Initial Lead Isotopic Composition Research Articles

Mineralogy and U–Th–Pb age of a uranium-bearing jasperoid vein, Sunshine Mine, Coeur d'Alene district, Idaho, USA

The age of the Mesoproterozoic Belt Supergroup hosted silver and base metal vein deposits of the Coeur d'Alene mining district in northern Idaho has long been controversial. Evidence for Precambrian mineralization comes largely from many ∼ 1400 Ma Pb model ages for galena throughout the district, and several problematic 1100–1300 Ma 207Pb/ 206Pb ages reported in the 1950s on uraninite from the Sunshine Mine. Field studies, on the other hand, have generally related mineralization to Mesozoic and Cenozoic tectonic and igneous events. A possible explanation of the galena model age enigma is that lead from metal-enriched Belt rocks was remobilized with minimal isotopic modification into younger deposits. The availability of a new occurrence of the uranium-bearing jasperoid vein exposed in the 5700-foot level of the Sunshine Mine allows reinvestigation of the uraninite using modern electron microprobe and isotope analysis techniques. Most of the uranium occurs in a prismatic UTi 2O 6 mineral, commonly in aggregates of 5–50 µm long lath-like crystals, identified as originally being brannerite. Subsequently, the brannerite has exsolved into extremely fine-grained uraninite and a TiO 2 polymorph. Rare, small crystals of zircon discovered with the electron microprobe, however, proved to be an even more useful geochronometer in unraveling the vein's complex history. Three morphological varieties of zircon were identified: (1) rounded, purple grains of obvious detrital nature, (2) simple euhedral crystals inferred to have originated in an igneous(?) rock or quartziferous vein precursory to the present jasperoid vein, and (3) crystallographically complex crystals attributed to hydrothermal growth in the jasperoid vein itself. The first two varieties lend themselves to rather straightforward age interpretations, while the third held the key to resolving the problematic uraninite ages. Four detrital zircon grains give discordant but projected concordia ages of 1.5–1.8 Ga, which indicate derivation from Paleoproterozoic crystalline rocks that supplied sediments to the Belt basin. Two zircon crystals yield essentially concordant ages of ∼ 1350 Ma that significantly postdates deposition of the host St. Regis Formation, and appear to have crystallized during a widespread orogenic event occurring soon after deposition of the uppermost Belt Supergroup. Unlike the detrital and ∼ 1350 Ma-old zircon, six grains of the younger, presumably hydrothermal zircon have a relatively high common lead content and give no consistent age pattern. For example, a Coeur d'Alene-type initial lead ( 206Pb/ 204Pb = 16.20, 207Pb/ 204Pb = 15.36, 208Pb/ 204Pb = 35.90) yields highly discordant results that range in 207Pb/ 206Pb age from 556–170 Ma and suggest variable 20–80% radiogenic lead loss. Similar discordance with an even greater range in 207Pb/ 206Pb age is shown by brannerite crystal aggregates, a euhedral pyrite crystal, and the whole-rock. Despite the uncertainty in initial lead isotopic composition, the rather tight clustering of 206Pb/ 238U ages around 120–150 Ma combined with the much older 207Pb/ 206Pb ages strongly hints at an anomalous, old radiogenic lead inherited by younger crystals. Indeed, some choice of initial lead lying along a secondary isochron defined by radiogenic lead grown between 1350 Ma and ∼ 130 Ma with an original Coeur d'Alene isotopic composition could be selected for every mineral analysis so as to make it concordant at between 124 and 139 Ma. Alternatively, a 3-D plane can be fit to the mineral analyses on a 238U/ 206Pb– 207Pb/ 206Pb– 204Pb/ 206Pb plot, yielding concordia intercept ages of 1344 ± 77 Ma and 136 ± 2 Ma (MSWD = 0.54). A scenario is proposed whereby a siliceous, uranium-enriched rock of uncertain but probable igneous or hydrothermal origin was introduced at ∼ 1350 Ma into the St. Regis Formation, and later contributed an isochronously related initial lead to a second generation of minerals at 136 ± 2 Ma. This enigmatic rock appears to have initially been emplaced during a contemporaneous episode of Mesoproterozoic faulting and metamorphism well-documented to the south in the Central Idaho thrust belt. The degree to which the precursor rock was altered and remobilized in forming the present jasperoid vein is poorly understood, but thorough re-crystallization of its original minerals seems to be required. The remobilization is most likely associated with renewed tectonism along the Lewis and Clark line caused by the Early Cretaceous docking of exotic terranes to the North American continent. Silver and base metal veins in the Sunshine mine are even younger that the reconstituted jasperoid veins, and, thus, cannot be assigned a Mesoproterozoic age.

Lead isotopes in sulfides from the Stillwater Complex, Montana: evidence for subsolidus remobilization

Isotopic ratios of Pb in sulfide minerals (primarily pyrrhotite, chalcopyrite, and pentlandite) from a suite of samples from the platiniferous J-M Reef of the Stillwater Complex were measured to elucidate the temporal and genetic relationship between sulfides and host silicate minerals. Results indicate that sulfides and coexisting plagioclases are generally not in isotopic equilibrium, that both sulfides and feldspars record highly radiogenic initial ratios at 2.7 Ga, and that a component of “post-emplacement” radiogenic Pb has mixed with common Pb in the sulfides. A model involving introduction of radiogenic Pb carried by fluids derived from sources external to the complex is favored. Analyses of the lead isotopic composition of sulfides in veins which cut the complex indicate that a significant fraction of the radiogenic lead which was added to the sulfides was externally derived during an extensive hydrothermal episode, associated with Proterozoic regional metamorphism around 1.7 Ga. The possibility that some fractions of the radiogenic Pb may have been derived from primary minerals altered during the low-grade metamorphism cannot be discounted. The amount of radiogenic lead added is variable and in some cases negligible. There is a good correlation between the lead isotope composition and the nature of the secondary mineral assemblage. Sulfides and plagioclases in samples that show little or no alteration of the primary minerals are generally in isotopic equilibrium and preserve isotope ratios consistent with magmatic crystallization at 2.7 Ga. Samples with the most radiogenic sulfides contain abundant secondary minerals (serpentine, talc, actinolite, chlorite and zoisite) associated with greenschist facies metamorphism. Some of the radiogenic Pb in the sulfides can be removed by progressive stepwise leaching. However, in most samples recrystallization of sulfides during metamorphism has mixed common Pb and radiogenic Pb throughout the crystal structure such that, in these samples, stepwise leaching does not recover initial Pb isotopic ratios. Plagioclases are much more resistant to low temperature recrystallization and in almost all cases, stepwise leaching reveals the initial lead isotopic composition. The reactivity of sulfides over a wide temperature range enhances their utility in understanding not only the processes involved in their formation at the time of magmatic emplacement but also postmagmatic processes which were important in the redistribution and enrichment of platinum group elements (PGE) within the ore zone.

Geochemistry and radiometric dating of a Middle Pleistocene peat

Uranium, lead, and sulphur data for a Middle Pleistocene interglacial peat deposit from Norfolk, UK, suggest that uptake of these elements was synchronous and confined to a single early diagenetic episode, probably coeval with peat formation. Sulphur isotope data indicate that reducing conditions have been maintained within the deposit throughout its history. Both uranium and lead concentration profiles show a marked discontinuity near the middle of the bed, probably indicating an environmental change, possibly emergence. The lead isotope data are compatible with a single lead component below the discontinuity and two components above. Groundwater is thought to be the dominant source of lead with an additional airfall component present in the upper peat. The uranium and lead concentration profiles below the discontinuity and the sulfur isotope profile throughout the peat support the view that these elements were sequestered from upwelling groundwaters. The organic material is particularly suitable for 230 Th 238 U dating because it contains a negligible allogenic mineral component and very low 232Th activity. A sequence of consistent ages through the peat profiel (mean 317 ± 14 ka) over a wide range of uranium concentrations (7–65 mg g −1, strongly suggests that a discrete, short-lived, uranium-uptake event has been dated and that subsequent differential isotopic migration has not occurred. One sample, from immediately below the discontinuity, has an infinite apparent age, but there is strong evidence for sequestration of uranium from the peat into adjacent wood fragments found along the discontinuity. Calculated initial 234 U 238 U values of 1.2–1.3 support a groundwater origin for the uranium, rather than a marine origin resulting from a subsequent rapid transgression. The very restricted range of U/Pb ratios in the lower part of the peat bed, and the heterogeneity of the initial lead isotopic composition in the upper part, preclude UPb isochron dating. 210Po measurements (as a proxy for 210Pb) also indicate possible post-depositional migration of 222Rn which, if active over a significant period, would bias any UPb age estimate. The 230 Th 238 U ages are consistent with deposition during oxygen isotope Stage 9.

Scapolite: a tracer for the initial lead isotopic composition in sulfide deposits with later additions of radiogenic lead

The initial lead isotopic composition of metamorphosed and tectonically reworked sulfide deposits is not always preserved, as sulfides easily change their lead isotopic composition through incorporation of lead derived from external fluids or redistribution and recrystallization of the deposit. Sulfide trace-lead and in cases even galena-lead from such deposits may show exceedingly radiogenic lead isotopic compositions. Thus, the initial lead isotopic composition has to be estimated from other minerals. Scapolite, which is a common phase in alteration haloes associated with epigenetic sulfide deposits in northern Sweden, has very low uranium-contents. Therefore, its trace-lead contents could preserve the initial isotopic composition of the ore-forming fluids. As scapolite is more resistant to recrystallization, it is more likely to reflect the original lead isotope signature of the deposit. This is illustrated using scapolite and sulfides from the Pahtohavare Cu-Au deposit in northern Sweden, which is hosted by Palaeoproterozoic mafic tuffites and graphitic schists and was affected by a mild thermal metamorphism during the Caledonian orogeny.

Lead isotope variations within the Bushveld complex, Southern Africa: a reconnaissance study

Variations in initial 87 Sr 86 Sr through the mafic cumulates of the Bushveld complex have long been used to trace the nature and relative influence of different parental liquids in the development of the magma chamber. The authors report the results of the first systematic investigation of the lead isotopic variation in the complex and provide information on both silicate and sulphide phases through the cumulate succession. Marginal and cumulate lithologies indicate that the initial lead isotopic composition (represented by model source 238 U 204 Pb ratios) varies systematically in a manner analogous to that detected with strontium isotopic ratios. Low μ2 ratios are evident in the Lower Zone and increase up the cumulate pile to the Main Zone, which apparently had homogeneous initial lead isotopic ratios. Unlike 87 Sr 86 Sr , however, considerable μ2 variation is detected in the Upper Zone. Cumulate lithologies also exhibit a greater range of μ2 values than those noted in the Marginal suite representatives, comparable to the situation for strontium isotopes. Most of the model μ2 results are higher than values expected for mantle-derived magmas, suggesting the incorporation of material with a crustal pre-history. Analyses of a detailed sampling of the Merensky and Bastard cyclic units at the Atok mine, eastern Bushveld are also presented and those reveal extreme values of μ2 while the variation in μ2 do not mirror the 87 Sr 86 Sr variations reported for the same samples by Lee and Butcher (1990). Elevated μ2 in some cumulate samples are impossibly high for a magmatic origin and presumably reflect the introduction of extra-magmatic components during sub-solidus hydrothermal alteration.

The 4.56 Ga U-Pb age of the MET 780058 ureilite

The U-Th-Pb and Sm-Nd isotopic systematics of mineral separates from the Ca-rich ureilite MET 78008 are reported. Extensive acid leaching procedures were used to eliminate terrestrial contamination because the amounts of U, Th, Pb, Sm, and Nd are extremely low in the meteorite. The three residues without terrestrial Pb contamination yield a Pb-Pb age of 4.563 ± 0.021 Ga. On a U-Pb concordia diagram, the residue data corrected for an initial lead isotopic composition calculated with μ = 11.7 for 3 Ma from starting Canõn Diablo troilite Pb in indicate a 2O7Pb∗/206Pb∗ model age of 4.563 Ga. The Sm-Nd isotopic data are scattered around the 4.56-Ga chondritic isochron. However, the Sm-Nd model age for handpicked augite is 4.573 ± 0.029 Ga, assuming an initial CHUR composition. Two separates show a positive 142Nd anomaly similar to those found in other meteorites of the same age (4.5–4.6 Ga). These Sm-Nd isotopic results support the Pb-Pb and U-Pb ages of this meteorite. Therefore, it is very likely that the formation age of ureilites is consistent with the so-called “4.56-Ga canonical age of meteorites.” It is also possible that extensive igneous processes for generating the ultramatic mineralogy of ureilites took place in the parent body within a few million years after formation of the solar nebula.

Initial lead isotopic composition of silicate minerals from the Mulcahy layered intrusion: Implications for the nature of the Archean mantle and the evolution of greenstone belts in the Superior Province, Canada

Samples from six localities in the late Archean Mulcahy layered intrusion of the Wabigoon greenstone belt have been analysed for their U and Pb concentrations and their lead isotopic composition in order to estimate the initial isotopic composition of the intrusion. Leaching experiments have been performed on plagioclase fractions and seven out of nine residues have yielded very similar calculated initial compositions ( 206 Pb/ 204 Pb= 13.32 ± 0.05; 207 Pb/ 204 Pb= 14.44 ± 0.04). These seven plagioclas residues also define a U-Pb isochron corresponding to an age of 2764 ± 20 Ma from which initial 206 Pb/ 204 Pb and 207 Pb/ 204 Pb ratios of 13.33 and 14.43, respectively, are obtained, indicating that the Mulcahy intrusion had a homogeneous initial lead isotopic composition. Plagioclase and whole-rock fractions from a titanomagnetite-rich layer near the top of the middle zone of the intrusion has yielded a Pb-Pb age of 2722 ± 4 Ma, which is only slightly younger than the reported U Pb zircon age of 2733 ± 1 Ma (MORRISON et al., 1985) for the same sample. Plagioclase, clinopyroxene, orthopyroxene, and magnetite mineral separates from the five other samples, however, yield older internal Pb Pb ages, ranging between 2.76 and 2.80 Ga, and all of the data lie on a single line in the Pb Pb isochron diagram, corresponding to an age of 2787 ± 14 Ma. This is believed to represent the crystallization age of the bulk of the Mulcahy intrusion. The 2733 Ma event may be the age of a small influx of magma intruded late after the bulk of the intrusion had crystallized or of a sill intruded within the middle zone and possibly associated to the adjacent 2732 Ma Atikwa batholith. When compared to available initial lead isotopic composition of various rocks from other greenstone belts of the Superior Province, the Mulcahy intrusion has one of the least radiogenic initial lead isotopic compositions, identical to that of komatiite lava flows from the Abitibi belt. These rocks are believed to represent melts of a late Archean depleted mantle component, which was homogeneous at that time, at the scale of greenstone belts of the Superior Province. Syn- to late-tectonic rocks in the Wawa, Wabigoon, and Abitibi belts have homogeneous and more radiogenic 207 Pb/ 204 Pb initial ratios than the depleted mantle values, indicating the melting of an enriched component of the mantle at that time, whereas post-tectonic plutons chiefly resulted from the melting of crustal precursors in a thickened lithosphere in response to the juxtaposition of greenstone belts and metasedimentary-plutonic-gneiss belts.

Initial lead isotopic composition of silicate minerals from the Mulcahy layered intrusion: Implications for the nature of the Archean mantle and the evolution of greenstone belts in the Superior Province, Canada

Samples from six localities in the late Archean Mulcahy layered intrusion of the Wabigoon greenstone belt have been analysed for their U and Pb concentrations and their lead isotopic composition in order to estimate the initial isotopic composition of the intrusion. Leaching experiments have been performed on plagioclase fractions and seven out of nine residues have yielded very similar calculated initial compositions (206Pb/204Pb= 13.32 ± 0.05; 207Pb/204Pb= 14.44 ± 0.04). These seven plagioclas residues also define a U-Pb isochron corresponding to an age of 2764 ± 20 Ma from which initial 206Pb/204Pb and 207Pb/204Pb ratios of 13.33 and 14.43, respectively, are obtained, indicating that the Mulcahy intrusion had a homogeneous initial lead isotopic composition.Plagioclase and whole-rock fractions from a titanomagnetite-rich layer near the top of the middle zone of the intrusion has yielded a Pb-Pb age of 2722 ± 4 Ma, which is only slightly younger than the reported UPb zircon age of 2733 ± 1 Ma (MORRISON et al., 1985) for the same sample. Plagioclase, clinopyroxene, orthopyroxene, and magnetite mineral separates from the five other samples, however, yield older internal PbPb ages, ranging between 2.76 and 2.80 Ga, and all of the data lie on a single line in the PbPb isochron diagram, corresponding to an age of 2787 ± 14 Ma. This is believed to represent the crystallization age of the bulk of the Mulcahy intrusion. The 2733 Ma event may be the age of a small influx of magma intruded late after the bulk of the intrusion had crystallized or of a sill intruded within the middle zone and possibly associated to the adjacent 2732 Ma Atikwa batholith. When compared to available initial lead isotopic composition of various rocks from other greenstone belts of the Superior Province, the Mulcahy intrusion has one of the least radiogenic initial lead isotopic compositions, identical to that of komatiite lava flows from the Abitibi belt. These rocks are believed to represent melts of a late Archean depleted mantle component, which was homogeneous at that time, at the scale of greenstone belts of the Superior Province. Syn- to late-tectonic rocks in the Wawa, Wabigoon, and Abitibi belts have homogeneous and more radiogenic 207Pb/204Pb initial ratios than the depleted mantle values, indicating the melting of an enriched component of the mantle at that time, whereas post-tectonic plutons chiefly resulted from the melting of crustal precursors in a thickened lithosphere in response to the juxtaposition of greenstone belts and metasedimentary-plutonic-gneiss belts.

Lead isotope signature of early Proterozoic ore deposits in western Africa; comparison with gold deposits in French Guiana

Lead isotope analyses of samples from ore deposits of the West African shield trace the metallogenic evolution of the Early Proterozoic or Birimian period. The selected deposits, which include the Loulo, Poura, and Banora gold deposits, the Faleme iron deposit, and the Perkoa, Gan, Hanhui, and Pilimpikou base metal deposits, are representative of each stage of the so-called Eburnean evolution. The study was completed by an analysis of gold deposits in the French Guianan Shield, which is a rifted remnant correlative of the West African craton.The isotopic compositions of most of the galenas cluster in a narrow field ( 206 Pb/ 204 Pb = 14.60-15.15, 207 Pb/ 204 Pb = 14.96-15.23, 208 Pb/ 204 Pb = 34.34-34.71) corresponding to the initial lead-isotopic composition of Birimian mineralization. Secondary isochrons, obtained on cogenetic lead-depleted sulfides, all converge toward the same narrow field; it is likely that they reflect an in situ addition of radiogenic lead to older lead from the Eburnean event. The calculated ages (2236-2741 Ma) have no clear geologic significance.The elongated field of initial lead compositions requires a mixing between juvenile, mantle-derived lead as expressed in the syngenetic deposits (Perkoa, mu = 9.11), and a more radiogenic component, such as old (>2.150 Ma) crustal lead, as expressed in the late orogenic deposits (Dorlin-THR stockwork, mu = 10.06). This increase in mu values reflects a gradual increase in the older recycled crustal lead component, favored by the tectonic change from D 1 thrusting to D 2 gold-bearing vertical strike-slip faulting. The age of the old recycled crust would almost certainly be earliest Proterozoic (2300-2145 Ma, Dabakalian); no perceptible Archcan lead influx is required to explain the genesis of these deposits. Geochronological data from other sources bracket most of the Eburnean metallogenesis between 2153 + or - 13 Ma (2Sigma ) age of the Dabakalian metamorphism, and 1949 + or - 55 Ma (2Sigma ) age of the last post-tectonic granites. Pb/Pb model ages, respectively 2113 Ma for the early so-called syngenetic ore deposits and 2014 Ma for the late-orogenic mesothermal gold deposits, are consistent with these independent geochronologieal data and probably reflect fairly well the approximate geologic ages of these deposits.

Lead isotopic heterogeneities within alkali feldspars: Implications for the determination of initial lead isotopic compositions

The mobility of uranium during metamorphism and alteration places constraints on the use of lead isotopic compositions as petrogenetic tracers in older rocks. The problem of uranium mobility may be partially circumvented by analysis of alkali feldspar which, because of its low U:Pb ratio, should have a lead isotopic composition similar to the initial lead isotopic composition of the rock. Previous leaching and volatilization experiments, however, have demonstrated that alkali feldspars contain radio-genic lead which can be removed preferentially to nonradiogenic lead. This paper reports on the results of progressive acid leaching of a single alkali feldspar phenocryst and on albitic perthites separated from the phenocryst. This experiment suggests that a number of distinct lead isotopic reservoirs are present within the alkali feldspar, including a nonradiogenic reservoir approximating the initial lead isotopic composition of the system, a radiogenic reservoir corresponding to albitic exsolution lamellae within the alkali feldspar, loosely bound labile lead presumably associated with uranium and thorium occupying fractures, and finally a radiogenic component corresponding to unidentified uranium-thorium rich phases. The commonly observed decrease in radiogenicity with progressive leaching of alkali feldspars is interpreted to be largely the result of preferential leaching of the albitic perthites relative to the host alkali feldspar containing the nonradiogenic reservoir. The results of this experiment confirm that initial lead isotopic compositions may be estimated from alkali feldspars through severe acid leaching; however, this experiment also illustrates the complexity of the lead isotopic systematics of alkali feldspars. The determination of the initial lead isotopic composition of an alkali feldspar requires experimental procedures which recognize and compensate for the presence of multiple lead reservoirs within typical alkali feldspars.

U-Th-Pb and Rb-Sr systematics of Allende and U-Th-Pb systematics of Orgueil

U-Th-Pb systematics study of Allende inclusions showed that U, Th and Sr concentrations in Ca, Al (pyroxene)-rich chondrules and white and pinkish-white aggregate separates of Allende are five to ten times higher than those of the matrix, whereas Mg (olivine)-rich chondrules have U and Th concentrations about twice as high as the matrix. Th concentrations are extremely high in white aggregates and in pinkish-white (spinel-rich) aggregates while U and Sr concentrations in white aggregates are more than twice as high as those in pinkish-white aggregates. Large enrichment of these refractory elements in the white aggregates indicates that they contain high-temperature condensates from the solar nebula. The Pb concentrations in the inclusions are less than half of those in the whole rock and matrix, indicating that the matrix is a lower-temperature condensate. The isotopic composition of lead in the matrix is less radiogenic than that of the whole meteorite, whereas lead in Ca- and Al-rich chondrules and aggregates is extremely radiogenic. The 206Pb/ 204Pb ratio reaches as high as 55.9 in a white aggregate separate. The lead of Mg-rich chondrules is moderately radiogenic and the 206Pb/ 204Pb ratio ranges from 18 to 26. A striking linear relationship exists among leads in the chondrules, aggregates and matrix on the 207Pb/ 204Pb vs 204Pb/ 204Pb plot. The slope of the best fit line is 0.6188 ± 0.0016, yielding an isochron age of 4553 ± 4 m.y. The regression line passes through primordial lead values obtained from Canyon Diablo troilite. The data, when corrected for Canyon Diablo troilite Pb and plotted on a U-Pb concordia diagram, show that the pink and white aggregates and the Ca-Al-rich and Mg-rich inclusions have excess Pb and define a chord which intersects the concordia curve at 4548 ± 25 m.y. and 107 ± 70 m.y. The intercepts might correspond to the agglomeration age of the meteorite and a time of probably later disturbance, respectively. The matrix and some chondrules which contain less radiogenic lead did, however, not fit on the chord. The Rb-Sr data of Allende did not define an isochron suggesting that the Rb-Sr system was also disturbed by a later event, as suggested by the U-Pb concordia data. The lowest observed 87Sr/ 86Sr ratio in Allende inclusions is similar to the initial ratio of the Angra dos Reis achondrite (Papanastassiou, Thesis, 1970). The initial Pb isotopic composition of Orgueil calculated by a single-stage evolution model is more radiogenic than that of Canyon Diablo troilite. To reconcile the U-Pb data of Orgueil and Allende, we propose that the initial lead isotopic composition of the carbonaceous chondrites was slightly different from that of Canyon Diablo troilite Pb.

Lead isotopic systematics and ages of Archaean acid intrusives in the Kalgoorlie-Norseman area, Western Australia

Lead-lead mineral isochrons consisting of K-feldspar + plagioclase + total rock have been determined for eight intrusive bodies in the Kalgoorlie-Norseman area of Western Australia. The ages found range from 2630 to 2760 m.y. Variation of initial lead isotopic composition within plutons has been identified in two cases. For the remaining plutons, whole rock lead-lead isochrons give the same age as the mineral isochrons. Attempts to determine uranium-lead ages reveal large-scale loss of uranium from surface rocks in recent times. The initial lead isotopic composition of the intrusions, as estimated from age corrected K-feldspar data, shows that the source regions for the plutonic magmas must have experienced multistage histories prior to magma generation. The time integrated μ value for the source region in all cases exceeds the mean μ value for the Earth. Multistage model calculations show that continental type rocks must have been present in the Norseman area as early as 3300 m.y. ago. Original formation of continental crust near Kalgoorlie either occurred much later than at Norseman, or consisted of more basic rock types.