MEGUMA LODE GOLD DEPOSITS: POST-METAMORPHIC, LATE SYN-FOLDING VEIN ARRAYS

Acadian deformation of the Meguma Group metasedimentary rocks of the Meguma Terrane is characterized by kilometer-scale box- and chevron-style folds, reflecting the highly anisotropic character of the Meguma Group rocks, in particular the metasandstone-dominated Goldenville Formation. Such folds typically initiate with little layer-parallel shortening and evolve by flexural folding. Over 60 past producing Meguma gold deposits (MGD) occur within the Meguma Group, most characterized by vein arrays within anticlinal hinges. The MGD are dominated by bedding-concordant veins confined to slate intervals, including laminated bedding-parallel veins, en-echelon vein arrays and saddle-reef veins. Slickenstria, duplex geometry, en echelon vein geometry and offset of discordant veins are consistent with flexural shear. Additional flexural shear structures lacking veins (e.g., bedding-parallel movement horizons, limb thrusts) confirm a flexural shear environment. These observations are consistent with a saddle-reef type model in which veins are introduced into fold-related structures recording flexural-shear strain. High flexural-shear strain is recorded within slate intervals by en echelon veins and offset of discordant veins. However, this strain occurred late in the folding history and, therefore, represents only a small change in limb dip. Abundant discordant veins are generally sub-parallel to the ac plane. The mutual cross-cutting relationships of all vein types along with similar isotopic signatures indicates synchronous formation of all veins and a post-metamorphic age for this is indicated by several features: (1) cleaved fragments within veins; (2) veins truncating cleavage; (3) localization of veins to brittle structures; (4) minimal shorting of discordant veins; and (5) restriction of veins recording layer-parallel shorting to fold hinges. In addition, Ar/Ar dating of hydrothermal vein minerals and whole-rock slate within veins support a late orogenic timing (i.e., ca. 375 Ma) for vein emplacement. The presence of strong ductile strain features in veins is consistent with a syn-folding origin. Geochemical studies, including radiogenic (Pb, Sr) and stable isotopes (C, O, D), indicate vein-forming fluids originated in a non-Meguma Group reservoir with variable amounts of interaction with the host rocks.