We describe a unique occurrence of a world-class Proterozoic ore deposit, Century, affected by an Ordovician meteorite impact, the Lawn Hill impact structure. The meteorite excavated a complex crater with a Proterozoic core surrounded by an annulus filled with Cambrian carbonate rocks. The Century deposit is located at the southwestern edge of the crater and is bounded by postore faults that indicate an originally larger orebody. It is overlain by breccias, which contain evidence of impact-related textures that we interpret as fallback suevite. (Suevite is a rock consisting partly of melted material, typically forming breccia containing glass and crystal or lithic fragments, formed during an impact event.) Above the suevite are slumped Cambrian carbonate and Proterozoic shale, including a million-tonne block of Century ore detached from the main orebody and contained in the carbonate breccias. The Cambrian rocks were partially consolidated prior to impact, became fluidized on impact, and then resurged into the crater from surroundings areas. The resurge process resulted in a fivefold thickening of carbonates in the annulus, preserved to 600-m depth. Beneath the suevite, in the more competent orebody, fracturing and slab formation were the dominant responses to the impact. A restoration of the displacements on the impact-related faults suggests that the orebody was moved toward the crater. The inference is that, prior to impact, the deposit continued for several hundred meters beyond the current faulted northern boundary of the orebody and that this missing segment was displaced and may be buried within the annulus.

The Mt Isa-McArthur region is renowned for a range of commodities and deposit types of world-class proportions. The region is described here in the context of a 'mineral system,' through consideration of processes that operate across a range of scales, from geodynamics and crustal architecture, to fluid sources, pathways, drivers and depositional processes. The objective is to improve targeting of Pb-Zn, Cu and Cu-Au deposits. Repeated extension and high heat flow characterise much of the history prior to 1640 Ma. The pre-Barramundi Orogeny (pre-1.87 Ga) metamorphic basement was the substrate on which a volcanic arc developed, focussed along the Kalkadoon-Leichhardt Belt. This is related to an inferred east-directed subduction between 1870 and 1850 Ma. From 1755 to 1640 Ma, three successive volcano-sedimentary basins developed, the Leichhardt, Calvert and Isa Superbasins, in an interpreted distal back-arc environment. The Isan Orogeny, from 1640 to 1490 Ma, overlapped with Isa Superbasin sedimentation, suggesting a transition from back-arc to a foreland basin setting. Most crustal thickening occurred in the Eastern Fold Belt, an area earlier characterised by thinned crust and deep marine environments. This region was deformed into nappe-like structures with high-temperature-low-pressure regional metamorphism and associated granites; the latter are absent from the Western Fold Belt. Metal deposition mainly occurred late in the history, with all known (and preserved) major base metal occurrences either hosted by Isa Superbasin rocks or formed during the Isan Orogeny. Earlier superbasins were potential fluid source regions. Sedimentary formation waters, metamorphic and magmatic fluids were present at prospect scale, while meteoric and possibly mantle sources are also implicated. The spatial distribution of metallogenic associations (i.e. iron oxide-copper-gold, Pb-Zn-Ag, U, Au) across the inlier may result from differences in the geodynamic make-up and evolution of the pre-1.87 Ga tectonic elements. Penetrative faults are interpreted as predominantly steeply dipping and to have acted as pathways for fluids, both in extension and compression. Fluid mixing was a potentially significant ore deposit control. Examples are drawn from the Ernest Henry iron oxide-copper-gold-related hydrothermal breccias in the east and from the Mt Isa Copper deposit in the west. Stress switching during late-stage deformation appears to have triggered a fluid mixing event that led to formation of the major copper deposits.

Magnetotelluric soundings were obtained along two traverse lines to the north and west of the Century mine in northwest Queensland. The survey was designed to cross the Termite Range Fault, a major structure on the Lawn Hill Platform, and to provide insights into the crustal-scale architecture that may have controlled the location of this world-class zinc deposit. The projected surface trace of the Termite Range Fault is coincident with a major change in resistivity character that extends to a significant depth. A relatively flat-lying, stacked series of resistive/conductive layers occurs on the northeastern side of the fault , while on the southwestern side the resistive/conductive layers are much less evident. The major contrast in resistivity is interpreted as due to a steep northeast-dipping Termite Range Fault that may extend to 20 km depth. To the southwest of the Termite Range Fault, a second major fault, the Riversleigh Lineament, is inferred from geology and gravity data, although there is no corresponding resistivity contrast seen across this fault in the magnetotelluric-derived model. This fault is interpreted as a buried structure, as distinct from the reactivated Termite Range Fault, and the two faults together may have created a wide damage zone (with an associated strike change) in the crust. A regional-scale 3D geological model of the Lawn Hill Platform provides a basis for correlating the resistive/conductive layers with major lithological units in the area. The stacked layers in the 2D resistivity inversion model of the Termite Range Fault hangingwall are reasonably well correlated with lithological units, particularly in the near-surface. A key point is that although similar geological units occur on either side of the Termite Range Fault, the contrasting electrical properties of these units are pronounced and their source is not well constrained; increased carbonaceous material in the Termite Range Fault hangingwall units is implied. In addition, there is a strong gradient in the Bouguer gravity field in the region of the Termite Range Fault and Riversleigh Lineament structures. This gradient provides supporting evidence for a northeast-facing fault structure in the basement and cover architecture. Newly acquired seismic data in the area has yet to be evaluated and compared with the magnetotelluric model. These results demonstrate an important role for magnetotelluric soundings in determining resistivity contrasts relating to the configuration of geological units and the architecture of deep-seated mineralising faults.

The three-dimensional crustal architecture of the eastern part of the Mount Isa Inlier is investigated from serial cross-sections constructed using geological map data, revised chronostratigraphy, gravity, magnetics, worms (multiscale wavelet edges of potential field data) and seismic data. The top part of the crust consists of rift and platform type metasediments that were deposited in three cover sequences from 1850 to 1610 Ma. These rocks constitute the Mount Isa Eastern Succession, and they were intruded by mafic–felsic plutons, dykes and sills of various ages before and during the Isan Orogeny (ca. 1.6–1.5 Ga). The Eastern Succession overlies a felsic metamorphic basement, which in turn sits on a tonalitic–gabbroic lower crust. The depositional basin architecture for the Eastern Succession was controlled by major N–S trending structures that penetrated the lower crust, and accommodated E–W extension. These structures also underlie major upper crustal structures such as the Mitakoodi Culmination and Snake Creek Anticline that were formed by contraction in the Isan Orogeny. Positive inversion may therefore have been a key process in the evolution of the eastern part of the inlier, and governs its architecture at the crustal scale. Inversion involved reactivation of basement-penetrating structures, which localised contractional structures in the cover sequences above, as well as influencing pluton emplacement. The felsic metamorphic basement may have been penetratively deformed during inversion. The spatial association between the basin-controlling and contractional structures suggests that either early extensional displacements were completely reversed by later contraction, or that much of the Eastern Succession has remained essentially parauthochthonous relative to the basement.

We explore three- and two-dimensional, regional- and local-scale numerical models for the development of Irish Zn-Pb-Ag mineralization. The calculations examined one of the proposed genetic models for the deposits in which mineralization was associated with mixing of two fluids of different salinities in the hanging walls of faults undergoing active deformation during mineralization. The numerical models involve coupling between volume changes arising from elastic-plastic deformation, single-phase fluid flow, and mixing of fluids with different salinities together with thermal transport both by conduction and by advection. We evaluated critical parameters that could optimize the rate of mineralization in three dimensions at the regional scale and in two dimensions at the local scale. Topographic relief is proposed as one driver of fluid flow. The models indicate a competing process, through convection of high-salinity fluids, mainly within faults and originating as partially evaporated seawater, which mix with lower salinity fluids driven by topographic relief. At the regional scale, although there was substantial fluid flow through the basinal sediments, considerable flow of basin-derived fluids also occurred through the basement, with subsequent focusing through faults. The effect was enhanced if the basement permeability was increased by fracturing. Basinal fluids (fluid 1, initially in the basin or introduced from meteoric sources in the adjacent highlands) were extensively exposed to the basement and became focused into both northward- and southward-dipping faults. A second, more saline fluid (fluid 2) was driven down into the rock column by convection; it had most exposure to the basinal sediments and, in some situations, to the basement before mixing with fluid 1. The positions of the fluid mixing sites involved a balance between topographically driven flow and convection in three dimensions at a regional scale. Changes in porosity and permeability induced by plastic deformation influenced the rates of mineralization at the local scale and local Darcy flow rates at mineralizing sites. Mineralization at the local scale resulted from the interplay of fault dip direction, displacement, deformation-induced dilation, seal thickness, and integrity. The modeling results are compatible with empirical data from the Irish ore field, suggesting that north-facing faults, dipping away from the inferred position of topographic highs, preferentially host economic deposits. There is nothing intrinsic in the modeling that constrains the timing of the mineralization relative to compressional or extensional events, although a body of research favors mineralization overlapping with extension, as at the Navan, Silvermines, and Tynagh deposits. In contrast, at the Lisheen deposit, mineralization postdates the main extensional fault offsets. We suggest that a switch to shortening is a possible driver for this mineralizing system.

At a regional, terrane scale, it is often found that the long strike length faults and related potential field gradients correlate with elevated metal distributions. This is linked to the concept that large dimension structures are intrinsically weaker, wider damage zones that act as pathways for fluids at different crustal levels. A GIS-based technique is described here that uses the strike length of fault-related structures as an area selection filter. This methodology is especially relevant when exploring in under-cover regions using potential field (gravity, aeromagnetic) data which has been processed using edge detection routines. If applied at an early stage in the exploration process, this can lead to a significant reduction in the search area, and appears to increase the chance of discovery.

A 150 x 150 km area of western Victoria has been modelled in three dimensions to a depth of similar to 20 km. This was constructed through integrated analysis and serial cross-sections of geological and geophysical datasets, utilising mapped positions of major faults, intrusive bodies and lithostratigraphic packages as primary inputs. These are extrapolated to depth and under cover through interpretation of upward continued multiscale wavelet edges of aeromagnetic and gravity data, inversion of the gravity field and the positions of acoustic boundaries. The objective was to develop an understanding of crustal structure in the context of gold mineralisation potential. The upper crustal structure is modelled as comprising a planar array of northwest-trending, steep to moderate inclined mainly east-dipping faults (Moyston, Pleasant Creek). These are interpreted to merge with a basal detachment (Western Fault), This elongate dome-shaped detachment overlies a buried wedge of inferred Proterozoic basement. We suggest that gold distribution in the upper crust may be influenced by the position in the mid-crust of the leading edge of the wedge and its interface with the mafic substrate that largely encloses it. The Coongee Fault appears to be a first-order regional-scale control on the localisation of gold deposits adjacent to basaltic dome prospects in the Stawell corridor. It represents a backthrust, with superimposed sinistral transpression, and is interpreted to have developed above the mid-crustal ramp detachment, Cross-faults that intersect the Coongee Fault may have high exploration potential for localising both orogenic- and intrusion-related gold.

3D models and computer-based numerical simulations have been used in the exploration industry for some time to visualise the geometry and mechanisms resulting in the formation of orebodies, However, due in part to computational limitations, few numerical simulations have been run on complex (real) geometries in order to predict the location of new ore systems. Presented here are the results of an exploration program developed by the Predictive Mineral Discovery Cooperative Research Centre (pmd*CRC) and MPI (now Leviathan Resources) in the orogenic-gold system of western Victoria that utilised 3D modelling and numerical finite-element simulations to successfully target several new orebodies and predict their geometries and extent. Existing drillcore databases were utilised to constrain the geometries of known deposits and associated mafic domes, the effects of known post-mineralisation faulting was systematically removed and syndeformation fluid flow was then modelled within the system. The results of these simulations were compared with the known geometry of the mineralised systems about these deposits in order to test the simulation parameters and accuracy. 3D models were also developed of poorly constrained target domes in regions with no outcrop utilising potential-field datasets and limited drilling data. Simulations were then run on these model geometries using the tested parameters in order to predict the likelihood of mineralisation in these systems, its geometry and (most importantly) its location. These targets were then drilled resulting in the discovery of previously unknown gold deposits associated with the Kewell Dome northwest of Stawell.

This paper presents an analysis of the Global Mineral Occurrences Database (GMOD), a zinc deposits database built by Pasminco Australia Ltd to cover zinc mineral occurrences over the entire globe. The database contains over 14 000 records of which 1700 contain resources information. A resource is taken as the total in-ground resource, including past production and current resources. An economic resource classification has been applied to all deposits with resources which divides them into four categories, high grade zinc deposits, low grade zinc deposits, deposits where Cu–Au is economically more significant than Zn–Pb–Ag and minimum grade zinc deposits. Analysis of the data shows that the total world endowment of zinc metal, defined in resources, is over 881 Mt. North America has the greatest endowment of zinc metal and the most deposits. Africa has the least. Canada, the US, China and Australia are the best endowed countries with Australia having larger and higher grade deposits. The Palaeoproterozoic and the Upper Palaeozoic were together the two greatest zinc producing periods in earth history accounting for 41% of global zinc metal. VHMS deposits are the most numerous ore deposit type, totalling 38% of all deposits in the world with zinc. By comparison, shale-hosted deposits are not numerous but are large and high grade, hosting over 18% of the world's zinc. Zinc oxide deposits (both primary and secondary) host less than 4.5% of the world's zinc. Carbonatehosted deposits of the MVT class provide the highest grade and cleanest (low iron) zinc sulphide concentrates, but include some of the lowest grade zinc deposits and also those with the lowest silver values. Analysis of contained metal and the grade of zinc deposits show that four deposit classes are potentially 'superior' from an economic perspective. These are shale-hosted, Irish-type carbonate-hosted, intrusionrelated mantos and Broken Hill type deposits. Together these four classes account for 32% of the world's zinc metal in only 108 deposits or 12% of deposits with resources. Because zinc oxide deposits represent such a small proportion of the world's zinc, future exploration will still have to focus on zinc sulphide deposits and with a significant emphasis on the Palaeoproterozoic and Upper Palaeozoic terranes that have already provided a high proportion of the worlds best zinc deposits.

The Global Zinc Project was developed by Pasminco Australia Ltd to provide a world view of zinc mineralisation and a methodology to prioritise geological provinces for exploration and the discovery of potentially economic ore deposits. The project grew in response to increasing risk from falling global discovery rates, maturing of exploration terranes in traditional areas, and the uncertainties of quantifying the risks for mineral investment in newly emergent countries. Components of the project involved the digital integration of a global mineral occurrences database, delineation of zinc provinces, ore deposit models, reverse economic studies, plate tectonic reconstructions and country risk analysis. The data were analysed in two separate modules. The Prospectivity Rating Module provides a quantitative comparison between the observed geology of a province and the mappable features of ore deposit models. It ranks the prospectivity of a province for the potential occurrence of one or more orebodies of a particular type. The Combined Rating Module incorporates the prospectivity rating with other risk factors, geological, commercial and sovereign to provide an overall rating for the mineral province. Four major questions encompass the range of risk elements: (i) is there an undiscovered and potentially economic orebody in the province; (ii) can it be found by available exploration technology; (iii) will the company get access to the ground to find it; and (iv) what is the sovereign risk? This paper focuses on methodology, with some example outputs. The methodology has the advantage of being able to compare data from many different countries, mineral provinces and deposit styles on a level playing field. It provides a template for rigorous data collection and assessment, allows for the identification of missing data, and key risk issues in each mineral province. It also provides a basis for presenting the exploration decision making process to non-technical stakeholders. An additional outcome of the Global Zinc Project was the confirmation that four classes of zinc deposits have a superior ability to yield high grade and potentially economic orebodies. These are the Broken Hill type, shale-hosted deposits, Irish-type carbonate hosted deposits and intrusion-related mantos.

The phyllitic mudrocks, quartz wacke sandstones and mélange units of the Annascaul Formation in the SE Dingle Peninsula, SW Ireland have been previously assumed to be Silurian in age, similar to overlying fossiliferous Wenlock rocks of the Ballynane Member. The contact between the Ballynane Member and the underlying Annascaul beds is either faulted or unconformable , and the Annascaul mudrocks contain two or more cleavages not present in the Silurian Ballynane and younger strata. The Ballynane Member is therefore differentiated from the Annascaul Formation by upgrading the proven Wenlock Ballynane beds to formational status. Grey and black mudrocks in the Annascaul Formation have yielded diagnostic palynomorph assemblages indicative of an Early Ordovician age. The Annascaul Formation invites comparison with similar Early Ordovician rocks, south of the putative trace of the Iapetus Suture, in SE and eastern Ireland, in the English Lake District, and in central Newfoundland.

A newly discovered brachiopod fauna from the Hilltown Formation (lower Llanvirn), Bellewstown Terrane, eastern Ireland, contains elements more typical of sites within the Iapetus Ocean than those associated with the bordering platform provinces. The association of Paralenorthis, "Ahtiella," and Jaanussonites suggests a comparison with the more diverse fauna of the Summerford Group on New World Island, central Newfoundland, and correlation of the Bellewstown Terrane of the Irish Caledonides with parts of the Dunnage and, possibly, Gander terranes of the northern Appalachians.

The Iapetus suture in Ireland and Britain is that line which separates Caledonian rocks of the Laurentian and Avalonian continents. The suture is cryptic: nowhere is there an exposed fault-zone containing ophiolite remnants, blue-schist melanges, or trench deposits. Instead a suture line may be traced with varying degrees of confidence through a series of faults that traverse the Iapetus suture zone, which contains two or more tectonostratigraphic terranes. Several data sets are utilized to constrain this trace. The distribution of faunal provinces caused by oceanic separation can be used most reliably to define early Ordovician terranes. Fauna1 intermingling in the mid-Ordovician reduces provinciality and the confidence of terrane identification using faunal data. The late Ordovician and particularly Silurian tectonostratigraphic histories of terranes either side of the suture suggest that amalgamation of the terranes, elimination of the Iapetus Ocean and development of the suture zone had begun by the Ashgill. Therefore the resolution of the suture trace becomes unreliable where Ordovician rocks do not crop out, because Silurian turbidite fans of Laurentian provenance may have dispersed across a significant width of Avalonian crust. Similarly, the precision of structural identification of the suture zone, by correlating the suture with a major fold-facing confrontation, may be weakened by late overthrusting. Tectonic interleaving of crustal flakes in the zone is supported by geophysical evidence which suggests that some major boundaries in the lower crust do not always coincide with their inferred projections in outcrop. Tectonic and sedimentary mixing of crustal blocks and sediments in the zone is further reflected by Nd and Pb isotopic patterns; the Rb-Sr patterns of granites record a stronger distinction between north and south of the suture. Despite these difficulties, we depict a possible suture trace in Ireland that departs significantly from the traditionally placed line by following a NE-SW-aligned trace through the Slane fault and the Navan-Tipperary lineament, and thence through a dog-leg to pass north of the Dingle Peninsula.

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A suite of microconglomerates is recognized in Silurian rocks which occur on both sides of the proposed line of the Iapetus suture in Ireland. Clast composition and palaeocurrent data show that these conglomerates, which grade into the typical quartz-rich Silurian turbidites, were derived from two compositionally similar magmatic arc terranes which lay on either side of the present Silurian outcrop. In the Llandovery, derivation was from both the south and the north. In the Wenlock, derivation was from the north and sedimentation prograded southwards across the ‘suture’ and onto the southern margin. The source terrain in the south was probably the Ordovician Wexford–Lake District arc. We identify the northern source as another arc (Cockburnland) which has since been cut out by sinistral strike-slip against the Ordovician Northern Belt. These data imply that arc activity ceased synchronously on either side of Iapetus during the late Ordovician and this leads us to speculate that subduction of oceanic crust ended at that time. Closure was associated with deformation and uplift of the bounding Ordovician terrains. These rocks then contributed detritus to the Silurian infill of a successor basin. Regional sinistral transpression finally deformed and reorganized these units between the end Silurian and the early Devonian and led to the complete closure of the remaining Silurian seaway.

Research Article| December 01, 1986 Comments and Replies on "Is the Southern Uplands of Scotland really an accretionary prism?": REPLY F. C. Murphy; F. C. Murphy 1Department of Geology, University College, Dublin 4, Ireland Search for other works by this author on: GSW Google Scholar D.H.W. Button D.H.W. Button 2Department of Geological Sciences, University of Durham, Durham DH1 3LE, England Search for other works by this author on: GSW Google Scholar Author and Article Information F. C. Murphy 1Department of Geology, University College, Dublin 4, Ireland D.H.W. Button 2Department of Geological Sciences, University of Durham, Durham DH1 3LE, England Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1986) 14 (12): 1044. https://doi.org/10.1130/0091-7613(1986)14<1044:CAROIT>2.0.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Permissions Search Site Citation F. C. Murphy, D.H.W. Button; Comments and Replies on "Is the Southern Uplands of Scotland really an accretionary prism?": REPLY. Geology 1986;; 14 (12): 1044. doi: https://doi.org/10.1130/0091-7613(1986)14<1044:CAROIT>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract No Abstract Available. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

AbstractTransected F1 fold structures in eastern Ireland are associated with subhorizontal stretching in the S1, cleavage whereas axial planar cleavage contains a vertical elongation direction. This suggests that the non‐axial planar cleavage was influenced by a distributed strike‐slip ductile shear.A major NE‐SW trending F1 syncline is described in which the minor F1 folds show systematic variations in cleavage transection parameters. On the steep limb of the major syncline the cleavage transects the minor F1 folds in a consistently clockwise sense, whereas on the normal limb anticlockwise transected folds are seen. Axial planar cleavage occurs at the core of the major syncline.Fold profile analysis indicates that the buckling of the layers began before the initiation of the cleavage. Open, parallel folds at the major synclinal hinge zone are progressively ‘flattened’ on the steep limb towards a major D1 sinistral transcurrent fault. The angular transection, A, attains a maximum of 15° clockwise which diminishes to <5° at higher strains adjacent to the major fault. Incremental fibre growth in pressure shadows show a two‐stage tectonic strain superposition of vertical pure shear followed by sinistral transcurrent simple shear during the development of the clockwise transecting cleavage. Anticlockwise transected folds were influenced by local dextral strike‐slip on the southern margins of a rigid terrane. As a regional feature, the clockwise transection is explained by a sinistral transpressive deformation of end‐Silurian age.