Pan Gem Resources (Aust) Pty Ltd 2011
Total Page：16
File Type：pdf, Size：1020Kb
Pan Gem Resources (Aust) Pty Ltd 2011 Pan Gem Resources (Aust) Pty Ltd Gemstone Exploration Across Australia Recent Advances to the Syntectonic Model and its Applicability to Opal Exploration along the Collarenebri Antiform By Dr. Simon R. Pecover Managing Director Pan Gem Resources (Aust) Pty Ltd Abstract Recent advances to the Syntectonic Model of opal deposit formation within the Great Australian Basin (GAB) have resulted from a detailed study of micro-structural features evident in opal veins within gently warped interbedded sandstones and claystones of the Angledool Antiform. This study has revealed opal vein textures that preserve repeated episodes of fluid flow injection by viscous Non Newtonian fluids, hyper-saturated with amorphous silica. The preserved textures within these opal veins also show evidence of repeated episodes of opal hardening and brittle fracture deformation of earlier injected viscous fluids. These textures are interpreted to have been formed by multiple viscous opal fluid injection and hydraulic extension fracturing depositional events, resulting from opalising fluid flows under pressure, moving along progressively developing and evolving opal vein array systems. Given the generally horizontal nature of the vein opal deposits studied, and their juxtaposition to facies change boundaries that have been subjected to faulting, generating relay-zone ―flats‖ and ―ramps‖ fault architectures, then these deposits could be classified as stratabound fault-controlled vein-type ore depositional systems. From a regional perspective, the vein systems studied were found to mainly coincide with areas of high intensity faulting within very specific parts of mapped antiformal and domal structures, where compressional dewatering of silica-rich clay facies reservoir rocks appears to have provided highly localised sources of opalising fluid flows into nearby structural (i.e. both tectonic and sedimentary) trap sites and vein systems. The Syntectonic structural opal formational analogue and paradigm provided by the opal depositional environment of the Angledool Antiform, has been applied to exploring the adjacent Collarenebri Antiform. Numerous structural targets have been identified along the Collarenebri Antiform, where high intensity faulting, suitable opalising fluid-source reservoir rocks and extensive silicification has been identified. Introduction Regolith genesis researchers have long advocated simple gravity-driven vertically-downward-moving meteoric groundwater flows, as the principal mechanism for carrying dissolved amorphous silica to depth across near-surface Great Australian Basin (GAB) sedimentary lithologies, with the resulting opal deposits said to have been formed by the evaporation of silica-rich waters, passively residing for millions of years in pre-existing open cavities. Additionally, some workers have even claimed a dominant role for microbes in the precipitation of opalising silica, co-genetic with the deposition of Cretaceous sediments across the GAB. However, a new study of micro-structural geological features, preserved in opal veins formed after the deposition of the Cretaceous sediments of the GAB, has revealed a complex range of textures that preserve dynamic fluid flow and kinematic relationships that are interpreted to be indicative of multiple episodes of viscous Non-Newtonian opalising fluid flows at relatively high pore pressures. Complex patterns of intermixed potch and precious opal have been observed in some of the vein systems studied, which suggests once vigorous, dynamic and turbulent fluid flows through vein networks, flowing from areas of higher pore pressure to areas of lower pore pressure. Copyright Dr. Simon R. Pecover, July 2011 Page 1 Pan Gem Resources (Aust) Pty Ltd 2011 Multiple episodes of opalising fluid injection, opal hardening and brittle fracture deformation, forming complex in-vein opal breccias, is considered indicative of multiple generations of opal formation, within a seismically active and vigorous fluid-flow-driven, hydraulic-extension-fracturing, Syntectonic environment. The vein systems studied, were found to be located in areas of high intensity faulting and fracture- mesh development, within discrete lateral and vertical parts of the Angledool Antiform. The geological setting of these vein opal deposits within the Angledool Antiform has now provided a suitable analogue and paradigm for Pan Gem Resources and its joint venture partners, to progress opal exploration across the nearby Collarenebri Antiform, and is considered to have applicability across the entire opal prospective area of the GAB. Core Tenants of the Syntectonic Model of Opal Formation in the Great Australian Basin The ―Syntectonic Model‖ of Pecover (1996), advocates a core process in which the vein opal deposits of the GAB were formed rapidly through a process of fault controlled, seismic-fluid-pumping and hydraulic extension fracturing of host rocks, by silica-rich fluids derived from the compressional overpressuring and dewatering/silica-stripping of silica-laden claystones, during antiformal buckling of interbedded Cretaceous sandstones and claystones. The precipitation of opal from these silica- super-saturated fluids is thought to have occurred through the polymerisation of dissolved silica, which then formed viscous gelatinous silica/water mixtures hyper-supersaturated with colloidal silica spheres. The potential contribution of fluids to the sedimentary pile of the GAB, from other sources, including hot artesian waters, is not excluded from the Syntectonic Model, and is supported by zirconium mobilisation research work carried out at Macquarie University (Liddicoat 2003). At its core, the Syntectonic Model conforms to the well understood processes by which most mineral veins are thought to have been formed in nature. These processes typically involve hydraulic extension of fractures that become filled with mineralising fluids which move along pressure gradients within the structural architecture of the geologic system, with relatively rapid precipitation of minerals occurring within these fractures, during periods of depressurisation, leading to mineral vein formation. As processes of this type are commonly multi-cyclic, then it is not surprising that several generations of mineralisation can occur within a given vein system. Thus, the core geologic and structural tenants of the ―Syntectonic Model‖ of opal genesis in the GAB (Figure 1), may be summarised as:- Kinematic In-Veins Stress Controlled Syntectonic Geotectonic Setting of Opal Deposits along the Angledool Antiform Opal mined across the Angledool Antiform commonly occurs as potch and much rarer precious opal, in horizontal to sub-horizontal veins, interstitial infillings between mineral grains in some sedimentary lithologies, isolated nodules, ironstone concretion cavity infill’s, and as pseudomorphic replacements of fossil remains in generally clay-rich facies rocks. At many widely-spaced locations along the Angledool Antiform, within the Narran-Warrambool Opal Mining Reserve (Figure 2), potch and precious opal occurs in highly faulted and fractured Cretaceous claystones and clayey sandstones. Fault and fracture-controlled deformation bands (known locally as Copyright Dr. Simon R. Pecover, July 2011 Page 2 Pan Gem Resources (Aust) Pty Ltd 2011 ―biscuit band‖) have also been found to be an important source of minable opal, at or close to the surface. Within the Reserve, the opal-bearing Cretaceous sediments have been gently warped into low, generally NE-SW trending, ridges, forming low-amplitude antiformal and domal tectonic structures. In many areas, opal-bearing country is commonly overlain by hard caps of silicified Tertiary sands and gravels, which crop out as silcretes. In these extensively silicified areas, distinctive and discrete mounds of silcrete rubble, are interpreted by experienced prospectors, to be the surface expression of ―blows‖, and are considered valuable surface indications of opal prospective country (Aracic 1996). It is well known by experienced opal miners’ across the GAB, that there is an absolute and intimate association between the location of faults and sites of opal deposition. Opal typically occurs close to the hanging wall and foot wall sides of these faults. Faulting observed in open-cut and underground exposures across the Angledool Antiform, exhibits complex mixtures of fault types, including normal, reverse and oblique-slip faults, commonly arranged in conjugate sets, resembling wedge-like structures that severely disrupt the "level". Faults cross- cutting claystone "levels", may flatten appreciably, forming layer-parallel slip surfaces concordant to bedding, particularly at the contact between sandstone and claystone. These layer-parallel slip surfaces may be related to complex linkages within fault relay-zones, comprising horizontal fault ―flats‖ connected to nearby inclined fault ―ramps‖, forming discrete fault damage zones, in which dilational and hydraulic extension fractures within the ―flats‖ host opal vein arrays. Such vein arrays would be expected where slip surfaces show significant undulation, and a high degree of roughness. When multiple fault relay-zones are arranged en-echelon to one another in areas of intense fault clustering, opal veins may occur over considerable lateral distances, in a stepwise vein-array fashion. In these sub-horizontal fault damage zones, complex layering and intermixing of bedding-parallel brecciation, fault gouge and networks of opal veins