Rare-earth element evidence for the genesis of the metalliferous sediments of Troodos, Cyprus

Abstract

Summary The origin of metalliferous sediments associated with active ocean ridges continues to be debated. The various types of metalliferous sediment that occur on and within the lavas of the Troodos massif, Cyprus, which is accepted by many workers as a slice of Mesozoic ocean crust formed at a constructive plate margin, enable the field relationships of such sediments to be studied at first hand. Umbers, which are chemically and mineralogically comparable with the metalliferous sediments of present-day active ocean ridges, overlie the uppermost lavas of the massif. Beneath the umbers there are extensive zones of veined, brecciated and chemically altered lavas, which are interpreted as channelways for ascending hydrothermal solutions, and within which metalliferous sediments occur. Manganese-poor metalliferous sediments known as ochres occur intimately associated with the massive sulphide ores of Troodos. The rare-earth element (REE) contents and, hence, relative fractionation of samples of these metalliferous sediments and of some associated lavas and sulphides have been determined by instrumental neutron activation analysis in an attempt to identify the sources of these materials and/or the different igneous and sedimentary geochemical processes that give rise to these materials. All the metalliferous sediments are enriched in the REE relative to chondrites and in the light relative to the heavy REE. In the case of umbers this is to a greater degree than normal pelagic clays and the bentonitic clays, which overlie the umbers and are thought to be continental in origin, are enriched. All these sediments also exhibit a marked negative Ce anomaly, the size of which decreases with the increasing lithogenous content of the sediments. Such a marked negative Ce anomaly is characteristic of the fractionation undergone by the REE in sea water and is a feature of the East Pacific Rise metalliferous sediments, but contrasts with the positive anomaly of manganese nodules. The REE contents of the lavas analysed are difficult to interpret, but are enriched in La relative to the other REE, probably because of alteration. Preliminary analyses show that, as would be expected, the REE contents of the sulphides are very low (generally less than 1 ppm), and therefore, as yet, no data comparable with those for the metalliferous sediments have been obtained. The REE contents of the Troodos metalliferous sediments appear to have been incorporated into the sediments from sea water. This supports the idea that the major components of the sediments, which are iron and, in the case of the umbers, some manganese oxides and hydroxides, were derived by the pervasive leaching by sea water of the igneous rock sequence. The REE contents of the interstitial sediments imply deep penetration by sea water far into the pillow lava succession, in agreement with the results of oxygen and hydrogen isotope analyses. * All the differing types of metalliferous sediment may be interpreted as related to differing redox conditions and events in the evolution of the Troodos ocean ridge (Fig. 1). In contrast, evidence from present-day ocean ridges and DSDP investigations indicates that the reducing conditions necessary to bring about sulphide deposition are not usually found in present-day median valleys and have not generally existed in the past; moreover, the redox conditions that lead to the deposition of iron but not managanese are only found locally around the mouths of channelways carrying hydrothermal solutions. † Finally, it may be noted that the umbers from above the massive Troodos sulphides apparently have anomalously high copper contents: such copper-enriched umbers may therefore be indicators of sulphides at depth.