Alteration and Mineralization Products of the Zannone Giant Pockmark (Zannone Hydrothermal Field, Central Tyrrhenian Sea)

Abstract

<jats:p>The Zannone Giant Pockmark (ZGP) is a shallow-water (&lt;−150 m) giant depression located on the shelf off Zannone Island (Pontine Archipelago, central Tyrrhenian Sea, Italy), hosting active hydrothermal vents. The ZGP seabed displays different fluid-venting morphologies (pockmarks, lithified pavements, mounds, and cone-shaped structures) and widespread bacterial communities. In this study, we analyzed ROV (Remote Operated Vehicle) images to gain information on seabed geology and the textural, mineralogical, and geochemical composition of authigenic crusts and gravel-sized clasts sampled close to active emissions. ROV images show authigenic dome-shaped crusts composed of native sulfur associated with barite, gypsum, amorphous silica, and secondary hydrothermal minerals (illite–montmorillonite). The gravel-sized clasts are mostly rhyolites strongly affected by hydrothermal alteration (Alteration Index &gt; 88; depletion of some mobile elements and enrichment of some base metals), causing feldspar-destruction, silicification, formation of hydrothermal phyllosilicates, and precipitation of disseminated pyrite. More intense alteration implying the complete obliteration of the primary mineralogy or fabric is represented by quartz-pyrite samples. ZGP seabed morphology and petro-geochemical features of deposits point to the possible occurrence of a sulfide system linked to the degassing of magma similar to that feeding the Pleistocene products of Ponza Island.</jats:p>

From Fossil to Active Hydrothermal Outflow in the Back‐Arc of the Central Apennines (Zannone Island, Italy)

“…Within the ZGP (Figure 2b), about 2 km to the east of ZI, crusts of native sulfur and altered rocks document sulfur-rich hydrothermal alteration (Conte, Di Bella, et al, 2020) which is similar to what is documented in Ponza and the Pontian Archipelago in general (Figure 2a; Ylagan et al, 1996Ylagan et al, , 2000. Mixing of hydrothermal fluids with seawater leads to the cooling and neutralization of fluids in an active low-sulfidation hydrothermal system (Conte, Di Bella, et al, 2020; Figure 12b). The widespread occurrence of hydrothermal activity shows that post-volcanism magmatic intrusions are the source of heat and magmatic fluids and actually feed the whole 60 km 2 wide Tyrrhenian back-arc area surrounding ZI (Figure 12b).…”

“…Currently, sulfur springs and hydrothermal vents occur along NE-SW-and NW-SE-striking normal faults and fractures, where fluid-escape morphologies such as cones, mounds, and pockmarks are reported (Italiano et al, 2019;Martorelli et al, 2016;Figures 2a, 2b and 12b). Within the ZGP (Figure 2b), about 2 km to the east of ZI, crusts of native sulfur and altered rocks document sulfur-rich hydrothermal alteration (Conte, Di Bella, et al, 2020) which is similar to what is documented in Ponza and the Pontian Archipelago in general (Figure 2a; Ylagan et al, 1996Ylagan et al, , 2000. Mixing of hydrothermal fluids with seawater leads to the cooling and neutralization of fluids in an active low-sulfidation hydrothermal system (Conte, Di Bella, et al, 2020; Figure 12b).…”

“…The control of faulting on the generation of hydrothermal mineralizations is clearly indicated by their development along steep vertical faults that interrupted the continuity of impermeable (or low‐permeability) horizons, allowing the mixing between magmatic and meteoric fluids (and not with downward percolating seawater, as in Kuroko‐type processes; Ohmoto, 1996) and the deposition of high‐temperature (monazite, rutile, and chalcopyrite) to low temperature (barite and neo‐formed smectite and pyrite) ore deposits. The active mineralizations in the ZGP, where sulfide deposits derive from mixing of hydrothermal fluids with seawater (Conte, Di Bella, et al., 2020) can be instead considered as an analog of more mature (but still immature), in the definition of Fouquet et al. (1993) submarine back‐arc conditions, produced by more extensive crustal stretching.…”

Hydrothermal Vent Complexes Control Seepage and Hydrocarbon Release on the Overriding Plate of the Tyrrhenian-Ionian Subduction System (Paola Basin)

“…For example, the morphology of seabed and nature of the gas is similar to active seabed doming and gas discharge in the Gulf of Naples (Figure 1A), where He and CO 2 are sourced from mantle melts and decarbonation reactions of crustal rocks (Passaro et al, 2016). Other large fluid-escape depressions and seafloor mounds have been observed on the continental shelf of the Pontine Archipelago (Figure 1A), where hydrothermal sulfides collected on the seabed and sub-seabed point to the possible degassing of magma similar to the one feeding the latest volcanic activity occurred on the islands in the Middle Pleistocene (Conte et al, 2020). Hydrothermal fluids discharged at this location are CO 2 -rich and show a mantle-derived signature indicating that cooling magmas are still releasing enough thermal energy to feed an active hydrothermal system (Italiano et al, 2019).…”

Are CO2‐rich seafloor pockmarks a suitable environment for ostracod assemblages? The example of the Zannone Giant Pockmark (central‐eastern Tyrrhenian)

“…Within this area, a crater‐like depression with a surface of 0.5 km 2 , the Zannone Giant Pockmark (ZGP) was identified (Ingrassia, Martorelli, et al, 2015). It is an active fluid emission site, formed by at least five smaller craters developed in water depth ranging from 110 to 130 m. Hydrothermal activity, linked to a deep residual magma body, leads to the release of high temperature (60°C, temperature value recorded in the northern sector of the ZGP) venting fluids enriched in CO 2 (concentration >90% from vents inside ZGP), with minor amounts of CH 4 and H 2 S (Italiano et al, 2019; Martorelli et al, 2016), and to formation of native sulphur crusts and secondary hydrothermal minerals (Conte et al, 2020). ROV videos of the pockmarks’ activities (Ingrassia, Di Bella, et al, 2015; Ingrassia, Martorelli, et al, 2015) highlighted different discharge rates: continuous or intermittent bubble streams from single vent points; dispersed emission over lithified pavements; violent expulsions generating pockmarks and craters.…”