Interaction of submarine volcanic and high-temperature hydrothermal activity proposed for the formation of the Agrokipia, volcanic massive sulfide deposits of Cyprus

Abstract

The results of drilling near the spreading-ridge-type, volcanic-hosted, massive sulfide deposits of Agrokipia, Cyprus, are described. Mineralization and associated argillic hydrothermal alteration occur over intervals of 5–130 m and at depths of 80–230 m beneath the original surface of the oceanic crust. Mineralization occurs in massive flows that probably represent a locally ponded sequence up to 300 m thick. Abundant glass–aphanitic basalt transitions are present from about 100 m below the surface of the ponded sequence, with glass abundances locally reaching 60% of the section. A novel hypothesis, involving the presence of active, high-temperature hydrothermal vents beneath the cooling ponded sequence, with the passage of hydrothermal fluids through the still molten lava, is proposed to account for the observations. While this hypothesis is reasonable, the inferred processes have not, as yet, been demonstrated under either laboratory or field conditions. The seafloor expression of this system was probably one of widely distributed, low-temperature, fluid emission over the surface of a lava pond in the axial graben of a spreading ridge.

Deep‐sea magnetic vector anomalies over the Hakurei hydrothermal field and the Bayonnaise knoll caldera, Izu‐Ogasawara arc, Japan

“…Magnetic anomalies can provide vital information on the subsurface structure of hydrothermal systems, because the extent of the hydrothermal alteration zone or the upflow zone can be evaluated by the areal extent of the low‐magnetization zone in the crust, which results from the destruction of magnetic minerals. This effect of hydrothermal activity on the magnetic properties of a host rock has been confirmed by studies of ancient deposits associated with ophiolitic rocks in Cyprus and Oman [e.g., Richards et al , ; Johnson et al , ; Hall , ], and seafloor hydrothermal deposits are now recognized as the modern analog. The lateral scale of the venting sites or alteration zones is rather small, typically a few hundreds of meters or less.…”

Heat Flow and Near‐Seafloor Magnetic Anomalies Highlight Hydrothermal Circulation at Brothers Volcano Caldera, Southern Kermadec Arc, New Zealand

“…Hydrothermal alteration processes can transform these primary magnetic minerals into less magnetic, or nonmagnetic, minerals, such as pyrite, effectively reducing the magnetization to negligible values (e.g., Ade-Hall et al, 1971;Fujii et al, 2018;Rona, 1978;Watkins & Paster, 1971;Wooldridge et al, 1990). For example, studies of the magnetic signature of fossil hydrothermal systems in ophiolites have clearly shown how areas of reduced crustal magnetization are associated with the mineralized stockwork zones (e.g., Hall, 1992;Johnson et al, 1982;Richards et al, 1989).…”

Interpretation of aeromagnetic anomalies in terms of hydrothermal alteration in Cretaceous normal polarity superchron extrusives of the Troodos, Cyprus, ophiolite

“…The first step in accounting for the variability in source layer thickness is to compare the RTP map of the area with the distribution of known geological features (Figure 9) Elsewhere [Hall et al, 1989;Hall, 1992] it has been proposed that the presence of a thick package of predominantly sheet flows, rather than the pillowed flows found elsewhere, represents the arrival of a large magma batch in the crust, with subsequent high magma supply rate to the seafloor and enhanced high-temperature hydrothermal circulation. If this interpretation is correct, then a geological explanation for the anomalies can be given.…”