Footwall Rocks Research Articles

Chromitite formation—a key to understanding processes of platinum enrichment

The mafic layered suite of the 2050 m.y. old Bushveld Complex hosts a number of substantial platinum-group element (PGE)-bearing chromitite layers, including the UG2, within the Critical Zone, together with thin chromitite stringers of the platinum-bearing Merensky Reef. Until 1982 only the Merensky Reef was mined for platinum, although it has long been known that chromitites also host PGE-bearing minerals (PGM). Three groups of chromitites occur: (i) a Lower Group of up to seven major layers hosted in feldspathic pyroxenite; (ii) a Middle Group with four layers hosted by feldspathic pyroxenite or norite; and (iii) an Upper Group, usually of two chromitite packages, hosted in pyroxenite, norite or anorthosite. There is a systematic chemical variation from the bottom to the top chromitite layer in terms of Cr : Fe ratio and the abundance and proportion of PGE.Detailed studies of 87Sr/86Sr isotope variations undertaken on interstitial plagioclase from chromitites and different silicate host rocks show that the magma from which the chromitites formed (interstitial plagioclase Sri <0.7099) usually differed radically from the resident liquid from which the immediate footwall rocks crystallized (Sri, ca 0.7060-0.7064). These high Sr isotope ratios can only have been produced by sudden and extensive contamination by an extremely radiogenic component. The only viable source for this component in the chamber is the felsitic roof rocks or a granophyric roof-rock melt. It is suggested that such contamination occurred when a new magma influx penetrated the residual liquid and interacted with the overlying roof rock as well as mixing with the resident liquid. It is envisaged in this model that chromite cascaded to the floor together with a small amount of magma adherent to the chromite or entrained within the slurry to produce interstitial silicates with enriched isotopic ratios.The close correspondence of chromitite and PGE enrichment strongly suggests that the contamination process that resulted in chromite formation also triggered precipitation of the PGE. The base of each major chromitite layer marks the point where there was a substantial injection of new magma into the chamber, which resulted in erosion of the cumulate pile, interaction with roof rocks and inflation of the chamber. Thus, the major PGM and chromitite ore deposits of the Bushveld Complex are unconformity-related and are associated with mixing of new magma, coupled to simultaneous contamination by granophyric roof-rock melt. The chromitites represent, therefore, the products of a roof contamination and magma mixing process.

Implications of middle Eocene epizonal plutonism for the unroofing history of the Bitterroot metamorphic core complex, Idaho-Montana

Petrologic and geochronologic limits on the progression of east-directed extensional unroofing of the Bitterroot metamorphic core complex provide a means to compare rates of denudation across the footwall during the earlystages of extensional denudation. U-Pb and 4 0 Ar/ 3 9 Ar data from epizonal plutons in the northwestern and southwestern Bitterroot metamorphic core complex demonstrate that much of the western footwall was exhumed to shallow levels (1-5 km) by ca. 49 Ma, while eastern footwall rocks remained at high temperatures following an episode of isothermal decompression. U-Pb ages of epizonal plutons along the western margins of the footwall suggest that as much as 13 km of unroofing occurred between anatectic melting of Belt Supergroup metasedimentary rocks in the western footwall no later than 56 Ma and emplacement of the adjacent epizonal Lolo Hot Springs batholith at ca. 49 Ma in the northwestern Bitterroot Range. Tectonic denudation during this interval removed roughly the same amount of overburden from eastern footwall rocks, suggesting that the initial stages of unroofing may have been accommodated by a planar, moderately east-dipping (30°-50°) shear zone. 4 0 Ar/ 3 9 Ar cooling data show that cooling in the western footwall was synchronous with unroofing. In contrast, we suggest that a significant delay in cooling of eastern footwall rocks may have resulted from a slight increase in geothermal gradient following this unroofing stage. Exposure of a west-dipping mylonitic shear zone along the western limb of the Bitterroot complex may provide a key for interpreting the postmylonitic structural evolution of the core complex. If this shear zone can be correlated to the Bitterroot mylonite zone to the east, then strong evidence for flexural uplift consistent with postmylonitic passage of a rolling hinge emerges.

The continuity of active fault systems in Greece

Summary Evidence from geomorphology, the distribution of large earthquakes, and geodetic measurements suggests that the active faulting in mainland Greece and the north Aegean Sea is concentrated into a small number of discrete, linear zones that bound relatively rigid blocks. On land, the zones are most clearly identified where the faulting is associated with large escarpments, particularly in hard footwall rocks. The zones can become indistinct and diffuse near their ends, sometimes because they enter high, unstable topography where landsliding obscures the fault morphology, but mostly because the extension rates decrease along strike as a consequence of the relative rotation of the blocks they bound. The main graben systems of Chalkidiki, southern Thessaly and the North Gulf of Evia all seem to connect with the strike-slip faulting in the offshore Aegean and to die out in the west, a configuration related to clockwise rotations in east-central Greece. By contrast, the Gulf of Corinth, which is the fastest opening graben system in Greece, opens more rapidly in the west than the east. Both these features of the Gulf of Corinth are consequences of the motion of the south Aegean and Peloponnese as a single block. Late Pliocene and Quaternary geology and geomorphology indicate that the boundaries of the rigid blocks in central Greece have changed over that time, with faulting migrating into the hanging walls, sometimes changing in orientation. These changes are probably related to the way faulting in the seismogenic layer adapts to block rotation, so as to maintain the general features of a velocity field that is controlled by larger scale effects, such as buoyancy forces and the seaward migration of the subducting slab beneath Crete. The image of Greece as a mosaic of rigid blocks whose boundaries change with time is a useful framework for seismic hazard evaluation, but we emphasize that some moderate-sized earthquakes do occur away from the main fault zones, within the relatively rigid blocks and especially near their diffuse ends.

Transition from Contraction to Extension in the Northeastern Basin and Range: New Evidence from the Copper Mountains, Nevada

Abstract New mapping, structural analysis, and 40Ar/39Ar dating reveal an unusually well‐constrained history of Late Eocene extension in the Copper Mountains of the northern Basin and Range province. In this area, the northeast‐trending Copper Creek normal fault juxtaposes a distinctive sequence of metacarbonate and granitoid rocks against a footwall of Upper Precambrian to Lower Cambrian quartzite and phyllite. Correlation of the hanging wall with footwall rocks to the northwest provides an approximate piercing point that requires 8–12 km displacement in an ESE direction. This displaced fault slice is itself bounded above by another normal fault (the Meadow Fork Fault), which brings down a hanging wall of dacitic to rhyolitic tuff that grades conformably upward into conglomerate. These relationships record the formation of a fault‐bounded basin between 41.3 and 37.4 Ma. The results are consistent with a regional pattern in which volcanism and extension swept southward from British Columbia to southern Ne...

New age data from the Lesnaya Group: A key to understanding the timing of arc‐continent collision, Kamchatka, Russia

Abstract The Lesnaya Group is part of a thick, poorly dated turbidite assemblage that sits in the footwall of a regionally extensive collision zone in which the Cretaceous–Paleocene Olutorsky island arc terrane was obducted onto continental margin basin strata. Nannoplankton from 18 samples from the upper part of the Lesnaya Group yield Paleocene through Middle Eocene assemblages. Detrital zircons from nine sandstone samples have a young population of fission‐track ages that range from 43.7 ± 3.4 to 55.5 ± 3.5 Ma (uppermost Paleocene to Middle Eocene). The deformed footwall rocks of the Lesnaya Group and the overlying thrusts of the Olutorsky arc terrane, are unconformably overlain by neoautochthonous deposits which are Lutetian (lower Middle Eocene) and younger. Together, these new data indicate that thrusting, which is inferred to have been driven by collision of the Cretaceous–Paleocene island arc with north‐eastern Asia, took place in the mid‐Lutetian, at about 45 Ma.

A Textural, Mineralogical, and Statistical Study of the Footwall Breccia within the Strathcona Embayment of the Sudbury Structure

The footwall breccia is one of the main Ni-Cu-(PGE) ore-hosting units of the Sudbury camp. This polymict, matrix-supported, contact-metamorphosed breccia occurs between the Sudbury Igneous Complex and the footwall rocks along the west, north, and east margins of the Sudbury structure. In areas where there are depressions or embayments in the footwall, the footwall breccia and basal units of the Sudbury Igneous Complex are thickened, and the footwall breccia is variably mineralized. Mineralogical and textural variations of footwall breccia samples from the Strathcona embayment in proximity to the Fraser mine suggest that the mineralogy and texture of this unit vary as a function of proximity to the Sudbury Igneous Complex, proximity to ore, and location within the embayment. The most mafic footwall breccia mineralogy is present proximal to the base of the Sudbury Igneous Complex within a transition zone. The presence of relatively mafic, hydrous, and Cl-rich silicate minerals (e.g., biotite and amphibole) characterize the footwall breccia matrix around ore zones and form an alteration halo of up to 5 m thick. Textural variations include a coarsening of quartz and feldspar grains with increasing proximity to the Sudbury Igneous Complex contact. Textural coarsening also occurs from the margins of the Strathcona embayment to the center. Together, these mineralogical and textural variations influence the color of the footwall breccia matrix and can explain some of the gray colors that are typical of mineralized footwall breccia. These variations also constrain the timing of formation of this unit and suggest that at least three phases of footwall breccia exist.

Global comparisons of volcanic-associated massive sulphide districts

Abstract Although volcanic-associated massive sulphide (VMS) deposits have been studied extensively, the geodynamic processes that control their genesis, location and timing remain poorly understood. Comparisons among major VMS districts, based on the same criteria, have been commenced in order to ascertain which are the key geological events that result in high-value deposits. The initial phase of this global project elicited information in a common format and brought together research teams to assess the critical factors and identify questions requiring further research. Some general conclusions have emerged. (1) All major VMS districts relate to major crustal extension resulting in graben subsidence, local or widespread deep marine conditions, and injection of mantle-derived mafic magma into the crust, commonly near convergent plate margins in a general back-arc setting. (2) Most of the world-class VMS districts have significant volumes of felsic volcanic rocks and are attributed to extension associated with evolved island arcs, island arcs with continental basement, continental margins, or thickened oceanic crust. (3) They occur in a part of the extensional province where peak extension was dramatic but short-lived (failed rifts). In almost all VMS districts, the time span for development of the major ore deposits is less than a few million years, regardless of the time span of the enclosing volcanic succession. (4) All of the major VMS districts show a coincidence of felsic and mafic volcanic rocks in the stratigraphic intervals that host the major ore deposits. However, it is not possible to generalize that specific magma compositions or affinities are preferentially related to major VMS deposits world-wide. (5) The main VMS ores are concentrated near the top of the major syn-rift felsic volcanic unit. They are commonly followed by a significant change in the pattern, composition and intensity of volcanism and sedimentation. (6) Most major VMS deposits are associated with proximal (near-vent) rhyolitic facies associations. In each district, deposits are often preferentially associated with a late stage in the evolution of a particular style of rhyolite volcano. (7) The chemistry of the footwall rocks appears to be the biggest control on the mineralogy of the ore deposits, although there may be some contribution from magmatic fluids. (8) Exhalites mark the ore horizon in some districts, but there is uncertainty about how to distinguish exhalites related to VMS from other exhalites and altered, bedded, fine grained tuffaceous rocks. (9) Most VMS districts have suffered fold-thrust belt type deformation, because they formed in short-lived extensional basins near plate margins, which become inverted and deformed during inevitable basin closure. (10) The specific timing and volcanic setting of many VMS deposits, suggest that either the felsic magmatic-hydrothermal cycle creates and focuses an important part of the ore solution, or that specific types of volcanism control when and where a metal-bearing geothermal solution can be focused and expelled to the sea floor, or both. This and other questions remain to be addressed in the next phase of the project. This will include in-depth accounts of VMS deposits and their regional setting and will focus on an integrated multi-disciplinary approach to determine how mineralisation, volcanic evolution and extensional tectonic evolution are interrelated in a number of world-class VMS districts.

Geochronology of clasts in allochthonous Miocene sedimentary sequences on Mykonos and Paros Islands: implications for back-arc extension in the Aegean Sea

In the Cyclades, low-angle normal faults juxtaposed Miocene sedimentary rock units lying over Alpine blueschist- and greenschist-facies metamorphic rocks and Miocene granites. The sedimentary units in the hanging wall were deposited in fault-bounded basins while their footwalls progressively emerged through the ductile and brittle crust. The sedimentation in the basins evolved from marine turbidites in the Early Miocene to shallow/continental conglomerates during the Late Miocene. The transition from marine to fan delta sedimentation was coeval with widespread magmatism and possibly reflects true crustal uplift. It is inferred that the sequence is no younger than 8 Ma. K–Ar and 40 Ar/ 39 Ar geochronology, petrology and petrography of the clasts deposited on the hanging wall provide proof of progressive exhumation of the footwall and reveal the nature of the overburden that covered the Cycladic blueschist belt during Alpine orogeny in the Tertiary. Abundant metamorphic clasts yielding mica cooling ages between 80 and 100 Ma occur throughout the sedimentary section and probably pertain to a vast Pelagonian-type rock mass that covered the internal Hellenides from the Olympos to the central Cyclades. In addition, volcanic clasts dated at 10 Ma reveal the existence of a hitherto unknown volcanic province of that age in the central Aegean. Miocene crustal extension and exhumation of granitic plutons is recorded in the detrital sequence in a concentration of 10 Ma granitic clasts restricted to the top of the conglomerate sequence. A group of metamorphic clasts that yielded ages of 13–16 Ma possibly represents exhumation of mid crustal levels. Clasts similar to the currently-exposed Cycladic Blueschist Unit, such as 40 Ma old blueschists, eclogites and marble, were not found, thus indicating the very late exposure of these footwall rocks.

Geochemistry and Paleotectonic Setting of Felsic VolcanicRocks in the Finlayson Lake Volcanic-Hosted Massive Sulfide District, Yukon,Canada,

The Finlayson Lake volcanic-hosted massive sulfide (VHMS) district represents one of Canada’s most recent VHMS discovery regions with ~34 million metric tons (Mt) of massive sulfide mineralization found since the mid 1990s. Felsic volcanic rocks are associated with three units: the Fire Lake unit, the Kudz Ze Kayah unit, and the Wolverine succession. Significant accumulations of polymetallic felsic VHMS deposits (Kudz Ze Kayah, GP4F, and Wolverine) have only been discovered in the Kudz Ze Kayah unit and Wolverine succession to date. In the hanging wall of the Money Creek thrust, felsic volcanic and high-level intrusive rocks in the Fire Lake unit have calc-alkalic and tholeiitic affinities with low high field strength element (HFSE) contents and intermediate Zr/Sc (9.4‐43.4) and Zr/TiO2 (254‐864) ratios. These rocks are interlayered with mafic rocks with arc geochemical signatures, to date devoid of significant VHMS mineralization, and represent bimodal magmatism within an evolving Devonian-Mississippian continental-arc system. The Kudz Ze Kayah unit in the footwall of the Money Creek thrust stratigraphically overlies the Fire Lake unit and consists of felsic volcanic rocks with high HFSE contents, within-plate (A-type) signatures, and high Zr/Sc (15.3‐190.3) and Zr/TiO2 (630‐2,185) ratios. The Kudz Ze Kayah unit felsic rocks are crosscut and overlain by alkalic mafic rocks, are associated with abundant carbonaceous sedimentary rocks, and represent magmatism within a Devonian-Mississippian ensialic back-arc rift-basin environment. The Wolverine succession unconformably overlies the Kudz Ze Kayah unit. Felsic rocks below the Wolverine deposit have geochemical attributes similar to the Kudz Ze Kayah unit with high HFSE contents, within-plate (A-type) signatures, and high Zr/Sc (29.9‐84.2) and Zr/TiO2 (391‐1,220) ratios. In contrast, aphyric rhyolite flows in the hanging wall of the deposit have much lower HFSE contents and the lowest Zr/Sc (3.5‐27.7) and Zr/TiO2 (181‐591) ratios in the district. All the felsic rocks of the Wolverine succession are interlayered with abundant carbonaceous sedimentary rocks and are overlain by midocean ridge basalt (MORB)-like basaltic rocks. The Wolverine succession is interpreted to have formed within an Early Mississippian ensialic back-arc basin environment that eventually evolved to sea-floor spreading. The variation in the HFSE budgets of the felsic rocks of the Finlayson Lake district likely reflects variations in the source and/or temperature of crustal melting. In particular, felsic rocks of the Fire Lake unit have higher Nb/Ta and lower Ti/Sc ratios than other volcanic rocks in the district, suggesting possible derivation from mafic crustal sources and/or lower crustal fusion temperatures. The Kudz Ze Kayah unit and footwall rocks to the Wolverine deposit are inferred to have formed from high-temperature partial melting of continental crust. The hanging-wall aphyric rhyolites from the Wolverine deposit may have formed from either lower temperature continental crustal melting or may have been derived from the mixing of HFSE-depleted N-MORB mafic magmas and evolved continental crust. Polymetallic felsic volcanic-associated, VHMS deposits within the Finlayson Lake district are preferentially associated with HFSE-enriched felsic rocks with high Zr/Sc (15.3‐190.3) and Zr/TiO2 (391‐2,105) ratios. The HFSE and rare earth element (REE) systematics of VHMS-associated felsic rocks of the Finlayson Lake district are different from prospective felsic rocks from Archean VHMS environments in the Superior province and are displaced toward higher Zr/Y and La/Ybn ratios. Their HFSE and REE systematics are similar to many Phanerozoic VHMS environments, in particular those at least partially to fully underlain by evolved continental crust. The geochemical differences between the felsic rocks of the Finlayson Lake district and those from Archean VHMS environments most likely reflect differences in the substrates from which the felsic rocks were derived (e.g., evolved versus juvenile).

Tectonic controls on a large landslide complex: Williams Fork Mountains near Dillon, Colorado

An extensive (∼ 25 km 2) landslide complex covers a large area on the west side of the Williams Fork Mountains in central Colorado. The complex is deeply weathered and incised, and in most places geomorphic evidence of sliding (breakaways, hummocky topography, transverse ridges, and lobate distal zones) are no longer visible, indicating that the main mass of the slide has long been inactive. However, localized Holocene reactivation of the landslide deposits is common above the timberline (at about 3300 m) and locally at lower elevations. Clasts within the complex, as long as several tens of meters, are entirely of crystalline basement (Proterozoic gneiss and granitic rocks) from the hanging wall of the Laramide (Late Cretaceous to Early Tertiary), west-directed Williams Range thrust, which forms the western structural boundary of the Colorado Front Range. Late Cretaceous shale and sandstone compose most footwall rocks. The crystalline hanging-wall rocks are pervasively fractured or shattered, and alteration to clay minerals is locally well developed. Sackung structures (trenches or small-scale grabens and upslope-facing scarps) are common near the rounded crest of the range, suggesting gravitational spreading of the fractured rocks and oversteepening of the mountain flanks. Late Tertiary and Quaternary incision of the Blue River Valley, just west of the Williams Fork Mountains, contributed to the oversteepening. Major landslide movement is suspected during periods of deglaciation when abundant meltwater increased pore-water pressure in bedrock fractures. A fault-flexure model for the development of the widespread fracturing and weakening of the Proterozoic basement proposes that the surface of the Williams Range thrust contains a concave-downward flexure, the axis of which coincides approximately with the contact in the footwall between Proterozoic basement and mostly Cretaceous rocks. Movement of brittle, hanging-wall rocks through the flexure during Laramide deformation pervasively fractured the hanging-wall rocks.

Large‐magnitude extension along metamorphic core complexes of western Arizona and southeastern California: Evaluation with paleomagnetism

Cordilleran metamorphic core complexes (MCCs) and their associated structures (detachment faults and mylonites) are the products of large‐magnitude Cenozoic extension. Paleomagnetic data from the Harquahala, Harcuvar, Buckskin, and Whipple MCCs of western Arizona and southeastern California (∼1900 samples; 191 of 278 sites accepted for inclusion in group and grand means) allow us to estimate the magnitude of footwall tilting associated with extension. All MCC crystalline rocks sampled are in the footwalls of associated brittle detachment faults. Rocks sampled include synkinematic, Miocene intrusions (dikes and plutons), Late Cretaceous granites, and Proterozoic host rocks. These rocks (with different levels of success at the site level) yield dual polarity, high unblocking temperature, and high to moderate coercivity magnetizations. Demagnetization behavior reveals that those sites with well‐grouped magnetizations and high laboratory unblocking temperatures and/or high to moderate coercivities have characteristic remanent magnetizations carried by either magnetite, with minor hematite, or by hematite, with minor magnetite. Results of baked contact tests show that these footwall rocks possess thermoremanent magnetizations or high‐temperature thermochemical remanent magnetizations acquired early in their cooling history, either before or during extension. Our overall interpretation of the paleomagnetic data is that during Cenozoic extension, MCC footwalls tilted (flattened) only a modest amount (&lt;15°) in the direction of tectonic transport. We also interpret Harcuvar and Whipple MCC paleomagnetic data to indicate tectonic tilting (steepening) of the mylonitic zone to form a back dipping mylonitic front. The Whipple‐Bullard‐Eagle Eye detachment fault system and associated mylonites are interpreted to have originated with a low‐angle dip (10° to 25° NE). We relate the small amount of footwall flattening in the direction of tectonic transport and the steepening of the back dipping mylonitic front to the buoyant rise of exhumed footwalls according to rolling‐hinge/isostatic uplift models. Our interpretation refutes the widespread applicability of models that predict MCCs to represent strongly tilted crustal blocks, originally bounded by high/moderate‐angle normal faults. Additionally, a fold test of Harquahala MCC data yielded “negative” results. This suggests that antiformal footwall arches represent the oldest structures of MCCs that developed prior to, or during mylonitization of footwall rocks (before acquisition of the remanent magnetizations carried by these rocks).

Geochronologic and thermobarometric constraints on the evolution of the Main Central Thrust, central Nepal Himalaya

The Main Central Thrust (MCT) juxtaposes the high‐grade Greater Himalayan Crystallines over the lower‐grade Lesser Himalaya Formation; an apparent inverted metamorphic sequence characterizes the shear zone that underlies the thrust. Garnet‐bearing assemblages sampled along the Marysandi River and Darondi Khola in the Annapurna region of central Nepal show striking differences in garnet zoning of Mn, Ca, Mg, and Fe above and below the MCT. Thermobarometry of MCT footwall rocks yields apparent inverted temperature and pressure gradients of ∼18°C km−1 and ∼0.06 km MPa−1, respectively. Pressure‐temperature (P‐T) paths calculated for upper Lesser Himalaya samples that preserve prograde compositions show evidence of decompression during heating, whereas garnets from the structurally lower sequences grew during an increase in both pressure and temperature. In situ (i.e., analyzed in thin section) ion microprobe ages of monazites from rocks immediately beneath the Greater Himalayan Crystallines yield ages from 18 to 22 Ma, whereas late Miocene and Pliocene monazite ages characterize rocks within the apparent inverted metamorphic sequence. A Lesser Himalayan sample collected near the garnet isograd along the Marysandi River transect contains 3.3±0.1 Ma monazite ages (P ≈ 0.72 GPa, T ≈ 535°C). This remarkably young age suggests that this portion of the MCT shear zone accommodated a minimum of ∼30 km of slip over the last 3 Ma (i.e., a slip rate of &gt;10 mm yr−1) and thus could account for nearly half of the convergence across the Himalaya in this period. The distribution of ages and P‐T histories reported here are consistent with a thermokinematic model in which the inverted metamorphic sequences underlying the MCT formed by the transposition of right‐way‐up metamorphic sequences during late Miocene‐Pliocene shearing.

Discovery of the West 45 Volcanic-Hosted Massive Sulfide Deposit Using Oxygen Isotopes and REE Geochemistry

A number of innovative geochemical techniques were tested around the Thalanga volcanic-hosted massive sulfide (VHMS) deposit in North Queensland, with the objective of being able to recognize a near miss in the exploration drilling in and around the mine. Whole-rock oxygen isotope analyses were used to determine temperature gradients in the footwall rocks just beneath the ore horizon. By assuming a water 6180 value of 0 per mil, consistent with a seawater-dominated hydrothermal system, an alteration temperature was calculated for each sample. A quantitative determination of the mineralogy was used to work out a weighted fractionation factor for each sample so that the variance attributable to differences in mineralogy could be taken into account. Comparison of the temperatures to a sodium depletion index indicates that the 6180 halo is comparable in size to an alteration mineralogy halo, but the distribution of Na is somewhat unpredictable in the distal parts of the hydrothermal system. This study identified a hot spot i km west of the limit of the mine workings. The Mount Windsor Volcanics, which host the Thalanga deposit, contain a large number of stratiform quartz-hematite lenses (red jaspers). Rather than drill test all of these occurrences, a geochemical index was determined to assist in ranking them. Elevated Ba, S, and Pb along with positive chondrite-normalized Europium anomalies were found to be characteristics of proximal jaspers as compared to barren jaspers. A red jasper outcrop west of Thalanga, coincident with the 6180 hot spot, was the most anomalous sample in this study. Subsequent drill testing of this target led to the discovery of the West 45 VHMS deposit.

The Origin of Chlorite-Tremolite-Carbonate Rocks Associated with the Thalanga Volcanic-Hosted Massive Sulfide Deposit, North Queensland, Australia

The Thalanga polymetallic massive sulfide deposit is hosted by felsic volcanics of the Cambro-Ordovician Mount Windsor subprovince in North Queensland. The sheetlike sulfide deposit consists of several semiconnected lenses, totaling 6.6 million metric tons (Mt), which lie at a single stratigraphic level between underlying subaqueously emplaced rhyolite and overlying dacitic to andesitic lavas, sills, and volcaniclastic rocks. The ores and host rocks have been deformed and metamorphosed to upper greenschist facies. Associated with the West Thalanga orebody are strata-bound lenses composed of chlorite, tremolite, dolomite, and calcite that previously have been interpreted as exhalites. They extend up to a few hundred meters along the mineralized horizon and locally also exist in the stratigraphic footwall around the fringes of the deposit. Rhyolitic volcanics beneath the deposit are extensively altered. A pervasive sericite ± chlorite ± pyrite zone that extends greater than 200 m into the footwall envelops semistratiform proximal quartz + pyrite ± chlorite stringer zones. Major and trace element geochemical data indicate that Zr, Ti, and Al remained essentially immobile in the footwall rocks despite intense hydrothermal alteration involving significant mass transfers of mobile elements. The chlorite + tremolite + carbonate assemblages associated with ore lenses have immobile element ratios identical to those of the altered rhyolites. Carbonate-rich assemblages retain some relict spheroidal and rhombic textures in dolomite, which suggest formation at an early hydrothermal stage in a matrix of chlorite or in open spaces. The chlorite + tremolite + carbonate rocks probably represent metamorphic equivalents of hydrothermal chlorite + quartz + dolomite + calcite (or Mg smectite + quartz + dolomite + calcite) assemblages. The formation of tremolite by metamorphic reaction of quartz and dolomite is consistent with regional metamorphic conditions. With allowance for metamorphic decarbonation, carbon and oxygen isotope data from dolomite and calcite are consistent with precipitation of primary dolomite at 170° to 250°C from seawater-dominated fluid. The chlorite + tremolite + carbonate rocks at Thalanga are consequently reinterpreted as altered rhyolitic volcaniclastics, containing variable proportions of hydrothermal carbonate. They probably formed in a zone of mixing of hydrothermal fluid and seawater, in a permeable substrate close to the paleosea floor. Siliceous ironstones that are present in distal parts of some Thalanga ore lenses have very low immobile element contents and are probably true exhalites.

Rock deformation studies in the Mineral Mountains and Sevier Desert of west-central Utah: Implications for upper crustal low-angle normal faulting

The Cave Canyon detachment, a low-angle normal fault that crops out in the Mineral Mountains, west-central Utah, has been interpreted as a hanging-wall splay of a much larger structure (the Sevier Desert detachment) that was influential in development of the idea that low-angle normal faults play a role in crustal extension. The Cave Canyon detachment provides expectations for the deformational features that might be expected along the hypothesized Sevier Desert detachment, which is not exposed in outcrop and is inferred to exist primarily on the basis of seismic reflection data. The footwall of the Cave Canyon detachment is characterized by a 200-m-thick granite cataclasite, which exhibits a clear decrease in grain size and increase in microfracture density as the fault surface is approached. Undulatory extinction in quartz and feldspar and abundant quartz deformation lamellae at distances more than 200 m from the fault surface are interpreted as related to cooling of the Miocene granite rather than to normal faulting. Although mylonitic textures have previously been described in the granite, we found no evidence for mylonitization in the footwall rocks. The hanging wall of the detachment is characterized by 9 m of deformed, partially dolomitized limestone, with a 2-m-thick carbonate mylonite at the contact. Deformation features include dynamic recrystallization, grain-size reduction, development of twinning with a strong preferred orientation, some grain-size layering, and undulatory extinction close to the fault. Static recrystallization overprints fossils and ooids at distances greater than 9 m. Drill cuttings and some core recovered at similar distances above and below the hypothesized Sevier Desert detachment show no evidence for localized deformation (ARCO Hole-in-the-Rock No. 1, ARCO Meadow Federal No. 1, and Argonaut Energy Federal No. 1 wells). Fossils and ooids are undeformed in Paleozoic carbonate rocks within 3 m below the contact, and sandstone and conglomerate with rounded clasts lacking more than background levels of microfracturing are found in samples within 3 m above the contact. These features contrast markedly with those of the Cave Canyon detachment, which was active at a considerably shallower and cooler level in the crust (∼5 km and <300 °C) than is implied for Paleozoic rocks beneath the Sevier Desert, once hanging-wall rocks are restored along the hypothesized detachment (9−14 km and 280−425 °C at the locations studied). The very different character of the two surfaces reinforces our earlier suggestion that beneath much of the Sevier Desert basin, the base of the Tertiary section is an unconformity rather than a low-angle normal fault.

Crustal Contamination and Fluid–Rock Interaction during the Formation of the Platreef, Northern Limb of the Bushveld Complex, South Africa

The Platreef is the main platinum group element (PGE)-bearing and contains some of the most important magmatic ore deposits yet discovered. The intrusion covers an area of facies of the northern limb of the Bushveld complex, but unlike the Merensky Reef of the eastern and western limbs, it is in direct roughly 65 000 km (e.g. Tankard et al., 1982) and lies almost entirely within the bounds of the Transvaal Basin contact with the country rock. Mineral separate O values for samples from the Upper Zone and Main Zone of the northern limb (Fig. 1). The layered igneous rocks (the Rustenburg Layered Suite, SACS, 1980) can be divided into eastern indicate crystallization from a well-mixed, already contaminated, magma having a O value of 7·5‰. Pyroxenes from the pyroxenites and western limbs of approximately the same size, and a smaller northern limb. The northern limb differs in facies of the Platreef at Sandsloot Mine have O values that are up to 2·4‰ higher than pyroxenes from the Upper and Main several important aspects from the more well-known eastern and western limbs (e.g. Van der Merwe, 1976; Zones. These differences can be explained by additional assimilation of up to 18% dolomite, which is in contact with the intrusion at Buchanan et al., 1981; Cawthorn et al., 1985; Eales & Cawthorn, 1996). First, the layered rocks show a this locality. Samples from the Platreef that have plagioclase and pyroxene not in oxygen isotope equilibrium appear to have interacted pronounced transgression across the floor rocks from a with fluids during slow cooling. Quartz veins with granophyric level above the Magaliesberg Quartzite Formation south margins have O values between 10·1 and 12·2‰, which of the town of Potgietersrus, across the whole Pretoria suggest that the fluid that passed through the cooling Platreef had Group, and the banded iron formations and dolomites a O value of 7–9‰. These data, together with hydrogen isotope of the older Chuniespoort Group, onto Archaean granitic data from minerals and whole-rock samples ( D of biotite −60 rocks (Fig. 1). Second, the igneous layering shows an to −88‰) of the Platreef suggest interaction with magmatic fluid apparent transgressive relationship with respect to the at low water/rock ratios. Interaction of this fluid with the calcsilicate contact, such that ultramafic rocks of the Lower Zone footwall rocks lowered their O values. abut against the contact south of Potgietersrus, whereas to the north, rocks forming the base of the intrusion are progressively higher in the stratigraphic succession of the

Regional oxygen isotope systematics of felsic volcanics; a potential exploration tool for volcanogenic massive sulphide deposits in the Iberian Pyrite Belt

Regional oxygen isotopic sytematics have been performed mainly on the felsic volcanic footwall rocks of the orebodies but also on purple schist characteristic of the hanging wall series, around two giant VMS deposits in the Spanish Iberian Pyrite Belt, Riotinto and La Zarza. As the terranes of the Iberian Pyrite Belt, these two giant deposits have been affected by the Hercynian tectono-metamorphic events, strongly modifying their geometry. About 60 and 40 samples were collected over a 10×4 km 2 area at Riotinto and a 3×2 km 2 area at La Zarza, respectively. Whole-rock powders were analysed for oxygen by CO 2-laser fluorination. At both sites, a same type of low- δ 18O anomaly down to +3.6‰, well differentiated from the regional background (up to 20‰), was identified near the orebodies. The lowest δ 18O values (+4 to +11‰) correspond to the chlorite hydrothermal halo, essentially restricted to the feeder zones of the orebody. Intermediate δ 18O values (+9 to +15‰) correspond to the sericite hydrothermal halo, mostly developed laterally to the orebody until 0.5–1 km. The regional background (+16 to +20‰) is represented by spilitised volcanic rocks. A same kind of low anomaly, but with less contrast, was defined in purple schist in the immediate hanging wall of the orebodies. All these results demonstrate that, despite high geometrical modifications of the orebodies related to the Hercynian tectonics, oxygen isotopic anomalies recorded by volcanic host rocks during the emplacement of the mineralising hydrothermal systems are still identified. This strongly suggests that oxygen isotopic systematics could be useful to identify target areas in the Iberian Pyrite Belt, as already demonstrated on other VMS targets in the world.

Limited syntectonic fluid flow in carbonate‐hosted thrust faults of the Front Ranges, Canadian Rockies, inferred from stable isotope data and structures

Samples collected from five carbonate‐hosted thrust faults in the Front Ranges (Canadian Rockies) were isotopically and structurally analyzed in order to document syntectonic fluid‐rock interaction. The most deformed rocks are adjacent to the thrust contacts, corresponding to a general increase in (micro)fractures, twins, foliation development, crystallographic preferred orientation fabric, and veins toward the thrusts. Subhorizontal veins that accommodated subvertical extension are present locally in the thrusts and help to define the foliated fabric of the fault rocks. The δ18O and δ13C values of fault rocks meter(s) away from the thrusts are typical of Paleozoic carbonates, while many rocks adjacent to the thrusts have lower values. There are not strong correlations between these values and the type and/or density of structures in the thrusts. Far from the thrusts, most syntectonic veins have isotopic values similar to their host rocks. Only within decimeters to a meter of the thrusts, do some veins have lower values. There is little isotopic evidence for significant fluid flow across the faults between hanging wall and footwall rocks. The structural data are consistent with the fault rocks having deforming by a variety of deformation mechanisms that probably resulted in both crystal plastic and brittle behavior. Transient high pore fluid pressure in the faults must have occurred locally and episodically in order to account for the formation of the subhorizontal veins. The isotopic data are consistent with a limited amount of fluid contained in very narrow zones adjacent to the thrusts. If true, this fluid must have been maintained in a semi‐closed system and not continuously replenished by large quantities of fluids from the thrusts' footwalls or from the hinterland. The long‐term rheology of the fault rocks in the Front Ranges was not necessarily dominated by pressure‐dependent frictional sliding, and the role of high pore fluid pressure in thrust movement might have been less important than is widely believed.

Controls on the Formation of Contact-Type Platinum-Group Element Mineralization in the East Bull Lake Intrusion

The East Bull Lake intrusion is the type example of a series of 2.48 to 2.49 Ga layered mafic intrusions that developed within the rifted southern margin of the Superior craton, central Ontario. The East Bull Lake intrusion contains contact-type PGE mineralization, e.g., strata-bound, PGE-rich, disseminated sulfide mineralization occurring within 200 to 300 m of the contact with footwall and sidewall units. Although the mineralization locally occurs in massive plagioclase cumulates (anorthosite zone), most of the sulfides are concentrated within an underlying, heterolithic, inclusion-rich unit (inclusion-bearing zone) that grades into an intrusive breccia. Disseminated sulfides also occur in fine-grained gabbroic veins within brecciated footwall rocks (border zone). Geochemical characteristics of the contact-type sulfide mineralization and its host rocks include: (1) high-average Pd abundances throughout the East Bull Lake intrusion, including an average of ca. 200 ppb in the Lower Series; (2) low-average S contents (typically Contact-type PGE mineralization in the East Bull Lake intrusion reflects flow-enhanced gravitational settling of a Cu-rich sulfide liquid followed by local fractionation of Ni-rich monosulfide solid solution to produce minor amounts of Cu- and Pt-rich, residual sulfide liquid. Most of the observed compositional variation in the sulfides can be modeled using variable silicate liquid/sulfide liquid mass ratios. Field and geochemical observations suggest that the parental magmas to the mineralized parts of the East Bull Lake intrusion were S saturated and PGE rich at the time of their emplacement. These fertile parent magmas most likely originated as buoyant, feldspathic residual magmas that escaped from a deeper, more primitive subchamber. Although first described from the East Bull Lake intrusion, contact-type PGE mineralization is now recognized in all of the major East Bull Lake suite intrusions. Similar styles of magmatic sulfide mineralization are developed in the contact environments of the Bushveld Igneous Complex (Platreef deposits) and the Duluth Complex, Minnesota (e.g., Dunka Road deposit). These contact-type deposits are part of the family of basal accumulation magmatic sulfide deposits. The economics of low sulfide tenor, contact-type, PGE-Cu-Ni mineralization, such as that developed in the East Bull Lake intrusion will be most favorable in systems in which (1) the parent magmas possessed high initial PGE tenors and reached S saturation prior to emplacement; (2) the parent magmas experienced vigorous convection in the early stages of chamber development; and (3) segregated sulfide was not diluted by coprecipitating silicate minerals—a scenario that is most likely to occur in feldspathic magmas in which plagioclase is the principal high-temperature liquidus mineral.

Late Alpine evolution of the central Menderes Massif, western Turkey

The central Menderes Massif (western Turkey) is characterized by an overall dome-shaped Alpine foliation pattern and a N–NNE-trending stretching lineation. A section through the southern flank of the central submassif along the northern margin of Buyuk Menderes graben has been studied. There, asymmetric non-coaxial fabrics indicate that the submassif has experienced two distinct phases of Alpine deformation: a top-to-the N–NNE contractional phase and a top-to-the S–SSW extensional event. The former fabrics are coeval with a regional prograde Barrovian-type metamorphism at greenschist to upper-amphibolite facies conditions. This event, known as the main Menderes metamorphism, is thought to be the result of internal imbrication of the Menderes Massif rocks along south-verging thrust sheets during the collision of the Sakarya continent in the north and the Anatolide–Tauride platform in the south across the Izmir–Ankara suture during the (?)Palaeocene–Eocene. Top-to-the S–SSW fabrics, represented by a well-developed ductile shear band foliation associated with inclined and/or curved foliation, asymmetric boudins, and cataclasites, were clearly superimposed on earlier contractional fabrics. These fabrics are interpreted to be related to a low-grade (greenschist?) retrogressive metamorphism and a continuum of deformation from ductile to brittle in the footwall rocks of a south-dipping, presently low-angle normal fault that accompanied Early Miocene orogenic collapse and continental extension in western Turkey. A similar tectono-metamorphic history has been documented for the northern flank of the dome along the southern margin of the Gediz graben with top-to-the N–NNE extensional fabrics. The exhumation of the central Menderes Massif can therefore be attributed to a model of symmetric gravity collapse of the previously thickened crust in the submassif area. The central submassif is thus interpreted as a piece of ductile lower-middle crust that was exhumed along two normal-sense shear zones with opposing vergence and may be regarded as a typical symmetrical metamorphic core complex. These relationships are consistent with previous models that the Miocene exhumation of the Menderes Massif and Cycladic Massif in the Aegean Sea was a result of bivergent extension.