Kula C. Misra

Understanding mineral deposits

Preface. 1. Introduction. 2. Formation of Mineral Deposits. 3. Interpretation of Mineral Deposits - I. 4. Interpretation of Mineral Deposits - II. 5. Chromite Deposits. 6. Nickel (-Copper) Sulfide Deposits. 7. Platinum-Group Element (PGE) Deposits. 8. Porphyry Deposits. 9. Skarn Deposits. 10. Volcanic-Associated Massive Sulfide (VMS) Deposits. 11. Sediment-Hosted Massive Zinc-Lead Sulfide (SMS) Deposits. 12. Sediment-Hosted Stratiform Copper (SSC) Deposits. 13. Mississippi Valley-Type (MVT) Zinc-Lead Deposits. 14. Uranium Deposits. 15. Precambrian Iron-Formations. 16. Gold Deposits. References. Index.

Sulfur isotope microanalysis of sphalerite by SIMS: constraints on the genesis of Mississippi valley-type mineralization, from the Mascot-Jefferson City district, East Tennessee

Abstract The Mascot-Jefferson City (MJC) district is the most productive zinc district in East Tennessee. The deposits are of Mississippi Valley-type (MVT), hosted by carbonate rocks and dominated by sphalerite mineralization in strata-bound breccia bodies. We have utilized the high spatial resolution (20–30 μm) of the ion microprobe to obtain in situ sulfur isotopic analyses from discrete growth zones of sphalerite and analyses of associated pyrite. Two types of pyrite were noted: pre-sphalerite, diagenetic pyrite ( δ 34 S of −16.1‰ and −20.0‰) and syn-sphalerite pyrite that is intergrown with sphalerite ( δ 34 S of 31.3‰ to 33.7‰). Two textural varieties of sphalerite mineralization (banded and non-banded) were characterized. Banded sphalerite exhibits fine (μm to cm) banding that has grown around a carbonate substrate. Banded sphalerite has δ 34 S values from 27.8‰ to 51.0‰, high Cd contents (up to 0.96 wt.%) and dark areas that are likely due to minute inclusions of organic carbon. The non-banded sphalerite has δ 34 S values from 20.2‰ to 39.5‰, high Fe content and no organic inclusions. Regardless of the textural variety of sphalerite mineralization, our results show that the sulfur isotopic composition within a single polished thin section is heterogeneous and can vary by as much as 15‰. The δ 34 S values recorded in this study are among the heaviest ever reported for MVT deposits. The microscale δ 34 S variations and presence of high δ 34 S values have been previously undocumented for East Tennessee. The data presented here suggest multiple sulfur sources and sulfide precipitation by fluid mixing. The most probable scenario involves significant sulfur input from a sulfate- and metal-bearing fluid of variable δ 34 S composition mixing with a gas cap containing H 2 S of relatively homogeneous δ 34 S composition. The gas cap provided lesser amounts of sulfur to the system. Mixing of two isotopically different sulfur sources of variable proportions can account for the observed microscale variation in δ 34 S.

Deposition and dolomitization of Upper Knox carbonate sediments, Copper Ridge district, East Tennessee

The upper part of the Cambro-Ordovician Knox Group in the fault-bounded Copper Ridge zinc district, East Tennessee, consists of interbedded limestones and dolostones, with very minor sandy and cherty horizons. The nature of allochems suggests that the limestones formed in marine peritidal environments. Other features, such as granular interallochem cements, dolomite inclusions in calcite, low Na content, and low Sr/Cr ratio, suggest stabilization of the limestones in the presence of fresh water. There are two textural varieties of dolostone: (1) fine grained and (2) medium to coarser grained. Fine-grained dolostones, composed of unzoned 0.02- to 0.05-mm dolomite crystals, represent penecontemporaneous dolomitization of calcareous sediments in upper intertidal to supratidal environments. Evidence for this includes the presence of continuous cryptalgal laminae, mud cracks, birdseyes and oolitic units, the probably former presence of evaporites, and the absence of normal marine fauna. The oolite units may be analogous to modern shoreline occurrences in Laguna Madre and Baffin Bay, Texas. The low Na and Sr concentrations, as well as the nearly stoichiometric Ca/Mg ratio of the fine-grained dolostones, compared to Holocene supratidal dolomites, are attributed to the formation or neomorphism of the fine-grained dolostones in the presence of fresh water. The medium and coarser grained dolostones consist of 0.1- to 0.8-mm dolomite crystals that show textural and compositional zoning. These dolostones formed by early diagenetic replacement of limestones due to dilution of marine pore water by fresh water. A mixing-zone environment of dolomitization is indicated by the following features of the constituent dolomite crystals: textural zoning with cloudy centers and clearer rims, concentric luminescent zoning, increase of Fe content toward the rims, and low Na contents and Sr/Ca ratios untypical of the marine environment. The depositional setting within the area of study was a coastal, tidal flat environment toward the northwest, adjacent to a very shallow marine environment dotted with tidal islands toward the southeast. In the rock record, the former presence of these islands is indicated by thickening of fine-grained dolostones and a corresponding thinning of underlying limestone units.

Amphibolites of the Ashe and Alligator Back Formations, North Carolina: Samples of Late Proterozoic-early Paleozoic oceanic crust

The amphibolites of the Late Proterozoic-early Paleozoic Ashe and Alligator Back Formations in western North Carolina represent metamorphosed tholeiitic basalts, unrelated to the mafic dikes of continental tholeiitic affiliation that intrude the Grenville-age basement rocks. On the basis of their geochemistry, the amphibolites are divisible into three compositional groups (I, II, and III), the protoliths of which appear to have evolved from different parental magmas at a spreading center. Group II (intermediate-Ti) amphibolite, the predominant variety in the study area, constitutes a highly fractionated suite (Zr ≃ 40-200 ppm, TiO2 ≃ 0.7-2.3 wt.%) with geochemical characteristics similar to those of N-type MORB formed from a heterogeneous mantle source. Group III (high-Ti) amphibolite is enriched in high-field-strength incompatible elements (Zr, Nb, Ti, Y, REEs), especially Ti (TiO2 = 2.8-3.4 wt.%), and is interpreted as T-type MORB on the basis of its Zr/Nb and Y/Nb ratios. Group I (low-Ti) amphibolite, restricted to the Ashe Formation, is characterized by extremely low abundances of incompatible elements, including Ti (TiO2 < 0.45 wt.%), and U-shaped REE profiles, requiring the mixing of a depleted MORB-type mantle source with a fluid phase enriched in LREEs. The juxtaposition of depleted, low-Ti basalt (group I amphibolite) and MORB-like basalts (group II and group III amphibolites) is suggestive of a back-arc basin setting, as has been postulated for many ophiolites, but it can also be accounted for by multi-stage melting of an upper-mantle source adjacent to a mantle plume in a mid-oceanic-ridge environment. In either case, the Ashe and Alligator Back amphibolites represent metamorphosed oceanic crust material generated at a spreading center.

Genetic implications of the trace element distribution pattern in the upper knox carbonate rocks, copper ridge district, East Tennessee

Abstract The Lower Ordovician, Upper Knox Group rocks (the Kingsport and Mascot formations) in the Copper Ridge district consist predominantly of fine-grained dolostones, medium and coarser grained dolostones, and limestones. Dolomite crystals of medium and coarser grained dolostones show up to eight cathodoluminescent zones of variable width and intensity. Electron microprobe analyses indicate that the zoning is related to variation in Fe/Mn ratios, the brighter luminescent zones corresponding to lower ratios. Superposed on this growth zoning is a compositional zoning characterized by a general increase in Fe from core to rim of individual dolomite crystals. Field and petrographic studies (Churnet, 1979; Churnet et al., 1981) indicate that the fine-grained dolostones formed in supratidal to upper intratidal environments, whereas the precursor lime muds of the limestones as well as of the medium and coarser grained dolostones formed in shallow subtidal to lower intertidal environments. The large areal extent of the dolostones must have required a regionally abundant source of Mg such as marine water. Yet, both limestones and dolostones have low Na and Sr contents suggestive of their formation in solutions more dilute than normal marine water. It is proposed that the fine-grained dolostones formed by aggradation of initially very fine-grained dolostones in presence of fresh water, and that the limestones stabilized and the medium and coarser grained dolostones formed in environments of mixed marine and fresh waters. Considered in the light of ordering of partition coefficients, such a mixing model can account for the observed correlation pattern of trace elements (especially, SMn and SrFe) as well as the Fe distribution in the zoned dolomite crystals. Variation of the partition coefficient of Mn due to fluctuations in the relative proportions of fresh and marine waters in the diagenetic solution may explain the different Fe/Mn ratios observed in the growth zones (luminescence bands) of zoned dolomite crystals.

Sweetwater BaFZn district, eastern Tennessee: a paleomagneticage for dolomitisation from fluid flow

Abstract Paleomagnetic analysis of 382 specimens from 27 sites in the Sweetwater Ba F Zn district in the southern AppalachianValley and Ridge province of eastern Tennessee indicates that peak dolomitisation or recrystallisation of the host rocks occurred during Mississippian time at 334 ± 8 Ma. This age is coeval with the 334 ± 14 Ma paleomagnetic age for host rock dolomitisation in the Mascot-Jefferson City Mississippi Valley-type (MVT) district ∼ 70 km to the northeast. These ages suggest that a single fluid flow event at 334 ± 8 Ma, either topographically or thrust-fault-driven, caused a regional chemical remagnetisation of the carbonates hosting MVT mineralisation in eastern Tennessee, indicating an early onset of the Alleghenian Orogeny in the southern Appalachians.

Mississippi Valley-Type (MVT) Zinc-Lead Deposits

Low-temperature, carbonated-hosted, strata-bound, Zn-Pb±fluorite±barite deposits are generally referred to as Mississippi Valley-type (MVT) deposits in recognition of the occurrence of many such deposits within the drainage basin of the Mississippi River in the central United States, where they were first studied in detail. MVT deposits contain a substantial proportion of the world’s reserves of zinc and lead. They are the main source of these metals in the United States and contribute significantly to the production of lead and zinc in Canada and Europe.

Sediment-Hosted Massive Zinc-Lead Sulfide (SMS) Deposits

This class of Zn-Pb sulfide (±barite±Ag±Cu) deposits constitutes a major global resource of zinc (>50%) and lead (>60%), and contributes 31% and 25%, respectively, of world’s primary production of zinc and lead (Tikkanen 1986). This deposit class has been variously referred to as: Sullivan-type massive sulfide deposits (Sawkins 1976a); subclasses of stratiform sulfides of marine and marine-volcanic association (Stanton 1972); exhalative sedimentary group (Hutchinson 1980); sediment-hosted submarine exhalative deposits (SEDEX) (Large 1980, 1981, Carne and Cathro 1982); sediment-hosted Pb-Zn deposits (Badham 1981); sedimentary-type stratiform ore deposits in flysch basins (Morganti 1981); sediment-hosted stratiform lead-zinc deposits (Lydon 1983); syngenetic and diagenetic lead-zinc deposits in shales and carbonates (Edwards & Atkinson 1986); and shale-hosted deposits of Pb, Zn, and Ba (Maynard 1991b). By analogy with the volcanic-associated massive sulfide (VMS) deposits discussed earlier (Ch. 10), the descriptive term sediment-hosted massive sulfide (SMS) deposits is preferred, because it emphasizes the lithologic association of the deposits and excludes any genetic constraint. Most of the deposits included here are dominantly stratiform (i.e., the deposits are composed of sulfide layers parallel to the bedding of the host sedimentary rocks), but some are not, particularly those that have been highly deformed (McClay 1983). In addition, many deposits either contain, or are associated with, mineralization that is not stratiform. As has been pointed out by Large (1983), the term massive sulfide, which loosely describes mineralization containing more than 50% sulfides, separates this class from other classes of sediment-hosted sulfide deposits, such as the sediment-hosted (stratiform) copper deposits (see Ch. 12) and the Mississippi Valley-type Pb-Zn deposits (see Ch. 13); there are also significant differences in the lithologic association, nature of mineralization, and metal ratios among these three classes of sediment-hosted deposits. Also excluded from the present discussion are sediment-hosted barite deposits without significant base metal enrichment, such as those of the barite districts in Arkansas and Nevada (USA), although both deposit types are regarded as exhalative in origin. In contrast to SMS deposits, which are hosted by basinal elastics in dominantly intracratonic rift settings, barite deposits display geochemical signatures that indicate the influence of oceanic crust and appear to have formed in compressional continental margin settings (Maynard 1991b), perhaps from cooler and shallower hydrothermal systems.

Volcanic-Associated Massive Sulfide (VMS) Deposits

Massive Cu-Zn-Pb sulfide deposits in predominantly volcanic terranes have been variously termed as ‘volcanic-associated’, ‘volcanic-hosted’, and ‘volcanogenic’. The term ‘volcanic-hosted’ is not entirely appropriate, because the deposits included in this class are not consistently hosted by volcanic rocks, nor do the deposits necessarily form as an integral part of the volcanic process as the term ‘volcanogenic’ implies. The term ‘volcanic-associated’ is considered more appropriate as it accommodates not only the deposits enclosed entirely within volcanic strata, but also those, such as the Besshi deposits (Japan), hosted by sedimentary rocks formed in a dominantly volcanic regime. The most important requirements of the volcanic-associated class of deposits are that penecontemporaneous volcanism must have accompanied the formation of the deposits, and that volcanic rocks must comprise an essential part of the immediate stratigraphic sequence (Franklin et al., 1981).

Platinum-Group Element (PGE) Deposits

The platinum-group elements (PGE) comprise a geochemically coherent group of siderophile to chalcophile metals that includes osmium (Os), iridium (Ir), ruthenium (Ru), rhodium (Rh), platinum (Pt), and palladium (Pd). Based on association, the PGE may be divided into two subgroups: the Ir-subgroup (IPGE) consisting of Os, Ir, and Ru and the Pd-subgroup (PPGE) consisting of Rh, Pt, and Pd.