Evolution of transient permeability and fluid flow in disseminated ores of Zaozigou orogenic gold deposit

Orogenic gold deposits are among the most important sources of gold globally. Ore occurs either in veins or disseminated within the host rock. While the dynamic permeability enhancement and fluid flow processes associated with vein-type ores have been extensively studied, the specific physical and chemical processes associated with fluid flux in disseminated ores have been largely overlooked. Here, we combine structural analysis, microstructural observation, and thermodynamic modeling of disseminated mineralized dacite to investigate the permeability evolution and the associated fluid flow characteristics in the Zaozigou orogenic gold deposit. The mineralized dacite is bordering an extensional quartz-stibnite vein that formed in response to rock implosion triggered by fluid pressure drops associated with co-seismic dilation on a nearby fault segment. The subsequent lateral alteration zonation on either side of the vein can be divided into four distinct zones (Z1-Z4) based on local geochemical and mineralogical variations:: Z1 is characterized by the enrichment of invisible gold, pyrite, arsenopyrite, sericite, albite, and dolomite; Z2 shows the occurrence of siderite, sericite, albite, and dolomite; Z3 can be defined by the concentration of chlorite and sericite; Z4 represents the least-altered dacite composed of quartz, biotite, and feldspar. Interconnected cracks observed in weakly altered dacite (Z3) reflect fluid pressure-induced grain-scale microcracking. In addition, the grain size reduction associated with fully altered minerals (Z1-Z2) results in the development of numerous new fluid pathways (grain boundaries) and a gradual increase in permeability. Pyrite, arsenopyrite, sericite, siderite, and chlorite are primarily distributed along the cleavage planes of biotite, while sericite and albite align with newly formed pores in feldspar. Additionally, dolomite is also observed around feldspar grains in Z1 and Z2. The spatial distribution of these hydrothermal minerals indicates that fluid flow predominantly occurred along pre-existing cleavage planes and newly formed microcracks and pores. The precipitation of hydrothermal minerals observed in altered dacite (Z1-Z3) indicates that the early increased permeability was eventually destroyed. Thermodynamic models based on our microstructural and geochemical investigations suggest that sulfidation reactions led to gold precipitation in the altered dacite and the subsequent compositional changes in reactive fluid flow are the predominant driver for the formation of the lateral alteration zoning.