IGEBA   23946
INSTITUTO DE GEOCIENCIAS BASICAS, APLICADAS Y AMBIENTALES DE BUENOS AIRES
Unidad Ejecutora - UE
artículos
Título:
Remanence, self-demagnetization and their ramifications for magnetic modelling of Iron Oxide Copper-Gold deposits: an example from Candelaria, Chile
Autor/es:
AUSTIN, J.; GEUNA, S. E.; CLARK, D.A.; HILLAN, D.
Revista:
JOURNAL OF APPLIED GEOPHYSICS
Editorial:
ELSEVIER SCIENCE BV
Referencias:
Lugar: Amsterdam; Año: 2014 vol. 109 p. 242 - 255
ISSN:
0926-9851
Resumen:
Magnetic modelling can be a powerful tool for
understanding the architecture of numerous types of mineralized systems; e.g.,
iron ore, IOCG and porphyry deposits. In such modelling, the induced component
is generally assumed to be dominant, whereas remanent magnetization is often
neglected and, furthermore, the effects of self-demagnetization are commonly
ignored.We present rock property measurements (magnetic susceptibility and
remanent magnetization) from the Candelaria IOCG deposit in northern Chile. The
results demonstrate that remanence is relatively weak (b20% of induced) and
that the causative lithologies have very high magnetic susceptibilities (3?4 SI),which makes
them highly prone to self-demagnetization. The rock property results were used
to constrain a simplified forward model in which the causative bodies are
modelled as a series of sub-horizontal highly magnetic sheets, corresponding to
?mantos?. These ?mantos? occur north and
south of Candelaria, sub-perpendicular to a splay off the Atacama Fault Zone.
We demonstrate that Candelaria's unusual magnetic anomaly is due to a
combination of its highly magnetic sub-horizontal architecture, and
self-demagnetization effects. A further simplified model was used
to calculate two synthetic anomalies, one ignoring and the other incorporating
the self-demagnetization effect. These synthetic anomalies demonstrate that the
magnetic anomaly amplitude is suppressed by up to approximately 50% at
Candelaria due to selfdemagnetization, and that the induced magnetization is
also slightly rotated from the regional inducing field towards the
plane of the ?mantos?. The dominant paleomagnetic component recorded by the
Candelaria deposit and host rocks is a normal polarity remanence of moderate to
high stability which is interpreted to have been acquired during the
mid-Cretaceous alteration andmineralisation event(s) that generated the
magnetic minerals (predominantly magnetite). However, the presence of a
reversed polarity overprint component in some samples suggests that the
Candelaria deposit and its immediate environs have experienced a post 83 Ma
thermal or thermochemical event that has not been previously recognised. The
remanence directions of both polarities are rotated clockwise with respect to
the expected directions for mid-Cretaceous/Early Tertiary fields, indicating clockwise
rotation of the Candelaria area, including the adjacent batholith, through at
least 45° since the acquisition of the normal and reversed remanence components,
i.e. since 83 Ma. This case study illustrates the importance of understanding
the magnetic behaviour of different ore types, and incorporating
self-demagnetization into modelling procedures for highly magnetic targets in
mineral exploration.