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

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 geochronologyand 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 dioriticrocks give U-Pb ages of 2094±6, 2092±11 and 2096+11/−10 Ma. Magmatic titaniteand 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 titaniteand 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 ∼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- Amazoniandeformation in central Guyana. Previous fluid inclusionstudies 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±5 Ma age is interpreted as a late hydrothermal overprintthat formed titaniteand reset rutile. Zircon and baddeleyitefrom a thick gabbrodyke 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 underplatingas previously proposed for the Superior province.