Structural Corridors Research Articles

Integrated geological mapping approach and gold mineralization in Buhweju area, Uganda

Deeply dissected mountainous ridges, paucity of exposure, deep weathering, extensive regolith, dense vegetation cover and poor accessibility, are detrimental to conventional field geological mapping. Conditions such as these, characterize our current study area; the Buhweju area in Uganda. The latter is poorly explored and known to host alluvial gold. In this work, an integrated geological mapping approach aiming at updating the geology, building relationships between the lithology and structure and assessing its control on mineralization was tested using Landsat TM, shuttle radar topographic mission (SRTM) and potential filed datasets (radiometrics and magnetics) subsequently validated by field investigations. Several image processing and enhancement techniques were applied that improved the aesthetic quality of the various remotely sensed datasets for visualization and interpretation. Among the image enhancement techniques, image fusion and pan-sharpening of the radiometric composite image with multispectral data and SRTM digital elevation model (DEM) have resulted in better lithological differentiation. In general, the present work, found complementarity in lithology with the legacy geological map. On the basis of similar radiometric signature however, various basement rock units such as pelitic schist, quartzo-feldspathic gneiss and granite were delineated and mapped; some in areas which were not reported before. These were validated during field investigation. Similarly, the aeromagnetic data depicted worthwhile information regarding litho-magnetic domains and tectonics of the Buhweju study area.Three prominent structures trending NNW, ESE and NNE determine the tectonic grain of the area. These orientations are consistent with the two major and important regional tectonic trends known in the area; the NNE to the western Rift while the NNW attest to the regional Aswan shear zone. The control of the Buhweju gold field was also assessed in terms of the improved geological units and interpreted lineaments of the area. Accordingly, most gold occurrences are localized in preferred orientations and host rock. Density analysis carried out on surface and subsurface lineaments reveals the Buhweju group rocks and the undifferentiated schist and amphibolites are highly affected by surface lineaments whereas the basement rocks are affected mainly by subsurface lineaments. Two major clusters of gold occurrences were identified in the Buhweju gold field. The gold cluster found at Kitaka mine is characterised by a low magnetic anomaly and high lineament density, while the gold occurrences found at Katonga is hosted by mudstone and Lubare quartzite and underlain by a high magnetic anomaly. These gold clusters are spatially controlled by the NNW and NNE trending faults and their intersection determines the localization of most of the Buhweju gold occurrences. The two trends of structures in Buhweju could also be interpreted in terms of the Geita and Twangiza structural corridors (gold trends) known from the work of others. The NNW trending Geita structural corridors control gold mineralization in Tanzania while the NNE trending Twangiza corridor controls gold mineralization in the Democratic Republic of Congo (DRC).

Structural controls on the primary distribution of mafic–ultramafic intrusions containing Ni–Cu–Co–(PGE) sulfide mineralization in the roots of large igneous provinces

Deposits of Ni–Cu–Co–(PGE) sulfide often occur in association with small differentiated intrusions that reside within local transtensional spaces in strike-slip fault zones. These faults often develop in response to incipient rifting of the crust and the development of large igneous provinces due to far-field stresses generated by plume-induced continental drift. We review the geology of a number of large and small nickel sulfide deposits and the associated intrusions, and show that the geometry of the host intrusion and localization of the mineral zones can be classified into three main groups. Further, we show that the morphology of each is controlled by space created in response to deformation on structures.One group of intrusions has the plan shape of an asymmetric rhomboid with the long axis sub-parallel to a fault zone, and contacts which have often been structurally modified during and/or after emplacement of the magma. The typical cross section is a downward-closing cone shape with curved walls and often a dyke-like keel at the base. This morphology is found in the Ovoid and Discovery Hill Zones of the Voisey's Bay Deposit (Canada), the Jinchuan, Huangshan, Huangshandong, Hongqiling, Limahe, Qingquanshan, and Jingbulake (Qingbulake) Intrusions in China, and the Eagle and Eagle's Nest deposits in the USA and Canada, respectively.A second group of deposits is associated with conduits within dyke and sheet-like intrusions; these deposits are often associated with discontinuities in the dyke which were created in response to structural controls during emplacement. Examples include the Discovery Hill Deposit and the Reid Brook Zone of the Voisey's Bay Intrusion, where there are plunging domains of thicker dyke which control the mineralization inside the dyke, and thin discontinuous segments of the dyke which are associated with structurally controlled mineralization in the surrounding country rock gneisses. The Oktyabrysk, Taimyrsk, Komsomolsk, and Gluboky Deposits in the Noril'sk Region of Russia are localized at the base of thicker parts of the Kharaelakh Intrusion which appear to be a conduit that follows synformal features in the country rocks located west of the Noril'sk–Kharaelakh Fault. Other examples of dyke-like bodies with both variation in width and the development of discontinuities are the Copper Cliff and Worthington Offset Dykes which radiate away from the Sudbury Igneous Complex (Canada). The distribution of ore bodies in these Sudbury Offset Dykes is principally controlled by variations in the thickness of the dyke, interpreted to reflect the presence of conduits within the dyke.A third group of mineralized intrusions located within structural corridors have the geometry of oblate tubes; examples include Kalatongke in China, Northeastern Talnakh and Noril'sk 1 in Russia, Babel–Nebo in Australia, and Nkomati in South Africa. Sometimes these oblate tube-like intrusions form in bridging structures between larger intrusions hosted in the more significant structures. Examples include the Tamarack Intrusion in Minnesota, USA, and the Current Lake Complex in Ontario, Canada, both of which contain magmatic Ni–Cu sulfide mineralization.In all of these deposits, the intrusions appear to be open system magma pathways, and so the term “chonolith” can be applied to describe them as a group. All of these intrusions are characterized by a high ratio of sulfide/silicate; there are 1–3 orders of magnitude more sulfide in the intrusion than the magma contained in the intrusion is capable of dissolving. The formation of these deposits is considered to have taken place in open system magma conduits. It is possible that the metal tenor of the sulfides were upgraded by equilibration of successive batches of silicate magma passing through the conduit, and equilibrating with a stationary pool of magmatic sulfide. At Voisey's Bay there appears little doubt that the sulfides were injected through a conduit dyke into higher level magma chambers. A similar model has been proposed for the formation of the deposits at Jinchuan and Noril'sk–Kharaelakh. Economically significant nickel sulfide deposits that tend to be high in Ni tenor, are often related to the late injection of magma that form distinct parts of the intrusion, and the localization of mineralization tends to be related to changes in the geometry of the magma chamber. Strongly deformed and metamorphosed komatiite-associated deposits (e.g. Pechenga, Thompson, and the Yilgarn komatiite associations) appear to be the remains of open system magma conduits which are now represented by segmented and boudinaged ultramafic bodies as a result of more than 4 phases of post-emplacement deformation.LIP activity at craton margins has long been recognized as a key control on the genesis of magmatic sulfide deposits; we show that the principal regional controls of strike-slip tectonics underpin the local geometry of the intrusions, and we provide an explanation for why so many of the global nickel sulfide ore deposits are associated with intrusions that share common morphologies and characteristics. This model provides a framework for more detailed structural investigations of nickel sulfide deposits, and it is a predictive framework for mineral exploration.

Response Variables for Evaluation of the Effectiveness of Conservation Corridors

Many studies have evaluated effectiveness of corridors by measuring species presence in and movement through small structural corridors. However, few studies have assessed whether these response variables are adequate for assessing whether the conservation goals of the corridors have been achieved or considered the costs or lag times involved in measuring the response variables. We examined 4 response variables-presence of the focal species in the corridor, interpatch movement via the corridor, gene flow, and patch occupancy--with respect to 3 criteria--relevance to conservation goals, lag time (fewest generations at which a positive response to the corridor might be evident with a particular variable), and the cost of a study when applying a particular variable. The presence variable had the least relevance to conservation goals, no lag time advantage compared with interpatch movement, and only a moderate cost advantage over interpatch movement or gene flow. Movement of individual animals between patches was the most appropriate response variable for a corridor intended to provide seasonal migration, but it was not an appropriate response variable for corridor dwellers, and for passage species it was only moderately relevant to the goals of gene flow, demographic rescue, and recolonization. Response variables related to gene flow provided a good trade-off among cost, relevance to conservation goals, and lag time. Nonetheless, the lag time of 10-20 generations means that evaluation of conservation corridors cannot occur until a few decades after a corridor has been established. Response variables related to occupancy were most relevant to conservation goals, but the lag time and costs to detect corridor effects on occupancy were much greater than the lag time and costs to detect corridor effects on gene flow.

Potential corridors and barriers for plague spread in central Asia

BackgroundPlague (Yersinia pestis infection) is a vector-borne disease which caused millions of human deaths in the Middle Ages. The hosts of plague are mostly rodents, and the disease is spread by the fleas that feed on them. Currently, the disease still circulates amongst sylvatic rodent populations all over the world, including great gerbil (Rhombomys opimus) populations in Central Asia. Great gerbils are social desert rodents that live in family groups in burrows, which are visible on satellite images. In great gerbil populations an abundance threshold exists, above which plague can spread causing epizootics. The spatial distribution of the host species is thought to influence the plague dynamics, such as the direction of plague spread, however no detailed analysis exists on the possible functional or structural corridors and barriers that are present in this population and landscape. This study aims to fill that gap.MethodsThree 20 by 20 km areas with known great gerbil burrow distributions were used to analyse the spatial distribution of the burrows. Object-based image analysis was used to map the landscape at several scales, and was linked to the burrow maps. A novel object-based method was developed – the mean neighbour absolute burrow density difference (MNABDD) – to identify the optimal scale and evaluate the efficacy of using landscape objects as opposed to square cells. Multiple regression using raster maps was used to identify the landscape-ecological variables that explain burrow density best. Functional corridors and barriers were mapped using burrow density thresholds. Cumulative resistance of the burrow distribution to potential disease spread was evaluated using cost distance analysis. A 46-year plague surveillance dataset was used to evaluate whether plague spread was radially symmetric.ResultsThe burrow distribution was found to be non-random and negatively correlated with Greenness, especially in the floodplain areas. Corridors and barriers showed a mostly NWSE alignment, suggesting easier spreading along this axis. This was confirmed by the analysis of the plague data.ConclusionsPlague spread had a predominantly NWSE direction, which is likely due to the NWSE alignment of corridors and barriers in the burrow distribution and the landscape. This finding may improve predictions of plague in the future and emphasizes the importance of including landscape analysis in wildlife disease studies.

Supergene Enriched, Intrusion Related Low Suphidation Deposit, Binebase-Bawone, North Sulawesi, Indonesia

DOI: 10.17014/ijog.v7i4.150Gold mineralization at East Asia Minerals’ Binebase Prospect on Sangihe Island displays controls to mineralization typical of other Pacific Rim Intrusion related low sulphidation systems. Better gold grades are localized on EW dilatant structures at the intersection of NW trending structural corridors. Mineralization at Binebase was previously interpreted as being hosted in a high sulphidation style of alteration system. This assumption was based on surface observations of alunite and what was thought to be residual vuggy silica. Recent observations indicate that the alunite is a supergene mineral in nature and that the vuggy silica is the boxworked remnants of silica + pyrite alteration. Precious metal grades decline moving away from feeder structures and there has been significant supergene enrichment of gold in the oxide zone. Exploration drilling completed in 2008 by East Asia Minerals Indonesia resulted in an inferred resource of approximately 835,000 oz of contained gold at a cut off grade of 0.25 g/t Au. This resource included both oxide and hypogene sulphide mineralization. Drilling was reinitiated in 2011 to change the inferred to indicated resource, that drilling programme continued at the time of this publication. Various geophysical techniques were employed at both prospect and regional scales to aid exploration. Ground magnetics defined hydrothermal alteration and magnetite destruction which was coincident with mineralization at both Binebase and Bawone and an Induced Polarisation survey also produced an anomaly which was coincident with disseminated fine sulphide associated with clay alteration within the hydrothermal alteration. The southern part of the island was covered by an airborne geophysical survey in an attempt to understand the regional structural controls on mineralization and to define additional areas of hydrothermal alteration.

Deep transverse basement structural control of mineral systems in the southeastern Canadian Cordillera1Geological Survey of Canada Contribution 20110294.

Intermittent reactivation of deep Paleoproterozoic- and Archean-aged transverse structures in the North American cratonic basement beneath the southeastern Canadian Cordillera exerted an important influence on the subsequent sedimentary, intrusive, deformation, and mineralization history. Two regional crossbelt transverse structural corridors recognized by recurrent episodic changes in sediment thickness and facies, and anomalous transverse structural trends, occur within the thrust and fold belt. Palinspastic restoration of the exposed transported structural corridors showed that the northern and southern structural corridors are cover expressions of the Red Deer zone and Vulcan Low structures in the cratonic basement, respectively. Transverse basement-controlled structural corridors enhanced the mineral potential and mineralization systems along them by localization of sedimentary facies favourable for mineralization, hydrothermal flow, intrusions, and favourable structures for mineralization. Greatest enhancement of hydrothermal flow and mineralization occurred near the intersection of basin-parallel and transverse structures. Greater opportunity for mineralization and intrusion occurred not only along the exposed corridors, but also in the west, above their undeformed “root” in the basement. The processes that intensified mineralization and localized intrusions along transverse basement-controlled structural corridors in the southeastern Canadian Cordillera should have occurred in thrust and fold belts elsewhere. The niobium-bearing Aley carbonatite complex in the eastern Cordillera of northeastern British Columbia may be an example of this.

Volcanic reconstruction of Paleoproterozoic arc volcanoes: the Hidden and Louis formations, Flin Flon, Manitoba, CanadaThis is a companion paper to DeWolfe, Y.M., Gibson, H.L., and Piercey, S.J. 2009. Petrogenesis of the 1.9 Ga mafic hanging wall sequence to the Flin Flon, Callinan, and Triple 7 massive sulphide deposits, Flin Flon, Manitoba, Canada. Canadian Journal of Earth Sciences, 46: this issue.

The hanging wall to the Flin Flon, Callinan, and Triple 7 volcanogenic massive sulphide deposits of the Flin Flon district is composed of the Hidden and Louis formations. The contact between these formations is marked by mafic tuff that represents a hiatus in effusive volcanism. The formations form a composite volcanic edifice that was erupted and grew within a large, volcanic–tectonic subsidence structure (hosting the deposits) that developed within a rifted-arc environment. The formations are evidence of resurgent effusive volcanism and subsidence following a hiatus in volcanism marked by ore formation since they consist of dominantly basaltic flows, sills, and volcaniclastic rocks with subordinate basaltic andesite and rhyodacitic flows and volcaniclastic rocks. The Hidden formation is interpreted to represent a small shield volcano and the Louis formation a separate shield volcano that developed on its flank. Both the Hidden and Louis volcanic edifices were constructed by continuous, low-volume eruptions of pillow lava. A gradual change from a dominantly extensional environment during the formation of the footwall Flin Flon formation to a progressively more dominant convergent environment during the emplacement of the hanging wall suggests that the Hidden and Louis formations are unlikely to host significant volcanogenic massive sulphide-type mineralization. However, synvolcanic structures in the formations define structural corridors that project downwards into the footwall where they encompass massive sulphide mineralization, indicating their control on ore formation, longevity,and reactivation as magma and fluid pathways during the growth of the Hidden and Louis volcanoes.

Plate Interactions, Evolving Magmatic Styles, and Inheritance of Structural Paths: Development of the Gold-Rich, Miocene El Indio Epithermal Belt, Northern Chile

Ore deposits constitute singularities in the Earth crust, and form in response to a combination in time and space of a variety of geological processes. We here explore the complex setting that led to formation of an outstanding belt of precious-metal epithermal deposits in northern Chile, including the world class Au-Cu-As El Indio deposit. We discuss the formation of the El Indio belt in terms of Oligocene-Miocene plate tectonic and magmatic evolution of the Nazca-South America margin. The analysis of Landsat images allows recognition of a highly fractured domain involving a major Oligocene NW-SE-trending fault zone, with associated R1-R2-type structures. We suggest that a set of time-coincidental factors that occurred between ~10 and 6 Ma may have led to formation of the Late Miocene El Indio belt. Late Miocene subduction of the Juan Fernández Ridge (JFR) coincided with a substancial shift in magmatic emplacement style, linked to a compositional change from andesitic to dacitic, which resulted in the passage from stratovolcanoes to dome and dike complexes. It is not the composition alone, but the style that may be the crucial element here to understand why mineralization developed. Although stratovolcanoes easily vent volatiles and metals (via violent eruptions and quiescent outgassing), intrusions tend to retain these, and therefore, can generate volatile-, metal-rich hydrothermal solutions. We further propose that subduction of the JFR may have involved increased plate coupling, and the reactivation of older NNE trending R2 type shears (inherited structural corridors), along which the belt would have formed preferentially.

Mapping landscape corridors

Corridors are important geographic features for biological conservation and biodiversity assessment. The identification and mapping of corridors is usually based on visual interpretations of movement patterns (functional corridors) or habitat maps (structural corridors). We present a method for automated corridor mapping with morphological image processing, and demonstrate the approach with a forest map derived from satellite imagery of northern Slovakia. We show how the approach can be used to differentiate between relativelynarrow (‘line’) and wide (‘strip’) structural corridors by mapping corridors at multiple scales of observation, and indicate how to map functional corridors with maps of observedor simulated organism movement. An application to environmental reporting is demonstrated by assessing structural forest corridors in relation to forest types in northern Slovakia. # 2006 Elsevier Ltd. All rights reserved.

TRACE ELEMENT GEOCHEMISTRY AND PETROGENESIS OF FELSIC VOLCANIC ROCKS ASSOCIATED WITH VOLCANOGENIC MASSIVE Cu-Zn-Pb SULFIDE DEPOSITS

Volcanogenic massive Cu-Zn-(Pb) sulfide (VMS) deposits occur primarily in subaqueous rift-related environments (e.g., oceanic, fore-arc, arc, back-arc, continental margin, or continental), are hosted primarily by bimodal, mafic-felsic volcanic successions, and are typically associated with felsic volcanic rocks with specific geochemical characteristics. FI alkalic dacites and rhyodacites, despite being abundant in the rock record, are typically barren. Some FII calc-alkalic rhyodacites and rhyolites host VMS deposits, but most are barren. FIII tholeiitic and FIV depleted rhyolites and high silica rhyolites are much less abundant in the rock record but commonly host VMS deposits, regardless of age, and FIII rhyolites appear to host the largest deposits. Most petrogenetic models proposed for the formation of FII and FIII-FIV felsic volcanic rocks link felsic magma genesis to fractionation processes in high-level magma chambers now represented by associated subvolcanic intrusions, where the magma is also interpreted to have supplied the heat and/or metals required to generate and sustain the VMS-forming convective hydrothermal system. However, the relatively constant compositions of FII and FIII-FIV felsic volcanic rocks within individual areas, the high eruptive temperatures (at or above liquidus) of FIII rhyolites, and the bimodality of VMS-hosting volcanic successions indicate that fractional crystallization within subvolcanic intrusions could not have generated or significantly modified the compositions of FII and FIII-FIV magmas. This, coupled with detailed geological, geochemical, and geochronological studies indicates that many of these subvolcanic intrusions were emplaced in multiple phases and that the later, most voluminous phases often cut ore-associated, hydrothermally altered rocks. A reassessment of the physical conditions responsible for producing the geochemistry of ore-associated FII and FIII-FIV felsic volcanic rocks and a review of the compositions of felsic volcanic rocks associated with VMS deposits that range in age from Mesoarchean to Cenozoic provide important constraints on models for VMS-associated felsic volcanic rocks and their relationship to mineralization. The compositions of felsic volcanic rocks may be explained by low to moderate degrees of partial melting of mafic sources at a range of depths within rift environments where the mineralogy and composition of the source regions, modes, and degrees of partial melting, pressure and temperature of melting, and, to a lesser extent, subsequent fractionation processes, account for the compositional variations from FI through FII to FIII-FIV. Long-lived, enhanced heat flow and structural permeability of rift environments that allows partial melting to form some FII rhyolites at midcrustal levels (10–15 km) and FIII-FIV rhyolites at shallow crustal levels (<10 km), both within the zone of brittle fracture permeability, are essential to sustain the high-temperature convective hydrothermal systems that are required to form large VMS deposits and camps. Rift environments contain long-lived, thermal, magmatic, and structural corridors that focus magma ascent, heat flow, high-temperature convective hydrothermal systems, and emplacement of subvolcanic intrusions that are favorable environments for the formation of VMS deposits and FII and FIII-FIV felsic volcanic rocks.

Calc-alkaline arc granitoid activity in the Sierra Norte-Ambargasta Ranges, central Argentina

A petrological and geochemical study of an area of approximately 750 km 2 has been carried out on representative lithological units of the extensive Sierra Norte-Ambargasta batholith (SNAB), in the Sierra Norte-Ambargasta range of the easternmost Sierras Pampeanas system of central Argentina. This batholith has intruded a dominantly metasedimentary basement of Precambrian age, which is mainly composed of quartzofeldsphatic-biotite or sericite-chlorite schists, and minor quartz-arenites of local distribution. The most primitive rocks of the magmatic sequence are diorite to tonalite microgranular enclaves (ME), which are included in the granodiorite-monzogranite (GM) basement. A large differentiated dacite-rhyolite (DR) stock intruded the earlier granitoids. All of these units were later intruded by highly evolved subalkaline granitoids (HESG) composed of miarolitic monzogranites, granite porphyries and large aplite dykes, which emplacement was controlled by old regional structural corridors. Geochemical parameters show that these granitoids series were derived from crystal fractionation of calc-alkaline magmas of metaluminous to weakly peraluminous composition. Discriminatory diagrams for tectonomagmatic settings, and the petrological and geochemical signature of the studied lithologies indicate that most granitoids of the intrusive sequence are the result of I-Type calc-alkaline magmatism. These granitoids are typical representatives of a precollisional volcanic arc environment, spatially related to an active continental margin. A significant chemical gap along the fractional crystallization trend separates the most evolved silica-rich granitoids from the less evolved units. This gap is attributed to the absence of representative magmatism of the syncollisional stage within the orogenic cycle. Silica and K-feldspar rich-rocks of the HESG-group are the result of intrusions that occured after the main magmatic activity, in a late-to post-orogenic tectonic setting. The occurrence of molybdenum, copper and other base-metal sulfides, associated with alteration assemblages that are common in porphyry systems, are in line with I-Type dacitic-rhyolitic magmatism in a volcanic arc geotectonic regime. Barren quartz-seriate greisen alteration is characteristic of the late-stage evolution of miarolitic monzogranites of the HESG-group. Radiometric ages for the SNAB granitoids suggest Upper Precambrian to Lower Paleozoic magmatic activity. Calc-alkaline arc magmatic activity of this age in the Sierra Norte of Córdoba, the easternmost block of the Oriental Sierras Pampeanas, raises the important question whether to assign the main magmatic activity of this region to the Pampean or to the Famatinian orogenic cycles. Our data suggest that the main igneous basement of the Sierra Norte-Ambargasta batholith consist of precollisional granitoids related to an orogenic cycle that most likely overlaps the G1 magmatic cycle of the Pampean orogen.

Heterogeneous brittle deformation in the Devonian carbonate rocks of the Pillara Range, Canning Basin: Implications for the structural evolution of the Lennard Shelf

Deformation documented in the Upper Devonian carbonate rocks of the Lennard Shelf shows a complex history involving discrete synsedimentary structures and faults. The reefal carbonate successions of the Lennard Shelf were deposited along the northeastern margin of the extensional Fitzroy Trough in the Canning Basin. Extensional deformation on the regional scale is concentrated along normal faults trending northwest parallel to the shelf margins, and in structural corridors that are oblique to the strike of the shelf. In the Pillara Range‐Limestone Billy Hills region, a corridor of northeast‐trending deformation is associated with dextral offset of the reef margins, and is interpreted to represent an accommodation zone, comprising individual en echelon sinistral oblique‐slip faults. Synsedimentary deformation controlled reef and platform facies distribution, and caused fracturing with extensive development of neptunian dykes. Later structures, which post‐date marine, radiaxial fibrous cements, are spatially associated with the earlier synsedimentary structures, and are interpreted to represent reactivation of these structures. Meso‐scale brittle and brittle‐ductile structures of this later phase include faults associated with extensive fault breccias, matrices of sparry calcite and ore sulfides, tectonic stylolites, and en echelon tension gashes filled with spar calcite. Importantly, this deformation is brittle and discrete, such that the wall rocks adjoining the deformation structures are undeformed, preserving complete and unstrained fossils and primary sedimentary structures and textures. The geometry and kinematics of these structures are consistent with sinistral oblique‐slip movement on a regional scale along northeast‐trending basement‐hosted faults. The northeast‐trending faults are contained in a northeast‐trending zone that accommodated offset of the regionally extensive basin‐bounding, normal, listric faults of the Fitzroy Trough. Within this zone there was reorientation of the regional stress field, in which σ1 (maximum compressive stress) was initially vertical, to one in which σ1 was locally subhorizontal and broadly north‐northwest to south‐southeast trending. Deformation commenced in the late Givetian, during reef development, and continued through to at least the Early Carboniferous. The reactivation appears to have been most pronounced in the very Late Devonian or Early Carboniferous, and all deformation had ceased prior to deposition of the Permo‐Carboniferous rocks in the study area. Post‐Permo‐Carboniferous deformation occurred in the Fitzroy Trough, but left the Lennard Shelf unaffected.

Structural corridors in Landsat lineament interpretation

In plots of symbolised geo-data which have variations in density of symbol population, structural corridors are defined by relatively narrow belts in which the density of symbol population is in marked contrast to that of surrounding areas. Some corridors are of high population density (HPD), some of low population density (LPD), whilst others are serial composites. In geology, corridors are to be seen in ancillary plots of gravimetrics, magnetics, photolineaments, Landsat lineaments and allied data. Those related to ore deposits are commonly 100–200 km in length, with aspect ratios (length/breadth) from 20 to 40. They are distinguished from conventional anomaly trends by not necessarily defining absolute highs or lows in individual symbol measurements. Rather do they reflect dislocations in the pattern of material distributions which the symbols represent, and are therefore related more to gradient functions. They must be distinguished from random trends which pervade ordinary patterns of material distributions. Corridors defined for the one area by different sets of geo-data frequently do not coincide, and this is anticipated by differences in the particular qualities measured, and the geological times at which these qualities were established. Field observations show structural corridors to correspond to subsurface dislocations, and, depending on the type of geological environment, either edges or centres of corridors are significantly related to mineral and oil deposits. In airphoto and Landsat lineament plots, the best data for revealing corridors are the smaller features at the high frequency end of the symbol dimensional range. In such cases, structural corridors are hard to preconceive, and emerge only after the assembly of data is complete. This greatly reduces the problem of subjectivity of the interpreter. Photolineament corridors in the Kalgoorlie goldfield of Western Australia are discussed and compared with Landsat corridors related to metal and oil occurrences in North America.