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| Titel |
Mineralized and Barren Tourmaline Breccia at Río Blanco-Los Bronces Copper Deposit, Central Chile |
| VerfasserIn |
Michael Hohf, Thomas Seifert, Lothar Ratschbacher, Osvaldo Rabbia, Joachim Krause, Sabine Haser, Patricio Cuadra |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250089209
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| Publikation (Nr.) |
 EGU/EGU2014-3405.pdf |
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| Zusammenfassung |
| The Río Blanco-Los Bronces porphyry copper-molybdenum cluster (14.8-4.3 Ma) in central
Chile is one of the largest mining districts of the world with more than 200 Mt of contained
Cu; almost 30% of these resources are hosted by hydrothermal breccias. These breccia
complexes are tourmaline-, biotite-, chlorite-, or iron oxide-cemented and are widespread in
the Paleo-Eocene and Mio-Pliocene porphyry Cu-Mo belt of the central Andes. The ongoing
research project aims to understand the time–space relationships between the different
breccia bodies and the multiple porphyry intrusions. For this, two cross sections in the
southern part of the deposit (Sur-Sur and La Americana areas) are studied. Most
interesting from the economic/genetic point of view is the intermineral breccia
(tourmaline- and biotite-cemented), which have high copper grades. It is under debate
whether there is a vertical mineralogical zonation of the cement of the breccia body
from tourmaline-rich at the top to biotite-dominated at the bottom, or there are two
superimposed breccia formation events. Textural and mineralogical observations of
benches- and tunnels-outcrops, drill cores, and polished-thin sections support the first
hypothesis.
Our work has been focused on tourmaline chemistry due to its high resistance to
alteration and weathering, which allows this mineral to retain its original isotopic signature.
Preliminary results of 127 microprobe measurements of tourmaline chemistry from the early
mineralized breccia (BXT) and the late barren one (BXTTO) show that all the tourmalines
belong to the alkali group and the composition ranges between the dravite-schorl end
members.
There is a pronounced negative correlation between Fe (ferric?) and Al, probably due to
exchange at the Z octahedral position. The backscatter images of tourmaline show oscillatory
and sector zonings, i.e., alternating light bands/zones (high CaO, FeO, Na2O) and darker ones
(enriched in Al2O3 and MgO). There is no significant difference between the concentrations
of the major constituent elements (SiO2, Al2O3, FeO, MgO), but a slight enrichment of TiO2
is recognized in the BXTTO.
Previous works attempted to determine the age of the brecciation-mineralization event,
dating sericite and biotite. The ages were interpreted to record thermal resetting of
the K-Ar system by later intrusions. Therefore, we are attempting Ar-Ar dating of
tourmaline (closure temperature ~600°C) albeit it’s low K2O contents (average 0.04 wt.
%).
We will complement the tourmaline major element measurements (EMPA) with minor
and trace elements data (LA-ICPMS) to find some discriminating factor between fertile and
barren crystallization environment. We are also conducting B-isotopic analyses to identify the
source/s of boron in these fluids (magmatic vs non-magmatic -evaporitic- origin). |
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