2.2. The Proposed Mineral System Definition Workflow
The workflow below and in Table 1
is a step-by-step guide to conduct the Mineral System Definition stage of a REE-HFSE project. Applying it will provide the user with a robust foundation to subsequently: (a) Gain geological processes-based perspective to explore for mineral resources, (b) predictively determine the most favorable mineral system branches to target/research, (c) probabilistically risk prospects during portfolio building, (d) have analogues for other, less-defined, mineral systems, and (e) compare and rank REE-HFSE mineral systems globally. It draws from the geological processes-based workflow to define any petroleum system in 4-D [49
], and applies the mindset to hypothesize commodity migration from a system’s initial source zone to all resultant accumulations, which has been economically successful standard practice for petroleum exploration for decades. This process-based petroleum system mindset has already been adapted by some mineral industry workers, e.g., References [24
]. We have applied this mineral system definition workflow to the Southwest Germany part of the Central European Volcanic Province (Section 3
(1) Task 1. Organise and Summarise All Province Framework Stage Data and Knowledge
All available data-knowledge about the host province’s 4-D evolution and mineralization—including all historic exploration and production data—needs to be summarized and organized geospatially. This will include a province-scale geological setting and evolution summary (tectonic, structural, magmatic, metamorphic, hydrothermal, climatic, erosional and sedimentological). This helps contextualize and predict the timings, pathways, locations and preservation potential of the pervading mineral system. There is no minimum data to commence mineral system definition; documentation of the geological uncertainties will incorporate the data-knowledge availability.
(2) Task 2. State the Mineral System’s Critical Components and the Project’s Investigation Products
The four critical components we currently recommend for province-scale carbonatite- or alkaline igneous-associated REE-HFSE evaluations are fertility, whole-lithosphere configuration, transient geodynamics, and exhumation-preservation (modified from Reference [23
]). Our research into the critical components at district and deposit-scales is ongoing, for example trapping and priming for further REE-HFSE enrichment. These critical components must have occurred in the correct order, and overlapped, in time and space (Figure 2
) for a chance of mantle-derived REE-HFSE deposits being at/near surface today. A mineral system only exists where all the critical components are known to have overlapped. In addition, a mineral system may exist where there is encouraging to favorable probability of them overlapping. Evaluating these four components is valuable to (a) distil a targeting model down to fundamental parameters [24
] and, (b) enable multiplicative, probability-based risking [40
] (and following standard petroleum industry best practice) for a province to have generated and preserved a mineral deposit. The critical components will be different at district and deposit scales, and in the different facies associations, so will require differing mappable criteria, e.g., Reference [37
]. For example, lithospheric-scale strike slip fault systems would be included on province-scale mineral system maps whereas magma chamber roof zone outcrops would be detailed on deposit-scale maps.
The following investigation products should be generated to summarize a mineral system in 4-D, to visualize areas for focus and identify potential mineralized plays:
Correlations between mineralized occurrences.
Correlations between mineralized occurrences and their commodity provenance.
Table of produced and unrecovered mineral resources, to calculate the known mineral endowment (size) of the mineral system.
Map of the mineral system’s geographic extent.
Cross section of the mineral system’s stratigraphic extent.
Mineral system events chart showing the mineral system’s temporal extent.
Mineral system preservation history chart.
(3) Task 3. Identify the Mineral System
Before a REE-HFSE mineral system can be examined, demonstration that it exists requires at least one REE-HFSE mineralization that is geochemically anomalous in the context of the local lithologies, i.e., significantly above the region’s surface baseline concentration (average bulk continental crust has 125 ppm total REE [56
]). This would be evidence that mineral system processes elevated the concentrations of the commodity [57
], and could be a historic mine, ore deposit, mineralized outcrop, or mineralization observation in a drill core. Next, the genetic relationships between near-surface REE-HFSE occurrences need to be formalized by establishing petrological, mineral chemical, isotopic, and geochronological correlations, and integrating them with mapped crustal structures. This enables: (a) Mapping the known mineral system extent, (b) a framework to incorporate future discoveries, (c) mapping mineralized plays and mineralization episodes within that mineral system’s duration. Subsequently, genetic relationships between the near-surface commodity occurrences and their sub-lithospheric provenance region need to be formalized. Mineralogical, geochemical, isotopic, or geochronological signatures are used to map the spatial extent of the provenance, to then infer where overlying districts may be underexplored/unknown. Petrological studies of near-surface occurrences help infer redox conditions, temperature, and pressure of the commodity provenance during commodity expulsion, e.g., Reference [58
]. Geological, geophysical, and structural maps are integrated to correlate mineral occurrences and mantle provenance to lithospheric-scale REE-HFSE conduit structures.
(4) Task 4. Hierarchically Organize the Mineral System’s Sub-Divisions
Mapping, modelling and quantifying the space-time span of a mineral system—from mantle expulsion period to the most distal deposit location—is improved by organizing a mineral system “tree” into a hierarchy of exploration targeting sub-divisions (Figure 3
). This has already commenced to some extent in a small section of the mineral system literature. For example, Reference [25
] split uranium mineral systems into: (a) Magmatic-related, (b) metamorphic-related, and (c) basin and surface-related, “families of mineralizing systems”, with each family containing, “deposit styles”. In contrast, Reference [42
] classified the REE mineral system concept into four, “Mineral-system associations” containing numerous “Deposit types” [42
]. The latter is a mix of depositional environments, depositional processes, deposit types and rock names (Figure 4
). To standardize mineral system sub-divisions and visualize their relationships we recommend a scale-based hierarchy of mineral system sub-divisions that aligns with the Linnaean classification scheme [59
] and the petroleum system divisions [60
] and their logic. This will also allow easy extrapolation into other commodity mineral systems. Figure 3
displays the hierarchy of sub-divisions we recommend for any mineral system whilst Figure 4
exhibits how carbonatite- and alkaline igneous rock-associated REE “deposit types” [42
] can be more effectively divided. Figure 5
demonstrates in cross-section the hierarchy of a mineral system’s sub-divisions, and how deposits and prospects link to form plays. Figure 6
uses the Mount Weld Carbonatite Complex and the Ponton Creek Complex (Australia) to demonstrate how to place deposits and prospects within the proposed mineral system hierarchy, and summarize the causes of economic success or failure.
Facies Associations and Enrichment Vectors
A mineral system’s REE-HFSE load may—after expulsion from the provenance—be either focused or dispersed during crustal and surface migration. Commodity concentration can occur by magmatic processes between provenance and upper crust (Figure 3.3 of Reference [42
]), then further ‘enriched’ by subsequent metamorphic, weathering, and basinal processes. Hypothesizing all the possible REE-HFSE concentrating processes along the lithospheric-scale geodynamic evolution cycle is therefore vital, because the formation of an economic deposit requires commodity concentration by orders of magnitude. A carbonatite or alkaline igneous-associated REE-HFSE mineral system may extend over as many as four facies associations (Figure 4
; Figure 3.3 of Reference [42
]) that the initial commodity load could be distributed in today (adapted from Reference [42
Magmatic facies association, with deposit types formed by fractionation of REE-HFSE-enriched mantle melts and/or associated hydrothermal fluids, e.g., the Mountain Pass REE deposit [8
Weathering facies association, with deposit types formed by weathering of REE-HFSE-bearing magmatic rocks and further chemical concentration of commodities in the residual material, e.g., the Mount Weld palaeo-regolith Central Lanthanide REE Deposit (Figure 6
and Figure 7
; Reference [42
Basinal facies association, with deposit types formed by physical concentration during sedimentary processes, e.g., the lacustrine Crown Tantalum-Niobium Deposit in the Mount Weld Carbonatite Complex (Figure 6
and Figure 7
; Reference [42
Metamorphic facies association, where deposit enrichment may occur due to regional or contact metamorphic fluids, heat, and pressure, e.g., Bayan Obo Fe-REE deposit [11
These facies associations are a logical foundation towards identifying mineralized plays and conducting Play Analysis. They can also be useful exploration indicator vectors. For example, a magmatic mineralization may be used as a locus for proximal fluvial deposits, whilst lateritic deposits may indicate a location of under cover magmatic mineralization. These four facies associations incorporate the lithofacies associations concept and methodology of Reference [52
], and maintain alignment with “magmatic-related”, “metamorphic-related”, and “basin- and surface-related” “mineralizing system families” [25
]. They also parallel the four REE “mineral system associations” [42
], except that we expand those authors’ Regolith “Mineral-system association” [42
] to Weathering, to accommodate the possibility of karst-associated mineralization in weathered carbonatite bodies.
REE-HFSE mineral systems in spatially or temporally disparate provinces could acquire fundamentally similar mineral deposit characteristics due to similar geodynamic evolution cycle stages, palaeo-geographies or palaeo-climates. Therefore, analyzing facies associations in one REE-HFSE mineral system can then provide analogues for genesis, richness, and value of under-evaluated districts in another REE-HFSE mineral system.
Few historically and currently mined REE-HFSE deposits are magmatic facies association deposits, despite the volume of magmatic REE-HFSE mineralization research. Most of the mined deposits represent weathering and basinal facies associations, i.e., lateritised carbonatites, lateritic ion-adsorption clays, and sediment placers (e.g., Reference [9
]). Focusing on potential REE-HFSE basinal and weathering deposition environments could therefore enhance exploration interest and probability of global geological success. Additionally, because REE metallurgic processing can be complex and expensive, and for numerous minerals still in test phase (e.g., References [65
] and references within,) summarizing a REE-HFSE mineral system’s facies associations can help envisage where and how geological processes may have conducted some natural beneficiation.
Each of the four REE-HFSE facies associations is a group of REE-HFSE play types (deposition processes):
Magmatic facies play types are: Orthomagmatic, magmatic-hydrothermal.
Weathering facies play types are: Supergene zone lateritic, residual zone lateritic, karstic.
Basinal facies play types are: Sedimentary, diagenetic-hydrothermal.
Metamorphic facies play types are: Research is ongoing.
Recognizing play types enables a mineral system to be investigated as a potential stack of chronostratigraphic groups (plays), with each group containing process-specific deposit types. For example, the Mount Weld ore deposit (Figure 6
and Figure 7
) is a stack of three ore deposit types [42
], each of a different play type and age, yet all sourced from one Palaeoproterozoic, metamorphosed, carbonatite body:
The Eocene, basinal, sedimentary, Crown Tantalum-Niobium Deposit.
The Late Mesozoic-Early Cenozoic, weathering, supergene palaeo-lateritic, Central Lanthanide Deposit.
The Late Mesozoic-Early Cenozoic, weathering, residual palaeo-lateritic, Swan Phosphate Deposit.
A carbonatite or alkaline igneous-associated REE-HFSE play type may contain one or more mineralized plays. A play is here defined as a group of geologically related ore deposits, mineral deposits and untested prospects within a chronostratigraphic unit (adapting the standard hydrocarbon play definition, e.g., Reference [51
]). Play is most appropriate entity and term as it brings chronologic and genetic context to mineralization zones. A play is not synonymous with a deposit. Each mineralized play could comprise several deposit types. As one ore deposit is a product of the processes that generated a play, understanding how a proven mineralized play operated provides predictive capability and appropriate analogues, to seek and develop other prospects within that play. To build a probabilistic REE-HFSE prospect portfolio the base unit of targeting should be a chronostratigraphically-bound play, rather than a deposit. For example, organizing the Mount Weld REE-HFSE complex into chronostratigraphic play types reveals three proven plays (each containing mineral deposits) and four potential plays (Figure 6
and Figure 7
) spanning a variety of processes and ages. Assigning mineral deposits into their host plays would also help clarify the literature’s mix of deposit settings, processes, and locations under the umbrella term ‘Deposit types’ (Table 2
and Table 3
Mineralization Types and Host Lithologies
Assigning each play’s mineralization types into ore deposits, mineral deposits, untested prospects, and mineralization observations clarifies the extent and exploration maturity of each play. Categorizing the mineralization types into their host lithologies can indicate optimal lithologies (mappable proxies) for investigation priority.
An example of successful exploration-production along an entire mineral system’s length and in its various plays is from the diamond industry of South Africa and Namibia [67
]. It has yielded success in several facies associations, play types and plays i.e. diamond discoveries from ‘primary’ kimberlite sources to younger, downstream Orange River and offshore Atlantic plays, e.g.,:
Upper Cretaceous kimberlite pipes.
Miocene Proto-Orange alluvial river gravels and Plio-Pleistocene Meso-Orange river terraces.
Mid Pleistocene-Holocene palaeo-shoreline and palaeo-beach sediments.
Plio-Pleistocene raised palaeo-beach deposits.
Pleistocene-Holocene shallow marine submerged beach deposits and seafloor bedrock.
To explore and discover diamonds, it is wise to explore all along the mineral system and not just the magmatic facies association, because less than 1% of Earth’s 6400 kimberlite occurrences will yield an economically-viable diamond deposit [70
], and some economic beneficiation occurs by diamond transport and redeposition. Although REE-HFSE minerals are not as durable as diamonds, a similar holistic approach could add success to REE-HFSE targeting.
(5) Task 5. Estimate the Known Mineral Endowment (Size) of the Mineral System
The size of one mineral system is the total sum of produced and unrecovered REE-HFSE mineral resources yielded from the system’s provenance, and estimated in compliance with a resource reporting code (Table 2
). The size will only represent the availability of known mineral resource and production data at the time of mineral system investigation, and not the system’s total mineralization/exploration potential. Documenting the size is crucial to communicate the extent of knowledge and uncertainty ranges about the system’s mineralization and historic exploitation. A list of mineral occurrences without a formal mineral resource category should also be created to emphasize known remaining potential.
The mineral system’s size is one way to compare mineral systems. It can also be used to determine the system’s yield–mineralization efficiency: the ratio (as a percentage) of the total known REE-HFSE mass accumulated (as opposed to dispersed to background) versus the estimated total mass of REE-HFSE yielded from mantle provenance partial melting (adapted from Reference [49
]). This ratio could be used for basic yet-to-find estimates, to subsequently estimate and compare the remaining potential in mineral systems. We recognize that a method to estimate the total mass of REE-HFSE mobilized during a provenance partial melting and expulsion event may not yet be available, and recommend the REE-HFSE research community consider how it could be formulated and tested.
(6) Task 6. Name the Mineral System
Geological entities, e.g., rock units, fossil species, orogenic zones and basins are assigned unique names to aid identification. Each mineral system also needs a name to distinguish it from other mineral systems. We recommend a standardized naming convention with several parts (adapted from Reference [49
The geological province/region.
The certainty of the correlation between the commodity provenance and occurrences (Table 3
The event most likely to have enriched the provenance in the commodity.
The age and name of the play containing the largest volume of initially in-place resource, or data.
An example is the Southwest Germany, Tentative, Variscan-Miocene Carbonatite REE-HFSE mineral system that contains the much-published Kaiserstuhl Volcanic Complex (KVC).
(7) Task 7. Map the Mineral System’s Known Extent
Mapping a mineral system’s known geographic, stratigraphic and temporal extents—across all facies associations—is conducted to identify possible play extensions and new play concepts that could contain additional deposits and value. In addition, a mapped mineral system provides an objective foundation to determine exploration risk of its plays and prospects (modified from Reference [50
]). A map is used to show the geographic extent, a cross section for the stratigraphic extent (depth and age range) and a preservation history chart to determine the mineralization critical moment and temporal extent. For example, a magmatic facies association critical moment is the geologically short “moment” when the critical components overlapped to enable the mineral system’s largest REE-HFSE resource proportion to migrate to the initial magmatic emplacement environment, i.e., peak mineralization (modified from Reference [49
]). The preservation time begins at the primary expulsion–migration–emplacement period and must extend to the present day for any deposit to still exist. It spans all physical or chemical alteration, concentration, re-migration and re-deposition events after REE-HFSE magmatic emplacement (modified from [49
(8) Task 8. Summarize the Mineral System’s Favorability for Undiscovered Mineral Endowment
Listing under-evaluated mineral plays and their mineral occurrences indicates how the remaining mineral system prospectivity may be distributed. These plays will need comparison to appropriate, economically viable, mineral system analogues to sense-check if they may be realistic and feasible. To indicate if a carbonatite- and alkaline igneous-associated REE-HFSE mineral system has the potential to yield more commodity volume and prospective play opportunities that warrant further evaluation, the favorability of each critical component should be qualitatively summarized for each facies association, as demonstrated for the magmatic facies association here.
Fertility. The sub-lithospheric mantle provenance received sufficient enrichment and focus of REE-HFSE and ligands (by metasomatic processes, e.g., Reference [74
]), then later received sufficient heat or confining pressure reduction (for low degrees of partial melting) to expel melts that can yield a REE-HFSE mineral deposit to the upper crust.
Whole-lithosphere configuration. Pre-existing, sub-vertical, deformation networks were prone to transient, lithospheric-scale failure [31
] and permeability (e.g., weak zones at the margins of cratonic blocks [38
]) to facilitate migration of mantle-derived primary magmas.
Transient geodynamics. A transient period occurred for prevailing geodynamics to favor Self-Organised Critical System processes [31
] to organize and focus fluid flux, e.g., transient compressional anomalies, incipient extension, or changes in far-field stress [23
Exhumation-preservation. An intrusive mineral deposit needs to have been sufficiently exhumed to enable near-surface mining, yet sufficiently preserved from erosion to retain an attractive commodity mass. For example, the Mount Weld carbonatite REE-HFSE deposit is viable whilst the same province and age Ponton Creek carbonatite is not viable due to the latter’s eroded pre-Eocene palaeo-regolith [63
Critical mineral system components with encouraging favorability are not, by themselves, an exploration targeting model. To prioritize data acquisition, an exploration targeting model or research investigation needs to list targeting criteria that are mappable in available, or realistically obtainable data sets [37
]. We recommend the four-step process of [37
] that translates a mineral system’s: (1) Critical components into, (2) constituent processes into, (3) targeting elements reflected in a province’s geology into, and (4) mappable targeting criteria used to verify that the critical components occurred. This translation is crucial for subsequent Play Analysis and Prospect Maturation stages, and is the link between Mineral system science to Targeting science (Figure 1
). This four-step translation will be conducted again at both district and deposit scales as the geological processes, project aims and key data types vary at each targeting scale [37
]. The translation of province-scale magmatic facies critical components into mappable, targeting criteria for carbonatite- and alkaline igneous-associated REE-HFSE mineral systems is ongoing in our research. Once the critical components of each facies association are translated into mappable targeting criteria, the maximum possible extent for the entire mineral system is sketched to visualize the mineral system’s under-evaluated extent. If an exploration company does not have sufficient expertise in all facies associations it should expand its team, otherwise pursue only the facies associations aligned to the company’s strengths and consider farming-out acreage dominated by the other facies associations. Conversely, a geological survey should attempt to research all a mineral system’s facies associations to maximize a nation’s REE-HFSE understanding.