Antofagasta, Chile
Antofagasta, Chile

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Riveros K.,University of Atacama | Riveros K.,Católica del Norte University | Veloso E.,Católica del Norte University | Campos E.,Católica del Norte University | And 2 more authors.
Mineralium Deposita | Year: 2014

Fluid–rock interaction related to the circulation of hydrothermal fluids can strongly modify the physicochemical properties of wall rocks in porphyry Cu deposits. These processes can also produce compositional and textural changes in ferromagnetic minerals, which can be quantified using magnetic methods. In the Escondida porphyry Cu deposit of northern Chile, each hydrothermally altered lithology is characterized by a discrete assemblage of Fe–Ti oxide minerals. These minerals have distinctive bulk magnetic susceptibility (Kbulk), temperature-dependent magnetic susceptibility, and magnetic hysteresis parameters. Selectively altered rocks (i.e., potassic and chloritic alteration types) exhibit the highest Kbulk values (>3.93×10−3 SI units), and their hysteresis parameters indicate multidomain magnetic mineral behavior. This suggests that these rocks are composed of the coarsest magnetic grain sizes within the deposit. Optical analyses and susceptibility–temperature curves confirm that the magnetic signals in selectively altered rocks are mainly carried by secondary magnetite. In contrast, pervasively altered rocks (i.e., quartz-sericite and argillic alteration types) exhibit low Kbulk values (2589309_ARTICLE_003.xml1.93×10−4 SI units) and contain smaller pseudo-single domain magnetic grain assemblages. This is consistent with the destruction and/or reduction in size of magnetite under acidic conditions. The results therefore demonstrate a genetic relationship between the hydrothermal alteration processes, Fe–Ti oxide minerals, and magnetic properties of the wall rock in the Escondida deposit. These magnetic methods can be considered a sensitive and efficient petrophysical tool for the identification and semiquantification of alteration assemblages, and facilitating the recognition and mapping of discrete hydrothermal zones during exploration and operation of porphyry Cu deposits. © Springer-Verlag Berlin Heidelberg 2014.


Romero B.,Católica del Norte University | Romero B.,Minera Escondida Ltd | Kojima S.,Católica del Norte University | Wong C.,Minera Escondida Ltd | And 3 more authors.
Resource Geology | Year: 2011

Molybdenum is an economically important subproduct of North Chilean porphyry-type deposits, and thus spatial and temporal distribution of molybdenite as the primary Mo-bearing mineral in the Escondida and Escondida Norte deposits were characterized using several mineralogical and chemical techniques and the Re-Os dating method. Molybdenum is distributed extensively in the two deposits, and high molybdenum concentrations (>500 ppm) are recognized particularly in the chlorite-sericite transitional zone between the potassic and sericitic zones. Two modes of occurrence of molybdenite are observed in the Escondida deposit: aggregates with Cu-Fe-sulfide minerals in fine veinlets (sulfide-veinlet type), and monomineralic microveinlets associated with NE-trending faults. The former and the latter yielded ages of 36.1 ± 0.2 Ma and 35.2 ± 0.2 Ma, respectively. Re-Os dating of Escondida Norte molybdenites also show two distinct episodes, at 37.7 ± 0.3 Ma and a younger episode at 36.6 ± 0.2 Ma. These data indicate that the Escondida Norte is older than the main Escondida deposit. The Re-Os age data combined with those of the porphyry emplacement suggest that the molybdenite mineralization in the Escondida district occurred as several short episodic pulses during the late-magmatic to hydrothermal transition, and that the Cu-Mo deposits were formed in a variable overall period spanning 1 to 5 m.y. © 2010 The Authors. Resource Geology © 2010 The Society of Resource Geology.

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