SEG 45 Final_qx4 - Society of Economic Geologists

14 SEG NEWSLETTER No 53 • APRIL 2003
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Hydrothermal Monazite and Xenotime Geochronology (Continued)
depositional age (1863 ± 7 Ma) of the
host graywacke. Older detrital zircons
were also recorded. Another group of
concordant zircon analyses was interpreted
to be of hydrothermal grains,
because their mean 207 Pb/ 206 Pb age of
1817 ± 16 Ma is similar as that
obtained from monazite and xenotime
grains (1810 ± 10 Ma). Interestingly, the
monazite analysed by Compston and
Matthai (1994) has a wide range of
both U concentrations (350 ppm to over
2,500 ppm) and individual ages, which
include older (~1900 Ma) and younger
(1831–1624 Ma) analyses (see discussion
in Şener et al., in press). Despite
this variability, and in the absence of
petrographic control, all phosphate
grains were interpreted by the authors
to belong to the same generation and to
be related to gold mineralization. The
similarity of the dates of the interpreted
hydrothermal zircon, monazite, and
xenotime with that of the Cullen
Batholith (1835 Ma to 1800 Ma; Stuart-
Smith et al., 1993) was used to support
the genetic relationship between gold
mineralization and felsic magmatism.
More recent dating of hydrothermal
monazite (Şener et al., in press) does not
support the results obtained by
Compston and Matthai (1994).
Monazite associated with pyrite and
gold in a quartz vein (Fig. 3B) from the
Goodall gold deposit was analysed in
situ. The Goodall monazite grains are
generally low in U (110–730 ppm), and
the mean 207 Pb/ 206 Pb age of ca. 1.75
Ga, despite a large uncertainty (±40 Ma
due to the low U contents), indicates
that gold mineralization postdates the
Cullen batholith. In the light of these
recent results, Şener et al. (in press) suggest
that the phosphate grains analysed
by Compston and Matthai (1994) could
have included older contact-metamorphic
grains that were incorporated into
the vein during its formation. Contactmetamorphic
monazite and xenotime
from this area have been analyzed by
SHRIMP and have similar mean
207Pb/ 206 Pb ages (1833–1814 Ma;
Rasmussen et al., 2001) to that
obtained by Compston and Matthai
(1994).
Ghana
Gold in Ghana is produced from either
paleoplacer (e.g., Tarkwa) or orogenic
gold deposits, including the giant
Obuasi deposit (e.g., Oberthür et al.,
1994; Allibone et al., 2002). Initially, it
was suggested that the paleoplacer deposits
were produced from weathering
and erosion of the lode gold deposits
(e.g., Kesse, 1985). However, detailed
structural and geochronological studies
negate this (e.g., Allibone et al., 2002).
At the Damang gold deposit, epigenetic,
orogenic-style gold mineralization
clearly overprints low-grade paleoplacer
hematite-magnetite gold occurrences in
Tarkwaian-age conglomerates (Pigois et
al., in press). Hydrothermal xenotime
(Fig. 1E) intergrown with hydrothermal
biotite and Fe carbonate in alteration
halos around auriferous quartz veins in
both conglomerate and basalt has been
dated using SHRIMP. The xenotime date
(2063 ± 9 Ma; Pigois et al., in press) provides
the first precise age constraint for
orogenic gold in Tarkwaian rocks.
IMPLICATIONS FOR EXPLORATION
Monazite and xenotime have been
shown to provide robust, precise geochronometers
applicable to the dating
of orogenic gold deposits. Understanding
the temporal framework for ore-systems
contributes significantly to genetic and
exploration models, and provides a
sound basis for comparative studies
between different ore systems. A precise
age for any mineralization event also
provides a means for direct correlation
with the local and regional geological
and tectonic history. It can form a basis
for identification of critical associations
that can be used in exploration programs
at all scales, and the prospectivity
of terranes, rock sequences, and
structures of different ages can be better
assessed. For example, a consequence of
the lack of temporal relationship
between gold mineralization and felsic
magmatism in the Pine Creek orogen
(Şener et al., in press) is that exploration
should not be restricted to contact
aureoles of granites such as the
Cullen batholith. In other cases, confirmation
of a temporal relationship
between mineralization and a particular
suite of granitoids might provide a
critical exploration indicator.
Chemical classification of phosphate
minerals can also allow correlation of
xenotime of different ages with different
events within the same area. As such,
there is the potential for unravelling,
both temporally and genetically, complex
multistage deposits.
CONCLUSION
Hydrothermal monazite and xenotime
are common accessory minerals in
orogenic gold deposits formed under a
range of temperatures and pressures,
and are commonly intimately associated
with ore or ore-related alteration
minerals. In many places monazite and
xenotime coexist, thus allowing for a
multimineral approach to dating of
mineralization. Such an approach can
confirm the isotopic robustness of either
phase, and provide extra confidence in
the final calculated date. Phosphate
dating complements existing
geochronometers and adds an important
and common mineral group to the
list of datable minerals currently used
to constrain the timing of ore formation
in a variety of settings.
ACKNOWLEDGMENTS
The authors thank S.M. Brown, G.C.
Dawson, G.L. England, C. Grainger, N.
Kositcin, J-P. Pigois, K.C.J. Pires, B.P.
Salier, A.K. Şener, F.H.B. Tallarico, and
D.A. Vallini for access to results prior to
publication. Staff of the CMM at UWA,
Marion Marshall, AMIRA and ARC support
is acknowledged. The manuscript
benefited from detailed and constructive
comments by J.K. Mortensen, J.
Richards, R. Vielreicher, N. White, and
an anonymous reviewer.
REFERENCES
Aleinikoff, J.N., and Grauch, R.I., 1990, U–Pb
geochronologic constraints on the origin of
an unique monazite-xenotime gneiss, New
York: American Journal of Science, v. 290,
p. 522–546.
Allibone, A.H., McCuaig, T.C., Harris, D.,
Etheridge, M., Munroe, S., Byrne, D.,
Amanor, J., and Gyapong, W., 2002,
Structural controls on gold mineralization
at the Ashanti deposit, Obuasi, Ghana, in
Goldfarb R.J., and Nielsen, R.L., eds.,
Integrated methods for discovery: Global
exploration in the twenty-first century:
Society for Economic Geologists Special
Publication 9, p. 65–93.
Brown, S.M., Fletcher, I.R., Stein, H.J., Snee,
L.W., and Groves, D.I., 2002, Geochronological
constraints on pre-, syn- and post-mineralization
events at the Cleo deposit, Eastern
Goldfields province, Western Australia:
Economic Geology, v. 97, p. 541–559.
Bruhn, F., Moeller, A., Sie, S.H., and Henson,
B.J., 1999, U-Th-Pb chemical dating of
monazites using the proton microprobe:
Nuclear Instruments and Methods in
Physics Research, v. B158, p. 616–620.