Reply to Norini and Groppelli's comment on “Estimating the depth and evolution of intrusions at resurgent calderas: Los Humeros (Mexico)” by Urbani et al. (2020)

Structural studies in active caldera systems are widely used in geothermal exploration to reconstruct volcanological conceptual models. Active calderas are difficult settings to perform such studies mostly because of the highly dynamic environment, dominated by fast accumulation of primary and secondary volcanic deposits, the variable and transient rheology of the shallow volcanic pile, and the continuous feedbacks between faulting, secondary porosity creation, and geothermal fluid circulation, alteration and cementation that tend to obliterate the tectonic deformation structures. In addition, deformation structures can be also caused by near- and far-field stress regimes, which include magmatic intrusions at various depths, the evolving topography and regional tectonics. A lack of consideration of all these factors may severely underpin the reliability of structural studies. By rebutting and providing a detailed discussion of all the points raised by the comment of Norini and Groppelli (2020) to the Urbani et al. (2020) paper, we take the opportunity to specify the scientific rationale of our structural fieldwork and strengthen its relevance for geothermal exploration and exploitation in active caldera geothermal systems in general and, particularly, for the Holocene history of deformation and geothermal circulation in the Los Humeros caldera. At the same time, we identify several major flaws in the approach and results presented in Norini and Groppelli (2020), such as (1) the lack of an appropriate ranking of the deformation structures considering an inventory method for structural analysis; (2) the misinterpretation and misquoting of Urbani et al. (2020) and other relevant scientific literature; and (3) irrelevant and contradictory statements within their comment.

Forming an economic bentonite resource in a volcanic arc environment: Milos island, Greece

<p>Volcanoes in island arcs can undergo edifice evolution that includes submarine and subaerial volcanism, providing a dynamic environment of magmatic heat and volatiles that drives hydrothermal fluid flow with potential inputs from sea and/or meteoric water. This, in turn, can generate significant hydrothermal alteration that can result in economic deposits of industrial minerals. One example includes bentonite, a smectitic rock composed dominantly of montmorillonite.</p><p>Economically viable bentonite deposits are typically only 0.5 – 5 meters thick and<strong> </strong>although Wyoming-type bentonites comprise 70% of the world’s known deposits, they are commonly no thicker than 8 m. The island of Milos is Europe’s largest and actively mined calcium bentonite resource from volcanic piles exceeding 80 m thickness. Here, we use the Milos island example to understand how magmatism, volcanic edifice evolution and hydrothermal activity interact. We integrate field relationships of volcanic stratigraphy and alteration zones, with clay mineralogy (XRD), stable (S, O and H) isotope analysis and high precision geochronology (CA-ID-TIMS zircon U-Pb, and alunite Ar-Ar) to elucidate the timescales, thermal drivers and fluid components that lead to the development of a globally important bentonite resource.</p><p>A vertical transect through bentonite-altered volcanic stratigraphy indicates multiple magmatic pulses ca. 2.8 Ma with a submarine andesitic cryptodome and accompanying pepperitic hyaloclastite. Cumulative volcanic and sub-volcanic processes occurred over ca. 170 kyrs, resulting in a vertically and laterally extensive volcanic pile overlain by an episode of magmatic quiescence and brackish-water diatomaceous sediments. It is overlain by a silicic pyroclastic flow host to pervasive silica-alunite-kaolinite alteration. Stable isotopic analyses of bentonite indicate a hydrothermal origin at around 70°C with the fluid being sourced from sea and meteoric waters. The timing of formation is defined by a maximum duration of ca. 170 kyrs, with clear geological evidence that a significant period of alteration occurred within < 20 kyrs at ~ 2.64 Ma. Alunite sulfur isotope compositions reflect steaming ground activity that could be interpreted as the oxidised, shallower level counterpart to a boiling geothermal system linked to development of extensive bentonite. However, the timing of alunite can be clearly resolved to > 1.5 myrs after bentonite formation to ~ 1.0 Ma, supporting a later overprint origin due to relatively recent steam heating of groundwater after emergence of the submarine system.</p><p>This study identifies key parameters that have resulted in the formation of an economic-scale bentonite resource on the emergent island of Milos. We conclude that the hydrology needed to form a bentonite deposit is not constrained to the marine environment and can be connected to emergent parts of the volcanic edifice. High precision geochronology indicates bentonite development happens on volcanic timescales (10 to 100 kyrs). A cumulative volcanic and sub-volcanic pile coeval with the formation of bentonite suggests multiple magmatic episodes over narrow timeframes provide and sustain the thermal driver for significant bentonite development. After emergence and development of a groundwater system, the subsequent steam heating is deleterious to grade and results in the development of alunite-kaolinite overburden.</p>

Structural studies in active caldera geothermal systems. Reply to Comment on Estimating the depth and evolution of intrusions at resurgent calderas: Los Humeros (Mexico) by Norini and Groppelli (2020)

Structural studies in active caldera systems are widely used in geothermal exploration to reconstruct volcanological conceptual models. Active calderas are difficult settings to perform such studies mostly because of the highly dynamic environment, dominated by fast accumulation of primary and secondary volcanic deposits, the variable and transient rheology of the shallow volcanic pile, and the continuous feedback between faulting and geothermal fluid circulation/alteration that tend to obliterate the tectonic deformation structures. In addition, deformation structures can be also caused by near- and far-field stress regimes, which include magmatic intrusions at various depths (volumes and rates), the evolving topography and regional tectonics. A lack of consideration of all these factors may severely underpin the reliability of structural studies. By rebutting and providing a detailed discussion of all the points raised by the comment of Norini and Groppelli (2020) to the Urbani et al. (2020) paper, we take the opportunity to specify the scientific rationale of our structural fieldwork and strengthen its relevance for geothermal exploration/exploitation in active caldera geothermal systems in general, and, particularly, for the Holocene history of deformation and geothermal circulation in the Los Humeros caldera. At the same time, we identify several major flaws in the approach and results presented in Norini and Groppelli (2020).

Geochemistry of Karoo basalts and dolerites in the northeastern Orange Free State: Recognition and origin of new Karoo basalt magma types. NE Free State.xls

One of the most significant results emerging from the Karoo Volcanics Project of the NGP ls the recognition of a number of geochemically distinct basalt magma types occurring within the lower part of the Karoo volcanic pile in the Northeastern Cape and Southern Lesotho.<div>NE Free State<br></div>

Geochemistry of Karoo basalts and dolerites in the northeastern Orange Free State: Recognition and origin of new Karoo basalt magma types. NE Free State.xls

One of the most significant results emerging from the Karoo Volcanics Project of the NGP ls the recognition of a number of geochemically distinct basalt magma types occurring within the lower part of the Karoo volcanic pile in the Northeastern Cape and Southern Lesotho.<div>NE Free State<br></div>

Forming an economic industrial mineral resource in a volcanic arc environment: timescales, fluids and thermal drivers of Europe’s largest bentonite resource

&lt;p&gt;Volcanoes in island arcs can undergo edifice evolution that includes submarine and subaerial volcanism. This provides a dynamic environment of magmatic heat and volatiles that drives hydrothermal fluid flow with potential inputs from sea and/or meteoric waters. This, in turn, can generate significant hydrothermal alteration that can result in economic deposits of industrial minerals such as bentonite and kaolinite. The island of Milos is Europe&amp;#8217;s largest and actively mined calcium bentonite resource, with production capacities exceeding 400,000 tons per year. Here, we use the Milos island example to understand how magmatism, volcanic edifice evolution and hydrothermal activity interact to generate important bentonite mineralisation. We integrate field relationships of volcanic stratigraphy and alteration zones, with clay mineralogy (XRD), stable (S, O and H) isotope analysis and high precision geochronology (CA-ID-TIMS zircon U-Pb, and alunite Ar-Ar) to elucidate the timescales, thermal drivers and fluid components that lead to the development of a globally important bentonite resource.&lt;/p&gt;&lt;p&gt;A vertical transect through bentonite-altered volcanic stratigraphy indicates multiple magmatic pulses initiated at ca. 2.8 Ma with a submarine andesitic cryptodome and accompanying hyaloclastite carapace that display quenched and peperitic contacts. Cumulative volcanic and sub-volcanic processes occurred over ca. 170 kyrs, resulting in a volcanic pile exceeding 80 m. This period included an episode of magmatic quiescence and diatomite formation in a shallow submarine environment and is overlain by a silicic pyroclastic flow. In this upper unit, a pervasive alunite-kaolinite alteration assemblage was developed. Stable isotopic analyses of bentonite (&gt; 85% montmorillonite) indicate a hydrothermal origin at around 125&amp;#176;C with the fluid being sourced from sea and meteoric waters. The timing of formation is defined by a maximum duration of ca. 170 kyrs, with clear geological evidence that a significant period of alteration occurred within &lt;20 kyrs at ca. 2.64 Ma. Sulfur isotope analysis on alunite indicates a steaming ground origin that could be interpreted as the oxidised, shallower level counterpart to a boiling geothermal system linked to development of extensive bentonite. However, the timing of alunite can be clearly resolved to &gt; 1 Ma after bentonite formation to 1.2 Ma, supporting a later overprint origin due to relatively recent steam heating of groundwater after emergence.&lt;/p&gt;&lt;p&gt;This study identifies new key parameters that have resulted in the formation of an economic-scale bentonite resource on the emergent island of Milos. In addition to the requisite appropriate protolith, we conclude that in an emergent volcanic arc setting the hydrology needed to form a bentonite deposit is not constrained to the marine environment and can be connected to emergent parts of the volcanic edifice. High precision geochronology indicates bentonite development happens on volcanic timescales (10 to 100 kyrs). A cumulative volcanic and sub-volcanic pile coeval with the formation of bentonite suggests multiple magmatic episodes over narrow timeframes provide and sustain the thermal driver for significant bentonite development. Once the volcanic edifice has completely emerged and developed a groundwater system, the steam heating of groundwater is deleterious to grade and results in the development of alunite-kaolinite overburden.&lt;/p&gt;

U–Pb (zircon) ages and provenance of the White Rock Formation of the Rockville Notch Group, Meguma terrane, Nova Scotia, Canada: evidence for the “Sardian gap” and West African origin

The White Rock Formation is the lowermost formation of the Rockville Notch Group, an assemblage of Silurian–Devonian rocks preserved in five areas along the northwestern margin of the Meguma terrane of Nova Scotia. The formation consists mainly of mafic and felsic metavolcanic rocks, interlayered with and overlain by marine metasedimentary rocks. Felsic metatuff has now been dated from four locations near both the bottom and top of the volcanic pile and yielded a narrow age range (with errors) of about 446–434 Ma. These dates confirm a 30 Ma hiatus after deposition of the Early Ordovician Hellgate Formation in the underlying Halifax Group. This hiatus is coeval with the “Sardian gap” in the Lower Palaeozoic of peri-Gondwanan Europe. The metavolcanic–metasedimentary assemblage is overlain by mainly metasiltstone with abundant quartzite and metaconglomerate lenses; some of the latter were previously interpreted to be Ordovician tillite, an interpretation no longer viable. New detrital zircon data from metasedimentary samples indicate that the major sediment sources for the White Rock Formation have ages of ca. 670–550 and ca. 2050 Ma, similar to ages from the underlying Goldenville and Halifax groups. A smaller population of Mesoproterozoic zircon grains indicates that the Meguma terrane interacted with a terrane composed mainly of Mesoproterozoic crust during the Silurian and Devonian. The occurrence of the “Sardian gap” and the detrital zircon record constrain the palaeoposition of the Meguma terrane to have been close to Cadomia and West Africa in the Early Cambrian to Early Silurian.

Seismological constraints for the dyke emplacement of the July-August 2001 lateral eruption at Mt. Etna volcano, Italy

In this paper we report seismological evidence regarding the emplacement of the dike that fed the July 18 - August 9, 2001 lateral eruption at Mt. Etna volcano. The shallow intrusion and the opening of the eruptive fracture system, which mostly occurred during July 12, and July 18, were accompanied by one of the most intense seismic swarms of the last 20 years. A total of 2694 earthquakes (1 £ Md £ 3.9) were recorded from the beginning of the swarm (July 12) to the end of the eruption (August 9). Seismicity shows the upward migration of the dike from the basement to the relatively thin volcanic pile. A clear hypocentral migration was observed, well constraining the upwards propagation of a near-vertical dike, oriented roughly N-S, and located a few kilometers south of the summit region. Earthquake distribution and orientation of the P-axes from focal mechanisms indicate that the swarm was caused by the local stress source related to the dike intrusion.

Faciologia da Associação Vulcano-Plutônica Taquarembó, Cambriano do Escudo Sul-rio-grandense, RS - Brasil

The Taquarembo Plateau, situated in the southwestern part of Sul-rio-grandense shield, is part of Camaquã Basin, that was formed at the end of the post-collisional stage in Brasiliano/Pan-Africa event, Neoproterozoic in age. This plateau is composed of volcanic and shallow intrusions, related to a silica-saturated alkaline series, ranging from alkaline metaluminous basalt to comendiitic flows, known as Taquarembo Volcano-Plutonic Association. The faciological identification leads to the reconstruction of the volcanic pile, formed from the base to the top by: particulated flows (ignimbrites and reo-ignimbrites); ressedimented syneruptive deposits; surge and fall horizons, closely associated with alkaline metaluminous lava flows (basalts, mugearites, rhyolites); and peralkaline lava flows (comendiites), and shallow intrusions of quarz monzonite and syenite that crosscut the volcanic pile. The facies identification suggests a highly explosive volcanic regime, under sub-aerial condition, in a close association with the feeder necks.