Mineralogy and geochemistry of the Masa Valverde blind massive sulphide deposit, Iberian Pyrite Belt (Spain)

Abstract

Masa Valverde is a blind, volcanic-hosted massive sulphide deposit recently discovered in the Spanish sector of the Iberian Pyrite Belt (IPB). The deposit is more than 1 km long and is located at depths between 400 and 850 m, in direct spatial association with a residual gravimetric anomaly. The ore body is concealed beneath Culm sedimentary rocks which cover the western extension of a Hercynian age anticline, a few kilometers west of an area where older massive sulphide workings existed. The Masa Valverde deposit consists of two main ore bodies composed dominantly of massive and banded pyrite which are hosted by a volcano-sedimentary sequence made up of felsic tuffs interlayered with shale, siliceous exhalite and radiolarian chert. The abundance of sedimentary host rocks to the deposit is a feature shared by other large massive sulphide deposits in the eastern sector of the IPB and is interpreted as significant with respect to the origin of the deposit (a break in volcanic activity). The thickness of the upper massive sulphide orebody varies between a few meters and 70 m, and it consists of lenses and blankets of massive sulphides with interbedded tuff and shale and occasionally stockwork zones. The lower orebody is smaller and thinner than the upper one, but this may be due to incomplete drilling. Two types of stockwork occur beneath the massive sulphide bodies, the more common type consists of irregular and anastomosing sulphide veinlets and irregular blebs formed by replacement. Stockwork with cross-cutting, generally straight-sided, sulphide veins is, by contrast, suggestive of formation by hydraulic fracturing. The mineralogy and alteration processes of the ore and host rocks at Masa Valverde are analogous to those of other IPB massive sulphides. The average temperature of formation of the components of the ore zones estimated on the basis of chlorite thermometry are as follows: siliceous exhalites (290 °C), massive sulphides (325 °C) and stockworks (305 °C). Rare earth element measurements indicate that the lowest metal concentrations occur in chloritites, which occasionally host Cu-rich stockwork and are characterised by a strongly negative Eu anomaly. By contrast, rocks hosting the sulphide lenses, where silicification, sericitization and carbonatization are more intense, are enriched in LREE. Two main stages of hydrothermal activity are suggested for the Masa Valverde deposit. During an early stage, ore fluids would have discharged onto the sea floor resulting in the deposition of pyrite with sphalerite and galena. Later on, with continued input of high temperature fluids (290 to 315 °C), chalcopirite precipitated, especially in the stockworks and at the base of the sulphide lenses. At Masa Valverde, this late Cu-rich hydrothermal event would have produced an ascending Cu-front which overpassed the stockwork zone and reached a higher temperature (325 °C) at the base of the massive sulphide lenses. The combination of geological, mineralogical and geochemical features of the Masa Valverde deposit provide guidelines for exploration for new blind orebodies in this part of the IPB, where tuffs and sedimentary rocks are the dominant host lithologies. The Masa Valverde deposit can be considered as a VHMS subtype transitional to SHMS.