Footwall Rocks Research Articles

Character of the UG2 Chromitite and Host Rocks and Petrogenesis of Its Pegmatoidal Footwall, Northeastern Bushveld Complex

The UG2 chromitite and its immediate footwall have been mapped at the centimeter scale in a characteristic section exposed in a wall in the Middelpunt mine, northeast Bushveld Complex. In this region the UG2 chromitite and its coarse-grained footwall rocks are underlain and overlain by plagioclase pyroxenite. The UG2 chromitite itself is a massive, 70-cm-thick layer composed of 75 to 90 percent chromite with plagioclase as the dominant interstitial mineral. Immediately beneath the chromitite is the so-called mixed layer, a previously undescribed but distinct, mappable layer, 50 to 70 cm thick, consisting of chromite-bearing pegmatoidal pyroxenite and chromitite intimately mixed at scales of centimeters to decimeters. The mixture is chaotic with no megascopic fabric and contains in bulk about 15 percent chromite. The contact between the mixed layer and overlying chromitite is well defined but sinuous at scales of centimeters to decimeters. Based on observations of drill core and outcrop, the mixed layer appears to exist throughout the central sector of the eastern Bushveld and may be ubiquitous throughout the entire Complex. The mixed layer usually rests directly on the UG2 pegmatoid, which is a well known and widespread pegmatoidal plagioclase pyroxenite. The pegmatoid is much coarser grained than the underlying pyroxenite, but the two lithologic units are similar in terms of mineral modes and compositions. The contact between them is irregular at scales of decimeters to meters, with vertical fingers of pegmatoid extending into the pyroxenite. The mixed layer and UG2 pegmatoid contain rare lenses of massive anorthosite encased by continuous, centimeter-thick rims of chromitite. A model is proposed to account for the footwall sequence of the UG2 chromitite. In this model, the UG2 pegmatoid formed when hydrous, interstitial melt percolated up through the partially molten crystal pile, accumulated beneath the preexisting, compacted, massive chromitite, and metasomatized and partially melted the pyroxenite protolith, which upon cooling recrystallized to a coarse-grained rock. On the basis of published experiments illustrating the effect of water on melting in mafic systems, it is shown how the hydrous melt was able to penetrate the pyroxene-bearing assemblages below the chromitite but unable to partially melt the chromitite itself, which in consequence remained relatively impermeable. The anorthosite lenses in the pegmatoid mark local areas of relatively high degrees of hydration partial melting, with the anorthosite representing the crystalline residue. The mixed layer formed by gradual collapse of the base of the UG2 chromitite into the less dense, relatively fluid footwall. The mixed layer and UG2 pegmatoid are diagnostic of the UG2 chromitite, and the three lithologic units should be considered a single petrologic unit.

Three-dimensional model and late stage warping of the Plattengneis Shear Zone in the Eastern Alps

The Plattengneis shear zone is a 250–600 m thick, flat lying, Cretaceous, eclogite facies, mylonitic shear zone, with north-over-south transport direction, that is exposed over almost 1000 km 2 in the Koralpe region along the eastern margin of the Alps. Although the shear zone is one of the largest in the Alps, its role in the Eoalpine metamorphic evolution and the subsequent exhumation of the region, remain enigmatic and its large-scale geometry is not well understood. The outcrop pattern suggests that the shear zone is made up of a single sheet that is folded into a series of open syn- and antiforms with wavelengths of about 10 km. Eclogite bodies occur above, within and below the shear zone and there is no metamorphic grade change across the shear zone. In the south, the fold axes strike east–west and plunge shallowly to the east. In the north, the fold axes are oriented in north–south direction and form a dome shaped structure of the shear zone. Total shortening during this late stage warping event was of the order of 5%. Indirect evidence constrains this folding event to have occurred between 80 and 50 Ma and the fold geometry implies that the final exhumation in the Koralpe occurred somewhat later than further north. Interestingly, the shear zone appears to strike out of the topography in the south and dip into the topography in the north, so that north of the shear zone only hanging-wall rocks are exposed and south of it only foot-wall rocks. Possibilities for the geometric relationship of the Plattengneis shear zone with the surrounding south dipping detachments are discussed.

Structural setting and tectonic evolution of the Apennine Units of northern Calabria

Abstract A new structural–stratigraphic synthesis of the Apennine units of northern Calabria is presented. The Meso-Cenozoic successions are grouped into two tectonic units, named Pollino–Ciagola Unit (PCU) and Lungro–Verbicaro Unit (LVU), comprising terrains formerly attributed to five different tectonic units. Fe Mg carpholite and blue amphibole record HP–LT metamorphism in the LVU, followed by progressive decompression leading to final greenschist facies re-equilibration during dominantly extensional deformation. Final tectonic emplacement of the LVU over the PCU post-dated the metamorphism of the LVU and was accompanied by intense ductile deformation along zones of strain localisation in footwall rocks. All of the units were later affected by folding and minor thrusting during subsequent Apennine tectonics. To cite this article: A. Iannace et al., C. R. Geoscience 337 (2005).

Are turtleback fault surfaces common structural elements of highly extended terranes?

The Death Valley region of the U.S.A. contains three topographic surfaces resembling the carapace of a turtle. These three surfaces are well exposed along the Black Mountain front and are named the Badwater, Copper Canyon, and Mormon Point Turtlebacks. It is widely accepted that the turtlebacks are also detachment surfaces that separate brittlely deformed Cenozoic volcanic and sedimentary rocks of the hanging wall from the strongly mylonitic, ductilely deformed pre-Cenozoic rocks of the footwall. We have found a turtleback-like detachment surface along the southern margin of the Alasehir (Gediz) Graben in western Anatolia, Turkey. This surface qualifies as a turtleback fault surface because it (a) is overall convex-upward and (b) separates brittlely deformed hanging wall Cenozoic sedimentary rocks from the ductilely to brittlely deformed, strongly mylonitic pre-Cenozoic footwall rocks. The surface, named here Horzum Turtleback, contains striations that overprint mylonitic stretching lineations indicating top to the NE sense of shear. This suggests that the northeasterly directed Cenozoic extension in the region resulted in a ductile deformation at depth and as the crust isostatically adjusted to the removal of the rocks in the hanging wall of the detachment fault, the ductilely deformed mylonitic rocks of the footwall were brought to shallower depths where they were brittlely deformed. The turtleback surfaces have been considered unique to the Death Valley region, although detachment surfaces, rollover folds, and other extensional structures have been well observed in other extended terranes of the world. The presence of a turtleback fault surface in western Anatolia, Turkey, suggests that the turtleback faults may be common structural features of highly extended terranes.

Mesozoic–Tertiary structural evolution of an extensional gneiss dome—the Kesebir–Kardamos dome, eastern Rhodope (Bulgaria–Greece)

The tectonic evolution of the Rhodope massif involves Mid-Cretaceous contractional deformation and protracted Oligocene and Miocene extension. We present structural, kinematic and strain data on the Kesebir–Kardamos dome in eastern Rhodope, which document early Tertiary extension. The dome consists of three superposed crustal units bounded by a low-angle NNE-dipping detachment on its northern flank in Bulgaria. The detachment separates footwall gneiss and migmatite in a lower unit from intermediate metamorphic and overlying upper sedimentary units in the hanging wall. The high-grade metamorphic rocks of the footwall have recorded isothermal decompression. Direct juxtaposition of the sedimentary unit onto footwall rocks is due to local extensional omission of the intermediate unit. Structural analysis and deformational/metamorphic relationships give evidence for several events. The earliest event corresponds to top-to-the SSE ductile shearing within the intermediate unit, interpreted as reflecting Mid-Late Cretaceous crustal thickening and nappe stacking. Late Cretaceous–Palaeocene/Eocene late-tectonic to post-tectonic granitoids that intruded into the intermediate unit between 70 and 53 Ma constrain at least pre-latest Late Cretaceous age for the crustal-stacking event. Subsequent extension-related deformation caused pervasive mylonitisation of the footwall, with top-to-the NNE ductile, then brittle shear. Ductile flow was dominated by non-coaxial deformation, indicated by quartz c-axis fabrics, but was nearly coaxial in the dome core. Latest events relate to brittle faulting that accommodated extension at shallow crustal levels on high-angle normal faults and additional movement along strike-slip faults. Radiometric and stratigraphic constraints bracket the ductile, then brittle, extensional events at the Kesebir–Kardamos dome between 55 and 35 Ma. Extension began in Paleocene–early Eocene time and displacement on the detachment led to unroofing of the intermediate unit, which supplied material for the syn-detachment deposits in supra-detachment basin. Subsequent cooling and exhumation of the footwall unit from beneath the detachment occurred between 42 and 37 Ma as indicated by mica cooling ages in footwall rocks, and extension proceeded at brittle levels with high-angle faulting constrained at 35 Ma by the age of hydrothermal adularia crystallized in open spaces created along the faults. This was followed by Late Eocene–Oligocene post-detachment overlap successions and volcanic activity. Crustal extension described herein is contemporaneous with the closure of the Vardar Ocean to the southwest. It has accommodated an earlier hinterland-directed unroofing of the Rhodope nappe complex, and may be pre-cursor of, and/or make a transition to the Aegean back-arc extension that further contributed to its exhumation during the Late Miocene. This study underlines the importance of crustal extension at the scale of the Rhodope massif, in particular, in the eastern Rhodope region, as it recognizes an early Tertiary extension that should be considered in future tectonic models of the Rhodope and north Aegean regions.

Age resetting of hanging wall rocks above a low-angle detachment fault: Tinos Island (Aegean Sea)

The central Aegean region has been affected by extensional tectonics since at least the Oligo-Miocene. On Tinos Island (Cyclades, Greece), a low-angle normal fault places low-grade metamorphic rocks (the Upper Unit) above high-pressure rocks (the Cycladic Blueschist Unit). K-Ar and 40Ar/ 39Ar ages were obtained by us on white-micas, amphiboles, and whole-rock samples separated from various metamorphic rocks representing different structural levels of the hanging wall. The hanging wall Upper Unit is commonly thought to have resided at shallow crustal levels since Late Cretaceous times, but our geochronological data, supported by previously published Rb/Sr dating [Bröcker, M., Franz, L., 1998. Rb-Sr isotope studies on Tinos island (Cyclades, Greece): additional time constraints for metamorphism, extent of infiltration-controlled overprinting and deformational activity, Geol. Mag., 135 (3), 369–382], indicate severe age resetting of the base of the hanging wall during Tertiary times. Amphiboles from the top of the hanging wall section yielded Late Cretaceous ages which correlate well with other Cretaceous Upper Unit rocks in the Cyclades. Their preservation implies that Tertiary age resetting was mainly confined to the lower part of the Upper Unit, while upper parts retained older ages. The base of the hanging wall section, where age resetting and ductile deformation are most pronounced, is taken to represent a part of a thick extensional shear zone that operated in low-greenschist-facies conditions during Tertiary time. The results of one-dimensional thermal modeling suggest that warm footwall rocks, emerging from great depth subsequently to HP/LT metamorphism, may have supplied the heat required for the metamorphism and ductile deformation at the base of the hanging wall. The model also indicates that only minor cooling of the footwall during decompression is allowed if the footwall is to warm up the hanging wall. This is a priori consistent with the isothermal decompression path of footwall blueschists and eclogites deduced from previous petrological studies on Tinos.

Formation of bifurcating chromitite layers of the UG1 in the Bushveld Igneous Complex, an analogy with sand volcanoes

Chromitite layers occur frequently within the Critical Zone of the Bushveld Complex. In particular, Dwars River in the eastern Bushveld is well known for the intimate association of chromitite and anorthosite layers. An explanation has been deduced for bifurcations of chromitite layers and other features frequently found in exposures of the UG1 and its footwall. It is proposed that the unconsolidated footwall anorthosite experienced liquefaction possibly caused by major magma influx and associated seismicity. This resulted in structures analogous to those seen in sediments that undergo liquefaction, including sand volcanoes or boils. Bifurcations of chromitite layers occur where there is ‘background sedimentation’ of chromite at the same time as periodic extrusion of plagioclase plus magma slurry at the magma–cumulate-pile interface. These extrusions form ‘waterlilies’ of anorthosite that build up on top of each other to form domes. Simultaneously, chromite accumulates and forms multiple layers, which vary in thickness from those formed by a single layer of chromite grains to those over 0.5 m thick. Subsequent post-depositional structures suggest continued liquefaction in the footwall rocks, mobility of residual anorthositic magma and brittle deformation of chromitite layers.

Borehole Radar Delineation of the Ventersdorp Contact Reef in Three Dimensions

A large portion of South Africa‘s gold reserve comes from the Ventersdorp Contact Reef (VCR). The VCR is a thin, tabular ore body, generally less than 1.2 m thick. The geometry of the VCR is mainly controlled by slopes, terraces, and faults. The gold is concentrated in palaeo-river channels situated on river terraces and not on the confining terrace risers or slopes, so there is a good correlation between topography and grade. The geophysical problem is to determine the reef geometry before mining begins, in order to plan optimal ore extraction.Borehole radar has already been shown to be a suitable technique for mapping topography on the VCR because of the favourable electrical property contrast between the footwall rocks and the overlying lavas. Here, a forward model is used to resolve the directional ambiguity inherent in low-frequency borehole radar data and to accurately position the target reflection in space. The data from five boreholes within a 200 m square block of ground are combined in a single model to estimate the topography of the ore body within the block. The illumination lines from the radar forward model are positioned in 3D. They are incorporated with other ore-body elevation data from mapping and drilling to improve the geological model of the ore body within the block.The spatial resolution of the geological model is substantially improved with borehole radar. Small faults and other changes in topography are accurately positioned, giving warning of oncoming changes in mining conditions. Terraces and slopes are mapped, facilitating planning of the optimal extraction of the gold within the block. In areas where exploration drilling is primarily undertaken to determine the structure of the ore body rather than its grade, borehole radar provides an effective alternative to conventional drilling.

Tectonics of the late Mesozoic wide extensional basin system in the China–Mongolia border region

AbstractThe China–Mongolia border region contains many late Mesozoic extensional basins that together constitute a regionally extensive basin system. Individual basins within the system are internally composed of a family of sub‐basins filled with relatively thin sedimentary piles mostly less than 5 km in thickness. There are two types of sub‐basins within the basins, failed and combined, respectively. The failed sub‐basins are those that failed to continue developing with time. In contrast, the combined ones are those that succeeded in growing by coalescing adjacent previously isolated sub‐basins. Thus, a combined sub‐basin is bounded by a linked through‐going normal fault that usually displays a corrugated trace on map view and a shallower dip on cross‐section. Along‐strike existence of discrete depocenters and alternation of sedimentary wedges of different types validate the linkage origin of combined sub‐basins. Localized high‐strain extension resulted in large‐amount displacement on linked faults, but contemporaneously brought about the cessation of some isolated fault segments and the formation of corresponding failed sub‐basins in intervening areas between active linked faults. Some combined sub‐basins might have evolved into supradetachment basins through time, concurrent with rapid denudation of footwall rocks and formation of metamorphic core complexes in places. A tectonic scenario of the broad basin system can be envisioned as an evolution from early‐stage distributed isolated sub‐basins to late‐stage focused combined or/and supradetachment sub‐basins bounded by linked faults, accompanied by synchronous cessation of some early‐formed sub‐basins.Initiation of the late Mesozoic extension is believed to result from gravitational collapse of the crust that had been overthickened shortly prior to the extension. Compression, arising from collision of Siberia and the amalgamated North China–Mongolia block along the Mongol–Okhotsk suture in the time interval from the Middle to Late Jurassic, led to significant shortening and thickening over a broad area and subsequent extensional collapse. Pre‐ and syn‐extensional voluminous magmatism must have considerably reduced the viscosity of the overthickened crust, thereby not only facilitating the gravitational collapse but enabling the lower‐middle crust to flow as well. Flow of a thicker crustal layer is assumed to have occurred coevally with upper‐crustal stretching so as to diminish the potential contrast of crustal thickness by repositioning materials from less extended to highly extending regions. Lateral middle‐ and lower‐crustal flow and its resultant upward push upon the upper crust provide a satisfying explanation for a number of unusual phenomena, such as supracrustal activity of the extension, absence or negligibleness of postrift subsidence of the basin system, less reduction of crustal thickness after extension, and non‐compression‐induced basin inversion, all of which have been paradoxical in the previous study of the late Mesozoic basin tectonics in the China–Mongolia border region.

Constraints on deformation conditions and the origin of oceanic detachments: The Mid‐Atlantic Ridge core complex at 15°45′N

Deformed rocks sampled from a corrugated detachment fault surface near the Mid‐Atlantic Ridge (15°45′N) constrain the conditions of deformation and strain localization. Samples recovered in situ record deformation restricted to the cold (shallow) lithosphere (greenschist facies), with no evidence for significant high‐temperature deformation either at the fault zone or in the footwall near it. High‐temperature deformation (∼720–750°C) is observed only at two sites, and cannot be directly linked to the detachment. Detachment faulting was coeval with dyke intrusions that cross cut it, as demonstrated by the presence of undeformed and highly deformed diabase found in shear zones, and by the presence of chill margins in diabase against fault rock. Basalts are very scarce and restricted to clasts in breccias, with no evidence of pillows or extrusive structures. Gabbros crop out along mass‐wasted and fault scarps structurally below the detachment. Footwall rocks show little or no deformation, due to strain localization along a narrow shear zone (<200 m) with fluid flow, as required to form talc‐ and amphibole schists after an ultramafic protolith. We speculate that the alteration front in a heterogeneous lithosphere may be a rheological boundary that may localize deformation during long periods of time. Our observations and other geological evidence elsewhere suggest that this detachment model limited to the cold (shallow) lithosphere is applicable to other corrugated surfaces along slow‐ and intermediate‐spreading ridges. These observations preclude detachment models rooting in melt‐rich zones (i.e., Atlantis Bank, Southwest Indian Ridge) or recording high‐temperature deformation. We infer that oceanic detachment faults (1) localize strain at T < 500–300°C, (2) persist during active magmatism, and (3) root at shallow rheological boundaries, such as a melt‐rich zone or magma chamber (“hot” detachments) or an alteration front (“cold” detachments).

Chemical, Isotopic, and Fluid Inclusion Evidence for the Hydrothermal Alteration of the Footwall Rocks of the BIF‐Hosted Iron Ore Deposits in the Hamersley District, Western Australia

Abstract. The petrography, chemical, fluid inclusion and isotope analyses (O, Rb‐Sr) were conducted for the shale samples of the Mount McRae Shale collected from the Tom Price, Newman, and Paraburdoo mines in the Hamersley Basin, Western Australia. The Mount McRae Shale at these mines occurs as a footwall unit of the secondary, hematite‐rich iron ores derived from the Brockman Iron Formation, one of the largest banded iron formations (BIFs) in the world. Unusually low contents of Na, Ca, and Sr in the shales suggest that these elements were leached away from the shale after deposition. The δ18O (SMOW) values fall in the range of + 15.0 to +17.9 per mil and show the positive correlation with calculated quartz/sericite ratios of the shale samples. This suggests that the oxygen isotopic compositions of shale samples were homogenized and equilibrated by postdepositional event. The pyrite nodules hosted by shales are often rimmed by thin layers of silica of varying crystallinity. Fluid inclusions in quartz crystals rimming a pyrite nodule show homogenization temperatures ranging from 100 to 240d̀C for 47 inclusions and salinities ranging from 0.4 to 12.3 wt% NaCl equivalent for 18 inclusions. These fluid inclusion data give direct evidence for the hydrothermal activity and are comparable to those of the vein quartz collected from the BIF‐derived secondary iron ores (Taylor et al, 2001). The Rb‐Sr age for the Mount McRae Shale is 1,952 ± 289 Ma and at least 200 million years younger than the depositional age of the Brockman Iron Formation of ∼ 2.5 Ga in age. All the data obtained in this study are consistent with the suggestion that high temperature hydrothermal fluids were responsible for both the secondary iron ore formation and the alteration of the Mount McRae Shale.

Fluid conduits in carbonate‐hosted seismogenic normal faults of central Italy

We studied the structures and stable isotope geochemistry of carbonate fault rocks in four normal faults of central Italy. The faults juxtapose Meso‐Cenozoic carbonates of the footwalls against continental basins of the hanging walls. Footwall rocks exposed along fault scarps have been exhumed from depths of ∼1 km. The fault rocks are systematically arranged in each fault and can be separated into five distinct domains. Farthest from the main fault contact are undeformed host rock and fractured host rock (domain 1). Progressively closer to the fault contact, in the core of the fault, are gouge (domain 2), cataclasite (domain 3), and cement‐dominated horizons with planar slip surfaces (domain 4). Thin horizons of brecciated hanging wall sediments (domain 5) are adjacent, and locally accreted, to the footwall. Structural and stable isotope data are consistent with compartmentalization of fluid in the faults during exhumation. The data are most consistent with these fluids being predominantly evolved meteoric water rather than fluids from the mantle, crustal magmas, and/or devolatilizing carbonate rocks. Meteoric water infiltrated domains 4 and 5 of the faults, either from hanging wall sediments or directly at the land surface. The gouge and cataclasites of domains 2 and 3 were impermeable barriers to movement of meteoric water into domain 1 and undamaged host rocks of the footwall. The results of this study do not support models of earthquake nucleation and rupture that envision large volumes of deep‐seated fluids passing upward through shallow portions of these seismogenic faults.

Post-deformation, sulphide-quartz vein hosted gold ore in the footwall alteration zone of the Palaeoproterozoic Långdal VHMS deposit, Skellefte District, northern Sweden

The Palaeoproterozoic, c. 1.88 Ga old Långdal VHMS deposit is situated in the eastern part of the Skellefte District, northern Sweden. In the stratigraphic footwall to the VHMS ore a sulphide-quartz vein system with high gold grades was mined in the second half of the 1990′s. The Långdal VHMS ore is hosted by the uppermost part of the Skellefte Group volcanic rocks, close to the contact with an overlying fine-grained sedimentary unit. Regional structural studies indicate that bedding surfaces in volcanic rocks are parallel to the contact between the volcanic and the sedimentary rocks. Based on the differences in structural style on each side, the contact is interpreted as a major structural break. The Långdal ore is situated close to this break that may have focussed fluid flow during metamorphism and deformation. The orientation of the contact indicates that it either is a D2 structure or that it was at least active during D2. The structural development in the altered footwall rocks to the Långdal VHMS ore indicates that gold-bearing sulphide and sulphide-quartz veins both pre- and post-date the first deformation. Gold associated with the vein system can thus not only be attributed to syngenetic exhalative or replacement processes. The close spatial relationship with the massive sulphide deposits suggests, however, that the gold was remobilized from these syngenetic systems. It is concluded that sulphides were introduced at several stages during the geological evolution of the area as: a) syngenetic disseminations of sulphide and folded, pre-S1 stringer sulphide±quartz veins in the footwall related to the syngenetic VHMS deposit, b) syn-S1 sulphide veins in the footwall gold ore, c) main, post-S1, sulphide-quartz veins associated with the gold ore in the footwall rocks to the Långdal VHMS deposit, and d) post-S1 to pre-S2 galena and sphalerite rich veins post-dating the main, post-S1, sulphide-quartz vein system in the footwall to the Långdal ore. From these relationships it is suggested that gold was re-mobilized from the sulphide rich parts of the VHMS system into post-D1 structures during or slightly after the peak metamorphism. The timing of this event is poorly constrained to post-date the syngenetic ore emplacement by 20–80 m.y.

A Special Issue Devoted to the Mineral Deposits of the Sudbury Basin

The magmatic Ni-Cu-platinum-group element (PGE) deposits in the Sudbury area were discovered in 1883 (Giblin, 1984). As Ni has grown from a chemical curiosity through early use in armor plating to its present-day pervasive role in modern industry, resources have continued to be discovered around the periphery of the Sudbury Igneous Complex and even richer ore zones have been identified in the footwall rocks, making Sudbury one of the world’s largest and most productive mining camps. The premining resource (production + current resource) of the Sudbury mining camp has been estimated to be >1,648 million metric tons at ~1.2 percent Ni and ~1.0 percent Cu (Naldrett and Lightfoot, 1993). As of 2000, the camp had produced 9.69 million metric tons of Ni, 9.59 million metric tons of Cu, 69.6 thousand metric tons of Co, 116 metric tons of Au, 319 metric tons of Pt, 335 metric tons of Pd, 37.6 metric tons of Rh, 23.3 metric tons of Ru, 11.5 metric tons of Ir, 3,710 metric tons of Ag, 3,002 metric tons of Se, and 256 metric tons of Te from 781 million metric tons of ore (Ontario Ministry of Northern Development and Mines, Information and Marketing Services Section, 2002). Sudbury remains one of the world’s largest producers of Ni, with an annual production of ~114 thousand metric tons, plus ~137 thousand metric tons of Cu, ~1.8 metric tons of Au, and ~11.4 metric tons of total platinum-group minerals (Ontario Ministry of Northern Development and Mines, Information and Marketing Services Section, 2000). Because of its great economic significance, the Sudbury basin has been studied intensively by local mining companies, federal and provincial survey geologists, and numerous university researchers. Landmark papers include the post-war synthesis by Stevenson (1963), the first recognition of the impact origin of the structure by Dietz …

The East Bull Lake Intrusive Suite: Remnants of a 2.48 GaLarge Igneous and Metallogenic Province in the Sudbury Area of the CanadianShield

The 2.49 to 2.475 Ga intrusions of the East Bull Lake intrusive suite occur in an east-northeast-trending discontinuous belt along the presently exposed boundary between the Archean Superior and the Proterozoic Southern provinces of the Canadian Shield near Sudbury, Ontario. The East Bull Lake intrusive suite is part of a regional Paleoproterozoic magmatic event, extending from 2.49 to 2.44 Ga, which also includes bimodal volcanic rocks, felsic plutons, and the regionally extensive Hearst and Matachewan dike swarms. This regional magmatic event is thought to be the result of a mantle plume-driven, intracontinental rifting event that led to the development of a major basin to the south that was subsequently filled by sedimentary rocks of the Huronian Supergroup. The East Bull Lake intrusive suite appears to have been emplaced along a major axial-rift fault related to this rifting event. Compelling field and geochemical evidence summarized here indicates that the three largest East Bull Lake suite intrusions, the East Bull Lake, Agnew Lake, and River Valley intrusions, crystallized from similar, low Ti, high Al tholeiitic parent magmas that originated in deeper, more basic chambers. It is proposed that the primary magmas for the East Bull Lake suite intrusions were second-stage melts derived from a sublithospheric depleted mantle source that had been modified by Neoarchean subduction and accretion events. Most of the petrological characteristics of the East Bull Lake suite intrusions reflect plagioclase-dominated fractional crystallization that generated a pronounced Fe enrichment trend in the residual magmas. Orthopyroxene is the high-temperature pyroxene and occurs as a cumulus phase through most of the stratigraphy of each intrusion of the suite. Local olivine-rich cumulates, present in all of the major East Bull Lake suite plutons, in some instances appear to be comagmatic with associated plagioclase-rich cumulates. A distinctive marginal facies, comprising brecciated and locally thermally recrystallized and/or partially melted footwall rocks, is a common feature of the East Bull Lake suite plutons and indicates that a high-energy flow regime existed during the initial stages of emplacement. This marginal unit typically grades into a heterolithic, inclusion-rich gabbronorite that contains erratically distributed mafic autoliths and footwall-derived xenoliths. Chemical contamination of the mineralized matrix of these inclusion- bearing rocks can be extreme at a local scale. The inclusion-rich zone is overlain by a thick interval of undifferentiated, plagioclase-rich cumulates that locally display spectacular glomerocrystic textures. Rhythmically and irregularly, modally layered leucogabbronorite and gabbronorite make up much of the middle parts of the stratigraphy. Massive to crudely layered ferrogabbro and ferrosyenite only occur in the uppermost part of the Agnew Lake intrusion. Varitextured gabbro and pegmatitic gabbro occur as irregular layers and local veins and pods throughout the stratigraphy. The East Bull Lake suite is currently being explored for contact-type platinum-group element (PGE)-Cu-Ni mineralization that has a clear spatial association with orthopyroxene-rich cumulates which are otherwise poorly represented in these intrusions. Disseminated PGE- and Cu-rich sulfide mineralization is concentrated within the inclusion-bearing zones but extends upward into overlying plagioclase cumulates and appears to have collected along the lower margins of these intrusions through the combined action of vigorous convection of the resident magma and downward percolation of dense, mafic residual liquid. The resultant narrow zones of sulfide mineralization were thus derived from a much larger volume of parent magma. Feeder dikes to the mineralized parts of the intrusions appear to have been saturated in sulfide upon emplacement and had relatively high background PGE contents compared to those for other known magmatic PGE deposits; however, the economic potential of the contact-type mineralization hinges on the efficacy of the physical concentration process, which upgraded the metal content and size of the original sulfide particles inside the magma chambers. Based on the proposed basal accumulation model, the best prospective mineralization is most likely to occur in embayments along the sidewall and floor of the intrusions.

Geology, Mineralization, and Emplacement of the Foy Offset Dike, Sudbury Impact Structure*

The Foy offset dike is the longest of the known radial dikes associated with the 1.85 Ga Sudbury Igneous Complex. The Foy dike extends for at least 30 km from the complex into the North Range Archean and Proterozoic footwall rocks of the Sudbury impact structure. The dike ranges in width from 400 m, where it connects to the Sudbury Igneous Complex, to 50 m at its most distal known exposure. We divide the dike into a proximal segment and a distal segment, based on where it intersects the concentric Hess offset dike. Three main dike lithologies are distinguished, all referred to as varieties of quartz diorite. Inclusion-rich pyroxene-quartz diorite occurs in the embayment, adjacent to the base of the main mass of the Sudbury Igneous Complex. This constitutes the sublayer. Northward, the sublayer gives way to amphibole-biotite inclusion-bearing quartz diorite and amphibole-biotite inclusion-poor quartz diorite. The dike is mineralized and deposits in the proximal segment have been mined. Mineralization is associated with the inclusion-rich dike phase. At the Nickel offset mine the dike contains inclusion- bearing massive Ni-Cu sulfides that also host platinum-group minerals. The orebodies occurred as strike-parallel sheets within the central parts of the dike in association with inclusion-bearing quartz diorite. The geochemistry of the dike lithologies indicates that both the inclusion-poor and inclusion-bearing quartz diorite varieties are close in composition to the postulated bulk composition of the Sudbury Igneous Complex. This implies that the dike was emplaced prior to any significant fractionation of the main melt body and, hence, very early in the evolution of the Sudbury Complex. We envisage dike emplacement rapidly following rebound and central uplift formation as part of the impact process. Early dike emplacement implies that the ore was also introduced early in the evolution of the melt sheet when the inclusion-poor quartz diorite phases were still unconsolidated. Uplift would have caused extension in the target rocks and so dilated the footwall fractures and faults that then became hosts, primarily via gravitationally driven intrusion, to the primitive polymict impact melt breccias and sulfide ores. The chilled margin compositions of the Foy inclusion-poor quartz diorite are considered good indicators of the original bulk composition of the Sudbury Igneous Complex; this idea is supported by comparison with calculated estimations of the bulk composition. A shift in geochemical trends, a grain size increase, and a decrease in inclusion content occurs ~22 km along the length of the dike, which we believe corresponds to a fault system that downfaults to the north. Given that the dense ore would have preferentially settled toward the base of the Foy dike-fracture system, postimpact downfaulting of the distal segment of the dike may have displaced any mineralized zones present to deeper levels relative to the proximal segment.

Mineral-chemical studies of metamorphosed hydrothermal alteration in the Kristineberg volcanogenic massive sulfide district, Sweden

The massive sulfide deposits of the Kristineberg area, Sweden, occur within a 2- to 3-km-thick succession of felsic volcaniclastic rocks belonging to the Skellefte Group. The volcanic pile is intruded by a synvolcanic Jorn-type granitoid (Viterliden intrusive complex) and is overlain by a thick sequence of metasedimentary rocks (Vargfors Group). Mineralization occurs at two main stratigraphic levels, at the base of the felsic volcanic succession and at the contact with the metasedimentary rocks of the Vargfors Group. The Kristineberg Cu–Zn mine is the largest deposit (approximately 21 Mt) and occurs at the base of the volcanic pile, close to the contact with the Viterliden intrusive complex. Four smaller deposits (Ravliden, Ravlidmyran, Horntrask and Nyliden) occur along the upper ore horizon. These deposits are thought to be related to a late intrusive phase of the Viterliden complex which cuts the altered volcanic rocks at the Kristineberg deposit. Within an area of about 50 km2 surrounding the Kristineberg deposit, felsic volcanic rocks between the two ore horizons are affected by extensive albite-destructive alteration (sodium depletion) and development of chlorite and muscovite (strong co-enrichment in magnesium and potassium). The Kristineberg deposit is enveloped by a large and partly transposed quartz–chlorite alteration zone, approximately 2 km in diameter, and a distal but coherent pyrite–quartz–muscovite alteration zone extending as far as 4 km from the deposit. Chlorite(±talc) in the mine area is notably magnesium-rich and contains anomalous F, Ba, Zn and Mn. High fluorine is also present in coexisting muscovite and phlogopite. The magnesium-rich chlorite alteration contrasts sharply with the iron enrichment observed in many other felsic, volcanic-hosted Precambrian massive sulfide deposits. This may indicate fixation of iron by large amounts of pyrite in the section or entrainment of large amounts of seawater in the hydrothermal upflow zones. Kyanite is developed locally in the chlorite-rich pipe at Kristineberg in response to regional thermal metamorphism of highly aluminous alteration in the immediate foot-wall rocks. Spectacular, andalusite-bearing quartz–muscovite schists and quartz–biotite–cordierite schists also occur where the altered felsic volcanic rocks are intruded by the late Revsund granite. However, similar metamorphic mineral growth is not observed where the volcanic rocks at the contact are less altered. Deposits near the top of the felsic volcanic succession are characterized by magnesium-rich chlorite alteration in the foot wall and proximal calc-silicate assemblages (dolomite, calcite, tremolite, ±garnet, ±margarite) where the host sedimentary rocks are carbonate-rich. In general, the calc-silicate alteration is restricted to the immediate hanging wall and zones lateral to the deposits and does not represent a regionally extensive exploration target. The two main ore horizons in the Kristineberg area are not linked by any obvious discordant structures or alteration zones. However, mineral-chemical studies highlight several possible fluid flow pathways leading from the Kristineberg deposit to the Ravliden ore horizon, more than 2 km upsection. Overprinting regional metamorphic minerals have inherited the hydrothermal signature of the ore-related alteration. Electronic Supplementary Material is available at http://dx.doi.org/10.1007/s00126-002-0299-y. On that page (frame on the left side), a link takes you directly to the supplementary material.

Thermal evolution and interaction between impact melt sheet and footwall: A genetic model for the contact sublayer of the Sudbury Igneous Complex, Canada

The Sudbury Igneous Complex (SIC) and associated Ni‐Cu‐PGE mineralization has been interpreted in terms of a large meteorite impact event. In this study, the thermal relationship between the large cooling melt sheet and the surrounding country rock is examined in terms of its role in an evolving thermal gradient rather than as a passive receptacle for the melt sheet above. Thermal modeling of this environment is undertaken using physical and thermal constraints appropriate to the SIC and assuming heat dissipation from the 2.5‐km‐thick superheated melt sheet (≥1800°C) by either diffusion with zero convection or by rapid convection within the melt sheet. With zero convection, basal cooling produces a solid base, which lowers conductivity such that the immediate footwall rocks reach ≤1000°C, producing partial melting that extends 200 m into the footwall. In a rapidly convecting melt sheet the initial footwall chill is remelted and high temperatures maintained within the sheet close to the contact. This results in higher temperatures being attained in the immediate footwall (1100–1200°C), inducing complete melting of proximal footwall and partial melting to depths of 500 m below the melt sheet. Proximal footwall consists of Paleoproterozoic Huronian basalts, granitoids and sediments, exposed in the south range, overlying Archaean gneisses and granitoids. Total and partial melting of this material early in the cooling history of the melt sheet and the subsequent gravitational accommodation of these melts according to density would produce a basalt‐dominated basal liquid corresponding to the so‐called contact sublayer. The thermal aureole predicted by our models is consistent with that preserved around the north range of the SIC assuming ∼800 m of thermally induced erosion at the contact.

Effects of normal faulting on fluid flow in an ore-producing hydrothermal system, Comstock Lode, Nevada

Normal faults in the Comstock Lode mining district, NV, USA, are delineated by low 18O belts and strong isotopic gradients perpendicular to strike. These faults perturb and also offset the regional pattern of δ18O contours produced by pervasive hydrothermal alteration of the Alta formation, and have preserved the down-dropped hanging wall rocks while exposing the footwall rocks to greater erosion. Fluid–rock interaction was greatly enhanced within >200-m-wide zones centered on the normal faults where permeability was increased and fluids were focused parallel to the fault plane. The effects of displacement and permeability enhancement around the faults are investigated using oxygen isotope profiles through the Comstock and Occidental faults and across steeper secondary normal faults such as the Coryell fault that may have formed after the principal faults were rotated to more shallow dips.Oxygen isotope profiles record both post-hydrothermal displacement and syn-deformational enhancement of fluid flow around the faults. Fluid focusing along normal faults perturbs the δ18O values by increasing the water–rock ratio. This isotopic effect can be mathematically described for any distance x from the fault using the formula:Δf=A·e−bx2which is analogous to the fundamental solution for diffusion away from a plane source. The maximum magnitude of 18O depletion, A, depends on the effects of permeability, temperature, mineralogy, fluid isotopic composition and integrated fluid flux. The constant b is related to the scale of exchange enhancement around the fault. The characteristic width (w=1/√b) of the fluid focusing effects is remarkably similar at 100 m for all three faults examined. Coupling this isotope exchange effect with a step-function, which accounts for fault displacement, provides a good fit to the observed 18O fault profiles. The 18O distribution around faults can be used to infer relationships and timing among the processes of structural deformation, hydrothermal convection, and ore deposition in the Comstock district.

Structural evolution of the Gurla Mandhata detachment system, southwest Tibet: Implications for the eastward extent of the Karakoram fault system

Field mapping and geochronologic and thermobarometric analyses of the Gurla Mandhata area, in southwest Tibet, reveal major middle to late Miocene, east-west extension along a normal-fault system, termed the Gurla Mandhata detachment system. The maximum fault slip occurs along a pair of low-angle normal faults that have caused significant tectonic denudation of the Tethyan Sedimentary Sequence, resulting in juxtaposition of weakly metamorphosed Paleozoic rocks and Tertiary sedimentary rocks in the hanging wall over amphibolite-facies mylonitic schist, marble, gneisses, and variably deformed leucogranite bodies in the footwall. The footwall of the detachment fault system records a late Miocene intrusive event, in part contemporaneous with top-to-the-west ductile normal shearing. The consistency of the mean shear direction within the mylonitic footwall rocks and its correlation with structurally higher brittle normal faults suggest that they represent an evolving low-angle normal-fault system. 4 0 Ar/ 3 9 Ar data from muscovite and biotite from the footwall rocks indicate that it cooled below 400 °C by ca. 9 Ma. Consideration of the original depth and dip angle of the detachment fault prior to exhumation of the footwall yields total slip estimates between 66 and 35 km across the Gurla Mandhata detachment system. The slip estimates and timing constraints on the Gurla Mandhata detachment system are comparable to those estimated on the right-slip Karakoram fault system, to which it is interpreted to be kinematically linked. Moreover, the mean shear-sense direction on both the Karakoram fault and the Gurla Mandhata detachment system overlap along the intersection line between the mean orientations of the faults, which further supports a kinematic association. If valid, this interpretation extends previous results that the Karakoram fault extends to mid-crustal depths.