Hafnium isotopes in zircon: A tracer of fluid-rock interaction during magnetite–apatite (“Kiruna-type”) mineralization Peter M. Valley a, ⁎, Christopher M. Fisher a , John M. Hanchar a , Rebecca Lam b , Michael Tubrett b a Department of Earth Sciences, Memorial University of Newfoundland, St. John's, NL Canada A1B 3X5 b INCO Innovation Centre, MicroAnalysis Facility, Memorial University of Newfoundland, St. John's, NL Canada A1C 5S7 abstract article info Article history: Received 31 August 2009 Received in revised form 23 March 2010 Accepted 7 May 2010 Editor: B. Bourdon Keywords: Magnetite–apatite (“Kiruna-type”) deposits IOCG deposits Kiruna Hf isotopes Zircon REE Trace elements Garnet Fluid alteration Fluorine Adirondack Mountains Lyon Mountain granite Accessory minerals LA-MC-ICPMS LA-ICPMS Apatite Magnetite In situ analyses of hafnium isotopes in zircon combined with U–Pb zircon ages, and trace element data, provide new information on the characterization and evolution of magnetite–apatite (“Kiruna-type”) deposits and the tectonic environments in which they occur. The Lyon Mountain granite in the Adirondack Mountains of New York is the host to numerous zircon-bearing magnetite–apatite deposits. Hafnium isotopic compositions and rare earth element contents in individual zircon crystals were measured in situ in both the host granites and the ore bodies by laser ablation inductively coupled plasma-mass spectrometry. Hafnium isotopic compositions in the ore zircon can be divided into two groups: those that have initial εHf (t) values that are indistinguishable from those of the host granites (e.g., εHf (t) less than +7) and are typical of relatively juvenile Proterozoic crust, and those that have extremely radiogenic εHf (t) values (as high +40). Two models are proposed to explain the observed εHf (t) values in the zircon crystals: 1) early-formed ore bodies containing magnetite, apatite, and clinopyroxene were remobilized by secondary fluid alteration, releasing Zr and Hf for the crystallization of new zircon; or alternatively, 2) fluids responsible for ore formation have interacted with garnet-bearing rocks during retrograde metamorphism, scavenging rare earth elements and radiogenic Hf. Previous work done to determine the U–Pb ages from the same zircon crystals, which were analyzed for the Hf isotopic composition in this study, revealed that ore bodies record a mineralizing event that is 20 to 60 m.y. younger than the age of granite emplacement. This age discrepancy, plus the highly radiogenic εHf (t) values in the ore zircon crystals, suggests that the fluids responsible for this younger event could not have been derived from the granite hosts. These data argue that magnetite–apatite deposits in the LMG have multiple mineralizing events superimposed upon one another and that early-formed deposits may be reworked, modified and redeposited by fluids subsequent to magma crystallization. © 2010 Elsevier B.V. All rights reserved. 1. Introduction The Hf isotopic composition of zircon combined with U–Pb age data and trace element data from the same region in the crystal provides an unparalleled way to measure the Hf isotopic composition of the fluid or melt from which the zircon crystallized, in a temporal context (Kemp et al., 2005; Hawkesworth and Kemp, 2006). The ability of zircon to record the Hf isotopic composition of the fluid or melt in which it crystallized is due to the refractory nature of zircon, as well as low Lu/Hf (e.g., Lu is a trace element whereas Hf is a minor element and 176 Lu/ 177 Hf are typically less than 0.002; Kinny and Maas, 2003) intrinsic to zircon, resulting in minimal growth of 176 Hf (from the decay of 176 Lu) through time. Additionally, the Lu/Hf offers a proxy for the heavy rare earth element (HREE) to high field strength element (HFSE) ratio in source materials, which varies significantly between minerals and rock types. This means that Lu–Hf isotopic systematics in zircon can potentially be a more sensitive indicator of the source region of melts and fluids than isotopic systems such as Sm/Nd that not only combine two geochemically similar elements (Patchett et al., 1981) but also occur in much lower abundances than Lu or Hf. Typical crustal rocks have 176 Lu/ 177 Hf of ∼ 0.01 (granite) to ∼ 0.03 (basalt); however these averages represent a composite Lu/Hf of their constituent minerals, many of which strongly fractionate Lu from Hf. For example, garnet has high Lu/Hf (0.1 to 8.0, (e.g. Scherer et al., 1997; Duchéne et al., 1997) resulting in very radiogenic 176 Hf/ 177 Hf over geologically short periods of time, but relatively low total Hf concentration (b 0.01 wt.%, e.g., Hickmott et al., 1987; Sisson and Bacon, 1992). Conversely, zircon has extremely low Lu/Hf with high Hf contents (typically between 1– 4 wt.% HfO 2 ; e.g., Hoskin and Schaltegger, 2003). This range of Lu/Hf Chemical Geology 275 (2010) 208–220 ⁎ Corresponding author. E-mail address: pvalley@mun.ca (P.M. Valley). 0009-2541/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.chemgeo.2010.05.011 Contents lists available at ScienceDirect Chemical Geology journal homepage: www.elsevier.com/locate/chemgeo