LETTER PGE fractionation in seafloor hydrothermal systems: examples from mafic- and ultramafic-hosted hydrothermal fields at the slow-spreading Mid-Atlantic Ridge Jan Pašava & Anna Vymazalová & Sven Petersen Received: 16 October 2006 / Accepted: 19 December 2006 / Published online: 4 January 2007 # Springer-Verlag 2007 Abstract The distribution of platinum group elements (PGEs) in massive sulfides and hematite–magnetite±pyrite assemblages from the recently discovered basalt-hosted Turtle Pits hydrothermal field and in massive sulfides from the ultramafic-hosted Logatchev vent field both on the Mid- Atlantic Ridge was studied and compared to that from selected ancient volcanic-hosted massive sulfide (VHMS) deposits. Cu-rich samples from black smoker chimneys of both vent fields are enriched in Pd and Rh (Pd up to 227 ppb and Rh up to 149 ppb) when compared to hematite– magnetite-rich samples from Turtle Pits (Pd up to 10 ppb, Rh up to 1.9 ppb). A significant positive correlation was established between Cu and Rh in sulfide samples from Turtle Pits. PGE chondrite-normalized patterns (with a positive Rh anomaly and Pd and Au enrichment), Pd/Pt and Pd/Au ratios close to global MORB, and high values of Pd/Ir and Pt/Ir ratios indicate mafic source rock and seawater involvement in the hydrothermal system at Turtle Pits. Similarly shaped PGE chondrite-normalized patterns and high values of Pd/Pt and Pd/Ir ratios in Cu-rich sulfides at Logatchev likely reflect a similar mechanism of PGE enrichment but with involvement of ultramafic source rocks. Keywords Platinum-group elements . Massive sulfide mineralization . Turtle Pits . Logatchev . Mid-Atlantic Ridge Introduction Layered igneous complexes are the principal source of platinum group elements (PGEs); however, PGE enrich- ments also occur in the sedimentary marine environment (Crocket et al. 1973; Hodge et al. 1985, 1986; Goldberg et al. 1988). This includes manganese crusts (Clarke et al. 1985; McMurtry et al. 1989; Halbach et al. 1990; Hein et al. 1990b; Stuben et al. 1999), manganese nodules (Haynes et al. 1985; Mannheim 1986; Hein et al. 1990a, Stuben et al. 1999), and volcanogenic seafloor exhalites (Crocket 1990; Cave et al. 2003, Pan and Xie 2001, Pašava et al. 2004). There are only a few studies on the distribution of PGEs in modern seafloor hydrothermal systems, which represent modern analogues for ancient volcanic-hosted massive sulfide (VHMS) deposits. Crocket (1990) determined Au, Pd, and Ir in hydrothermal deposits from the Juan de Fuca and Mid-Atlantic Ridges, where he reported very low Pd values (3.2–3.5 ppb) but rather high Au (up to 18,900 ppb) and Ir values (7.6±1.4 ppb) in two massive Cu–Fe sulfides from the central black smoker complex of the TAG hydrothermal area (26°N). Pašava et al. (2004) described anomalous Pd (up to 356 ppb) and Rh (up to 145 ppb) values in Cu-rich samples from inactive and active chimneys at Satanic Mills (PACMANUS hydrothermal field, eastern Manus basin) and suggested leaching of back-arc volcanic rocks together with the addition of magmatic volatiles into the convecting hydrothermal system as the most important factor for PGE enrichment. Torokhov and Lazareva (2003) reported on the study of ferromanganese crusts and massive sulfides hosted in Miner Deposita (2007) 42:423–431 DOI 10.1007/s00126-006-0122-2 Editorial handling: B. Lehmann J. Pašava (*) : A. Vymazalová Czech Geological Survey, Klárov 131/3, 118 21 Praha 1, Czech Republic e-mail: pasava@cgu.cz S. Petersen Leibniz-Institut für Meereswissenschaften, IFM-GEOMAR, Wischhofstr. 1-3, 24148 Kiel, Germany