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
Kategori