Download Adakites and the Origin of Cu, Au and Mineralisation

IAPETUS
Adakites and the Origin of Cu, Au and Mineralisation
Project reference IAP/13/31. Please quote this reference when applying.
Durham University, Earth Sciences
In partnership with Scottish Universities Environmental Research
Centre
Supervisory Team
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Key Words
Dr Colin Macpherson (Durham University),
[email protected]
Prof Jon Davidson (Durham university)
Prof Adrian Boyce (SUERC)
Dr Chris Dale (Durham University)
Subduction, magmatism, mineralisation, differentiation.
Overview
Chalcophile and siderophile elements, like copper,
gold and indium, provide resources vital to developed
societies and are an important part of the financial
engine for the development of emerging economies.
Many of these elements have a fundamental magmatic
genesis, but can we predict which intrusions/volcanics
are likely to host enrichments of these elements, and,
if so, what makes these host rocks so special? These
are the questions at the heart of this PhD.
Several recent studies have suggested that deposits of
Cu, Au and related metals are linked with formation
of adakitic rocks (Thieblemont et al., 1997; Mungall,
2002; Dreher et al., 2005). Adakites are rocks formed
from intermediate to evolved subduction zone magma
bearing a signature of equilibration between melt and
either garnet and/or amphibole at relatively high
pressure. Where exactly this occurs, however,
remains controversial. Therefore, in addition to
exploring the processes that control the distribution
of chalcophile and siderophile elements in adakitic
rocks, this project will also attempt to shed further
light on the processes that form this type of
magmatism. Case studies will be conducted for rocks
from a variety of locations to compare and contrast
different possible modes of formation. Primary targets
will be rocks from Greece, the Philippines and Chile.
Methodology
Fieldwork will be conducted to collect adakitic
samples from the Aegean, where this style of
magmatism appears to be associated with rapid
extension of arc lithosphere. Although originally
attributed to slab melting, later analysis suggests that
these rocks may, in fact, reflect re-melting of basaltic
rock stored in the crust. New fieldwork and
geochemical analyses will be used to explore this
relationship further. The Aegean also hosts numerous
Au deposits of various ages. Determining the spatial
and temporal relationship between these and adakite
magmatism will be a primary objective of this work.
Fig 1. Adakitic rocks at Oxilithos, Evia, Greece.
Northern Mindanao, in the Eastern Philippines, hosts
substantial Cu and Au mineralisation that
accompanied adakite formation and so provides
another opportunity to explore the geochemical and
spatial relationships between magmatism and
mineralisation. The East Philippine adakitic magmas
are interpreted as forming through crystallisation of
garnet and amphibole from basaltic melt in the
shallow mantle (Macpherson et al., 2006). They are
also contemporaneous with more “normal” arc
magmatic rocks that differentiated at shallower levels
in the arc crust. Therefore, Mindanao provides an
excellent opportunity to test models that advocate a
greater metal endowment of adakitic versus nonadakitic magmatism. The magmatism in Mindanao
occurred in a very young subduction zone and so
offers the chance to explore how both mineralisation
and adakite genesis may be related to the onset of
subduction.
elements during differentiation of the various suites.
Differentiation processes have been studied
previously in the Philippine and Cook Island rocks but
quantitative modelling is required to explore the
conditions under which this occurred. New samples
will be collected during fieldwork in Greece and
major and trace element analyses will be conducted to
constrain their origin. Modelling of petrogenesis in all
three datasets will be conducted in parallel with
incorporation of the siderophile element data
generated by this work to understand their origins.
This will provide a perspective on whether storage
and evolution in the arc crust can influence these
elements in magmas and, in turn, will provide a means
to evaluate if and how the distribution of these
elements varies between different suites of adakitic
magmas generated in the end-member settings.
The Andean Austral Volcanic Zone (AAVZ), southern
Chile, lies above a subducted spreading centre
between the Nazca and Antarctic Plates. Adakitic
magmatism in the AAVZ has been attributed to
melting of the hot crustal rocks associated with the
subducted spreading centre (Stern and Killian 1996). If
this is true then equilibration of magma with garnet
would have occurred in the subducted slab itself.
Cook Island, at the southern end of the AAVZ,
displays an extreme adakitic signal. As a strongly
supported possible example of slab melting, Cook
provides an opportunity to explore an end-member
mode of adakite formation that contrasts with those
proposed for Mindanao and the Aegean. Although
mineralisation is not known close to (the poorly
explored) Cook Island site, adakitic rocks occur at
several localities throughout the Andes and are
frequently located with, or in close vicinity to, a range
of mineralised bodies. The results of this part of the
study will thus have wider application in this region.
The student will, again, use a pre-existing sample suite
from Cook Island to compare with results from
adakite-related ore bodies throughout the Andes.
To understand this range of processes the student will
undertake geochemical studies of the chalcophile and
siderophile elements and of isotope ratios of sulphur,
the main complexing agent for economic metals in arc
magmatic settings. This will require relevant training in
techniques in state-of-the-art geochemical facilities at
Durham University and at the Scottish Universities
Environmental Research Centre (SUERC). The
student will have opportunities to interact with
several major, international, subduction-oriented
research programmes at Durham and with mineral
and metal experts at both SUERC and Durham.
Timeline
Year 1. Develop and analyse databases for Philippines
and Chile. Conduct fieldwork in Greece
Year 2. Conduct geochemical analysis of samples.
Year 3. Continue analytical work and integrate
findings from case studies to develop general model.
Year 4. Complete compilation of thesis.
Training & Skills
The primary objective of this project is to explore the
distribution of chalcophile and siderophile elements in
the different examples of adakitic magmatism. First, it
will be necessary to constrain the behaviour of these
References & Further Reading
Dreher ST, Macpherson CG, Pearson DG, Davidson
JP, 2005. Re-Os isotope studies of Mindanao
adakites: Implications for sources of metals and
melts. Geology 33, 957-960.
Macpherson CG, Dreher ST, Thirwall MF, 2006.
Adakites without slab melting: High pressure
differentiation of island arc magma, Mindanao, the
Philippines. Earth and Planetary Science Letters
243, 581-593.
Mungall JE, 2002. Roasting the mantle: Slab melting
and the genesis of major Au and Au-rich Cu
deposits. Geology 30, 915-918.
Stern CR, Killian R, 1996. Role of the subducted slab,
mantle wedge and continental crust in the
generation of adakites from the Andean Austral
Volcanic Zone. Contributions to Mineralogy and
Petrology 123, 263-281.
Thieblemont D, Stein G, Lescuyer JL, 1997. Epithermal
and porphyry deposits: the adakite connection.
Comptes Rendus Acad. Sci. Ser II-A 325, 103-109.