Precambrian Research 102 2000 69 – 86
U-Pb and Pb-Pb age constraints on Paleoproterozoic magmatism, deformation and gold mineralization in the
Omai area, Guyana Shield
Christine Norcross
a
, Donald W. Davis
b,
, Edward T.C. Spooner
a
, Alison Rust
a
a
Department of Geology, Uni6ersity of Toronto,
22
Russell Street, Toronto, Ont., Canada M
5
S
3
B
1
b
Earth Science Department, Royal Ontario Museum,
100
Queen
’
s Park, Toronto, Ont., Canada M
5
S
2
C
6
Received 16 February 1999; accepted 3 December 1999
Abstract
The Omai intrusion-centred Au-quartz vein system, the largest Au producer presently operating in the Guyana Shield, was sampled for detailed U-Pb and Pb-Pb geochronology and petrological investigation. The age of a
metavolcanicsub-volcanic unit in the host rock sequence is 2120 9 2 Ma. Zircon analyses from the main body dioritic rocks give U-Pb ages of 2094 9 6, 2092 9 11 and 2096 + 11 − 10 Ma. Magmatic titanite and apatite that grew in
hornblende-rich peripheral phases of the intrusion define a consistent Pb-Pb age of 2094 9 1 Ma, in agreement with the zircon data. Colourless titanite and rutile from strongly altered phases of the intrusion, along with low-U apatite
and feldspar, define a significantly younger Pb-Pb isochron age of 2002 9 5 Ma. The igneous ages agree with data from similar units in the Guyana Shield and West Africa, showing that 2100 Ma was a time of significant intrusive
activity. The ages obtained for the deformed metavolcanic and undeformed intrusion at Omai define a 26 9 2 Ma bracket for Trans-Amazonian deformation in central Guyana. Previous fluid inclusion studies indicate that the
mineralizing solutions at Omai were too CO
2
-rich to form titanite, and the titanite-bearing sample is unmineralized, suggesting that it was not altered by gold-bearing solutions. Therefore, the 2002 9 5 Ma age is interpreted as a late
hydrothermal overprint that formed titanite and reset rutile. Zircon and baddeleyite from a thick gabbro dyke of the Avanavero Suite, which cuts the Omai pluton, define an age of 1794 9 4 Ma, ruling out the dyke as a source for the
late thermal effects. The hydrothermal age may record the passage of fluids released by deep crustal metamorphism due to late-stage tectonic underplating as previously proposed for the Superior province. © 2000 Elsevier Science B.V.
All rights reserved.
Keywords
:
U-Pb; Zircon; Gold; Geochronology; Guyana; Trans-Amazonian www.elsevier.comlocateprecamres
1. Introduction
The Precambrian of South America contains two large cratonic regions, the Amazonian and
Corresponding author. Tel.: + 1-416-5865811; fax: + 1- 416-5865811.
E-mail address
:
dondrom.on.ca D.W. Davis 0301-926800 - see front matter © 2000 Elsevier Science B.V. All rights reserved.
PII: S 0 3 0 1 - 9 2 6 8 9 9 0 0 1 0 2 - 3
Sa˜o Francisco cratons. The Amazonian craton is further subdivided by the Amazon river basin into
the Guyana and Guapore´ Shields Fig. 1, to- gether covering eastern Venezuela, Guyana, Suri-
name, French Guyana, the northeastern part of Brazil and a small eastern section of Colombia.
The geochronology of the South American Pre- cambrian is broadly known, and in many areas in
the cratons of Brazil it is well constrained by detailed studies e.g. Machado et al., 1996a,b;
Teixeira et al., 1996. The focus of this work is the Trans-Amazonian orogeny in Guyana. This was a
major crust-forming episode that has been dated in the approximate range 1900 – 2200 Ma Gibbs
and Barron, 1993. It is broadly correlative with the Eburnean orogeny in West Africa Milesi et
al. 1992. The nature and timing of Trans-Amazo- nian activity has not been described in detail
Fig. 1. Geology of South America. Inset shows Precambrian geology of the Guiana Shield. The Omai intrusion-centred Au-quartz vein hosted deposit is located in the Barama-Mazaruni greenstone belt. Modified from Wirth et al., 1990.
Fig. 2. Geology of the Omai intrusion-centred Au-quartz vein hosted deposit. a is a plan section at 122 m below surface, showing approximate locations of samples boxed numbers = OM95-x. Sample OM95-27 is a drill core obtained from beneath the Wenot
Lake zone and the location is not shown. b is a cross-section of the deposit. Modified from Bertoni et al., 1991
across the continent. Were events coincident in time in different areas, or did they occur di-
achronously? The large scale of the Trans-Amazo- nian, and its apparent duration of several
hundred million years, makes detailed reconstruc- tion of its activity essential to understanding the
geologic evolution of South America. The first step in this work is to define periods of specific
Trans-Amazonian activity in specific areas. Rela- tively little work has been done in the northern
cratonic regions, such as the Guyana Shield, where minimal development and access combined
with heavy tropical cover and weathering have made reliable geological sampling difficult. The
Omai mine provides an important opportunity to extend coverage of Trans-Amazonian rocks in this
area.
The Guyana Shield has been a producer of alluvial and eluvial gold for over a century and, as
such, makes an excellent exploration target. Sev- eral deposits are presently being mined in the
granite – greenstone terranes. The Omai diorite in- trusion-centred Au-quartz vein system in Guyana
Fig. 2 is the largest current open-pit gold pro- ducer in the Guyana Shield, with estimated re-
serves of four million ounces of gold. This open pit operation, owned by Cambior, Golden Star
Resources and the Government of Guyana, pro- vides what is probably the largest hard-rock out-
crop in the Guyana Shield at the present time.
The purpose of this paper is to present a de- tailed geochronological study of volcanism and
plutonism in the Omai area that constrains ages of Trans-Amazonian deformation and gold min-
eralization at Omai. A deformed rhyolite unit hosting the pluton and several phases of the late-
tectonic Omai intrusion were sampled for zircon, as well as a variety of other igneous and hy-
drothermal minerals including titanite, rutile and apatite Fig. 2. A late mafic dyke was also sam-
pled in order to test whether it may have been a heat source for mineralizing hydrothermal pro-
cesses. Results will help to understand igneous and hydrothermal processes in the Omai area and
put its development in the context of Trans-Ama- zonian and Eburnean events in other areas.
2. Background
