Formation Of Massive Sulfide Deposits Research Articles

Evidence of hydrothermal fluid migration pathways in pores and microfractures based on nuclear magnetic resonance and electron microscopic analyses from the Southwest Indian Ridge

The formation of massive sulfide deposits on the seafloor is closely related to sub-seafloor hydrothermal circulation. Hydrothermal circulation depends on the connected pores and fractures. Most previous studies have focused on hydrothermal circulation at the macroscopic scale; however, the transport pathways of hydrothermal fluids have rarely been studied at the microscopic scale. This study showed that multiple measurement techniques could characterize the pores of rocks and the mineral fraction surrounding these pores. Nuclear magnetic resonance (NMR) was used to measure the porosity of fresh basalt, altered basalt, massive sulfide, and sulfide chimneys obtained from the Southwest Indian Ridge. Meanwhile, the distribution of pore radius and pore throats in these samples was obtained using the NMR transverse relaxation time distribution. The scanning electron microscopy results and polarizing light microscope images showed that the pores of altered basalts are more developed than those of fresh basalts, which is consistent with the NMR measurement results that altered basalts have a large pore size distribution. In addition, metallic minerals such as pyrite were found to be precipitated in these pores, and microfractures were observed in altered basalts. This demonstrates that the pores and microfractures in these altered basalts provide pathways for fluid transport and are enriched with metal minerals. Altered basalt is closely related to the formation of massive sulfide deposits and is an important ore-prospecting indicator. Therefore, the study of rock pore structures and components can provide a better understanding of hydrothermal circulation and contribute to the exploration of mineral resources.

Mineral formation conditions in acid magmatic systems related to the formation of massive sulfide deposits of the Urals and Altai-Sayany area

Studies of melt inclusions in quartz indicate the similarity of acid magmatic systems of massive sulfde deposits in the Urals and Altai-Sayany region. The melts of normal alkalinity corresponding to rhyodacite and rhyolite compositions and related to the tholeiitic series are dominant in all the deposits considered. The magmas are characterized by the same type evolution with a decreasing content of main oxides (TiO2, Al2O3, FeO, MgO, CaO, Na2O, K2O) and an increasing SiO2 content. Our results show the accumulation of Cu in relatively low-H2O acidic melts of ancient (Cambrian) deposits of the Altai-Sayany region and low metal contents in the intermediate (Silurian–Devonian) and H2O-saturated magmas of the Urals. The youngest (Devonian) magmas of Siberia evolve simultaneously along these two directions. The analysis of melt inclusions in quartz suggests that the minimum contents of trace and rare earth elements are characteristic of the Silurian-Devonian acid melts of the Urals, with their maximum contents in the youngest (Devonian) magmas and the intermediate contents of ancient (Cambrian) magmatic systems of the Altai-Sayany region. The features of rare and rare earth element patterns in melt inclusions in quartz indicate the similarity of acid magmatic systems of massive sulfde deposits in the Urals and Altai-Sayany region with present-day suprasubduction melts in the ocean-continent transition zones. Computational modeling using data on melt inclusions in quartz confrms our previous conclusions (Simonov, Maslennikov, 2020) that the occurrence of contrasting (basic and felsic) volcanic complexes with massive sulfde deposits in the Urals and Altai-Sayany region is a result of evolution of basaltoid magmas. Keywords: conditions of mineral crystallization, basaltic-rhyolitic complexes, massive sulfde deposits, melt inclusions, quartz, acidic melts.

Geoscientific model of a seafloor hydrothermal system associated with the formation of massive sulfide deposits

Geoscientific model of a seafloor hydrothermal system associated with the formation of massive sulfide deposits

PALEOVOLCANIC CONDITIONS OF MASSIVE SULPHIDE MINERALIZATION FORMATION OF AKZHARSKOYE ORE FIELD (SOUTHERN URALS)

The article considers the geological structure and paleovolcanic conditions for the formation of massive sulphide deposits and ore mineralizations of the Akzharskoye ore field located within the Dzhusinsko- Dombarovsky paleovolcanic belt. It is shown that three strata participate in the structure of the ore-bearing volcanogenic complex: lower basaltic, rhyolite-basalt and upper basaltic. Paleovolcanological analysis of the complex made it possible to reconstruct the following volcanic structures: 1) Akzharskoye shield volcano; 2) Akzharskaya caldera; 3) rhyolite-basalt volcanic apparatus; 4) Alimbayskaya depression; 5) Tyulkubayskaya depression. The history of the development of these structures is detailed in the work. It is shown that the structures are well mapped in physical fields - the volcanic accumulation region differs by an elevated gradient of gravity, its negative values characterize areas with a significant thickness of terrigenous deposits. The assignment of massive sulphide objects of the Akzharskoye ore field to the Urals type is proved. Moreover, they possess a number of specific features similar to those of the dombarovsky type. The ores were formed during the activity of acidic volcanism and are located on a single stratigraphic level marked by rhyodacitic bodies. It is established that the paleostructure of volcanic complexes is due to the combination of three-order synvolcanic structures. The largest of them are the structures of the first (shield volcanoes, relics of volcanic ridges) and the second (caldera depressions, separate apparatus of the central type) of orders, correspond respectively to volcanogenic complexes and ore fields. Placing of pyrite deposits and ore bodies is controlled by third-order structural elements - volcanic-tectonic depressions, local basalt-rhyodacite structures and vertex calderas of volcanicdome apparatus.

Geodynamic conditions of formation of massive sulfide deposits in the Magnitogorsk Megazone, Southern Urals, and prospection criteria

The zoned composition changes of the massive sulfide deposits in the major massive sulfide zone of the Southern Urals such as the Magnitogorsk Megasynclinorium are considered. The zoning is expressed as the trend of Ni–Co–Cu → Zn–Cu → Cu–Zn → Au–Ba–Pb–Cu–Zn. This trend is related to two basic factors: (1) the subduction process with the slab’s eastward subsidence preconditioned the formation (from the west to the east) of the following massive sulfide zones: accretionary prism, frontal island arc, developed island arc, inter-arc spreading zone, split back arc, and back-arc spreading; (2) the longitudinal zoning of the massive sulfide paleovolcanic belts related to changes in the thickness of the crust’s basaltic layer and an inclination of the subducting plate in transverse blocks of the belt. The first factor affects the general paleovolcanic and metallogenic latitudinal zoning of the studied region, while the second factor defines the local meridional zoning. The composition of ore-bearing solutions is dependent on the formation depth of the subduction fluids, magma differentiation type, and the ratio of deep fluids to solutions of near-surface convective cells. The combination of the geodynamic factors expressed in the composition of ore-bearing volcanic complexes and the specific geological settings defines the massive sulfide mineralization composition and productivity criteria. The most productive structures include the frontal island-arc and inter-arc spreading zones and within them, the central-type volcanic edifices whose basalts are referred to as the island-arc tholeiite series and are characterized by the minimum TiO2 and Zr content and low La/Yb ratios.

Genesis of massive sulfide deposits in the Verkhneural’sk ore district, the South Urals, Russia: Evidence for magmatic contribution of metals and fluids

Melt inclusions and aqueous fluid inclusions in quartz phenocrysts from host felsic volcanics, as well as fluid inclusions in minerals of ores and wall rocks were studied at the Cu-Zn massive sulfide deposits in the Verkhneural’sk ore district, the South Urals. The high-temperature (850–1210°C) magmatic melts of volcanic rocks are normal in alkalinity and correspond to rhyolites of the tholeiitic series. The groups of predominant K-Na-type (K2O/Na2O = 0.3–1.0), less abundant Na-type (K2O/Na2O = 0.15–0.3), and K-type (K2O/Na2O = 1.9–9.3) rhyolites are distinguished. The average concentrations (wt %) of volatile components in the melts are as follows: 2.9 H2O (up to 6.5), 0.13 Cl (up to 0.28), and 0.09 F (up to 0.42). When quartz was crystallizing, the melt was heterogeneous, contained magnetite crystals and sulfide globules (pyrrhotite, pentlandite, chalcopyrite, bornite). High-density aqueous fluid inclusions, which were identified for the first time in quartz phenocrysts from felsic volcanics of the South Urals, provide evidence for real participation of magmatic water in hydrothermal ore formation. The fluids were homogenized at 124–245°C in the liquid phase; the salinity of the aqueous solution is 1.2–6.2 wt % NaCl equiv. The calculated fluid pressure is very high: 7.0–8.7 kbar at 850°C and 5.1–6.8 kbar at 700°C. The LA-ICP-MS analysis of melt and aqueous fluid inclusions in quartz phenocrysts shows a high saturation of primary magmatic fluid and melt with metals. This indicates ore potential of island-arc volcanic complexes spatially associated with massive sulfide deposits. The systematic study of fluid inclusions in minerals of ores and wall rocks at five massive sulfide deposits of the Verkhneural’sk district furnished evidence that ore-forming fluids had temperature of 375–115°C, pressure up to 1.0–0.5 kbar, chloride composition, and salinity of 0.8–11.2 (occasionally up to 22.8) wt % NaCl equiv. The H and O isotopic compositions of sericite from host metasomatic rocks suggest a substantial contribution of seawater to the composition of mineral-forming fluids. The role of magmatic water increases in the central zones of the feeding conduit and with depth. The dual nature of fluids with the prevalence of their magmatic source is supported by S, C, O, and Sr isotopic compositions. The TC parameters of the formation of massive sulfide deposits are consistent with the data on fluid inclusions from contemporary sulfide mounds on the oceanic bottom.

Physico-chemical parameters from melt inclusions for the formation of the massive sulfide deposits in the Altai–Sayan Region, Central Asia

Studies of quartz-hosted melt inclusions from rhyolitic dacite and andesite rocks showed that high-temperature (910–1180°C) felsic melts of normal alkalinity, which correspond to a tholeiitic K–Na series, played an active role during the formation of volcanogenic massive sulfide deposits of the Altai–Sayan Region. High temperatures can be explained by felsic melts degassing in open volcanic systems and heating by ascending basaltic magma. Considerable variation in the H2O content in magmatic systems of the Altai–Sayan volcanogenic massive sulfide deposits (0.08–4.30 wt%) has been established. REE patterns of quartz-hosted inclusions from volcanic rocks of the Altai–Sayan volcanogenic massive sulfide deposits display distinct negative Eu anomalies, which suggest plagioclase fractionation during differentiation of the melt. They are similar to the patterns of quartz-hosted inclusions of the Yaman–Kasy deposit (marginal sea setting) in the South Urals and to island arc rhyolites. Analyses of melt inclusions showed high values of Cu—up to 4100 ppm at the Yubileinoe deposit (Rudny Altai) and up to 3230 ppm at the Kyzyl–Tashtyg deposit (East Tuva). Comparison of data on melt inclusions has shown that some physical and chemical parameters of the magmas that generate volcanogenic massive sulfide and porphyry Cu deposits can be quite similar within certain limits.

Rochas ricas em turmalina da formação Morro da Pedra Preta: relações com os turmalinitos portadores de ouro do Grupo Serra do Itaberaba, SP

Tourmaline is a common mineral in the Serra do Itaberaba Group (Guarulhos-Brazil), especially in its basal, volcanic-sedimentary sequence, the Morro da Pedra Preta Formation. Tourmalinites, basic metatuffs, intermediate to acid metavolcaniclastic rocks, metapelites and quartz veins are among the tourmaline-rich rocks. The presence of gold disseminated in a tourmalinite of the Tapera Grande prospect led to the investigation of the relation between the environments of tourmalinite genesis and mineralizations in the Serra do Itaberaba Group. The investigation was extended to tourmaline-rich basic metatuffs, intermediate to acid metavolcaniclastic rocks, and metapelites that also show some silicification, carbonatization, potassification, chloritization and sulfidation. The tourmaline chemistry showed that even in the regions proximal to the exhalative center, the fluid/rock ratio is low because the tourmaline preserves most of the host rock's characteristics rather than those of the boron-rich fluids themselves. Such hydrothermal-metamorphic fluids promoted the redistribution of synsedimentary or syngenetic gold. The composition (aqueous-carbonic, low-salinity) of these fluids, however, did not favor the formation of massive sulfide deposits.

Controls on Heat Flow, Fluid Migration, and Massive Sulfide Formation of an Off-axis Hydrothermal System-- the Lau Basin Perspective

A numerical model has been developed to investigate heat and fluid migration in a modern off-axis seafloor hydrothermal system, that is, hydrothermal activity and fluid discharge distal to an axial magma chamber emphasizing model geometry, rock/fault properties and fault distribution. The model is based on geophysical data and seafloor observations of the Lau back-arc basin and results suggest a different hydrothermal convection scheme than axial hydrothermal systems. Major hydrothermal activity is predicted to occur at topographic highs due to significant fluid migration along inferred basement topography off-axis with associated permeability differences. Major hydrothermal fluid discharge occurs at off-axis topographic elevated positions with temperatures (150°C - 450°C) and exit fluid velocities (∼4 m/s), in good agreement with seafloor observation and theoretical calculations. Heuristic mass calculations pertaining to the formation of massive sulfide deposits imply that a significant base metal sulfide deposit (5 Mt at 10% Cu + Zn) may form in less than 6,000 years, assuming a fluid containing a maximum of 10 ppm base metals and a deposition efficiency of 10 percent. The size and distribution patterns of massive sulfide deposits are determined primarily by fault distribution, provided that adequate fluid flow pathways and heat supply exist.

THE IDENTIFICATION OF THE FOSSIL BLACK SMOKER CHIMNEY AND ITS IMPLICATION FOR SCIENTIFIC RESEARCH

The present day black smoker chimneys and mounds have been widely found in the settings of mid-oceanic ridge, backarc basin, shallow sea and continental rift, which result in the formation of massive sulfide deposits at the vent sites. Moreover, these deep vents support chemosynthetic metabolizing bacteria. The modern sulfide chimneys commonly show evident columnar to conical geometry, characterized with the concentric mineralogical zonation around a central conduit. The black smoker chimney is formed when the mineral laden hydrothermal fluid is mixed with the surrounding ocean water. It begins to grow by the instant precipitation around outer wall, followed by the crystallization of polymetal sulphide on inner wall of conduits. The deeply seated magma as heat source, hydrothermal fluid concentrated within fissures and continuous eruption are favorable to the creation of a giant black smoker chimney. The drilling in sulphide mounds at the ocean bottom and their comparison with VMS (volcanogenic massive sulfide) indicate that they have similar internal structures and mineral zonation. The sulphide mounds of economic value are built with the accumulation of collapsed chimney and breccias, reworked by replacement and recrystalization. Finally, the identification of chimney structures within VMS is very important for the understanding of the process of minerallization and the origin of life.

Discussion on extensional tectonics and magmatism as drivers of convection leading to Iberian Pyrite Belt massive sulphide deposits?

G. Digges La Touche writes: Boulter’s (1996) paper on the mechanisms leading to the formation of massive sulphide deposits in the Iberian Pyrite Belt, at Rio Tinto, interprets them as a sill sediment complex, with magmatism and extensional tectonics acting as the driving forces behind fluid convection. From the evidence presented by Boulter (1996) this interpretation appears plausible, although there do appear to be several inconsistencies in the ore-fluid convection model proposed. Boulter (1996) invokes two mechanisms for convective fluid flow. The first of the mechanisms put forward is an adaptation of the sea water convection model of Russell (1978) . It is proposed that during extensional tectonics sea water flowed down syn-sedimentary faults to be heated at depth and then flowed upwards again due to buoyancy through fracture networks or some other high permeability zone. The problem with this model is that it is essentially a low-volume system with flow down and up descrete fault and fracture zones. In order to achieve the ore-fluid volumes necessary to create the Rio Tinto massive sulphides unrealistically large long lived fluxes would be necessary. It is not suggested that this mechanism was not occurring, just that it would not have contributed significantly to the metal budget. Additional problems with the extension driven convective cell mechanism exist in that it is hard to understand why in the model of Russell (1978) sea water should flow down extensional faults, considering that the sediments in which the syn-extensional faults are forming would already be fully saturated

The internal structure of an active sea-floor massive sulphide deposit

THE hydrothermal circulation of sea water through permeable ocean crust results in rock–water interactions that lead to the formation of massive sulphide deposits. These are the modern analogues of many ancient ophiolite-hosted deposits1–4, such as those exposed in Cyprus. Here we report results obtained from drilling a series of holes into an actively forming sulphide deposit on the Mid-Atlantic Ridge. A complex assemblage of sulphide–anhydrite–silica breccias provides striking evidence that such hydrothermal mounds do not grow simply by the accumulation of sulphides on the sea floor. Indeed, the deposit grows largely as an in situ breccia pile, as successive episodes of hydrothermal activity each form new hydrothermal precipitates and cement earlier deposits. During inactive periods, the collapse of sulphide chimneys, dissolution of anhydrite, and disruption by faulting cause brecciation of the deposit. The abundance of anhydrite beneath the present region of focused hydrothermal venting reflects the high temperatures ( > 150 °C) currently maintained within the mound, and implies substantial entrainment of cold sea water into the interior of the deposit. These observations demonstrate the important role of anhydrite in the growth of massive sulphide deposits, despite its absence in those preserved on land.

Temperature, density and buoyancy fluxes in “black smoker” plumes, and the criterion for buoyancy reversal

Measurements of the temperature and composition of effluent from vents on the sea floor can be used to deduce the in-situ density of this fluid, which is required for calculations of flow in the chimneys and through their porous walls. This density is, however, not directly relevant when calculating the buoyancy flux in the plume above a smoker. It is the asymptotic buoyancy flux, following extensive dilution with seawater, which is required when estimating the height of rise of plumes in a stably stratified ocean, and when calculating the criterion for reversal of buoyancy due to non-linear mixing effects. The results of mixing calculations show that the effluent from hydrothermal vents on the sea floor will exhibit reversing buoyancy if the ejected fluid has a temperature of 300°C and a salinity greater than ∼ 8 wt.% NaCl. If the temperature of the effluent is 200°C the salinity required for reversing buoyancy falls to ∼ 5.5 wt.% NaCl. Measurements of temperature and salinities of sea-floor hydrothermal fluid suggest that fluids with the characteristics required to form reversing plumes are ejected at the sea floor. The possibility that reversing plumes may be found has important implications for the formation of massive sulfide deposits.

Formation of massive sulfide deposits on oceanic ridge crests: Incremental reaction models for mixing between hydrothermal solutions and seawater

Equilibrium path calculations have been used to model mixing between hot (350°C) hydrothermal solutions and ambient seawater, in an attempt to simulate mineral precipitation at seafloor vents. These calculations predict temperatures of precipitation, paragenetic sequence of minerals, and chemical composition of chimney deposits associated with vents on the seafloor at 21°N, EPR. Assuming sulfate-sulfide disequilibrium during mixing, the paragenetic sequence revealed is: chalcopyrite, anhydrite, pyrrhotite, pyrite, sphalerite, graphite, and barite. When sulfate-sulfide equilibria is permitted during mixing, however, reduction of small amounts of sulfate results in early precipitation of pyrite and a sequence of Cu-rich sulfide minerals (chalcopyrite-bornite-chalcocite-covellite). This sequence is analogous to that observed in thin chimney walls. The calculations indicate that sulfide mineral precipitation occurs in response to both cooling and change in composition of the hydrothermal solutions as a result of mixing. Varying the amount of mixing with respect to temperature, simulating conductive heating of seawater prior to mixing, results in only minor variations in the sequence and abundance of precipitated phases. Anhydrite precipitation during mixing occurs early, which is consistent with formation of an anhydrite leading edge of chimney structures. Similarly, extrapolation of warm spring data from Galapagos to zero SO 4 concentration suggests anhydrite formation due to mixing with seawater beneath the seafloor, most likely below the level of reactive calcareous sediments. Subsequent interaction of the mixed hydrothermal solution with those sediments results in elevated and variable Ca concentrations estimated for end-member solutions from the Galapagos. Precipitation of Mg hydroxide sulfate hydrate in the walls of the vent chimneys at 21°N, EPR, occurs as a result of conductive heating of ambient seawater with only very minor amounts of mixing. In contrast, precipitation of amorphous silica in the vents must be due to conductive cooling of the hydrothermal solutions. Thus, incremental reaction calculations demonstrate that reactions occurring in and associated with venting ridge crest hydrothermal solutions can be effectively modeled using the thermodynamic data and reaction modeling codes available today. Departures from equilibrium required to accurately model the mixing process are easily accommodated and consistent with data from the vents and vent forming materials.

Convective processes caused by a dense, hot saline source flowing into a submarine depression from above

The fluid dynamic processes which occur when dense hot saline fluid flows along the sea floor into a local depression (reservoir) are examined in this paper. After some time, the reservoir fills with salty fluid which then flows out of the reservoir over a sill. A sharp double-diffusive fluid interface develops at the level of the sill forming a ‘cap’ to the reservoir and the new source fluid flows horizontally just below this interface, continually losing heat (and a little salt) to the seawater above. A steady-state balance is eventually achieved where the total fluxes of heat and salt which are lost across the double-diffusive interface are equal to the corresponding advective fluxes due to the source fluid being warmer and more saline than the fluid which spills over the weir. The horizontally moving gravity current of new source fluid entrains some reservoir fluid from below and this entrainment is shown to have a very weak influence on the steady-state values of the reservoir temperature and salinity. This fluid model is applied to the formation of massive sulphide deposits in submarine depressions on the sea floor. McDougall (1984, Deep-Sea Research, 31, 145–170) has considered the case where the source is located below the height of the weir and in this paper we assume that the hot saline source is located above this level.

Flow rates in the axial hot springs of the East Pacific Rise (21°N): Implications for the heat budget and the formation of massive sulfide deposits

The first in-situ measurements of flow rates in submarine hot springs (temperatures between 275° and 350°C) were made in the hot springs of the East Pacific Rise at 21°N during November, 1981. The flow rates ranged between 0.7 and 2.4 m/s. We estimate that the total rates at which heat and mass are transported from the three hot spring areas—OBS, National Geographic, and Southwest Vents—are ca. 2.2×10 8 W and 150 kg of fluid/s respectively. The very high rate of heat loss precludes the existence of hot springs as steady state features of the East Pacific Rise at 21°N. We estimate that hot springs are active for a maximum of ca. 40,000 years. Chemical and/or mechanical clogging of the vents can drastically reduce the lifetime of the hot springs. Using a simple model of plume development [1], we calculated that less than 3% of the sulfide particles that are entrained in the hydrothermal plumes at 21°N settle from the plumes before dispersal by a lateral submarine current at a height of ca. 250 m above the seafloor [2]. The time that is required to form the sulfide mounds that surround the hot springs by the accumulation of sulfide particles that settle from the plumes is ca. 70–85 years. The current rate at which sulfide particles settle from the hydrothermal plumes and accumulate near the hot springs appears to be too small to lead to the formation of large massive sulfide deposits such as the Mavrouni deposit on Cyprus [3], within the time limits that are suggested by our thermal energy balance calculations.

Submarine hot springs: Origin of life?

The observations of hydrothermal systems along midoceanic spreading centers by the deep‐diving submarine, Alvin, have led to numerous geological theories to explain phenomena ranging from heat flow to the formation of massive sulfide deposits. Unusual life in the forms of giant tube worms and mussels (Eos, Dec. 29, 1981) have been found to live along the submarine hot springs in chemically reducing and normally toxic sulfurous environments. Analyses of data over the past year or two have formed the basis of new life‐evolution schemes. J.B. Corliss, J.A. Baross, and S.E. Hoffman have outlined a process by which concentrations of methane, ammonia, hydrogen, and metals may have reacted, in several steps, to produce living organisms within or adjacent to submarine hydrothermal systems (Oceanol. Acta, 59–69, 1981).

The formation of massive sulfide deposits on the sea floor

Salinity and temperature data from fluid inclusions in Cyprus and Kuroko massive sulfide deposits indicate that the ore solutions were buoyant on entering the sea water. They probably rose from the sea floor like axisymmetric conical plumes derived from a finite source or as line plumes from a linear source. The height to which they rose depended on the magnitude of the density gradients in the ambient sea water. However, in the region immediately above the outlet pipe, the effect of interference between the many vents making up the pipe and the effect of the anticipated very low flow rate probably caused necking of the plume and a drastic retardation of the normal loss of vertical velocity and buoyancy caused by entrainment of sea water.Solubility data for copper sulfides indicate that virtually all the copper was precipitated by quenching immediately above the vent. Precipitation of sphalerite (+ or - galena) took place higher in the plume and the very fine grained particles of these phases were probably entrained in the plume, carried to considerable heights, and in many cases dispersed. During the inevitable slowing of the flow rate and cooling of the hydrothermal system, sphalerite (+ or - galena) was precipitated immediately above the vent. This was probably the most important way of forming vertically zoned Cyprus and Kuroko deposits.Thin, sheetlike, well-banded deposits like Rosebery and many in the Bathurst area of Canada, may have a different form because they developed in relatively shallow water. In this situation the plume, stagnating at the water surface, spreads laterally for considerable distances, raining sphalerite and galena to the sea floor. This hypothesis predicts such deposits will have greater metal contents than other massive sulfides, higher Zn/Cu ratios, a lack of sulfur isotope equilibration between minerals, and wide geochemical halos.

Intrapartum sensitisation of human infants to the Rhesus-D antigen.

A study is reported of the numbers of nonimmunoglobulin-bearing lymphocytes specifically binding to Rhesus-D positive erythrocytes in the blood of Rhesus-dd infants born to either Rhesus-dd or Rhesus-Dd mothers, at birth and at 6 months of age. There was a significant increase in the number of such antigen-binding cells by age 6 months in babies born to Rhesus-Dd mothers. It is concluded that the Rhesus-negative offspring of Rhesus-positive mothers may be sensitised against the Rhesus-D antigen at birth and that protection of such female infants by the injection of antiRhesus-D antibody should be considered.

Lead isotope events of the Canadian Shield, ad hoc solar galactic orbits and glaciations

Abstract Lead isotope events of the Canadian Shield which may represent the time of formation of massive sulfide deposits correlate unexpectedly with known and predicted Precambrian glaciations. The predictions of Precambrian glaciations were made on the basis of a simultaneously published controversial ad hoc galactic mode). An attempt is made to discuss the meaning of these correlations, but they are essentially not understood. There is the possibility that the ad hoc galactic model, which was used more as a conceptual tool, may at present be of marginal concern, and that the more topical question is why the two oldest dated Precambrian glaciations (940–950 Ma and 2290 Ma) correlate with two of the six lead isotope events of the Canadian Shield.