Petrology and Geochemistry of Mafic-Ultramafic Fragments from the Aguablanca NiCu Ore Breccia , Southwest Spain

Aguablanca (southwest Spain) is the first economic Ni-Cu-(PGE) deposit found in southern Europe. Two features make it an unusual example of magmatic sulfide ore: it is related to the development of an Andean-type continental magmatic arc, and it is hosted by a subvertical magmatic breccia. The structural style and the geodynamic context of the deposit contrast with most plutonic Ni-Cu-PGE deposits elsewhere, which occur at specific levels of layered mafic intrusions in rift environments. The Ni-Cu deposit is hosted by the Aguablanca intrusion, a mafic body composed of gabbronorite and minor quartz-diorite, gabbro, and norite. Sulfides are concentrated in a gabbronorite matrix along a subvertical (dip of 70o–80o N), funnel-like magmatic breccia that contains barren or slightly mineralized ultramafic-mafic cu-mulate fragments. Modal compositions of the fragments reflect a wide variety of rock types, including peri-dotite (hornblende-rich werhlite, dunite, and hornblende-rich harzburgite), pyroxenite (orthoand clinopyrox-enite), gabbro (gabbro, gabbronorite, and hornblende gabbro), and anorthosite. The primary silicate assemblage includes olivine (Fo91–Fo79), orthopyroxene (Mg no. 0.85–0.73), clinopyroxene (Mg no. 0.93–0.62), plagioclase (An99–An38), amphibole (Mg no. 0.87–0.68) and phlogopite (Mg no. 0.89–0.64). The wide range of rock types and the Fe-enrichment trends in the primary ferromagnesian silicates suggest magmatic differenti-ation processes from the parent melts, with the fragments representing different stages of cumulate formation. The ore-bearing breccia contains both semimassive and disseminated sulfides in the gabbronorite matrix. Textures vary between mesoand orthocumulate, and the rock-forming magmatic silicates are orthopyroxene (Mg no. 0.83–0.74), clinopyroxene (Mg no. 0.89– 0.78), plagioclase (An50-An77), and intercumulus amphibole (Mg no. 0.86–0.70), phlogopite (0.84–0.69) and minor quartz. The gabbronorite in the matrix of the breccia is petrographically and chemically very similar to that of the unmineralized parts of the main Aguablanca intru-sion and exhibits a similar differentiation trend, suggesting that the matrix of the ore-bearing breccia and the unmineralized rocks belong to a same magmatic suite. The local presence of mafic-ultramafic fragments in the barren Aguablanca intrusion supports this suggestion. The presence of highly Ni depleted olivine, whole-rock Cu/Zr ratios below 1, and the local occurrence of dis-seminations of magmatic sulfides in the peridotite fragments point to sulfide segregation before and/or during the formation of the peridotite cumulates. Mantlenormalized incompatible trace element patterns of the frag-ments along with published sulfur isotope data are consistent with crustal contamination, suggesting that addi-tion of crustal sulfur from pyrite-bearing black slates led to sulfide saturation. These results support a model in which sulfides segregated and settled during the differentiation of an unexposed mafic-ultramafic complex, now sampled as fragments in the breccia, whereas the overlying silicate magma, most probably fed by succes-sive fresh magma injections, underwent fractional crystallization, giving rise to this cumulate sequence. The emplacement of the ore breccia took place at temperatures above the (monosulfide solid solution (mss) solidus but below the olivine and pyroxene solidus, likely owing to the explosive injection of a new pulse of magma into the chamber, which mingled with the sulfide liquid and disrupted the overlying cumulate sequence. As a consequence, fragments reached their current position in the breccia, injected along with the sulfide and the sili-cate melts, which subsequently formed the sulfide-rich gabbronorite. † Corresponding author: e-mail, rpinagar@geo.ucm. es Hitura, and Kotalahti deposits (Frietsch et al., 1979). Other known examples include Råna in Norway (Boyd and Mathiesen, 1979) and Ivrea-Verbano in Italy (Garuti et al., 1986). Owing to the scarcity of this type of deposit, the discovery of the economic Aguablanca ore deposit in southwest Spain (lat. 37o57' N, long. 6o11' W) caused an important change in the outlook for the supply of Ni and PGE in southwest Europe. The Ni-Cu sulfide mineralization was discovered in 1993 by Presur-Atlantic Copper S.A. during a regional exploration program which revealed an Ni geochemical anomaly related to a gossan (Lunar et al., 1997; Ortega et al., 2000, 2004). The company carried out detailed work to evaluate the economic potential of the mineralization, with satisfactory results, and the project was subsequently acquired by Rio Narcea Gold Mines Limited in 2001. Reserves of 15.7 million tonnes (Mt), grading 0.66 wt percent Ni, 0.46 wt percent Cu, 0.47 g/t platinum group minerals (PGM), and 0.13 g/t Au were established, and the exploitation of the deposit started at the end of 2004. The discovery of this deposit has promoted an extensive and ambitious exploration program in the region which has led to the identification of more than 100 targets with anomalies similar to those of Aguablanca. Sulfide ores are hosted by the Aguablanca intrusion within a subvertical magmatic breccia, mainly composed of gabbronorite rocks containing disseminated and semimassive NiCu-Fe sulfides which include numerous barren or slightly mineralized mafic-ultramafic cumulate fragments. Tornos et al. (2001) suggested that fragments came from a hidden mafic-ultramafic complex containing interbedded sulfides which were brecciated by residual silicate melts. More recently, Ortega et al. (2004) examined the mineralogy and geochemistry of the sulfide ore, defining an assemblage of PGM hosted within sulfides composed mainly of (Pd, Pt)-bismuthotellurides. Ortega et al. (2004) proposed a genetic model for the sulfide ores consisting of magmatic crystallization and subsolidus reequilibration of a sulfide liquid, accompanied by the formation of a magmatic PGM assemblage exsolved from the sulfides. Although the evolution of the sulfide ore has been well defined, the origin and nature of the breccias, their genetic relationship to the sulfide-bearing matrix, and their role in the geologic processes involved in the genesis of the ore have not been studied in detail. In this paper we examine the petrography, mineralogy, mineral chemistry, and bulk rock composition of the fragments and the sulfide-bearing matrix of the breccia in order to better understand the mechanisms of crystal fractionation and sulfide-melt segregation in the source chamber of the deposit and the shallow crustal emplacement of the sulfide melt. Geology of the Aguablanca Intrusion