Nd and Sr isotope study of hydrothermal scheelite and host rocks at Omai, Guiana Shield: implications for ore fluid source and flow path during the formation of orogenic gold deposits

Omai is a high tonnage, low-grade, world-class gold deposit located in the Paleoproterozoic Guiana Shield. It is the second most important gold deposi t in the Guiana Shield (after Las Cristinas, Venezuela), and one of the lar gest in South America (4.0 million oz.). Sm-Nd and Sr isotope data are pres ented for host rocks and for scheelite from auriferous quartz-carbonate-sch eelite-sulfide-telluride veins from the Omai deposit. Gold-bearing veins ar e hosted by the Paleoproterozoic Barama-Mazaruni Supergroup, a greenstone b elt sequence consisting of mafic volcanic rocks interbedded with sedimentar y rocks that are intruded by quartz-feldspar porphyry and rhyolite dikes. T his lithologic sequence was folded and metamorphosed to lower greenschist f acies during the Paleoproterozoic Trans-Amazonian orogeny. The volcano-sedi mentary unit was intruded by a post-tectonic quartz monzodiorite-diorite-ho rnblendite stock. Initial Nd isotope ratios for the Omai volcanic rocks ran ge from epsilon(Nd) = +2.1 to +4.2. These values suggest that this part of the Guiana Shield was a site of new crust formation during the Paleoprotero zoic and was not contaminated by older (Archean), reworked continental crus t. Initial Nd isotope ratios for the Omai stock range between +0.5 and +2.3 , which suggest limited contamination with previously formed continental cr ust. Although the Nd isotopic ratios of gold-related scheelites overlap wit h those of the host rocks, particularly the tholeiitic basalts at the inter preted time of vein emplacement, the lack of both isotopic mixing and signi ficant Nd movement during the hydrothermal process suggest that: the Nd iso tope composition call be used to determine the isotopic characteristics of the ore fluid source area. At Omai, the ore fluid is largely derived from a radiogenic Nd source, represented by mantle or lower crustal reservoirs. S trontium isotope ratios for the scheelites cluster tightly between 0.7019 a nd 0.7021. The Sr isotope data suggest that unlike Nd, Sr was significantly mobile during the hydrothermal process. The fluids responsible for the Oma i deposit may have picked up Sr along the flow path. The constant low Sr is otope values of scheelites probably reflect the key role that the local tho leiitic basalts played as the main source of Sr in the fluids. Whereas Nd i sotopes identify the fluid source area, the Sr isotopes map the fluid flow paths.