The Miocene Brahma porphyry Cu-Mo prospect in Central Chilean Andes (35°45′S): Geology, geochronology (U-Pb, Re-Os) and geochemistry

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Highlights

  • Brahma is a Miocene porphyry Cu-Mo prospect located in the Central Chilean Andes.

  • U-Pb and Re-Os geochronology indicates multiple magmatic-hydrothermal events.

  • Geochemistry of igneous units indicates a complex deep feeding magmatic reservoir.

  • Southerly expansion of the Mio-Pliocene Metallogenic Belt of Central Chile.

Abstract

The Brahma prospect is a Neogene porphyry copper deposit located in the Chilean Andes at ~ 35°45′S, beyond the southerly extension of the previously defined Mio-Pliocene metallogenic belt of Central Chile. We present the results of a comprehensive study of this prospect based on drill hole logging, petrography, geochemistry, and geochronology, with the aim of constraining its paragenesis and formation within the metallogenic background of the region. The prospect is hosted by dioritic and granodioritic equigranular units mineralized by fluids from at least two hydrothermal pulses in turn related to the emplacement of porphyritic intrusives and hydrothermal breccias. Existing exploration drill holes only intercepted the phyllic and argillic zone of the system, consistent with the paucity of potassic alteration and the dominance of pyrite, sphalerite, chalcopyrite and molybdenite mineralization. Zircon U-Pb determinations of the main igneous units present yielded ages of ~ 17 Ma, 14 Ma, and 9 Ma. Mineralization ages of ~ 14 Ma, 12 Ma, and 10 Ma were obtained by Re-Os determinations on molybdenite from samples of different paragenetic stages. These data indicate that (i) the prospect is composed of at least three main magmatic events, (ii) early mineralization is associated with the ~ 14 Ma magmatic pulse, and (iii) economic mineralization occurred at ~ 10 Ma and lasted until at least 9 Ma, as indicated by the presence of a younger mineralized porphyritic unit of this age. The obtained ages are comparable to those reported for the Los Pelambres deposit, a Cu-Mo porphyry located at 31°43′S and belonging to the Mio-Pliocene Metallogenic Belt of Central Chile. For the main intrusive units recognized in the Brahma prospect, whole rock geochemical analyses reveal arc affinities, juvenile signatures, and the involvement of amphibole in the fractionating assemblages. In a whole, our results support the existence of a complex deep magmatic reservoir that fed the magmatic-hydrothermal system for a long period, of at least 4 to 5 m.y., a common characteristic in other Miocene-Pliocene deposits in Chile and also in porphyry Cu deposits worldwide. In addition, our results highlight an interesting exploratory potential for the Brahma prospect, a long-lived Cu-Mo porphyry deposit of early Miocene age which southerly expands until ~ 36°S the Mio-Pliocene Metallogenic Belt of Central Chile.

Introduction

The Chilean Andes margin corresponds to one of the most richly endowed copper provinces in the world and most of its resources are from Porphyry Cu (-Mo, -Au) deposits. These include the largest ones discovered up to date in the world, as Chuquicamata (22°16′S), Río Blanco-Los Bronces (33°12′S), and El Teniente (34°05′S), along with numerous other comparatively smaller deposits currently under exploitation (Fig. 1a; e.g. Camus, 2003, Maksaev et al., 2007). Such characteristic has historically made the Chilean margin particularly attractive for exploration oriented to the discovery of porphyry Cu deposits. Given the vigorous and long-standing development of such activities, currently there seems to be no exposed resources left, which has led to both: (i) a change in the exploration techniques towards a more penetrative and integral approach and (ii) the search for new target areas among those previously disregarded as highly fertile.

Porphyry Cu deposits correspond to large volumes of hydrothermally altered rocks centered on porphyry stocks associated to late volcano-plutonic arc activity (e.g. Richards, 2003, Richards, 2005, Sillitoe, 2010). They are inferred to result from the favorable confluence of a series of specific geological factors and/or processes, which operating altogether, prompt their formation (e.g. Richards, 2003, Richards, 2005, Sillitoe, 2010). Throughout its evolution, regional scale formation of porphyry deposits has occurred repeatedly along the Chilean Andes margin, but during relatively short time spans and at specific moments during the lifetime of the coeval hosting magmatic arc (for reviews see Camus, 2003, Maksaev et al., 2007). As resulting from such specific episodes, porphyry deposits in the area have been classically grouped in metallogenic belts that distribute along strike of the Chilean margin (for reviews see Camus, 2003, Maksaev et al., 2007). The youngest of these corresponds to the Miocene-Pliocene metallogenic belt, which hosts highly endowed Cu-Mo porphyry deposits between ~ 32° and 34°S and Au-rich porphyry and epithermal deposits further north (Fig. 1a; e.g. Camus, 2003, Maksaev et al., 2007). The southern termination of this belt is marked by the El Teniente mega-deposit (34°05′S); however, the push to explore in previously underestimated areas has attracted interest in the extension of this metallogenic belt in the region further south. In such region, several prospects have been found (Fig. 1a), but they remain under early exploration activities and their full metallogenic and economic potential remains to be determined. One of these prospects and the focus of the present study corresponds to the Miocene Brahma porphyry Cu-Mo prospect, located in the Andean main range in the Maule region at ~ 36°S (Fig. 1).

The Brahma prospect is located on the western slope of the Chilean Andes, at 35°46′S, in the southern shore of the Maule river, and 200 km south-southwest of the El Teniente Cu-Mo mega-deposit (Fig. 1a). In this contribution we present the results of a comprehensive study of this prospect based on drill hole logging, petrography, U-Pb and Re-Os age determinations, Lu-Hf isotopic compositions and whole-rock geochemical analyses. Our results indicate that this corresponds to a Miocene porphyry Cu-type mineralization event, whose economic potential is yet to be determined, therefore suggesting an important southerly extension of the Chilean Miocene belt.

Section snippets

Tectonic setting and geology of the study area

The Chilean Andes runs along an active continental margin corresponding with the eastward subduction of the Nazca oceanic plate beneath the South American plate (Fig. 1a). Such a configuration has been continuous, but not steady, at least since the early Jurassic and is the first order control on the geological evolution of the upper continental plate (e.g. Charrier et al., 2007, Charrier et al., 2015). The area where the Brahma prospect is located corresponds to the Southern Central Andes,

Sampling and analytical procedures

All samples collected for geochronology in this study were taken from drill cores recovered from boreholes BRA002, BRA003, BRA004 and BRA006, these represented on the cross-section A-A' (Fig. 2). In addition, samples from the BRA005 and BRA007 boreholes were also collected for geochemical analyses (Fig. 2a).

Geology of the deposit

The Brahma prospect covers an area of around 62 km2 and was discovered through a geochemical stream sediment study. There are only a few previous studies performed on this prospect, all of which are exploration technical reports (Culbert, 2007, Burns et al., 2008, Sanchez and Motta, 2013). In this study a lithologic and alteration cross-section was constructed from observations made during detailed logging of drill cores from four boreholes (BRA002, BRA003, BRA004, and BRA006) supplemented by

Main geological features of the Brahma prospect

The Brahma hydrothermal system covers an area of 6 km2 and is open laterally and at depth (Burns et al., 2008; Fig. 2). Outcrops and drill cores from exploration boreholes show the presence of Cu, Mo, Zn and Pb mineralization hosted in intrusive porphyritic and equigranular rocks, and hydrothermal breccias. The feldspar-destructive alteration assemblages (sericitic and sericite-chlorite, as defined in Sillitoe, 2010, phyllic as defined in Corbett and Leach, 1998) are best developed within the

Summary and conclusions

(1) The Brahma prospect corresponds to a porphyry type deposit in which mineralization is hosted in and is related to intrusive rocks. The main intrusive units that comprise it correspond to: Granodiorite (GRD), Feldspar and Rounded Hornblende Porphyry (FRHP), Tonalite (TON) and Feldspar and Hornblende Porphyry (FHP). In addition, related to the emplacement of the latter intrusives there are numerous syn-mineralization igneous and hydrothermal breccias that are mostly polymict with rock flour

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was supported by CONICYT (Comisión Nacional de Investigación Científica y Tecnológica, Chile) through the projects Fondecyt 1161360 and 11140012. Additional support from the CONICYT projects PAI79160139 and AFB180004, the Advanced Mining Technology Center (AMTC) and the Unidad de Vinculación del Departamento de Postgrado – Universidad de Chile is gratefully acknowledged. We specially thank Patrick J. Burns and Miguel Peral of Southern Cross Minerals S.A for allowing access to the

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