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Ore-Forming Processes in Geologic History

By
Charles Meyer
Charles Meyer
Department of Geology and Geophysics, University of California, Berkeley, California 94720
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Published:
January 01, 1981

Abstract

The transfer of the earth’s interior heat to the surface, mainly by igneous activity, shapes the tectonic evolution of the crust. The effects of orogenesis are modified by erosion and sedimentation, which are powered chiefly by the sun. Interaction between these two sources of energy is continuous in the region of steep thermal and chemical gradients near the earth’s surface, where reactions between lithosphere, atmosphere, hydrosphere, and the biomass produce the extreme geochemical anomalies of most ore deposits.

The cumulate ores of chromite appear to show the least variation over the whole range of geologic time. Cumulates range from layers of the Fiskenaesset Complex in Greenland, more then 3,700 m.y. old, to the strongly deformed podiform chromite ores of Phanerozoic ophiolite suites. The morphological character of the podiform bodies indicates that they were deformed, along with their parent peridotite, before crystallization was complete. Their abundance in the Phanerozoic, scarcity in the Archean, and virtual absence from the Proterozoic relates to the tectonic evolution of the crust.

Layered intrusions are also host to nickel-bearing sulfide segregations over much of geologic time, though the best sulfide nickel ores are not in the same intrusions with the best chromites. Sudbury, about 1,840 m.y. old, is unique in size and richness, whatever the origin of its magmas. It contains as much copper as nickel, a sign of progressing geochemical evolution.

Kambalda-type nickel deposits are poor in copper, with high Ni/Fe ratios in the sulfides. The orebodies are interlensed with komatiite ultramafic lava flows and sheets in thick successions of greenstone-type basalts. Kambalda-type deposits appear to be most prevalent in the Archean, though later deposits of nickel sulfide in amphibolites, where caught up in intense deformation, are difficult to categorize. Significantly, no important concentrations of nickel sulfide have been found in Phanerozoic ophiolite assemblages that host podiform chrome cumulates.

Archean volcanic assemblages are well preserved in the greenstone belts, most of which range in age from about 3,400 to 3,000 m.y. in the southern African cratons and from 3,200 to 2,600 m.y. elsewhere. Gold-quartz veins and Algoma-type banded iron-formations are present in nearly all greenstone belts, but volcanogenic massive sulfide deposits are most abundant in some of the younger Archean belts such as the Canadian. Archean massive sulfides are the largest of their class, accumulating over submarine fumarolic alteration pipes atop the felsic rocks which close an eruptive cycle. A typical greenstone volcanic pile shows repeated, cyclical mafic to felsic assemblages, locally containing more than one ore zone. The abundant concentration of zinc in addition to copper corresponds to the concentration of salic components in the magmas, possibly through second-cycle partial melting of older crust.

Gold-quartz veins in the greenstone belts are at least crudely stratabound. They are secondary concentrates in gash veins and were dependent on the deformation of the rocks under the heat flux of magmatic or metamorphic thermal fronts. The crust foundered under the greenstone volcanic and clastic piles, probably even while accumulation was in progress. Close folding, with steeply plunging fold axes and penetrative deformation, characterizes the greenstone belts, but the thermal level of metamorphism rarely exceeded greenschist grade.

Greenstone belts, gold-quartz veins, and massive sulfide deposits all diminished in frequency of occurrence with increasing stabilization of the crust, as evidenced by the development of the large, slowly sinking basins characteristic of the Proterozoic, beginning 3,000 m.y. ago in Africa and 2,500 m.y. ago elsewhere. These basins received enormous clastic accumulations, as in the Witwatersrand, and widespread chemical sedimentation of Lake Superior-type banded iron-formations, which reached climactic rates during the period about 2,200 to 2,000 m.y. ago.

A few Archean-style massive sulfide deposits were still added later, possibly at continental margins, until about 1,800 m.y. ago, which also marked the approximate end of important banded iron-formation deposition. Iron and silica for the banded iron-formations were probably supplied in part by chemical weathering which produced soluble ferrous iron on the outcrop. But also, perhaps more importantly, they were derived from greenstone volcanic activity. A diminishing volcanic supply of reduced iron, sulfur, and other reductants, which had provided oxygen sinks for proliferating, photosynthesizing blue-green algae, led to more rapid increases in the free oxygen supply in the atmosphere and the sulfate content of the oceans. The oxygenated atmosphere was then able to mobilize uranium on the outcrop as it demobilized iron. While high grade vein-type uranium concentrations were formed below surfaces of prolonged weathering about 1,800 m.y. ago, the gold content of marine placers diminished during the lower Proterozoic, possibly signaling the increased solubility of gold in more oxygenated saline systems.

The mid-Proterozoic transition (1,800 ± 100 m.y. ago) is also marked by the advent of stratiform sediment-hosted sulfide deposits of the base metals, beginning with a giant copper deposit at Udokan. Later, the world’s first and some of its finest major concentrations of lead were formed, about 1,700 m.y. ago. But whereas the sulfur of these deposits appears to be reduced from sulfate, the reason for the accelerated precipitation of the lead is less clear; the inversion of the carbon dioxide/oxygen ratio in seawater and other surface systems may have influenced its change of behavior across the mid-Proterozoic transition.

The mid-Proterozoic transition involved profound changes in ore-forming processes that were dependent on the chemistry at the surface of the earth, without being marked by major sharp changes in tectonic style. Nevertheless, it might have been triggered by the decrease in volcanism accompanying the tectonic transition from Archean greenstone-style convection-driven crustal dynamics to the rift-controlled tectonics of the late Proterozoic. Rift-controlled kimberlites and carbonatites entered the scene at about 2,000 m.y. ago, together with major doleritic dike swarms.

Bedded-sulfate evaporites entered as stratigraphic units before the major copper-rich carbonate-shale deposits were formed in the African copper belt. There were also smaller concentrations of copper formed during the same time interval (1,000 ± 200 m.y.) on other continents. As in the iron-formation basins, a slow, uniform rate of sinking was compensated for by a uniform fine-grained carbonate-rich shale sedimentation in shallow water, thus producing great stratigraphic thicknesses. Metal supply could have been from rifts bordering the slowly sinking basins.

Except in a few areas like the Coronation geosyncline, there is little evidence for large-scale marginal plate override and consumption over long strike lengths before the closing of the proto-Atlantic Ocean to initiate the great orogenic belts of the Phanerozoic. Large-scale plate tectonic recycling of ocean crust greatly increased the number and variety of ore-forming environments by generating long chains of volcanic island arcs around the margins of the continents, and by providing rift-bordered and back-arc basins, as well as large. shallow epicontinental seas.

Consequently, the Phanerozoic assemblage of ore types includes both Archean volcanic types and Proterozoic sedimentologic types, each modified in detail as chemical and tectonic evolution progressed. Additionally, there are a few new types that depend on accreting plate mechanisms or on extreme geochemical evolution of siliceous magmas.

Phanerozoic volcanogenic massive sulfide deposits are smaller on the average than their Archean counterparts. Lead became an increasingly important metal in these deposits as time advanced through successive orogenic periods to the Miocene Kuroko deposits of Japan. These deposits are associated with abundant sulfate sediments and with locally strong hydrogen ion metasomatism owing to penetration by large amounts of fumarolic sulfur in oxidizing seawater. Such chemical characteristics contrast sharply with the sulfate-free, lead-poor, magnesium and ferrous ion metasomatic characteristics of more reduced Archean massive sulfides. Gold-quartz veins are generated in black shales originally sedimented behind some of the Phanerozoic arcs. The veins themselves resemble the Archean-style gash veins.

Copper, silver, and zinc deposits in black shales and lead deposits in carbonate rocks were formed widely during the Paleozoic, but they are somewhat less abundant in later basins. Clinton and Minette iron ores were concentrated (without silica) in local oxidizing sinks on the continents. Uranium responded to oxidation-reduction cycles in continental arkoses to form sandstone-type ores.

Cyprus-type copper-pyrite deposits and alpine-type podiform chromite deposits in ophiolite assemblages first appear in the Phanerozoic. They probably formed at accreting plate margins directly from primitive igneous asssemblages. The scarcity of zinc and lead in the Cyprus-type massive sulfides corroborates the hypothesis that these metals require reprocessing of crustal rocks for concentration to ore grades. Hydrothermal leaching of the ridge basalts seems to have extracted chiefly copper, iron, and sulfur.

The largest base metal concentrations of the Phanerozoic are the subvolcanic porphyry coppers. Whether their habitat is continental or island arc, their alignment parallel to consuming plate boundaries clearly suggests a genetic relationship to the volcanism released by plate subduction. This does not mean, however, that their parent magmas are necessarily wholly attributable to partial melting of the descending plate itself. Like the Kuroko deposits, the porphyry copper deposits also reflect Phanerozoic near-surface chemistry through their abundant sulfate and moderate to strong hydrogen ion metasomatism.

Molybdenum, tin, and tungsten were effectively concentrated to ore grade on a large scale beginning in the Phanerozoic, probably as a result of increasing concentration of siliceous residues during repeated magmatic recycling.

The Archean to Proterozoic and the Proterozoic to Paleozoic transitions in ore types relate to important steps in tectonic evolution of the earth. The mid-Proterozoic transition relates more to changing chemistry at the earth’s surface.

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Contents

Economic Geology Publishing Company

Seventy-Fifth Anniversary Volume

Brian J. Skinner
Brian J. Skinner
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Society of Economic Geologists
ISBN electronic:
9781934969533
Publication date:
January 01, 1981

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