The Granny Smith gold deposit: the role of heterogeneous stress distribution at an irregular granitoid contact in a greenschist facies terrane |
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| Authors: | V J Ojala J R Ridley D I Groves G C Hall |
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| Institution: | (1) Key Centre for Teaching and Research in Strategic Mineral Deposits, Geology Department, The University of Western Australia, 6009 Nedlands, WA, Australia;(2) Placer Exploration, P.O. Box 764, 6105 Cloverdale, WA, Australia |
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| Abstract: | The Granny Smith gold deposits formed late in the structural history of the Yilgarn Block at a high crustal level in a largely
brittle structural régime. 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. |
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