Basalt-Sediment Relationships on a Mesozoic Ocean Ridge

Cyprus umbers: basalt-sediment relationships on a Mesozoic ocean ridge

A. H. F. ROBERTSON

SUMMARY Overlying the upper Pillow Lavas, the latest emplacement of stratiform cupriferous sul- extrusives of the Troodos Massif, interpreted phides. as Cretaceous ocean floor, there occur umbers: Umbers originated in the Campanian due iron, manganese and trace-metal enriched to volcanism on an ocean ridge. The sub-umber mudstones of volcanic exhalative origin. veining and brecciation was produced by Umber deposits occur in hollows, underlain late-stage hydrothermal activity, associated, by thin lava breccias, also by deeper zones of initially, with a brief episode of violent vol- intensely veined and fragmented pillow lavas. canism, then with more quiescent discharge Occasional thicker umber deposits are located of submarine thermal springs from a geother- in elongate fault-controlled depressions; within mal system. Large volumes of dilute metal- and above thick lava breccias, restricted to liferous brines were released into open marine the south margin of the Troodos Massif; and waters whereupon umbers were rapidly pre- also in depressions inherited from earlier cipitated.

RECENTLY, much research has gone into an understanding of sedimentation on ocean ridges, especially metalliferous sediments generated by activity of submarine geothermal systems. With the wide acceptance of the Troodos Massif of Cyprus as a fragment of Mesozoic ocean crust (Gass & Masson-Smith I963, Moores & Vine I97i), the relationships of the basalt lavas to the overlying sediments become widely relevant; it provides a rare opportunity to study ocean floor sediments now exposed well preserved on land. Within and immediately above the latest extrusives there are umbers, sediments which are strongly enriched in Fe, Mn and trace elements, analogous to the metalliferous sediments of Recent ocean ridges. The umbers were formed by a geothermal system active during the waning stages of volcanism; field relations yield information about the mechanism and extent of interaction of seawater with freshly extruded ocean floor lavas and the influence of these processes on pelagic sedimentation, subjects of major current interest (Hart I973, Spooner & Fyfe I973). I, Previous interpretations For long the Cyprus umbers were viewed as secondary products, e.g. of hydrothermal alteration, either of volcanic ash (Wilson 1959), or of clays formed long after the end of volcanism (Cullis & Edge i927). Alternatively, circulating meteoric waters were invoked to leach metals from deeply weathered pillow lavas in an attempt to explain umbers as altered clays (Bear I96O ). With time, a consensus began to emerge that umbers were primary sediments, variously interpreted as due to slow lateritic weathering of subaerially exposed lavas (Morel i96o), slow sea floor erosion of cold pillow lavas (Constantinou & Govett

Jl geol. Soe. Lond. vol. x3x, x975, pp. 511-53 I, 8 figs. Printed in Northern Ireland.

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i972 , Lapierre I972), or the result of direct interaction of freshly erupted lava with seawater (Elderfield et al. 1972). Others invoked biological extraction of metal from seawater (Bagnall I96o), even the activity of sulphide-reducing bacteria (Zomenis I972 ). An alternative school attempted to relate umbers to formation of the massive sulphides and their associated sediments (ochres), which occur in the lavas (Hutchinson I965, Corliss et al. I972 , Searle I973, Williams I973, Pantazis I973). For long the manganese deficient ochres were confused with the much more widely distributed ferro-manganoan umbers. Various depositional settings have been proposed to explain the patchy and restricted nature of umbers which, to many, suggested that they formed in shallow water (e.g. Constantinou & Govett I972); lagoons were the favourite setting (Wilson I959, Bear I96O, Gass I96O, Kluyver x969). Alternatively, the presence of Radiolaria in some umbers suggested an open marine depositional environment (Pantazis x967, Mantis i97o ). On the basis of field data, umbers are here interpreted as chemical precipitates which were rapidly deposited in seawater from submarine thermal springs active during the latest stages of volcanism on a Tethyan ocean ridge.

2. Nature and stratigraphy of umbers In the field the umbers are pale, chestnut coloured or almost black, fine grained mudstones, low in density and almost free of calcium carbonate. They are found in hollows in the surface of the upper Pillow Lavas, the uppermost igneous unit of the Troodos Massif (Fig. I). Goethite and poorly crystalline manganese oxides dominate the mineralogy of umbers (Elderfield et al. i972), but small amounts of quartz and calcium apatite also occur. Relative to the overlying sediments, umbers are strongly enriched in many trace metals, e.g. Ba, Co, Cu, Ni, Pb, V, Zn and Zr (Robertson & Hudson I973a ). StratigraphicaUy, umbers have been assigned to the Parapedhi Formation (Wilson I959). Upwards, they are succeeded by Campanian radiolarites and radiolarian mudstones, then by Maastrichtian and Tertiary chalks. Details of the locations of the umber outcrops and a measured section at Drapia are given in Supplementary Publication No. SUP I8OIi (6 pages), deposited with the British Library Lending Division at Boston Spa, Yorkshire, U.K. and The Geological Society Library.

3. Types of umber deposit Outcrops of umber in Cyprus can conveniently be divided into five distinct, although sometimes overlapping, types of deposit, characteristic of a variety of depositional settings. I) Small-hollow umbers: encountered in small hollows on the upper Pillow Lava surface. 2) Interlava umbers: intercalated between the uppermost lava flows. 3) Fault-controlled umbers: substantial umber deposits related to major faults in the upper Pillow Lavas. 4) Umbers associated with lava breccias: they overlie thick volcanic breccias, restricted to the S. margin of the Troodos Massif. 5) Umbers spatially associated with the massive sulphide ores.

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Umbers are extensively quarried for use as pigment in the leather tanning industry. Many of the geologically most informative outcrops are in danger of destruction as reserves are small.

UMBERS IN SMALL HOLLOWS Umbers most frequently occur in small hollows in the surface of the upper Pillow Lavas where they are almost always finely laminated and undisturbed. At 23 of these outcrops umbers vary in thickness from 0. 5 m to 7 m, the average is 2. 5 m. Almost invariably there is an upward gradation from pure dark brown umber to grey clay-rich umber, itself ranging from 0. 5 m to 4"5 m, with an average thickness of 2- 7 m (Fig. 2). The clay-rich umber is succeeded by pink, carbonate-free radiolarite and radiolarian mudstone, then sometimes by variable thicknesses of bentonitic clay (Robertson & Hudson I974). Upwards, there is a sharp transition over a few cm to white marl or chalk, of Maastrichtian or later age. Good examples of small umber hollows occur at Kinousa, Margi, Pyrga and Zakharia (Fig. I). In several of these small hollows, as at Kambia and Ayia Marina, the basal few cm of umber are bright orange (analysis shows it to be Fe-rich but Mn-deficient) ; upwards, there is a sharp change to typical dark brown umber; the contact cuts obliquely across fine sedimentary laminations, which shows that the iron-rich base is not a primary feature, but is related to post-depositional migration and

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loss of Mn. The small-hollow umbers rarely contain any admixed tuffaceous debris; exceptions occur at Pyrga where pieces of fragmental lava occur in the base of the umbers, and at Palaeomandra where tuff beds several centimeters thick have been disrupted by post-depositional slumping. Dark brown or almost black vitreous splintery cherts sometimes occur in the small-hollow umbers. West of Kotchati, the cherts in umbers form irregular lenticular masses up to 0.6 m thick, whereas in the Tremithios River cherts of similar lithology are bedded and laterally continuous. Small-hollow umbers are usually underlain by pale grey, soft, friable, highly altered and decomposed lava breccias. The lava fragments, mostly Io-x 5 cm in diameter, occur in a matrix of orange iron-rich fine grained sediment. They form a zone I to to m thick in which the most highly altered lava breccias occur immediately below umbers. These lava breccias reach much greater thicknesses along the S. margin of the Troodos Massif. Near Ayia Marina, N. of the Troodos Massif (Fig. I), umbers over 7 m thick are preserved as an erosional outlier. The umbers occur in a steep-sided hollow floored, not by brecciated pillow lavas as elsewhere, but instead by yellow or pale grey lava pillows of normal size, mostly several metres in diameter. Above the basal umbers, which are orange, there is a sharp change to dark brown umber in which there are numerous horizons rich in small elongate jet-black manganese concretions, mostly 2 to 5 cm in length. These sometimes coalesce to form laterally continuous thin beds of dense splintery pyrolusite. These concretions as well as the orange basal layer testify to the mobility of manganese. The finely laminated nature of umbers and their upward unbroken sedimentary transition to other lithologies indicate that they are primary sediments. They are neither massive nor homogeneous as believed by Constantinou & Govett (x972). Evidence of post-depositional migration of manganese is inconsistent with the views of Searle (I973) and Elderfield et al. (I972), that the orange basal layer of some umbers is a primary sedimentary feature, which resulted from original geochemical separation of iron and manganese.

INTERLAVA UMBERS Umbers within the lavas are significant because of the light they throw on the supposed long time interval between lava extrusion and umber formation. This misconception arises from the early view that the Troodos Massif was Triassic (e.g. Gass & Masson-Smith r963) , whereas the base of the sedimentary succession, including the umbers, was correctly assigned to the upper Cretaceous on micro- palaeontological evidence (Henson et al. I949). Belief in this long time interval (e.g. Constaninou & Govett I972 ) survived radiometric dating of the Troodos Massif, which confirmed an upper Cretaceous age (Lapierre 1972, Gass personal communication). Field evidence of the close association of umbers with the upper Pillow Lavas is incontrovertible; umbers are found not only above, but also within, the upper levels of the upper Pillow Lavas. Fig. 3 a for example shows black umbers overlain by basalt and then by pillowed lavas with interstitial umber. This umber first accumulated in a small hollow which was then inundated with lava to form a lava pool which cooled slowly 6

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enough to allow the development of crude columnar jointing, rare elsewhere in the upper Pillow Lavas. Occasionally, umberiferous sediments occur at greater depths in the upper Pillow Lavas. Near Kambia there is a small outcrop of discontinuous lenticular black argillaceous umber, up to o.6 m thick (Fig. 3b). Orange umber fills the interstices between the immediately underlying pillow lavas, which are themselves pale and altered to within six feet of the overlying umber. Above this, red baked umber is overlain by columnar-jointed lavas which probably originated as a lava pond. Bear (i96o) took the umbers to mark the boundary between the lower and upper Pillow Lavas, but recent work suggests that they lie within the upper unit (Smewing, pers. comm. t972 ). Elsewhere, e.g. Malounda, there is a clear stratigraphical break between the upper and lower Pillow Lavas, but umbers did not accumulate during this pause in volcanism, as stated by Elderfield et al. (x972) . They are almost totally absent from the lower Pillow Lavas and the Basal Group; the only exception occurs near Kataliondas where o.x m of laterally discontinuous umber is found. Along the S. margin of the Troodos Massif, tufts and local angular unconformities provide abundant evidence of breaks in lower Pillow Lava volcanism, but not associated with umbers. More frequently, interlava umbers occur intercalated with the lava breccias which underlie almost all umber hollows. A good example occurs near Zakharia where o-6--I m of laminated interlava umbers, with dark manganese-rich segregations, are overlain by xo m of lava breccias which consist of a mixture of fragmented pillow lavas and small pale pillows, mostly io-i 5 cm in diameter, with intact chilled margins. The lava breccias are overlain by more umbers, restricted to small hollows in the lava breccia surface. These field relations show that most interlava umbers were formed during, and immediately after, the final phase of upper Pillow Lava volcanism; nowhere was there a long time break between volcanism and umber formation•

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Fxo. 3 a. Diagrammatic cross-section through interlava umbers, near the entrance to Troulli Mine, Larnaca District. 3b, Diagrammatic cross-section through interlava umberiferous sediments which occur at depth in the upper Pillow Lavas, Pedieos River, Nicosia District.

FAULT-CONTROLLED UMBER DEPOSITS Umbers sometimes reach substantial thicknesses in fault-controlled depressions which were favoured sites of umber deposition. Seawater is believed to have penetrated downwards along these faults to react with still hot lavas beneath to produce the geothermal solutions from which umbers were precipitated. (i) Lymbia Umber Pit. This shows slumping of pillow lavas into a major depression in the sea floor where umbers were rapidly accumulating. In detail, quarrying has revealed umbers in a elongate trough, about 13 ° m long, 20 m wide and up to 15 m deep (Fig. 4a). The S. wall of the quarry shows large masses of disrupted pillow lavas floating in umber, structureless apart from thin white tuff beds, locally highly deformed. In the N., two inward dipping beds are composed of rounded, abraded and roughly size-graded pillow lava fragments in a matrix of structureless brown umber. These observations reveal a complex tectonic history. Umber with thin tuff beds first accumulated in a steep-sided elongate fault-controlled trough. At the N. of the quarry, large masses of pillow lavas slide into umber; individual pillows disintegrated along original coofing cracks and were then grooved, abraded and roughly graded in size before they came to rest on the floor of the original hollow as a submarine scree. This was probably the result of continued fault movement. Subsequently, after more umber sedimentation, masses of lava again slumped into umber at the S. end of the hollow which, as a result, became highly con- torted. Probably somewhat later, the whole outcrop was again faulted; the eastern margin of the pit was upthrown c. 1o m, and the adjacent umber deformed and slickensided. The thick umbers and the extensive veining and brecciation of the surrounding pillow lavas at Lymbia exemplify the importance of faults as lines of weakness which allowed seawater to penetrate deep into the lavas. (ii) Troulli Umber Pit. Umbers accumulated in an elongate fault-controlled depression which was later deformed into a single V-shaped syncline (Fig. 4b). The underlying lavas are, as at Lymbia, altered and veined with interstitial orange iron-rich sediment. At the foot of the original depression angular fragments of pillow lavas float in structureless umber. At the NE. end of the pit finely laminated umbers, over IO m thick, are interbedded with 3-6 cm beds of pale tuffaceous sediment. Each grades up from medium grained tuffaceous, to finely laminated brown umber. Other small-scale sedimentary structures include micro- cross-lamination and small-scale sediment flames. In places, the umber is lenticular and individual umber beds are thinned tectonically against the lavas. At the W. of the pit, the upper part of the umber succession shows 5 m of umber rhythmically interbedded with pink or grey radiolarite beds lO-15 cm thick. Upwards, this radiolarite becomes paler, more thinly bedded and the intervening umber beds become more argillaceous. The umber of Troulli was initially transported into a steep-sided fault- controlled depression, already floored by fragmented pillow lavas. The origin of the thin tuffaceous beds is problematical. Possibly they are true tufts, the product of subaerial extrusion, for which, otherwise, there is no evidence. More probably,

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they represent silt-sized volcaniclastic detritus. The grading can then be envisaged as gravity differentiation of successive influxes of umber, in effect, 'umber micro- turbidites', triggered off by continuing tectonic instability. Subsequently, there was en masse slumping of parts of the umber. Later still, after consolidation, more disturbance led to tectonic attenuation of the umbers adjacent to the lavas, an effect of the extreme competency difference between these two lithologies. It was probably this same event which produced slickensiding and boudinage of the thin white tuffaceous beds, as well as deformation of the whole deposit into a single V-shaped syncline. (iii) Arsos Umber Pit. Pillow lavas beneath the umbers here, as elsewhere, are extremely altered and brecciated. Near the pit entrance umbers are over io m thick, in places conspicuously lenticular with lenses up to I m thick. Arsos exposes the immediately overlying sediments, mostly eroded at Lymbia and Troulli. These are grey bentonitic clays, 5 m thick, interbedded with volcaniclastic sediments comprising weathered lava fragments in a silty clay-rich matrix. A conspicuous feature of the Arsos pit is an E.-W. trending vertical fault which has thrown the umbers, and the immediately overlying sediments, down to the N. by over I o m. The umber lenses, as at Troulli, originated in repeated post-depositional slumping of umbers prior to their consolidation. Continued disturbance in this area is reflected by the much later vertical fault, probably a reactivation of an earlier line of weakness associated with the original umber deposition. The Arsos umbers throw light on the nature and extent of post-depositional migration of manganese. Very dark Mn-rich umber alternates with paler orange umber. The contact between these two types is not sharp but sinuous; it bears no relationship to bedding. Above black umber the clay is grey, but above places where the umber is orange, the basal clay, up to 3"5 m thick, is pink. Apparently Mn (possibly together with other trace elements) was locally mobilized within the umbers, transported upwards and redeposited in the immediately overlying clays. In contrast, Lilliequist (i969) attributed these colour variations to primary lithological variations.

UMBERS ASSOCIATED WITH THICK LAVA BRECCIAS Along the south margin of the Troodos Massif occur very thick and extensive volcanic breccias overlain by much the thickest and most numerous umber deposits in Cyprus (Fig. 5a). The lava breccias crop out extensively between Vavla and Ayios Mamas, roughly parallel to the Arakapas Fault Zone, interpreted by Moores & Vine (i 97 i) as an inactive transform fault (Fig. I). (i) Drapia. Below the umbers are lava breccias over IOO m thick consisting of grooved and abraded pillow lava fragments, mostly less than 5 cm, but including a few blocks up to I m across. Within 2 m of the overlying umber, the average size of the lava fragments decreases progressively to less than 2 cm, and the lava fragments are roughly bedded. The umber which immediately overlies these breccias is highly atypical. A measured section is given in Supplementary Publication SUP No. I8Ol I. There is a basal zone up to I- 5 m thick which con- sists of dark slaggy umber peppered with small black vertical pipe-like structures

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o.5-t cm in diameter, up to 20 cm long. This zone has been much disturbed by post-depositional slumping and small faulting of the basal umber. Above, numerous graded tuffaceous beds occur; a pale coarser grained tuffaceous base is succeeded by brown very fine grained, finely laminated umber. Towards the middle of the section is a series of shallow trough-shaped lenses of umber, usually less than i m thick but one is up to 4 m thick and over 25 m wide. Upwards, the dip of these lenticular umbers progressively decreases from c. 2o ° to near horizontal. Cherts occur both as weakly silicified grey-brown umber beds

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FIo. 5 a. Isopachyte map of umbers E. ofPetra, Kalavassos area, Limassol District. Flat-lying urnbcrs fill a hollow in the surface of the 'Mavridhia Agglomerates'. Diagram by courtesy of Mr G. Maliotis, Hcllcnlc Mining Company. 5b, Tongue of pillow lavas extending into disrupted umber; the overlying radiolarianrnudstoncs are updomcd and silicificdto form a 3o cm thick chert along the urnbcr-radiolarian rnudstonc contact, N. of Asgata, Limassol District. 5 c, Simplified geologicalsketch- map of North Mathiati Mine showing the relationshipsof the sulphidcs, ochres, and umbers. Modified after Scarlc 1972.

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(rich in Radiolaria), and as brown splintery and sometimes brecciated vitreous chert. Unlike the intervening lenticular umber, the cherts are mostly laterally continuous; they never occur in the umber lenses although, near the margins of several, the cherts have been highly deformed. They were evidendy disturbed when still plastic, by slumping of the umbers. The uppermost 5 m of the Drapia umbers are highly disturbed; tuffaceous beds have been disrupted and the silicified umbers brecciated. The nature of the Drapia umbers points to very rapid deposition during a period of continuing tectonic instability. The umber lenses were produced by intermittent depositional slumping in response to progressive tilting of a hollow in the surface of the lava breccias. The cherts are likely to represent periods of reduced umber sedimentation which allowed Radiolaria to accumulate. The basal pipe-like structures resemble pipe amygdales and are thought to result from degassing of the lavas after umbers began to accumulate.

(ii) Asgata. Thick lava breccias are overlain, first by pale iron-rich umbers, then by a single bed of massive brecciated hard splintery chert, 0. 9 m thick. Interestingly, the silicification appears to be associated with post-depositional slumping of umbers. The features shown in Fig. 5 b are best explained by local extrusion oflavas into umbers after the radiolarites had begun to accumulate.

(iii) Mavridhia ouaier. Southwest of Mavridhia Mine, clearly demonstrates an intimate association of umbers with the thick lava breccias (Fig. 6). Intercalated umber beds are restricted to the uppermost 3 ° m of the lava breccia. The lowest of these is less than I "5 m thick, discontinuous and confined to hollows in the underlying lava breccias. Above, there is again lava breccia with brown interstitial umber, then more umber, thickest in small fault-generated depressions. In turn, this umber is overlain by more lava breccia; this outcrop is wedge-shaped so that the immediately overlying pink radiolarite rests partly on breccia and partly on umber, forming an angular unconformity. Angular unconformities of this type are numerous along the S. margin of the Troodos Massif, but rare elsewhere. Over a wide area around Kalavassos, the upper Pillow Lavas are often steeply inclined, whereas the overlying sediments are almost flat. In the past, the lava breccias have been interpreted as gradual accumulations from prolonged submarine erosion of pillow lavas, a view which implied that the umbers were deposited long after the final cessation of volcanism, and were themselves erosion products oflavas (Constantinou & Govett 1972). The numerous local angular unconformities were cited as supporting evidence. Here the lava breccias are interpreted as due to violent eruption during the culminating phases of upper Pillow Lava volcanism, the time when umbers first began to accumulate. The angular unconformities are merely an index of numerous and continuing tectonic movements, a persistent feature of the South Troodos transform fault zone. After the area was finally stabilized, the radiolarites and chalks accumulated free of tectonic disturbance.

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UMBERS ASSOCIATED WITH STRATIFORM CUPRIFEROUS SULPHIDES Sulphide deposits of various sizes crop out in the pillow lavas around the margins of the Troodos Massif. Their origins, even their basic stratigraphical setting, have long been controversial. Searle (:973) argued that mineralization occurred solely within the time interval between extrusion of the basal and lower Pillow Lava units, but Pantazis (:973) and many earlier authors (e.g. Bear I96O , Bagnall i96o , Gass I96O) maintained that mineralization occurred much later, during the closing stages of volcanism, contemporaneously with, or at the end of, extrusion of the upper Pillow Lavas. According to this view, the sulphides and umbers were interpreted as elements of a single, continuous volcanic cycle (e.g. Corliss et al. :972, Williams I973, Elderfield et al. :972). The present study strongly supports the views of Constantinou & Govett (I972), Vine (I973) and Maliotis (pers. comm. t973) that all major sulphide mineralization preceded extrusion of the upper Pillow Lavas; formation occurred during the time interval between lower and upper Pillow Lavas, and as such is not directly related to umber deposition. In the past, the main complicating factors have been the obscure stratigraphical relationships of mineralized pillow lavas and the confusion of umbers with

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FIo. 6. Geological sketch map of part of the Mavridhia Outlier. Umbers axe intercalated with thick lava breccias, separated by an unconformity from the overlying radiolarian mudstones and chalks, Kalavassos area, Limassol District. Imet shows area in relation to the whole outlier.

ochres, which cap many of the sulphide orebodies. Critical evidence comes from Mousoulos, North Mathiati, Kokkinoyia and Agrokipia A and B orebodies, where the sulphides are overlain by unmineralized upper Pillow Lavas.

(i) North Mathiati. A large tilted, fault-bounded block of sulphides and ochres is overlain by unmineralized upper Pillow Lavas (Fig. 5c). The ores have been discussed by Searle (i973) and the ochres by Constantinou & Govett (I972). The umbers occur in hollows in the surface of the upper Pillow Lavas which blanket the sulphides (Fig. 5c). Beneath these umbers, the lavas are cut by numerous sub-vertical veins, each several cm thick, which are filled with fine- grained orange iron-rich sediment, which also occurs interstitially to the pillow lavas (see later discussion). In field appearance, the Mathiati umber is in- distinguishable from numerous other deposits in the surrounding area which all overlie the upper Pillow Lavas; all were deposited long after sulphide emplacement.

(ii) Skouriotissa. In general, as at Kambia, the basal umber is bright orange; locally it contains flattened nodules of vitreous chert, also occasional thin beds of pale grey bentonitic clay. Above, the pale Mn-deficient umber is abruptly succeeded by black umber, homogeneous, or only weakly laminated. Upwards, this umber becomes increasingly argillaceous and over a few cm grades into grey bentonitic clay, overlain by Maastrichtian marls. The exceptionally dark colour (analysis reveals up to x9.5% MnO2) and paucity of sedimentary structures distinguishes the Skouriotissa umbers from all other thick umber deposits.

Evidence for the origin of umbers within the pillow lavas

SUB-UMBER VEINS AND BRECCIAS Veining and brecciation of the lavas beneath umbers is ubiquitous, e.g. beneath many of the umber hollows, as in the Margi area, the lavas are cut by numerous anastomosing sub-vertical veins of orange iron-rich sediment, generally calcium carbonate-free (Fig. 7)- There are also numerous veins of calcite and pyrolusite which cross-cutting relationships show to be mostly later in origin. The orange sediment is uniformly fine grained; it resembles the ferruginous basal umbers seen, e.g. at Kambia and Skouriotissa. Although highly ferruginous, relative to umber, the vein sediment is depleted in manganese and most trace elements. Mineralogically, like umber, poorly crystalline geothite and smectite are the only phases normally identifiable. In most areas the veining persists well down into the upper Pillow Lavas. At Margi, several hundred metres stratigraphically below the umbers, the upper Pillow Lavas are intensely veined along parallel zones about 3 ° m wide and several hundred metres apart. The veined and indurated zones stand up as ridges in contrast to the softer lavas in between. Along the ridges the lavas, which are shot through with veins up to 12 cm wide, have locally been strongly brecciated. Some veins contain blebs up to 4 cm across of glassy lava, introduced with the vein sediment. Most veins contain orange iron-rich sediment or calcite; several,

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which are black, are highly manganoan. In contrast, the intervening lavas between the ridges are free from veins except for a few which are sinuous and follow the margins of individual lava pillows. The vertical zones of veined lavas can be traced down into the lower Pillow Lavas, where they progressively narrow to a width of only a few m. Elsewhere, thick columnar-jointed celadonite-rich flows of lower Pillow Lavas are cut by vertical breccia zones which are filled with red highly altered lava fragments scattered through a calcite-rich matrix. In the Maroulena River, S. of Malounda, such breccia zones extend from the upper Pillow Lavas to near the base of the lower Pillow Lavas where they finally disappear.

INTERSTITIAL IRON-RIGH SEDIMENT Fine grained, bright orange, iron-rich sediment of the type already described from the sub-umber veins, is also abundant in the interstices between lava pillows, mostly in the upper zones of the upper Pillow Lavas. This sediment was formed earlier than the vertical ferruginous veins which normally cut the pillow lavas with interstitial sediment. A critical observation is the presence of numerous blebs of glassy lava in the interstitial sediment which proves the deposition of this ferruginous sediment was coeval with the later stages of upper Pillow Lava volcanism, rather than being intruded subsequently. With continuing eruption new lava pillows were violently quenched into a thin superficial cover of previously deposited iron-rich sediment which was then

F xo. 7. Field sketches of sub-umber interstitial iron-rich sediment, veining and brecciation. (A) Normal-sized upper Pillow Lavas (U.P.L.) veined with iron-rich sediment. The interstitial material is spalled-off volcanic glass, uppermost U.P.L., near Margi. (B) Normal-sized U.P.L. as in Fig. 7A which have been cut through by a breccia zone in which locally derived pillow lava fragments (angular) and blebs of glassy lavas ('micro-pillows') are strewn through a matrix of orange iron-rich sediment (white). Uppermost U.P.L. beneath umber, near Kokkinoyia Mine, Nicosia district. (C) Irregularly shaped U.P.L. in a matrix of orange iron-rich sediment (white) and blebs of glass lava (black) spalled-off during eruption. The iron-rich sediment penetrates the pillows along cooling cracks. The glassy chilled margins (thick black lines) are generally confined to the upper pillow surfaces. Uppermost U.P.L., near Margi, Nicosia District. (D) Chaotically arranged, atypically small pillow lavas, with broad chilled margins (thick black line), floating in a matrix of orange iron-rich sediments (stippled), glassy, 'micro-pillows' (black) and occasional fragments of originally larger pillows (angular). Uppermost U.P.L. below umbers, near Kokkinoyia Mine, Nicosia District. (E) Atypically small pillows with chilled margins (thick black lines), near Kato Drhys, Larnaca District. (F) Mavridlfia Agglomerate, broken and abraded large pillows with occasional original chilled margins preserved (thick black lines); small pillows with thin, but well formed chilled margins (rounded outlines); and blebs of glassy 'micro-pillows' (dark spots). These components are strewn through red-brown earthy matrix (clear) with later stage calcite veirfing. Above Mavridhia opencast mine, Kalavassos area, Limassol District.

redeposited; often it was injected into individual pillows along cooling cracks (Fig. 7). This mode of formation is closely analogous to the resedimented inter- pillow limestone described by Garrison (I973). The frequent association of this iron-rich sediment with small-hollow umbers along the top surface of the upper Pillow Lavas suggests that both are genetically related. The inter-pillow sediment probably represents an early stage of deposition

.,.~--. ::': ~T.:, '; ,.;:: /~- :.:

• . ~¢ ~i.,:',~ i ~" .,...':"

•

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of ferruginous 'umber' controlled by geochemical separation of iron from manganese, which resulted from the short time available for sediment deposition during continuing upper Pillow Lava volcanism.

THE SOUTH TROODOS LAVA BRECCIAS Along the S. margin of the Troodos Massif, lava breccias are developed on a large scale associated with unusually thick deposits of umber. These lava breccias have in the past been interpreted as volcanic agglomerates, slump deposits or, most often, as fault breccias (Maliotis, pers. comm. 1973). Here, these breccias are shown to be eruptive agglomerates genetically related to umbers. At their type locality, Mavridhia, an opencast sulphide mine near Kalavassos, the lava breccias, over 150 m thick, consist of rounded and abraded fragments of originally large lava pillows, also of small pillows, mostly IO-I 5 cm in diameter, with intact chilled margins (Fig. 7). This material is set in a red or red-brown matrix, clay-rich fine grained sediment which sometimes contains blebs of glassy or partly devitrified lava, normally less than 5 cm in diameter. At Asgata and Kato Dhrys, complete spectra are visible between intact large pillows, and a chaotic jumble of lava fragments as seen at Mavridhia. Locally the exposed upper surfaces of the lava breccias have been eroded to produce roughly bedded volcaniclastic sediments. Near Trimiklini, in the S. Troodos fault zone, umbers are locally underlain by bedded, in places cross- laminated, medium to coarse grained volcaniclastic sediment up to 4 m thick, of which the basal zones are rich in umber.

LAVA BRECCIAS IN THE ARAKAPAS FAULT ZONE Close to the fault zone the lava breccias become more complex; they are sometimes interbedded with volcaniclastic sediments (never umbers), some of which probably originated as submarine screes derived from local fault scarps (Sim- monian, pers. comm. 1973).

ORIGIN OF THE SUB-UMBER BRECCIAS The South Troodos lava breccias were produced by a violent culminating episode of upper Pillow Lava volcanism associated with powerful and extensive hydrothermal activity. During the closing stages of volcanism, hydrothermal solutions and gases surged upwards disrupting and brecciating freshly erupted upper Pillow Lavas. As a result these lavas were first brecciated, then the fragments were rounded by in situ abrasion of the fragments in contact with each other. The interstitial red-brown clay-rich matrix of the breccias probably represents ground, pulverized and then oxidized material produced by abrasion of the disrupted lava pillows. Overall, the intensity of brecciation and volume of interstitial sediment decreases upwards. Intermittent lava extrusion continued throughout brecciation; pulses of magma fountained upwards into seawater where upon rapid quenching gave rise to the atypically small pillow lavas. The blebs of altered or glassy lava were carried upwards and intruded interstitially by the ascending hydrothermal solutions. At Mavridhia, phases of lava brecciation and fountaining continued intermittently to produce the almost structureless

jumble. Umbers first began to accumulate during pauses in the later stages of this terminating episode of violent volcanism. Locally developed volcaniclastic sediments at the top of the breccias formed by rapid localized erosion of the adjacent lava breccias, which occurred contemporaneously with the earliest umber sedimentation. The Mavridhia breccias vary in thickness over short distances; small hills of lava breccia were rites of later slumping and rapid erosion to produce bedded volcaniclastic sediments.

Discussion of the formation of umber All the field evidence discussed above is consistent with an origin of umbers as chemical precipitates which originated in the widespread activity of submarine thermal springs during the waning stages of upper Pillow Lava volcanism. Large volumes of metalliferous solutions were exhaled into oxidizing seawater, followed by umber precipitation. The higher zones of the upper Pillow Lavas are vesicular, sometimes highly, so the lavas may have been extruded into relatively shallow water, probably less than 400 m deep (cf. Moore 1965, Jones 1969). This means that, to considerable depths in the pillow lavas, confining pressures were insufficient to suppress segregation of volatiles from solution (e.g. for pure NaG1 the critical point is 408 ° and 304 bars-Sourirayan & Kennedy, quoted by Spooner & Fyfe 1973). Hence the driving force behind the ubiquitous veining and brecciation of the lavas may be the upward passage of hydrothermal solutions from a super- to a sub-critical state with violent gas release accompanied by vigorous boiling of brines (Fig. 8). In keeping with studies of active geothermal systems (e.g. in New Zealand,

l UMBER UPPER PILLOW LAVA LOWER PILLOW LAVA BRECCIA ZONES l~//'/I VEINS OF IRON-RICH l~(iJ SEDIMENT&CALCITE ~.[ SUB-UMBER LAVA BRECCIA F.IOO M 50 O r I I lOOM

SCALE D DYKES D D

FIo. 8. Diagrammatic representation of the sub-umber veining and breccia zones beneath small-hollow umber deposits in the iargi area, Nicosia District.

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Brown & Ellis i97o , and Iceland, Elder I965) ferruginous sediments in the veined and brecciated lavas (the interstitial iron-rich sediment) may be attributed to elevation of Eh (less crucially also of pH) as the umber solutions moved upwards, where they are likely to have come into contact with downward moving more oxygenated seawater. The vein-filling calcite appears to have been precipitated later. The umber-bearing solutions were exhaled from numerous widely scattered discharge zones represented by the small-hollow umbers. After an initial relatively violent phase of discharge of gaseous solutions represented by the sub-umber breccias, there was more quiescent exhalation of large volumes of metalliferous solutions, and the umbers were precipitated and then accumulated on the floors of depressions in the pillow lava. The pipe-like structures at Drapia are indicative of continued upward per- colation of solutions and gases after the umbers had begun to form. The most extensive precipitation of umbers, around the flanks of the largest umber hollows, was marked by successive micro-turbiditic influxes of umberiferous sediments and repeated downward slumping of umber into the deepest parts of the hollows (e.g. Lymbia, Arsos, Drapia) ; hydrothermal activity was especially active along fault zones (e.g. Lymbia), even more so along the South Troodos Arakapas Fault zone. At Skouriotissa, where the thick umbers are unusually dark (rich in Mn, Robertson & Hudson I973a ) and lack sedimentary structures, they probably accumulated more slowly than elswhere, without disturbance, in a pre-existing hollow inherited from the block faulting, which followed emplacement of the massive sulphides. This explains the gradual upward transition through argillaceous umber into bentonitic clay which contrasts with the relatively sharp transition seen, for example, at Drapia. The oxidized nature of the umbers implies that, unlike the ochres, conditions of positive Eh were maintained throughout umber deposition. Once precipitated, umbers are likely to have been transported by currents and deposited elsewhere. For example, the umbers at Ayia Marina rest on pillow lavas which lack any signs of the usual sub-umber breccia zone. The unusually thick and extensive nature of the South Troodos lava breccias is problematical. Their general spatial association with the tectonically unstable Arakapas transform fault shows that within this fault zone, late stage hydrothermal activity was much more violent than elsewhere. Seawater, on penetration far down into the lavas, may have come into contact with high level magmas, to produce cataclysmic hydrothermal activity along the transform fault, a major zone of crustal weakness. In origin, these lava breccias may be akin to violent submarine eruptions observed off Hawaii (Steams i965). Wider implications of the lava-umber-sulphide relationships The stratiform cupriferous sulphides are thought to owe their origin to fumarolic exhalations in ealdera-like depressions (Constantinou & Govett I972), interpreted by Spooner & Fyfe (i972) as massive discharge zones of metalliferous solutions, into a highly reducing, oxygen-deficient environment in hollows near the ridge crest. Accordingly, the lower Pillow Lavas, found beneath most

massive sulphide bodies, are interpreted as axial extrusives (Moores & Vine i97I ). Although the upper Pillow Lavas deviate geochemically from samples dredged from ocean floors--they are depleted in many elements (Pearce 1975, Smewing et al. in press)--field relations are consistent with an origin on the block-faulted, but not deeply tiffed flanks, not far from the spreading axis of the Troodos ocean ridge. The often highly vesicular nature of these lavas may imply extrusion in waters shallower than the crests of Recent ocean ridges (cf. Sclater et al. 1971). The umbers, like the sulphides and related ochres, also resulted from ridge hydrothermal activity, as evidenced by breccia zones traversing both lower and upper lavas, but the metalliferous solutions were released into the more oxidising enviroment of the elevated flanks of the ridge. Hydrothermal processes were most intense close to the Arakapas transform fault. Accordingly, axial sulphide emplacement extrusion and metamorphism of both the lower and upper Pillow Lavas, and deposition of umbers may all have occurred simultaneously, related to events in the construction and modification of a submarine lava ridge.

Umbers in space and time. The sediments by far the most closely comparable to umbers, in physio- graphical setting, chemistry and mineralogy, are the metalliferous accumulations in the axial zones and the flanks of actively spreading ocean ridges (Skornyakova 1964, Bonatti & Joensuu I966), especially the East Pacific Rise (e.g. Bostr~m & Peterson 1966 , Bostr6m et al. I969c). Umber-like sediments are also accumulating on Banu Wuhu, an Indonesian andesite-dacite island arc volcano. There, discharge of jets of hot water from the lavas become cloudy and turbid about one metre above the sea floor, as iron and manganese oxyhydroxides are precipitated (Zelenov I964) , exactly as envisaged for umbers. In the caldera of Santorini, late stage volcanism is represented by emission of warm volcanic solutions from which ferric hydroxide-rich metalliferous sediments are precipitated (Bonatti et al. 1973, Puchelt 1963). Unlike these, the Cyprus umbers probably accumulated under a deeper and permanent cover of seawater. The sediments in which oxide phases predominate were all formed in freely circulating oxidizing seawater. In contrast, release of chemically comparable brines into a restricted environment may lead locally to reducing conditions and the formation of sulphides, as is observed along the margins and floor of a bay (Baia di Levante) of the Mediterranean island of Volcano (Honnorez et al. 1973). The highly metalliferous sulphide-rich brines of the Red Sea bottom (Degans & Ross 1969) , in the Afar Depression (Bonatti et al. 1972) and Salton Sea (White et al. 1963) are less comparable to umbers because of their probable association with evaporites and circulating meteoric waters. Unlike umbers they are found in a tectonic setting of early continental rifting. ACS~OWLEDOEM1Z~. For useful discussions I thank Mr M. Mantis, Mr G. Maliotis, Mr C. Xenofontos, Dr G. Constantinou, Dr H. Lapierre, Dr J. E. Pearce, Mr J. Smewing and Mr K. Simmonian. I thank Dr J. D. Hudson, for advice and guidance throughout this work, undertaken at the University of Leicester during the tenure of a N.E.R.C. studentship.

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Received 20 November I974; revised typescript received 22 February I975; read 4 June x975.

ALAS'r~aR H. F. ROBERTSON, Geology Department, Sedgwlck Museum, Downing Street, Cambridge CB2 3EQ.

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