Abstract

1.1. Ore deposit vs. Mineral deposit

The term “mineral deposit” is defined slightly differently depending on the purpose of use. However, the most applicable and wide-covering definition of the term “mineral deposit” is the one given e.g. by Cox et al. (1986):

“A mineral deposit is a mineral occurrence of sufficient size and grade that it might, under the most favourable of circumstances, be considered to have economic potential.”

Further, Cox et al., (1986) state that an “ore” is:

“A mineral deposit that has been tested and is known to be of sufficient size, grade, and accessibility to be producible to yield a profit.”

The classification of the mining industry into different sectors varies from country to country. The only consistency is that the term “mining industry” is globally used only for metallic ores. The other sectors related to mineral production are typically viewed as more simple “quarrying” operations. In many cases the term “ore” is also used solely for metallic mineral deposits AND when there is an economic concentration of the metal in question. This is also seen in international reporting standards for mineral resources and mineral reserves (“The CRIRSCO International Reporting Template,” 2019), where the term “ore” is synonymous to “mineral reserve”. In legal terms, the Norwegian Minerals Act (Mineralloven, 2009) relates the term “ore” with the “Minerals owned by the State“ which are minerals containing metals with a density of > 5 g/cm³ and “ores of such metals”.

The term unconventional mineral deposits is used for non-metallic industrial mineral deposits, aggregates (sand, gravel, and crushed rock), and also for metallic deposits in the deep sea, which include polymetallic manganese nodules (PMN), cobalt-rich manganese crusts (CRC), and seafloor massive sulphides (SMS) deposits. Although the latter may be regarded as a conventional deposit, only in an unconventional setting. It must also be noted that currently, now deep sea mineral deposits, are or have ever been mined commercially.

Industrial minerals are diversified as a sector, where the extraction may be in the form of a simple quarry, and only a few people to handle the entire operation while producing low-cost industrial minerals. The opposite extreme of an industrial mineral operation is the millions of tonnes per year run-of-mine (ROM) operation, involving a large workforce, running complex mining and highly specialized mineral processing operations that produce high-purity or high-quality speciality products for downstream customers. The former type of operation typically extracts and sells the rock as is, with low costs related to exploration and resource evaluation, and would rely on the fact that the requested quality of the mineral or rock is still present after the next blast. The low value of their products will not allow for long transport distances. Quality requirements are not very strict and often related to particle size distribution (PSD) or some chemical parameters to be met. The latter type of operations is, on the contrary, where detailed knowledge about geological, mineralogical and other parameters is crucial for the day-to-day planning of the operations, both in the mine and the mineral processing plant. The final quality or purity of the end products (concentrates) from the mineral processing plant is the result of costly planning and processing. In such operations, detailed knowledge about mineralogical constraints through the application of process mineralogy is crucial for success Case studies are presented that illustrate fundamental parameters for different unconventional mineral deposits, where the grades are distinctly different from traditional metallic ores. These examples include; Metallurgical grade quartz deposits that are mined as a source for Si, the quartz content in the deposit must typically exceed 98 wt-%. However, for the quality requirement of the deposits, quartz grades are less important, the main quality parameter is related to the maximum content of critical impurities. Other important aspects of metallurgical grade quartz deposits could be the thermo-mechanical strengths, which are crucial for optimal furnace operation. Calcite as a source for precipitated calcium carbonate (PCC) is aimed at calcium carbonate fillers for the paper industry. Quality requirements are mostly concerned with the whiteness of the final concentrate, rather than the calcite grades in feed and concentrate. Often, concentrate whiteness and calcite grade in the feed may correlate strongly, the calcite grade is not merely a result of the content of any mineral impurity.

1.2. Process mineralogy

Process mineralogy as a field has been around for decades (Bradshaw, 2014, Malvik, 2014) and is acknowledged for its ability to aid optimization and troubleshooting of mineral processing plants for base- and precious metals. There are publications dating back to the 1960 s but still there are discrepancies in the definitions (Malvik, 2014). The origin of process mineralogy as mentioned above, is related to the conventional ore types producing metal, or more specifically mineral, concentrates. Further, many of the definitions of process mineralogy are related to the metallic content. Such as the definition by Bradshaw (2014) which states that process mineralogy can be defined as:

the practical study of minerals associated with the processing of ores, concentrates and smelter products for the development and optimisation of metallurgical flowsheets, including waste and environmental management considerations.

Bradshaw (2014) relates “mineralogy” to the studies of minerals as with applied mineralogy, and “process” to mineral processing. However, more general definitions, such as by Malvik (2014), open for including any mineral deposit under process mineralogy, including those that may be regarded “unconventional”:

Process mineralogy relates the physical, chemical, mineralogical and textural properties of the mineral raw materials to their behaviour in the process, to product quality and the utilization of the mineral products.

Hence, whereas Bradshaw (2014) clearly links the field to metallic ores, Malvik (2014) includes any mineral deposit type within the field, and also links the field closer to the term Industrial Mineralogy as defined by e.g. Chang (2002) and Mukherjee (2011). Although, contrary to Malvik, 2014, Chang, 2002 and Mukherjee (2011) exclude metallic ores from industrial mineralogy.

Mineralogical analyses are fundamental when understanding the processing behaviour of any mineral deposit. Typical fundamental parameters in process mineralogy are e.g. mineral identification and modal mineralogy, mineral liberation and lockings, particle and grain sizes, mineral associations, and elemental deportment.

There are few publications in international journals that discuss the use of process mineralogy on industrial minerals and other unconventional mineral deposits. Most of these appear to be originating from the environment at the Norwegian University of Science and Technology (NTNU), most of these again from a geometallurgical perspective, such as (Aasly et al., 2007a, Aasly et al., 2007b, Aasly and Ellefmo, 2014, Ellefmo et al., 2019, Lang et al., 2018b, Lang et al., 2018a, Mena Silva et al., 2018, Mena Silva et al., 2020). A series of PhD theses have been prepared during the last two decades, focusing on any part of the process mineralogy related to unconventional mineral deposits, (e.g. Aasly, 2008, Bunkholt, 2015, Hestnes, 2013, Lang, 2020, Moen, 2006, Silva, 2019, Watne, 2001).

1.3. Unconventional mineral deposits

So, what are those unconventional mineral deposits? This paper has already attempted to define the term “mineral deposit”. We all know what these are, but the mining industry tends to be focused on metallic mineral deposits. Further, defining the term “unconventional” related to mineral deposits. These are elements that will be discussed in this paper before examples of application and challenges related to process mineralogy on unconventional mineral deposits are presented.

To answer the question, it might be better to first turn the question around and ask, “What is a conventional mineral deposit?”. Conventional means something that conforms or adheres to accepted standards, or something which is ordinary (Dictionary.com, 2024). Hence, unconventional mineral deposits must refer to mineral deposits that do not adhere to what has been included as accepted standards for being known as mineral deposits by the mining industry. A more clear definition of the term “unconventional mineral deposit” has not been published, although at the widely known website for mineral information, https://www.mindat.org, a search for the phrase provides a definition reading:

“A mineral deposit of such unusual grade, mineralogy, or geologic setting that experienced mining personnel would not consider it to be similar to any known deposit type” (Mindat.org, 2024). At the Mindat.org webpage, the citation is to “Barton”, without more detailed reference. However, “Barton” is most likely referring to Barton (1983).

In this paper, the term “unconventional mineral deposits” will be used for any mineral deposit not typically associated with the field process mineralogy. And a typical process mineralogist is a minerals engineer (or geoscientist) who on a day-to-day basis works on base- and precious metal operations and follows a project by constantly surveying the operation in the mineral processing¹ plant by diagnosing the mineralogical and textural characteristics of one or several material streams in the plant. It could be the feed or final concentrate; it could be the concentrate from the scavenger flotation, or the middlings from a wet high-intensity magnetic separation (WHIMS). Typically looking for modal mineralogy, mineral liberation, mineral associations, etc. by analysing these parameters and monitoring the periodic (or none-periodic) variations, the process mineralogist can help predict or correct a potential critical change, such as in the concentrate grade, or losses to tail, or deleterious elements in the concentrate.

Although mineral identification and modal mineralogy will be important when looking at any mineral deposit, other parameters can deviate significantly from the conventional process mineralogy parameters when working on unconventional mineral deposits. Examples may be thermal strength index for metallurgical grade quartz, or whiteness for calcium carbonates.

2.1. Metallurgical quartz for silicon production

Norway is among the largest producers of silicon (Si) and ferrosilicon (FeSi) (U.S. Geological Survey, 2022). The raw material for Si in these types of commodities is quartz, either from vein quartz (hydrothermal quartz) or from quartzites, depending on the required purity. Quartz is produced as lump-size (typically 30–100 mm) feed, for charging the carbothermic furnace, together with coal/charcoal and wood chips (Schei et al., 1998). In the case of FeSi, scrap iron, iron pellets or iron ores are typically used as the source of iron. Quartz is mined as typical quarrying operations from hard-rock deposits involving drilling and blasting, but there are several placer deposits for gravel quartz being mined globally. Hard-rock deposits will require crushing and screening operations to produce the correct size fractions. Because of the size of the feed, quartz qualities must already be identified at the mining stage, based on exploration results and geological modelling. Only in specific cases, sorting of lump quartz based on sensor-based sorting may be used (Aasly, 2008, Robben and Wotruba, 2019) to improve, or upgrade, the quality of the final product.

There are several requirements to quartz raw materials beyond their chemical purity. The main requirement is related to the thermal strength of the quartz (Schei et al., 1998) where the lump-size quartz is charged to the carbothermic furnace (Schei et al., 1998) and experiences shock heating to temperatures up to > 1300 °C. The effect of the shock heating may be catastrophic decrepitation of the large quartz fragments into many small fragments that will lead to reduced gas flow through the furnace charge and reduce furnace efficiency. The silicon producers use different types of tests to measure the effect of the shock heating on the quartz and one of these tests is the Thermal Strength Index (e.g. Aasly, 2008). Several attempts have been carried out to understand the mechanisms behind the decrepitation of quartz as a result of shock heating as summarized by Aasly (2007b).

Research related to understanding the high-temperature decrepitation of quartz (Aasly et al., 2007a) involved experiments with shock heating of different quartz types, and sources as seen in Fig. 1. In the shock-heating experiments, equal-sized quartz specimens were inserted to a muffle furnace at 1300 °C and kept at this temperature for one hour before cooled at room temperature. The example in Fig. 1 shows the starting point in a) and b), and the result of the shock heating in c) and d). Huge differences were observed between different sources and types of rock.

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Fig. 1

In order to explain why different quartz responds differently to shock heating, a thorough investigation of related mineral textures was needed (e.g. Aasly et al., 2007a). Fig. 2 shows mineral textures in one specimen before and after shock heating. Fig. 2a) and 2b) show microphotographs of the typical quartz grains in the sample before and after heating. The micrographs show well-defined quartz grains (Fig. 2a) that, as an effect of heating, have more defined grain boundaries (Fig. 2b), and the individual mineral grains have been affected by the shock heating. Preparation of the thin sections using fluorescent epoxy resin allows for the investigation of the same samples using UV light. c) and d) show the same area on the thin section, only now the fluorescent epoxy illuminates the permeable part of the quartz, where the epoxy resin has been able to penetrate. A clear difference between the unheated c) and the heated d) sample can be observed. The shock heating seems to have affected all quartz grains, causing microcracking and the formation of larger cavities along grain boundaries, indicating that some volume expansion of the quartz specimen must have occurred.

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Fig. 2

These effects of internal expansion and fracturing vary between different sources and types of quartz. According to Aasly (2008), these effects may be related to the phase transitions of alpha-quartz to other high-temperature silica polymorphs. The formation of cristobalite as a result of the shock heating is expected to play a significant role as the phase transition from quartz to cristobalite has a > 15 % volume expansion (Salmang and Scholze, 1982).

As shown, understanding the process mineralogy of metallurgical quartz differs significantly from the conventional mineral deposit. Rather than relying on properties such as mineral liberation, locking, and mineral association, more important parameters are textural parameters such as grain boundaries and mineralogical properties related to phase transitions.

2.2. Calcite marble for PCC

The paper industry is in constant need of mineral fillers and coatings to improve the optical and physical properties of paper (e.g. Bunkholt, 2015). The minerals improve printing properties, such as the whiteness of the paper, and are beneficial environmentally and economically, over the cellulose fibres. High-quality coated papers have reached more than 50 % mineral filler. To provide the required white appearance of the coated paper, the mineral filler must adhere to strict requirements for the whiteness of the mineral filler. Hence, all calcite marble is produced from high-quality deposits such as the Tromsdalen quarry, Norway (Lang, 2020) and other calcite marble deposits supplying the paper industry (Bunkholt, 2015).

The quality of calcite marble for precipitated calcium carbonates (PCC) is evaluated based on the resulting PCC product. The typical particle size of a feed to the PCC process is 30 – 100 mm (Lang et al., 2018a) and limited upgrading of the chemical quality is relevant before the PCC process. Hence, mineral impurities play an important role for the quality of the calcite marble, together with the processing performance (Lang et al., 2018a). From a geometallurgical perspective, several properties affect the processing performance. Inherent mineral textures and the location of the mineral impurities are crucial. Fig. 3shows different examples of marble types and the appearance of the most critical impurities, which are the sulphides and iron oxides. The mineralogical information is used to classify the marble into marble types, or ore types, which may be used to predict the processing performance of the different ore types.

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Fig. 3

The process mineralogy of calcite marbles for PCC is mostly related to understanding how mineral impurities occur in the calcite marble. With limited possibilities for upgrading the chemical quality of the feed, using modal mineralogy and mineral textures to predefine ore types is crucial. It may also be discussed if the mineral liberation properties of the main impurity minerals may contribute to minor upgrading of the chemical quality of the calcite marbles during crushing. If the main mineral impurities have significantly smaller grain sizes than the calcite and are texturally located along grain boundaries, rather than as mineral inclusions in calcite, this may be beneficial for selective liberation of mineral impurities. During crushing, these liberated mineral impurities will report to the fine fraction.

2.3. Deep sea mineral deposits

Deep sea mineral deposits have not yet been extracted as part of commercial mining operations. However, there are expectations that some exploitation permits will be awarded within the next decade or two. This may happen, either in international waters by the International Seabed Authority (ISA), or by some national government within a natiońs 200 nautical mile border. Deep sea mineral deposits may include many types of deposits (Petersen et al., 2016). In this context, deep sea mineral deposits have been used for mineral deposits, typically found outside the continental slopes. They are typically related to tectonic events, such as mid-oceanic spreading ridges, where the formation of seafloor massive sulphides (SMS) occurs. Or they can be related to the deep abyssal planes, where the polymetallic nodules are found. A third type of interesting mineral deposit is related to polymetallic accretions as well but occurs as a polymetallic crust on the bedrock that forms sea mounts. While the SMS deposits bear great similarities with the onshore traditional volcanic-hosted massive sulphides (VMS) (Franklin et al., 2005), polymetallic nodules and crusts cannot be related to any known onshore analogue.

2.4. Seafloor massive sulphides

Seafloor massive sulphides (SMS) are the recent equivalents to the volcanogenic massive sulphide (VMS) deposits mined onshore. Hence, one could claim that seafloor massive sulphides also represent potential future conventional mineral deposits, as they most likely will be processed using a similar approach as for VMS deposits. However, SMS deposits are found in significantly different geographic environments and have been exposed to nearly any metamorphic processes compared to conventional VMS deposits. Thus, they may still be regarded as unconventional mineral deposits, as they are found in an unconventional setting. Currently, there are no SMS deposits that are mined commercially (Boschen-Rose et al., 2022), and the only known pilot-scale mining has been done in the Japanese territories (e.g. Kawano and Furuya, 2022). Future commercial activities on these SMS deposits are expected to be carried out on the extinct hydrothermal systems (in geological time scale these are still recent deposits). Little has been done concerning the process mineralogy of SMS deposits (Ochromowicz et al., 2021, Snook et al., 2018). One of the reasons that little has been done in this field, may be the fact that there is limited access to sample material for performing metallurgical test work and related process mineralogy. Kawano and Furuya (2022) reported from metallurgical test work on material collected from the Japanese area during pilot testing. Part of the metallurgical test work included the characterization of the relevant SMS samples. The deposit mined by Kawano and Furuya (2022) is compared to the Kuroko-type VMS deposits, although also significantly different when it comes to the smaller grain sizes and lower crystallinity of minerals. On the other hand, Kawano and Furuya(2022) report high contents of iron sulphides and lower contents of silicates.

Fig. 4 shows one of the chimneys at the Lokís Castle hydrothermal vent field at the Arctic Mid-Ocean Ridge. During the MarMine expedition in 2016 (Ludvigsen et al., 2016), fragments from collapsed chimneys were collected and were later studied at NTNU. Fig. 5 shows a polished block from one of these fragments. A quartz/silica-rich matrix with iron sulphide zones is the dominating feature of the sample. An SEM-based automated mineralogy (AM) overview scan collected with a 10 µm step size on the QemScan AM-system, shows the details of the relationship between the quartz matrix and the iron sulphides. A small vein of Cu-sulphides can be found, as seen in Fig. 5 c). The Cu-sulphides are enveloped by sphalerite. The detailed false colour mineral map in Fig. 5 c) (2 µm step size) shows that there are more than one Cu-sulphide mineral present.

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Fig. 4

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Fig. 5

Detailed investigations of the copper and zinc minerals in the sample show that the main copper mineral is isocubanite and that chalcopyrite occurs as structurally controlled exsolutions in isocubanite Fig. 6. Additionally, sphalerite shows signs of the so-called chalcopyrite disease (Barton and Bethke, 1987) as seen in the top left sphalerite grain in Fig. 6.

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Fig. 6

The mineralogy of the Cu and Zn minerals is crucial for the metallurgy of the SMS deposits that will be mined in the future. The SMS material sampled from the Lokís Castle area contains typically very fine-grained sulphide minerals (Snook et al., 2018), hence achieving optimal mineral liberation through comminution is challenging. Additionally, complex intergrowth structures between chalcopyrite and isocubanite and the well-known chalcopyrite disease (Barton, 1978) and the high iron content in sphalerites, create selectivity issues when using conventional flotation. since the different sulphides are difficult to separate into individual concentrates (Ochromowicz et al., 2021). However, most sample material collected from SMS mineralizations to date are collected from collapsed chimney fragments. It is expected that the interior of the hydrothermal vent systems represents the more massive parts of the SMS mineralizations. Thus, more work is needed, both in sampling and characterization, of seafloor massive sulphides.