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Distribution of platinum-group and chalcophile elements in the Aguablanca Ni–Cu sulfide deposit (SW Spain): Evidence from a LA-ICP-MS study

Piña García, Rubén and Gervilla, Fernando and Barnes, Sarah-Jane and Ortega Menor, Lorena and Lunar, R. (2012) Distribution of platinum-group and chalcophile elements in the Aguablanca Ni–Cu sulfide deposit (SW Spain): Evidence from a LA-ICP-MS study. Chemical geology, 302-303 . pp. 61-75. ISSN 0009-2541

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Abstract

The concentrations of platinum-group elements (PGE) and chalcophile elements Ni, Co, Au, Ag, Se, Re, Cd, Bi,
Te and As have been determined by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICPMS)
in base metal sulfide minerals (BMS) from the Aguablanca Ni–Cu deposit, SW Spain. The main aim was to
constrain the role played by the BMS as hosts of PGE as this reveals important information regarding the
processes controlling the distribution of these elements in the deposit. The BMS (pyrrhotite, pentlandite,
chalcopyrite and minor pyrite) occur as semi-massive, disseminated and minor chalcopyrite-veined ores. On
the basis of whole rock metal abundances and BMS mineralogy, these ore types have been interpreted to be
the result of the fractionation and crystallization of an immiscible sulfide liquid.
Platinum-group and chalcophile element concentrations vary as a function of the BMS and ore types. The
partitioning behavior of some of these metals during the fractional crystallization of the sulfide liquid largely
governed their distribution in the ore. Rhenium, Os, Ir, Ru, and Rh occur mostly in solid solution in pyrrhotite
and pentlandite from the semi-massive ore which has been interpreted to represent monosulfide solid
solution (mss) cumulates. The mss crystallization gave rise to minor Cu-rich sulfide liquid in the form of
chalcopyrite veinlets with relatively Pd-, Au- and Ag-enriched chalcopyrite, and minor Re-, IPGE- and Rhdepleted
pyrrhotite and pentlandite. Platinum-group element contents in the BMS from the disseminated ore,
interpreted to represent an original unfractionated sulfide melt, are approximately intermediate to the semimassive
and chalcopyrite-veined ores.
Palladium and Pt occur mostly associated with Bi, Te, and As forming platinum-group minerals (PGM, Pd–Pt
bismuthotellurides and Pt arsenides) within individual BMS grains. This preferential location along with the
textures adopted (usually rounded grains and laths) and the temperatures of crystallization (inferred below
500 °C) suggests that Pd and Pt, initially dissolved in the BMS, were exsolved along with Bi, Te and As to form
the PGM assemblage present in the ore. Some Pd (approximately 30% of the bulk) remains in solid solution in
pentlandite for the three ore types. The presence of Pd in pentlandite is likely a combined effect of limited
sulfide fractionation with some of Pd remaining in mss and Pd diffusion into pentlandite from the mss and Curich
portions on cooling.
Two textural types of pyrite hosting distinct PGE concentrations have been described: (1) large idiomorphic
pyrite and (2) ribbon-like pyrite. Idiomorphic pyrite is the unique BMS hosting Pt (with contents as high as
15 ppm) and also contains relatively high Rh concentrations (4–31 ppm). By contrast, ribbon-like pyrite has
no Pt and hosts similar Os, Ir, Ru and Rh concentrations (30–360 ppb) to those of the host pyrrhotite to that it
replaces. The origin of the idiomorphic grains, whether exsolution products from mss or alteration products of
pyrrhotite, is not well known and further work will be necessary to constrain this point. Nevertheless, the
presence of PGE hosted by pyrite reveals that this sulfide should not be overlooked as a potential carrier of PGE
in Ni–Cu–(PGE) ore deposits.

Item Type:Article
Uncontrolled Keywords:Platinum-group elements; Aguablanca; Laser ablation; Base-metal sulfides; Pyrite
Subjects:Sciences > Geology > Crystallography
ID Code:17055
Deposited On:07 Nov 2012 12:41
Last Modified:07 Nov 2012 12:41

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