Abstract:

The Donalda gold deposit in the southern part of the Archean Abitibi greenstone belt consists mainly of a subhorizontal goldquartz vein perpendicular to subvertical shear zones. The 0305 m thick vein is characterized by veinparallel banding structures indicating multiple episodes of fracture opening and mineral precipitation. Measurement of the caxis of primary growth quartz indicates that quartz preferentially grew perpendicular to the fracture, suggesting open space filling and/or extensional nature of the fracture. Measurement of the orientations of microfractures, veinlets and fluidinclusion planes (FIPs) crosscutting primary growth quartz indicates that the vein minerals were subject to a vertical maximum principal stress (σ1), which is inconsistent with the subhorizontal σ1 inferred from the regional stress field with NS shortening. This apparent discrepancy is explained by invoking episodic fluid pressure fluctuation between supralithostatic and hydrostatic regimes accompanied by episodic opening and closing of the subhorizontal fracture. When fluid pressure was higher than the lithostatic value, the fracture was opened and primary growth minerals were precipitated, whereas when fluid pressure decreased toward the hydrostatic value, the hanging wall of the fracture collapsed, causing collision of protruding primary growth minerals from both sides of the fracture and resulting in formation of veinparallel deformation bands. The columns where the two facing sides of the fracture collided were subject to higherthanlithostatic stress due to the bridging effect and reduced support surface area, explaining the development of vertical σ1. This hypothesis is consistent with the faultvalve model, and explains the flipping of σ1 without having to change the regional stress field.

Key words: hydrodynamics, fluidstructural relationship, orogenic gold deposit, fluid inclusion planes, microstructures