recent advances to the syntectonic model and its applicability to opal exploration along the...

Pan Gem Resources (Aust) Pty Ltd 2011

Copyright Dr. Simon R. Pecover, July 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.



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



―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 collectively define the ―opal horizon‖. These

damage zones may also show significant horizontal to sub-horizontal slicken-sided surfaces,

indicative of prolonged fault movement. In some areas, the opal horizon may also be developed at the

base of a claystone unit immediately overlying a sandstone unit. This development of "top" and

"bottom" opal-bearing horizons may also be accompanied by the stacked succession of several opal

levels, each hosting opal mineralization at repetitive sandstone/claystone interfaces.

The fault and bedding-controlled vein systems within the Angledool Antiform, commonly exhibit

complex meshwork patterns of branching and anatomising veinlets, similar to those seen in fault-

controlled net-vein fracturing geological environments. The opal veins typically show pinch and swell

morphologies when viewed in cross-section. Lateral extent, like thickness, is highly variable and

ranges from small centimetre-scale pods to undulating vein arrays that can be traced over tens of

metres.

In all the opal fields occurring across the Angledool Antiform, extensive vertical fracturing of both

sandstones and claystones is evident. These vertical fracture sets resemble Hill-Type fracture

meshes, commonly developed in many parts of the world, where fluid flow, induced by seismic-fluid

pumping processes, has occurred in horizontally bedded sandstones and shales (Sibson 1987, 1989

& 1994).

Across the Angledool Antiform, these fracture meshes may be associated with normal and reverse

faults, which may also host brecciated stockworks and vertical pipe-like ―chimneys‖ of brecciation

(known locally as ―blows‖). These conduits provide evidence that fluids have travelled vertically

upwards under pressure through the sedimentary pile. This process has given rise to a variety of

geological structures, ranging from simple net vein fracturing of host rock lithologies, to angular clast-

supported jig-saw-fit breccias, to matrix-supported breccias with rounded clasts; with the latter

resulting from the prolonged fluidisation of the breccia column. While not all such breccia bodies have



provided conduits for opalising fluids (as clearly, many breccias in GAB rocks are not silicified), those

that are indurated and hardened by significant amounts of amorphous silica, are more likely (in the

authors view) to be associated with areas of significant opal vein mineralization (Pecover 1996, 1999,

2003, 2005, 2007 & 2010).

The close association of those breccia pipes that display intense opal silicification and kaolinisation

with nearby opal vein arrays, suggests an important genetic link between breccia formation, fault and

fracture development, and significant lateral and vertically-ascending, opalising fluid flows.

Thus, given the generally horizontal nature of the vein opal deposits studied, and their juxtaposition to

facies change boundaries, that have been subjected to faulting that has generated significant ―flats‖

and ―ramps‖ relay-zones, then the opal deposits within the ―flats‖ could be classified as stratabound

fault-controlled vein-type ore depositional systems (Pecover 2010).

Recent Advances to the Syntectonic Model through the Study of Opal Vein Textures

To investigate the processes of opal vein formation in the GAB at the micro-scale level, samples of

opal veins comprising multiple generations of potch, as well as mixtures of potch and precious opal,

have been studied in detail.

This research has revealed a range of textures and features that demonstrate the dynamic processes

by which opalising fluids have deposited multiple generations of potch and precious opal into evolving

vein systems. This investigation also provides the basis for new lines of research into how viscous

opalising fluids have behaved to produce the myriad of opal patterns and textures encountered in opal

mines all across the GAB.

This research also supports the proposition, that the opal deposits of the GAB were primarily formed

during the Tertiary Period, by tectonically-driven near-surface fluid flow processes associated with the

antiformal warping, faulting and fracturing of water-saturated overpressured silica-rich clayey reservoir

rocks, within the upper sedimentary sequence of Cretaceous rocks making-up the GAB, as advocated

by the ―Syntectonic Model‖ of Pecover (1996).

Pan Gem Resources Opal Vein Research in the GAB

The following discusses the opal vein textural features identified during current opal genesis research

carried out by the Author, with the financial support of Pan Gem Resources (Aust) Pty Ltd.

Potch Opal Vein Textures

The opal vein textures studied, record histories of incremental and multiple episodes of opal

deposition via the injection of viscous opalising fluids. Earlier formed generations of opal in the veins

studied show evidence of hardening and subsequent brittle fracture deformation. Parallel and cross-

cutting relationships to previously formed veins were also found to be common, while fragments,

slivers and blocks of rotated wall rock clasts, incorporated into the composite structure of the veins,

resulted in complex wall rock and opal vein brecciation textures (Figure 3).

Complex patterns of cross-cutting, and vein-wall-parallel brittle-fracture micro-faulting, was also found

to be common in the veins studied. Small-scale dyke-like intrusions of opal, comprising fluidised

mixtures of both viscous and previously hardened opal clasts were found to occur along fractures in

previously hardened opal (Figure 4). Some of these micro intrusions show evidence of dissolution

and erosion/corrosion along the walls of the dykes, suggesting a vigorous and chemically corrosive

siliceous fluid flow environment, even at the micro level.



Clear textural evidence for the viscous nature and Non Newtonian fluid flow behaviour of the opalising

fluids that formed the veins examined, can be seen where injecting fluids (now frozen) display

distinctive curved fluid-flow-front patterns of movement, resulting from fluid drag along vein and

internally intrusive micro-dyke walls (Figures 3, 4 & 5). Compelling textural evidence for turbulent

viscous fluid flow of once liquid opal, which has become hardened and then fractured in a brittle

manner through hydraulic extension fracturing, leading to new opal vein formation, can be seen in

Figure 6.

Composite Potch and Precious Opal Vein Textures

Evidence of complex and dynamic processes in the formation of opal in GAB rocks, can be seen in

opal veins displaying vein-wall-parallel stratification of both potch and precious opal. Particularly

intriguing are those veins that show ―colour bars‖ sandwiched between layers of potch (Figures 7 &

8).

In some of the veins studied, the colour bars were found to be composed of precious opal comprising

distinctive close-packed fibrous crystalline structures (Pecover 1996), consistent with the formation of

colloidal photonic crystals (Colvin 2001). These crystals were found to have grown as closely-

packed, sub-parallel crystal aggregates, with individual crystals displaying extreme length to width

ratios, and orientated at right angles to vein stratification (Figure 7). In contrast, the potch layers in

these veins, on either side of the ―colour bars‖, showed both laminar and turbulent viscous fluid flow

textures.

In many of the stratified potch/precious opal veins examined, bands of potch and precious opal

display simple to very complex interlayer laminar and turbulent viscous fluid flow relationships,

involving the fluid mixing, diffusion and intrusion of multiple generations of potch and precious opal.

In these types of veins, the distinctive, undisturbed close-packed parallel aggregates of photonic

colloidal crystals of precious opal, orientated normal to vein walls, is lacking, with laminar and

turbulent fluid flows having generated a flowing "crystal mush" of broken and re-distributed photonic

colloidal crystals, creating intriguing patterns of liquid-crystal-fire (Figure 8).

Why Precious Opal is so Rare in GAB Rocks

As the growth of photonic colloidal crystals of precious opal, presumably requires relatively

stable and quiescent conditions to form (e.g. Figure 7, Specimen A.), then the textures evident in

specimens B, C & D of Figure 7, indicate that these conditions can be severely disrupted by later

invading fluid flows of viscous liquid potch opal. Furthermore, the laminar and turbulent liquid-opal-

fluid-flow conditions affecting silica gels containing disordered accumulations of generally non-

equidimensional sized silica spheres, within dynamically evolving Syntectonic opal vein arrays, would

tend to work against the orderly arrangement of close-packed silica spheres to form photonic colloidal

crystals, and accordingly, the creation of effective light diffraction gratings; so necessary for the

formation of precious opal.

Thus, in a dynamically forming and evolving Syntectonic opal vein depositional system,

channelized flowing "rivers" of viscous siliceous fluids, carrying a jumbled mass of silica spheres, from

areas of high pore pressure to areas of low pore pressure, in response to fault-controlled seismic fluid

pumping processes, would greatly favour the formation of potch opal over precious opal (i.e. because

the silica spheres in such a dynamic fluid-flow environment would have virtually no opportunity to

arrange themselves into orderly close-packed light-diffracting arrays).

In contrast, precious opal would likely only form in those "quiet" parts of the opal depositional system

(e.g. within sealed fluid-filled compartments), that allowed for the stable and orderly assembladge of



close-packed equidimensional-sized silica spheres, and the subsequent growth of photonic colloidal

crystals.

Therefore, in such a dynamic liquid-opal-fluid-flow geological environment, the formation of precious

opal would be rare and the formation of potch opal would be very common.

Opal Vein Formation

The textural features described above, together with the inclusion of comminuted wall rock clasts

within the veins examined, strongly suggests a tectonically-driven multi-cyclic process of opal vein

formation in the GAB. Such a process would likely involve cyclic fracture-induced permeability,

coupled with multiple fluid injection events associated with elevated pore pressures.

The Non Newtonian fluid flow textures preserved in the veins studied, also show the apparent mixing

of different types of viscous fluids, with some veins preserving liquid flow patterns of precious opal

intermixed with various types of potch opal. These features also suggest a cyclic formational process,

involving episodically induced fracture permeability coupled with self-sealing along dynamically

evolving fluid migration pathways (i.e. the vein arrays), and are thus unlikely to be the result of

weathering and microbial formational processes.

Given that precious opal is likely to require quiescent conditions to achieve orderly packing of silica

spheres to form a light diffracting array, then the stratification described above and the turbulent

intermixing of potch and precious opal in the veins examined, points to a far more complex and

dynamic fluid flow environment of opal deposition than has previously been contemplated by those

advocating simple passive weathering and microbe geo-formational process models.

The opal vein textures described above, clearly show that passive rock weathering processes and the

activity of so-called microbes (assuming of course that such filamentous structures reported from

Australian opal, are indeed the fossil remains of real microbes, and not simply colloidal ligands and

microbe-like silica nanoparticles, associated with colloidal chemistry processes) could not have

formed the opal veins studied, and that complex textures preserving both viscous fluid flow and brittle

fracture deformation are more likely to have been formed by fluids under pressure, associated with

tectonic processes during opal vein formation, that were imposed on the near-surface Cretaceous

sedimentary rocks of the GAB, after they were laid down.

Pan Gem Resources Opal Vein Formation Research

Applicable to Exploration along the Collarenebri Antiform

The knowledge gained from the study of opal vein textures described above, has been used by Pan

Gem Resources to better predict the depositional environments for opal deposition within GAB

antiformal structures. Those areas which display stratigraphic and structural geometries favouring the

formation of trap sites for opalising fluid accumulation, particularly within the ―flats‖ component of fault

relay-zones, are considered by the author to be high priority opal exploration targets.

A number of such prospective sites associated with high concentrations of faults and surficial deposits

of silcrete have now been identified during opal exploration along the Collarenebri Antiform. These

sites also show many of the surface indications described by Aracic (1996) that are associated with

known opal deposits across the Angledool Antiform.

Structural Setting of the Collarenebri Antiform

Structurally, the Cretaceous sediments of the Collarenebri Antiform (Figure 9) are part of the Byrock

Lightning Ridge Composite Block (Figures 10 & 11). This structural block is bounded by large



regional scale lineaments, which separate the block from the Boomi Trough in the east, the

Coonamble Embayment in the south, and the Cunnamulla Shelf to the west. The block also coincides

with the southerly extension of the Dirranbandi Syncline (Exon and Den Hertog 1972). The

Cretaceous ridge country at Collarenebri is bounded by two very large continental-scale ENE - WSW

trending lineament fracture systems. These are the Darling River Lineament to the west, which

controls the drainage of the Darling River to the south-west and the Culgoa River to the north-east,

and the Cobar-Inglewood Lineament, which controls the drainage of the Barwon River from Walgett,

north-east to where it joins the Macintyre River, west of Goondiwindi. The Darling River Lineament

and the Cobar-Inglewood Lineament are also broadly conformable to a pervasive ENE-WSW

structural trend that is evident across most of NSW (Scheibner 1973 and 1979).

At Collarenebri, the regional structural setting is dominated by the Weemelah Block and the Cobar-

Inglewood Kink Zone, with much of the area covered by opal Exploration Licences 6738 and 7650,

occurring between the south-eastern edge of the Cobar-Inglewood Kink Zone, and the north-western

edge of the Weemelah Block (Figures 10 & 11).

Opal prospective WNW-ESE-trending structures, comprising joints and fractures lie within ENE-WSW-

trending structural corridors across outcropping Cretaceous sediments of the Collarenebri Antiform.

As at Lightning Ridge, these structures appear to be prospective for the location of opal fields.

However, the aeromagnetic mapping over the Collarenebri Antiform does not support the existence of

a so-called ―Opal Corridor‖, such as the one proposed for the Angledool Antiform (which curiously

does not seem to include the opal fields of Lightning Ridge proper).

Opal Exploration Targets Across the Collarenebri Antiform

Geological interpretation of Landsat and aerial photo imagery, coupled with ground-based geological

mapping, has identified a large number of highly prospective structural and litho-stratigraphic targets

for more detailed opal exploration across the Collarenebri Antiform. In particular, faults and fractures,

striking at 0o to 10o, 30o to 45o, 60o to 90o and 325o to 355o respectively across the Collarenebri

Antiform are considered highly prospective structural targets for opal mineralisation (Figure 12).

Within this structural architecture, current exploration has located a number of areas of intense fault

clustering associated with zones of brecciation, and argillic/silicic alteration of Cretaceous sediments,

which are considered prospective for opal mineralisation at depth (Figure 13).

The most interesting of the prospective areas identified to-date, is Opal Prospect A, located in the

northern part of the Collarenebri Antiform, where both potch and precious opal have been identified

from drill hole samples. The prospect contains a number of brecciated silcrete outcrops associated

with areas of significant argillic alteration located within a fault-cluster-complex that includes both

normal and reverse faults. Prospective claystone reservoir rocks overlain by sandstone beds occur

within the prospect area; with the stratigraphy displaying Hill-Type fracture sets containing vertical

zones of brecciation and pervasive silicification (Figure 14).

Future exploration will concentrate on better defining the structural and stratigraphic architecture of

the prospective areas identified along the Collarenebri Antiform, as well as characterising the

mineralogy of the opal depositional system, and in particular, the nature and distribution of opal-

bearing dilatational jogs and vein systems within fault relay-zone ramp and flat structural settings.

Concluding Remarks

During the last 15 years, since the Syntectonic Model of opal formation in the GAB was first proposed

(Pecover 1996), considerable complimentary evidence has been presented by a number of

researchers (Glass - van der Beek 2003; Liddicoat 2003, Verberne 2004; Rey, Verberne and Glass-

van der Beck 2005; Rey & Dutkiewicz 2010).



Much of this evidence is consistent with the fault-valve tectonic processes of mineralising fluid flow

formation, proposed by Sibson (1987, 1989, 1990, 1994, 1996 & 2000), associated with fault-fracture

mesh development, and the seismic pumping of geo-fluids in ore deposit settings across the planet.

Importantly, such processes are known to be applicable to the formation of fault-controlled vein-type

ore deposits in a variety of geological settings (Micklethwaite and Cox 2004).

However, many ―old-school‖ proponents of traditional weathering and biological processes still

advocate opal forming mechanisms which do not adequately explain the formation of opal veins in the

GAB.

As the head of a successful, privately funded, gemstone exploration and mining company, that has

been in business for over 15 years, I for one would not invest in opal exploration and resource

development projects, based on the formational mechanisms advocated by both the deep weathering

and microbe models of opal deposit genesis in the GAB.

From a practical gemstone explorationist perspective, I believe that geo-formational models that

provide little predictive value for targeting drill holes to discover new opal deposits are of no

commercial value to the Opal Industry going forward.

Rather (and in deference to the on-going debate), in the absence of the application of any particularly

agreed geological model to aid in the search for opal deposits across the GAB, newcomers need only

to initially acquaint themselves with the published works of long-time opal miner, Mr. Stephen Aracic

(1996), to understand and appreciate the most important and practical indications to observe when

exploring for and mining opal deposits throughout the Great Australian Basin.

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Diagram Modified After Micklethwaite & Cox 2004

Figure 1. Geological characteristics consistent with the Syntectonic Modelof opal formation in the Great Australian Basin.

Opalising fluids derived from

overpressured reservoir rocks,

such as compressed claystones

Dr. Simon R. Pecover, 2011

Figure 2. Angledool Antiform, within the Narran-Warrambool Opal Mining Reserve.Modified after NSW Mineral Resources opal fields Landsat imagery.

Figure 3. A. Horizontally laminated vein of potch opal, cross-cut by multiple fracturesthat have been in-filled by later generations of opal, all showing viscous fluid flow andbrittle fracture deformation textures. B. Complex vein of intermixed potch and preciousopal, showing curved viscous fluid flow fronts around brittle-fractured wall rock clasts.

A

B

Micro faults, fractures and opalveins cross-cutting earlier

formed potch layers

Layer-parallel fracturesin-filled by later a

generation of potch

Anastomosing fracturein-filled by a possible

4th generation of potch Micro potchbreccias

Multiple generations of potchshowing viscous fluid flow textures

Potch vein containingfragments of earlier-formed opal

Curved viscous preciousopal fluid-flow-fronts

Sandy claystone clastsripped from vein wallsand carried along in

a viscous liquidopal fluid flow

Opal veinletshowing sense of

hydraulic extension

Mixing of potchand precious opal

viscous fluids

Brecciated wallrock clasts

Curved viscous opalfluid-flow-fronts

Sandy claystone wallrock host to opal veins

Fluid mixingfront

Fluid mixingfront


Dr. S. R. Pecover Specimen & Photo


Figure 4. A. & B. Composite veins of potch showing numerous complex relationshipsbetween multiple generations of opal, showing a variety of viscous fluid-flow behaviors.Dyke-like intrusions are clearly evident in both specimens, as are well developed curvedviscous fluid-flow-fronts of once liquid opal. Rheological differences between opalgenerations, ranging from plastic to brittle fracture deformation, are also clearly evidentin these veins.

A

B

Curved viscous fluid flow-frontsof once liquid opal

Intrusiveopaldyke

Sandy claystone wall rock

Wall rock brecciation

Intrusiveopal dyke

Patterns ofturbulent viscous

fluid-flowBrecciated

opal

Curved viscousfluid-flow-fronts

of once liquid opal

Intrusiveopaldyke

Laminar viscousfluid flow

Laminar viscousfluid-flow

Laminar viscousfluid-flow

Fluid mixingfronts




Figure 5. A. & B. Composite veins of potch showing numerous complex relationshipsbetween multiple generations of opal, showing a variety of viscous fluid-flow behaviors.Dyke-like intrusions are clearly evident in both specimens, as are well developed curvedviscous fluid-flow-fronts of once liquid opal. Rheological differences between opalgenerations, ranging from plastic to brittle fracture deformation, are also clearly evidentin these veins.

A

B

Curved viscousfluid-flow-fronts

of once liquid opal

Sandy claystone wall rock

Large clast of earlier-formed potchshowing laminar & turbulentviscous fluid-flow patterns

Patterns ofturbulent viscousfluid-flow within

laminar flow

Brecciated opal & sandy claystone wall rock clasts

Brittle-fracturedclasts of potch

Fractured opal veins& sandy claystone wall rock

Micro normal fault

En echelon micro reversefaults with intrusive

potch dykes

En echelon microreverse faults

Fractured opal veins& sandy claystone wall rock

Rotated potchclast

Patterns ofturbulent viscous

fluid-flowFluid mixing

fronts




Figure 6. Turbulent and laminar viscous fluid-flow of potch opal, cross-cut by hydraulic extension fractures in-filled bya later generation of potch opal. The turbulent fluid flow patterns evident in this specimen, record classic Non Newtonianviscous flow behavior. While ever these processes are at work within a developing and evolving Syntectonic opal veindepositional system, precious opal is prevented from forming (which is why precious opal is so rare).

Laminar viscous fluid-flowof liquid potch opal

Turbulent viscous fluid-flowof liquid potch opal

Hydraulic extension fracturesin-filled with a later generationof viscous liquid potch opal,forming a younger opal vein

within an older opal vein



A B

C D

Figure 7. A. Colour bar within translucent potch, comprising parallel photonic colloidalcrystals of precious opal that have grown at right angles to the prevailing opalising fluidlamination. B. Colour bars of precious opal within wavy viscous fluid flow layers of potch.C. Colour bars of precious opal being invaded, corroded and disaggregated by viscousfluid flow layers of potch. D. Photonic colloidal crystals of precious opal breaking apartand moving within laminar and wavy viscous fluid flows of potch opal.

Len Cram Photo

Potch

Potch

Photonic colloidalcrystals of precious opal

Potch

Potch

Potch

Potch

Potch

Photonic colloidal crystalsof precious opal breaking-up

in laminar & viscous fluid flowingpotch opal

Laminar & wavy viscous fluid

flowing potch opal

Corrosion & disaggregationof precious opal layer by

potch opal fluid flow

As the growth of photonic colloidal crystals of precious opal presumably requiresrelatively stable and quiescent conditions to form, then the textures evident in samplesB, C & D suggest that these conditions can be disrupted by later invading fluid flows ofviscous liquid opal. Furthermore, the laminar and turbulent conditions of liquid opal fluidflow within dynamically evolving Syntectonic opal veins would tend to prevent the orderlyclose-packing of silica spheres and the creation of light diffraction gratings.

Thus, in a dynamically forming and evolving Syntectonic opal vein depositional system,containing flowing "rivers" of viscous liquid opal, moving from areas of high pore pressureto areas of low pore pressure, in response to fault-controlled seismic fluid pumpingprocesses, potch opal formation would be greatly favoured over the formation of preciousopal, with precious opal only being able to form in those "quiet" parts of the opaldepositional system that allowed for the stable and orderly assembly of close-packedequidimensional sized silica spheres, and the subsequent progressive growth of photoniccolloidal crystals.

Therefore, in such a geological environment, the formation of precious opal would be rareand the formation of potch opal would be very common.


A B

C D

Figure 8. A.-D. Laminar and turbulent viscous fluid-flow patterns of multiple generationsof potch and precious opal.

Horizontal layers ofpotch and precious opal.

Curved viscous fluid-flow-frontsof once liquid precious opal

Fluid-flow direction

Len Cram Photo

Laminar to turbulentviscous fluid flowsof precious opal,

comprisingbroken

fragmentsof photonic

colloidal

crystals

Laminar to turbulentviscous fluid flows

of potch opal,intruding

"crystal mush"of precious

opalPrecious opal"crystal mush"

Precious opal"crystal mush"

Fluid flow patternsin potch opal



Fluid flow patterns

in potch opal

Fluid-flow-frontsbetween potch

andprecious opal

Fluid-flow-frontsbetween potch

andprecious opal

In all of the above samples, complex patterns of intrusion and mixing of different generations of viscous liquidpotch and precious opal are evident. Distinctive, undisturbed close-packed parallel clusters of photoniccolloidal crystals of precious opal, orientated normal to vein walls, are lacking in these specimens, with

laminar and turbulent fluid flows having generated flowing "crystal mushes" of broken andre-distributed photonic colloidal crystals, creating intriguing patterns of liquid-crystal-fire.


Figure 9. Landsat image of the Collarenebri Antiform covered by opalExploration Licence 6738.


Figure 10. Structural settings of the Angledool and CollarenebriAntiforms.

Figure 11. Structural and magnetic settings of the Angledool andCollarenebri Antiforms.


Figure 12. Structural architecture of the Collarenebri Antiform, showingmajor interpreted lineaments and faults.


Figure 13. Opal exploration targets across the Collarenebri Antiform,located in areas of high intensity faulting (Note: Many more targets havebeen identified than just the ones shown here).


Figure 14. Stratigraphy of Opal Prospect A, showing vertical Hill-Typefractures containing breccia pipes in the NE sector of the prospect.A high degree of silicification and argillic alteration of the fault damagezones are evident in this sector, which are overlain by thick deposits ofsurficial silcrete. Both potch and precious opal have been recorded fromthis Prospect.
