The Granny Smith gold deposit: the role of heterogeneous stress distribution at an irregular granitoid contact in a greenschist facies terrane

Abstract

Abstract. The Granny Smith gold deposits formed late in the structural history of the Yilgarn Block at a high crusta~ level in a largely brittle structural r6gime. Gold mineralisation is located along a N-S striking fault which wraps around the contact of a small granitoid intrusion. In different sections of the fault, mineralisation may be developed in the granitoid, in the adjacent sedimentary sequence and/or along the contact between them. In the granitoid, gold mineralisation is in conjugate networks of thin carbonate-quartz veins and their alteration halos. Small displacements along veins are common. In contrast, veins and faults in the sedimentary rocks are subparallel to bedding. Spatial variations in the conjugate vein orientations indicate that the local stress field was heterogeneous and controlled by the shape of the granitoid contact. The greatest variations in vein and implied stress orientations occur in zones where the contact is most irregular. These are also the areas of richest mineralisation. Fluid flow was thus focused in a regional-scale low mean-stress region created by the geometry of the granitoid intrusion. Its irregular contact caused deposit scale variations in fluid flow and resulted in heterogeneous gold grades along the contact zone.

Carboniferous Orogenic Gold Deposits at Pataz, Eastern AndeanCordillera, Peru: Geological and Structural Framework, Paragenesis, Alteration,and 40Ar/39Ar Geochronology

“…Mineralization styles, however, vary from high-grade quartz sulfide in faults and extensional veins in diorite, granodiorite, and hornfels, to weakly mineralized, muscovite-altered shear zones in coarse-grained monzogranite, and narrow and branching, generally bedding-concordant, veinlets in turbidite and phyllite sequences. Similar observations concerning the passive but crucial behavior of a pluton in determining the style of orogenic gold systems have been made in several other intrusion-hosted orogenic gold systems, in particular in the French Massif Central , the Yilgarn craton (Cassidy and Bennett, 1993;Ojala et al, 1993;Cassidy et al, 1998), the Jiaodong Peninsula, China (Wang et al, 1998), and in Alaska . Despite the magmatic component in their model, Vidal et al (1995) and Macfarlane et al (1999) also pointed to the structural similarities between the Parcoy gold deposits and shear zone-hosted deposits, particularly those hosted by granitic rocks.…”

Les minéralisations aurifères au sein de la tonalite de La Grande-Sud Baie-James Québec

“…De même, il est possible de caractériser géochimiquement l'altération hydrothermale ayant affecté une roche pour laquelle une connaissance minimale de la composition chimique d'origine est disponible (Gresens, 1967 ;Grant, 1986 ;MacLean, 1990 ;MacLean et Barrett, 1993). Certains phénomènes importants dans la compréhension d'événements de déformation dans les intrusions sont présentés dans des articles tel celui de Paterson et al (1989) traitant des critères d'identification de foliations magmatiques ou tectoniques dans les granitoïdes ou encore de l'effet des dykes sur le développement de cisaillements aurifères dans les intrusions (Belkabir et al, 1993 ;Ojala et al, 1993). D'autres publications traitent quant à elles de la déformation des .…”

Coupled Models of Brittle-plastic Deformation and Fluid Flow: Approaches, Methods, and Application to Mesoproterozoic Mineralisation at Mount Isa, Australia

“…The local increase of stress difference during the mechanical loading of the model is therefore used as proxy for the spatial distribution of permeable regions, and fluid flow as such is not simulated. Models In both cases there is a potential to drive flow upward in regions where fluid pressure departs from hydrostatic gradient, but the high pore pressure below the seal (in b) may favour tensile failure of the rock at the top of the reservoir are either purely elastic (Cox and Ruming 2004;Micklethwaite and Cox 2004), based on damage mechanics concepts (Sheldon and Micklethwaite 2007), or simulate plasticity by slip along interfaces of discrete elements representing weak fault systems (Ojala et al 1993;Holyland and Ojala 1997;Mair et al 2000). Other workers have used numerical models based on coupling Mohr-Coulomb plasticity with Darcy flow as implemented in the finite difference codes FLAC (Itasca 2000) and FLAC 3D (Itasca 2003;Cundall and Board 1988), to simulate dilation in complex multi-material geometries (Jiang et al 1997;Gow et al 2002;Ord et al 2002;Ord and Oliver 1997;Schaubs and Zhao 2002;Sorjonen-Ward et al 2002;Gessner et al 2006;McLellan et al 2004;Sheldon and Ord 2005;Robinson et al 2006;Schaubs et al 2006;Vos et al 2007;Zhang et al 2006aZhang et al , b, 2007Potma et al 2008).…”