Reversely discordant data have only rarely been observed from zircon. Lack of equilibration be-
tween spike and sample can produce spurious reversely discordant data, but this is unlikely to be
the case with analysis 27 because its
207
Pb
206
Pb age is older than those of the near concordant
data and the three data define a statistically con- sistent regression with a fairly old lower concordia
intercept age. The reversely discordant grain showed a relatively high U concentration, and
lower concordia intercepts near 1000 Ma are typi- cal for high U zircons from mafic rocks, probably
because they begin to accumulate radiation dam- age-induced Pb loss earlier than low U zircons. It
appears that this may be a real case where U loss from zircon exceeded Pb loss. Unfortunately, the
few remaining zircon grains from this sample are quite altered and, since they probably underwent
pronounced secondary Pb loss, they are likely to give complex discordia that would not clarify the
cause of the reverse discordance.
6. Discussion
6
.
1
. Ages of magmatism and deformation The age of one metavolcanic rock in the Omai
area is defined by this work to be 2120 9 2 Ma Fig. 3. The most precise age obtained for
metavolcanics in the Guyana Shield before this study was on the Pastora Group in Venezuela,
less than 500 km from the Omai location Day et al., 1995. This unit gave an age of 2131 9 10 Ma,
in agreement with the metavolcanic age from Omai. This suggests that greenstone belts in the
Guyana Shield may have formed over a short time span despite having been derived from differ-
ent volcanic centres. However, U-Pb dating by Gibbs and Olszewski 1982 on the Barama-
Mazaruni assemblage and Sm-Nd work by Gruau et al. 1985 on the Paramaca series in French
Guyana defined ages for metavolcanics of 2250 9 106 and 2210 9 90 Ma, respectively. These ages
are significantly older than in this work although their large errors make it difficult to define the
total age span of volcanism. The five samples obtained from the Omai plu-
ton are representative of all the main intrusive phases. The discordance of much of the zircon
data results in relatively large errors 6 – 10 Ma. However, the agreement of zircon results from
three samples 2094 9 6, 2092 9 11 and 2096 + 11 −
10 Ma indicates that the ages obtained are probably reliable indicators of primary crystalliza-
tion. The igneous titanite and apatite data give an average
207
Pb
206
Pb age of 2094 9 1 Ma Fig. 5: 29 probability of fit, corrected using the most
primitive feldspar datum. This age agrees well with the less precise zircon ages and is probably
the best estimate for crystallization of the pluton, particularly because textural evidence indicates
that the titanite grew as part of the magmatic assemblage. The only known similar U-Pb age for
a granitoid in the Guyana Shield is a 2087 9 21 Ma zircon age from a granite in a granite – green-
stone terrane south of El Dorado, Venezuela P. Klipfel, Placer Dome Exploration, personal com-
munication, November 4, 1998; Table 1.
The ages obtained in this study for the metavol- canic sample 2120 9 2 Ma and the intrusion
2094 9 1 Ma fall within the accepted period of the Trans-Amazonian tectonothermal episode
1900 – 2200 Ma. The metavolcanic units were affected by one or more of the Trans-Amazonian
deformational episodes. The Omai pluton does not show the deformation seen in the surrounding
metavolcanic units and clearly cross-cuts their steep tilting and ductile strain fabric. The contact
of the pluton margin with the metavolcanics is equally undeformed, so the pluton must have
been intruded as a member of the Younger Gran- ite group of intrusions after the end of the last
Trans-Amazonian ductile deformational episode. Thus, the ages obtained for the metavolcanic unit
and the intrusion bracket the time period for Trans-Amazonian deformation in this region of
Central Guyana to 26 9 2 Ma.
6
.
2
. Pb loss from zircon The zircon analyses from the metavolcanic sam-
ple are concordant despite the fact that the sample was taken very close to the permeable saprolite
zone. This indicates that near-surface water circu-
lation does not necessarily result in Pb-loss due to leaching. The zircons from the metavolcanic sam-
ple are fresher than those from the plutonic sam- ples, which are generally cracked, cloudy, and
small. The pluton-derived zircons may be more zoned in U than the metavolcanic zircons. Radia-
tion damage in high U zones causes an increase in crystal volume, cracking the crystals and allowing
penetration of water and subsequent alteration Krogh and Davis, 1974, 1975. In contrast, the
lack of cracks in the metavolcanic zircons may have prevented water from entering and altering
the crystals thus reducing Pb loss from the interi- ors of these grains.
The ca. 500 Ma lower concordia intercept age from OM95-1 probably records an average of
multiple or prolonged ancient Pb loss events. Its ambiguous upper intercept age interpretation is
probably due to the fact that data from different grains lie on discordia with slightly different lower
intercepts. In contrast, regressions of zircon analyses from two other Omai intrusion samples,
OM95-11 and OM95-28, have near-zero age lower concordia intercepts. The near-zero lower inter-
cept age from these samples indicates that, while they may have undergone ancient alteration and
Pb-loss as shown in sample OM95-1, Holocene tropical weathering may have caused extensive
leaching of Pb from altered zones in the zircons, resetting them to give consistent primary
207
Pb
206
Pb ages, despite increasing the discordance of their data. Thus, paradoxically, the influence of
recent weathering may have improved recovery of primary age information by erasing any previous
history of complex lead loss. If so, laboratory hydrothermal leaching of altered domains could
potentially be used to achieve the same effect.
6
.
3
. Hydrothermal mineral ages Data on suspected hydrothermal mineral frac-
tions from strongly altered samples, with the ex- ception of one colourless titanite fraction that
may have contained some of the magmatic phase analysis 7, regress within error of an isochron on
a Pb-Pb diagram Fig. 4 and define an age of 2002 9 5 Ma 29 probability of fit. This age
agrees with a less precise Sm-Nd age of 1995 9 140 Ma on scheelite from gold-quartz veins at the
Omai mine Voicu et al., 1997. The thermal closure temperature for titanite is in excess of
500°C Mezger et al. 1991, whereas that of rutile is about 400°C Mezger et al. 1989. Therefore,
the relatively young age defined by these minerals is unlikely to be due to diffusional Pb loss during
regional metamorphism, which did not go above greenschist facies.
Although the precision of the isochron age may not reflect its accuracy, the relative radiogenicity
of the titanites and the apparent agreement of their Pb-Pb systematics with a variety of other
minerals indicates that some hydrothermal event affected the rocks substantially later ca. 100 mil-
lion years than intrusion of the Omai pluton and the end of ductile deformation at 2094 9 1 Ma.
This event is clearly much older than emplace- ment of the mafic dike at 1794 9 4 Ma. The dike
age shows that the Avanavero Suite is probably related to a thermal event that occurred long after
the Trans-Amazonian orogeny.
If the 2002 9 5 Ma age dates gold, then miner- alization would be unrelated to local igneous
events. Such a case has been argued for anoma- lously young titanite, rutile, muscovite and
scheelite ages in Superior province gold deposits e.g. Jemielita et al., 1990; Wong et al., 1991;
Haynes et al., 1992; Anglin et al., 1996. However, the interpretation of these ages is controversial
e.g. Kerrich, 1994. An example of a greenstone- hosted Au-quartz vein system where rutile gave
ages younger than the estimated age of mineral- ization is the Kerr Addison-Chesterville system in
the Superior province. At Kerr Addison, rutile in dykes gave relatively young ages of 2630 – 2580
Ma, but mutual cross-cutting relationships be- tween the veins that host the mineralization and a
system of ‘albitite’ dykes that contain the rutile indicate that mineralization at that time was not
possible, as the age of the dykes is unlikely to be younger than 2670 Ma Spooner and Barrie,
1993.
Although it can be shown that the wall rock rutile at Omai formed only in close proximity to
mineralized veins and as fine fibres in auriferous vein quartz Rust, 1996, it cannot be ruled out
that it was subsequently recrystallized. In this
case, the age defined by the rutile would reflect the time of recrystallization and be a minimum esti-
mate for gold. Fluid inclusion data from mineral- ized quartz veins at Omai indicate that the
mineralizing solutions were of a H
2
OCO
2
, low to moderate salinity composition Rust, 1996. Flu-
ids of this type generally do not form titanite at moderate temperatures since any Ca
2 +
in the system would be used by the excess CO
2
in solu- tion to form carbonate minerals, with the Ti
4 +
being left to form rutile e.g. Clark et al., 1986. Thus the secondary titanite, whose data largely
control the age interpretation, could not have formed from the same fluids as the gold-associ-
ated rutile. This suggests that the secondary titan- ite was formed by an overprint that recrystallized
the rutile. This hydrothermal event appears not to have affected all of the Omai samples. For exam-
ple, OM95-4 apparently contains only one genera- tion of colourless titanite, which gave an igneous
age. A fluid flow episode would be most likely to generate the variable effects seen in the Omai
rocks. In any case, the data constrain the age of gold mineralization to between 2002 9 5 and
2094 9 1 Ma the intrusion age of the Omai plu- ton. The question of how the late hydrothermal
event might fit into the context of Trans-Amazo- nian crustal development is discussed below.
6
.
4
. Correlations with West Africa and comparisons with the Superior pro6ince
Since the pioneering work of Hurley et al., 1967 based on early radiometric dating, the
Guyana Shield and West African cratons have been widely accepted as having originally been
part of a single unit e.g. Marcoux and Mile´si, 1993. Considerably more geochronology and
mapping have since been done in West Africa, yet there is still controversy surrounding the timing of
the orogeny that generated, deformed and meta- morphosed ca. 2.1 – 2.2 Ga greenstone belts and
plutons. In West Africa this orogeny is known as the Eburnean, and is believed to correlate with the
Trans-Amazonian of South America. Granitoid intrusions in West Africa are often divided into
‘belt-type’ and ‘basin-type’, depending on whether they intrude greenstone belts or metasedimentary
‘basins’. Most belt-type plutons in Ghana, for example, give dates around 2170 Ma, whereas the
basin-type plutons are younger, in the range of 2120 to 2090 Ma Hirdes et al., 1992; Davis et al.,
1994a. Hirdes et al. 1996 separated the West African region into eastern and western sub-
provinces. The eastern subprovince is said to in- clude slightly older volcanics 2185 – 2150 Ma
than the western subprovince 2105 Ma. The 2120 Ma age of the volcanics at Omai falls be-
tween the ages of these two subprovinces.
Similar to descriptions of the Trans-Amazonian orogeny, descriptions of the timing of the
Eburnean orogeny vary considerably with loca- tion. The entire event is considered to have oc-
curred broadly between 2200 and 1980 Ma e.g. Sidder and Mendoza, 1995. In the sedimentary
basins of Ghana, Eburnean plutonism, deforma- tion and metamorphism have been more tightly
constrained to between 2120 and 2080 Ma Oberthu¨r et al., 1998. In southern Mali, Lie´geois
et al. 1991 constrained Eburnean deformation by dating volcanic and granodiorite rocks to be-
tween 2098 9 5 Ma and 2074 + 9 − 8 Ma, respec- tively. This age spread is similar to that found in
Guyana ca. 25 million years although the vol- canic and plutonic rocks dated in Mali are some-
what younger.
Gold-associated rocks in Ghana, West Africa, show similar ages to those at Omai. Gold-bearing
granitoid intrusions in the Ashanti gold camp have been dated at 2105 Ma, and the late-tectonic
Banso granitoid in the Ashanti belt 2097 9 2 Ma is coeval within error with the Omai intrusion
Oberthu¨r et al., 1998. In Ghana, gold can be bracketed in the range 2098 – 2105 Ma, based on
ages of hydrothermal rutile Oberthu¨r et al., 1998.
In general, the EburneanTrans-Amazonian orogeny shows a similar pattern of crustal devel-
opment to the late Archean Superior province. This is characterized by relatively early greenstone
belts and associated plutons followed by a period of deformation associated with voluminous sedi-
mentation, emplacement of syn- to late-tectonic plutons in the volcanic belts and finally intrusion
of later syntectonic and synmetamorphic, often peraluminous, plutons in the sedimentary basins.
Both crust-forming episodes resulted in juxtaposi- tion of greenstone and metasedimentary terrains.
These patterns are likely the result of accretionary orogenies that operated at different times but
underwent broadly similar processes, as discussed by Davis et al. 1994a. The analogy with late
Archean processes is reinforced by ages of rutile and other hydrothermal minerals in gold deposits
of the late Archean Superior province which, as at Omai, are found to post-date their host green-
stones by up to 100 Ma e.g. Jemielita et al., 1990; Wong et al., 1991; Anglin et al., 1996, but corre-
spond to ages from less robust geochronological systems such as Rb-Sr and K-Ar ages on miner-
als. Krogh 1993 presented evidence that deep crustal metamorphism resulting from late tectonic
underplating of the Superior craton caused an influx of hot fluids that were focused along shear
zones in the upper crust. In some cases, these may have deposited gold, while in others, pre-existing
gold deposits may have been overprinted by these events. If so, similar processes may have affected
the Amazonian-West African shield much later, during an analogous stage of its development.
This suggests that continental crustal growth mechanisms typical of the late Archean were still
operating at 2 Ga.
7. Conclusions
