28 research outputs found
Re-assessing the resource potential of seafloor massive sulfide deposits
No full textJamieson, J.W., Hannington, M.D., Petersen, S., and Monecke, T
Analysis of Sulfides for Gold and Associated Trace Metals by Direct Neutron Activation With a Low-Flux Reactor
No full textPotential mineral resources in historically dismissed volcanogenic massive sulfide (VMS) deposits of the Emilia Romagna region (Italy): petrological and geochemical study for Critical Raw Materials (CRMs) exploration and exploitation
No full textThe last update on the Critical Raw Materials Act (2023) drawn up by the European Union identified
54 Critical Raw Materials (CRMs), i.e., minerals, elements, or materials that are fundamental to supply for
technology and strategic for the green transition, but subjected to fickle supply, e.g., for the fragile geopolitical
contest (Kiss et al., 2023). This led many European countries, including Italy, to focus on the metal recovery
from dismissed mines, mine wastes, and landfills to accomplish the circular economy politics. Italy has
more than 100 historically dismissed mining sites just in the North, and, among them, the ones in the Emilia
Romagna region are now under investigation for the various volcanogenic massive sulfide (VMS) deposits
(Zaccarini & Garuti, 2008). These are a type of metal sulfide ore deposits that occur as a result of underwater
volcanic eruptions, associated with hydrothermal events in submarine environments, and are divided based
on ore composition (Cu, Cu-Zn, Cu-Zn-Pb group) and environment formation (Cyprus, Kuroko, Besshi, as
mentioned by Zaccarini & Garuti, 2008). In the Emilia Romagna region, these deposits occur as pods within
small bodies of ophiolitic basalts cropping out as olistoliths in the Northern Apennine External Ligurian units
and owe their origin to the metal-rich hydrothermal circulation which developed quartz-sulfide veins when
mixed with seawater through a fissures network (Saccani, 2015; Kiss et al., 2023). These ophiolites represent
Jurassic Alpine Tethys oceanic crust fragments obducted in the continental crust (Zaccarini & Garuti, 2008).
The stratigraphy of the area is characterized by sequences of pillow lavas associated with serpentine and gabbro
breccias, radiolarian cherts, limestones, and abundant serpentinized subcontinental mantle peridotites (Kiss et
al., 2023). Basalts, then, show Ocean Continent Transition Zone (OCTZ) chemical features with transitionalMORB affinity and a garnet signature (Dyn/Yb0: 1.2-1.4, Saccani, 2015), in agreement with Cyprus-type
VMS deposits (Zaccarini & Garuti, 2008). Major and trace elements bulk rock geochemical analyses were
performed in a group of basalts of the Boccassuolo ophiolite and compared with the previous results (e.g.,
Barrie & Hannington, 1999; Zaccarini & Garuti, 2008; Kiss et al., 2023): the VMS deposits in the Emilia
Romagna region belong to the Cu and Cu-Zn types (Cu up to 5818 ppm, 200 times Upper Continental Crust,
UCC, composition; Zn up to 7941 ppm, 118*UCC), low to very low Pb contents (< 1 ppm, max. 0.42*UCC).
These preliminary results provide the first relevant geochemical information to map trace metal enrichment
distribution in the main rocks of the area. Radiogenic (Sr-Nd-Pb) and stable (S-C) isotopic analyses, as well as
mineralogical and in-situ analyses, will provide additional information on the enrichment and distribution of
VMS deposits in the Region.
Barrie C.T. & Hannington M.D. (1999) - Classification of Volcanic-Associated Massive Sulfide Deposits Based on
Host-Rock Composition. In: Barrie C.T. & Hannington M.D. (Eds), Volcanic-Associated Massive Sulfide Deposits:
Processes and Examples in Modern and Ancient Settings, Rev. Econ. Geol., 8, 2-10.
Kiss G.B. et al. (2023) - Tracing the Source of Hydrothermal Fluid in Ophiolite-Related Volcanogenic Massive Sulfide
Deposits: A Case Study from the Italian Northern Apennines. Minerals, 13, 8.
Saccani E. (2015) - A new method of discriminating different types of post-Archean ophiolitic basalts and their
tectonic significance using Th-Nb and Ce-Dy-Yb systematics. Geosci. Front., 6, 481-501, https://doi.org/10.1016/j.
gsf.2014.03.006.
Zaccarini F. & Garuti G. (2008) - Mineralogy and chemical composition of VMS deposits. Mineral. Petrol., 94, 61-83,
https://doi.org/10.1007/s00710-008-0010-9
Submarine magmatic-hydrothermal systems at the Monowai volcanic centre, Kermadec Arc
No full textThe Monowai volcanic center is located at the midpoint along the ?2,530-km-long Tonga-Kermadec arc system. The Monowai volcanic center is comprised of a large elongate caldera (Monowai caldera area ?35 km2; depth to caldera floor 1,590 m), which has formed within an older caldera some 84 km2 in area. To the south of this nested caldera system is a large composite volcano, Monowai cone, which rises to within ?100 m of the sea surface and which has been volcanically active for the past several decades. Mafic volcanic rocks dominate the Monowai volcanic center; basalts are the most common rock type recovered from the cone, whereas basaltic andesites are common within the caldera. Hydrothermal plume mapping has shown at least three major hydrothermal systems associated with the caldera and cone: (1) the summit of the cone, (2) low-temperature venting (<60°C; Mussel Ridge) on the southwestern wall of the caldera, and (3) a deeper caldera source with higher temperature venting that has yet to be observed. The cone summit plume shows large anomalies in pH (a shift of ?2.00 pH units) and ?3He (?358%), and noticeable H2S (up to 32 ?m), and CH4 (up to 900 nm). The summit plume is also metal rich, with elevated total dissolvable Fe (TDFe up to 4,200 nm), TDMn (up to 412 nm), and TDFe/TDMn (up to 20.4). Particulate samples have elevated Fe, Si, Al, and Ti consistent with addition to the hydrothermal fluid from acidic water-rock reaction. Plumes extending from ?1,000- to 1,400-m depth provide evidence for a major hydrothermal vent system in the caldera. The caldera plume has lower values for TDFe and TDMn, although some samples show higher TDMn concentrations than the cone summit plume; caldera plume samples are also relatively gas poor (i.e., no H2S detected, pH shift of ?0.06 pH units, CH4 concentrations up to 26 nm). The composition of the hydrothermal plumes in the caldera have higher metal contents than the sampled vent fluids along Mussel Ridge, requiring that the source of the caldera plumes is at greater depth and likely of higher temperature. Minor plumes detected as light scattering anomalies but with no 3He anomalies down the northern flank of the Monowai caldera most likely represent remobilization of volcanic debris from the volcano flanks.We believe the Monowai volcanic center is host to a robust magmatic-hydrothermal system, with significant differences in the style and composition of venting at the cone and caldera sites. At the cone, the large shifts in pH, very high ?3He% values, elevated TDFe and TDFe/TDMn, and the H2S- and CH4-rich nature of the plume fluids, together with elevated Ti, P, V, S, and Al in hydrothermal particulates, indicates significant magmatic volatile ± metal contributions in the hydrothermal system coupled with aggressive acidic water-rock interaction. By contrast, the caldera has low TDFe/TDMn in hydrothermal plumes; however, elevated Al and Ti contents in caldera particulate samples, combined with the presence of alunite, pyrophyllite, sulfide minerals, and native sulfur in samples from Mussel Ridge suggest past, and perhaps recent, acid volatile-rich venting and active Fe sulfide formation in the subsurface
Fluid inclusion studies as a guide to the temperature regime within the TAG hydrothermal mound, 26°N, Mid-Atlantic Ridge
No full textThe active Trans-Atlantic Geotraverse (TAG) hydrothermal mound is a mature submarine massive sulfide deposit at the slow-spreading Mid-Atlantic Ridge at 26°N. Fluid inclusion measurements were conducted on quartz and anhydrite from six boreholes drilled in different areas of the mound to characterize the fluids responsible for the deposition of sulfide-silica breccias and anhydrite and to investigate the vertical and horizontal temperature zonation within an actively forming hydrothermal system. Fluid inclusions in both host minerals are generally two phase liquid/vapor inclusions that homogenize into the liquid phase. Trapping temperatures for quartz and anhydrite from the TAG mound range from 212° to 390°C. Salinities vary from 1.9 to 6.2 wt% NaCl equivalent for anhydrite and from 4.0 to 6.0 wt% NaCl equivalent for quartz. This salinity variation is probably best explained by supercritical phase separation at temperatures above 450°C with subsequent remixing of the liquid and the vapor phase during ascent. A zone of anhydrite-rich precipitates recovered at 20 to 35 mbsf below the central Black Smoker Complex (TAG-1) is characterized by trapping temperatures averaging 348°C for anhydrite and 358°C for quartz, which is slightly below the exit temperature of hydrothermal fluids presently venting at the Black Smoker Complex (360°-366°C). Breccias in the stockwork zone underlying the anhydrite zone were formed at slightly higher temperatures ranging from 327°-
381°C for quartz and from 349° to 384°C for anhydrite. Trapping temperatures vary strongly between different areas of the mound. Fluid inclusions in quartz and anhydrite from the central Black Smoker Complex are characterized by a narrow range of trapping temperatures, whereas other areas drilled on the mound were influenced by lower temperature hydrothermal fluids percolating through the mound or by local entrainment of seawater into the mound. White smokers venting on the southeastern side of the TAG mound are characterized by exit temperatures of 270°-300°C, (Kremlin area, TAG-2). Fluid inclusion measurements in quartz and anhydrite from this area give trapping temperatures in the range of 266°-375°C with a distinct peak around 340°C, only somewhat lower than results for the Black Smoker Complex. Trapping temperatures in anhydrite-hosted fluid inclusions in this area show a strong vertical temperature increase. The west side of the mound (TAG-4) is characterized by trapping temperatures ranging from 212° to 390°C showing evidence for seawater entrainment or overprinting by lower temperature hydrothermal events at the sulfide/basalt interface. Samples from the northern side of the mound (TAG-5) exhibit trapping temperatures in the range from 258°-383°C with a strong vertical temperature increase, indicating additional hightemperature upflow at the northern margin of the mound outside the central Black Smoker Complex
Geochemistry and sulfur-isotopic composition of the TAG hydrothermal mound, Mid-Atlantic Ridge, 26°N
No full textEighty-five bulk samples consisting of varying proportions of pyrite, silica, and anhydrite and 82 mineral separates (pyrite, chalcopyrite) from the TAG hydrothermal mound were analyzed using Neutron Activation Analyses (INAA), Inductively Coupled Plasma Emission Spectrometry (ICP-ES), Inductively Coupled Plasma Mass Spectrometry (ICP-MS), and sulfur-isotopic methods. The samples were collected from five different areas of the Trans-Atlantic Geotraverse (TAG) mound during Ocean Drilling Program Leg 158. The chemistry of the bulk samples is dominated by high Fe (average 30.6 wt%, n = 57) and S concentrations (average 42.0 wt%, n = 50), reflecting the high amount of pyrite in these rocks. High Ca (up to 11.5 wt%, n = 57) and SiO2 values (up to 49.8 wt%, n = 50) indicate the presence of anhydrite-rich zones in the center of the mound, and pyritesilica breccias, silicified wallrock breccias, and paragonitized basalt breccias deeper in the system. The Cu and Zn concentrations vary from <0.01 to 12.2 wt% Cu (average 2.4 wt%, n = 57) and from <0.01 to 4.1 wt% Zn (average 0.4 wt%, n = 57), with highest values commonly occurring in the uppermost 20 m of the mound. Most trace-element concentrations are relatively low compared to other mid-ocean ridge hydrothermal sites and average 0.5 ppm Au, 43 ppm As, 234 ppm Co, 2 ppm Sb, 14 ppm Se (n = 85), 9 ppm Ag, 11 ppm Cd, and 59 ppm Pb (n = 57). Gold, Ag, Cd, Pb, and Sb behave similarly to Cu and Zn and are enriched close to the surface of the mound. This is interpreted as evidence for zone refining, a process in which elements that are mobilized from previously deposited sulfides in the interior of the mound by later hydrothermal fluids are transported to the surface, where they reprecipitate as a result of mixing with ambient seawater. The trace-element composition of pyrite and chalcopyrite separates is similar to the bulk geochemistry. However, down to about 50 mbsf, Au, As, Sb, and Mo values in pyrite separates are generally higher than in bulk samples and chalcopyrite separates. Below this depth, these elements appear to be enriched in chalcopyrite separates. Cobalt is typically more enriched in pyrite than in chalcopyrite throughout. A major difference between pyrite and chalcopyrite separates is the strong enrichment of Se in chalcopyrite at the top of the mound, whereas pyrite separates show a moderate increase of Se with depth. Sulfur-isotopic values for bulk sulfides from the interior of the TAG mound vary from +4.6‰ to +8.2‰, with an average of +6.4 ‰ d34S (n = 49). These values do not change significantly downhole, but samples collected from the top of the mound appear to have somewhat lower d34S values than samples from the interior. The average d34S value for TAG sulfides is about 3‰ higher than for most other sulfides generated at sediment-free mid-ocean ridges (average 3.2‰, n = 501). This is largely attributed to thermochemical sulfate (anhydrite) reduction by hightemperature hydrothermal fluids upwelling through the interior of the TAG mound
The Current State of Global Activities Related to Deep-sea Mineral Exploration and Mining
No full textDeep-sea mining is seen as a potential way to provide future secure metal supply to global markets. The
current rush to the seafloor in areas beyond national jurisdiction indicates that sound knowledge of the
geological characteritics of the various commodities, a realistic resource assessment, and a social and
political discussion about the cons and pros of their exploitation that is based on facts, not myths, is
required. This contribution provides the most recent information on global deep-sea mineral resources and
sets the stage for detailed talks in this session
Comparison of the TAG mound and stockwork complex with Cyprus-type massive sulfide deposits
No full textDrilling of the active Trans-Atlantic Geotraverse (TAG) deposit indicates that the size of the mound-stockwork complex is approximately 3.9 million t, including 2.7 million t of massive and semi-massive sulfide (~2% Cu) at the seafloor and 1.2 million t of mineralized breccias (~1% Cu) in a subseafloor stockwork. Quartz-pyrite veining in the stockwork zone extends from about 40 meters below seafloor (mbsf) to a depth of 95 mbsf. Siliceous wallrock breccias in the lower part of the stockwork grade abruptly into chloritized basalt breccias at the margins of the mineralized zone, and massive sulfides at the flanks of the deposit onlap relatively unaltered, partially hematized basalts. The pipe-like dimensions of the stockwork zone do not exceed the diameter of the sulfide mound. Comparisons with samples collected during earlier dive series confirm that the vent complexes at the surface of the mound are not representative of the bulk composition of the deposit. Steep vertical metal zonation within the mound suggests that a long history of hydrothermal reworking has effectively stripped the constituents that are soluble at lower temperatures from the massive sulfides and concentrated them at the top of the deposit through a process of zone refining. The bulk of the mound is composed of massive pyrite and anhydrite-cemented breccias. The massive anhydrite (~165,000 t) occupies a high-temperature zone, immediately beneath the central Black Smoker Complex and above the quartz-rich stockwork. Fracturing in the underlying quartz-pyrite stockwork also has resulted in anhydrite veining at considerable depths in the stockwork zone. Despite the abundance of anhydrite in the mound, the amount of seawater penetrating the region of hightemperature upflow is small in comparison to the total mass flux of hydrothermal fluid. The anhydrite has been deposited by conductive heating of a small amount of entrained seawater at the margins of high-temperature conduits, and little or no mixing has occurred with the end-member fluids. Collapse of the anhydrite-supported portion of the mound following major episodes of hydrothermal upflow has caused extensive in situ brecciation of the mound and is an important mechanism for the formation of “breccia ores” in the deposit. Although anhydrite is not well preserved in the geologic record, given its retrograde solubility, it has likely played an important role in the development of similar ore types in ancient massive sulfides. The morphology, size, and bulk composition of the TAG mound-stockwork complex is identical to that of some of the largest Cyprus-type massive sulfide deposits in the Troodos ophiolite. Typical Cyprus-type deposits comprise massive brecciated pyrite ores, underlain by a vertically extensive quartz-pyrite-chlorite stockwork. Sandy pyrite or conglomeratic ore, similar to that found in the TAG mound, is characteristic of the upper parts of Cyprus-type deposits. Textures in these ores, previously attributed to seafloor weathering and erosion, are most likely the result of anhydrite dissolution. Massive, granular pyrite (hard, compact ore), with abundant vuggy cavities lined by idiomorphic pyrite and quartz, occur below the conglomeratic ores and closely resemble sections of massive pyrite and pyrite-silica breccias from the TAG mound. At TAG, seafloor oxidation of the sulfides is currently taking place, even as the deposit is forming. Fe-oxide gossans have developed at the surface of the mound as a result of weathering of chimney debris. These deposits are modern analogs of the
extensive ochers that typically overlie the massive sulfide deposits in Cyprus. By analogy with TAG, a number of the weathering features of Cyprus-type deposits (e.g., red clays, leached lavas), previously thought to be products of acid alteration by meteoric groundwaters, may have formed while the deposits were still on the seafloor. Low-temperature venting through this material has locally produced distinctive red cherts (silicified Fe oxides). This material is common within the mound and in the underlying basalts and closely resembles the red jaspers found throughout the pillow lava sections in Cyprus. Silicification in the upper part of the TAG mound also has produced a cherty, sulfide carapace at the top of the deposit that inhibits further degradation of the mound by seafloor weathering. This may have important implications for the long-term preservation of the deposit, although dissection of the mound along active fault scarps may eventually expose its interior to seafloor oxidation. An estimated growth rate for the TAG deposit, based on a total accumulation of 2.7 million t of massive sulfides and a cumulative venting history of 5 to 10 k.y., is between 500 and 1,000 t per yr. This is consistent with observed growth rates for the central Black Smoker Complex and with estimates of mass fluxes from heat and fluid flow at black smoker vents on the East Pacific Rise. Although TAG is among the largest of the known mid-ocean ridge deposits, grade-tonnage models for Cyprus-type massive sulfides world-wide suggest that much larger deposits are likely forming elsewhere on the mid-ocean ridges and at similar, slow-spreading centers in extensional back-arc basins
Origin of fluids and anhydrite precipitation at the sediment-hosted Grimsey hydrothermal field north of Iceland
No full textThe sediment-hosted Grimsey hydrothermal field is situated in the Tjörnes fracture zone (TFZ) which represents the transition from northern Iceland to the southern Kolbeinsey Ridge. The TFZ is characterized by a ridge jump of 75 km causing widespread extension of the oceanic crust in this area. Hydrothermal activity occurs in the Grimsey field in a 300 m×1000 m large area at a water depth of 400 m. Active and inactive anhydrite chimneys up to 3 meters high and hydrothermal anhydrite mounds are typical for this field. Clear, metal-depleted, up to 250 °C hydrothermal fluids are venting from the active chimneys. Anhydrite samples collected from the Grimsey field average 21.6 wt.% Ca, 1475 ppm Sr and 3.47 wt.% Mg. The average molar Sr/Ca ratio is 3.3×10−3. Sulfur isotopes of anhydrite have typical seawater values of 22±0.7‰ δ34S, indicating a seawater source for SO42−. Strontium isotopic ratios average 0.70662±0.00033, suggesting the precipitation of anhydrite from a hydrothermal fluid–seawater mixture. The endmember of the venting hydrothermal fluids calculated on a Mg-zero basis contains 59.8 μmol/kg Sr, 13.2 mmol/kg Ca and a 87Sr/86Sr ratio of 0.70634. The average Sr/Ca partition coefficient between the hydrothermal fluids and anhydrite of about 0.67 implies precipitation from a non-evolved fluid. A model for fluid evolution in the Grimsey hydrothermal field suggests mixing of upwelling hydrothermal fluids with shallowly circulating seawater. Before and during mixing, seawater is heated to 200–250 °C which causes anhydrite precipitation and probably the formation of an anhydrite-rich zone beneath the seafloor
