Geology and mineralization Directory UMM :Data Elmu:jurnal:P:Precambrian Research:Vol102.Issue1-2.2000:

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