2. Background
The Guyana Shield can be divided into four principal Precambrian terranes: inset, Fig. 1 the
Archean Imataca
Complex, Paleoproterozoic
greenstone belts, the Uatuma˜ Group and sedi- mentary sequences such as the Roraima forma-
tion. The Imataca Complex in northeastern Venezuela includes granulite gneiss terranes, iron
formations
and metasediments.
This al-
lochthonous unit is thought to be at least 3.4 Ga old, and suffered major deformational events at
: 2.7 and 2.0 Ga Wirth et al., 1990.
The first major continental crustal development in the Shield occurred during the early Protero-
zoic at : 2.3 – 2.1 Ga. This created a series of greenstone belts and associated gneisses and am-
phibolites that are similar to Archean granite – greenstone complexes found in shield regions
around the world. The greenstone sequence in the Guyana Shield generally changes from low-K
basalts through intermediate and felsic volcanics to volcanic and chemical sediments. Most of the
volcanism is thought to be of submarine origin from multiple centres Gibbs and Barron, 1993.
Greenstone belts across the Guyana Shield in- clude the Pastora group in Venezuela, the
Barama-Mazaruni group in Guyana, the Marowi- jne group in Suriname, and the Maroni group in
French Guyana.
Following volcanism and associated plutonism, all the existing crustal fragments were assembled
during the Trans-Amazonian orogeny. This tec- tonothermal episode was originally defined by
Hurley et al. 1967, based on a large cluster of K-Ar and Rb-Sr radiometric ages around 2000
Ma. Since then, it has been found that throughout the cratons of the continent there is an abundance
of U-Pb, Rb-Sr, K-Ar and Sm-Nd radiometric data that cluster in the range 1900 – 2200 Ma,
indicating that this period was a time of signifi- cant deformation, metamorphic and intrusive ac-
tivity, followed by crustal cooling e.g. Cordani and de Brito Neves, 1982; Gibbs and Barron,
1993. A limited number of reliable geochronolog- ical studies of the granite – greenstone terranes of
the Guyana Shield have been published to date and are summarized in Table 1. Ages range from
1850 to 2350 Ma for metavolcanics and 1900 to 2250 Ma for syn- to post-orogenic plutons, in-
cluding errors. As in other Trans-Amazonian ter- rains, the Rb-Sr and K-Ar ages form a cluster
around 2000 Ma. However, Rb-Sr and K-Ar methods are generally considered unreliable to
date crystallization ages due to the likelihood of isotopic resetting during late stages of the Trans-
Amazonian orogeny.
Within the Trans-Amazonian period of activity, two major stages of intrusion can usually be
recognized. The first stage produced pre- and syntectonic intrusions, occasionally associated
with greenstone belt volcanism. In the Guyana Shield, these rocks were affected by cataclastic
deformation in WNW and ENE directions Gibbs and Barron, 1993. Following the final stages of
deformation, a second phase of intrusive activity created more potassic granitic rocks and other
intrusions ranging from quartz syenite and diorite to tonalite in composition. In Venezuela and
northern Guyana these are termed ‘younger gran- ites’ and are thought to have followed the main
Trans-Amazonian deformation because they do not show the same deformational characteristics
as their host metavolcanics Gibbs and Barron, 1993.
The mid-Proterozoic saw the formation of the Uatuma˜ group of felsic volcanics and granitoid
intrusions from 1.7 to 1.9 Ga, followed by the development of sedimentary sequences, such as
the Roraima formation. These sequences were later intruded by mafic dykes such as the
Avanavero suite, formerly known as the Roraima Intrusive Suite e.g. Gibbs and Barron, 1993; Sid-
der and Mendoza, 1995.
3. Geology and mineralization
The Omai Au-quartz vein system is located at 58° 45W and 5° 28N on the west bank of the
Essequibo River. It is located in the north-central Guyana Shield, in the Mazaruni arm of the
Barama-Mazaruni Supergroup inset, Fig. 1; Fig. 2. This Supergroup consists of felsic to mafic
volcanic and sedimentary units within three gran- ite – greenstone belts. The greenstone rocks are
thought to be mostly of submarine origin based on textural, chemical and mineralogical evidence
Gibbs, 1987. In the vicinity of the Omai deposit, the metavolcanics have undergone regional ductile
deformation and greenschist facies metamor- phism. The Omai pluton is a lobate intrusion of
dioritic affinity, with margins that clearly cross- cut the ductile deformational fabric in the host
metavolcanics. The margins of the Omai pluton are marked by hornblende-rich diorite and horn-
blendite rocks, whereas the main body ranges in composition from diorite to quartz diorite Fig.
2. Hydrothermal alteration is pervasive through- out the intrusion, but varies in intensity with
location. Gold is mainly found in the quartz diorite. The marginal hornblende-rich phases and
metavolcanics are rarely mineralized. However, a zone of gold enrichment in saprolite above pri-
mary gold mineralization, termed the Wenot Lake zone, is located in metavolcanics about 500 m
south of the intrusion, and is presently being mined.
Gold, as free grains of the native metal, is hosted largely by quartz veins disseminated
throughout the main body of the intrusion, but can also be found as micro-inclusions in pyrite
within the wall rock Bertoni et al., 1991. The auriferous vein stockwork consists mainly of
quartz, ferroan carbonate, sulphides and scheelite, ranging in size from stringers up to 5 cm in width
to occasional larger 1 m veins. The Omai deposit also contains auriferous tellurides and bis-
muthinides.
Hydrothermal alteration
shows phases that are spatially and temporally related to
gold mineralization. These phases include carbon- ate alteration, sulphidation and silicification. Car-
bonate alteration resulted in the formation of ferroan carbonate both in veins and wall rock
Table 1 Ages of greenstone belts and intrusions in the Guiana Shield
Rock type Reference
Method Age, Ma
Location Pastora group
Day et al., 1995 Metavolcanic
2131 9 10 U-Pb, zircon
Venezuela Metagraywacke
2250 9 106, Barama-Mazaruni
Gibbs and Olszewski, 1982 U-Pb, zircon
2244 9 43 group Guyana
Priem et al., 1982 Metavolcanics
Marowijne group 1950 9 150
Rb-Sr Suriname
Gruau et al., 1985 Metavolcanics
Paramaca series 2210 9 90
Sm-Nd French Guiana
U-Pb, zircon 2227 9 39
Gibbs and Olszewski, 1982 Barama-Mazaruni
Bartica gneiss group Guyana
Lerouge et al., 1996. Pb-Pb, zircon
2123 9 11, Pegmatite, yaou
Paramaca series French Guiana
granite 2127 9 10
Voicu et al., 1997 Sm-Nd
2171 9 140 Barama-Mazaruni
Omai intrusion
a
group Guyana and metavol-
canics P. Klipfel, personal communication,
Pastora group KM24 granite
2087 9 21 U-Pb, zircon
November 4, 1998. Venezuela
Paramaca series, Granites
2030 9 65, Teixeira et al., 1996 from Gibbs and Bar-
Rb-Sr Pb-Pb K- ron, 1993.
2083 9 39, Ar model
French Guiana 2032 9 61
Barama-Mazaruni 2015 9 80
Younger granite
a
Snelling and McConnell, 1969 K-Ar
group Guyana Younger granite
a
1945 9 100 Barama-Mazaruni
K-Ar Williams et al., 1967
group ? Guyana Marowijne group ?
Granitoids and acid Priem et al., 1971
1810 9 40 Rb-Sr
volcanics Suriname
a
Intrusions that are not deformed, and therefore post-date the Trans-Amazonian deformational events.
Fig. 3. U-Pb concordia diagram of zircon analyses from sample OM95-27, the felsic metavolcanic. Numbers refer to
analyses in Table 2. Error ellipses are 2s.
on several characteristics, as follows. Attempts were made to select zircons that were clear, crack-
free and lacking inclusions, cores and over- growths. However, the poor quality of samples
often made selection of cracked and cloudy grains necessary. Titanite fragments were divided based
on colour and selected for internal clarity and lack of cracking. Apatite grains are prismatic
whereas the feldspars are generally anhedral. Both were selected for clarity and lack of cracking.
Rutiles were chosen for crystal shape, colour and luster an indicator of freshness. Rutile aggre-
gates were washed in HF for 15 min in an ultra- sonic cleaner to remove surrounding silicate
minerals. Selected zircons were abraded Krogh, 1982, but many were left unabraded due to the
possibility of shattering because of internal cracks. No other minerals were abraded. Large
titanite, apatite and feldspar fractions were weighed, whereas weights for zircon, baddeleyite
and rutile fractions were estimated by eye and are likely accurate to about 9 50. Zircon, baddeley-
ite and rutile were digested in bombs, while titan- ite, apatite and feldspar were digested in Savillex
capsules. For zircon, U and Pb were separated using HCl with 50 ml anion exchange columns
following the method of Krogh 1973. Titanite, apatite, rutile and feldspar were passed through
500 ml anion exchange columns with HBr, follow- ing the method of Corfu 1988. Pb blanks are 1
pg for small column and 2 pg for large column chemistry. U blanks are taken to be 0.1 pg. U and
Pb were loaded onto Re filaments using H
3
PO
4
and silica gel. Isotopic analysis was carried out on a VG354 mass spectrometer in peak jumping
mode, with either a Faraday collector for large signal samples or Daly detector for small signal
samples. The mass discrimination correction for the Daly detector is 0.40 per AMU and the
thermal mass discrimination factor is 0.10 per AMU. Common lead in the apatite, titanite and
rutile samples made ages variably dependent on the isotopic composition defined for the initial
common Pb. Data from the feldspar from OM95- 2 were used as a common lead correction on all
regressions for these minerals. Data are plotted with two sigma error ellipses on U-Pb and Pb-Pb
diagrams Figs. 3 – 9. Regressions are calculated during pre-gold and gold-forming stages. Sul-
phides are mostly pyrite, but also include minor sphalerite, galena, and chalcopyrite. Pyrite miner-
alization occurred before, during and after gold deposition, as did silicification. Increased sulphi-
dation and silicification correlate with increased veining and gold mineralization.
A several hundred metres thick gabbro dyke, believed to be a member of the Avanavero Suite,
cuts the mineralized pluton at a depth of : 200 – 300 m Bertoni et al., 1991. Members of this
group of mafic rocks occur throughout the Guyana Shield in the form of sills, dykes and
other irregular bodies and have been dated by K-Ar, Rb-Sr and Ar-Ar methods. Sidder and
Mendoza 1995 estimated the age at between 1650 and 1850 Ma, based on compiled data from
several sources.
4. Analytical methods