位于加拿大Abitibi绿岩带南部的Donalda金矿床主要由近水平的、与剪切带近垂直的含金石英脉组成,矿脉厚度0.3～0.5m,含大量与脉壁平行的条带,指示多期次裂隙扩张和矿物沉淀。原生石英c-轴方位测量显示石英生长方向垂直于脉壁,指示断裂属张性,矿物沉淀属空间充填。显微裂隙、细脉及流体包裹体面（FIPs）的方位测量指示原生矿物受到了近直立的最大主压应力（σ1）作用,与区域性的、近水平的南北向挤压应力不符。这一矛盾现象可用流体压力在超静岩和静水压力之间周期性变化及近水平断裂的周期性张开与闭合来解释。当流体压力大于静岩压力时,断裂张开,原生矿物沉淀,而当流体压力减小时,断裂上盘往下垮,两盘突出的矿物碰撞、碾压形成与脉壁平行的变形条带。由于断裂两盘岩石不是沿整条断裂都互相接触,两盘相顶部分承受的垂直应力大于水平方向的挤压应力,这就解释了为什么显微构造指示近直立的最大主压应力（σ1）。这个流体-构造模式支持断层阀模式,并且无须改变区域应力场就可解释脉体内局部主压应力的变化。%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 0.3-0.5 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（σ 1）,which is inconsistent with the subhorizontal σ 1 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 σ 1.This hypothesis is consistent with the fault-valve model,and explains the flipping of σ 1 without having to change the regional stress field.
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