A Semipermeable Interface Model for the Genesis of Subseafloor Replacement-Type Volcanogenic Massive Sulfide (Vms) Deposits

Abstract

Subseafloor replacement-style volcanogenic massive sulfide (VMS) deposits are a subset of VMS deposits where sulfides have replaced unconsolidated volcanic, volcano-sedimentary, and sedimentary material. These deposits are anomalously large and are important global sources of metals. They have distinct textures at the sulfide-ore interface, including bed-by-bed replacement of sedimentary layers, and typically fill void space between unconsolidated volcaniclastic detritus or fractures in flows or intrusions. At the microscale, metalbearing sulfides have partially to fully replaced framboidal (bacteriogenic) sulfides, or the framboidal sulfides have acted as nuclei upon which additional metalliferous massive sulfide is deposited.The textures presented are reconciled within a semipermeable interface model for replacement. In this model unconsolidated sediment, volcaniclastic rocks, or fractured coherent volcanic rocks provide a permeable to semipermeable interface that allowed ingress of cold seawater into the pore spaces of the stratigraphic sequence prior to and during lulls in hydrothermal activity. Seawater sulfate in the pore water is partially reduced by bacteria to provide reduced sulfur (H2S) as well as framboidal pyrite in the host sequence(s). The reduced sulfur and framboidal pyrite, as well as the cool pore water, provided a thermal, redox, and chemical gradient in which upwelling hydrothermal fluids interact. In such an environment rising hydrothermal fluids mix with cold water, not only at the seawater interface leading to exhalative sulfide deposition, but also in the subseafloor leading to sulfide precipitation via replacement. The upwelling hydrothermal fluids can also interact with bacterial H2S in the pore spaces of the unconsolidated material, resulting in additional subseafloor precipitation of metal sulfides. The fluids also result in replacement of framboidal pyrite nuclei pseudomorphous after the original framboidal masses. This semipermeable interface also favors enhanced zone refining, assuming the hydrothermal system is sufficiently long lived, leading to upgrading of the tenor of the sulfides with well-developed metal zoning, as observed in many ancient replacement-type deposits. Furthermore, the precipitation of a significant subseafloor sulfide mineralization results in greater trapping of metals from upwelling fluids and larger tonnage deposits with greater contained metal.This model may also be applicable to other replacement-type deposits in broadly similar geologic and hydrothermal environments (e.g., sediment-hosted and Irish-type Zn-Pb deposits). Additional, critical tests are required to validate and refute the model and potential tests are presented herein.

Microbial sulfate reduction plays an important role at the initial stage of subseafloor sulfide mineralization

“…2B and 2C). This progression from framboidal to colloform and euhedral texture has also been reported in VMS deposits (Piercey, 2015;Velasco-Acebes et al, 2019). In drill cores from the Original site, paragenesis of framboidal and euhedral pyrite is observed in thin section ( Fig.…”

“…That framboidal pyrite is commonly replaced by other sulfide minerals such as chalcopyrite and galena (Fig. 2D) as well as sphalerite (Piercey, 2015) indicates that framboidal pyrite provides component material for other sulfide minerals. Our evidence suggests that framboidal pyrite incorporating S derived from microbial sulfate reduction serves as a nucleus for subsequent mineral growth at the initial stage of subseafloor sulfide mineralization.…”

“…In drill cores from the Original site, paragenesis of framboidal and euhedral pyrite is observed in thin section ( Fig. 2D), where framboidal pyrite is commonly replaced by chalcopyrite and galena, indicating that framboidal pyrite formed at an initial stage of sulfide mineralization (Halbach et al, 1993;Piercey, 2015). Chimneys from the Original site exhibit two distinct pyrite textures, colloform and/or spherical on the outer side and euhedral and/ or acicular on the inner side ( Figs.…”

“…Volcanogenic massive sulfide (VMS) deposits are ancient and/or fossil examples of SMS deposits presently observed on land (Ohmoto, 1996;Piercey, 2011); both are formed by seafloor hydrothermal activity related to volcanism in mid-ocean-ridge and arc-back-arc settings (Tivey, 2007;Tornos et al, 2015). Sulfur isotope (δ 34 S) analyses of VMS deposits (Lode et al, 2017;Slack et al, 2019;Velasco-Acebes et al, 2019) have shown that an initial mineralization process characterized by the formation of framboidal pyrite (Piercey, 2015) is closely associated with microbial sulfate reduction. However, VMS deposits are subject to diagenesis and metamorphism during their emplace-ment on land, which can obscure the petrological (mineralogical) and geochemical records of their mineralization process.…”

Subseafloor sulphide deposit formed by pumice replacement mineralisation

“…Previous research cruises employing dive surveys and subseafloor drilling have yielded basic knowledge of two different emplacement mechanisms of SMS deposits 7 – 10 , which are (1) exhalative deposition, building chimney and mound structures on the seafloor, and (2) subseafloor replacement of relatively unstable material and volcanic glass by sulphide minerals. VMS deposits on land show evidence of both mechanisms 6 , 11 - 13 , but few examples document the entire formation and mineralisation process, including the nascent stage of sulphide formation 12 , 13 , because subsequent hydrothermal, diagenetic, metamorphic and tectonic activity often has erased any relevant evidence. The subseafloor replacement process is considered to promote the trapping of metals from upwelling hydrothermal fluids and produce large deposits with sizable tonnage 11 , 13 .…”