Crustal-scale Fault Systems Research Articles

Posteruptive Thermal History of the Proterozoic Basaltic North Shore Volcanic Group of the Midcontinent Rift: Evidence from K/Ar Data of Celadonite

Abstract This study reports three K/Ar ages on celadonite, a dioctahedral K-Fe mica, in the Proterozoic North Shore Volcanic Group (NSVG) of the Midcontinent Rift in northeastern Minnesota. Celadonite formed during beginning posteruptive, low-temperature conditions at temperatures<100°C and with input of meteoric water. K/Ar ages between 1062±16 Ma and 955.0±12 Ma document a remarkably long posteruptive thermal history of >100 myrs in a thick continental basaltic sequence. In the stratigraphically lower part of the NSVG, celadonite formation occurred at 1062±16 Ma in an amygdule or a vesicle filled with celadonite, while another celadonite amygdule in a stratigraphically higher flow was dated at 1039.4±14 Ma. Both flows are overprinted by a later multistage lower zeolite-phyllosilicate facies assemblage (laumontite-albite-corrensite±chlorite±smectite±prehnite±pumpellyite). In the stratigraphically higher part of the sequence, celadonite crystallization at an amygdule rim is followed by upper zeolite facies conditions (stilbite-heulandite-smectite assemblage) and was dated at 955.0±12.4 Ma. The constrained time frame of 107 myrs indicates a long-lived, probably not continuous and locally occurring, posteruptive thermal alteration process. The data suggest that alteration was depth-controlled and temporally and spatially inhomogeneous and implies the progression of the sequence from a close-to-the-surface alteration mode with input of meteoric water to a burial metamorphic mode and with locally occurring hydrothermal activity due to continuous magmatic activity. Volcanism in the Midcontinent Rift system is supposed to have lasted between 1109 Ma and 1083 Ma based on U/Pb zircon ages. The first crystallization of celadonite is recorded in the lower part of the NSVG and occurred ca. 30 myrs after the emplacement of the Silver Bay aplite intrusion in the upper part of the NSVG. Burial rates are determined to be 0.04 km·Ma-1and 0.10 km·Ma-1. The hydrothermal alteration under low-temperature burial conditions clearly postdates the rift-related alkaline and tholeiitic magmatism of the Midcontinent Rift and overlaps with the depositional window of the sedimentary rocks that overlie the Midcontinent Rift volcanics, as well as crustal-scale fault systems that were active during Grenvillian tectonic uplift after the cessation of magmatic activity.

Regional and local controls on Archean rock-hosted cobalt mineralization at the McAra deposit, southern Superior Province, Ontario, Canada

The McAra deposit is in eastern Ontario, Canada, and is hosted in an Archean inlier to the Paleoproterozoic Huronian basin. It is currently estimated to contain ∼2.4 million pounds of cobalt at an average grade of 1.25%. New drill data show the mineralized zone comprises glaucodot–cobaltite veins and breccias that transect a mafic–siliciclastic volcanogenic massive sulfide (VMS) deposit. The high cobalt grade and host stratigraphy at the McAra deposit contrast with five-element (Ag–Co–Ni–Bi–As) deposits at the Cobalt and Gowganda camps in the region that produced high-grade silver and by-product cobalt from veins spatially associated with Nipissing Gabbro intrusions. However, geochemical data from recent core samples alongside fluid inclusion and mineralogical data suggest the cobalt zone at McAra and the five-element veins share a similar metal assemblage and were deposited from similar fluids. The mafic–siliciclastic VMS deposit at McAra contains anomalous amounts of cobalt, suggesting the Archean host stratigraphy was the source for the high-grade cobalt zone. Basin brines in the Paleoproterozoic are interpreted to have leached cobalt from Archean rocks and then redeposited it through oxidation–reduction reactions along synvolcanic faults that controlled earlier VMS deposit formation. High-resolution aeromagnetic data show that McAra is immediately adjacent to a mafic dike that transects the Huronian basin along a northwest-striking, crustal-scale fault system. These data, alongside observations from field mapping, also suggest the deposit is on the margin of a sub-basin that contains an 80 km2 Nipissing sill that may have originally overlain the deposit area and been a hydrologic seal during mineralization. The new deposit- and regional-scale data and interpretations are used to create a model for the McAra deposit and provide evidence for why it is cobalt-rich relative to other five-element veins. The model and data can be used to guide exploration for additional cobalt-rich deposits in the region and similar settings globally.

Late Miocene topographic inversion in southwest Tibet based on integrated paleoelevation reconstructions and structural history

Investigations of the deformation history of the Himalayan orogen support interpretations of rapid and striking changes in the landscape of the Tibetan Himalaya and High Himalaya. We examine this issue by integrating oxygen isotope-based paleoelevation reconstructions of the Zada basin in southwestern Tibet with information on the structural evolution of the High and Tibetan Himalaya between 79°E and 84°30′E. δ 18O psw values were calculated from δ 18O cc values from pristine fluvial Miocene gastropod shells. Analyses comparing the most negative δ 18O sw and reconstructed δ 18O psw values to Δδ 18O sw versus elevation relationships based on both thermodynamic models and an empirical data set suggest a decrease in the mean watershed elevation of 1 to 1.5 km since the Late Miocene. Geologic mapping and structural data from crustal scale fault systems in the Zada region and regions to its east indicate a phase of arc-normal shortening and vertical thickening since the Middle Miocene, followed by ongoing arc-parallel extension and vertical thinning. These results suggest that regions in this part of the orogen transitioned from undergoing arc-normal shortening to arc-parallel extension in the Late Miocene, and that arc-parallel extensional structures root deeply within the Himalayan thrust wedge. When combined with data on the distribution, age, and provenance of sedimentary basins, our geologic mapping, structural data, and paleoelevation results suggest that this transition from shortening to extension was accompanied by a topographic inversion from mountains to basins in < 4 m.y. These observations can be explained by a foreland propagating fault system that accommodates outward radial expansion of the Himalayan orogen.

The St Ives mesothermal gold system, Western Australia—a case of golden aftershocks?

The late orogenic evolution of the Archaean greenstone sequence in the Eastern Goldfields Province of the Yilgarn Craton in Western Australia is characterised by strike-slip tectonics and locally very high, fault-controlled, fluid fluxes. Fluid flow was associated with the formation of many fault-hosted and shear-zone-hosted gold deposits, which are commonly clustered adjacent to high displacement faults or shear zones. In the St Ives goldfield, near Kambalda, fluid flow in a gold-producing hydrothermal system was localised within arrays of low displacement faults and shear zones, which form part of the NNW trending, crustal-scale, Boulder–Lefroy fault system (BLFS). The numerous ore-hosting structures are kinematically related to sinistral to sinistral–oblique slip on the Playa Fault, which is a 20-km-long splay of the 200-km-long Boulder–Lefroy Fault. Most of the known gold mineralisation at St Ives occurs within an area of 20 km 2 immediately south-west of the Playa Fault. The distribution of low displacement faults and shear zones that host gold mineralisation is related to the presence of a kilometre-scale contractional jog (the Victory jog) and an associated imbricate thrust array on the Playa Fault. By analogy with modern seismogenic systems, the low displacement structures that localised fluid flow and gold mineralisation in the St Ives goldfield are interpreted as aftershock structures whose development was driven by major slip events on the BLFS. For large slip events on the BLFS, and mainshock rupture arrest at the Victory jog, modelling of co-seismic static stress changes indicates that most ore-hosting structures are localised within a crustal volume whose stress state was closer to failure as a consequence of Coulomb stress transfer. The modelling supports an interpretation that aftershock networks can form a high permeability damage zone that localises fluid flow and gold mineralisation within particular parts of crustal-scale fault systems. Both co-seismic stress transfer and time-dependent changes in fluid pressures, during post-seismic fluid redistribution, are implicated in driving the growth of low displacement, gold-hosting fault networks in the St Ives goldfield. Stress transfer modelling has application for area selection in exploration programs targeting mesothermal gold systems. Clustering of deposits hosted by aftershock fracture networks is favoured by the presence of major, long-lived jogs or bends that can repeatedly arrest ruptures propagating along high displacement faults.

Spatiotemporal evolution of the Central Apennines fault system (Italy)

A variety of models show that crustal deformation is a self-organized process on long (geologic) timescales. In this paper, we analyse an active seismogenic crustal-scale fault system (the Central Apennines Fault System or CAFS) with the aim of assessing the spatial and temporal characteristics of fault development and related earthquake activity. The basic properties of the CAFS, as derived from our study, are then compared with those of other fault systems worldwide in order to validate/constrain the results of available statistical physics models based on self organized criticality (SOC).

Review of recent results from continental deep seismic profiling in Australia

The Australian Geological Survey Organisation regularly collects 450–500 km of onshore deep seismic reflection data and up to 4500 km offshore each year in Australia. These recordings are made in a wide range of tectonic provinces, including, in the last few years, late Palaeozoic-Mesozoic intracontinental and Palaeozoic-Mesozoic-Cenozoic continental margin extensional basins, moderately deformed Palaeozoic transtensional basins and compressional fold belts, and Archaean greenstone terranes. Several of these provinces are major petroleum exploration provinces, whereas others contain significant mineral deposits. The primary purpose of the deep seismic profiling program is to resolve the tectonic history of the Australian continent, and thereby to encourage exploration for hydrocarbons and mineral resources in Australia. On the northwest Australian continental margin, major basin systems including the Bonaparte Basin, formed as a result of complex interactions since the Carboniferous, involving episodes of extension followed by strike-slip movements and inversion, which reactivated both the primary extensional and ancient basement structures. Off southeastern Australia, basins such as the Gippsland Basin formed as part of a linked transtensional system related to movement on a common mid-crustal detachment complex. On continental Australia, the Bowen Basin, in the northeast, was deformed by thrust faults that root in a major E-dipping detachment that flattens in the middle crust. The Cobar Basin, in the southeast, is a case where the seismic data support a detachment model in which the upper plate displacement vector can be calculated by plate reconstructions linking the geometry of the detachment surface with that of the basin. The greenstone terranes within the Eastern Goldfields region of Western Australia show crustal-scale fault systems that are planar and steep dipping, more in keeping with those interpreted in data from other Precambrian provinces rather than those of the Palaeozoic provinces.

Geochemistry of Archean shoshonitic lamprophyres from the Yilgarn Block, Western Australia: Au abundance and association with gold mineralization

Spatial and temporal associations between Archean mesothermal gold deposits, shoshonitic minor intrusions (e.g. lamprophyre dikes), and crustal-scale fault systems are well recognized features of some Archean terranes. It has been proposed that the association may be due to a combination of genetic factors, including intrinsic Au enrichment of shoshonitic magmas, and tectono-structural factors arising from crustal-scale orogenic activity in the Late Archean. To determine the nature of the association in the highly mineralized Archean Yilgarn Block, the major, trace and precious metal geochemistry of a suite of 49 lamprophyres and related microdiorite porphyries, covering a range of alteration states and proximities to gold mineralization, were investigated. The lamprophyres exhibit rock fabrics indicative of partial to extensive metamorphic recrystallizatio, range from primitive to more evolved compositions (MgO∼9to<5wt%) and have geochemical signatures typical of Phanerozoic subduction-related magmas. Variable mobile lithophile element (K, Rb, Ba, Sr) concentrations and anomalously high δ 18O signatures of the lamprophyres reflect their interaction with hydrothermal±metamorphic fluids. Lamprophyres emplaced in proximity to gold deposits are commonly affected by carbonation, have enhanced S and Au contents and have Au/Pd ratios that exceed primitive mantle values by up to several orders of magnitude. In contrast, lamprophyres emplaced in locations remote from gold mineralization tend to be depleted in S and Au and have low Au/Pd ratios. High Au contents were mostly acquired by interaction with Au-mineralizing fluids, whereas very low Au contents are the result of fluid leaching in lamprophyres remote from gold deposits. However, some lamprophyres of high F content display small intrinsic enrichments in Au of ≈2to3 times typical igneous rock abundances. The F, S and CO 2 contents of the Yilgarn lamprophyres can effectively discriminate mineralized lamprophyres from non-mineralized samples. This study shows that shoshonitic lamprophyres are unlikely to have contributed significant Au or other components to Yilgarn mesothermal gold deposits.

Environment and structural controls on the intrusion of the giant rare metal Greenbushes Pegmatite, Western Australia

The Greenbushes pegmatite district has been the main center of production of alluvial tin in Western Australia since the beginning of this century, and more recently, for the production of tin, tantalum, and lithium from one of the largest rare metal pegmatite deposits in the world. The intrusive rocks in the Greenbushes pegmatite district are concentrated along the ancient crustal-scale, Donnybrook-Bridgetown shear zone, analogous to the present San Andreas fault system, and are characterized by steeply dipping planar mylonitic fabrics with horizontal stretching lineations, asymmetric folds, asymmetric pressure shadows, and shear bands suggesting sinistral strike-slip movement. The Greenbushes pegmatite occurs in a higher temperature and higher pressure metamorphic terraine than would be expected for pegmatites containing rare element mineralization. The pegmatite contains the same shear fabrics as its host rocks and has evidence for syntectonic crystallization of minerals such as tourmaline, fantalite, garnet, and cassiterite.It is proposed that intrusion of the Greenbushes pegmatite magma was controlled by the Donnybrook-Bridgetown shear zone. Any melts or fluids present during movement along the shear zone would have been channeled into it, leading to intrusion of the pegmatite. The fluid pressure in the pegmatite magma may then have increased causing further failure, zones of structural weakness, and further intrusion of pegmatite generally parallel to the mylonitic fabric in the shear zone.According to present models and classifications, the Greenbushes pegmatite group should be unmineralized. However, it is apparent from this study that giant rare metal pegmatites can occur in higher grade metamorphic terranes and that these types of pegmatites need not have obvious parental granitoids. They may contain a variety of mineralization and are likely, at least in the Archean, to be associated with tectonism along crustal-scale fault systems.

Craton-scale distribution of Archean greenstone gold deposits; predictive capacity of the metamorphic model

Genetic models should not only attempt to explain features of Archean gold mineralization on the scale of mines or districts but should also explain its marked spatial and temporal heterogeneity in greenstone belts. The application of genetic models developed with such a philosophy can have predictive capacity in gold exploration.The metamorphic model, one of a number of current genetic models for Archean gold deposits, involves generation of auriferous H 2 O-CO 2 fluids by devolatilization of the stratigraphically lowermost greenstones during metamorphism and broadly synchronous synkinematic granitoid intrusion. Crustal-scale fault-shear systems are required to induce channeled fluid flow and to focus ore fluids toward suitable host rocks and structural sites. Declining temperature below the amphibolite-greenschist transition, combined with sulfidation of Fe-rich host rocks, is a particularly efficient Au-depositing mechanism. The model suggests that the major controlling factors (in declining order of importance) are: (1) high temperature-low pressure, generally low-strain, metamorphic regimes, (2) crustal-scale fault systems, (3) unaltered Fe-rich rocks (tholeiites, banded iron-formation) at a suitable stratigraphic-metamorphic level, and (4) abundant Au-enriched komatiites and S-rich sediments in source regions.The low-pressure metamorphic conditions combined with the occurrence of elongate high strain zones in most, if not all, generally low strain greenstone belts may explain the ubiquitous occurrence of gold deposits in Archean terranes. All four favorable parameters appear to act in concert to produce the largest gold deposits and most abundant gold mineralization in 2.7 + or - 0.1-b.y.-old rift-phase greenstone belts, interpreted to have formed under conditions of high crustal extension and extensive crustal thinning (i.e., with predicted high geothermal gradients, high fault density, limited synvolcanic alteration, and abundant komatiites and sulfidic sediment horizons). More widespread, commonly older, platform-phase greenstone belts, which formed under conditions of lesser crustal extension and thinning, are less mineralized, as predicted by the metamorphic model.The ability of the model to explain the regional distribution of known Archean gold deposits lends credence to its predictive capacity. For example: (1) greenstone belts in any one craton can be broadly ranked in order of likely relative gold potential, (2) the likelihood of discovery of large deposits in poorly exposed or poorly explored belts of known type can be estimated, (3) areas with the highest potential can be defined in rift-phase greenstone belts by the coincidence of critical parameters such as specific host-rock sequences and metamorphic facies, (4) prospective structural sites for gold mineralization can be postulated in platform-phase greenstone belts affected by diapiric tectonics, and (5) the possible spacing of large deposits can be estimated. Further, the potential use of the model in exploration emphasizes serious gaps in our knowledge of parameters crucial to understanding greenstone belt metallogeny, including alteration patterns, metamorphic P-T paths, and regional-scale structural regimes.