Pb-isotope data base for sulfides from Alaska, March, 1987
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Open-File Report↗
Geology topics
M.H. Delevaux
Publications and source records attributed to M.H. Delevaux.
The plumbotectonics of the West Shasta mining district, eastern Klamath Mountains, California
The tectonic setting comprising the West Shasta mining district has often been compared with that of primitive island arcs. Concentrations of uranium, thorium, and lead and lead isotope compositions were determined for Devonian ores and rocks of the West Shasta district, eastern Klamath Mountains, California, to help evaluate the tectonic classification. The lead isotope pattern is found to be complex. From comparison of the data with those on younger ores and rocks in the region and with those isotopic patterns found in modern tectonic terranes, however, a number of conclusions are possible. A lead isotope point for the Devonian oceanic mantle is now well established from this study on the West Shasta district. This isotopic composition is in agreement with that suggested by Slawson in 1983--i.e., from a sample of massive sulfide ore from the Golinsky deposit--with a 206 Pb/ 204 Pb of 17.830, a 207 Pb/ 204 Pb of 15.450, and a 208 Pb/ 204 Pb of 37.313. If the Devonian mantle is analogous to the modern mantle, the determined value is probably at the more radiogenic end of the Devonian oceanic mantle array.Some samples of volcanic rocks and ores that have elevated values of 207 Pb/ 204 Pb and 208 Pb/ 204 Pb indicate that subducted pelagic sediments or interaction of the magmas with other sediments downsection have been involved to varying degrees in the generation of magmas. The high ratios were not attained by addition of lead to the volcanic rocks or ores during subsequent magmatic events, such as intrusion of the Permian Pit River stock to the east or of the Cretaceous Shasta Bally batholith that crops out to the south and west and may underlie part of the district. Some values of 207 Pb/ 204 Pb are sufficiently high as to suggest that West Shasta may have developed nearer a continent than was previously supposed. Lead isotope data for Quaternary volcanic rocks are somewhat more radiogenic than for Cretaceous ores and plutonic rocks. Permo-Triassic ores are again a bit less radiogenic. The isotopic differences between the Permo-Triassic and Quaternary data could be accounted for by a value for 238 U/ 204 Pb of 10.42 in common source material. Using this value of 238 U/ 204 Pb to calculate even further back to 400 m.y., the resulting ratios are found to fall very close to the "best value" for the lead isotope mixing line between the Devonian mantle and pelagic sediments. We interpret this intersection to be the "mean value" for the assimilated Devonian sediment lead--18.250 for 206 Pb/ 204 Pb and 15.582 for 207 Pb/ 204 Pb. As deduced for the lead isotope composition for the Devonian mantle at this location, the pelagic sediment, lead isotope composition also is toward the more radiogenic end of the values expected for pelagic sediments in the Devonian. Lack of sediments in the observed volcanic section of the West Shasta district suggests a submerged volcanic-arc setting (as interpreted by others for the Troodos and Samail ophiolites) rather than an island-arc situation (such as the Green Tuff area of Japan) or an incipient spreading ridge setting (that might have grown to be a subaerial Iceland). The elevated values of 207 Pb/ 204 Pb for some samples suggest a submerged volcanic are whereas the exceedingly low values of thorium--which is not easily mobilized during alteration events--for both basaltic andesites and plagiorhyolites is suggestive of depleted source material such as is found for most oceanic spreading centers. Elevated values of 207 Pb/ 204 Pb are exceedingly rare in midocean ridge basalts and low contents of thorium are known in island arcs, so a submerged volcanic arc is the preferred interpretation. Some similarity exists between the 206 Pb/ 204 Pb values of Devonian ores having elevated values of 207 Pb/ 204 Pb and those of the Permo-Triassic ores of the East Shasta district, as mentioned by Slawson in 1983. This similarity is now confirmed to be a coincidence. We find it due first of all to some incorporation of sediment lead in the magmas, followed later by some migration of radiogenic lead--probably locally derived--into the ores during subsequent events. Because there was some postdepositional lead migration into the ores, some migration of copper also into the ores cannot be excluded. Lead, especially radiogenic lead, however, is more mobile than copper. As the migrating lead is deduced to have been locally derived from the observed section rather than exotically derived, any copper added was probably locally derived also.
Economic Geology↗
U-Th-Pb isotope chronology of sulfide ores and rocks in the early Proterozoic metavolcanic belt of northern Wisconsin
Lead was probably derived from 3.7-b.y.-old source material about 1.8 to 1.9 b.y. ago. Data on metavolcanic and granitic rocks from the belt show that the U-Th-Pb systems in most were reset about 1.6 b.y. ago; a time of shearing and retrogressive metamorphism. A concordia plot of the whole-rock lead data shows that many of the rocks lost lead relative to uranium and thorium during the interval 200 to 400 m.y. ago. Similar ores might be sought between the northern Wisconsin metavolcanic belt and the Upper Mississippi Valley because the lead lost from the basement there might be available for ore formation in overlying units. The lead in the early Proterozoic metavolcanic belt of northern Wisconsin seems to have been derived from the same mantle parent as the late Archean metavolcanic belts of northeastern Minnesota and southern Ontario.
Economic Geology↗
A lead isotope study of mineralization in the Saudi Arabian Shield
New lead isotope data are presented for some late Precambrian and early Paleozoic vein and massive sulfide deposits in the Arabian Shield. Using the Stacey Kramers (1975) model for lead isotope evolution, isochron model ages range between 720 m.y. and 420 m.y. Most of the massive sulfide deposits in the region formed before 680 m.y. ago, during evolution of the shield. Vein type mineralization of higher lead content occurred during the Pan African event about 550 m.y. ago and continued through the Najd period of extensive faulting in the shield that ended about 530 m.y. ago. Late post-tectonic metamorphism may have been responsible for vein deposits that have model ages less than 500 m.y. Alternatively some of these younger model ages may be too low due to the mineralizing fluids acquiring radiogenic lead from appreciably older local crustal rocks at the time of ore formation. The low207Pb/204Pb ratios found for the deposits in the main part of the shield and for those in north-eastern Egypt, indicate that the Arabian craton was formed in an oceanic crustal environment during the late Precambrian. Involvement of older, upper-crustal material in the formation of the ore deposits in this part of the shield is precluded by their low207Pb/204Pb and208Pb/204Pb characteristics. In the eastern part of the shield, east of longitude 44??20???E towards the Al Amar-Idsas fault region, lead data are quite different. They exhibit a linear207Pb/204Pb-206Pb/204Pb relationship together with distinctly higher208Pb/204Pb characteristics. These data imply the existence of lower crustal rocks of early Proterozoic age that apparently have underthrust the shield rocks from the east. If most of the samples we have analyzed from this easterly region were mineralized 530 m.y. ago, then the age of the older continental rocks is 2,100??300 m.y. (2??). The presence of upper crustal rocks, possibly also of early Proterozoic age, is indicated by galena data from Hailan in South Yemen and also from near Muscat in Oman. These data are the first to indicate such old continental material in these regions. ?? 1980 Springer-Verlag.
Contributions to Mineralogy and Petrology↗
Genesis of ore deposits in the San Juan Volcanic Field, Southwestern Colorado-lead isotope evidence
In the San Juan volcanic area of southwestern Colorado, the isotopic composition of lead in ores and ore prospects of Cenozoic age ranges widely: 17.72 to 21.13 for 206 Pb/ 204 Pb; 15.50 to 15.81 for 207 Pb/ 204 Pb; and 37.21 to 38 for 208 Pb/ 204 Pb. Examination of the lead isotope data indicates that once deposition of lead minerals begins, further exchange of lead between fluid and wall rock is insignificant. This conclusion is supported by the relatively constant isotopic composition of the lead in these ores, which is not affected by the grade of ore mineralization or type of wall rock. The values of 206 Pb/ 204 Pb in some vein-type deposits exceed the maximum value known for all Mesozoic and Cenozoic igneous rocks of the Rocky Mountain region. These isotopic relations show that if ore-forming solutions are composed of meteoric water, as indicated in studies of light stable isotopes, they must have penetrated deep enough to acquire lead from Pre-cambrian rocks or sediments derived from them. The fact that some of these vein ores are now in Cenozoic igneous rocks indicates the ore fluid had an upward vertical component to its movement. The lead isotope data therefore support a circulating cell hypothesis for these kinds of ores, as suggested by many recent studies of light stable isotopes as related to mineralization. Some other deposits (Summitville, Jasper, Red Mountain district) have values of 206 Pb/ 204 Pb similar to those of the large volumes of altered rock that enclose them ( 206 Pb/ 204 Pb ∼ 18.5), suggesting that in places the lead may have been locally derived by leaching of the adjacent rocks or from magmatogenic fluids. When the lead isotope data are treated in detail, the rocks and galenas of the Platoro caldera complex, of the central San Juan caklera complex, and of the Baughman Creek center appear to contain significant components derived from 1,400- to l,500-m.y.-old and 1,700- to 1,800 m.y.-old source materials. These also are the ages of the two main groups of rocks that comprise the Precambrian basement under the San Juan volcanic area. Although the data from the western San Juan caldera complex scatter considerably, the only obvious source for the lead seems to be the 1,700- to l,800-m.y.-old rocks or detritus of such age in Phanerozoic sediments. Where the involvement of 1,400- to l,500-m.y.-old sources is greatest, the Th/U ratio in the source material is small (calculated to be about 0.7), whereas the Th/U ratio of the 1,700- to l,800-m.y.-old source material appears to be 2.3 to 3.3. The largest Th/U values are from the western San Juan caldera complex. © 1979 Society of Economic Geologists, Inc.
Colorado↗
The potential source of lead in the Permian Kupferschiefer bed of Europe and some selected Paleozoic mineral deposits in the Federal Republic of Germany
New lead isotopic compositions have been measured for Paleozoic bedded and vein ore deposits of Europe by the high precision thermal emission (triple filament) technique. Eleven samples have been analyzed from the Upper Permian Kupferschiefer bed with representatives from Poland to England, three samples from the Middle Devonian Rammelsberg deposit and one from the Middle Devonian Meggen deposit, both of which are conformable ore lenses and are in the Federal Republic of Germany (FRG); and also two vein deposits from the FRG were analyzed, from Ramsbeck in Devonian host rocks and from Grund in Carboniferous host rocks. For Kupferschiefer bed samples from Germany, the mineralization is of variable lead isotopic composition and appears to have been derived about 250 m.y. ago from 1700 m.y. old sources, or detritus of this age, in Paleozoic sedimentary rocks. Samples from England, Holland, and Poland have different isotopic characteristics from the German samples, indicative of significantly different source material (perhaps older). The isotopic variability of the samples from the Kupferschiefer bed in Germany probably favors the lead containing waters coming from shoreward (where poor mixing is to be expected) rather than basinward (where better mixing is likely) directions. The data thus support the interpretation of the metal source already given by Wedepohl in 1964. Data on samples from Rammelsberg and Meggen tend to be slightly less radiogenic than for the Kupferschiefer, about the amount expected if the leads were all derived from the same source material but 100 to 150 m.y. apart in time. The vein galena from Ramsbeck is similar to that from Rammelsberg conformable ore lenses, both in rocks of Devonian age; vein galena from Grund in Upper Carboniferous country rocks is similar to some bedded Kupferschiefer mineralization in Permian rocks, as if the lead composition was formed at about the same time and from similar source material as the bedded deposits. Although heat has played a more significant role in the formation of some of these deposits (veins and Rammelsberg-Meggen) than in others (Kupferschiefer), there is no indication of radically different sources for the lead, all apparently coming from sedimentary source material containing Precambrian detritus. One feldspar lead sample from the Brocken-Oker Granite is not the same in isotopic composition as any of the ores analyzed. ?? 1978 Springer-Verlag.
Contributions to Mineralogy and Petrology↗
Preliminary lead isotope investigations of brine from the Red Sea, Galena from the Kingdom of Saudi Arabia, and galena from United Arab Republic (Egypt)
The isotopic composition of lead in Red Sea chloride brine containing 0.5 ppm Pb is found to be similar to that of some Cenozoic ore leads such as galena at Rabigh in Saudi Arabia that may have formed during mineralization accompanying Tertiary rifting. Bir Ranga galena in Miocene sediments from United Arab Republic (Egypt) is also isotopically similar to lead in Red Sea brine. The chlorine brine must be considered a possible mineralizing fluid. Lead isotopes show promise for use in mineral prospect evaluation in that galena from Samrah is isotopically similar to that from Mahd adh Dhahab, which has been the only ore producer in Saudi Arabia since 1945. Drilling at Samrah does indicate a possible economic mineralization. The lead isotope data coupled with available geologic knowledge and geochronometry are used to tentatively divide the ore prospects of the Kingdom of Saudi Arabia into relative categories of mineralization age. Two Mesozoic and Cenozoic mineralizations are distinguished on the basis of a 207 Pb/ 204 Pb difference; an early Paleozoic mineralization grouping is outlined; and a late Precambrian mineralization period is suggested.
Earth and Planetary Science Letters↗
Isotopic study of galenas from the upper Mississippi Valley, the Illinois-Kentucky, and some Appalachian Valley mineral districts
Analyses of lead isotopes in galena in carbonate rock ore deposits collected from several mineralized districts in the Central and Eastern Interior of the United States support previous studies in establishing that the galena is anomalously radiogenic ("J"type")- This study , using care-fully selected samples to test some géologie relationships, shows region-wide and distrietwide systematic variations in isotope ratios. The genetic implications of thèse variations are discussed. Two mechanisms of origin are considered to explain thèse variations in isotopic ratios. One involves mixing of leads from multiple sources, the other calls for variable lead-isotope ratios originating from a single source. Several source materials for the lead ores have been suggested; whether a single source has predominated, or two or more main sources have predominated is still unresolved. Slopes from Pb206/Pb204 versus Pb207/Pb204 plot are compatible with a contribution of lead by the 1300 ± 300 m.y. basement rocks that underlie the deposits. The spatial relationships of alkaline igneous rocks to several of the minerai districts in the Mississippi Valley région suggest a possible mag-matic source for the lead. Isotopic analyses of galenas closely associated with several of the intrusive rocks are similar in isotopic composition to adjacent ore leads, but genetic implications are ambiguous. Galena oc-curring in shales in the Mississippi Valley région outside mineralized districts contains ordinary lead. Théories that call upon such shales to be the major source of the lead in the depoaits of the Mississippi Valley type must explain this distinct différence in isotopic compositions. Galena collected from districts within the Appalachian Ridge and Valley Province contains notably less radiogenic lead than that from the Mississippi Valley minerai districts , and the lead is of the ordinary type.
Illinois, Kentucky↗
Lead isotope variation with growth zoning in a galena crystal
A large crystal of lead sulfide from Picher, Oklahoma, has significant differences in isotopic composition of lead in successive growth zones. Lead isotope ratios in the parent ore-fluid evidently changed with time during crystal growth. The growth history of this crystal, interpreted quantitatively, points to a tentative hypothesis of genesis of Mississippi Valley deposits of lead and zinc.
Oklahoma↗