Michael M. Kimberley

Exhalative origins of iron formations

Iron formations are stratigraphic units which are largely composed of iron-rich chemical sedimentary rock, here called ironstone. Most aspects of iron formations continue to be controversial and so one must read voluminous literature to appreciate either the range of iron-formation characteristics or the conflicting interpretations of those characteristics. Most protagonists in the ongoing debate may be classified into two groups, i.e. those who support a shallow weathering source for the iron (weathering of land or surficial seafloor sediment) and those who invoke deep weathering (hydration of new crust or late diagenesis of sediments) followed by exhalation of ferriferous fluids through the seafloor. The present review concludes that deep weathering has been the source of all iron formations. Cherty iron formations are attributed to hydration of new crust. Noncherty iron formations are attributed to exhalation of late-diagenetic fluids which have been driven through a continental margin by seismic pumping. n nIron-formation controversies are reviewed herein through the development of flow charts which illustrate relationships among the many controversies. Preferred routes through these flow charts are suggested for both cherty and noncherty iron formations but the reader readily may select other routes. The mode of iron supply (deep or shallow weathering) is the most fundamental among many other issues, e.g., the mechanism for long-term maintenance of abundant dissolved iron within a large water body. The iron in any extensive iron formation which is consistently thicker than 10 m is attributed to a long-lasting suboxic mass of seawater which lacked H2S. The paucity of H2S either has been due to a paucity of all sulfur species or to an inhibition of sulfate reduction under reducing conditions, as in the modern Orca Basin under the Gulf of Mexico. Water in the Orca Basin contains up to 20 ppm Mn2+ just below the oxic-suboxic interface and an average of 1.6 ppm Fe2+ throughout the suboxic region. n nCherty iron formations are attributed to low-temperature (<300°C) hydration of newly formed igneous crust by seawater. Peak production of cherty iron formations, e.g. during the beginning and end of the Proterozoic, is attributed to particularly rapid crustal accumulation in opening rifts, followed by abrupt failure of the rift and low-temperature hydration of the new crust. Rifts presumably opened, failed, and became sheared by transform faults more rapidly on a radioactively hotter young planet. Broad submarine transform fault zones are characterized by seismic pumping of seawater. Exhalative sedimentation of small cherty iron formations within rifts has continued into the Phanerozoic and a partial modern analog exists in the Red Sea. n nNoncherty iron formations are attributed to seismic pumping of seawater through an ophiolite-bearing sedimentary pile along a continental margin. Iron-dissolving fluids are hypothesized to have been hypersaline because of pumping through evaporites or cooling plutons within the sedimentary pile. The production rate of noncherty iron formations has not changed much through Earth history. A modern analog exists in the continental margin of Venezuela where the soft-sediment equivalent of ferrous-silicate ironstone is accumulating near Cabo Mala Pascua. Ferrous-silicate (berthierine) ironstone is accumulating where exhalations rise quickly and reach the shallow ocean before precipitating iron. More slowly rising coastal and many deep-water exhalations in Venezuela precipitate glauconite just below the sediment-water interface. If all iron formations have formed by exhalation, then manganese and phosphate deposits probably also are exhalative.

Chemistry and mineralogy of Precambrian paleosols at the base of the Dominion and Pongola groups (Transvaal, South Africa)

Abstract We have studied alteration zones found between sedimentary rocks of the (lowest Proterozoic) Dominion and (Archean) Pongola Groups and underlying granitic rocks (Transvaal, South Africa). The alteration zones have a transitional lower boundary, primary igneous minerals are gradually destroyed upward toward the contact with overlying sedimentary rocks. Ca, Mg, Na, P, and Mn are progressively lost upwards in the zone. The chemical and mineralogical variations, and available stratigraphic evidence are all consistent with the interpretation that these zones are paleosols formed by Precambrian weathering 2.8-3.0 Ga ago. After formation the paleosols underwent metasomatism and metamorphism. Sericite now present in the paleosols was probably formed by low temperature potassium metasomatism of original clays. Andalusite in the pre-Pongola paleosol was probably formed by metamorphism of original clays in impermeable zones which prevented groundwater flow and potassium metasomatism. The Dominion paleosol was investigated at three localities. The effects of weathering were similar at the three sites except for the behavior of iron. One site lost about 20% of iron whereas the other sites lost 75% and 80% of initial iron content. The differences in iron loss may be due to variation in soil ventilation and diffusion of atmospheric gases as a function of grain size and topography. Variations in iron loss may be used to place some constraints on the composition of the Precambrian atmosphere. Given constraints imposed by Dominion paleosols, overlying uraniferous conglomerates, requirements for chemical weathering and the greenhouse effect, and possible presence of non-diffusing photochemical oxidants, the partial pressures of oxygen and carbon dioxide gases probably are in the ranges: PO1 - 0.02% to 0.5% PAL (Present Atmospheric Level) and PCO2 - 5 to 30 PAL.

Early Organic Evolution

This volume presents both an overview and final synopsis of the work performed between 1978-1989 by an international task force of geologists, geochemists, and paleontologists rallied around the charter of IGCP-Project 157 (Early Organic Evolution and Mineral and Energy Resources). Apart from several review papers summarizing the state of the art in selected facets of the subject, the bulk of the contributions reflects current research activities in the field (including modern analogs of ancient processes) ranging from early evolution of life on this planet to questions of Precambrian weathering, metallogeny and petroleum formation. Dealing with processes at the interface between evolutionary biology, organic geochemistry and economic geology, the book gives an updated summary of the current state of one of the most challenging frontiers in Earth Sciences.

Origin of Oolitic Iron Formations

ABSTRACT The oldest genetic hypothesis for oolitic iron formations based upon thin-section study, that of Henry Clifton Sorby (1856), is defended and amplified. Sorby (1856) proposed that oolitic iron formations originated as beds of calcareous oolite, which were covered by ordinary mud rich in organic matter. Iron was leached from the mud by pore water bearing organic-decay products, resulting in ferruginization of the calcareous sediment. All characteristics of oolitic iron formations and partially ferruginized limestone appear to support this, as does quantitative modeling of present processes to explain the youngest oolitic iron formation, a voluminous deposit less than 5 m. yr. old.

Profiles of elemental concentrations in Precambrian paleosols on basaltic and granitic parent materials

Abstract Profiles of elemental concentrations are interpreted for four Precambrian paleosols, two developed on basalt and two on granodiorite. All four paleosols appear to be the erosional remnants of originally thick soil-saprolite regoliths. The granitic paleosols are in South Africa where they underlie the 2.9-3.0 Ga Pongola Supergroup and the 2.8-2.9 Ga Dominion Reef Conglomerate. One of the basaltic paleosols also occurs in South Africa where it caps the Ventersdorp Basalt and underlies the 2.3 Ga Black Reef Quartzite. The other basaltic paleosol underlies sandstone of the 2.7 Ga Timiskaming Group in the Abitibi belt of the Canadian Shield. All four paleosols exhibit pronounced upward loss of sodium but no consistent loss of the heavier alkali elements, rubidium and cesium. Iron decreases upward above iron-rich basaltic parent rocks but there is no consistent loss of iron above iron-poor granitic parents. The rare-earth elements are less fractionated than during intensive modern weathering. Uranium locally has been fractionated from thorium, possibly due to oxidative dissolution during Precambrian weathering.

Nomenclature for iron formations

This paper explains the terminology and a classification scheme which are used in an accompanying paper about the genesis of iron formations. An iron formation is considered to be a stratigraphic unit which consists mostly of iron-rich chemical sedimentary rock, specifically rock with more than 15% Fe.∗ The rock is called ironstone. Iron formations may be classified into six groups on the basis of the sedimentary environment in which they accumulated. n nSome previous authors have used the terms, ironstone and iron formation, in more restricted ways. For example, “ironstone” has been used to name one of the six environmental types of iron formations and “iron formation” has been used exclusively for chert-rich iron formations. These restrictions are avoided herein because they are not consistent with the simplicity of the names. By analogy with limestone, ironstone is simply an iron-rich chemical sedimentary rock. The word, formation, clearly refers to a stratigraphic unit and so “iron formation” is applied to any stratigraphic unit which is composed predominantly of ironstone. Alternative usages of the terms, ironstone and iron formation, have been sufficiently varied that none of the usages can be defended on the grounds of overwhelming support, independent of semantic arguments.

Origin of ultramafic-hosted vein magnesite deposits

Abstract Large deposits of magnesite (MgCO3) occur either as carbonated ultramafic rocks or as sedimentary beds. The origins of both sedimentary and ultramafic-hosted deposits remain controversial, as does the relationship between them. Ultramafic-hosted deposits are reviewed herein, with an emphasis on veins rather than massive bodies. Magnesium apparently has been supplied to magnesite veins by their ultramafic host. Uncertainties concerning ultramafic-hosted deposits include the source, state, and mode of transportation of carbon, the cause of its precipitation, and the mineralogy of the initial precipitate. Genetic questions about ultramafic-hosted vein deposits are summarized with the aid of flow charts. The preferred route through these charts attributes most vein magnesite to an influx of metamorphic-degassing CO2 and CH4 into groundwater which flows through serpentinite or peridotite. The source of CO2ue5f8CH4 is considered to be deeper than 10 km because metamorphism (>300°C) is required to release carbonaceous volatiles of the type which apparently induced observed magnesite mineralization. However, vein magnesite precipitation generally has occurred within just a few hundred meters of the Earths surface. Vein deposits are considered to be genetically related to massive bodies but the massive bodies typically contain less 12C. The 12C-rich CO2, which has produced vein deposits, is attributed to a fluid phase (variably mixed CH4ue5f8COue5f8CO2) which rose until it reached upper-crustal groundwater. The hypothetical water-poor fluid presumably became slightly oxidized, hence richer in soluble CO2, as it encountered near-surface, sulfate-bearing water. CO2-enriched water is envisioned to have been expelled regionally due to tectonic processes, leaving little mineralogical evidence of its expulsion through non-ultramafic rock. CO2-rich water which happened to encounter either serpentinite or peridotite presumably has produced magnesite vein deposits and a common silicate byproduct, nontronite, according to the following reaction: 12 Mg3Si2O5(OH)4 (serpentine)+36 HCO3−+4 Fe3O4+O2→12 FeSi2O5(OH) (nontronite)+36 MgCO3 (magnesite)+36 OH−+18H2O.

Origin of ultramafic-hosted magnesite on Margarita Island, Venezuela

Ultramafic-hosted deposits of magnesite (MgCO3) have been studied on Margarita Island, Venezuela, to elucidate the source of carbon and conditions of formation for this type of ore. Petrographic, mineralogic, and δ18O data indicate that magnesite precipitated on Margarita in near-surface environments at low P and T. δ13C ranges from −9 to −16‰ PDB within the magnesite and −8 to −10‰ PDB within some calcite and dolomite elsewhere on the island. The isotopically light dolomite fills karst and the calcite occurs as stock-work veins which resemble the magnesite deposits. These carbon isotopic ratios are consistent with a deep-seated source rather than an overlying source from a zone of surficial weathering. However, there is not much enrichment of precious metals and no enrichment of heavy rare-earth elements, as would be expected if the carbon had migrated upward as aqueous carbonate ions. The carbon probably has risen as a gaseous mixture of CO2 and CH4 which partially dissolved in near-surface water before leaching cations and precipitating as magnesite and other carbonates. The process probably is ongoing, given regional exhalation of carbonaceous gases.

Geochemical distinctions among environmental types of iron formations

Abstract Classification of iron formations according to the sedimentary-volcanic environments in which they formed reveals that chemical compositions of major iron-formation types are largely independent of deposit age. No evidence is found in iron formations for atmospheric-hydrospheric compositional evolution. However, chronological variation in relative abundance among major environmental types may indicate an Archean to Phanerozoic tectonic-magmatic evolution from abundant shallow-volcanic platforms through predominantly non-volcanic shallow platforms or flat continental shelves to abundant inland seas. Shallow-volcanic-platform iron formations typically display large positive europium anomalies and vary widely in silica, phosphorus and alkali content, even within individual iron formations. Various minor-element abundances may be substantial, particularly where phosphatic or near sulfide ore bodies. Iron formations which formed on extensive, chemical-sediment-rich continental shelves or non-volcanic oceanic platforms are generally silica-rich, phosphorus-poor, alkali-poor and minor-element-poor. Europium anomalies are normally small and mostly negative. Oolitic-inland-sea iron formations are poor in silica, except where gradational to sandstone, and are typically enriched in phosphorus and several ferrides.

Composition of Middle Precambrian uraniferous conglomerate in the Elliot Lake-Agnew lake area of Canada

Abstract Multielemental instrumental neutron-activation analysis has been used to study uraniferous conglomerate and associated rocks in the Middle Precambrian Elliot Lake—Agnew Lake area of Ontario, Canada. Chemical compositions have been determined to elucidate the process of uranium mineralization. Compositional evidence is consistent with a genetic model which involves physical and/or chemical processes of concentration. Uranium content correlates closely with that of thorium, titanium, zirconium, tantalum, and rare-earth elements, possibly indicative of heavy-mineral concentration. However, consistently very large K/Na ratios and the paucity of magnetite and ilmenite indicate peculiar chemical processes. Abundant associated pyrite displays volcanic Co/Ni ratios and may have formed during Middle Precambrian volcanism. Weathering of such pyrite could have resulted in chemical concentration of uranium-thorium aggregates available for subsequent physical concentration.