A model for the origin of Al-rich efflorescences near fumaroles, Melos, Greece: enhanced weathering in a geothermal setting

Efflorescences in the geothermal field of SE Melos, Greece, contain significant amounts of hydrated Al sulphate, alunogen, which could represent the Melian alumen exploited in Roman times and commended by Pliny. The efflorescences at subaerial fumaroles are explained as follows: Sulphur crystallizes on oxidation of H2S emanating from depth. Weathering produces sulphuric acid enhancing groundwater alteration of volcanic rocks. The high geothermal gradient and arid climate stimulate efflorescences. Salts are recycled during wet and dry weather leading to Al-enrichment on loss of Fe(II,III) and other cations. δ34S‰ V-CDT values for sulphur in fumarole sublimates, solfatara soils and ‘veins’ range from —0.3 to 6.4‰, mean 3.8‰ (n = 8) while Al, Ca and Mg-sulphates in diverse settings range from —4.1 to 6.8‰ (n = 16). The values for sulphur indicate that the initial H2S had an igneous source and the signature is largely inherited by the sulphates.This study aims to underpin research into the exploitation of industrial minerals in the Roman period. When searching for early alumen workings, areas with evidence of acid sulphate alteration (white rocks) and sulphurous fumarole activity should be investigated.

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The Aghia Kyriaki, Melos survey: prospecting for the elusive earths in the Roman period in the Aegean

The Annual of the British School at Athens ◽

10.1017/s0068245400000654 ◽

1999 ◽

Vol 94 ◽

pp. 377-413 ◽

Cited By ~ 7

Author(s):

E. Photos-Jones ◽

A. J. Hall ◽

J. A. Atkinson ◽

G. Tompsett ◽

A. Cottier ◽

...

Keyword(s):

Energy Source ◽

Medieval Literature ◽

Geothermal Field ◽

Roman Period ◽

Archaeological Record ◽

Industrial Minerals ◽

Geological Surveys ◽

Scientific Analysis ◽

Material Evidence

The industrial minerals (IM) of antiquity, the ‘earths’ of the Classical sources, have for long eluded the archaeological record because of the limited material evidence they leave behind in the course of their extraction and processing. The extensive references in Classical and medieval literature and travellers' accounts have never previously been followed up by detailed surveys and by scientific analysis. In the course of April–May 1998 archaeological, topographic and geological surveys were carried out at the Roman site of Aghia Kyriaki, on the island of Melos, with the long term aim of putting the elusive ‘earths’ on the archaeological map of ancient industries. The present article outlines the work so far and highlights the complex factors that may contribute to the identification and characterisation of industrial minerals working in the absence of pyrotechnological waste. Furthermore, since IM processing requires a sustainable low temperature energy source, the hypothesis is put forward that Aghia Kyriaki may have been built on agriculturally inhospitable terrain mainly in order to take advantage of the area's still active geothermal field. If proven, this may constitute evidence for the first use of a ‘soft’ and environmentally friendly energy source during the Roman period in the Aegean.

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ON SHALLOW‐HOLE TEMPERATURE MEASUREMENTS—A TEST STUDY IN THE SALTON SEA GEOTHERMAL FIELD

Geophysics ◽

10.1190/1.1440728 ◽

1977 ◽

Vol 42 (3) ◽

pp. 572-583 ◽

Cited By ~ 10

Author(s):

Tien‐Chang Lee

Keyword(s):

Numerical Methods ◽

Ground Surface ◽

Nonlinear Least Squares ◽

Geothermal Gradient ◽

Geothermal Field ◽

The Other ◽

Temperature Measurements ◽

Salton Sea ◽

Depth Interval ◽

Average Value

Shallow‐hole (<13 m) temperature measurements made at various depths and/or times may yield reliable values of geothermal gradient and thermal diffusivity if the groundwater table is shallow (a few meters) such that the effect of time‐dependent moisture content and physical properties is negligible. Two numerical methods based on nonlinear least‐squares curve fitting are derived to remove the effect of annual temperature wave at the ground surface. One method can provide information on the gradient and diffusivity as a function of depth while the other gives average value over the depth interval measured. Experiments were carried in six test holes cased with 2 cm OD PVC pipes in the Salton Sea geothermal field. A set of 5 to 7 thermistors was permanently buried inside the individual pipes with dry sand. Consistent gradient determinations have been obtained with both numerical methods from six monthly observations. By linearly extrapolating the depths to the 100°C and 200°C isotherms from the calculated gradients and mean ground temperatures, we have found good agreement with the nearby deep‐well data for four holes. Discrepancy is found for two holes, one of which is located near the field of [Formula: see text] mud volcanoes and the other near the volcanic Red Hill, reflecting complicated local hydrologic conditions.

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Audiomagnetotelluric survey at the Bañitos-Gollete geothermal area, main Andes Cordillera of San Juan, Argentina.

Geologica Acta ◽

10.1344/geologicaacta2019.17.8 ◽

2019 ◽

Author(s):

H. BARCELONA ◽

G. PERI ◽

D. WINOCUR ◽

A. FAVETTO

Keyword(s):

Geothermal Gradient ◽

Geothermal Field ◽

Flow Path ◽

Fault System ◽

Conductive Heat ◽

Exploration Process ◽

Tectonic Environment ◽

Geothermal Potential ◽

Geologic Setting ◽

Andes Cordillera

The present research explores the Bañitos-Gollete geothermal field located in the Frontal Andes Cordillera over the Pampean flat-slab. We carried out an audiomagnetotelluric survey in order to define the underground geoelectrical structure and to understand the link between the geothermal fluid flow path and the main geological structures. 2-D audiomagnetotelluric models suggest that the deep-rooted N-S fault system controls the geothermal flow path. We propose a conductive heat-driven system, taking into consideration the geologic setting and the supposed low geothermal gradient of this tectonic environment. The mature Na-Cl waters from Gollete and an estimated reservoir temperature of ~140ºC are consistent with this conceptual model. Further investigations are required to assess the geothermal potential of the study area, and the present work likely represents only the first but necessary step in the exploration process.

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Origin of grain-coating chlorite by smectite transformation: an example from Miocene sandstones, North Sumatra back-arc basin, Indonesia

Clay Minerals ◽

10.1180/claymin.1994.029.4.21 ◽

1994 ◽

Vol 29 (4) ◽

pp. 681-692 ◽

Cited By ~ 18

Author(s):

B. Humphreys ◽

S. J. Kemp ◽

G. K. Lott ◽

Bermanto ◽

D.A. Dharmayanti ◽

...

Keyword(s):

Geothermal Gradient ◽

Upper Miocene ◽

The North ◽

Direct Precipitation ◽

Back Arc ◽

North Sumatra ◽

Textural Evidence ◽

High Geothermal Gradient

AbstractGrain-coating chlorite cements commonly occur within sandstones of late Middle and Upper Miocene age deposited in the North Sumatra back-arc basin. Chlorites from the Lower Keutapang Member contain Ca (maximum 0.75 wt% oxide) and show textural evidence for direct precipitation on grains. However, crystals are subhedral, showing curved faces and often ragged edges, and show a tendency to merge together. In overlying beds of the Upper Keutapang Member, grain-coating chlorite-smectite (20% smectite) cements display an identical morphology but are more siliceous, have a lower octahedral occupancy and contain higher total (Na + Ca + K). It is proposed that chlorite cements in the Keutapang Formation originated as smectite-rich cement rims whose initial precipitation was related to the breakdown of volcanic detritus in the sediments. Transformation to chlorite occurred subsequently during burial, facilitated by a high geothermal gradient in the back-arc basin.

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A rock magnetic fingerprint of hydrothermal alteration in volcanic rocks - An example from the Los Azufres Geothermal Field, Mexico

Journal of South American Earth Sciences ◽

10.1016/j.jsames.2019.02.018 ◽

2019 ◽

Vol 91 ◽

pp. 260-271

Author(s):

Kailasa Pandarinath ◽

Rajasekhariah Shankar ◽

E. Santoyo ◽

Shwetha B. Shetty ◽

America Yosiris García-Soto ◽

...

Keyword(s):

Hydrothermal Alteration ◽

Volcanic Rocks ◽

Geothermal Field

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Behavior of subducting sediments beneath an arc under a high geothermal gradient: Constraints from the Miocene SW Japan arc

GEOCHEMICAL JOURNAL ◽

10.2343/geochemj.37.503 ◽

2003 ◽

Vol 37 (4) ◽

pp. 503-518 ◽

Cited By ~ 18

Author(s):

Gen Shimoda ◽

Yoshiyuki Tatsumi ◽

Yuichi Morishita

Keyword(s):

Geothermal Gradient ◽

Gradient Constraints ◽

High Geothermal Gradient ◽

Sw Japan

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Acid-sulphate hydrothermal alteration of andesitic tuffs and genesis of halloysite and alunite deposits in the Biga Peninsula, Turkey

Clay Minerals ◽

10.1180/claymin.2008.043.2.10 ◽

2008 ◽

Vol 43 (2) ◽

pp. 281-315 ◽

Cited By ~ 34

Author(s):

Ö. I. Ece ◽

P. A. Schroeder ◽

M. J. Smilley ◽

J. M. Wampler

Keyword(s):

Hydrothermal Alteration ◽

Volcanic Rocks ◽

Geothermal Field ◽

Parent Material ◽

Thermal Waters ◽

Geothermal Waters ◽

Biga Peninsula ◽

The North ◽

Clay Deposits ◽

Nw Turkey

AbstractThe Biga Peninsula of NW Turkey is host to six major halloysite deposits in the Go¨nen, Yenice and Balya districts. Mineralization took place in areas of Permian limestone blocks where the Triassic Karakaya Complex is in contact with early Miocene calc-alkaline volcanic rocks. Hypogene halloysite mineralization was controlled by the intersection of minor faults in the vicinity of clay deposits. During the Pleistocene, activity of the North Anatolian Fault (NAF) brought ascending geothermal solutions through the fault zones to the surface, which led to hydrothermal alteration and halloysite formation. N-MORB normalized element values for each halloysite deposit and the volcanic rocks suggest genetic links. Alunite and halloysite were formed in the Turplu area where upwelling hydrothermal waters contained major H2S and SO2acids. Only halloysite mineralization occurred in outflow areas of the same fossil geothermal field.Pyrite and alunite samples from the Turplu deposits have δ34S values of 0.6–1.8% and 4.8–7.9%, respectively, with values for gypsum of 3.1–3.5%. The δ34S values of pyrite suggest that local meteoric waters had partially mixed with the dominant fluid during the closure stage of fossil hydrothermal activities. The range of δD values of halloysite samples from Turplu is –58.4 to –68.6%. The δ18O values for halloysite are in the range 16.7–18.1%. All halloysite deposits in the study areas are either overlying or adjacent to limestone blocks, and these provide excellent drainage for the discharging geothermal waters. Subsurface drainage systems in the karstic environment and the SO2-bearing thermal waters indicate the importance of acidic waters and the continuous leaching of elements in forming relatively pure hydrated halloysite. A steam-heated dissolution-precipitation model is proposed for the occurrence of all halloysite and alunite deposits. Sulphur gases (H2S-SO2) of hypogene origin rose from deep in the fault zone to the surface where they encountered oxygenated groundwater at the water table. The occurrence of H2SO4in this hydrothermal system enhanced the acidity of geothermal waters provoking advanced argillic alteration. Hypogene alunite deposits also have large P2O5contents, suggesting a parent material with a magmatic origin deeper than the alkaline tuffs. Halloysite is a fast-forming metastable precursor to kaolinite.

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Kaoline Mapping Using ASTER Satellite Imagery: The Case Study of Kefalos Peninsula, Kos Island

Materials Proceedings ◽

10.3390/materproc2021005076 ◽

2021 ◽

Vol 5 (1) ◽

pp. 76

Author(s):

Maria Kokkaliari ◽

Christos Kanellopoulos ◽

Ioannis Illiopoulos

Keyword(s):

Hydrothermal Alteration ◽

Satellite Imagery ◽

Volcanic Rocks ◽

Matched Filtering ◽

Industrial Minerals ◽

Geologic Structure ◽

Volcanic Arc ◽

The Mediterranean ◽

Eastern Edge

The present work aims to map kaolin occurrences on the Kefalos peninsula, SW Kos Island, Greece, through the elaboration of ASTER satellite imagery. The island of Kos is located on the eastern edge of the South Aegean Active Volcanic Arc (SAAVA) and is characterised by its complex geologic structure. During Plio-Pleistocene, the voluminous eruption of the Kos Plateau Tuff was recorded on Kefalos; the largest quaternary eruption in the Mediterranean. Kaolin is the product of hydrothermal alteration of the Pliocene volcanic rocks with rhyolitic composition. Our study emphasises the usefulness of satellite imagery combined with the Mixture Tuned Matched Filtering (MTMF) technique to detect occurrences of industrial minerals, kaolin-group minerals in this case, either in terms of raw mineral exploitation or by mapping hydrothermal alteration.

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