Mineralization and Alteration of a Modern Seafloor Massive Sulfide Deposit Hosted in Mafic Volcaniclastic Rocks

Abstract

Tinakula is the first seafloor massive sulfide deposit described in the Jean Charcot troughs and is the first such deposit described in the Solomon Islands—on land or the seabed. The deposit is hosted by mafic (basaltic-andesitic) volcaniclastic rocks within a series of cinder cones along a single eruptive fissure. Extensive mapping and sampling by remotely operated vehicle, together with shallow drilling, provide insights into deposit geology and especially hydrothermal processes operating in the shallow subsurface. On the seafloor, mostly inactive chimneys and mounds cover an area of ~77,000 m2 and are partially buried by volcaniclastic sand. Mineralization is characterized by abundant barite- and sulfide-rich chimneys that formed by low-temperature (<250°C) venting over ~5,600 years. Barite-rich samples have high SiO2, Pb, and Hg contents; the sulfide chimneys are dominated by low-Fe sphalerite and are high in Cd, Ge, Sb, and Ag. Few high-temperature chimneys, including zoned chalcopyrite-sphalerite samples and rare massive chalcopyrite, are rich in As, Mo, In, and Au (up to 9.26 ppm), locally as visible gold. Below the seafloor, the mineralization includes buried intervals of sulfide-rich talus with disseminated sulfides in volcaniclastic rocks consisting mainly of lapillistone with minor tuffaceous beds and autobreccias. The volcaniclastic rocks are intensely altered and variably cemented by anhydrite with crosscutting sulfate (± minor sulfide) veins. Fluid inclusions in anhydrite and sphalerite from the footwall (to 19.3 m below seafloor; m b.s.f.) have trapping temperatures of up to 298°C with salinities close to, but slightly higher than, that of seawater (2.8–4.5 wt % NaCl equiv). These temperatures are 10° to 20°C lower than the minimum temperature of boiling at this depth (1,070–1,204 m below sea level; m b.s.l.), suggesting that the highest-temperature fluids boiled below the seafloor. The alteration is distributed in broadly conformable zones, expressed in order of increasing depth and temperature as (1) montmorillonite/nontronite, (2) nontronite + corrensite, (3) illite/smectite + pyrite, (4) illite/smectite + chamosite, and (5) chamosite + corrensite. Zones of argillic alteration are distinguished from chloritic alteration by large positive mass changes in K2O (enriched in illite/smectite), MgO (enriched in chamosite and corrensite), and Fe2O3 (enriched in pyrite associated with illite/smectite alteration). The δ18O and δD values of clay minerals confirm increasing temperature with depth, from 124° to 256°C, and interaction with seawater-dominated hydrothermal fluids at high water/rock ratios. Leaching of the volcanic host rocks and thermochemical reduction of seawater sulfate are the primary sources of sulfur, with δ34S values of sulfides, from –0.8 to 3.4‰, and those of sulfate minerals close to seawater sulfate, from 19.3 to 22.5‰. The mineralization and alteration at Tinakula are typical of a class of ancient massive sulfide deposits hosted mainly by permeable volcaniclastic rocks with broad, semiconformable alteration zones. Processes by which these deposits form have never been documented in modern seafloor massive sulfide systems, because they mostly develop below the seafloor. Our study shows how hydrothermal fluids can become focused within permeable rocks by progressive, low-temperature fluid circulation, leading to a large area (>150,000 m2) of alteration with reduced permeability close to the seafloor. In our model, overpressuring and fracturing of the sulfate- and clay-cemented volcaniclastic rocks produced the pathways for higher-temperature fluids to reach the seafloor, present now as sulfate-sulfide veins within the footwall. In the geologic record, the sulfate (anhydrite) is not preserved, leaving a broad zone of intense alteration with disseminated and stringer sulfides typical of this class of deposits.

Subseafloor sulphide deposit formed by pumice replacement mineralisation

“…Anhydrite precipitated within the pumice layer or at the contact between pumice and hemipelagic sediment locally reduces the permeability and forms a cap layer that restricts the hydrothermal fluid to lateral flow and raises the temperature beneath it high enough to precipitate sulphide minerals within the pumice deposit. The existence of such an anhydrite cap layer is consistent with observations of a modern SMS deposit hosted in mafic volcaniclastic rocks 35 and numerical simulations of the seafloor hydrothermal field 36 . This process is similar to the initial formation of chimney structures on the seafloor 33 , 37 .…”

Potential for Volcanogenic Massive Sulfide Mineralization at the A6 Anomaly, North-West British Columbia, Canada: Stratigraphy, Lithogeochemistry, and Alteration Mineralogy and Chemistry

“…Based on variations in illite composition (i.e., I/S and close to illite endmember) and their stability at T = 200-360 • C (e.g., [63,65,119]), δ 18 O H 2 O composition of evolved seawater was most likely closer to 4‰ (Figure 27b). This 'heavier' oxygen composition in hydrothermal fluids has been observed in some VMSforming systems including Kuroko deposits (δ 18 O H 2 O = 6-10‰; [136,137]), Bathhurst mining Camp (δ 18 O H 2 O = 4‰; [138]) and seafloor massive sulfide systems (δ 18 O H 2 O ≈ 5‰; [128]). However, it is unclear if boiling, a magmatic component or both processes contributed to higher δ 18 O H 2 O composition at the A6 Anomaly based on the current data (e.g., unknown water depth; lack of bladed alteration phases).…”

“…The inverse temperature profile at the A6 Anomaly is related to zones of increased brittle deformation with abundant fractures and quartz veins that allowed for larger lateral fluid flow due to higher permeability and was not caused by lithological changes (i.e., coherent flows vs. overlying volcaniclastic rocks). Despite varying temperatures with depth, the alteration assemblage does not change significantly which contrasts with other hydrothermal systems (e.g., [128,129]) and displays the homogeneity of alteration assemblage over a relatively large area (ca. 0.5 km 2 ).…”

“…Additionally, variations in illite composition show that sericitization of feldspars occurred at increasing temperatures since both I/S and illite close to endmember composition are present (Figure 18). This process of illitization (i.e., sequential formation of illite commonly from smectite with increasing temperature; [66,107,124,125]) is well observed in hydrothermal systems including geothermal fields [126], Zn-Pb hosted Irish-type deposits [127], VMS and seafloor massive sulfide deposits (e.g., [128,129]), and porphyry deposits [130]) and is a function of temperature. Illite/smectite is stable at temperatures of <200-230 • C [119,124], whereas endmember illite stability is commonly achieved at temperatures of 230 to 360 • C [63,65,107,119,125].…”

“…Lower fluid temperatures (200-220 • C) coincide with I/S composition (e.g K = 0.50-0.69 apfu), whereas higher temperatures (230-240 • C) relate to illite composition close to endmember illite (K = 0.69-0.80 apfu). Although temperature commonly increases downwards in VMS systems (e.g., [81,86,118]), an inverse temperature gradient has also been observed locally on the modern Tinakula seafloor massive sulfide deposit in the Solomon Islands [128]. The inverse temperature profile at the A6 Anomaly is related to zones of increased brittle deformation with abundant fractures and quartz veins that allowed for larger lateral fluid flow due to higher permeability and was not caused by lithological changes (i.e., coherent flows vs. overlying volcaniclastic rocks).…”