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Mineralization and alteration of a modern seafloor massive sulfide deposit hosted in mafic volcaniclastic rocks

Melissa O. Anderson, Mark D. Hannington, Timothy F. McConachy, John W. Jamieson, Maria Anders, Henning Wienkenjohann, Harald Strauss, Thor Hansteen and Sven Petersen
Mineralization and alteration of a modern seafloor massive sulfide deposit hosted in mafic volcaniclastic rocks
Economic Geology and the Bulletin of the Society of Economic Geologists (August 2019) 114 (5): 857-896

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 approximately 77,000 m (super 2) and are partially buried by volcaniclastic sand. Mineralization is characterized by abundant barite- and sulfide-rich chimneys that formed by low-temperature (<250 degrees C) venting over approximately 5,600 years. Barite-rich samples have high SiO (sub 2) , 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 (+ or - 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 degrees C with salinities close to, but slightly higher than, that of seawater (2.8-4.5 wt % NaCl equiv). These temperatures are 10 degrees to 20 degrees 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 K (sub 2) O (enriched in illite/smectite), MgO (enriched in chamosite and corrensite), and Fe (sub 2) O (sub 3) (enriched in pyrite associated with illite/smectite alteration). The delta (super 18) O and delta D values of clay minerals confirm increasing temperature with depth, from 124 degrees to 256 degrees 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 delta (super 34) S values of sulfides, from -0.8 to 3.4ppm, and those of sulfate minerals close to seawater sulfate, from 19.3 to 22.5ppm. 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 m (super 2) ) 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.


ISSN: 0361-0128
EISSN: 1554-0774
Coden: ECGLAL
Serial Title: Economic Geology and the Bulletin of the Society of Economic Geologists
Serial Volume: 114
Serial Issue: 5
Title: Mineralization and alteration of a modern seafloor massive sulfide deposit hosted in mafic volcaniclastic rocks
Affiliation: University of Ottawa, Department of Earth and Environmental Sciences, Ottawa, ON, Canada
Pages: 857-896
Published: 201908
Text Language: English
Publisher: Economic Geology Publishing Company, Lancaster, PA, United States
References: 121
Accession Number: 2019-065666
Categories: Economic geology, geology of ore deposits
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. sects., 7 tables, geol. sketch maps
S12°12'25" - S12°11'55", E167°36'45" - E167°37'10"
Secondary Affiliation: Neptune Minerals, AUS, AustraliaMemorial University of Newfoundland, CAN, CanadaGEOMAR-Helmholtz Centre for Ocean Research Kiel, DEU, GermanyWestfaelische Wilhelms-Universitaet, Institute fuer Geologie und Palaeontologie, DEU, Germany
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2019, American Geosciences Institute. Abstract, Copyright, Society of Economic Geologists. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 201934
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