | Orange | Diamond mesh/ Pyramidal/cleavage |
| Molybdenite | Black or gray | Foliated |
Limitations and challenges
Despite the fact that they are useful for discovering deposits, these indicators face significant limitations. The first is that they can be misinterpreted, especially by inexperienced geologists. Secondly, there can be false positives, because it is important to remember that not all iron-rich outcrops are gossans. Lastly, extensive alteration can lead to false interpretations of gossans. To limit the damage, integration with AI, machine learning, and remote sensing to identify subtle gossan expressions is necessary.
Conclusion
Gossans remain invaluable indicators in the search for hidden mineral deposits. Their distinct textures, colors, and geochemical signatures offer crucial clues for targeting subsurface mineralization. Despite limitations, advancements in technology now enhance the interpretation and detection of gossans. Integrating traditional geological knowledge with modern tools strengthens exploration outcomes. In essence, gossans continue to bridge the gap between surface observations and buried economic potential.
Reference
Haldar, S. K. (2018). Chapter 5—Exploration Geochemistry. In S. K. Haldar (Ed.), Mineral Exploration (Second Edition) (pp. 85–101). Elsevier. https://doi.org/10.1016/B978-0-12-814022-2.00005-8
Ozdemir, A., & Sahinoglu, A. (2018). Important of Gossans in Mineral Exploration: A Case Study in Northern Turkey. International Journal of Earth Science and Geophysics, 4(1). https://doi.org/10.35840/2631-5033/1819
van Moort, J. C., & Swensson, C. G. (1984). The gossan at Broken Hill. Journal of Geochemical Exploration, 22(1), 355–356. https://doi.org/10.1016/0375-6742(84)90022-0
West, L., McGown, D. J., Onstott, T. C., Morris, R. V., Suchecki, P., & Pratt, L. M. (2009). High Lake gossan deposit: An Arctic analogue for ancient Martian surficial processes? Planetary and Space Science, 57(11), 1302–1311. https://doi.org/10.1016/j.pss.2009.05.011
