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Mineralium Deposita

, Volume 45, Issue 1, pp 23–41 | Cite as

Magnetic polarity zonation within the El Teniente copper–molybdenum porphyry deposit, central Chile

  • Natalia Astudillo
  • Pierrick RoperchEmail author
  • Brian Townley
  • Cesar Arriagada
  • Annick Chauvin
Article

Abstract

El Teniente porphyry copper deposit, the world’s greatest intrusion-related Cu–Mo ore body, is hosted within basaltic–andesitic volcanic and gabbroic rocks (mafic complex). This ore body is strongly affected by multiple events of alteration/mineralization with pervasive potassic and chloritic alteration and coetaneous with associated copper mineralization. We present paleomagnetic results obtained from oriented samples at four locations within the mine and from two drill cores, 200 and 400 m long, respectively. Samples are representative of all the main hydrothermally altered rock units, with emphasis on the mafic host rock and dacitic (Teniente dacite porphyry) and dioritic porphyry intrusions. Magnetic experiments [hysteresis loop, isothermal remanent magnetization (IRM), kT curves, thermal, and alternating field demagnetization] show the presence of prevailing magnetite. Microscope and SEM observations show two families of magnetite, (a) large multidomain magnetite grains, associated with biotite and chlorite of various different hydrothermal alteration events, and (b) abundant small to medium grain-size magnetite (<10 μm) contained within plagioclase, either related to an early Na–Ca–Fe alteration or included within plagioclase during magmatic crystal growth. While the Teniente dacite porphyry and the quartz diorite–tonalite have low magnetic susceptibility (<0.0005 SI) and low natural remanent magnetization (NRM, 10−4–10−3 Am−1), the mineralized mafic host rocks have usually high susceptibility (>0.01 and up to 0.2 SI) with NRM in the range 0.1–2 Am−1. Most mafic complex rock samples have univectorial magnetizations during alternating field or thermal demagnetization. Within the mine, the magnetic polarity is spatially distributed. In the northern part of the deposit, the Teniente dacite porphyry, the associated hydrothermal breccias, and the hosting mafic complex record a reverse polarity magnetization, also observed in the El Teniente sub-6 mine sector immediately to the east and southeast. In the eastern part of the deposit, a normal polarity is observed for samples of the mafic complex from the two long drill cores. There is no evidence for superimposed magnetizations of opposite polarities in samples of the mafic complex. Anhysteretic remanent magnetization (ARM) in a DC field of 40 μT and NRM have similar magnitude and comparable behavior upon alternating field demagnetization. The well-defined strong remanent magnetizations associated with high unblocking temperatures (>500°C) indicate an acquisition of remanent magnetization during mineralization by circulating high temperature fluids related with ore deposition. Paleomagnetic results and the recorded polarity zonation suggest multiple mineralization events occurred at El Teniente, each one with its own evolution stages, superimposed within the district. These results indicate that a simplified broad four-stage model for El Teniente, as presented and overly employed by many authors, divided in (1) late magmatic, (2) main hydrothermal, (3) late hydrothermal, and (4) posthumous stage, does not recognize various short-lived single mineralization events, some superimposed and some distinctly separated in time and space. There is no paleomagnetic evidence for post-mineralization deformation

Keywords

Paleomagnetism Magnetic polarity El Teniente Porphyry copper Chile 

Notes

Acknowledgments

We would like to thank the “Superintendencia de Geologia El Teniente” for their logistical support for sampling within the mine. Ludovina Burgos and other geologists from El Teniente are thanked for assistance for sampling the drill cores and discussion about the El Teniente geology. Reviews by F. Tornos, A. Rapalini, and an anonymous reviewer significantly improved the manuscript. We thank Andres Tassara and Miguel Faundez for their help during the initial stages of this project. Original work was financed by project DID-I009-99/2, University of Chile, and IRD, France.

Supplementary material

126_2009_256_MOESM1_ESM.pdf (256 kb)
ESM Figure 1 Detailed sampling maps (location in Fig. 2). Each star corresponds to an oriented block. The scale is given by the grid in mine coordinate (in mts) (PDF 255 kb).
126_2009_256_MOESM2_ESM.pdf (136 kb)
ESM Figure 2 Magnetic susceptibility versus temperature experiments (K-T). Pure magnetite (580°C) is the main mineral identified during K-T experiments (a,b,c,d). A second phase (400°C) is observed in some samples (b,d) upon heating but not during cooling. Thick (thin) lines correspond to heating (cooling) curves. Arbitrary units for the susceptibility not corrected for mass or volume. (e,f) K-T experiments for samples of the Teniente dacite porphyry. Upon heating above 400°C, there is a progressive increase of magnetic susceptibility due to mineralogical changes and formation of magnetite. (g,h,i) Variation of magnetic susceptibility measured at room temperature after each step of thermal demagnetization in air. (g,h) Samples from the mafic complex from drill cores SG-184 and SG-185, respectively. (i) Samples from the Central quartz diorite–tonalite stock in drill core SG-185 (PDF 135 kb).
126_2009_256_MOESM3_ESM.pdf (149 kb)
ESM Figure 3 (a) Back-field IRM experiments showing Hcr values in between 10 and 50 mT. Multidomain magnetite is the dominant magnetic phase in most samples with high magnetic susceptibility as shown by an example of hysteresis curve (b) and the Day plot (c) (PDF 149 kb).
126_2009_256_MOESM4_ESM.pdf (46 kb)
ESM Figure 4 Orthogonal plots of thermal and AF demagnetizations. Sample 03DT1601A: Teniente dacite porphyry; sample 00ETM1602A: CMET from the sub-6mine sector; sample 00ETE2302A: CMET from the Esmeralda mine sector. Open (filled) circles are projections in the vertical (horizontal) plane (PDF 45 kb).
126_2009_256_MOESM5_ESM.pdf (111 kb)
ESM Figure 5 Examples of orthogonal plots of thermal (a, b, d) and AF (c, e) demagnetization of samples from drill cores SG184 and SG185. Open (filled) circles are projections in the vertical (horizontal) plane. f) Variation of intensity of magnetization during thermal demagnetization for samples from the mafic complex in drill cores SG184 and SG185 showing the sharp unblocking temperature spectra above 500°C (PDF 111 kb).
126_2009_256_MOESM6_ESM.pdf (58 kb)
ESM Figure 6 Examples of orthogonal plots of thermal and AF demagnetization of samples from the sites outside the mine. Open (filled) circles are projections in the vertical (horizontal) plane (PDF 57 kb).

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Copyright information

© Springer-Verlag 2009

Authors and Affiliations

  • Natalia Astudillo
    • 1
    • 2
  • Pierrick Roperch
    • 2
    • 3
    Email author
  • Brian Townley
    • 1
  • Cesar Arriagada
    • 1
  • Annick Chauvin
    • 3
  1. 1.Departamento de Geología, Facultad de Ciencias Físicas y MatemáticasUniversidad de ChileSantiagoChile
  2. 2.LMTG, IRDUniversité Paul Sabatier Toulouse IIIToulouseFrance
  3. 3.Géosciences RennesUniversité de Rennes 1RennesFrance

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