The role of source rocks in the formation of Cyprus-type Volcanogenic Massive Sulphide

(VMS) deposits is not fully understood. In this paper we suggest that the formation of epidosites

– episode + quartz ± chlorite ± titanite rocks commonly found at the base of ophiolitic

sheeted dyke complexes – has the potential to release cobalt and zinc into active hydrothermal ore

forming systems. New geochemical and mineralogical data from the sheeted dyke complex of the

Troodos ophiolite indicates that progressive alteration of greenschist facies altered metabasalts to

end-member epidosites results in decreasing base metal concentrations (Zn, Co) in individual dyke

units. We believe this relationship provides the clearest evidence that epidosites act as source rocks

for VMS deposits, and, moreover, the process of epidositisation provides a method of mobilizing

metals such as Co and Ni that are considered generally immobile under typical greenschist grade alteration

Chenery, Simon R.N.

Coogan, Laurence A,

Jenkin, Garwen R.T.

Jowitt, Simon M.

Naden, Jon

English

NERC Open Research Archive

 Submitted for presentation at Society for Geology Applied to Mineral Deposits (SGA) 9th Biennial Meeting,  Dublin, Ireland, August 2007  Epidosites of the Troodos Ophiolite: A direct link between alteration of dykes and release of base metals into ore-forming hydrothermal systems? Simon M Jowitt, Gawen R T Jenkin Department of Geology, University of Leicester, University Road, Leicester, LE1 7RH, UK (smj9@le.ac.uk) Laurence A Coogan School of Earth and Ocean Sciences, University of Victoria, British Columbia, Canada Jon Naden, Simon R N Chenery British Geological Survey, Kingsley Dunham Centre, Keyworth, Nottingham, NG12 5GG, UK  ABSTRACT: The role of source rocks in the formation of Cyprus-type Volcanogenic Massive Sul-phide (VMS) deposits is not fully understood. In this paper we suggest that the formation of epi-dosites – episode + quartz ± chlorite ± titanite rocks commonly found at the base of ophiolitic sheeted dyke complexes – has the potential to release cobalt and zinc into active hydrothermal ore forming systems. New geochemical and mineralogical data from the sheeted dyke complex of theTroodos ophiolite indicates that progressive alteration of greenschist facies altered metabasalts to end-member epidosites results in decreasing base metal concentrations (Zn, Co) in individual dyke units. We believe this relationship provides the clearest evidence that epidosites act as source rocksfor VMS deposits, and, moreover, the process of epidositisation provides a method of mobilizing metals such as Co and Ni that are considered generally immobile under typical greenschist grade al-teration. KEYWORDS: VMS, basalt, hydrothermal alteration, ophiolite, Troodos  1 INTRODUCTION Understanding the source-deposit relationship for Volcanogenic Massive Sulphide (VMS) de-posits is fundamental both for future mineral exploration and to increase our understanding of ocean floor hydrothermal processes and ocean-crust fluxes. Although it has long been known that metals are stripped from the oceanic crust by high-temperature fluids and rede-posited in ore-bodies as the fluids cool, the de-tails of this process are poorly understood. For example, it has been proposed that metal de-pleted “epidosites” (dykes altered to an assem-blage of epidote + quartz ± chlorite ± titanite) are the source of metals in Cyprus-type VMS deposits (e.g. Richardson et al. 1987). How-ever, the movement of base metals within sub-seafloor hydrothermal systems and into ore de-posits has not been directly linked with the formation of these potential source rocks. The movement of metals traditionally considered immobile during sub-seafloor alteration, such as Co and Ni, but also found in ore deposits (e.g. Skouriotissa, ~6 Mt, with 2.5% Cu, 1% Zn, 0.35% Co and 0.1% Ni) also has not satis-factorily been addressed. 2 THE ROLE OF EPIDOSITES IN ORE FORMATION? Epidosites are found in the sheeted dyke com-plex of many supra-subduction zone ophiolites Figure 1. Photomicrograph showing typical end-member epidosite mineralogy (crossed polars). Note the dominance of anhedral quartz and euhedral epi-dote, and the lack of preservation of igneous texture. (e.g. Troodos [this study]), Samail [Nehlig et al. 1994], Josephine [Harper et al. 1995]) in dis-tinct zones up to 1 km or more across dyke strike. Epidosite zones are commonly spatially related to plutonic rocks and comprise a mix-ture of distinct epidosite and metabasalt units. The percentage of individual epidosite units present varies from around 40% to as high as 80%, but is generally around 60% (this study). 100% epidosite unit areas are very rare within lower sheeted dyke unit systems, and the ma-jority of epidosite appears to have selectively replaced entire dyke bodies. However, other distinct types of epidosite (e.g. haloes around quartz veins or intercalating with irregular con-tacts to metabasalt units) are often seen within epidosite zones. Previous workers used the generally depleted nature of these units in some base metals (using Cu and Zn as evidence) when compared to pro-tolith analyses to suggest that epidosites play a fundamental role as source rocks for Cyprus-type mineral deposits (e.g. Richardson et al. 1989). Oxygen isotopic evidence from the Marathasa and Solea valley areas of the Troo-dos ophiolite (Schiffman et al. 1990) also sug-gests that these epidosites were altered at much higher water-rock ratios than the background alteration represented by actinolite-albite-chlorite greenschist facies metabasalts.  Nevertheless, the formation and the role played by epidosites in ore forming systems is still a poorly understood process. This is further complicated by work undertaken by Bickle and co-workers (1998) who suggest, using stron-tium isotope analysis of samples from the Troodos ophiolite, that fluid fluxes which formed the epidosites were similar, if not iden-tical, to the fluid flux present in the background metabasalt. Also, although some metals are de-pleted (primarily Cu) in epidosite zones, other important metals such as Co, which grades up to 0.35% in some Troodos ore deposits, do not seem to be depleted. In contrast Cu appears to be generally depleted throughout the entire sheeted dyke complex. This conflicting evidence has cast doubt on the role of epidosites as source rocks in VMS systems. However, it is generally agreed that epidosite zones are thought to develop from in-termediately altered chlorite-quartz-epidote units and, through further interaction with hydrothermal fluids, progress towards an epi-dote-quartz end-member (Figure 1). This is supported by petrological investigations con-ducted during this study. However, quantifying the release of base metals into hydrothermal Figure 2. Co, Zn and Cu vs MgO for epidosite zone samples. Arrow denotes progressive alteration from transitional epidosite-greenschist facies to end-member epidosite facies. Chlorite percentages (%chl) corre-spond to MgO contents and are taken from semi-quantitative XRD analyses.  Submitted for presentation at Society for Geology Applied to Mineral Deposits (SGA) 9th Biennial Meeting,  Dublin, Ireland, August 2007  systems during progressive alteration of these units has not previously been investigated. 3 METHODOLOGY Mapping and sampling of metabasalts and epi-dosite units from epidosite zones within the Troodos ophiolite was undertaken during 3 separate field seasons. Sampling was on a vari-ety of scales from centimeter scale across indi-vidual epidosite and metabasalt units to km long transects over entire epidosite zones, sam-pling at 30-50 m intervals. A number of sam-ples from background altered metabasalt and fresh rocks were also taken for comparison to epidosite unit samples. In order to quantify chemical and minera-logical changes represented by epidosite facies alteration geochemical analysis was undertaken on samples obtained during these field seasons. Whole rock samples were analysed by XRF for major and trace elements at the University of Leicester and by solution ICP-MS for a selec-tion of trace elements at the British Geological Survey. Mineralogical analysis at the Univer-sity of Leicester was carried out using a combi-nation of thin section description and semi-quantitative XRD analysis using crushed whole rock powders and XRF analytical data.  4 RESULTS Whole rock geochemical analyses of epidosite samples are shown in Figure 2. These plots rep-resent the progressive alteration of epidosite units from transitional chlorite dominated sam-ples (“Least Altered”) to epidosite end-member (epidote-quartz) compositions (“Most Al-tered”).  5 DISCUSSION The new geochemical data presented in this paper indicate a direct relationship between the progressive alteration towards epidosites and the release of Zn and Co into the ore forming hydrothermal fluids (Figure 2). The decrease in modal proportion of chlorite correlates strongly with decreasing Zn and Co – two important metals in Cyprus-type ore deposits. These data are in direct contrast to data from fresh glass protolith analyses which, for example, show in-creasing Zn with decreasing MgO, an inversion of the epidosite relationship. A simple interpretation of these data is that Zn and Co are contained mainly in chlorite within these altered rocks. Alternatively, these elements may be distributed between epidote and chlorite with the correlation being due to progressive leaching of metal and a progressive decrease in modal chlorite, both of which could be attributed to increased fluid flux. These data are in direct contrast to the background green-schist facies alteration, where typical igneous incompatible–compatible relationships are pre-served within the sheeted dyke complex. Figure 3 shows a transect across a single dyke in the background greenschist altered sheeted dyke complex. The preservation of igneous relation-ships for metals such as nickel, cobalt and, somewhat surprisingly, zinc, suggests that they are generally immobile under typical sub-seafloor greenschist facies alteration – as seen in previous work undertaken on this style of al-teration (e.g. Baragar et al. 1990). This would suggest that another form of alteration is needed to mobilize these metals, epidosite for-mation being our favored method. In contrast to these metals, copper concen-trations do not correlate with mineralogy. In-stead, Cu is generally depleted to very low lev-els (<4 ppm) in most epidosite zone dykes compared to metabasalt dyke levels (average ~80 ppm, e.g. Baragar et al. 1990). However, both metabasaltic dykes and epidosites are lo-cally enriched in Cu (epidosites generally up to ~50-100 ppm, background up to 2500 ppm [e.g. Baragar et al. 1990]). This rare enrichment to protolith levels, and occasionally above, is in-Figure 3. Geochemical variation across a singlegreenschist altered dyke. Note the preservation ofincompatible (e.g. Zr) versus compatible (e.g. Ni) re-lationships across this dyke indicating immobilityfor Co, Zn, Zr and Ni. The generally low level ofcopper suggests that this metal, unlike other impor-tant base metals in Cyprus-type ore deposits wasmobilised by the background greenschist alteration.010203040506070800 10 20 30 40 50 60 70Distance (cm)Element (ppm) CoCuNiZnZrterpreted to indicate that Cu was initially mobi-lized during the early stages of hydrothermal al-teration but was locally re-deposited. The local-ized deposition of copper – unlike cobalt or zinc – may also indicate that copper was mobi-lised but not removed from the sheeted dyke complex during initial intrusion and intrusion-related hydrothermal metamorphism. Subse-quent re-mobilisation and removal of copper from the sheeted dyke system via incorporation into the hydrothermal system could then take place during the formation of epidosite units. 6 CONCLUSIONS Epidosite formation plays a key role in the formation of Cyprus-type VMS deposits. The mobilisation of cobalt and zinc by the forma-tion of epidosites is a process that enables the movement of these metals. This process does not occur within typical greenschist facies al-teration encountered in ophiolitic sheeted dyke complexes. This suggests that the only way to form Co- and Zn-bearing Cyprus-type VMS deposits is via the epidosite facies alteration at depth within the sheeted dyke complex. The presence of copper anomalies throughout the sheeted dyke complex also suggests that copper was locally mobile during the background greenschist alteration event and in many cases the copper did not leave the sheeted dyke com-plex. During epidosite formation any back-ground facies dykes with copper anomalies would also undergo epidosite alteration. This, whilst releasing zinc and cobalt into the hydro-thermal system, would also release the copper contained by the anomalous dyke units.  This documentation of a direct link between progressive alteration and the release of base metals confirms the importance of epidosite formation in forming Zn and Co-rich VMS de-posits, whilst also indicating a certain amount of ‘independence’ for Cu within these ore form-ing hydrothermal systems. These data also al-low us to explain the presence of Co in signifi-cant amounts within larger Cyprus-type VMS deposits, and suggests Co and associated ele-ments may have uses in the exploration and testing of VMS prospects and targets.  ACKNOWLEDGEMENTS  SMJ acknowledges funding and support for PhD research from the British Geological Sur-vey University Collaboration Scheme (BUCS) and from the University of Leicester. Fieldwork was funded by a Society of Economic Geolo-gists Student Research Grant, and by the Edgar Pam Fellowship of the Institute of Materials, Minerals and Mining (IoM3), both to SMJ. EMED Mining and the Geological Survey of Cyprus helped with logistics during fieldwork.  REFERENCES  Baragar WRA, Lambert MB, Baglow N, Gib-son IL (1990) The sheeted dyke zone in the Troodos Ophiolite In: Malpas J, Moores EM, Panayiotou A, Xenophontos C (eds) Ophiolites; oceanic crustal analogues; proceedings of the symposium "Troodos 1987". Minist. Agric. and Nat. Resour., Nicosia, Cyprus, pp 37-51.   Bickle MJ, Teagle DAH, Beynon J, Chapman HJ (1998) The structure and controls on fluid-rock interactions in ocean ridge hydrothermal systems; constraints from the Troodos Ophio-lite In: Mills RA, Harrison K (eds) Modern ocean floor processes and the geological record. Geological Society of London, London, United Kingdom, pp 127-152.  Harper GD (1995) Pumpellyosite and prehnitite associated with epidosite in the Josephine ophiolite - Ca metasomatism during upwelling of hydrothermal fluids at a spreading axis In: Schiffman P, Day HW (eds) Low-grade meta-morphism of mafic rocks. 187: 101-123  Nehlig P, Juteau T, Bendel V, Cotten J (1994) The Root Zones of Oceanic Hydrothermal Sys-tems - Constraints from the Samail Ophiolite (Oman). J. Geophys. Res.-Solid Earth 99: 4703-4713.  Richardson CJ, Cann JR, Richards HG, Cowan JG (1987) Metal-depleted root zones of the Troodos ore-forming hydrothermal systems, Cyprus. Earth and Planetary Science Letters 84: 243-253.   Schiffman P, Bettison LA, Smith BM (1990) Mineralogy and geochemistry of epidosites from the Solea Graben, Troodos Ophiolite, Cy-prus In: Malpas J, Moores EM, Panayiotou A, Xenophontos C (eds) Ophiolites; oceanic crustal analogues; proceedings of the sympo-sium "Troodos 1987". Minist. Agric. and Nat. Resour., Nicosia, Cyprus, pp 673-683. 

Epidosites of the Troodos Ophiolite: A direct link between alteration of dykes and release of base metals into ore-forming hydrothermal systems?

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Epidosites of the Troodos Ophiolite: A direct link between alteration of dykes and release of base metals into ore-forming hydrothermal systems?

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