On the oxygen isotope distribution among mineral triplets in igneous and metamorphic rocks

A compilation of ¹⁸O analyses of minerals separated from about 400 igneous and metamorphic rocks from published investigations reveals regularity in the fractionation of ¹⁸O among associated minerals, suggesting that an approach to isotopic equilibrium may be common. However, for only a minority of terrestrial rocks are these regularities sufficiently systematic to be compatible with the actual attainment and preservation of isotopic equilibrium among three minerals. Fractionations among triplets of quartz, calcite, feldspar, muscovite, and magnetite show some correspondence to those expected on the basis of experimental calibrations; however, there are also considerable deviations. The variability of natural data is such that less than half of the rocks analyzed to date would yield concordant ¹⁸O-derived temperatures. Of the additional 52 mineral triplets studied, plagioclase-pyrox-ene-ilmenite, plagioclase-pyroxene-magnetite, plagioclase-pyroxene-olivine, quartz-amphibole-garnet, pyroxene-ilmenite-magnetite, muscovite-biotite-magnetite, and quartz-muscovite-amphibole show the most systematic oxygen isotope fractionations. For 12 other mineral triplets a defined isotope fractionation relationship may be postulated to underlie the data; however for these a close approach to isotopic equilibrium is not commonly observed. For 33 of the mineral triplets an approach to isotopic equilibrium can be noted; however, the scatter of the available data is such that a systematic influence of a factor, such as temperature, on the size of the ¹⁸O fractionation could not be detected. In the past, regularities of oxygen isotope fractionations among three minerals have been used to establish secondary isotope geothermometers. Before this can be done with any reliability, however, the effects of possible retrograde isotope exchange and spurious correlation must be accounted for.     

On the radiogenic heat production of metamorphic,igneous, and sedimentary rocks

Sedimentary rocks cover-73% of the Earth's surface and metamorphic rocks account for approximately91% of the crust by volume. Understanding the average behavior and variability of heat production for these rock types are vitally important for developing accurate models of lithospheric temperature. We analyze the heat production of ～204,000 whole rock geochemical data to quantify how heat production of these rocks varies with respect to chemistry and their evolution during metamorphism. The heat production of metaigneous and metasedimentary rocks are similar to their respective protoliths. Igneous and metaigneous samples increase in heat production with increasing SiO_2 and K_2 O, but decrease with increasing FeO, MgO and CaO. Sedimentary and metasedimentary rocks increase in heat production with increasing Al_2 O_3, FeO, TiO_2, and K_2 O but decrease with increasing CaO. For both igneous and sedimentary rocks, the heat production variations are largely correlated with processes that affect K_2 O concentration and covary with other major oxides as a consequence. Among sedimentary rocks,aluminous shales are the highest heat producing(2.9 μW~(-3)) whereas more common iron shales are lower heat producing(1.7 μW m~(-3)). Pure quartzites and carbonates are the lowest heat producing sedimentary rocks. Globally, there is little definitive evidence for a decrease in heat production with increasing metamorphic grade. However, there remains the need for high resolution studies of heat production variations within individual protoliths that vary in metamorphic grade. These results improve estimates of heat production and natural variability of rocks that will allow for more accurate temperature models of the lithosphere.     

The oxygen isotope geochemistry of igneous rocks

Oxygen isotope analyses have been obtained for 443 igneous rock and mineral samples from various localities throughout the world. Detailed studies were made on the Medicine Lake, Newberry, Lassen, Clear Lake, S. E. Guatemala, Hawaii and Easter I. volcanic complexes and on the Bushveld, Muskox, Kiglapait, Guadalupe, Duluth, Nain, Egersund, Lac St. Jean, Laramie, Skaergaard, Mull, Skye, Ardnamurchan and Alta, Utah plutonic complexes, as well as upon several of the zoned ultramafic intrusions of S. E. Alaska. Basalts, gabbros, syenites and andesites are very uniform in O¹⁸/O¹⁶, commonly with δ-values of 5.5 to 7.0 per mil. Many rhyolite obsidians, particularly those from oceanic areas and the Pacific Coast of the United States, also lie in this range; this indicates that such obsidians are differentiates of basaltic or andesitic magma at high temperatures (about 1,000° C). They cannot represent melted sialic crust. The only plutonic granites with such low δ-values are some of the hypersolvus variety, suggesting that these also might form by fractional crystallization. Obsidians from the continental interior, east of the quartz-diorite line, have higher δ-values. This is compatible with their having assimilated O¹⁸-rich sialic crust. A correlation generally exists between the O¹⁸/O¹⁶ ratios of SiO₂-rich differentiates and the chemical trends in volcanic complexes. High O¹⁸/O¹⁶ ratios accompany those trends having the lower Fe/Mg ratios, while ferrogabbro trends are associated with depletion in O¹⁸. Variations in oxygen fugacity may be responsible for these effects, as abundant early precipitation of magnetite should lead to both O¹⁸-enrichment and Fe-depletion in later differentiates. Plutonic granites have higher O¹⁸/O¹⁶ ratios than their volcanic equivalents, because (a) their differentiation occurred at much lower temperatures, or (b) they are in large part derived from O¹⁸-rich sialic crust by partial melting or assimilation. Also, the oxygen isotope fractionations among coexisting minerals are distinctly larger in plutonic rocks than in volcanic rocks. This is in keeping with their lower crystallization temperatures and their longer cooling history, which promotes post-crystallization oxygen isotope exchange. Hydrated obsidians and perlites have δO¹⁸-values that are much different from their primary, magmatic values. A correlation exists between D/H and O¹⁸/O¹⁶ ratios in hydrated volcanic glass from the western U.S.A., proving that the isotopic compositions are a result of exchange with meteoric waters. The O¹⁸ contents of the glasses appear to be about 25 per mil higher than their associated waters; hence, these hydrated glasses have not simply absorbed H₂O, but they have exchanged with large quantities of it. The igneous rocks from Mull, Skye, Ardnamurchan and the Skaergaard intrusion are all abnormally depleted in O¹⁸ relative to “normal” igneous rocks. This is a result of their having exchanged at high temperatures with meteoric water that was apparently abundant in the highly jointed plateau lavas into which these igneous rocks were intruded. In part, this exchange occurred with liquid magma and in part with the crystalline rock; in the latter case the feldspar was more easily exchanged and has become much more depleted in O¹⁸ than has coexisting quartz or pyroxene. The later differentiates of the Muskox intrusion are markedly O¹⁸-rich, but this is not a result of fractional crystallization. It is in large part a result of deuteric exchange between feldspars and an oxygen-bearing fluid (H₂O ?) that was either O¹⁸-rich or had a relatively low temperature. This phenomenon was also observed in a number of granophyres from other localities, particularly those containing brick-red alkali feldspar. The exchanged feldspars in all these examples are turbid or cloudy, and may be filled with hematite dust. It is concluded that most such feldspar in nature is the result of deuteric exchange and is probably drastically out of oxygen isotopic equilibrium with its coexisting quartz.     

Experimental hydrogen isotope studies III: Diffusion of hydrogen in hydrous minerals,and stable isotope exchange in metamorphic rocks

Diffusion parameters for hydrogen diffusion in epidote-group minerals and micas have been measured under hydrothermal conditions, or calculated from existing experimental data, for bulk hydrogen isotope exchange experiments between hydrous minerals and water. Activation energies in the range 14 to 31 kcals/g-atom H are comparable to those derived by application of kinetic theory to experimental hydrogen isotope exchange data, and to those for oxygen diffusion in minerals under hydrothermal conditions. Diffusion of hydrogen in epidote is about four orders of magnitude faster than in muscovite, and about two orders of magnitude faster than in zoisite. Hydrogen diffusion in micas is about five orders of magnitude faster than oxygen diffusion, and hydrogen transport occurs dominantly parallel to the layers rather than parallel to the c-axis as for oxygen.Rapid hydrogen transport in minerals may proceed by hydrolysis of Si-O and Al-O bonds, followed by exchange of hydrolyzed oxygens with slower-diffusing (OH) or H₂O. Water appears to be essential for stable isotope exchange between minerals in slowly cooling metamorphic rocks.Stable isotope data for regional metamorphic mineral assemblages suggests that water is usually present in small amounts during cooling of prograde regional metamorphic systems, and estimated closure temperatures for cessation of stable isotope exchange are often more comparable to those calculated from diffusion data than to likely temperatures of metamorphism.Alpine deformation of the Hercynian Monte Rose Granite (Frey et al. 1976) permitted access of water and initiated stable isotope exchange amongst coexisting minerals. The diffusional behaviour of species in relict Hercynian muscovites is consistent with available experimental diffusion data.     

Stable carbon isotopes in selected granitic,mafic, and ultramafic igneous rocks

Noncarbonate (combustion) and carbonate (acid decomposition) carbon were separately analyzed in 18 granitic rocks from a group of related Tertiary intrusions near Crested Butte, Colorado, and 14 mafic and ultramafic rocks from various localities in the western United States. Among the granites, carbonate carbon ranges from nil to 0.76 per cent with δC¹³-values from ?5.6 to ? 9.0‰ (vs PDB); noncarbonate carbon varies from 32–360 ppm with δC¹³-values from ?19.7 to ?26.6‰, The mafic and ultramafic rocks have carbonate carbon contents ranging from 53 ppm to about 2 per cent with δC¹³-values from + 2.9 to ?10.3‰; noncarbonate carbon varies from 26 to 150 ppm with δC¹³-values of ?22.2 to ? 27.l‰ For these samples, carbonate carbon ranges from 12.0 to 29.4‰ heavier than coexisting noncarbonate carbon. This consistent difference between δC¹³ of carbonate and noncarbonate carbon may be an isotopic fractionation effect. Because the specific indigenous form of noncarbonate (combustion) carbon is in doubt, conclusive interpretations regarding isotopic equilibration and fractionation cannot be made.These results have bearing on the assessment of the isotopic composition of mantle carbon and consequently are germane to the question of the origin (source) and history of crustal carbon. If mantle carbon is isotopically similar to noncarbonate (combustion) carbon, i.e. δC¹³-values from ?19.7 to ? 27.1‰, then a simple mantle degassing source for crustal carbon is improbable. Such a result would indicate an additional source of crustal carbon such as from a primitive atmosphere or extra-terrestrial accretion.     

Rb-Sr isotope systematics in metamorphic rocks,Kongsberg sector,south Norway

Rb-Sr isotope data are presented for gneisses, migmatite neosome material and granitic and gabbroic intrusive rocks from the southern part of the Kongsberg sector, south Norway. The maximum age of the crust in this area appears to be ～1.6 AE. Two metamorphic episodes at ～1.5–1.6 AE and at ～1.1–1.2 AE are recognized. Initial ⁸⁷Sr/⁸⁶Sr ratios for the granitic rocks give evidence for reworking of sialic crust and indicate that approximately 1.6 AE old crust repeatedly acted as a source for granitic magmas for a timespan of ～0.5 AE.     

Estimation of joint trace length probability distribution function in igneous,sedimentary, and metamorphic rocks

To predict the behavior of structures in and on jointed rock masses, it is necessary to characterize the geomechanical properties of joints and intact rock. Among geometry properties of joints, trace length has a vital importance, because it affects rock mass strength and controls the stability of the rock structures in jointed rock masses. Since joint length has a range of values, it is useful to have an understanding of the distribution of these values in order to predict how the extreme values may be compared to the values obtained from a small sample. For this purpose, three datasets of joint systems from nine exposures of igneous, metamorphic, and sedimentary rocks are studied. Joint trace length is one of the most difficult properties to measure accurately, but it may be possible to record other geometrical properties of exposed joints accurately; thereby, support vector machine (SVM) model is used to predict the joint trace length. SVM is a novel machine learning method, which is a powerful tool used to solve the problem characterized by small sample and non-linearity with a good generalization performance. Consequently, goodness-of-fit (GOF) tests were applied on these data. According to these GOF tests, the lognormal distribution was found to be the best probability distribution function for representing a joint trace length distribution.     

西天山特克斯北中酸性火成岩地球化学特征及成因意义

新疆西天山特克斯县城北部伊特公路沿线和库勒萨依出露大量中酸性火成岩,伊特公路沿线为石英钠长斑岩,库勒萨依为石英闪长斑岩和花岗闪长斑岩。岩石地球化学和同位素组成研究表明,前者为典型的岛(陆)弧带火成岩,而后者具有埃达克岩的成分特征,两者均为古亚洲洋壳在俯冲过程中岩浆活动的产物。早先俯冲的较冷洋壳板片在深处脱水诱发上覆地幔楔熔融,熔体上升并经历壳幔相互作用等过程引发伊特公路一带弧岩浆活动; 由于洋壳持续俯冲,后来新形成的靠近洋脊的年轻板片由于高热在较浅处直接发生部分熔融形成埃达克岩浆,并上侵至库勒萨依一带。库勒萨依斑岩体SIMS锆石 U-Pb年龄为342.5±2.3Ma,属于早石炭世。两组中酸性火成岩的地球化学特征表明,古亚洲洋(南天山洋)在早石炭世还未完全闭合,洋壳向北的持续俯冲过程造成伊犁-中天山板块南缘广泛的岩浆活动,此时西天山陆壳增生方式主要为侧向增生,增生物质主要为洋壳板片(埃达克岩)和洋壳板片流体交代的地幔楔成分。     

Geochemistry of mafic igneous rocks of the northern Prince Charles Mountains,Antarctica

Rare mafic dykes, which intrude 1000 Ma high‐grade metamorphic rocks of the northern Prince Charles Mountains‐Mawson Coast area, are compositionally distinct from abundant early to middle Proterozoic tholeiite dykes, which are confined to Archaean or early Proterozoic terrains in the southern Prince Charles Mountains and elsewhere in East Antarctica, and which have therefore proved useful as stratigraphic markers. The younger dykes (and extrusive rocks) are a composition‐ally heterogeneous group with a wide range of ages (at least Cambrian to Eocene), although most are of K‐rich alkaline composition or have alkaline affinites. Their strong enrichment in highly incompatible elements (Rb, Ba, Th, Nb, K, Pb, Th and U) relative to less incompatible elements (La, Ce and P) suggests derivation by partial melting of more enriched mantle source regions than those of most of the Proterozoic tholeiite suites. However, unlike the latter, many incompatible element ratios have been significantly affected by fractional crystallisation and possibly also by the presence of residual minor phases during low degrees of melting.     

U-Th-Pb isotope systematics related to igneous rocks and ore Pb,Mount Isa,Queensland

This study is a search for a genetic relationship between Pb sulphide ore and igneous rocks in the region of Mount Isa, Queensland. The approach involves derivation of Pb isotope initial ratios by the whole-rock isochron method, and comparison of the initial ratios (Pb²⁰⁶/Pb²⁰⁴, Pb²⁰⁷/Pb²⁰⁴ and Pb²⁰⁸/Pb²⁰⁴) with the isotopic composition of the ore Pb. Data are reported for four igneous units; Kalkadoon granodiorite, Kalkadoon adamellite, Sybella granite and Eastern Creek volcanics. The results display considerable scatter for each of the units, and reveal the effects of recent surficial loss of U. The positioning of isochrons is aided by previous Rb-Sr geochronological data wherever possible. Comparison of initial ratios and ore Pb suggests that none of the igneous rock units is co-genetic with the ore deposit. Both phases of the Sybella Granite are more radiogenic and are apparently younger than the ore Pb. The Kalkadoon Granite is possibly related to the ore through some post-emplacement process of extraction and transport of Pb (e.g. by erosion or by anatectic magma generation) to the present site of the orebodies.     

大巴山区早古生代基性火成岩的形成时代

在大巴山陕西岚皋白崖垭的红椿坝-曾家坝深大断裂带一带,志留系的陡山沟组和吴家河组的含碳粉砂质页岩和笔石页岩与辉绿岩、火山集块岩、粗面质玄武岩岩层和岩体相互间隔出现,特别是在粗面质玄武岩和火山集块岩岩体之上,整合覆盖着一层厚约19m的生物碎屑灰岩、20m左右的黑色笔石页岩和30m的绢云母化片岩,在笔石页岩中产丰富的早志留世晚期和中志留世的笔石化石Oktavitesspiralis,Cyrtograpttuscentrifugus等.这一发现对确定大巴山区早古生代的岩浆活动的时限提供了可靠的化石证据,也为探讨秦岭岩浆活动的时代以及秦岭大地构造格局及其演化史具有重要意义.     

Petrogenesis of Franciscan glaucophane schists and associated metamorphic rocks,California

Rocks of the glaucophane-schist facies are widely though irregularly developed in the Franciscan formation of California. Minerals critical of the facies are lawsonite, aragonite, jadeite and omphacitic pyroxenes associated with quartz; amphiboles of the glaucophane-crossite series are almost ubiquitous. The most widely distributed rock, occurring over areas of many square kilometers, is jadeite-lawsonite metagraywacke, commonly veined with aragonite. More spectacular, but occurring mainly in isolated blocks are coarse-grained glaucophane-lawsonite Schists of many kinds. Commonly, but by no means invariably, they are closely associated with bodies of serpentinite. Also common in the vicinity of serpentinite masses are blocks of amphibolite and eclogite.All the metamorphic rocks are considered to be Franciscan sediments and basic volcanics metamorphosed and metasomatized in the deep levels of a folded geosynclinal prism. Experimental data on the stability fields of jadeite-quartz, aragonite, and lawsonite show that the glaucophane-schist facies represents metamorphism at pressures of between 5 and 10 kb and temperatures of 150–300° C. Such conditions could develop at depths greater than 15 km provided a very low geothermal gradient (10°/km) were maintained. The metagray-wackes are considered to represent a regional response to such conditions.The role of serpentinites in glaucophane-schist metamorphism is discussed in terms of a tentatively proposed model: — In very deep levels — perhaps at depths as great as 30 km, bodies of hot ultramafic magma develop restricted aureoles' in which temperatures of 400–600° C are maintained fer perhaps 100–1000 years. The products of metamorphism, which also involves desilication under the influence of the ultramafic magma, are eclogite and amphibolite. Later, and perhaps at higher levels serpentinization of the now solid ultramafic masses (near 400° C), causes renewed metamorphism at lower grades. Marginal development of glaucophane Schists and prehnite and hydrogarnet rocks, and retrogressive alteration of eclogite and amphibolite to glaucophane-schist assemblages is attributed to this period.     

Lead isotope systematics of some igneous rocks from the Egyptian shield

Lead isotope data on late Precambrian igneous rocks from the eastern desert of Egypt are presented. Previous work has indicated that this igneous suite is characterized by uniformly low initial ratios of ${}^{37}\text{Sr}{}^{86}\text{Sr}$ (< 0.7035). The Pb data define three groups, loosely corresponding to age. An older tonalite to granodiorite (OTG) suite, with ages in the range 610–710 Ma, has Pb isotope characteristics similar to modern, mantle-derived oceanic mafic rocks. The age-corrected initial Pb isotope ratios of the OTG group lie near the “ocean regression line” in correlation diagrams of ${}^{206}\text{Pb}{}^{204}\text{Pb}$ vs. ${}^{207}\text{Pb}{}^{204}\text{Pb}$ and ${}^{203}\text{Pb}{}^{204}\text{Pb}$. The isotope data imply an origin for the OTG group analogous to calc-alkalic igneous rocks in modern intra-oceanic island arcs.A younger suite of K-rich plutonic rocks (570–595 Ma) has similar ${}^{206}\text{Pb}{}^{204}\text{Pb}$ and ${}^{208}\text{Pb}{}^{204}\text{Pb}$ initial ratios relative to the OTG suite, but have higher ${}^{207}\text{Pb}{}^{204}\text{Pb}$ initial ratios when calculated from measured whole-rock U and Pb contents. However, K-feldspar and galena, associated with this suite, indicate initial Pb isotope ratios close to the OTG suite. We interpret this to indicate that the younger granites could have originated by anatexis of OTG-type material, but they probably experienced an episode of metasomatism after emplacement.A suite of volcanic rocks (the Dokhan Volcanics) from the central eastern desert are intermediate between the younger granites and OTG in both age (～ 610 Ma) and Pb isotope characteristics.Limited data on initial ${}^{208}\text{Pb}{}^{204}\text{Pb}$ ratios suggest that all the Egyptian samples originated from a source with anomalously low $\text{Th}\text{Pb}$ ratios.Although the relative abundance of granitic rocks in the Egyptian Shield decreases to the south, no evidence of north-south heterogeneity in the Pb isotope composition of the respective source regions is apparent in the data. However, the westernmost sample studied, from the Aswan area, contains distinctly more radiogenic Pb relative to the mean. This location may mark the boundary between the Late Precambrian ensimatic orogen now outcropping in the Egyptian and Saudi Arabian Shields, and an older sialic craton to the west.     

The oxygen isotope composition of granitoid and sedimentary rocks of the southern Snake Range,Nevada

Six diverse intrusive igneous types are exposed as discrete outcrops within an area of 900 km² in the southern Snake Range, White Pine County, Nevada. The previously recognized variety among these igneous types is reflected in the wide range of ¹⁸O values (–1.1 to 13.4 permil) found in these rocks. This range of ¹⁸O values probably results from differences in source material and post-crystallization history of the different intrusive types.The Jurassic intrusive of the Snake Creek-Williams Canyon area represents the chemical equivalent of a large part of a differentiation sequence, with the entire range of composition (63–76 percent SiO₂) exposed over a horizontal distance of about five km. The rather regular increase of ¹⁸O values from the most mafic to the most felsic parts of this pluton, together with ¹⁸O values determined for constituent minerals recovered from five of the samples, supports a fractional crystallization model. The high ¹⁸O values found (10.2–12.2 permil) indicate that the magma likely was derived from or assimilated sedimentary materials.Nine samples of the Cretaceous two-mica granite of the Pole Canyon-Can Young Canyon area have ¹⁸O values in the range 10.6–12.1 permil. These high ¹⁸O values, an initial⁸⁷Sr/⁸⁶Sr ratio of 0.7165, and the presence of muscovite along with an accessory mineral suite limited to monazite, apatite, zircon, and an allanite-like mineral, characterize this intrusive mass as an S-type granite. It probably formed through anatexis of late Precambrian pelitic rocks.The granitoid rock exposed in the Young Canyon-Kious Basin area is Tertiary (32 m.y.). Most of this intrusive has been cataclastically deformed as a result of late (18 m.y.) movement on the overlying Snake Range decollement. The undeformed portion of this intrusive has ¹⁸O values of 8.7–10.0 permil. However, the deformed portion of this intrusive has ¹⁸O values as low as –1.1 permil, apparently resulting from isotopic exchange between this rock and ground water at the time of cataclasis.Although the igneous types exposed in the southern Snake Range differ petrologically and range in age from Jurassic to Tertiary, most have relatively high ¹⁸O values compared with other granitoid rocks of the Basin-Range Province.     

REE distribution in corundum-bearing and other metasomatic rocks during the exhumation of metamorphic rocks of the belomorian belt of the Baltic Shield

The paper reports data on the distribution of REE in metasomatic rocks, including those with corundum, that were formed during the exhumation of the rocks of the Belomorian Belt at 1.9–1.75 Ga. This process is thought to have occurred concurrently with the horizontal extension and tectonic denudation of the upper crust, which, in turn, induced the massive release of fluids. The latter formed two major groups of silicic metasomatic rocks. The deepest-sitting corundum-bearing and other mafic metasomatic rocks are enriched in REE, alkalis, and alumina compared to the host rocks. The coeval acid metasomatic rocks such as orthotectites are, conversely, depleted in REE (with positive Eu anomalies), mafic elements, and HFSE but are also enriched in alumina. These complimentary rocks are thought to have been produced early during the exhumation of deep rocks under the effect of reduced fluids whose genesis was related to decompression. The silicic metasomatic rocks of the second group (with muscovite) have elevated REE concentrations (with negative Eu anomalies) and were formed by already oxidized fluids at shallower depths. The fact that the corundum-bearing metasomatic rocks are enriched in REE (in spite of the ultrabasic composition of these rocks) suggests that they were generated in an extensional environment with the participation of deep fluids, which enriched the rocks in Al, Na, K, Ba, Sr, Zr, and LREE.     

Cl,Br and I analyses of metamorphic and sedimentary rocks by isotope dilution mass spectrometry

Isotope dilution mass spectrometry with negative thermal ions was applied to determine Cl, Br and I in rocks which, in part, contained less than 100 ppb of Br and I. Two sets of samples have been investigated: a) a series of Al-rich metapelites of increasing metamorphic grade from the Damara Orogen, Namibia and b) fresh and hydrothermally altered greywackes from the Pb−Zn deposit Bad Grund in Germany. It was found that regional metamorphism of Al-rich metapelites causes no strong fractionation of the halides. The Br/Cl ratios in the metapelites are similar to those of sea water. The I/Cl and I/Br ratios, however, are 500 times higher. The I depleted in the ocean most probably is associated with organic matter stored in sediments. The unaltered greywackes from near the Pb−Zn vein have about the same concentrations of halides as the metamorphic Al-rich pelites. By contrast, the hydrothemally altered greywackes contain about twice as much Cl and I, and about 2–5 times as much Br as the unaltered samples. The element ratios, however, are similar for all three categories of rocks, thereby indicating that sea water played no role in the hydrothermal system.     

Sulfur isotope evidence for penetration of MVT fluids into igneous basement rocks,southeast Missouri,USA

Previous studies of galena and sphalerite from Paleozoic MVT deposits in the Viburnum Trend, southeast Missouri documented large variations in ³⁴S values throughout the ore-forming event. The present study of Cu-Fe-sulfides reveals a similar ³⁴S variation that reflects two end-member sulfur reservoirs whose relative importance varied both temporally and spatially. More ³⁴S-enriched sulfides (³⁴S approaching 25) indicate introduction of sulfur from basinal sedimentary sources, whereas more ³²S-enriched sulfides (³⁴S < 5) may reflect fluids moving through underlying granitic basement. Two areas containing Precambrian, igneous-hosted FeCu mineralization in southeast Missouri (West and Central Domes of Boss-Bixby) were investigated to elucidate their relationship to Cu-rich MVT orebodies hosted nearby within the overlying Cambrian Bonneterre Dolomite. Mineralization at Boss-Bixby is composed of an early phase of iron oxide deposition followed by Cu-Fe-sulfides. The Central Dome is faulted and its mineralization is more fracture-controlled than the typically podiform ores of the West Dome. The ³⁴S values of West Dome sulfides are 0.9 to 6.5 and pyrite-chalcopyrite indicate a temperature of 525° ± 50 °C. These data indicate an igneous source of sulfur during Precambrian ore deposition. In contrast, ³⁴S values of Central Dome sulfides are 9.4 to 20.0 and pyrite-chalcopyrite indicate temperatures of 275° ± 50 °C. Similar ³⁴S values are obtained for chalcopyrite from the overlying MVT deposits. We speculate that deeply circulating, basin-derived MVT fluids mobilized sulfur and copper from the underlying igneous basement and redeposited them in overlying Curich MVT orebodies, as well as overprinting earlier Precambrian sulfides of the Central Dome with a later, Paleozoic MVT sulfur isotope signature. Many models for MVT fluid circulation in the Midcontinent region of North America assume that igneous basement rocks are an impermeable boundary, but in southeast Missouri, evidence exists for structurally controlled MVT fluid movement > 600 m vertically through underlying Precambrian igneous rocks. Such basement involvement has been suggested for other carbonate-hosted base-metal districts (e.g. Irish base metal deposits) and should be considered an integral part of the ore-forming process in southeast Missouri.