Footwall Sequences Research Articles

The Role of Magma Mixing in the Genesis of PGE Mineralization in the Bushveld Complex: Thermodynamic Calculations and New Interpretations

The elegant magma-mixing model for the formation of the platinum-group elements (PGE) mineralization associated with chromite and sulfide in the Bushveld Complex proposed by Naldrett and von Gruenewaldt (1989) has recently been questioned by Cawthorn (1999). Based on his own calculation, Cawthorn concluded that the concave-upward sulfur solubility curve of a fractionated basaltic magma, which is central to the model, does not exist. Cawthorn (1999) pointed out that both the hanging-wall and footwall sequences of the Merensky reef formed from sulfide-undersaturated magmas instead of sulfide-saturated magmas as predicted by the model. Cawthorn argued that there is a mass-balance problem with the model if these rocks did not contribute PGE to the sulfides in the reef. Our thermodynamic calculations indicate that the concave-upward curve of S solubility, though different in detail from the original, is very much in evidence. Our calculations also indicate that the content of S dissolved in the residual magma may fall below the S solubility curve when the rate of S increase in the magma, due to the removal of silicate cumulates, is less than the rate of increase in its S solubility, due to the change in its temperature and composition during fractional crystallization. Mixing of such fractionated and yet sulfide-undersaturated magma with appropriate amounts of a primitive magma is still capable of achieving sulfide oversaturation in the hybrid. Once the immiscible sulfides settle out of the magma, the content of S dissolved in the hybrid may fall below its solubility curve again during further crystallization, thus no sulfide segregation is expected in the rocks crystallized subsequently. R-factor assessment needs to consider the consequence of sulfide suspension in magma. Small quantities of sulfide droplets formed during the normal course of fractional crystallization may remain suspended in magma until the next replenishment, and in this case they can be redissolved in the hybrid when the proportion of a new input is large enough. Because there is no early sulfide liquid segregation, both the residual and new magmas will not be depleted in PGE and are equally important for contributing PGE to the sulfide liquid segregated during a subsequent magma-mixing event.

Structure, mineralogy, and Pb isotopic composition of the As-Au-Ag deposit Rotgülden, Eastern Alps (Austria): significance for formation of epigenetic ore deposits within metamorphic domes

The abandoned As–Au–Ag mining district Rotgulden is located within the eastern Tauern window of the Eastern Alps and was reinvestigated in order to evaluate ore deposition during Alpine/late orogenic tectonic processes. Four major ore types have been recognized: (1) quartz-sulfide veins within Variscan basement rocks; (2) deformed metamorphic massive ores within fold hinge zones (“saddle reefs”) of Permian to Mesozoic cover sequences; (3) ores in tension gashes of the cover sequence; and (4) irregular replacement ore bodies in marbles of the cover sequence. Ore deposition was intimately related to late orogenic exhumation by stretching of footwall sequences within the Tauern metamorphic core complex during late Oligocene and Neogene. Hydrothermal systems developed and metals from apparently distinct sources were deposited under decreasing temperature conditions. Lead is distinctly radiogenic and resembles the lead in Au-quartz veins of the Mesozoic cover sequence of the Hohe Tauern.

Postmagmatic thermal activity in the Abitibi greenstone belt, Noranda and Matagami districts; evidence from whole-rock Pb isotope data

The Pb isotope compositions of the footwall sequences of the Noranda and Matagami volcanogenic massive sulfide deposits in the Abitibi greenstone belt have been examined. At Noranda, whole-rock Pb isotope data for the Flavrian, Northwest, Rusty Ridge, and Amulet Formations and the subvolcanic Flavrian intrusion scatter about a line that corresponds to a Pb-Pb age of 2638 + or - 14 Ma. Pb-Pb ages calculated for the individual units are all within error of this age but are significantly younger than the 2700 + or - 2 Ma U-Pb zircon age for the Flavrian intrusion. The Pb-Pb array passes above a tightly constrained field for the galena Pb from ore deposits within the Mine sequence. The younger Pb-Pb ages and the lack of collinearity with the sulfide ores reflect rehomogenization of the Pb isotope compositions in these rocks approximately 60 m.y. after their formation.At Matagami, whole-rock Pb isotope data fall along two arrays. The first, which includes the porphyritic metarhyolite of the Watson Lake Group from the footwall and the key tuffite, corresponds to a Pb-Pb age of 2584 (super +19) (sub -20) Ma. The second array includes the Bell River igneous complex and the overlying Wabassee Group basalts and corresponds to an age of 2647 + or - 14 Ma. These Pb-Pb ages are approximately 75 and 140 m.y. younger than corresponding zircon U-Pb ages of 2725 + or - 2 and 2725 (super +3) (sub -2) Ma. Both arrays pass above a tightly constrained field for galena Pb from several Matagami ore deposits and are probably the result of differences in the initial Pb isotope compositions of the respective rocks. The apparent initial Pb values most likely represent heterogeneities in the mantle source regions for these rocks. The younger Pb-Pb ages for the two arrays indicate Pb isotope homogenization in these rocks approximately 100 m.y. after their formation. The cause of the different ages for the two arrays remains controversial but may be the result of either incomplete isotopic homogenization of Pb or an earlier closure of the U-Pb systems in the rocks constituting the older array.The Pb homogenization implied by the young Pb-Pb ages may be the result of the pervasive circulation of hydrothermal solutions that may have been derived from the dewatering of the lower parts of a newly formed, thickened Archean crust. Such large-scale processes apparently occurred 50 to 150 m.y. after this crust was formed and assembled.

Lead isotope systematics of strata-bound sulfide deposits in the Caledonides of Norway

A lead isotope investigation has been carried out on approximately 40 strata-bound sulfide deposits in the Caledonides of Norway. The deposits occur in a wide range of palcotectonic enviromnents and lithostratigraphie units. A plot of their lead isotope data ( 206 Pb/ 204 Pb vs. 207 Pb/ 204 Pb) displays a linear trend, with volcanogenie deposits in an ocean-floor environment at the least radiogenic end and sediment-hosted deposits in an intraplate continental environment at the radiogenic end. This trend is probably a result of mixing mantle-derived lead with lead derived from an older radiogenic basement. Within the upper allochthon there is a general decrease in radiogenicity of the lead with higher teetono-stratigraphic position. This is paralleled by an increase in the proportion of volcanic rocks and is interpreted to reflect the diminished input of radiogenic lead from continentally derived sediments.The uniform character of the radiogenic end member indicates that lead in sediments, derived from a continental domain, was the immediate source and that the lead was homogenized during sedimentary transport. This is in agreement with the observed lithologies in the footwall sequences of the sampled deposits.Early obduetion of ophiolites and island ares onto Laurentia has been proposed as a model for the Scandinavian Caledonides. Although not ruled out by the lead isotope data from Norway, these data give no reason for invoking derivation of continental lead from the Laurentian side of the Iapetus ocean.A compilation of similar lead isotope data from the whole Caledonian-Appalachian orogen shows regional differences in the slopes of the mixing lines, reflecting different basement (continental) sources of lead. The lead isotope compositions of massive sulfide deposits may therefore become a useful tool in interpreting the origin of exotic terranes.

Geochemistry of the Cambrian volcanic-hosted massive sulfide-rich Mount Read Volcanics, Tasmania, and some tectonic implications

The Cambrian Mount Read Volcanics belt in western Tasmania hosts five major gold-rich massive sulfide deposits and remains the focus of intense exploration activity and geologic research. A geochemical study of least altered lavas and shallow intrusions, with emphasis on basaltic to andesitic compositions, was carried out to determine the primary magmatic affinities and tectonic setting of eruption of the Mount Read Volcanics, to aid in internal correlations within the belt, and to provide starting-material estimates for mineralization-related alteration studies.Three calc-alkaline suites (one extending to shoshonitic compositions) and two tholeitiic suites have been distinguished within the Mount Read belt. Suite I is voluminous and includes the Eastern sequence, Central Volcanic Complex, Tyndall Group, the intrusive quartz-feld-spar porphyries and granitoids, and the andesitic lavas of the Que-Hellyer footwall sequence. Suite I andesites are transitional, medium to high K calc-alkaline rocks ((La/Yb) N = 5-12, avg 8.1) and are the least light REE-enriched calc-alkaline lavas in the Mount Read Volcanics.Suite II comprises intrusive and extrusive, often hornblende-porphyritic andesites and dacites mainly from the upper part of the southern Central Volcanic Complex. They are more P 2 O 5 and light REE enriched ((La/Yb) N = 10-26, avg 16.7) than suite I and have high K calc-alkaline affinities.Suite III includes basaltic and andesitic lavas from the Que-Hellyer hanging-wall sequence in the northern part of the belt, the Lynch Creek basalts near Queenstown, and intrusive basalts in the Howards Plains area northwest of Queenstown. Suite III rocks show a striking compositional range, from low TiO 2 (0.4-0.5%), low P 2 O 5 (<0.1%) basalts with (La/Yb) N values from 8 to 12 and almost flat heavy REE patterns, to low TiO 2 (0.4-0.8%) but strongly to exceptionally P 2 O 5 and light REE-enriched basalts with up to 350 times chondritic La and (La/Yb) N values up to 34. The latter are regarded as shoshonites.Tholeiitic pillow basalts of the Henty fault wedge, and the closely related dikes of the Henty dike swarm, constitute suite IV: these have TiO 2 contents from 1.0 to 1.6 percent, very low Nb contents (<3 ppm), and only slightly light REE-enriched REE patterns ((La/Yb) N = 1.4-3.4). They are considered to be most similar to suprasubduction zone basalts erupted during the early phase of arc splitting and back-arc basin development and indicate a period of tension and rifting of the Mount Read belt in post-Central Volcanic Complex but pre-Tyndall Group time.Suite V is constituted by the Miners Ridge basalts and includes low TiO 2 (<0.7%) basalts with strong light REE depletion ((La/Yb) N <0.5) and higher TiO 2 (to 2.6%) lavas with only slight light REE depletion ((La/Yb) N = 0.9). Suite V basalts are tentatively correlated with rift tholeiite sequences associated with the Crimson Creek Formation, and therefore, are regarded as part of the pre-Mount Read Volcanics basement.The three calc-alkaline suites occur in a general stratigraphic order (with suites I and II being more or less coeval near Queenstown); they indicate a magmatic evolution from transitional medium to high K rocks through high K rocks, to strongly enriched shoshonitic rocks. Closest modern analogues are postcollisional volcanics in regions of arc-continent collision (e.g., East Papua and the Roman province).The occurrence of boninite-derived chromites (probably derived from the allochthonous mafic-ultramafic complexes outcropping just west of the Mount Read belt) in two sedimentary units within the Mount Read Volcanics sequence, below suite III basalts, supports a postcollisional origin for at least the major part of the Mount Read Volcanics. Other similar postcollisional sequences, particularly if formed in a submarine setting, must be considered highly prospective for VHMS deposits.

Volcanology and sedimentology of the host succession to the Hellyer and Que River volcanic-hosted massive sulfide deposits, northwestern Tasmania

The Hellyer and Que River high-grade polymetallic volcanic-hosted massive sulfide deposits of western Tasmania are hosted by a sequence of late Middle Cambrian subaqueous mafic to felsic volcanics and volcaniclastics known as the Que-Hellyer Volcanics. These volcanics occur toward the top of a thicker sequence of mainly felsic volcanics, also hosting mineralization, known as the Mount Read Volcanics.The base of the footwall sequence within the Que-Hellyer Volcanics is marked by massive to pillowed basaltic lavas and volcaniclastics of variable thickness. These are conformably overlain by a thick sequence of andesitic, basaltic, and minor dacitic lavas, and associated volcaniclastics. Eruption of these footwall sequences was rapid, with breaks in volcanism represented only by thin discontinuous epiclastic horizons.A hiatus in volcanism followed the emplacement of the footwall sequences. This hiatus is represented by the deposition of locally derived, coarse-grained mass flows. The distribution and geometry of these epiclastics suggest that they accumulated in local syndepositional depressions. Sedimentation was also accompanied by syngenetic Zn-Pb-Cu-Ag-Au-rich mineralization in the central or axial area of these depressions. Rapid burial by further mass flows assisted in the preservation of mineralization. Shallow intrusive to extrusive dacitic lava flows and domes in the host horizon were emplaced during and immediately after mineralization. Autoelastic material from these dacitic domes was then resedimented and incorporated in the epiclastic horizons overlying the orebodies.Eruption of massive to pillowed basaltic, and minor andesitic, lavas of the Hellyer basalt occurred soon after mineralization and was the final phase of volcanism in the Que-Hellyer Volcanics. The basalts were emplaced as extrusions and as shallow intrusions into wet, fine-grained sediments which began to accumulate above the coarse-grained epiclastic horizons. These basalts buried the remaining parts of the sulfide mound not covered by epiclastics. Immediately after the cessation of volcanic activity in the area, a 100-m-thick sequence of black shales and minor sandstones, known as the Que River Shale, was deposited.The general lack of products of explosive volcanism within the Que-Hellyer Volcanics is most likely a function of the composition of the lavas combined with their emplacement in deep water under high confining pressures. Although the absence of such products does not rule out the possibility of emplacement in shallow-water depths, the absence of abundant large-scale tractional sedimentary structures, the presence of the overlying black, anoxic Que River Shale, and the mass-flow characteristics of elastic units favor a relatively deep water setting for mineralization in the Que-Hellyer Volcanics.

Origin of culminations within the Southeast Oman Mountains at Jebel Ma-jhool and Ibra Dome

Abstract Structural culminations within the southeast Oman Mountains at Jebel Ma-jhool and the Ibra Dome developed through a process of multistage thrusting. This involved assembly and emplacement of the Semail and Hawasina thrust sheets over a footwall sequence of shelf carbonates and their pre-Permian basement, followed by progressive footwall collapse and breaching of the Hawasina thrust stack producing a hanging-wall anticline cored by shelf carbonates at Jebel Ma-jhool. At the Ibra Dome, foreland- and hinterland-dipping thrusts developed during the latter stages of thrusting forming a pop-up structure. Structural relations at Jebel Ma-jhool provide conclusive evidence that culminations within the Hawasina Complex are related to a decollement surface which lies at least as deep as the level of the shelf carbonates. Formation of the Ibra Dome and Jebel Ma-jhool structures was part of a widespread re-imbrication and culmination forming episode within the Oman Mountains. Major culminations, such as the Jebel Akhdar—Jebel Nakhl—Saih Hatat structures form significant morphological as well as structural highs. Slippage of the upper level Semail Ophiolite and Oman Melange sheets off these structures has resulted in truncation of the underlying Hawasina Complex thrust stack at Ibra Dome and Jebel Ma-jhool producing a break-back thrust sequence. The timing of initial thrusting, subsequent re-imbrication and culmination formation, and gravity-driven slippage of surficial thrust sheets is constrained to a 5 Ma period between the late Campanian and early Maastrichtian. This is based on the Coniacian to Campanian age of Aruma Group strata, the youngest rock unit affected by thrusting and the presence of the Maastrichtian Qahlah Formation unconformably overlying the flanks of both the Jebel Ma-jhool and Ibra Dome culminations. Broad scale folding and high-angle faulting of Maastrichtian and Tertiary strata in the southern Oman Mountains formed through regional mid-Tertiary compression and reactivation of Late Cretaceous culminations.

The Niflheim thrust: a tectonic contact between granulite and amphibolite facies gneisses, South-East Greenland

The Niflheim thrust forms part of the northern boundary zone of the Proterozoic mobile belt of the Ammassalik region and defines the southernmost extent of granulite facies gneisses north-west of Sermilik. The thrust sharply separates grey amphibolite facies gneisses (footwall) from a thick and extensive unit of brown granulite facies gneisses, suggesting considerable lateral as well as vertical transport of the brown gneisses. Above the contact, the brown gneisses have only been weakly affected by deformation, whilst below the contact intensely folded and sheared grey gneisses indicate strong deformation of the upper part of the footwall sequence during thrusting. Asymmetry of folds below the contact and the E–W trending, gently north dipping thrust contact indicate a sinistral transpressional sense of movement with an up-to-the-south main component of transport. Three undeformed, discordant basic Proterozoic dykes in the grey gneisses of the footwall are truncated by the thrust and the thrust plane has been gently folded during a late stage of the regional Proterozoic deformation. Contrasts between high-grade mineralogy of Proterozoic dykes in the northern part of the Ammassalik region and lower grade, high crustal-level dykes of the grey gneiss terrain in the south are related to the regional thrusting from the north.

Acadian remobilization of a Taconian ophiolite, Hare Bay allochthon, northwestern Newfoundland

The Hare Bay fault is a major subhorizontal detachment at the base of the ophiolitic St. Anthony Complex in the Hare Bay allochthon, northwestern Newfounland. The fault is a postmetamorphic brittle detachment that truncates footwall structures related to both initial Ordovician (Taconian) assembly of the allochthon and subsequent Silurian-Devonian (Acadian) deformation. Although previously mapped as a thrust, the fault has an extensional rather than a contractional geometry; it cuts downsection to the west in the direction of transport, and it juxtaposes a hanging-wall sequence that contains little or no Acadian deformation against a footwall sequence that was pervasively deformed during the Acadian orogeny. The St. Anthony Complex lies on the western margin of the Acadian deformed zone. Its final emplacement, through movement on the Hare Bay fault, probably occurred through extensional faulting during gravitational collapse of the Acadian mountain front.

Potholes of the Merensky Reef at Brakspruit Shaft, Rustenburg platinum mines; primary disturbances in the magmatic stratigraphy

Potholes are locations where parts of the magmatic stratigraphy are missing in the foot-wall sequence of the Merensky reef. In general, potholes are circular to elliptical in plan and funnel shaped in cross section. Within narrow limits, most potholes reveal the same three-dimensional orientation to the strike and dip of the magmatic layering. The updip pothole flank is steep to vertical; the downdip flank is horizontal. The pothole bottom generally coincides with one of the marker horizons of the footwall stratigraphy and is conformable with layering.Where the Merensky reef fills potholes, it is more variable in thickness than it is at its normal stratigraphic level. In the potholes, it changes over short distances from an unusually thin chromite-rich contact phase of the reef, to thick unlayered lenses of harzburgite pegmatite in deep-seated parts of the potholes. When the Bastard reef is observed in potholes, it sometimes grades into massive graphite-rich transgressive gabbroic pegmatite. The marker horizons in the anorthositic footwall units around potholes are often intensely mineralized with stratiform and crosscutting sulfides. Such marginal footwall mineralization is absent in the undisturbed footwall sequence away from potholes.Potholes are traditionally considered to be resorption pits due to the influx of a new hot primitive magma at the Merensky reef level or to represent scars of a late magmatic fumarole activity. The present study suggests that cumulate resorption played a minor role only. Potholes are sites of nondeposition, where the missing rock units did not crystallize. They formed where locally high concentrations of dissolved C-H-O-S volatiles lowered the liquidus temperature of the magma and suppressed the crystallization of cumulus plagioclase. Potholes seem to be concentrated above anticlines of the metasedimentary basement rocks in the footwall of the Bushveld Complex. Volatiles derived by pyrometamorphism were focused upward into the overlying melt above such topographic highs and were accumulated at the level of the Merensky reef.

Bidjovagge copper-gold deposit in Finnmark, Northern Norway

The Bidjovagge copper-gold deposit is located 40 km northwest of Kautokeino in Finnmark, northern Norway. The deposit occurs in the lower Proterozoic Kautokeino greenstone belt and consists of four ore deposits in albitic felsite and graphitic albitic felsite over a strike length of 2.5 km. The orebodies occur on the eastern limb of a north-south-striking anticline. The albitic felsite may represent strongly altered tuffite and diabase or partly metamorphosed chemical sediment. The alteration of the metadiabase in the footwall sequence is complex. Carbonatization is very extensive, but there are also zones with biotite, scapolite, and hematite alteration.The main ore is strata bound and occurs as veins, breccias, and low-grade disseminated mineralization of a more stratiform character. High gold values are usually related to late quartz-carbonate veins containing tellurides and are often associated with weak uranium mineralization. The ore minerals of economic significance are chalcopyrite and native gold. Other common metallic minerals are pyrite and pyrrhotite. Marcasite, magnetite, ilmenite, hematite, tellurides, rutile, sphalerite, galena, davidite, and pentlandite occur in accessory amounts. Preliminary isotopic analyses show that the lead isotope composition is markedly radiogenic ( 206 Pb/ 204 Pb = 22.2-23.8).The association of ore deposits with albitic felsites is also known to the north, in the Kvnangen tectonic window, where similar copper deposits but with low gold values occur in the Bergmark area and to the east near Masi in Big'geluobbal where U-V-Ti-rare earth element mineralization has recently been found. Bidjovagge is similar in its depositional environment to the Viscaria deposit in Sweden and the Pahtavuoma deposit in Finland.

Source and Time of Generation of Hydrocarbons in Fossil Basin, Western Wyoming Thrust Belt: ABSTRACT

Oil trapped in Triassic-Jurassic reservoir rocks in fields along the Ryckman Creek-Pineview structural trend on the Absaroka thrust plate was generated in source rocks in the footwall Cretaceous sequence which was overridden by the thrust plate. This conclusion is supported by data from a variety of analytic techniques used to make oil-to-oil and oil-to-source rock correlations. The source of hydrocarbons in Paleozoic reservoirs in the Whitney Canyon-Carter Creek trend on the Absaroka plate is more difficult to identify with certainty. The accumulations in the Paleozoic rocks are dominantly gas with some condensate, and a significant amount of H2S is present in contrast to the low-sulfur sweet oil and gas in the Ryckman Creek-Pineview trend. The identical sul ur content and chromatographic character of condensate from the Paleozoic and the Triassic-Jurassic reservoirs suggest that both are from the same source. Reconstruction of the maturation history and measurement of present levels of thermal maturation of source rocks in different structural settings in the Fossil basin demonstrate that peak generation and migration of hydrocarbons from Paleozoic source rocks predated the Absaroka fault which was formed about 75 m.y.B.P. Cretaceous source rocks beneath the Absaroka plate were immature when overridden by the fault and have reached a high level of maturity since that time. Most of the hydrocarbons were generated and expelled from source rocks in the footwall sequence during the last 60± m.y., after the hanging-wall structural traps involving both Mesozoic and Paleozoic reservoirs were formed. End_of_Article - Last_Page 800------------