- Abstract
- Affiliation
- All
- Authors
- Book Series
- DOI
- EISBN
- EISSN
- Full Text
- GeoRef ID
- ISBN
- ISSN
- Issue
- Keyword (GeoRef Descriptor)
- Meeting Information
- Report #
- Title
- Volume
- Abstract
- Affiliation
- All
- Authors
- Book Series
- DOI
- EISBN
- EISSN
- Full Text
- GeoRef ID
- ISBN
- ISSN
- Issue
- Keyword (GeoRef Descriptor)
- Meeting Information
- Report #
- Title
- Volume
- Abstract
- Affiliation
- All
- Authors
- Book Series
- DOI
- EISBN
- EISSN
- Full Text
- GeoRef ID
- ISBN
- ISSN
- Issue
- Keyword (GeoRef Descriptor)
- Meeting Information
- Report #
- Title
- Volume
- Abstract
- Affiliation
- All
- Authors
- Book Series
- DOI
- EISBN
- EISSN
- Full Text
- GeoRef ID
- ISBN
- ISSN
- Issue
- Keyword (GeoRef Descriptor)
- Meeting Information
- Report #
- Title
- Volume
- Abstract
- Affiliation
- All
- Authors
- Book Series
- DOI
- EISBN
- EISSN
- Full Text
- GeoRef ID
- ISBN
- ISSN
- Issue
- Keyword (GeoRef Descriptor)
- Meeting Information
- Report #
- Title
- Volume
- Abstract
- Affiliation
- All
- Authors
- Book Series
- DOI
- EISBN
- EISSN
- Full Text
- GeoRef ID
- ISBN
- ISSN
- Issue
- Keyword (GeoRef Descriptor)
- Meeting Information
- Report #
- Title
- Volume
NARROW
GeoRef Subject
-
all geography including DSDP/ODP Sites and Legs
-
Africa
-
Central Africa
-
Congo (1)
-
-
East Africa
-
Zambia (2)
-
-
Madagascar (1)
-
Southern Africa
-
Botswana (1)
-
South Africa
-
Bushveld Complex (2)
-
-
-
-
Asia
-
Far East
-
Indonesia (1)
-
-
-
Atlantic Ocean
-
North Atlantic
-
North Sea
-
Viking Graben (1)
-
-
-
-
Australasia
-
Australia
-
Western Australia (1)
-
-
New Zealand
-
Coromandel Peninsula (9)
-
Taupo volcanic zone (1)
-
Waihi New Zealand (1)
-
-
-
Copperbelt (1)
-
Europe
-
Western Europe
-
Ireland
-
Meath Ireland (1)
-
-
-
-
Indian Ocean Islands
-
Madagascar (1)
-
-
Mexico (1)
-
North America
-
Keweenawan Rift (1)
-
North American Cordillera (1)
-
-
North Island (9)
-
South America
-
Chile (2)
-
Peru (1)
-
-
South Island (1)
-
United States
-
Arizona (1)
-
Coeur d'Alene mining district (3)
-
Colorado Plateau (1)
-
Idaho
-
Kootenai County Idaho (1)
-
-
Michigan
-
Michigan Upper Peninsula
-
Keweenaw Peninsula (1)
-
Ontonagon County Michigan (1)
-
-
-
Montana (3)
-
Nevada
-
Humboldt County Nevada (1)
-
-
New Mexico (1)
-
-
White Pine Mine (2)
-
-
commodities
-
bitumens (1)
-
metal ores
-
base metals (1)
-
cobalt ores (1)
-
copper ores (12)
-
gold ores (14)
-
iron ores (1)
-
lead ores (5)
-
lead-zinc deposits (1)
-
molybdenum ores (2)
-
nickel ores (1)
-
platinum ores (3)
-
rare earth deposits (1)
-
silver ores (15)
-
uranium ores (1)
-
zinc ores (7)
-
-
mineral deposits, genesis (15)
-
mineral exploration (11)
-
mineral resources (3)
-
petroleum (3)
-
-
elements, isotopes
-
carbon
-
C-13/C-12 (4)
-
organic carbon (1)
-
-
hydrogen
-
D/H (1)
-
-
isotope ratios (8)
-
isotopes
-
stable isotopes
-
C-13/C-12 (4)
-
D/H (1)
-
O-18/O-16 (6)
-
S-34/S-32 (2)
-
Sr-87/Sr-86 (1)
-
-
-
metals
-
alkaline earth metals
-
strontium
-
Sr-87/Sr-86 (1)
-
-
-
platinum group
-
platinum ores (3)
-
-
precious metals (1)
-
-
oxygen
-
O-18/O-16 (6)
-
-
sulfur
-
S-34/S-32 (2)
-
-
-
geochronology methods
-
Ar/Ar (1)
-
Re/Os (1)
-
U/Pb (2)
-
-
geologic age
-
Cenozoic
-
Tertiary
-
Neogene
-
Miocene
-
upper Miocene (3)
-
-
Pliocene (1)
-
-
Paleogene
-
Eocene (1)
-
Paleocene (1)
-
-
-
-
Paleozoic
-
Devonian
-
Middle Devonian (1)
-
Upper Devonian (1)
-
-
Permian
-
Lower Permian (1)
-
-
-
Precambrian
-
Archean (1)
-
Nonesuch Shale (1)
-
upper Precambrian
-
Proterozoic
-
Mesoproterozoic
-
Belt Supergroup (4)
-
Ravalli Group (1)
-
Revett Quartzite (1)
-
-
Neoproterozoic (1)
-
Paleoproterozoic (2)
-
-
-
Waterberg System (1)
-
-
-
igneous rocks
-
igneous rocks
-
kimberlite (1)
-
plutonic rocks
-
granites (1)
-
granodiorites (1)
-
ultramafics
-
peridotites (1)
-
-
-
porphyry (2)
-
volcanic rocks
-
andesites (2)
-
dacites (2)
-
rhyolites (1)
-
-
-
-
metamorphic rocks
-
metamorphic rocks
-
metasedimentary rocks (3)
-
-
-
minerals
-
carbonates
-
calcite (1)
-
-
native elements
-
graphite (1)
-
-
oxides
-
goethite (1)
-
magnetite (3)
-
martite (1)
-
spinel group (1)
-
wustite (1)
-
-
silicates
-
chain silicates
-
pyroxene group
-
orthopyroxene (1)
-
-
-
framework silicates
-
feldspar group
-
alkali feldspar
-
adularia (4)
-
-
-
silica minerals
-
quartz (1)
-
-
-
sheet silicates
-
chlorite group
-
chlorite (1)
-
-
clay minerals
-
smectite (1)
-
-
illite (1)
-
mica group
-
muscovite (1)
-
-
sericite (3)
-
-
-
sulfates
-
anhydrite (1)
-
-
-
Primary terms
-
absolute age (3)
-
Africa
-
Central Africa
-
Congo (1)
-
-
East Africa
-
Zambia (2)
-
-
Madagascar (1)
-
Southern Africa
-
Botswana (1)
-
South Africa
-
Bushveld Complex (2)
-
-
-
-
Asia
-
Far East
-
Indonesia (1)
-
-
-
Atlantic Ocean
-
North Atlantic
-
North Sea
-
Viking Graben (1)
-
-
-
-
Australasia
-
Australia
-
Western Australia (1)
-
-
New Zealand
-
Coromandel Peninsula (9)
-
Taupo volcanic zone (1)
-
Waihi New Zealand (1)
-
-
-
bitumens (1)
-
carbon
-
C-13/C-12 (4)
-
organic carbon (1)
-
-
Cenozoic
-
Tertiary
-
Neogene
-
Miocene
-
upper Miocene (3)
-
-
Pliocene (1)
-
-
Paleogene
-
Eocene (1)
-
Paleocene (1)
-
-
-
-
clay mineralogy (1)
-
diagenesis (1)
-
Europe
-
Western Europe
-
Ireland
-
Meath Ireland (1)
-
-
-
-
faults (7)
-
fractures (1)
-
geochemistry (2)
-
geochronology (2)
-
geophysical methods (5)
-
hydrogen
-
D/H (1)
-
-
igneous rocks
-
kimberlite (1)
-
plutonic rocks
-
granites (1)
-
granodiorites (1)
-
ultramafics
-
peridotites (1)
-
-
-
porphyry (2)
-
volcanic rocks
-
andesites (2)
-
dacites (2)
-
rhyolites (1)
-
-
-
inclusions
-
fluid inclusions (6)
-
-
Indian Ocean Islands
-
Madagascar (1)
-
-
isotopes
-
stable isotopes
-
C-13/C-12 (4)
-
D/H (1)
-
O-18/O-16 (6)
-
S-34/S-32 (2)
-
Sr-87/Sr-86 (1)
-
-
-
lava (1)
-
metal ores
-
base metals (1)
-
cobalt ores (1)
-
copper ores (12)
-
gold ores (14)
-
iron ores (1)
-
lead ores (5)
-
lead-zinc deposits (1)
-
molybdenum ores (2)
-
nickel ores (1)
-
platinum ores (3)
-
rare earth deposits (1)
-
silver ores (15)
-
uranium ores (1)
-
zinc ores (7)
-
-
metals
-
alkaline earth metals
-
strontium
-
Sr-87/Sr-86 (1)
-
-
-
platinum group
-
platinum ores (3)
-
-
precious metals (1)
-
-
metamorphic rocks
-
metasedimentary rocks (3)
-
-
metamorphism (2)
-
metasomatism (10)
-
Mexico (1)
-
mineral deposits, genesis (15)
-
mineral exploration (11)
-
mineral resources (3)
-
mining geology (2)
-
North America
-
Keweenawan Rift (1)
-
North American Cordillera (1)
-
-
oxygen
-
O-18/O-16 (6)
-
-
Paleozoic
-
Devonian
-
Middle Devonian (1)
-
Upper Devonian (1)
-
-
Permian
-
Lower Permian (1)
-
-
-
paragenesis (3)
-
petroleum (3)
-
petrology (1)
-
Precambrian
-
Archean (1)
-
Nonesuch Shale (1)
-
upper Precambrian
-
Proterozoic
-
Mesoproterozoic
-
Belt Supergroup (4)
-
Ravalli Group (1)
-
Revett Quartzite (1)
-
-
Neoproterozoic (1)
-
Paleoproterozoic (2)
-
-
-
Waterberg System (1)
-
-
sedimentary rocks
-
chemically precipitated rocks
-
evaporites (1)
-
iron formations (1)
-
-
-
sedimentary structures
-
planar bedding structures
-
bedding (1)
-
-
-
South America
-
Chile (2)
-
Peru (1)
-
-
sulfur
-
S-34/S-32 (2)
-
-
tectonics (4)
-
United States
-
Arizona (1)
-
Coeur d'Alene mining district (3)
-
Colorado Plateau (1)
-
Idaho
-
Kootenai County Idaho (1)
-
-
Michigan
-
Michigan Upper Peninsula
-
Keweenaw Peninsula (1)
-
Ontonagon County Michigan (1)
-
-
-
Montana (3)
-
Nevada
-
Humboldt County Nevada (1)
-
-
New Mexico (1)
-
-
-
sedimentary rocks
-
sedimentary rocks
-
chemically precipitated rocks
-
evaporites (1)
-
iron formations (1)
-
-
-
volcaniclastics (1)
-
-
sedimentary structures
-
sedimentary structures
-
planar bedding structures
-
bedding (1)
-
-
-
-
sediments
-
volcaniclastics (1)
-
-
soils
-
paleosols (1)
-
Front Matter
Spatial Periodicity in Self-Organized Ore Systems
Abstract The development of more predictive models for the distribution patterns of large ore deposits and districts is critical for future discovery success in mineral exploration. Some studies have suggested that the distribution of orogenic Au and porphyry Cu deposits appears ordered with a periodic spacing in some mineral provinces, but it remains unclear if spatial periodicity is a common feature of diverse mineral systems. We present evidence for spatial periodicity of large mineral deposit clusters along 20 major structural corridors from nine world-class mineral provinces with five ore deposit types (orogenic Au, porphyry Cu, sediment-hosted Cu and Zn-Pb-Ag, and diamondiferous kimberlites). For orogenic Au deposit clusters, spatial periodicity commonly occurs around 30 to 40 km (range 19–50 km) in the Eastern Goldfields (Australia), Abitibi (Canada), and Sierra Foothills (United States) provinces. Periodicity of moderate- to giant-sized sediment-hosted Cu deposit clusters occurs around 27 km in the Central African Copperbelt (Zambia, Democratic Republic of Congo). Large porphyry Cu deposit clusters show periodicity around 65 to 122 km in the American Cordillera (United States, Mexico, Chile). Large shale-hosted Zn-Pb-Ag deposit clusters have a periodicity around 116 km in the Carpentaria province (Australia). Finally, kimberlite clusters have a spatial periodicity around 121 to 237 km in southern and central Africa. We also observed a dual periodicity along some structural corridors, with smaller deposits located at half the spacing of giant deposits. Whereas the mineral provinces studied were selected on the basis that they seemed to show spatial periodicity, many other provinces worldwide do not appear to display spatial periodicity of ore deposits. We link our results to the phenomenon of self-organization, which explains emergence of large-scale spatial (and temporal) order in complex systems as an effective mechanism to dissipate large energy gradients. As the best examples of spatial periodicity of ore deposits identified to date are associated with some of the world’s best endowed mineral provinces, it is possible that overall province endowment is linked to both the degree of self-organization and the magnitude of regional energy gradients. Further research is required to identify relevant underlying geologic causes for spatial periodicity. Nevertheless, we provide two case studies suggesting that the intersection of preexisting basement fault sets with at least semiregular spacing may be a common control on spatial periodicity of mineral deposits. Where spatial periodicity of mineral deposits is observed to occur, it can improve the predictive capacity of exploration models and ore discovery rates.
Abstract The critical metals are vital to modern life due to their use in a variety of domestic, green, and military high technology applications but have supplies that are inherently insecure. This study provides an overview of the concept of criticality as applied to the critical metals and outlines key issues around the resources and future supply of these metals. The methods used to quantify the criticality of critical metals have advanced over time, demonstrating that some metals are more strategically important than others, depending on the viewpoint of the organization considering criticality. However, global resources and reserves of a number of critical metals as well as their production statistics remain unclear. Methods exist to quantify the resources of critical metals with reasonable accuracy but these methods rely on information provided by the mining industry, indicating that better reporting practices would improve our knowledge of the global resources and cycling of these key commodities. Criticality can also be addressed in numerous ways, including the analysis of known mine supply chains to enable the economic extraction of critical metal by-products, the determination of the critical metal prospectivity of mining/mineral processing wastes (given a significant amount of critical metals currently deport to waste), increased amounts of recycling intermediates or end-use products containing critical metals, and the discovery of new and economic deposits of the critical metals. However, all of these approaches and the associated policy around them require more information in terms of mineral resource accounting, mineral economics, material flow analysis, mineral processing, as well as increased economic geology knowledge that would enable the making of future discoveries and increase the likelihood of critical metals being extracted as either primary or by-products. Without this information, significant parts of our knowledge base on the supply (and the security of this supply) of the critical metals will remain opaque.
The Power of a Systems Approach to Mineral and Petroleum Exploration in Sedimentary Basins
Abstract Petroleum systems (conventional and unconventional) and hydrothermal sedimentary rock-hosted copper, lead-zinc (clastic-dominated and Mississippi Valley-type), and uranium systems can be described in a common system framework comprising the critical processes of (1) establishing the fertility of source(s) of the commodity of interest and the transporting fluid, (2) geodynamic triggers for commodity movement and accumulation, (3) establishing an architecture for fluid movement, (4) accumulation by deposition of the commodity, and (5) preservation. To translate these commodity system models to effective exploration targeting models, they must correspond to business decisions. Exploration is an exercise in scale reduction and has a number of natural business decision points that map to scale: Regional-scale targeting—what basin has the potential of hosting a substantial mineral or petroleum system? Play-scale targeting—where within the basin could a number of deposits be clustered? Prospect scale targeting—where is there a deposit of sufficient quality within the play? Marrying the systems to the decision points involves identifying (1) constituent processes relevant at each scale, (2) the geology that can map the evidence of the processes occurring, and (3) the data or interpretative products that are best used as spatial proxies to map the evidence and guide area selection at the appropriate scale. A common change in focus is noted across spatial scales for all commodities: in basin selection, fertility is key, with lesser focus on other aspects of the system; in play analysis within a basin, all elements of the mineral system are fully considered; in prospect delineation the focus shifts toward accumulation and preservation. The similarity in the targeting workflow highlights that similar key data sets, tools, and interpretative products are required to assess each mineral system across scale, albeit looking for different features within those products, dependent upon the system being targeted. There are several key differences between mineral and petroleum systems. First, petroleum systems involve a mass trapping process with the transporting fluid as the commodity, whereas mineral systems involve mass scrubbing processes, with the transporting fluid having low concentrations of the commodity, thus requiring much fluid throughput. Second, petroleum systems require the entire system to remain reduced to maintain high-quality hydrocarbon, whereas most copper, lead-zinc, and uranium systems require the systems to remain oxidized until the site of deposition. Consideration of these commodity systems in the context of the Earth’s evolving atmosphere-hydrosphere-biosphere-lithosphere highlights the power of paleotectonic, paleogeographic, and paleoenvironmental reconstructions in the critical step of basin selection. Such consideration also highlights common gaps in understanding the commodity systems. These knowledge gaps constitute high-value research paths that would provide greatest leverage in area selection at the basin and play scales. These include improved knowledge of paleogeographic and paleoenvironmental reconstructions, basin hydrodynamics, and timelines of mass and energy flow through basins. For metal systems, better understanding is required of how metal extraction efficiency, solubility, mineral precipitation, permeability, and pressure and temperature gradients dynamically interact along flow paths during the evolution of basins.
Assessing and Mitigating Uncertainty in Three-Dimensional Geologic Models in Contrasting Geologic Scenarios
Abstract The management of uncertainty in three-dimensional (3D) geologic models has been addressed by researchers across a range of use cases including petroleum and minerals exploration and resource characterization, as well as hydrogeologic, geothermal energy, urban geology, and natural hazard studies. Characterizing uncertainty is a key step toward informed decision-making because knowledge of uncertainty allows the targeted improvement of models, is indispensable to risk analysis, improves reproducibility, and encourages experts to explore alternative scenarios. In the minerals sector there is not a unified approach to uncertainty characterization, nor its mitigation. Assessing and mitigating uncertainty in 3D geologic models is a growing field but quite compartmentalized among different subdisciplines within the geosciences. By comparing uncertainty analysis as implemented for three modeling scenarios: basins, regional hard-rock terranes, and mines; at different stages of their respective workflows, we can better understand what a future “complete” modeling platform could look like as applied to the minerals industry. We analyze uncertainty characterization during the different steps in building 3D models as a generic workflow that consists of (1) geologic and geophysical data acquisition followed by processing and inversion of geophysical data, (2) the interpretation of a number of discrete domains boundaries defined by stratigraphic and structural surfaces, (3) homogeneous or spatially variable properties infilling within each domain, and finally (4) use of the models for downstream predictions based on these properties, such as resulting gravity field, gold grade distribution, fluid flow, or economic potential. Although regional- and mine-scale modelers have much to learn from the basin modeling community in terms of managing uncertainty at different stages of the 3D geologic modeling workflow, perhaps the most important lesson is the need to track uncertainty throughout the entirety of the workflow. At present in the minerals sector, uncertainties have a tendency to be recognized within discrete stages of the workflow but are then forgotten, so that at each stage a “best guess” model is provided for further analysis, and all memory of earlier ambiguity is erased.
Abstract Times of metal-rich brine discharge into ancient ocean basins, associated with the formation of sedimentary-exhalative (sedex) Zn-Pb-Ba ore deposits, coincided with short-duration positive excursions (“spikes”) in the global marine Sr isotope record. While these spikes are unexplained by conventional oceanic models, chronostratigraphic correlations, combined with mass balance evidence and oceanographic modeling, suggest that the flux of radiogenic Sr from sedex brines during ore formation is sufficient to explain these previously enigmatic 87 Sr/ 86 Sr spikes. We review existing 87 Sr/ 86 Sr data and present new data as verification of these global 87 Sr/ 86 Sr spikes and their correlations with the formation of giant sedex ore deposits. Major events include an 1 ×10 −4 (~0.7078–~0.7079) excursion contemporaneous with formation of the Rammelsberg deposit at ~389 Ma; spikes on the order of 1 to 3 × 10 −4 , coeval with formation of the Meggen deposit at ~381 Ma, several ore deposits in the Macmillan Pass district at ~379 to 375 Ma, and the Silvermines deposits at ~352 Ma; and two >6 × 10 −4 spikes coincident with formation of the giant Navan deposit at ~346 Ma and Red Dog deposits at ~337 Ma. Moreover, the timing of peak 87 Sr/ 86 Sr spikes correlates with global δ 13 C and δ 18 O spikes, deposition of metal-rich black shales and ironstones, metal-induced malformation (teratology) of marine organisms, and mass extinctions. The relationships among these features were poorly understood, but our new model explains how the flux of key biolimiting nutrients and metals contained in sedex brines, demonstrably equivalent to or exceeding that of the total modern riverine flux to the ocean, spurred ocean eutrophication, which, ultimately, through a series of positive feedback mechanisms, may have triggered global chemical and biological events. If, as we hypothesize, sedex hydrothermal systems are recorded in the global marine isotopic, geologic, and biological records, our findings define a new approach to the study of and exploration for sedex deposits. We demonstrate that fluid inclusion solute chemistry and isotopic and stratigraphic studies of sedex deposits, coupled with chronostratigraphic correlation and high-resolution 87 Sr/ 86 Sr isotope chemostratigraphy, can be used to answer long-standing questions about geologic processes responsible for formation of these extraordinary deposits. This approach provides evidence for the age, duration, and fluxes of fluids and metals vented into the ocean by these giant hydrothermal systems. Accordingly, the marine 87 Sr/ 86 Sr curve constitutes a global exploration tool that could be applied to assess the mineral potential of sedimentary basins. To illustrate the potential of this tool to identify favorable stratigraphic ages and basins with potential for undiscovered giant sedex deposits, we highlight several spikes, on par with those characteristic of the Red Dog and Navan deposits, which have not been correlated with known metal deposits. Given these strong temporal correlations, mass balance estimates, and results of ocean chemistry modeling, our study suggests that further work is warranted to determine the extent to which periodic venting of hydrothermal basinal brines into the ocean has influenced the evolution of marine chemistry. Ultimately, these global signatures can be applied to the study of and exploration for sedex deposits.
Abstract Iron oxide copper-gold (IOCG) and Kiruna-type iron oxide-apatite (IOA) deposits are commonly spatially and temporally associated with one another, and with coeval magmatism. Here, we use trace element concentrations in magnetite and pyrite, Fe and O stable isotope abundances of magnetite and hematite, H isotopes of magnetite and actinolite, and Re-Os systematics of magnetite from the Los Colorados Kiruna-type IOA deposit in the Chilean iron belt to develop a new genetic model that explains IOCG and IOA deposits as a continuum produced by a combination of igneous and magmatic-hydrothermal processes. The concentrations of [Al + Mn] and [Ti + V] are highest in magnetite cores and decrease systematically from core to rim, consistent with growth of magnetite cores from a silicate melt, and rims from a cooling magmatic-hydrothermal fluid. Almost all bulk δ 1 8 O values in magnetite are within the range of 0 to 5‰, and bulk δ 56 Fe for magnetite are within the range 0 to 0.8‰ of Fe isotopes, both of which indicate a magmatic source for O and Fe. The values of δ 1 8 O and δ D for actinolite, which is paragenetically equivalent to magnetite, are, respectively, 6.46 ± 0.56 and −59.3 ± 1.7‰, indicative of a mantle source. Pyrite grains consistently yield Co/Ni ratios that exceed unity, and imply precipitation of pyrite from an ore fluid evolved from an intermediate to mafic magma. The calculated initial 187 Os/ 188 Os ratio (Os i ) for magnetite from Los Colorados is 1.2, overlapping Os i values for Chilean porphyry-Cu deposits, and consistent with an origin from juvenile magma. Together, the data are consistent with a geologic model wherein (1) magnetite microlites crystallize as a near-liquidus phase from an intermediate to mafic silicate melt; (2) magnetite microlites serve as nucleation sites for fluid bubbles and promote volatile saturation of the melt; (3) the volatile phase coalesces and encapsulates magnetite microlites to form a magnetite-fluid suspension; (4) the suspension scavenges Fe, Cu, Au, S, Cl, P, and rare earth elements (REE) from the melt; (5) the suspension ascends from the host magma during regional extension; (6) as the suspension ascends, originally igneous magnetite microlites grow larger by sourcing Fe from the cooling magmatic-hydrothermal fluid; (7) in deep-seated crustal faults, magnetite crystals are deposited to form a Kiruna-type IOA deposit due to decompression of the magnetite-fluid suspension; and (8) the further ascending fluid transports Fe, Cu, Au, and S to shallower levels or lateral distal zones of the system where hematite, magnetite, and sulfides precipitate to form IOCG deposits. The model explains the globally observed temporal and spatial relationship between magmatism and IOA and IOCG deposits, and provides a valuable conceptual framework to define exploration strategies.
Abstract The Central African Copperbelt is the world’s premier sediment-hosted Cu province. It is contained in the Katangan basin, an intracratonic rift that records onset of growth at ~840 Ma and inversion at ~535 Ma. In the Copperbelt region, the basin has a crudely symmetrical form, with a central depocenter maximum containing ~11 km of strata positioned on the northern side of the border of the Democratic Republic of Congo and Zambia, and marginal condensed sequences <2 km in thickness. This fundamental extensional geometry was preserved through orogenesis, although complex configurations related to halokinesis are prevalent in central and northern parts of the basin, whereas to the south, relatively high-grade metamorphism occurred as a result of basement-involved thrusting and burial. The largest Cu ± Co ores, both stratiform and vein-controlled, are known from the periphery of the basin and transition to U-Ni-Co and Pb-Zn-Cu ores toward the depocenter maximum. Most ore types are positioned within a ~500-m halo to former near-basin-wide salt sheets or associated halokinetic structures, the exception being that located in extreme basin marginal positions, where primary salt was not deposited. Stratiform Cu ± Co ores occur at intrasalt (Congolese-type), subsalt (Zambian-type), and salt-marginal (Kamoa-type) positions. Bulk crush-leach fluid inclusion data from the first two of these deposit types reveal a principal association with residual evaporitic brines. A likely signature of the ore fluids, the brines were generated during deposition of the basin-wide salt-sheets and occupied voluminous sub and intrasalt aquifers from ~800 Ma. Associated intense Mg ± K metasomatism was restricted to these levels, indicating that capping and enclosing salt remained impermeable for prolonged periods of the basin’s history, isolating the deep-seated aquifers from the upper part of the basin fill. From ~765 to 740 Ma, the salt sheets in the Congolese part of the basin were halokinetically modified. Salt was withdrawn laterally to feed diapirs, ultimately leading to localized welding or breaching of the former hydrological seal. At these points, deeper-level residual brines were drawn into the intrasalt stratigraphy to interact with reducing elements and form the stratiform ores. It is probable that salt welding occurred diachronously across the northern and central parts of the basin, depending upon the interplay of original salt thickness, rates and volumes of sediment supply during accumulation of salt overburden, and tectonism. The variable timing of this fundamental change in hydrologic architecture is poorly constrained to the period of halokinetic onset to the earliest stages of orogenesis; however, the geometry of the ores and associated alteration patterns demands that mineralization preceded the characteristically complex fragmentation of the host strata. Thus, while an early orogenic timing is permissible, mineralization during the later stages of extensional basin development was more likely. In situ reducing elements that host Zambian-type stratiform Cu ± Co ores were in continuous hydrological communication with subsalt aquifers, such that ore formation could have commenced from the ~800 Ma brine introduction event. The nonhalokinetic character of the salt in this region allowed the intact seal to have maintained suprahydrostatic pore pressures, facilitating fluid circulation until late stages of basin growth and possibly early stage orogenesis. Leachate data from ores positioned in the depocenter maximum and southern parts of the basin that underwent relatively high grade metamorphism record mixing of residual and halite dissolution-related brines. Salt dissolution was likely triggered by emergence of diapirs or thermally and/or mechanically induced increased permeability of halite. While it is certain that halite dissolution occurred during and after orogenesis, conditions favorable for salt dissolution may have existed locally during extension in the depocenter maximum. The permeability of salt increased to a point where it became the principal aquifer. The salt’s properties as an aquiclude lost, originally deep-seated residual brine mixed with new phases of evaporite dissolution-related brine to produce ores at middle levels of the basin fill. During the final stages of ore formation, recorded by postorogenic Pb-Zn-Cu mineralization in the depocenter maximum, the salinity of fluids was dominantly derived from the dissolution of remnant bodies of salt.
Abstract The northern limb of the Bushveld Complex of South Africa contains a diverse array of Cr, Ni-Cu-platinum group element (PGE), Fe-V mineralization in mafic-ultramafic rocks and Sn mineralization hosted in granites. The limb has historically been underexplored compared to other parts of the Bushveld Complex and currently represents one of the world’s most interesting exploration frontiers. Successful low-cost open-pit mining of the thick Platreef Ni-Cu-PGE deposit, coupled with rising costs and limited scope for mechanization associated with narrow reef-type deposits in the eastern and western Bushveld, have driven efforts to locate similarly wide magmatic sulfide orebodies at surface or at reasonably shallow depths elsewhere in the northern limb, including recent discoveries of the Flatreef- and Main zone-hosted PGE deposits in the troctolite unit, at Aurora, and in the lower (F) and upper (T) mineralized zones at Waterberg. The Flatreef is hosted within a more consistent series of stratigraphic units than the more varied Platreef located updip, and while it shows similarities in terms of rock types and some geochemical features with the upper Critical zone of the eastern and western Bushveld, strict time equivalence remains to be proven. The various styles of Main zone-hosted PGE mineralization, on the other hand, have no known equivalents in the other limbs of the Bushveld Complex and seem to represent processes and events confined to the northern limb. Potential links based on similar rock types and metal budgets between Aurora and the Waterberg T zone and between the troctolite unit and the Waterberg F zone are attractive but must remain speculative until it becomes clearer whether the northernmost compartment that contains the Waterberg mineralization is linked to the remainder of the northern limb. If both the Flatreef and the Waterberg deposits enter production as planned over the coming decade, they will have dramatic effects on the South African platinum industry and dramatically increase the amount of Pd relative to Pt produced by South Africa due to the Pd-rich nature of all of the northern limb PGE orebodies.
Abstract Sulfate assimilation by mafic to ultramafic melt is thought to be an important process in the genesis of magmatic PGE-Ni-Cu deposits. We consider petrological indicators and possible mechanisms of anhydrite assimilation by ultramafic melts of the northern limb of the Bushveld Complex. On farm Turfspruit, an anhydrite-bearing sedimentary raft of the Duitschland Formation separates the Platreef from underlying Lower zone peridotites. The proportion of anhydrite across the raft increases from negligible in corundum-sillimanite-magnetite hornfels at the base to 95 to 100% in anhydrite marble at the top. Underlying Lower zone peridotites lack anhydrite, whereas overlying Platreef pyroxenites contain both widespread interstitial to euhedral anhydrite as well as spherical to irregularly shaped anhydrite inclusions in association with olivine chadacrysts inside oikocrystic orthopyroxene. Olivine chadacryst compositions (Mg# 79–81 and 0.33–0.46 wt % NiO) support their pristine liquidus origin, although an association of Al-enriched orthopyroxene and interstitial anorthite indicates exchange reactions involving anhydrite and aluminosilicates from hornfels. Plagioclase from the anhydrite-contaminated rocks has an Sr isotope initial ratio (Sr i ) of 0.7047 to 0.7063, similar to the compositions of Bushveld early primitive magmas, in agreement with a relatively nonradiogenic signature of the anhydrite-bearing contaminant with Sr i of 0.7057 to 0.7094. The range of Sr i of plagioclase from the underlying Lower zone peridotites (0.7040–0.7067) and from the Turfspruit platinum reefs just below the Main zone contact (0.7068–0.7084) supports their correlation and synchronous emplacement with the Lower zone and the top of the Upper Critical zone in the western and eastern limbs of the Bushveld. The δ 34 S values of anhydrite (12.2–14.5‰) and a coexisting pyrrhotite-millerite-chalcopyrite sulfide assemblage (6.2–7.8‰) in a hornfelsed raft and overlying pyroxenites are interpreted to have resulted from open-system isotopic exchange, indicating closure temperatures of 750° to 820°C. The assimilation of sedimentary anhydrite is interpreted to be an important component of contact-style mineralization of the Platreef at Turfspruit that took place through the erosion and disintegration of footwall rocks by dynamic pulses of hot magmas. Chemical dissolution, thermal decomposition, and melting of sulfate-bearing rafts or xenoliths are viable assimilation processes that result in the saturation of silicate melt with sulfate, exsolution of immiscible sulfate melts, crystallization of cumulus and interstitial anhydrite, and precipitation of contact-style sulfide mineralization at the base of the intrusion. Reef-style mineralization at the top of the Platreef shows contrastingly negligible compositional and isotopic evidence of sulfate assimilation.
Regional- to Deposit-Scale Geologic Controls on Copper-Silver Mineralization in the Kalahari Copperbelt, Botswana
Abstract The Kalahari Copperbelt in northwestern Botswana is characterized by structurally controlled, stratabound, mineralogically zoned copper-silver deposits hosted along a major redox boundary within a late Mesoproterozoic rift succession. Copper-silver mineralized rocks occur on the limbs and in the hinge positions of regional-scale folds that characterize the Pan-African Ghanzi-Chobe zone fold-and-thrust belt. Regional facies changes along the base of the transgressive marine D’Kar Formation, the host to the majority of mineralized rocks, delineate a series of synsedimentary basin highs and lows. The facies changes were identified through both lithostratigraphic analysis of drill holes and along-strike variations in magnetic lithostratigraphy, a technique that correlates the magnetic fabrics of second vertical derivative aeromagnetic maps with changes in lithostratigraphy. Basin highs controlled the development and distribution of favorable lithostratigraphic and lithogeochemical trap sites for later sulfide precipitation. Major facies changes across the Ghanzi Ridge area straddle a significant crustal structure identified in gravity datasets that appears to have influenced extensional activity during basin development. During basin inversion, the basin highs, cored by rheologically stronger bimodal volcanic rocks, localized strain within mechanically weaker rock types of the Ghanzi Group metasedimentary rocks, leading to the development of locally significant permeability and the formation of structural trap sites for mineralization by hot (250°–300°C), oxidizing, metalliferous Na-Ca-Cl brines. Structural permeability was maintained within trap sites due to silicification and/or feldspar alteration during progressive deformation and associated hydrothermal mineralizing events.
Abstract Normal faults commonly represent one of the principal controls on the origin and formation of sedimentary rock-hosted mineral deposits. Their presence within rift basins has a profound effect on fluid flow, with their impact ranging from acting as barriers, causing pressure compartmentalization of basinal pore fluids, to forming conduits for up-fault fluid flow. Despite their established importance in controlling the migration and trapping of mineralizing fluids, we have yet to adequately reconcile this duality of flow behavior and its impact on mineral flow systems within basinal sequences from a semiquantitative to quantitative perspective. Combining insights and models derived from earthquake, hydrocarbon, and mineral studies, the principal processes and models for fault-related fluid flow within sedimentary basins are reviewed and a unified conceptual model defined for their role in mineral systems. We illustrate associated concepts with case studies from Irish-type Zn-Pb deposits, sedimentary rock-hosted Cu deposits, and active sedimentary basins. We show that faults can actively affect fluid flow by a variety of associated processes, including seismic pumping and pulsing, or can provide pathways for the upward flow of overpressured fluids or the downward sinking of heavy brines. Associated models support the generation of crustal-scale convective flow systems that underpin the formation of major mineral provinces and provide a basis for differences in the flow behavior of faults, depending on a variety of factors such as fault zone complexities, host-rock properties, deformation conditions, and pressure drives. Flow heterogeneity along faults provides a basis for the thoroughly 3D flow systems that localize fluid flow and lead to the formation of mineral deposits.
Abstract The porphyry copper mineralization at the Zaldívar deposit is confined to a NE-striking corridor of early- and late-intermineral granodioritic and dacitic porphyry intrusions and associated magmatic-hydrothermal breccia bodies. Country rocks comprise Early Permian rhyolite and andesite of La Tabla Formation plus comagmatic granitoids and Late Triassic andesite dikes. Middle Eocene andesitic rocks are common but of ill-defined distribution. Hydrothermal alteration consists of centrally located, magnetite-bearing potassic assemblages that are partially to completely overprinted by chlorite-epidote and sericitic alteration zones. The bulk of the hypogene metal resource was introduced synchronously with potassic alteration and A- and B-type veinlets during emplacement and evolution of multiple centers of biotite-bearing, early-intermineral porphyry and breccia bodies. Late-intermineral, hornblende-bearing dacite porphyry phases and associated breccia centers were emplaced later than the A- and B-veinlets but prior to multiple D-type veinlet generations and contributed additional, although lower grade, mineralization. Late-mineral dacite dikes are barren. Extensions to the east and northeast connect Zaldívar with Escondida Norte, and both can be considered as separate, coalescing porphyry copper deposits. Two discrete porphyry copper systems coexist at Zaldívar: Early Permian and late Eocene. The minor, copper-only Early Permian event (~290–285 Ma) was associated with an evolved, end-stage rhyolite porphyry phase of the La Tabla magmatism. The major late Eocene event (38.6–36.1 Ma) produced copper in addition to gold, molybdenum, and silver. Protracted Eocene porphyry copper alteration and mineralization, over ~2.5 m.y. as constrained by numerous U-Pb (zircon) and Re-Os (molybdenite) ages, was coincident with the high rates of uplift and denudation synchronous with contractional Incaic deformation. Earliest-stage porphyry intrusions at 39–38 Ma were probably associated with the terminal stages of a volcanic edifice, likely a dome complex, whose erosion products were deposited in contiguous, synorogenic basins. District-wide precursor magmatism of intermediate composition was active between 45 and 41 Ma. Oxidation and enrichment were active between ~17 and 15 Ma (supergene alunite), consistent with the chronology of supergene activity throughout the district and wider region.
Abstract At the Cerro Verde district in southern Peru, granodiorite porphyry stocks formed two adjacent porphyry copper-molybdenum deposits that collectively form one of the largest copper districts in the world, with current resources of ~17 million metric tonnes (Mt) of copper. The district is located within the Coastal batholith of western Peru. The Coastal batholith in the Cerro Verde area consists of the Cretaceous Tiabaya and the Paleocene Yarabamba granodioritic plutons. Granodiorite porphyry stocks associated with the porphyry coppermolybdenum deposits were emplaced into the Yarabamba plutons. The granodiorite porphyry stocks are composite, steep-walled cylinders. Breccia bodies of diverse textures are localized in the apical parts of both stocks. The breccia fragments are predominantly of stock composition proximal to the intrusion and grade outward to heterolithic fragments of stock and Yarabamba wall rocks. Hydrothermal matrix breccia bodies are widespread, situated in the outer areas of the breccia column, and contain tourmaline, chalcopyrite, and molybdenite within the matrix. New zircon U-Pb dating confirms and refines earlier work, indicating that magmatism and mineralization at Cerro Verde occurred about 61 Ma. The hypogene mineralization is bracketed by the Yarabamba batholith host (~62 Ma), well-mineralized stocks at Cerro Verde and Santa Rosa (~61 Ma), and postmineral plugs (~60–59 Ma). The U-Pb ages are consistent with all crosscutting relationships. Each granodiorite stock is associated with similar sequences of alteration and mineralization. Biotite veinlets and halos containing chalcopyrite formed in the deeper areas of both deposits and are cut locally by later quartz veins and quartz-K-silicate veinlets containing chalcopyrite and molybdenite. Tourmaline-bearing sulfide veins with K-feldspar and chlorite envelopes form an inverted cup-shaped shell that overlaps the medial and upper parts of quartz-K-silicate veinlets. In distal positions, tourmaline veins contain sericite and are bordered by sericitic alteration. Quartz-sericite-pyrite veins and envelopes form an expansive stockwork in the upper part of the deposits and are transitional to and overprint K-silicate alteration. The Cerro Verde and Santa Rosa stocks formed individual copper and molybdenum ore shells within K-silicate alteration that forms thick-walled cylinders in the medial and upper parts of the deposit. The shells merge at depth to form one NW-SE–oriented mass that is 4.6 × 1.6 km in size using a >0.2% Cu value. Copper grades of 0.7 to 0.4% result from chalcopyrite-dominant veinlets occurring with chlorite in quartz-K-silicate alteration and are localized proximal to the stocks. The copper grades into a zone of 0.4 to 0.2% Cu within the biotitealtered zone at depth. Two discrete molybdenum ore shells are contained within the copper ore shell and are located proximal to the granodiorite stocks. The highest abundance of molybdenite is inward toward the stock and is zoned outward to lower grades. Breccia pipes contain abundant chalcopyrite and molybdenite within the matrix and are the source for the highest-grade ores in the district. The pipes truncate the majority of the veins containing copper, molybdenum, and tourmaline veins and bottom within quartz-rich K-silicate veinlets. Supergene mineralization consists of zones of leached capping, oxide copper mineralization, and an enriched chalcocite blanket developed above the copper ore shells within sericitic alteration. The oxide copper deposits contain isolated brochantite with chrysocolla in tourmaline breccia bodies situated above a laterally continuous and deposit-wide chalcocite enrichment blanket. Whereas molybdenum contents are little affected by supergene processes, copper and silver are generally leached from sericitically altered rocks and concentrated downward in the sulfide enrichment blanket by a factor of 1.5 to 2 compared to the subjacent protore. The oxide ores reflect in situ oxidation of a mature enrichment blanket hosted within rocks lacking abundant pyrite. The proximal location and approximately synchronous formation of the Cerro Verde and Santa Rosa porphyry copper-molybdenum deposits formed an expansive K-silicate alteration system and related ore shells that encompass both intrusive centers. In detail, multiple episodes of hydrothermal alteration and metal introduction can be inferred spanning no more than about 1 m.y. Although hydrothermal activity began with the emplacement of the Yarabamba granodiorite, most metals were introduced with the composite Cerro Verde-Santa Rosa stocks, and activity waned with formation of late breccias and ceased in late barren porphyries that truncate ore. These patterns are quite similar to those of other giant Andean porphyry systems, notably El Salvador, Los Pelambres, and Toquepala.
Discovery, Geologic Setting, and Controls on Iron Mineralization, South Flank, Western Australia
Abstract South Flank is a ~1.8-billion-tonne martite-goethite iron deposit located in the Late Archean to Paleoproterozoic central Hamersley province, Pilbara craton, Western Australia—a district containing multiple giant iron deposits. A combination of detailed mapping, high-precision airborne magnetic and gravity gradiometer data, and resource range analysis, followed up by systematic drilling, was used to discover and fully define iron mineralization at South Flank. Exploration was targeted using a deposit-scale model, based on observed geologic controls on martite-goethite deposits in the South Flank district, combined with a systems approach, which identified key processes in the formation of iron mineralization at the camp scale, namely fluid pathways, controlling structures, potential host rocks, and ore preservation beneath detrital cover. Iron mineralization at South Flank is hosted by the Marra Mamba Iron Formation and occurs as a series of strata-bound tabular orebodies over a strike length of 25 km. Individual ore zones are up to 150 m thick and can extend to depths of 300 m. Martite-goethite-ochreous goethite ore is predominantly hosted by N2 and N3 subunits of the Mount Newman Member and is best developed in E-W–trending, upright to N-verging asymmetric synclines and associated low-angle reverse faults, which have caused substantial thickening of host rocks. Primary textures within banded iron formation are largely preserved within ore zones and can control location and grade of iron mineralization. Both unmineralized iron formations and ore zones are overprinted by recent extensive ferricrete, locally termed “hardcap.” Phosphorous, Al 2 O 3 , and volatile contents of ore co-vary with iron, albeit at low absolute abundances, whereas SiO 2 is strongly negatively correlated with Fe, reflecting the transition from iron formation (Fe = 30–35 wt %) to iron ore (Fe = 50–65 wt %). Premineralization host-rock composition is an important control on both ore geochemistry and mineralogy. Martite-goethite-ochreous goethite is the dominant style of iron mineralization in the Hamersley province, in terms of overall tonnage and contained Fe, and is also widely developed in iron formations in the Pilbara and Yilgarn cratons and in other major global iron ore districts (e.g., India and Brazil). In each of these regions, martite-goethite and ochreous goethite are commonly developed as a weathering-related supergene overprint of earlier-formed hypogene hematite mineralization. In contrast, South Flank and other major deposits in the central Hamersley province (e.g., Mining Area C, Hope Downs) show no evidence of hypogene iron mineralization and its commonly associated wall-rock alteration. These iron orebodies are characterized by common structural association with synclines and associated reverse faults, preferential host-rock settings within particular units of the Brockman and Marra Mamba iron formations, simple ore mineralogy and geochemistry, and absence of associated wall-rock alteration. The giant martite-goethite deposits in the Hamersley province, of which South Flank is a type example, potentially represent a distinct deposit style. While some of the geologic characteristics of iron mineralization at South Flank are compatible with a supergene origin, many factors relating to ore genesis are unknown or not adequately constrained, including timing and mechanisms of ore formation.
Abstract Flake graphite is a critical battery material due to its role as the primary anode component in lithium-ion batteries. With the shift to electrification of vehicles, it is forecast that in the next five years flake graphite’s number-one use will be in battery applications, overtaking its traditional industrial uses. The burgeoning demand for battery anode materials is anticipated to double the current natural flake graphite market of roughly 645,000 tonnes per annum by 2025, which will require new flake graphite sources like the Molo graphite deposit to come into production. The Molo graphite deposit is world class due to its large size (NI 43–101 measured resource of 23.62 Mt at 6.32% C, indicated resource of 76.75 Mt at 6.25% C, and inferred resource of 40.91 Mt at 5.78% C), high proportion of large and jumbo flake (46.4%), and high average flake carbon purity (97.27% C). The deposit was discovered in 2011 as the result of a regional exploration program initiated by NextSource Materials Inc. following their delineation of a vanadium deposit called the Green Giant. Graphitic mineralization in the Molo is bimodally distributed, with low-grade and high-grade zones having carbon cutoff grades of 2 and 4% C, respectively. High-grade mineralization is associated with metamorphosed siltstones and mudstones, while low-grade mineralization is associated with rocks interpreted to represent metamorphosed sandstones, which are interpreted to be more favorable hosts for large- and jumbo-flake graphite. The Molo graphite deposit appears to have resulted from many mineralizing events, which extended over a period of time that may range from ca. 900 to ca. 490 Ma. These include graphitization during the emplacement of anorthosite complexes, graphitization in a high-strain regime under high-pressure and high-temperature granulite facies metamorphism during the collision of the Androyen domain with the Vohibory domain, graphite refining and recrystallization believed to have taken place during East Gondwana and West Gondwana collision, and the formation of postcollisional hydrothermal vein graphite during orogenic collapse. The superimposition of the tectono-metamorphic history of southern Madagascar on a sedimentary sequence in which the protoliths were rich in organic carbon has resulted in world-class flake graphite mineralization with high carbon purities and large flake sizes.
Abstract In the first decade of the 21 st century, surface exploration drilling around the Boliden Tara mine at Navan, Ireland, aimed at ~1-km-deep targets, was becoming ineffective. During 2010, the extensive geologic knowledge of the existing Navan orebody was leveraged in an Experts Meeting to promote near-mine discovery. Two ideas, of many, were of relevance to this paper: (1) undiscovered mineralized fault-related zones were predicted south of the orebody, and (2) seismic surveys could locate subsurface faults. By late 2012, seven 2D seismic lines (totaling 101 km) had been acquired, processed, and initially interpreted. Pre-stack time migration images were used for interpretation, augmented by diamond drill core data where available. The seismic imaging proved a “game changer” in terms of subsurface visualization and a priority target was identified 2 km south of the mine on the footwall crest of a large south-dipping basin-margin fault. The first hole intersected 34 m of mineralized rock with 14% Zn + Pb, but at greater depth than anticipated. Follow-up drilling was initially successful but proved to be challenging. The first hole intersected a deep structurally complex section of the newly discovered zone that required more drilling to establish its location and attitude. Further drilling, utilizing extensive navigational deflection technology, outlined a mineralized zone similar in nature to the Navan 5 Lens at depths of 1 to 2 km. Inferred resources through 2016 were estimated at 10.2 Mt grading 8.5% Zn and 1.8% Pb. Underground exploration development of this zone commenced in April 2017, and will allow accurate delineation of this significant discovery.