Hydrothermal influences on zircon from the Kiruna iron apatite ore district
in the Norrbotten region of
northern Sweden: a comprehensive geochemical study
Anne Westhues1,
John M. Hanchar1, Christopher M.Voisey1, Martinj. Whitehouse2, George R. Rossman3, and Richard Wirth4
The iron
oxide apatite (IOA) deposits of Kiruna are
the type locality for this
kind of deposit, but the ore genesis
is poorly understood and suggested
to be either magmatic or
hydrothermal. Zircon crystals separated from these highly debated iron deposits
located in northern Sweden show
several distinct characteristics compared
to zircon from adjacent metavolcanic and intrusive rocks. Detailed zircon imaging by BSE and CL revealed complex textures in several
samples. Zircon from the iron ores
shows xenocrystic cores that are
overgrown by "spongy",
inclusion-rich rims. In contrast, zircon from the host rocks and a granite
intrusion exhibit typical igneous growth zoning.
Selected zircon crystals were analysed for major and trace elements using EPMA and LA-ICPMS. While zircon grains from the volcanic
host rock and granite are of near stoichiometric composition, zircon crystals from the ore contained
elevated Fe, P, and LREE
concentrations and low analytical totals.
EPMA elemental X-ray maps reveal Ferich
inclusions, veins and/or
zones in zircon crystals from the
ore and syenite. FTIR spectroscopy done on
selected zircon grains from the ore revealed that they contained up to several weight percent of H2O. TEM analyses show no evidence
of micro- or nano- inclusions
of a LREE-rich phase such as monazite
within high REE zircon, suggesting LREE occur within
the zircon structure. Combined with
zircon texture and their water content, it is proposed that these zircon grains
experienced hydrothermal fluid alteration.
The complex
nature of the zircon crystals required analyses with a high spatial resolution: U-Pb data (SIMS) suggest that the metavolcanic host rocks were emplaced and intruded by a syenite (ca. 1884 to
1880 Ma), before the ore was formed (ca. 1877 to 1874
Ma), close to the emplacement of the granite intrusion. In situ oxygen (SIMS)
and Hf (LA-ICPMS) isotopic composition show clear contrasts between ore zircon (δ18O ~7%o, and εHf1 = -5 to +3) on the one hand,
and zircon grains from metavolcanic host rocks and from
intrusions (δ18O -3%o, and εHf1
= -6 to -10) on the other hand. The oxygen isotopic signature suggests the involvement of high-T hydrothermal fluids and the Hf
isotopic compositions indicate a source region for the ore
that is different from its host
rocks. To explain all observations,
a high temperature (-600-700°C) magmatic fluid(s), at the magmatic to hydrothermal transition, seems the most likely heat and fluid source to have remobilized the iron and concentrated it in the massive iron oxide deposits
located at Kiirunavaara and smaller deposits in the vicinity.
Atlantic Geology, 2016, Volume 52
Geological
Association of Canada Newfoundland and Labrador Section
ABSTRACTS -
2016 Spring Technical Meeting doi:
10.4138/atlgeol.2016.005