Ore Horizon Research Articles

The extensive hydrocarbon-mediated fixation of hydrothermal gold in the Witwatersrand Basin, South Africa

There is a close spatial relation between high-grade gold mineralization in the Witwatersrand basin and carbonaceous nodules, veins and seams. Hydrocarbons thus may well have been essential in ore genesis. We have sampled four major gold-, uranium- and hydrocarbon-bearing ore horizons, namely the Carbon Leader, Vaal, B and Black reefs, to determine the role of hydrocarbons in the accumulation and hydrothermal fixation of gold. Our multipronged approach included high-resolution scanning and transmission electron microscopy (SEM, TEM), nanotomography with video clips, and geochemical modeling. Post-depositional hydrothermal activity at the peak of regional metamorphism produced an assemblage of quartz, phyllosilicates, brannerite, crandallite, florencite, monazite and gold in all four reefs. The gold, hydrocarbons and associated mineral assemblages are closely related on the micro to nano scale. Gold deposition occurred in interstices, as fracture fillings in detrital minerals, and on the surface of migrated solid hydrocarbon residues. The spherical to elliptical inclusions in the gold consist of an outer pyrobitumen phase and a central void space, partially associated with nanometric gold, uraninite, coffinite and silica. The hydrocarbon-bearing inclusion likely formed by the entrapment of a fossil liquid oil precursor during gold precipitation. The oil was subsequently thermally altered and converted into the final pyrobitumen and gaseous residues. Geochemical calculations to simulate the interaction of an invading hot hydrothermal fluid with the hydrocarbons in the reefs reveal that a very small amount of hydrocarbons will drastically decrease the aqueous solubility of gold and hence cause its instant precipitation.We extend our genetic model for the epigenetic formation of gold in the Witwatersrand. Regional metamorphism promoted the extensive and likely basin-wide circulation of hydrothermal fluids; these were capable of mobilizing substantial amounts of gold. The liquid, gaseous and solid hydrocarbons in the reefs acted as efficient chemical traps for the concentration of gold. Being strong chemical reductants, they caused the rapid precipitation and accumulation of gold on the surface of the fossil oil droplets and already solidified hydrocarbons. The release of the gases from accessible hydrocarbons into the sediments away from their source buffered the redox state of the hydrothermal solutions even at a considerable distance from the pyrobitumen seams and veins, likely resulting in the deposition of gold in the absence of visible hydrocarbons. Although our findings do not explain the ultimate origin and exceptional endowment of gold in the Witwatersrand, we do provide intriguing evidence for the large-scale hydrothermal mobilization, accumulation and fixation of gold mediated by hydrocarbons during post-depositional metamorphism.

A New Model for the Genesis of Carboniferous Mn Ores, Longtou Deposit, South China Block

AbstractThe lower Carboniferous Luzhai and Baping Formations (ca. 359 Ma) of the South China block, Guangxi Province, comprise an ca. 170-m-thick clastic-carbonate succession capped by Mn ore horizons near the town of Longtou. Excellent exposure of the stratigraphic succession provides an unparalleled opportunity to investigate the origin of carbonate-hosted Mn deposits, which are generally understudied. Lithofacies associations suggest inner and middle shelf clastic rocks accumulated with deposition of carbonates on a mesotrophic middle to outer shelf. In the Longtou region, carbonate deposition during marine transgression culminated with the precipitation of high-grade Mn deposits during maximum flooding. Mn ore horizons are composed of amalgamated alabandite-bearing rhodochrosite, Mn calcite, and braunite laminae. Mn carbonates have been largely interpreted as forming in oxic water columns via reduction of Mn oxides by organic matter. However, paragenetic relationships and δ13C values (similar to those of seawater) indicate the Mn carbonates of Longtou were formed during authigenesis by the emplacement of anoxic, Mn-rich water masses on the distal to middle shelf. Such anoxia is interpreted to have shut down the carbonate factory and diminished sedimentation, a prerequisite for the concentration and precipitation of Mn carbonates in pore water. This research supports the notion that areas of the Paleozoic deep ocean were persistently anoxic and periodically tapped by coastal upwelling to produce Mn- and Fe-rich deposits. Application of this emerging ore deposit model to other economically important carbonate-hosted Mn deposits may improve resource exploration.

П Р И М Е Н Е Н И Е Т Р Е Х М Е Р Н О Й С Е Й С М О Р А З В Е Д К И Д Л Я М О Д Е Л И Р О В А Н И Я Р У Д Н Ы Х З А Л Е

The article deals with the possibility of using seismic exploration in solving problems of ore geology in complex mining and geological conditions of Kazakhstan. The substantiation and characterization of geological and geophysical conditions for conducting experimental 3D seismic surveys at the deposits of the Zhilandinsky group of copper sandstones located in the Karaganda region of Central Kazakhstan are presented. The tasks are defined and the technique of conducting high-resolution 3D seismic exploration is developed, accompanied by modern processing and interpretation complexes for obtaining high-quality seismic materials for solving geological problems: studying the structural and tectonic structure of ore areas, identifying and refining ore-controlling structures, detecting and deep mapping of tectonic faults, volumetric mapping of intrusive massifs. The directions and technology of the process of modeling ore horizons and tracing them in space based on the results of a comprehensive interpretation of three-dimensional seismic exploration and geological and geophysical data, which will significantly increase the reliability coefficient of forecasting ore deposits, are indicated.

Structural and Morphological Features of the Participation of Microorganisms in the Formation of Nb–REE–Rich Ores of the Tomtor Field (Russia)

Data indicating the important role of microorganisms in the redistribution of REEs in the weathering crust and the decisive role in the concentration of REEs during the formation of ores in the upper ore horizon of the Tomtor field are obtained. The uptake of REEs was carried out by the community of microorganisms, such as phototrophs, methanogens, methanotrophs, and proteobacteria, which form the basis of the microbiocenosis for this paleoecosystem. The isotopic composition of C carbonates in all samples studied with fossilized microorganisms corresponds to the biogenic one, and the isotopic composition δ18ОSMOW (from 7 to 20‰) indicates the endogenous (hydrothermal) and, to a lesser extent, exogenous nature of the solutions. The low (87Sr/86Sr)I values of carbonates (~0.7036–0.7042) exclude the participation of seawater.

Ultrapotassic Rock from the Tomtor Complex of Ultrabasic Alkaline Rocks and Carbonatites (Arctic Siberia)

Among the rich REE–Nb ores of the upper ore horizon of the Tomtor deposit, under the Jurassic deposits (borehole 324, interval 47–62 m), ultrapotassiс rocks (feldspar porphyrite), consisting of more than 90% potassium feldspar (K-feldspar), are found. The rocks contain titanium–niobium oxide and rare grains of fersmite, Ce-bastnaesite, Ce-monazite, calcite, barite, and strontianite. K-feldspar is unevenly replaced by muscovite. By the type of REE and element distribution on the multidiagrams, the rock correlates well with the rich REE–Nb ores from the Burannyi site at the lower levels of contents. The Ar–Ar age of K-feldspar corresponds to 323.4 Ma and is correlated with the geological time of the formation of the rich REE–Nb ores from the Tomtor deposit, as well as the time of occurrence of magmatic and hydrothermal activity in this part of the region during this period.

Pre‐Klondikean oxidation prepared the ground for Broken Hill‐type mineralization in South Africa

AbstractNew Cu isotope data obtained on chalcopyrite from the Black Mountain and the Broken Hill deposits in the medium‐ to high‐grade metamorphic Aggeneys‐Gamsberg ore district (South Africa) require a revision of our understanding of the genesis of metamorphic Broken Hill‐type massive sulphide deposits. Chalcopyrite from both deposits revealed unusually wide ranges in δ65Cu (−2.41 to 2.84‰ NIST 976 standard) in combination with distinctly positive mean values (0.27 and 0.94‰, respectively). This is interpreted to reflect derivation from various silicate and oxide precursor minerals in which Cu occurred in higher oxidation states. Together with the observation of a typical supergene base metal distribution within the deposits and their spatial association with an unconformity only meters above the ore horizon, our new data are best explained by supergene oxidation of originally possibly SEDEX deposits prior to metamorphic sulphide formation, between the Okiepian (1,210–1,180 Ma) and Klondikean (1,040–1,020 Ma) orogenic events.

Major, trace and rare earth element (REE) geochemistry of the Oligocene stratiform manganese oxide-hydroxide deposits in the Nikopol, Ukraine

The Nikopol manganese deposit is situated in the west and north of Nikopol, Ukraine and one of the world's largest deposits among the sedimentary manganese deposits. The Nikopol Oligocene basin consists of two structural stages, Precambrian crystalline basement rocks and sedimentary cover rocks (Cretaceous to Quaternary). Nikopol Ore horizon is traced in a thickness varying from several cm to 4.5 m and a single stratum from the west to the eastwards to about 250 km. Nikopol manganese ore horizon was separated to three different units; carbonate ore, mixed carbonate-oxide ore, and oxide ore. The main minerals of supergene oxidation zone in Nikopol manganese deposits are: manganite, pyrolusite, todorokite, cryptomelane, birnessite, rancieite. The manganese oxide-hydroxide ores were analyzed for major oxides using ICP-AES and, trace and rare earth elements (REE) using ICP-MS. The mean major oxide contents of the manganese ores are 40.7 wt% MnO, 4.1 wt% Fe2O3, 15.6 wt% SiO2, 4.17 wt% for Al2O3, and 7.1 wt% for CaO. The average Mn/Fe, Co/Ni, Co/Zn, and Zr/Hf ratios in the Nikopol manganese deposit are 25, 0.54, 1.15, and57, respectively. The total REE contents are between 60 and 197 ppm, with a mean of 108 ppm. The PAAS-normalized REE patterns of the Nikopol manganese oxide ores have similar trends and show MREE (middle rare earth element) and HREEs (heavy rare earth element) enrichments. The Ce/Ce* values of manganese oxide-hydroxide ores collected from the study area vary from 0.88 to 1.43, with the mean of 1.16. Most of manganese samples have positive Ce anomalies except for a few samples, indicating that ore-forming rocks were primarily marine chemical or biogenic deposit. The Eu/ Eu* anomalies of the manganese oxide-hydroxide ores are close to 1 with a range of 0.68 to 1.12 (mean: 0.96). The Y concentrations vary from 9.1 to 47.1 ppm, with an average of 18.2 ppm and show negative Y anomalies, except for three samples. The Pb isotope ratios are 19.0 for 206Pb/204Pb, 15.7 for 207Pb/204Pb and 39.4 for 208Pb/204Pb isotopes. All these data indicate that the Nikopol manganese oxide-hydroxide ores formed rapidly within oxic/suboxic seawater as reflected by Ce anomalies close to 1 in compared to similar environments, low-oxygen fugacity in source of the hydrothermal fluids, volcanogenic input or hydrothermal contributions to seawater, and high Co, Ni, Cu, Sr, Ba and As and low V, Rb, Nb, Hf, U, Th, Pb and Zn values. Also, our results point out that the metals were transported from both a hydrothermal source in deeper water (chemical discrimination diagrams) and terrestrial sources (Pb isotopic data). The metals formed to the Mn-oxide ores syngenetically likely came from water depths deeper than the area where the carbonate-ore deposits formed.

Facies architecture and palaeoenvironmental constraints of subaqueous felsic volcanism in the Iberian Pyrite Belt: The Paymogo Volcano-Sedimentary Alignment

The Paymogo Volcano-Sedimentary Alignment (PVSA) is an essentially igneous sequence associated with VHMS deposits of the Iberian Pyrite Belt. Unlike other areas of this mining district, where mafic igneous rocks are abundant, in the PVSA these are essentially felsic. Two facies associations have been distinguished based on petrographic and chemical features: a lower dacite facies association and an upper rhyolite facies association. The main ore horizons appear related to volcanic breccias of the rhyolite facies association. A detailed petrographic study of these breccias suggests they are resedimented hyaloclastite deposits. The analysis of volcanic facies indicates that the volcanism developed at water depths deeper than 500 m, consistent with the minimum depth at which VHMS deposits can form. We conclude that detailed volcanological studies provide effective constraints to palaeoenvironmental interpretations, which are relevant in assessing regional stratigraphy and area selection for VHMS exploration.

Supergene nonsulfide Zn–Pb mineralization in the Mehdiabad world-class sub-seafloor replacement SEDEX-type deposit, Iran

The world-class Mehdiabad nonsulfide deposit consists of several accumulations of Zn–Pb–Fe nonsulfide minerals, derived from supergene of a vent-proximal sub-seafloor replacement SEDEX-type deposit hosted in lower Cretaceous clastic–carbonate rocks of the Taft and Abkuh Formations. The main sulfides are galena, sphalerite, pyrite, and minor amounts of chalcopyrite, which have been partially or completely transformed into nonsulfide minerals by supergene processes. Currently, the Zn mineralization in the Abkuh Formation (ore horizon II) is almost fully oxidized: the Mehdiabad deposit which can be assigned to both direct replacement and wall-rock replacement types. The time of the formation of the nonsulfide deposit has not been determined, but it is probable that the main supergene metallogenesis happened during Tertiary, possibly between post-Cretaceous to present. The mineralogy of the nonsulfide mineralization is generally complex and consists of smithsonite, hydrozincite, and hemimorphite as the main economic minerals, accompanied by cerussite, anglesite, iron–manganese oxy-hydroxides, sauconite, and Zn-rich clays. Commonly, nonsulfide minerals in this deposit consist of two types of ore: red zinc ore (RZO), rich in Zn, Fe, Pb-(As), and white zinc ore (WZO), typically with very high zinc grades but low concentrations of iron and lead. The proposed three-step scenario for the deposition of nonsulfide mineralization in the Mehdiabad deposit comprises: (1) the deposition of primary sulfides contemporaneous with lower Cretaceous clastic–carbonate host rocks to form SEDEX-type mineralization; (2) during the Late Cretaceous to present, regional uplift leads to deformation, folding, and thrusting of sulfide-bearing lower Cretaceous sequence; and (3) as a result, Zn–Pb–Fe sulfides hosted in carbonates experienced oxidation under an arid-warm climate to form the supergene nonsulfide ore mineralizations.

Geochemical Features and Mineral Associations of Differentiated Rocks of the Norilsk 1 Intrusion

The purpose of this study is to show the patterns of distribution of disseminated sulfide in layered rocks based on the numerous geochemical and mineralogical data obtained for eight boreholes of the Norilsk intrusion (southern part of the Norilsk 1 deposit). There is a common trend of sulfide liquid fractionation in the Main Ore Horizon, which is composed of picritic and taxite (or olivine) gabbro-dolerites: the Ni/Cu in both rock types decreases down all sections, indicating an increase in the degree of fractionation of the sulfide liquid from top to bottom. On the contrary, the Ni/Fe ratios in pentlandite increase in this direction due to an increase in sulfur fugacity. However, picrite and taxite/olivine gabbro-dolerites are very distinctly separated by Ni/Cu values: these values are >1 in picritic gabbro-dolerite while they are always <1 in taxite/olivine gabbro-dolerite. These rock types are distinguished by sulfide assemblages. The first includes troilite, Fe-rich pentlandite, chalcopyrite, cubanite, talnahite, bornite and copper (low sulfur association); the second one is composed of monoclinic pyrrhotite, chalcopyrite, Ni-rich pentlandite and pyrite (high sulfur association). A two-stage magma injection with different ore specializations is supposed for picritic and taxite/olivine gabbro-dolerites.

New insights from legacy seismic data: reprocessing of legacy 2D seismic data for imaging of iron‐oxide mineralization near Sishen Mine, South Africa

ABSTRACTTwo overlapping legacy seismic profiles, 130 km long end to end, were shot in the 1990s over the Kuruman Hills on the western margin of the Kaapvaal Craton in southern Africa. The 6‐s profiles were aimed at investigating the crustal structure of the western Kaapvaal Craton as well as to locate potential continuation of the Witwatersrand gold‐bearing horizons beneath the cover rocks, the latter of which was unsuccessful. In this study, the legacy seismic data are reprocessed and used to image the iron‐oxide (mainly haematite) mineralization found in the Kuruman Formation of the Griqualand‐West Supergroup, which outcrops along the two seismic profiles. The seismic profiles are located close to the Sishen open pit iron mine, where one of the world's largest iron ore concentrations (986 Mt) is mined. The reprocessed and merged seismic data are combined with magnetic, magnetotelluric, borehole and outcrop data to constrain the interpretation, and all indicate the mineralization host rocks to have ∼500 m thickness and 950 m depth. The seismic data further reveal seismic reflections associated with multiple iron ore horizons, which are affected by a first‐order scale syncline and numerous near‐vertically dipping (∼65–80°) normal and reverse faults of various orientations and throws, thus providing insight into the structurally controlled iron ore mineralization in the area. Seismic tomography and magnetotellurics characterize the sediments to have a velocity ranging between 5000 and 6000 m/s and a resistivity of <10 Ωm. The seismic imaging of the syncline and associated structural disruptions is important for future mining purposes and plans in the area as these structures might have preserved iron‐oxide mineralization from erosion. The reprocessed data thus provide information that could be incorporated in potential future underground mine planning in the area, improving the resource evaluation of the iron‐oxide deposit. Legacy seismic data are thus shown to hold intrinsic quality and possible untapped potential that can be realized via data reprocessing.

Geochemistry of Famennian to Visean Metapelites from the Iberian Pyrite Belt: Implications for Provenance, Paleo-Redox Conditions and Vectoring to Massive Sulfide Deposits

Massive sulfide deposits of the Iberian Pyrite Belt (IPB) document the progression of ore-forming processes in the SW Iberian Variscides, throughout uppermost Devonian to Early Carboniferous. Sulfide ores are usually hosted in volcanic-dominated or shale-dominated successions belonging to the Volcano-Sedimentary Complex (VSC; Late Famennian to Late Visean). Yet, several important orebodies at Neves-Corvo are sitting within upper sections (Famennian) of the early deposited Phyllite–Quartzite Group (PQG). Four key sectors of the Portuguese segment of IPB were revisited and 262 samples collected to characterize the geochemical features of metapelites from PQG and VSC sequences in barren and fertile settings. The selected sectors include active (Neves-Corvo and Aljustrel) and old (Lousal) mines, besides promising brownfield (Gaviao) and greenfield (Sesmarias) prospects, and other lithostratigraphic sections without known mineralization. The composition of PQG and VSC metapelites is similar and mainly controlled by a mixture of clayey-derived and quartz sandy-derived components largely resulting from different sources of granitic to granodiorite/quartz diorite composition. A third component of local volcaniclastic origin could be significant in many samples of upper VSC sections. Mineral transformations during diagenesis and/or hydrothermal alteration/mineralization partially overprinted the primary composition. During sedimentation/diagenesis, prevalent redox conditions were mostly confined to oxic environments, at places transiting to sub-oxic. Subsequent multistage interaction with reducing hydrothermal fluids (to which the sulfide mineralization is related) generated heterogeneous anoxic signs. Consistent increases in Fe2O3/TiO2, (Cu + Zn + Pb)/Sc and (As + Sb)/Sc ratios to values above 10 represent a valuable footprint toward ore horizons.

The Ab-Bagh Late Jurassic-Early Cretaceous sediment-hosted Zn-Pb deposit, Sanandaj-Sirjan zone of Iran: Ore geology, fluid inclusions and (S–Sr) isotopes

The Ab-Bagh clastic- and carbonate-hosted stratabound and stratiform Zn-Pb deposit in the Sanandaj-Sirjan zone, in the southeastern corner of the Malayer-Esfahan metallogenic belt of Iran, is typical of several sedimentary–exhalative (SEDEX) deposits in this metallogenic belt. The deposit is hosted in a late Jurassic-early Cretaceous sedimentary sequence. Based on position, there are two Zn-Pb-bearing stratigraphic ore horizons. Ore horizon 1 is hosted by late Jurassic-early Cretaceous black shale and siltstone. The wedge-shaped ore body is located close to a synsedimentary normal fault. Petrographic studies indicates that mineralization comprises three sulfide ore facies: stockwork, bedded and massive ore facies. Ore horizon 2 occurs in early Cretaceous carbonates and comprises a massive-replacement ore facies that is concordant with host rock layering; it is also underlain by a stockwork facies. Textures include framboidal, laminated, breccia, replacement, massive and vein-veinlet. Hypogene sulfide minerals are predominantly sphalerite, galena, pyrite and chalcopyrite. Calcite and quartz are major gangue minerals. Silicification and carbonatization are the main wallrock alteration styles; alteration intensity increases towards the feeder zone. Based on relationships between ore minerals and rock-forming minerals, sulfide mineralization in the Ab-Bagh deposit formed during two main stages: fine-grained sulfide bands (stage I) are intricately interlayered with organic matter-rich beds of black shale and siltstone. Sulfide bands exhibit classic sedimentary textures, such as laminations and bedding, indicative of a synsedimentary origin. Coarser-grained stage II base metal sulfides show breccia and vein-veinlet textures, and are considered to have formed by replacement during sub-seafloor fluid flow. Fluid inclusion microthermometry was carried out on calcite associated with sulfides of the stockwork ore facies. Homogenization temperatures are in the range of 180–301 °C, and the average salinity is 17.93 wt% NaCl eq. The size distribution of pyrite framboids of the bedded ore, geochemistry of host rock and presence of organic matter, collectively suggest anoxic to locally suboxic ambient conditions for the host basin. The δ34S composition of pyrite, sphalerite, and galena ranges from −5.08 to +4.51‰. The highest δ34S values are from massive and stockwork ore facies (+2.66 to + 4.51‰) and the lowest from bedded ore (−5.08 to −4.72‰). In the bedded ore facies, the main sulfur source was bacterially reduced seawater sulfate. Whereas in massive and stockwork ore facies, thermochemically reduced seawater sulfate was the main source of sulfur. The 87Sr/86Sr ratios of calcite indicate that origin of ore forming fluids is Paleozoic radiogenic basement. Collectively, the isotopic, sedimentological, textural, mineralogical, fluid inclusion, and geochemical data for the Ab-Bagh deposit support a vent-proximal sub-seafloor replacement SEDEX ore deposit.

ORE POTENTIAL OF THE ARAMINLAMPI MASSIF. THE QUESTION OF EXTENDING THE KOTALAHTI NICKEL BELT TO THE LAKE LADOGA AREA (PRILADOZHYE)

The Araminlampi massif is part of the Kaalamo clinopyroxenite-gabbro-norite-diorite complex dated at 1888.3 ± 5.2 Ma. This age is older than the ore-bearing intrusive units in the Kotalahti belt (1875–1885 Ma). Geodynamic reconstructions show that the Kotalahti belt is not traced into the Kaalamo intrusion area in the Russian territory, but it seems to extend only to the south of the Meieri thrust zone. The predominant petrotypes of Kaalamo rocks are diorite, gabbrodiorite and the less common metaclinopyroxenite; the Kotalahti intrusive units are dominated by peridotite, harzburgite, wehrlite, lherzolite, websterite, norite and gabbronorite. The distribution spectra of rare-earth elements and the Ce/Yb and Th/Yb ratios of Araminlampi rocks are similar to those of the ore-free massif differentiates in the Kotalahti belt. Olivin in Kaalamo ultramafics contains more iron (58–70 % forsterite) than Kotalahti (63–85 % forsterite; 78–85 % in ore-bearing rocks). The major ore minerals in the Kotalahti deposits are pyrrhotite, pentlandite and chalcopyrite, and the major ore mineral in the Araminlampi massif is pyrrhotite. In this structurally differentiated massif (olivine clinopyroxenite, clinopyroxenite and gabbro) pyrrhotite dissemination with minor quantities of other sulphides (chalcopyrite, pyrite, pentlandite, etc.) forms a ~ 10 m thick ore horizon with 0.1–0.2 % Cu, 0.03–0.1 % Ni, ΣPGM, 0.2–0.5 g/t Au and 70–90 g/t Sc in the olivine pyroxenite-pyroxenite transition zone. The 20–30 cm thick quartz veins cross-cutting the metapyroxenites near the ore horizon display abundant silver-bismuth mineralization (Ag – up to 550 g/t, Au – 0.2 g/t and Bi – 0.1 %). Palladium bismuthotellurids that predominate in the PGM-mineral association of the ore horizon were crystallized at a temperature (< 500 °C) below a typical sulphide solidus, and part of merenskyite phases under subsolidus conditions (~ 700 °С). All the rocks of the massif are highly amphibolized and cut by pegmatite veins. Primary minerals, such as diopside, olivine, chrome-spinels and bytownite, occur only as relics. Olivine, together with serpentinization, is altered to amphibole-magnetite symplectites. The Araminlampi ore occurrence displays signs of both late- and post-magmatic genesis, and the formation of molybdenite, scheelite, sphalerite, uraninite and uranothorianite, together with palladium bismuthotellurids at the final mineral formation stage suggests the involvement of fluids associated with the granites and pegmatites that cross-cut the massif.

How to form a giant epithermal precious metal deposit: Relationships between fluid flow rate, metal concentration of ore-forming fluids, duration of the ore-forming process, and ore grade and tonnage

Economic metal deposits associated with hydrothermal systems represent the end product of a series of complex processes the are related to the amount of ore fluid that flows through the system, the concentration of metal in the ore fluid, the depositional efficiency, and the duration of the ore-forming process. These factors, in turn, largely control the size (tonnage) and grade of the deposit that forms. Of these factor, the duration of the mineralizing event is one of the least understood aspects of ore genesis. We have used ore tonnage (produced + reserves) and grade information from a large number of epithermal precious metal (Au ± Ag) occurrences, combined with simplifying assumptions related to the metal solubilities and precipitation efficiency, to estimate the amount of fluid required to deposit the amount of the metal observed in the deposit. Total gold tonnage for 279 epithermal veins, prospects and deposits ranges from 0.0002 T (6 troy oz; Wharekirau-ponga, New Zealand) to 1089 T (35 million troy oz) in the giant Pueblo Viejo deposit, Dominican Republic, with a median value of 5.93 T Au (191,000 troy oz). Total silver tonnage for 246 epithermal deposits ranges from 0.0004 T (12 troy oz; Wharekirau-ponga, New Zealand) to 62,207 T (2.0 billion troy oz; Cerro Rico de Potosi, Bolivia), with a median value of 40.8 T (1.31 million troy oz) Ag. The median gold grade, based on 246 deposits, is 5.29 g/T, and the median silver grade for 216 deposits is 93.3 g/T. The reported metal content and grade data suggest that about ~1012 kg of fluid is required to transport and deposit the amount of Au or Ag observed in a median size deposit, and that the largest deposits may require 1014 to 1015 kg of metal-transporting fluid. The median reported flow rate compiled for 693 modern continental geothermal systems, representing modern analogs of the ore-forming environment for the epithermal deposits, is about 5.78 kg/sec with the highest reported flow rate of ~3000 kg/sec. Based on the median flow rate, the time required to form a median-sized epithermal Au deposit is ~12,900 years, and that required to form a median sized Ag deposit is ~7800 years. The largest “giant” deposits could have formed in ~105 years assuming the median flow rate, or in ~104 years at the maximum reported fluid flow rate of 3000 kg/sec.Results of this assessment of the relationship between precious metal grades and tonnage, combined with reasonable fluid flow rates and metal contents of the ore-forming solutions, indicate that the active mineralization event need not be longer than ~104 years to form a median-sized deposit, assuming continuous deposition. Field and laboratory data suggest, however, that mineralization in epithermal deposits is episodic rather than continuous and is associated with discrete pulses, perhaps triggered by hydrothermal eruption events that lead to fluid immiscibility and the concomitant quantitative precipitation of all metal in solution. As such, gold and/or silver deposition likely occurred over only a small fraction of the total lifetime of the hydrothermal system, as evidenced by discrete bands of high-grade ore separated by barren or lower grade bands within mineralized quartz veins. Results of this study suggest that giant deposits do not necessarily require ore-forming fluids with exceptionally high metal contents, but rather may simply represent hydrothermal systems that were active for longer periods of time or were characterized by higher fluid flow rates, or both, resulting in greater amounts of metal-bearing fluid flowing through the ore horizon.

An investigation into the 87Sr/86Sr radiogenic isotope geochemistry of the manganese ore of the Kalahari Manganese Field with a view on hydrothermal fluid flow and related rare earth element enrichments

Abstract The Paleoproterozoic Kalahari Manganese Field is the largest known land-based manganese (Mn) deposit on Earth and comprises five erosional relics of the iron- and manganese-rich Hotazel Formation. A total of 19 manganese ore samples from the lower manganese ore horizon of the Hotazel Formation were selected for analysis based on their relative metasomatic alteration states. These samples range from primary diagenetic, classic supergene enriched, hydrothermally enriched (Wessels-type ore) and thrusted manganese ore. When normalised to Post-Archaean Australian shale composites, rare earth elemental analyses on the selected samples indicate significant relative enrichments within the thrusted manganese ore, an ore type that hasn’t been studied from a geochemical point of view, so far. A comparison between these respective enrichments and 87Sr/86Sr ratios indicates a distinct link between REE enrichments and associated alteration events, and a progressive increase in 87Sr/86Sr isotope values. An isotopic relationship between fluid infiltration events and REE redistribution within the Hotazel Formation has been established, highlighting unique isotope signatures necessary in better defining and characterising metasomatism within the Paleoproterozoic Kalahari Manganese Field.

Ore mineralization and genesis of the Yuzhka gold–limonite occurrence, Elmus area, Karelia

Yuzhka gold-limonite occurrence is the first one to have been found in the linear-type brecciation and oxidation zone in the Archean greenstone rock oxidation zone, Elmus area, Central Karelia. The oxidation zone formed along the broken quartz-sericite-chlorite schist, which contains a pyrite ore horizon and broken quartz veins. The mineral limonite mixture was studied using Raman, thermal, X-ray, chemical, ICP-MS and microprobe analyses. The upper portion of the oxidation zone consists primarily of unconsolidated yellow-brown ochre, sintered packages and gossan (consolidated rocks). Iron oxides and hydroxides in the upper unconsolidated portion of the limonite zone were found to consist of goethite (hydrogoethite) and hematite. Pyrite ores of chalcopyrite-pyrite composition occur in lower horizons; they are broken and replaced by either hematite or both hematite and goethite. Broken quartz veins in the oxidized and ferruginized schist sequence form breccia with angular fragments of varied size, which contain scarce sulphide (pyrite, chalcopyrite, galena), sulphosalt and sulphoarsenide inclusions and are cemented by goethite and hematite. Persisting non-oxidized sulphosalts and sulphoarsenides in the ores and broken veins are composed of semseyite, tetrahedrite, jamsonite, plagionite, cobaltite and gersdorffite; melonite is occasionally encountered. The gold is fine-textured and high-grade (Ag 1–10 %). It is intergrown with goethite near the oxidized pyrite ore horizon; its concentration is 1–2.6 g/t. The geochemistry of the limonite zone (As, Sb, Pb, Ni, Cu, Co) reflects the composition of primary pyrite ores and quartz vein associations in the Elmus area, suggesting that these oxidized rocks were the source of the gold.

Association of platinum-group elements with chromitite within the Merensky reef, Western Limb, Bushveld Complex: Results of a high resolution mineralogical and geochemical study

We present the results of a high-resolution mineralogical and geochemical study conducted across three intersections of Merensky reef at Impala Platinum Mines on the Western Limb of the Bushveld Complex. We studied two intersections of non-pegmatoidal Merensky reef and one intersection of pegmatoidal Merensky reef in an attempt to reconstruct the petrogenesis of this economically important ore horizon and in order to establish the relationship between non-pegmatoidal and pegmatoidal reef intersections. Our results indicate a much stronger correlation between PGE and Cr than between PGE and S (or Cu as a proxy for sulfides), which favours a model in which chromite and associated PGEs were introduced from a deeper staging chamber to result in the formation of the chromitite layers of the Merensky reef. The geochemistry of chromite in the Merensky reef is suggestive of the fact that the bottom chromitite layer of the pegmatoidal reef is a lateral correlate of the chromitite of the non-pegmatoidal reef, in which only a single chromitite layer occurs. We propose that the Merensky reef at Impala Platinum Mines is a product of the introduction of chromite-laden basal flows into the Bushveld Complex magma chamber and that the relationship between pegmatoidal and non-pegmatoidal reef intersections is related to non-deposition, rather than erosion, as was shown to be the case at other localities within the Western Limb.

Quantification of fluid-mobile elements in white mica by LA-ICP-MS: From chemical composition to a potential micro-chemical vectoring tool in VMS exploration

The chemical variation of white mica, in particular fluid-mobile elements typically associated with base-metal mineralization, in the host rock alteration haloes enveloping massive sulfide deposits of the Bathurst Mining Camp is constrained by LA-ICP-MS. White mica from examined deposits hosts Tl (up to 744 ppm), Sn (up to 1069 ppm), Hg (up to 524 ppm), Sb (up to 3650 ppm), As (up to 17100 ppm), and In (up to 524 ppm). The occurrence of Tl, Sn, Sb, and probably As is predominantly controlled by crystal-chemical substitution, although the occurrence of micro- to nano-scale inclusions of other phases cannot be disqualified. White mica immediately adjacent (within 50 m) to the massive sulfides in both the hanging wall and footwall is enriched in Tl, Sb, Hg, and variably enriched in As, Sn, In, Se, Bi, and Cd and can be used to vector toward mineralization. In distal zones up to several hundred meters distant, the variation of fluid-mobile elements in white mica is recognized in lower concentrations, but detectable up to several hundred meters away from the ore horizons. Here, variation in the ∑ Tl + Sb + Sn + Hg can serve as a vectoring tool. This methodology can potentially serve as a complementary tool to other geochemical and geophysical exploration methods in prospecting for buried volcanogenic massive sulfide mineralization.

Integrated 3D Geological Modeling to Gain Insight in the Effects of Hydrothermal Alteration on Post-Ore Deformation Style and Strain Localization in the Flin Flon Volcanogenic Massive Sulfide Ore System

3D geological modeling of lithogeochemical and geological data provides insight into the role of the sulfide ore horizon and associated footwall hydrothermal alteration in localizing shear strain in the Flin Flon volcanogenic massive sulfide deposits, Canada, as deformation evolved from brittle-ductile to ductile regimes during collisional stages of the 1.9–1.8 Ga Trans-Hudson orogeny. 3D spatial characterization of hydrothermal alteration based on the Ishikawa index (AI) and normative corundum percentages outline sericite + chlorite-rich high strain zones, consisting of Al-enriched and Na-depleted felsic and mafic volcanic rocks in the footwall of the sulfide ore horizon. The hydrothermal vent complex, from which these sheared alteration zones originated, was stacked together with the ore horizon by W-vergent thrust faults during an early collisional deformation regime, imbricating molasse-type clastic sediments with the ore-hosting volcanic and volcaniclastic rocks of the Flin Flon arc assemblage. Chlorite-rich planar zones marked by high values of the Carbonate–chlorite–pyrite index (CCPI) are laterally more extensive and outline a later system of ductile shear zones, in which phyllosilicates, quartz and chalcopyrite in stringer zones localized shear strain and enhanced transposition of the hydrothermal vent stockwork. The contrasting deformation styles of these two thrusting events and their localization within the ore horizon and hydrothermal vent stockwork have important implications for vectoring towards undiscovered ore in this mature mining camp that are possibly also relevant to other strongly deformed VMS ore systems.