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This study aims to map the non-sulphide Zinc (Zn)-bearing gossans at the Gujer Zn deposit area, Central Iran, using Landsat 8 Operational Land Imager (OLI) and GeoEye-1 satellites. The colour composites, Principal Component Analysis (PCA), and Support Vector Machine (SVM) were adopted for image analysis. Zn-bearing gossans contain Fe-oxyhydroxide minerals displaying spectral characteristics in visible and infrared (IR) wavelengths. The application of colour composites using GeoEye-1 images resulted in the delineation of gossans (real target) and ferruginous sandstones (false targets) having the same colour tone in the study area. IR spectroscopy of ore samples showed that hemimorphite exhibits low absorption in shortwave infrared (SWIR) wavelengths. Consequently, the Crosta-PC analysis was conducted using bands 4, 5, SWIR-1, and SWIR-2 of Landsat OLI to enhance only ore gossans. Five target zones were specified using the Crosta technique. The SVM method was performed to increase the accuracy of image analysis using the Radial Basis Function (RBF) kernel. The SVM-RBF method accomplished enhancing ore gossans by defining a new target zone. According to the results, the application of the Crosta technique using bands 4, 5, SWIR-1, and SWIR-2 of Landsat OLI can specify ore gossans and eliminate the interfering effect of ferruginous sandstones in similar geological settings. The SVM-RBF can improve the results of image processing using PC entry of Landsat OLI bands. GeoEye-1 images are useful for the initial assessment of geological units in the region and for delineating the accurate boundary of ore gossans derived from Landsat 8 OLI data.

This study aims to map the non-sulphide Zinc (Zn)-bearing gossans at the Gujer Zn deposit area, Central Iran, using Landsat 8 Operational Land Imager (OLI) and GeoEye-1 satellites. The colour composites, Principal Component Analysis (PCA), and Support Vector Machine (SVM) were adopted for image analysis. Zn-bearing gossans contain Fe-oxyhydroxide minerals displaying spectral characteristics in visible and infrared (IR) wavelengths. The application of colour composites using GeoEye-1 images resulted in the delineation of gossans (real target) and ferruginous sandstones (false targets) having the same colour tone in the study area. IR spectroscopy of ore samples showed that hemimorphite exhibits low absorption in shortwave infrared (SWIR) wavelengths. Consequently, the Crosta-PC analysis was conducted using bands 4, 5, SWIR-1, and SWIR-2 of Landsat OLI to enhance only ore gossans. Five target zones were specified using the Crosta technique. The SVM method was performed to increase the accuracy of image analysis using the Radial Basis Function (RBF) kernel. The SVM-RBF method accomplished enhancing ore gossans by defining a new target zone. According to the results, the application of the Crosta technique using bands 4, 5, SWIR-1, and SWIR-2 of Landsat OLI can specify ore gossans and eliminate the interfering effect of ferruginous sandstones in similar geological settings. The SVM-RBF can improve the results of image processing using PC entry of Landsat OLI bands. GeoEye-1 images are useful for the initial assessment of geological units in the region and for delineating the accurate boundary of ore gossans derived from Landsat 8 OLI data. [ABSTRACT FROM AUTHOR]

Copyright of BSGF: Earth Sciences Bulletin is the property of EDP Sciences and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

As the depletion of zinc sulfide ores becomes more severe, investigations into the recovery of zinc from zinc oxide ores have aroused more interest. In this regard, acid-based hydrometallurgical treatment strategies have had great effectiveness. However, they are inadequate for low-grade zinc oxide ores. In this study, we examined the alkaline treatment of gossan for the recovery of oxide ores that bear zinc, such as siderite and limonite. Additionally, of particular note, the effects of a leaching agent, its concentration and time, temperature, liquid-to-solid ratio, as well as the agitation rate on the leaching of zinc from gossan were studied to evaluate the effects of these parameters on the kinetics of zinc dissolution. The results showed that the leaching of zinc is controlled by a single rate-controlling step with an activation energy of 4.458 kJ/mol before 120 min and 5.536 kJ/mol after 120 min, with zinc leaching efficiency less than 50% in all leachings. [ABSTRACT FROM AUTHOR]

Introduction Erosion and oxidation of massive sulfides when uplifted and exposed to the surface, commonly lead to the formation of gossans. In this process, surface water will dissolve soluble elements, and oxides and hydroxides of iron (goethite and hematite) will form on top of the volcanogenic massive sulfide (VMS) deposits. The main tectonic settings for Iranian VMS deposits are magmatic arcs, which can be subdivided into volcanic primitive arc, arc/intra-arc rift, and back-arc settings and Sanandaj-Sirjan zone is one of the structural zones that host many VMS deposits in Iran (Mousivand et al., 2018). The study area is located southwest of Jiroft, Kerman province. The main rock units include vitric tuff, pelagic sediments, volcano-sedimentary rocks, gabbro and intermediate to mafic dykes. Mineralization has occurred in volcano-sedimentary beds. The pelagic sediments which are composed of limestone, shale, sandstone, siltstone and interlayers of pillow lava, are the main hosts for mineralization. Surface oxidation of mineralized zones has led to conversion of primary sulfides to iron oxides and hydroxides to form gossan. This study contributes to mineralogical and geochemical composition and mineralization of gossans to demonstrate how surface oxidation of primary sulfides can play a role in locating VMS mineralization at depth. Materials and methods A geological map with a scale of 1:5000 was prepared during field and laboratory studies. Twenty polished section were studied to identify mineral distributions and textures, and some of them were chosen for scanning electron microscopic (SEM) examinations. Fifteen rock samples from the gossan horizons were chosen for geochemical studies. The samples were taken from across the mineralized horizon. Six rock samples were taken from old mining site outcrops to compare the geochemistry of gossans with other surface mineralization. All samples were sent to the laboratory for analysis by Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES). The rare earth element (REEs) values were measured by Inductively Coupled Plasma Mass Spectrometry (ICP-MS). X-ray diffraction (XRD) spectroscopy was used to identify mineralogy of 30 rock samples. All analyses were performed in the central laboratory of the Geological Survey and Mineral Exploration of Iran, in Tehran and Karaj. Results The ore and gangue minerals have massive, layered, disseminated, veinlet, breccia and replacement textures. Based on mineralography, XRD and SEM studies, the main minerals are hematite, goethite, quartz, and jarosite-group minerals. The upper horizon of gossan, with 13 meters thickness has large volume of hematite and gothite minerals. The enrichment of gold, arsenic, antimony, silver, lead and bismuth were observed in this zone. The lower horizon, with a thickness of about 1.5 meters show anomalies of copper and zinc elements. The highest amount of gold and silver were measured about 18.5 and 120 g/ton, respectively. The highest amount of lead element is 1.3 wt.%, which shows a positive correlation with silver variations. The other values are copper 0.16 wt.%, arsenic 0.61 wt.%, bismuth 580 g/ton, and antimony 280 g/ton. Discussion Trace and REEs geochemistry are useful in identifying gossans and probable sources (Scott et al., 2001). Geochemical studies also can be used to separate mature from immature gossans. Although the composition of gossans is influenced by early composition of the ore, gossans with high content of Pb (more than 4 wt.%) are usually considered immature. The average Pb measured in the studied gossans is about 2210 g/ton. The Ag content is also low (less than 150 g/ton) and there is a relatively linear relationship between increasing Ag and Pb content. High values of copper often refer to a lower degree of maturity. In the studied gossans, the average amount of Cu is about 2900 g/ton, which is much lower than the immature gossans with average 1.6 wt.%. Therefore, the results of chemical analysis indicate that these gossans are in the category of mature ore bearing gossan. The REE from La to Lu, is relatively consistent with the shape of REE profiles for volcanogenic massive sulfide mineralization and concurrent massive sulfide gossans (Peter et al., 2003; Volesky et al., 2017; Gieré, 1993). The pattern of distribution of REEs shows small positive Eu enrichment and zoning of precious mineral elements confirms the possibility of orebody under the gossans. Further exploration of volcanogenic massive sulfide deposits is recommended for this area. References Gieré, R., 1993. Transport and deposition of REE in H2S-rich fluids: evidence from accessory mineral assemblages. Chemical Geology, 110(1–3): 251–268. https://doi.org/10.1016/0009-2541(93)90257-J Mousivand, F., Rastad, E., Peter, J.M. and Maghfouri, S., 2018. Metallogeny of volcanogenic massive sulfide deposits of Iran. Ore Geology Reviews, 95: 974–1007. https://doi.org/10.1016/j.oregeorev.2018.01.011 Peter, J.M., Goodfellow, W.D. and Doherty, W., 2003. Hydrothermal sedimentary rocks of the Heath Steele Belt, Bathurst Mining Camp, New Brunswick: Part 2. Bulk and rare earth element geochemistry and implications for origin. In: W.D. Goodfellow, S.R. McCutcheon and J.M. Peter (Editors), Massive Sulphide Deposits of the Bathurst Mining Camp, New Brunswick, and Northern Maine, Littleton, CO, Society of Economic Geologists, pp. 391–415. https://doi.org/10.5382/Mono.11.17 Scott, K.M., Ashley, P.M. and Lawie, D.C., 2001. The geochemistry, mineralogy and maturity of gossans derived from volcanogenic Zn–Pb–Cu deposits of the eastern Lachlan Fold Belt, NSW, Australia. Journal of Geochemical Exploration, 72(3): 169–191. https://doi.org/10.1016/S0375-6742(01)00159-5 Volesky, J.C., Leybourne, M.I., Stern, R.J., Peter, J.M., Layton-Matthews, D., Rice, S. and Johnson, P.R., 2017. Metavolcanic host rocks, mineralization, and gossans of the Shaib al Tair and Rabathan volcanogenic massive sulphide deposits of the Wadi Bidah Mineral District, Saudi Arabia. International Geology Review, 59(16): 1975–2002. https://doi.org/10.1080/00206814.2017.1307789

Elliptical sulfate-rich features, stained red by the presence of iron oxides, are set within permafrost and carbonate rock at Borup Fiord Pass in the Canadian High Arctic. These features, which vary in diameter from ∼0.5 to 3 m, exhibit the co-localization of sulfur (S) and iron (Fe) minerals, with S and Fe both preserved in multiple oxidation states. Through application of x-ray diffraction (XRD) and Raman microspectroscopy, we find that these features appear to encapsulate central cores with abundant pyrite while S0, gypsum, and iron-bearing secondary minerals such as jarosite, goethite, and hematite have formed from oxidative weathering of the pyrite. These features appear similar to terrestrial gossans, mineral features representative of weathered sulfide ores in the near-surface. We use Raman microspectroscopy and scanning transmission x-ray microscopy (STXM) to show that organic carbon is associated with the minerals within these features. The origin of this carbon remains enigmatic; however, we consider the possibility that this carbon, along with etching of the pyrite grains and accumulation of encrusted filaments detected in the features, may be indicative of preserved biological activity during alteration of the pyrite. We also consider how such geological sulfide emplacements and their weathered surface expressions may provide a target for future surface and remote sensing studies of Mars and in the search for extraterrestrial life.

Mapping of regolith is significant to identify and discriminate geomorphic expression, regolith evolution, weathering histories and assistance in clarification of relationships between parent and surface materials. In this research work, the hydrothermally altered regolith of Western Jahajpur Belt, Bhilwara, India is studied using ASTER and AVIRIS imageries by PCA, decorrelation stretching supervised classification image processing methods. Results of decorrelation stretching of ASTER spectral bands 6, 7 and 9 showed the regolith in blue, green and red colour with the associated geological formations. Analyses of VNIR-SWIR spectral bands of ASTER by PCA showed the bed rocks associated to the regolith green, yellow, white and red colours, respectively. The minerals of the regolith and parent geological formations are identified using AVIRIS data by supervised classification and endmember spectral analyses. The study also identified six regolith-landforms which include the regolith (vegetation cover), crop land (vegetation cover), water, crop land with vegetation, alluvium and regolith (ferruginous and calcareous mineral zone). The discrimination of regoliths and geological formations are verified in the field and confirmed by XRF and ICP-MS geochemical analysis of samples. [ABSTRACT FROM AUTHOR]

Identification and delineation of alteration minerals and regolith play an important role in the discrimination of metallic and non-metallic minerals and are also significant for metallic and non-metallic mineralization zone identification. In this work, altered minerals and gossans have been identified in and around the Askot area in eastern Kumaon Himalaya using hyperspectral data. Analysis of the Hyperion data indicates alteration minerals in the alteration zones surrounding the Askot sulfide orebody. These minerals include alunite, axinite, illite, and pyrite. Besides, iron bearing minerals such as goethite and hematite are found in the gossans. These gossans are formed by surface weathering of sulfide orebodies and found in a northeast direction along the strike of the Askot orebody. Field surveys in two target prospective mineralized zones confirm the presence of gossan and altered minerals identified via the Hyperion hyperspectral data. This study demonstrates the ability of hyperspectral data and field investigations to identify hydrothermal alteration minerals, gossans, and potential mineralized zones in the Himalaya.

Copyright of Boletin de Geologia is the property of Universidad Industrial de Santander and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

In this study, we have analyzed the comparative potential of reflectance and emittance bands of advanced spaceborne thermal emission and reflection radiometer (ASTER) sensor for detecting surface exposures of Proterozoic rocks and gossan capping of gold mineralization. In this regard, few new image analysis products are derived. These are matched filtering factor-modified ratio (MFR) image and the inverse of a difference image of the ratio and matched filtering map (IDRM). These image products are used with the conventional ratio images, relative band depth images and index-based principal component (PC) images of reflectance and emittance bands of ASTER for delineating metasediments of the study area. We have found that reflectance band products are superior for mapping metasediments and gossan capping than the emissivity products, which are only suitable to enhance quartzite. Self-potential (SP) geophysical survey is carried out around the identified gossan site as SP is sensitive to sulphide mineralization. Results of the SP survey indicated presence of sulphide enrichment underneath the gossan. [ABSTRACT FROM AUTHOR]

Gossans are surficial deposits that form in host bedrock by the alteration of sulphides by acidic and oxidizing fluids. These deposits are typically a few meters to kilometers in size and they constitute important vectors to buried ore deposits. Hundreds of gossans have been mapped by field geologists in sparsely vegetated areas of the Canadian Arctic. However, due to Canada’s vast northern landmass, it is highly probable that many existing occurrences have been missed. In contrast, a variety of remote sensing data has been acquired in recent years, allowing for a broader survey of gossans from orbit. These include band ratioing or methods based on principal component analysis. Spectrally, the 809 gossans used in this study show no significant difference from randomly placed points on the Landsat 8 imageries. To overcome this major issue, we propose a deep learning method based on convolutional neural networks and relying on geo big data (Landsat-8, Arctic digital elevation model lithological maps) that can be used for the detection of gossans. Its application in different regions in the Canadian Arctic shows great promise, with precisions reaching 77%. This first order approach could provide a useful precursor tool to identify gossans prior to more detailed surveys using hyperspectral imaging.

Graphical abstract Highlights • The WV-3 VNIR data is able to discriminate iron oxide and hydroxide minerals. • The WV-3 VNIR data could generate large-scale mineral maps. • The band math and matched filtering were applied to map the target materials. • Image Spectra were extracted for iron oxide and hydroxide abundance mapping. • The output results were validated by microscopic studies, spectra and XRD analysis. Abstract Discrimination of iron oxide and hydroxide mineral assemblages is a key factor in porphyry copper exploration. This paper investigates the ability of the Worldview-3 VNIR data in mapping iron oxide / hydroxide minerals associated with the porphyry copper mineralization. The study area is situated in the central part of the Urumieh–Dokhtar magmatic arc, northeastern Isfahan, Iran, which hosts a number of important porphyry copper deposits. The copper mineralization is related to the porphyry intrusive complex that ranges in composition from diorite to granodiorite. Bands 2,3,4,5, and 8 of this satellite were used to enhance the pixels containing target minerals and to mask the effects of vegetation on these pixels. The best results in terms of mapping and discriminating target materials were acquired by the band math and the matched filtering (MF) methods. The MF output results enhanced pixels with the highest fractions of the target minerals. The field investigations, X-ray diffraction (XRD) analysis, and microscopic studies showed that the distribution pattern of target minerals correspond well to ground criteria and gossan caps associated with the porphyry copper mineralization. It was concluded that Worldview-3 has the efficient spectral bands in VNIR region, which could be proposed for the iron oxide / hydroxide mineral mapping. [ABSTRACT FROM AUTHOR]

Several gold deposits hosted mainly by Variscan granites and Precambrian to Palaeozoic metasediments occur in the northwestern part of Portugal. Most of these deposits were mined by the Romans (in the period I BC to II AD) as open pits and surface galleries. The Castromil-Serra da Quinta gold deposit is an important example of such a mined site; it occurs in the Dúrico-Beirã Au province located in the Central Iberian Zone (CIZ) in the western branch of European Variscan belt, mainly on the eastern flank of the Valongo anticline. Open pits and underground galleries at Castromil-Serra da Quinta exploited the gossan formed from the weathering of primary mineralization. The gossan is composed essentially of goethite, scorodite and clay minerals. A recent drilling campaign at Castromil-Serra da Quinta has provided samples of the primary mineralization below the oxidation level. Different modes of gold occurrence are defined based on metallographic studies of both the gossan and drill cores. Gold I occurs encapsulated in primary sulfide minerals, mainly arsenopyrite and pyrite; Gold II is also associated with the main primary sulfides, but occurs along grain boundaries and in microfractures of the sulfides or in associated quartz veins; and Gold III occurs as free gold particles in iron oxides within the gossan. In the gossan samples, it is difficult to distinguish whether the gold particles hosted in oxides correspond to Gold I, Gold II, or both, so these particles are described as Gold I--II and they are commonly surrounded by very much smaller particles of Gold III. Scanning electron microscope (SEM) and electron probe microanalysis (EPMA) data for the different gold particles reveal that Gold I is poorer in Ag (-15.5-39.76%) than Gold II (37.46--51.45%), whereas Gold III corresponds to native gold (<16.11% Ag). Gold III is thought to reflect gold enrichment in the upper level of the deposit, resulting from weathering processes that affected the primary Au (Bi) mineralization. [ABSTRACT FROM AUTHOR]

The mineralogical and chemical compositions of various ocher gossans from a gold mine in northeastern Thailand were investigated, including some heavy metals and other toxic elements. Mineralogical characteristics were carried out using X-Ray Diffractometer (XRD) and Scanning Electron Microscope (SEM) whereas chemical compositions were analyzed using Electron Probe Micro-Analyzer (EPMA). These ocher gossans can be classified, initially based on Munsell color, into five types: type-I (pale-yellow color), type-II (brownish-yellow color), type-III (yellowish-brown color), type-IV (dusky-red color) and type-V (red color). The primary silicate minerals (i.e., quartz, garnet epidote and amphibole) are found in type -I, -II, -III and -IV. They appear to be composed of skarn rock. On the other hand, the secondary minerals (i.e., goethite, jarosite, ankerite, montmorillonite, magnetite, gypsum and secondary quartz) are observed in types-II, -III, -IV and -V. As and Cu are found crucially in types-III, -IV and -V in which both elements can be adsorbed by goethite and/or jarosite. As the result, the gossan rocks in this area are natural adsorbents with high potential to reduce As and Cu contamination into the ecosystem. Therefore, the gossan, a natural attenuation material, is recommended for site remediation because of its low cost and local abundance. Feasibility studies should be conducted to further investigate the potential.

The Jbel Rhals deposit, located in the Oriental High Atlas of Morocco, hosts a polymetallic Fe-Mn-Cu ore. Large metric veins of goethite and pyrolusite cut through Paleozoic schists that are overlaid by Permian-Triassic basalts and Triassic conglomerates. The genesis of this deposit is clearly polyphased, resulting from supergene processes superimposed over hydrothermal phases. The flow of Permian-Triassic basalts probably generated the circulation of hydrothermal fluids through the sedimentary series, the alteration of basalts and schists, and the formation of hydrothermal primary ore composed of carbonates (siderite) and Cu-Fe sulfides. Several episodes of uplift triggered the exhumation of ores and host rocks, generating their weathering and the precipitation of a supergene ore assemblage (goethite, pyrolusite, malachite and calcite). In the Paleozoic basement, Fe-Mn oxihydroxides are mostly observed as rhombohedral crystals that correspond to the pseudomorphose of a primary mineral thought to be siderite; goethite precipitated first, rapidly followed by pyrolusite and other Mn oxides. Malachite formed later, with calcite, in fine millimetric veins cutting through host-rock schists, conglomerates and Fe-Mn ores. [ABSTRACT FROM AUTHOR]

Waste rocks from gold mining in northeastern Thailand are classified as sandstone, siltstone, gossan, skarn, skarn-sulfide, massive sulfide, diorite, and limestone/marble. Among these rocks, skarn-sulfide and massive sulfide rocks have the potential to generate acid mine drainage (AMD) because they contain significant amounts of sulfide minerals, i.e., pyrrhotite, pyrite, arsenopyrite, and chalcopyrite. Moreover, both sulfide rocks present high contents of As and Cu, which are caused by the occurrence of arsenopyrite and chalcopyrite, respectively. Another main concern is gossan contents, which are composed of goethite, hydrous ferric oxide (HFO), quartz, gypsum, and oxidized pyroxene. X-ray maps using electron probe micro-analysis (EPMA) indicate distribution of some toxic elements in Fe-oxyhydroxide minerals in the gossan waste rock. Arsenic (up to 1.37 wt.%) and copper (up to 0.60 wt.%) are found in goethite, HFO, and along the oxidized rim of pyroxene. Therefore, the gossan rock appears to be a source of As, Cu, and Mn. As a result, massive sulfide, skarn-sulfide, and gossan have the potential to cause environmental impacts, particularly AMD and toxic element contamination. Consequently, the massive sulfide and skarn-sulfide waste rocks should be protected from oxygen and water to avoid an oxidizing environment, whereas the gossan waste rocks should be protected from the formation of AMD to prevent heavy metal contamination. [ABSTRACT FROM AUTHOR]

Gossans (Fe-rich alteration zones) are the surface indications of massive sulfide, porphyry and skarn deposits in the Arabian Nubian Shield. They consist of limonite, goethite, hematite, malachite, and azurite. The Khunayqiyah gossans, Eastern Arabian Shield, Saudi Arabia, have been used to demonstrate the effectiveness of using the Landsat 8 OLI imagery for detecting and delineating of gossan zones in arid regions. The Khunayqiyah gossans have diagnostic spectral features, and they are often larger than the pixel size of the Landsat 8 OLI images. Remote sensing techniques in this study include pan sharpening, principal component analysis (PCA), minimum noise fraction (MNF), and band ratio. The RGB (red, green, blue) color composites of pan-sharpened original bands (4, 3, 2), PCA (PC3, PC2, PC1) and MNF (MNF2, MNF4, MNF3) images were found to be the most useful to delineate gossan/alteration zones in the Khunayqiyah district. The obtained results show reasonable matches between the spectra of collected samples and image-derived spectra from Landsat 8 data. Notably, the use of above-mentioned technique for mineral exploration is facilitated by the arid environment. The results of this study demonstrate Landsat 8 OLI images and the above-mentioned technique are useful in the exploration of new gossan occurrences in the Arabian Nubian Shield and other arid regions worldwide where little in situ geological data exist. [ABSTRACT FROM AUTHOR]

The Wadi Bidah Mineral District of Saudi Arabia contains more than 16 small outcropping stratabound volcanogenic Cu–Zn–(Pb) ± Au-bearing massive sulphide deposits and associated zones of hydrothermal alteration. Here, we use major and trace element analyses of massive sulphides, gossans, and hydrothermally altered and least altered metamorphosed host rock (schist) from two of the deposits (Shaib al Tair and Rabathan) to interpret the geochemical and petrological evolution of the host rocks and gossanization of the mineralization. Tectonic interpretations utilize high-field-strength elements, including the rare earth elements (REE), because they are relatively immobile during hydrothermal alteration, low-grade metamorphism, and supergene weathering and therefore are useful in constraining the source, composition, and physicochemical parameters of the primary igneous rocks, the mineralizing hydrothermal fluid and subsequent supergene weathering processes. Positive Eu anomalies in some of the massive sulphide samples are consistent with a high temperature (>250°C) hydrothermal origin, consistent with the Cu contents (up to 2 wt.%) of the massive sulphides. The REE profiles of the gossans are topologically similar to nearby hydrothermally altered felsic schists (light REE (LREE)-enriched to concave-up REE profiles, with or without positive Eu anomalies) suggesting that the REE experienced little fractionation during metamorphism or supergene weathering. Hydrothermally altered rocks (now schists) close to the massive sulphide deposits have high base metals and Ba contents and have concave-up REE patterns, in contrast to the least altered host rocks, consistent with greater mobility of the middle REE compared to the light and heavy REE during hydrothermal alteration. The gossans are interpreted to represent relict massive sulphides that have undergone supergene weathering; ‘chert’ beds within these massive sulphide deposits may be leached wall-rock gossans that experienced silicification and Pb–Ba–Fe enrichment from acidic groundwaters generated during gossan formation. [ABSTRACT FROM AUTHOR]

Under acidic, weathering conditions, silver (Ag) is considered to be highly mobile and can be dispersed within near-surface environments. In this study, a range of regolith materials were sampled from three abandoned open pit mines located in the Iberian Pyrite Belt, Spain. Samples were analyzed for Ag mineralogy, content, and distribution using micro-analytical techniques and high-resolution electron microscopy. While Ag concentrations were variable within these materials, elevated Ag concentrations occurred in gossans. The detection of Ag within younger regolith materials, i.e., terrace iron formations and mine soils, indicated that Ag cycling was a continuous process. Microbial microfossils were observed within crevices of gossan and their presence highlights the preservation of mineralized cells and the potential for biogeochemical processes contributing to metal mobility in the rock record. An acidophilic, iron-oxidizing microbial consortium was enriched from terrace iron formations. When the microbial consortium was exposed to dissolved Ag, more than 90% of Ag precipitated out of solution as argentojarosite. In terms of biogeochemical Ag cycling, this demonstrates that Ag re-precipitation processes may occur rapidly in comparison to Ag dissolution processes. The kinetics of Ag mobility was estimated for each type of regolith material. Gossans represented 0.6-146.7 years of biogeochemical Ag cycling while terrace iron formation and mine soils represented 1.9-42.7 years and 0.7-1.6 years of Ag biogeochemical cycling, respectively. Biogeochemical processes were interpreted from the chemical and structural characterization of regolith material and demonstrated that Ag can be highly dispersed throughout an acidic, weathering environment. [ABSTRACT FROM AUTHOR]

The Las Cruces VMS deposit is located at the eastern corner of the Iberian Pyrite Belt (SW Spain) and is overlain by the Neogene–Quaternary sediments of the Guadalquivir foreland Basin. The deposit is currently exploited from an open pit by Cobre Las Cruces S.A., being the supergene Cu-enriched zone the present mined resource. The Las Cruces orebody is composed of a polymetallic massive sulfide orebody, a Cu-rich stockwork and an overlying supergene profile that includes a Cu-rich secondary ore (initial reserves of 17.6 Mt @ 6.2% Cu) and a gossan cap (initial reserves of 3.6 Mt @ 3.3% Pb, 2.5 g/t Au, and 56.3 g/t Ag). The mineralogy of the Las Cruces weathering profile has been studied in this work. Textural relationships, mineral chemistry, deposition order of the minerals and genesis of the Las Cruces gossan are described and discussed in detail. A complex mineral assemblage composed by the following minerals has been determined: carbonates such as siderite, calcite and cerussite; Fe-sulfides including pyrite, marcasite, greigite and pyrrhotite; Pb–Sb sulfides and sulfosalts like galena, stibnite, fulöppite, plagionite, boulangerite, plumosite, and the jordanite–geocronite series, Ag–Hg–Sb sulfides and sulfosalts including miargyrite, pyrargyrite, sternbergite, acanthite, freibergite, cinnabar, Ag–Au–Hg amalgams; and Bi–Pb–Bi sulfides and sulfosalts such as bismuthinite, galenobismutite, others unidentified Bi–Pb-sulfosalts, native Bi and unidentified Fe–Pb–Sb-sulfosalts. Remains of the former oxidized assemblage appear as relicts comprised of hematite and goethite. Combining paragenetic information, textures and mineral chemistry it has been possible to derive a sequence of events for the Las Cruces gossan generation and subsequent evolution. In that sense, the small amount of Fe-oxyhydroxides and their relict textures replaced by carbonates and sulfides suggest that the gossan was generated under changing physico-chemical conditions. It is proposed that the Las Cruces current gossan represents the modified residue of a former gossan mineralization where prolonged weathering led to dissolution and leaching out of highly mobile elements and oxidation of the primary sulfides. Later, the gossan was subject to seawater-gossan interaction and then buried beneath a carbonated-rich cover. The basinal fluids-gossan interaction and the equilibration of fluids with the carbonated sediments brought to the carbonatization and sulfidation of the gossan, and thus to the generation of Fe-carbonates and Pb–Sb-sulfides. The Las Cruces mineral system likely represents a new category within the weathering class of ore deposits. [ABSTRACT FROM AUTHOR]

Gossan at the Perkoa deposit of stratabound lead and zinc ores (Burkina Faso) is composed of hematite, goethite, clay minerals, minerals of the brucite group and sulfates (corkite, alunite, natroalunite, hinsdalite and hidalgonite). Residual quartz contains inclusions of chalcopyrite, sphalerite and cinnabar. Compared with the primary ores , the gossan is especially enriched with As, Ba, Pb, P, and Sb. On the other hand, the contents of Ag, Co, Cu, Hg, Mo, Ni, and particularly Zn are significantly lower. However, compared with the barren ferruginous crust in the same area, the gossan is significantly enriched in most of chemical elements, namely As, Pb, Sb, and Zn and to lesser extent also in Ag, Ba, Cd, Mo, S, and Sr. The isotopic composition of lead in massive primary ore and in gossan is practically identical. However, the isotopic composition of lead in disseminated ores, the bleached gossan envelope, and in mineralized soils in the Perkoa area is very variable and reflects mixing of ore and lithogenic lead. The 206 / 207 Pb ratio in barren soils and ferruginous crusts is much higher compared with gossan and related altered rocks. Compared to massive ore (δ 65 Cu = + 1.91 to + 2.17‰ and δ 66 Zn = + 0.30 to + 0.36‰, respectively), the gossan is depleted in heavy isotopes of copper and zinc (δ 65 Cu = − 0.02 to − 0.58 and δ 66 Zn = − 0.10 to − 0.88‰, respectively). The chemical weathering of sulfide-rich rocks thus gives rise to considerable variations in Cu isotopes (median: − 2.36‰, Δ 65 Cu gossan–massive ore ), and minor changes in Zn isotopes (median: − 0.73‰, Δ 66 Zn gossan–massive ore ). Isotopic composition of copper in gossan and lateritic soil sampled in the vicinity of gossan is similar but differs from barren laterite duricrust which is more depleted in the 65 Cu isotope. In zinc, no differences were proved between the δ 66 Zn values in the gossan and those in the soil and ferruginous crust. [ABSTRACT FROM AUTHOR]

Owing to the links between gossans and mineral deposits, detecting gossans by remote sensing means is essential for mineral exploration. In northern regions, gossans can develop as thin oxidized surfaces, named thin gossans, that can be covered with lichens. This study investigates the effects of spectral mixing between such gossans with lichens and their rock substrates using laboratory spectroscopic data obtained from samples collected in the Cape Smith Belt of Canada. These observations are then scaled up to the airborne hyperspectral data obtained from the same area. Our laboratory results indicate that the presence of lichens on gossans induces a general spectral shift towards shorter wavelengths of the iron absorption typical of gossan spectra. The opposite shift is observed due to the influence of the rock substrates. These effects can thus impede classification of gossans based on the interpretation of iron oxide mineralogy from spectra. Our airborne spectral results suggest that thin gossans can be detected and discriminated from thick gossans, and further broken down into several classes according to their host rock substrates. The ability to define distinct classes of thin gossans is significant since the association of these gossans with specific rock substrates can be exploited for exploration. The ability to distinguish thin and thick gossans alone can contribute to mineral exploration since it can be either the former or the latter group of gossans that acts as an ore deposit vector. [ABSTRACT FROM AUTHOR]

The Las Cruces deposit is located at the eastern margin of the Iberian Pyrite Belt (IPB), beneath the Neogene-Quaternary deposits of the Guadalquivir Basin. These sediments covered and preserved the supergene profile somewhat after its generation until its recent discovering. For this reason, the Las Cruces ore deposits, together with Lagoa Salgada in Portugal, are the only known deposits in the IPB that preserve the entire weathering profile, including both the gossan and the enrichment zone. In that sense, the present study provides new data on the IPB ore deposits and their post-Palaeozoic evolution. The primary mineralization consists of massive and semi-massive polymetallic sulfides overlaying a cupriferous and pyritic stockwork. The supergene enrichment extends from the primary zone to gossan and consists of a thick cementation zone characterized by intense replacement of part of the primary sulfides by Cu-rich supergene sulfides. At present, the only economic resource of Las Cruces is the cementation zone, with initial reserves of 17.6 Mt @ 6.2% Cu, but the orebody also includes significant contents of Cu and Zn within the primary mineralization, and Au, Ag, and Pb in the gossan. Mineralogical and geochemical data confirm that the primary mineralization at Las Cruces resembles other IPB deposits in terms of distribution and evolution, however the gossan and cementation zone show an uncommon evolution for this kind of supergene profile. The major singularity that makes the Las Cruces supergene mineralisation noticeably different from others in the IPB or elsewhere resides in its unusual mineralogical composition, mostly comprised of newly-formed siderite, calcite, Fe-sulfides and galena. The genetic model proposed for the Las Cruces deposit includes three main stages: (1) ore genesis and evolution of primary mineralization, which seems to be comparable to other IPB deposits; (2) genesis of Cu-rich secondary mineralization and gossan by weathering, after Miocene exhumation, of the upper part of the massive sulfide deposit, under oxidizing and acidic conditions; and (3) late evolution of supergene profile below the carbonated-rich sediments of the Guadalquivir Basin, with the subsequent change in the redox conditions due to water–rock interaction between the supergene profile and basinal fluids. [ABSTRACT FROM AUTHOR]

Los materiales jarosíticos pueden encontrarse en estado natural (minerales de gossan) o generarse en circuitos hidrometalúrgicos. Para recuperar la plata contenida en estos materiales, se han propuesto procesos de descomposición alcalina-cianuración a temperaturas moderadas, elevadas o a presión, según el tipo de jarosita. En este trabajo se presenta un estudio sobre el tratamiento de soluciones sólidas jarosita-beudantita con sulfuros alcalinos a temperatura ambiente, seguido de cianuración convencional para la recuperación de la plata. Las transformaciones en estado sólido se han determinado mediante difracción de rayos X, microscopía óptica y electrónica (SEM-EDS) y microsonda electrónica. Durante la primera etapa del proceso, tiene lugar la descomposición de las fases jarosita, con formación de sulfuros de plomo y plata y un gel de hidróxido de hierro. La descomposición de potasiojarosita-beudantita tiene lugar a pH ≥ 12 y la de beudantita-plumbojarosita, precisa de un valor más elevado (pH = 14). La oxidación posterior para eliminar el exceso de sulfuro, seguida de cianuración convencional, permite la recuperación de la plata contenida.

Gossan Hill is located within the Minto Inlier in central Victoria Island, Northwest Territories (N 71.36697°, W 114.95155°). A study of the mineralogical associations and geological setting of this deposit indicates that it is an arrested hydrothermal system frozen in permafrost. From above, the hill stands out because of the topographic relief of 75 m and the orange-brown color of the surficial material. The surface of the hill is marked by areas of concentric color zonation up to 3 m across, with light gray centers surrounded by a yellow–orange ring that is surrounded by an orange–brown color that covers the rest of the surface of the hill. Trenches dug into these areas reveal that the central zone contains quartz and pyrite native sulfur in a loose aggregate of sand-sized grains. This central area is surrounded by a zone dominated by gypsum and quartz with some jarosite. Beyond this, the surrounding surface consists of quartz, hematite, and amorphous iron oxides. The radial arrangement of the mineral assemblage indicates an increase in oxidation of sulfur from the center outward. Analysis of isotopic composition of the sulfur indicates the source of sulfur could be the underlying strata. The hill is underlain by inter-bedded carbonate and sulfate-evaporite sedimentary rocks of the Kilian formation in the upper part of the Neoproterozoic Shaler Super group. The sedimentary rocks were intruded by diabase sills of the 720 Ma Franklin igneous event, which crop out 2 km to the south of Gossan Hill. The soft friable nature of the deposit and the topographic relief of the hill indicate a post-glacial (Pleistocene) age of formation. Permafrost has maintained the disequilibrium mineral assemblage since the cessation of fluid flow. Extraction of the permafrost ice from the central zone yields a liquid with a pH of 2.3. The observed long-term persistence of pyrite encased within the acidic permafrost indicates that oxidation and dissolution reactions common in mine waste are slowed, if not stopped, in such an environment. The predicted rise of Arctic temperatures will cause the active layer to move deeper and result in the release of the acidic solutions frozen in the permafrost. Water ice or frozen CO2 just below the Martian surface would also preserve such mineral disequilibrium for very long periods of time. No region exists on Earth where ice has existed continuously for millions of years, but the Gossan Hill deposit is an excellent terrestrial analogue. On Mars, the subsurface ice may be very old. Ancient reactive Martian mineral assemblages and the fluids associated with them will reflect conditions that existed in the past. [ABSTRACT FROM AUTHOR]