KEY FEATURES OF OPAL VEIN DEPOSITS AT LIGHTNING RIDGE, NSW, AUSTRALIA Dr. Simon R. Pecover Pan Gem Resources (Aust) Pty Ltd Photo by Sarah Pecover
Distribution of Opal Deposits in the Great Australian Basin
Regional Structural Setting of the Lightning Ridge Opal Fields
Geological Setting of Opal Vein Deposits at Lightning Ridge Great Australian Basin sediments of Cretaceous age, comprising interbedded sandstones and claystones, have been gently warped into NE-trending antiforms, during E-W compression in the Miocene. Opal veins, (and pseudomorphic replacements of fossils by opal) of Miocene age, typically occur within claystone/sandstone sequences that are close to the surface (0 m to ~50 m). The vein systems generally occur parallel to bedding contacts and are intimately associated with nearby sub-parallel and crosscutting faults, fracture meshes, zones of brecciation, and clastic pipes. Discrete areas of opal mineralisation that define individual opal fields, could be classified as Stratabound, Fault-Controlled Vein- Type Ore Deposits.
Diagram by Simon Pecover Subsurface Structural Setting of Vein Opal Deposits at Lightning Ridge Zone of Simple Fault Block Development Surface Fault Blocks Zone of Intense Fracturing Clays tone "level" Sandstone Clays tone "level" Conjugate Normal Faults Normal Fault Thrust Fault Fracture Meshes Breccia Pipes Fracture Meshes Sandstone Sandstone Claystone Major fault Fracture-controlled breccia pipe Fracture mesh Opal veins Simplified schematic geology of fault-hosted vein-type precious opal ore deposition al system at Lightning Ridge.
Opal-Producing Claystone Level Overlain by Sandstones & Siltstones Photo by Simon Pecover
Reverse Faulting of Sandstone & Claystone Photo by Steven Aracic
Normal Faulting of Sandstone & Claystone Photo by Simon Pecover
Clastic Pipes in Opal-Producing Country Cross-section of a tulip-shaped underground clastic pipe Plan-view of an underground clastic pipe Millions of these structures across the Great Australian Basin attest to the widespread vertical ascent of highly localised geofluids along fault-fracture meshes Cross-section of a tulip-shaped near-surface clastic pipe Photos by Simon Pecover
Fault-Fracture-Mesh-Controlled Clastic Pipes in Opal Mine at Lightning Ridge Photo by Simon Pecover
Vertical and Lateral Intrusion of Clastic Material Controlled by Fault-Fracture-Mesh Photo by Simon Pecover
Vertical Clastic Pipes Being Mined For Precious Opal Photos by Simon Pecover
Opal Veins Formed Within Sub-Horizontally Fractured Silty Claystones Along Layer-Parallel-Slip Fault Damage Zones
Opal Nobby Formed Within a Small Jog Along a Layer-Parallel-Slip Fault Damage Zone Photo by Simon Pecover
Complex Vein of Intermixed Potch and Precious Opal with Brecciated Wall Rock Clasts Photo by Simon Pecover
Horizontally Laminated Vein of Potch Opal, Cross-cut by Multiple Fractures that have been In-filled by Later Generations of Opal Photo by Simon Pecover
Complex Patterns of Multi-Generational Viscous Opaline Fluid Flow and Brittle-Fracture Deformation Photo by Simon Pecover
Complex Patterns of Multi-Generational Viscous Opaline Fluid Flow and Brittle-Fracture Deformation Photo by Simon Pecover
Brittle-Fracture Micro-Faulting Coupled with Micro- Intrusions of Later Generations of Potch Opal Photo by Simon Pecover
1st Generation Potch Opal Vein Showing Patterns of Turbulent Viscous Opaline Fluid-Flow, Cross-Cut by Fractures In-Filled with 2nd Generation Potch Opal Photo by Simon Pecover
Fossil Curved Viscous Fluid-Flow-Fronts in both Potch & Precious Opal Potch Opal Precious Opal Compare With Typical Flow Patterns in Viscous Lava
Complex Viscous Fluid Flow and Fluid Mixing Patterns of Potch and Precious Opal Photo by Len Cram
Why is Precious Opal so Rare in Great Australian Basin Sediment-Hosted Vein Systems? As viscous opaline fluid flow appears to have been the dominant fluid migration process in GAB opal veins, then the orderly packing of silica spheres to create light diffraction gratings, would have been restricted in these flows, leading to a depositional system dominated by potch opal.
Photos by Len Cram Formation of Colloidal Photonic Crystals of Precious Opal Regions of arrested viscous opaline fluid flow within opal vein systems, favours the growth of colloidal photonic crystals to form precious opal
Colloidal Photonic Crystal Growth in an Arrested Liquid Medium of Viscous Opaline Potch Photo by Len Cram
Fault-Hosted Fluid-Pumping Processes Applicable to Opal Vein Formation Within the Lightning Ridge Opal Fields But Near Surface <50 m Lower Opal Prospectivity Mature driving structures:- thick gouge, intermittent veining, large damage zone of minor faults + foliation (broad zone of interconnected but thinner diffuse, grain-scale porosity) Aftershock structures:- Small-displacement less mature faults with fracture and vein networks, breccias (narrow zones of connected open porosity) Higher Opal Prospectivity And Low Temperature <50 degrees C Diagram Modified After Micklethwaite & Cox 2004
Australian Opal Genesis - Concluding Remarks The formation of opal veins in Great Australian Basin rocks at Lightning Ridge IS NOT the result of weathering or microbial processes. The formation of opal veins in Great Australian Basin rocks at Lightning Ridge IS the result of syntectonically driven opaline fluid flow along hydraulic extension fracture pathways. Compressed and sheared, opal-phytolith-bearing claystones that host the opal veins, are considered the most likely source for the low temperature opaline silica-rich fluids that formed the opal vein deposits at Lightning Ridge. The laminar and turbulent viscous flow of opaline fluids along fracture pathways has resulted in the dominance of potch over precious opal in the vein systems. The growth of colloidal photonic crystals to form precious opal in the vein systems, required quiescent conditions to prevail in areas of arrested fluid flow.