Classification of Ancient Metal-Bearing Conglomerates
Abstract Generated abstract
This paper addresses the lack of a consistent geological concept for ancient metal-bearing conglomerates, whose mineralization has been variously interpreted as buried placer concentration or as related to later metamorphic processes. Drawing on comparative data from foreign deposits, it proposes a classification applicable to mineralization in ancient conglomerates regardless of the economic commodity, including gold and radioactive elements. The scheme distinguishes two principal types, with several subtypes, based on ore body form, pebble and cement composition, ore minerals, heavy-mineral assemblages, and structural setting. It emphasizes that the types are not rigid categories, since transitional forms occur between sulfide-rich Witwatersrand-type conglomerates, uranium-thorium and thorium-bearing varieties, and gold-bearing conglomerates.
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PROCEEDINGS
OF THE TOMSK ORDER OF THE RED BANNER OF LABOR
POLYTECHNIC INSTITUTE named after S. M. KIROV
Vol. 134 1968
CLASSIFICATION OF ANCIENT METAL-BEARING CONGLOMERATES
F. P. KRENDELEV (IGiG, Siberian Branch, USSR Academy of Sciences)
In the geological history of the Earth, conglomerates occur in the most diverse formations, including platform and geosynclinal formations, in molasse sequences of intermontane depressions and foredeeps, etc., and, in age, they range from the Archean to the gravels of modern streams and beaches. Large concentrations of gold and other elements are associated with conglomerates and gravel deposits; the richest of these are the Precambrian deposits of South Africa, Canada, and Brazil. The ores of such deposits are represented mainly by conglomerates, as in South Africa, but they may also be quartzite-like sandstones, as in Brazil. Between the two extreme types there are mutual transitions, and even within a single region and ore body they may pass facies-wise from one into the other. More precisely, one may speak of mineralization in clastic rocks, just as modern placers belong to one genetic type regardless of whether the gold is concentrated in gravels or in sands.
Among geologists there is no clear concept of what should be regarded as ancient ore-bearing conglomerates and their analogues. Some consider all conglomerates to be typical representatives of ancient buried placers; others link Precambrian mineralization with processes of metamorphism.
Since the identification of deposits of gold ores and radioactive raw materials is an economic rather than a geological task, and since gold and radioactive elements are present in all types of conglomerates, the proposed classification applies to all occurrences of mineralization in ancient conglomerates.
Among radioactive Precambrian conglomerates, in recent years several independent types and subtypes have already begun to be distinguished (V. A. Shkvorev and K. V. Kovaleva, 1961; F. P. Krendelev, 1965; P. K. Dement’ev, V. G. Kruglova, A. I. Bezugbov et al., 1965).
In the proposed classification, based on an analysis of data on foreign deposits, two main types are distinguished among ancient metal-bearing conglomerates; these are characterized by specific conditions of occurrence, by different compositions of the pebble material and cement, and by the useful minerals of the heavy-mineral concentrate. The classification is presented in the form of a table. There are transitions between individual types.
Classification of ancient conglomerates
| 1 | “Pyritic” or sulfide (Witwatersrand type) | Uranium–thorium | Thorium | Gold |
|---|---|---|---|---|
| Character of ore bodies | Siderite beds, gravels or conglomerates with subordinate content of useful components. Ore bodies reach a width of 150–300 m, a length of 500–1500 m, and a content of useful components within the ore bodies is high. The thickness of the bodies is insignificant, but rarely high (decimeters, rarely up to 2–3 m) | Lens-shaped bodies, frequently pinching out, wedging out, or interstratified with barren rocks. Usually they are displaced along faults. The ore bodies have the form of short lenses, stringers with a large number of barren rocks. The thickness is usually insignificant (from several centimeters to tens and more meters) | Lens-shaped bodies, frequently pinching out, wedging out, or interstratified with barren rocks. Usually they have the form of short lenses, stringers with a large number of barren rocks. The thickness is usually insignificant (from several centimeters to tens and more meters) | Lens-shaped bodies, frequently pinching out, wedging out, or interstratified with barren rocks. Usually they have the form of short lenses, stringers with a large number of barren rocks. The thickness is usually insignificant (from several centimeters to tens and more meters) |
| Content in ores and their percentage of the total volume of the ore | Less than 70%. Well sorted by size and composition. Oligomictic, chiefly quartz pebbles (including quartz, jasper, siliceous rocks, fragmentary basic rocks), pebbles and fragments of basic rocks, as well as jaspilites are present. | The percentage of pebbles in broad limits is very variable. Often a coarse fraction predominates. Pebbles are well rounded, including hornstones, quartzites, quartzites and other quartziferous rocks, diorites, and other igneous rocks; in addition, pebbles of underlying rocks are noted | The percentage of pebbles in broad limits is variable in size. Often a coarse fraction predominates. Pebbles are well rounded, including hornstones, quartzites, quartzites and other igneous rocks; in addition, pebbles of underlying rocks are noted | The percentage of pebbles in broad limits is variable in size. Often a coarse fraction predominates. Pebbles are well rounded, including hornstones, quartzites, quartzites and other igneous rocks; in addition, pebbles of underlying rocks are noted |
| Composition of cement | Quartz–sericite with pyrite, chlorite; rarely muscovite, pyrophyllite; sulfides of nickel, cobalt, lead, zinc, and other elements. Gold and cement sulfides occupy up to 5% or more of the total volume of the rock | Quartz–chlorite with native magnetite, hematite (black sand). Often there are inclusions and fragments of dark-colored minerals, sulfides—rarely | Quartz–chlorite with native magnetite, hematite (black sand). Often there are inclusions and fragments of dark-colored minerals, sulfides—rarely | Clayey–sandy cement, weakly cemented, almost completely unmetamorphosed. The sulfides are, as a rule, secondary, probably detrital |
Continuation of the table
| 1 | 2 | 3 | 4 | 5 |
|---|---|---|---|---|
| Ore minerals (in order of importance) | Uraninite, brannerite, gold, thucholite, uranium pitches | Zircon, cyrtolite, monazite, thorite, ferrithorite, rarely brannerite, uraninite | Zircon, cyrtolite, thorite, ferrithorite, monazite | Gold, rarely monazite, thorite, zircon |
| Minerals of the heavy concentrate | Monazite, zircon, rarely rutile, garnet, tourmaline, apatite, cassiterite, chromite, leucoxene, ilmenite, spinel, diamond | Zircon, rutile, ilmenite, magnetite, martite, chloritoid, apatite, tourmaline | Magnetite, martite, rutile, garnet, chloritoid, apatite, tourmaline, garnets | Magnetite, ilmenite, rutile, cassiterite, wood tin, wolframite, pyroxenes, amphiboles, chloritoid, chromite, garnets, spinels |
| Structural confinement | Precambrian marginal or superimposed geosynclinal troughs, most often in the framing of platforms or shields | Within shields, marginal troughs, on the slopes of platforms, in deposits of various ages | In folded regions of any age, predominantly in the framing of platforms | In folded regions of various ages, including Mesozoic, and also in internal and intermontane troughs, superimposed depressions and troughs |