Subseafloor sulphide deposit formed by pumice replacement mineralisation

Abstract

Seafloor massive sulphide (SMS) deposits, modern analogues of volcanogenic massive sulphide (VMS) deposits on land, represent future resources of base and precious metals. Studies of VMS deposits have proposed two emplacement mechanisms for SMS deposits: exhalative deposition on the seafloor and mineral and void space replacement beneath the seafloor. The details of the latter mechanism are poorly characterised in detail, despite its potentially significant role in global metal cycling throughout Earth's history, because in-situ studies require costly drilling campaigns to sample SMS deposits. Here, we interpret petrographic, geochemical and geophysical data from drill holes in a modern SMS deposit and demonstrate that it formed via subseafloor replacement of pumice. Samples from the sulphide body and overlying sediment at the Hakurei Site, Izena Hole, middle Okinawa Trough indicate that sulphides initially formed as aggregates of framboidal pyrite and matured into colloform and euhedral pyrite, which were replaced by chalcopyrite, sphalerite and galena. The initial framboidal pyrite is closely associated with altered material derived from pumice, and alternating layers of pumiceous and hemipelagic sediments functioned as a factory of sulphide mineralisation. We infer that anhydrite-rich layers within the hemipelagic sediment forced hydrothermal fluids to flow laterally, controlling precipitation of a sulphide body extending hundreds of meters.

Figure 1

Microphotographs showing pyrite textures under reflected light. All images are in plane-polarised light unless otherwise noted. (a,b) Framboidal pyrite (f-Py) in pumice fragments of the hanging wall of the underwater debris flow deposit, (c) framboidal pyrite with pyrite/marcasite replacing pyrrhotite pseudomorphs (Po-pseudo) in the blackish sulphidic vein of the debris flow deposit, (d) colloform pyrite/marcasite (c-Py/Mrc) with recrystallised framboidal pyrite in the upper part of the subseafloor sulphide body (cross-polarised light), (e,f) euhedral pyrite (e-Py) with sphalerite or chalcopyrite in the sulphide body of the Northern Mound. Ccp chalcopyrite, Mag magnetite, Sp sphalerite.

Figure 2

Scanned drill core images showing the contact between hemipelagic sediment and the sulphide body at Hole C9026A. The T-layer, composed of pyrite + barite + marcasite + sphalerite + galena, is 2 cm thick and is interpreted as a cap layer restricting hydrothermal fluid to lateral flow, controlling subseafloor sulphide mineralisation and hydrothermal alteration. Anh anhydrite, Brt barite, Chl chlorite, Gn galena, Ill illite, Kln kaolinite, Mnt montmorillonite, Mrc marcasite, Py pyrite, Qtz quartz; Sp, sphalerite.

Figure 3

Depth profiles of natural gamma-ray intensity in Holes C9025A and C9026A. Red lines are gamma-ray intensity measured by the PPS71 instrument in the drill holes and open circles are onboard multisensor core logger measurements of recovered drill cores. Areas shaded brown, green and pink represent intervals of the subseafloor sulphide body with intercalated hemipelagic sediment, continuous occurrences of K-feldspar and melting of the inner plastic liner at the high temperatures of the coring operation (interpreted as a hydrothermal fluid channel), respectively. Patterns of the gamma-ray intensities at both holes are quite similar (blue arrows and dotted lines).

Figure 4

Simplified subseafloor structure of the Hakurei Site, Izena Hole. The subseafloor sulphide body underlies the pumiceous underwater debris flow deposit within hemipelagic sediment and contains at least two hemipelagic sediment layers. The footwall of the sulphide body consists of hydrothermally altered clay with alteration minerals dominated by chlorite + illite ± K-feldspar. A layer of hydrothermally altered clay 10–15 m below the sulphide body contains pyrrhotite-cubanite veins. A deeper cap layer in Hole C9026A appears to confine hot (> 200 °C) hydrothermal fluid. Not to scale. See Supplementary Fig. S11 for more details. Cbn cubanite, Po pyrrhotite.