Sediment-Hosted Stratiform Deposits of Copper, Lead, and Zinc
Sediment-hosted stratiform deposits of copper, lead, and zinc are distinguished from volcanogenic massive sulfide deposits by a lack of associated volcanism. They are distinguished from Mississippi Valley-type deposits by an early timing of mineralization relative to deposition of host sediments, by greater conformity with their hosts, and by higher iron sulfide and silver contents. These two types, however, are probably part of a continuum in terms of characteristics and mechanism of formation. Characteristics of the diverse sediment-hosted stratiform deposit type are illustrated by 28 selected examples, which include several of the world s giant deposits. Metal ratios of sediment-hosted stratiform copper and lead-zinc deposits are distinctly separate from one another and unlike the range of ratios in volcanogenic deposits. Most sediment-hosted stratiform lead-zinc deposits contain abundant iron sulfide, in contrast to the copper deposits which are characterized by low sulfide iron contents. Barite is abundant and massive in several of the lead-zinc deposits but is sparse in stratiform copper deposits.
Sediment-hosted stratiform deposits of both copper and lead-zinc range in age from about 2,000 m.y. to recent and occur in tectonically active intracratonic settings, commonly in fault-controlled sedimentary basins. They do not appear to be related to orogenic events or to events at margins of tectonic plates. Local tectonic settings and relations to basin margins and faulting are diverse. We interpret their unifying features to be that each provides a situation in which basinal ground water could be moved to a shallow site of sulfide deposition. Regional stratigraphie settings include (1) first marine transgressions over red terrestrial sections (e.g., Zambian deposits, Kupferschiefer, White Pine, Boleo), (2) within marine (and lacustrine) sections containing red beds (e.g., Dzhezkazgan, Udokan, McArthur River, Redstone River, Spar Lake, Irish deposits), (3) terrestrial sections containing red beds (e.g., Corocoro, Nacimiento, Largentière), and (4) marine sections without red beds (e.g., Sullivan, Meggen, Rammelsberg). Evaporites (bedded or interstitial gypsum, anhydrite, or carbonate after sulfate) or other evidence of aridity (salt casts, desiccation features) are closely associated with deposits in all settings but the last. Although they lack any close association with volcanism, most deposits occur in basins with some regionally contemporaneous volcanic activity or with a significant quantity of volcanic rocks in underlying stratigraphie sections. Sedimentary facies are important local controls on sulfide distribution, but host lithology varies widely from district to district and commonly within districts and is among the least definitive features of these deposits.
Most sediment-hosted copper deposits (except in Shaba) are associated with bedded host or footwall units that retained high permeability after early diagenesis. The copper mineralization is invariably “disseminated” (including stockwork and breccia matrix). Most of the ore is in reduced sediments that contain (or can be inferred to have originally contained) abundant organic matter. Ore occurs at a redox interface with footwall or hanging-wall or laterally equivalent units that are hematitic (red now or before metamorphism) and barren of sulfide and organic matter. In contrast, most lead-zinc deposits are in sediments that became rather impermeable during early diagenesis. The ores are mostly either massive or banded (i.e., interbedded with sedimentary rock), although some are disseminated. Many deposits are also associated with evaporites and red beds, though less directly than the copper deposits, and the ore is not obviously controlled by the interface between oxidized and reduced conditions.
Zonation in copper deposits typically follows the sequence: barren (hematite but no sulfide) chalcocite → bornite → chalcopyrite → pyrite, expressed both outward and upward. In many deposits this zonation does not reflect the paleogeography directly. Zonation is commonly continuous across alternation of depositional environments and probably reflects post-depositional hydrologie regimes and chemical gradients. In deposits with lead-zinc as well as copper, the general zonal sequence Cu-Pb-Zn-Fe is observed. In several deposits this sequence evidently marks increasing distance along an ore solution pathway. Sedimentary iron-formation, both sulfide and oxide facies, appears to have formed in distal portions of some lead-zinc mineralizing systems. Zoning patterns are varied. The timing of metal fixation relative to deposition of host sediments and their diagenetic modification is crucial but rarely constrained by unambiguous evidence. Concretions and authigenic minerals provide some of the best evidence, indicating fixation of base metal sulfides after early diagenesis at McArthur River and Shaba, but much earlier at Rammelsberg. The lack of definitive detailed documentation of evidence for timing and of subtle nonsulfide zoning patterns in most deposits seriously limits our ability to interpret their origins. The lack of information on temperatures of mineralization in most deposits is also critical.
Unlike volcanogenic massive sulfide deposits, sulfur isotopes in sediment-hosted stratiform deposits do not reflect secular variations in the isotopic composition of seawater and appear to be dominated by processes at the site of ore deposition rather than the source of the sulfur. Narrow clustering of δ34S values in lead-zinc deposits appears to reflect systems dominated by mineralizing fluids, in contrast to scattered δ34S values in copper deposits which reflect sulfate reduction reactions at lower temperature and in systems dominated by the depositional environment. The data are ambiguous as to mechanisms and to definition of the ore-forming systems. At McArthur River, carbon and oxygen isotopes support an interpretation of mineralization dominated by a hydrothermal ore-forming fluid.
We speculate that most sediment-hosted stratiform deposits of both copper and lead-zinc formed early in the diagenetic history of their enclosing sediments from brines derived from the sedimentary basin itself. They are thus diagenetic in the broad sense, even though the metals may have been introduced epigenetically into the system defined by the present physical limits of mineralization. The wide and gradational range of deposit characteristics derives from more or less independent variation of factors linked in a source-transport-trap chain of coincidence. The stratiform copper deposits are probably formed from cool sulfate-rich brines, derived at an early stage of basin evolution and migrating updip to reducing sites of deposition. The lead-zinc deposits are probably formed from similar but more evolved brines that have been heated and probably chemically reduced deeper within the basin. Some of the latter may have been formed by exhalation of dense basinal brines on the sea floor.
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