U-Pb and Pb-Pb age constraints on Paleoproterozoic magmatism, deformation and gold mineralization in the Omai area, Guyana Shield

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 metavolcanic/sub-volcanic unit in the host rock sequence is 2120 +/- 2 Ma. Zircon analyses from the main body dioritic rocks give U-Pb ages of 2094 +/- 6, 2092 +/- 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 +/- 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 +/- 5 Ma. The igneous ages agree with data from similar units in the Guyana Shield and West Africa, showing that similar to 2100 Ma was a time of significant intrusive activity. The ages obtained for the deformed metavolcanic and undeformed intrusion at Omai define a 26 +/- 2 Ma bracket for Trans-Amazonian deformation in central Guyana. Previous fluid inclusion studies indicate that the mineralizing solutions at Omai were too CO2-rich to form titanite, and the titanite-bearing sample is unmineralized, suggesting that it was not altered by gold-bearing solutions. Therefore, the 2002 +/- 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 +/- 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. (C) 2000 Elsevier Science B,V. All rights reserved.