MEGUMA GOLD DEPOSITS: INSIGHTS INTO THE ROLE OF MULTIPLE AND PROTRACTED EARTH-SYSTEM PROCESSES DURING FORMATION OF A SEDIMENT-HOSTED, OROGENIC-TYPE GOLD SYSTEM

Although production from Meguma gold deposits (MGD) is small at (ca. 1.3 M oz Au), abundant research over multiple decades provides the means to assess the processes involved in formation of this sediment-hosted orogenic gold system; these data are used to review the origin of the MGDs in the context of broad scale earth-system processes relevant to gold deposit formation.

The MGDs occur in the Meguma terrane and throughout a large part of a 10 km thick sequence of late Neoproterozoic to Ordovician metasandstones and mudrocks that dominate it. Importantly, an internal stratigraphy for the strata is now recognized, including significant lithogeochemical (e.g,, Mn, S, CO₂) and isotopic (i.e., δ³⁴S) variations, but how this relates to gold formation is unclear. Deformation and metamorphism of the strata record docking of Meguma with Avalonia during the Neoacadian orogeny (410-380 Ma) which culminated in intrusion of large volumes of both mantle- and crust-derived magmatic suites with the latter dominating. Auriferous veins (quartz±-carbonate±sulfide±oxide±silicate) occur as a variety of types (i.e., en echelon, concordant, saddle) that reflect emplacement during a single, but protracted flexural-slip folding event in concert with fluid overpressuring, as evidence by crack-seal textures. Most veins, however, are considered to have been emplaced during late-stage fold tightening coincident with granite emplacement. Radiometric dating (Ar-Ar: Amp, Bt, Ms; Re-Os: Aspy) also indicates vein formation was protracted, over 10s Ma, which may also account for complex trace-element zoning and paragenesis patterns produced from elemental maps of arsenopyrite. The vein-forming fluids are dominantly H₂O-CO₂ type with XCO₂=0.1-0.2, but decrepitate mound analysis reveals several fluid types (Na, Na-K, Na-Ca), which is also supported by LA ICP-MS analysis which further indicates variation in metal abundances (e.g., As, Sb, W, Sn). That fluids interacted with and equilibrated with different reservoirs is also indicated from stable (O, S, C, D) and radiogenic (Sr) isotopes; exchange with the basement and felsic intrusions is suggested. Intriguingly, the recent discovery of metal- (wt. % As, Zn, Cu, Fe) and S- (<7 wt.%) rich carbon layers in some veins suggests that at least some metals and Au(?) may be locally sourced.