Occurrence of Orogenic and Coeur d’Alene-Type Mineral Systems

Both orogenic and Coeur d’Alene-type mineral systems are typically generated along dilatant structures during exhumation and dewatering of crustal metamorphic zones (Hofstra and Kreiner, 2020, and references therein). However, orogenic mineral systems occur in more reduced and sulfidic metamorphic zones that locally contain volcanogenic massive sulfide deposits and generate gold, antimony, mercury, and graphite deposits. Coeur d’Alene-type mineral systems occur in more oxidized metamorphic zones that locally contain basin brine path zinc-lead and copper deposits and generate polymetallic sulfide, antimony, and uranium deposits (Hofstra and Kreiner, 2020, and references therein).

Orogenic mineral systems have also been referred to as “orogenic gold systems,” “deformation and metamorphism mineral systems,” and “metamorphic-hydrothermal mineral systems.” Orogenic gold deposits represent one of the most important lode sources for gold in the world (Goldfarb and others, 2001; Frimmel, 2008). In the United States, most orogenic gold deposits are located within accreted terranes of the western cordillera of North America (fig. 1). The highest concentration of orogenic gold deposits is in Alaska, California, and Oregon (table 1), but deposits are also in Alabama, Arizona, Georgia, Idaho, Michigan, and South Dakota (in the Homestake mine). Low-sulfide gold-quartz veins in North Carolina, Virginia, and Wyoming may also be orogenic gold deposits, but further studies are needed to confirm their classification.

The abundant placer gold that fueled the California gold rush from 1848 to 1855 was derived from large, eroded orogenic gold deposits. Orogenic gold deposits were commonly identified as mesothermal or hypothermal gold deposits in the first half of the 20th century, emphasizing the inferred depth and temperature of deposit formation (Lindgren, 1933). More recently, orogenic gold deposits have been called low-sulfide gold-quartz veins for Phanerozoic examples and Homestake gold deposits for Phanerozoic to Archean types (Cox and Singer, 1986). Some have been called turbidite-hosted gold deposits (Keppie and others, 1986) or characterized by their location, such as Mother Lode gold (not shown in fig. 1; Goldfarb and others, 2005).

Coeur d’Alene-type polymetallic sulfide deposits are important sources of silver, lead, and zinc in the United States. The Coeur d’Alene-type mineral systems are named after the Coeur d’Alene district in Idaho and Montana, which is one of the largest silver producers in the world (Long, 1998b). These systems are also referred to as “mesothermal base and precious metal systems” (Harrison and others, 1986), “silver-lead-zinc veins in clastic metasedimentary terranes” (Beaudoin and Sangster, 1992), and “orogenic base-metal systems” (Skirrow and others, 2013). Although Coeur d’Alene-type mineral systems concentrate different commodities, they share broad geometric similarities with large kilometer-scale ore shoots.

In the United States, Coeur d’Alene-type mineral systems are largely in deformed and faulted metasedimentary rocks of the Proterozoic Belt Basin of Idaho and western Montana (Leach and others, 1998), the Paleozoic Arkoma basin in southwest Arkansas (not shown in fig. 1; Howard, 1979), and the Spruce Creek sequence in the Katishna Hills at Quigley Ridge in Alaska (not shown in fig. 1; Bundtzen, 1981; fig. 1; table 2).

Formation of Orogenic Mineral Systems

A temporal relationship exists between the accretion of juvenile crustal terranes and the formation of orogenic mineral systems (Goldfarb and others, 2001). Orogenic mineral systems form late during the collisional orogenic process and after regional metamorphism of the accreted terranes (Goldfarb and others, 2005). Deformation and prograde metamorphism of midcrustal rocks during an accretionary orogeny can release large volumes of fluid (Elmer and others, 2006; Phillips and Powell, 2010), bisulfide (HS^–) ligands (Tomkins, 2010), and gold and other metals (Pitcairn and others, 2006, 2010) as a result of metamorphic reactions near the greenschist to amphibolite facies transition. These gold-rich hydrothermal fluids are subsequently focused and flow upwards along dilatant structures in concert with the earthquake cycle. High fluid pressure can induce fault ruptures and swarm seismicity, during which enormous volumes of metamorphic hydrothermal fluid can channel and flow upwards along major structures that only display small displacement (for example, Sibson, 1987; Sibson and others, 1988; Cox, 2016).

As the metamorphic hydrothermal fluids are pumped towards shallower depths, ore deposits may form within favorable chemical and structural traps. Metals are introduced by ascending hydrothermal fluids, but additional metals may also be derived from local aquifers and host rocks through fluid–fluid and fluid–rock reactions at the deposit site. The metals within orogenic systems are dependent upon the types of rocks undergoing metamorphism at depth, the immediate host rock of the shear zone and veins, and the depth of formation (fig. 2; Goldfarb and others, 2005). A continuum of deposits within this system goes from deeper level gold through shallower mercury deposits. The major commodity within orogenic gold deposits is gold. Other critical minerals that may be enriched in these deposits include arsenic, tellurium, and tungsten. Additionally, antimony, mercury, and graphite deposits may be present in other parts of the system; antimony and graphite are critical minerals (Hofstra and Kreiner, 2020).

The energy source required for forming an orogenic gold deposit is orogenesis and its associated metamorphism. Fluids, ligands, and metals are all sourced from supracrustal rocks undergoing metamorphism in the midcrust. Reduced oceanic volcanic arc and associated sedimentary rock domains containing pyrite (FeS₂) are likely to be a favorable protolith (source rock) that release fluids (Elmer and others, 2006; Phillips and Powell, 2010), ligands (Tomkins, 2010), and metals (Pitcairn and others, 2006, 2010), particularly during the greenschist to amphibolite facies metamorphic transition.

Fluid inclusion studies demonstrate the predominance of low-salinity water (H₂O)–carbon dioxide (CO₂; ± methane [CH₄], dinitrogen [N₂]) fluids in orogenic gold deposits (Goldfarb and Groves, 2015); there are rare examples containing only miniscule amounts of CO₂ (Taylor and others, 2021). Generation of an H₂O–CO₂ fluid composition would be expected through metamorphic devolatilization reactions occurring in the midcrust (Fyfe and others, 1978; Powell and others, 1991), and the general consistency in fluid characteristics between different orogenic gold deposits is suggestive of a common fluid source. Within accretionary belts such as the California Mother Lode (not shown in fig. 1), accreted oceanic volcanic arc and associated sedimentary rocks are the likely source of metamorphic fluids associated with orogenic gold deposits through dehydration reactions such as the conversion of chlorite to amphibole (Elmer and others, 2006; Phillips and Powell, 2010).

Although numerous potential ligands can be found in the hydrothermal fluid, bisulfide (HS^–) is the most important ligand for gold complexing (Loucks and Mavrogenes, 1999). Desulfidation reactions of a reduced rock containing pyrite as the sulfur source are necessary to generate these gold-complexing ligands. The pyrite to pyrrhotite (Fe_(1–X)S) reaction that releases sulfur occurs at the greenschist to amphibole facies transition, which is the same important fluid-generating transition detailed in the previous paragraph (Tomkins, 2010). Major ore metals—including gold, arsenic, bismuth, tellurium, and tungsten—are released through prograde metamorphic reactions in metasedimentary rocks, particularly at the greenschist to amphibolite facies transition (Pitcairn and others, 2006). These ore metals are largely stripped from pyrite (Pitcairn and others, 2010), and although sedimentary pyrite is an important source for gold, basalt may also provide a source for the ore metals (Pitcairn and others, 2015).

Orogenic gold deposits form in association with crustal-scale shear zones that act as fluid pathways that link the metamorphic area of fluid, ligand, and metal generation with the mineralization site and shallower outflow region (fig. 3). The spatial association between major orogenic gold deposits and crustal-scale regional fault zones has been established (for example, Turneaure, 1955), although the sites of mineralization are typically in lower order faults where more efficient structural and chemical traps allow gold mineralization. Geopressurized fluids are channeled along the permeable first-order structures during seismic events (Sibson and others, 1988; Cox and others, 2001; Cox, 2016). In California, these major structures commonly mark boundaries between various amalgamated terranes.

Within orogenic gold deposits, gold may be in its native form, as gold telluride minerals, or as invisible gold found as nanoinclusions or structurally bound within sulfide minerals (generally, pyrite or arsenopyrite). Ore deposition is postulated to occur through various physicochemical processes that lower gold solubility within the ore fluid. Fluid–rock interaction is of primary importance, especially for nanoinclusions or structurally bound gold associated with sulfides. During fluid interaction of rocks with high iron (Fe) to iron plus magnesium (Mg; Fe+Mg) ratios, gold will become destabilized from its bisulfide ligand, and pyrite will form from the reaction of iron in the host rock with sulfur in the fluid along with concomitant deposition of gold (Phillips and Groves, 1983; Böhlke, 1988). In addition to iron-rich rocks, carbon-rich rocks are also important reductants of the fluid that can lead to gold mineralization (Cox and others, 1991). Hydraulic fracturing, an important process in the formation of orogenic gold deposits, can trigger pressure fluctuations within the ore-forming environment from supralithostatic to hydrostatic pressure conditions. Pressure changes alone are an effective mechanism that alter gold solubility and cause gold precipitation (for example, Groves and others, 1987; Loucks and Mavrogenes, 1999). Agglomeration of colloidal gold in the hydrothermal fluid resulting from pressure changes may result in large sponge- and dendrite-like masses of gold (Taylor and others, 2021). Additional mechanisms of lesser importance that may result in gold precipitation include pH decrease of the hydrothermal fluid, fluid mixing, changes in oxygen fugacity (ƒO₂), and temperature changes.

Hydrothermal alteration related to gold precipitation occurs at the deposit site due to fluid–rock interaction and may occur pre-, syn-, and postmineralization. Early alteration may increase the competency of the host rock, allowing brittle fractures to develop that host gold mineralization (for example, Poulsen and others, 1986). The composition and structure of the wall rock plays a significant role in what alteration products are present. Quartz-sericite (muscovite)-pyrite and carbonate (calcite, ankerite, ferroan dolomite, magnesite, and siderite) alteration are fundamental alteration types. Talc and fuchsite may substitute for sericite or muscovite within ultramafic wall rocks, and magnesite and siderite are the accompanying dominant carbonate minerals.

In addition to gold and silver, other ore minerals may include pyrite, arsenopyrite (FeAsS), galena (PbS), sphalerite (ZnS), chalcopyrite (CuFeS₂), stibnite (Sb₂S₃), scheelite (CaWO₄), graphite (C), rutile (TiO₂), tetrahedrite ([Cu,Fe,Zn,Ag]₁₂[Sb,As]₄S₁₃), and various telluride minerals (fig. 4); of these ore minerals, arsenopyrite (arsenic), stibnite (antimony), scheelite (tungsten), graphite, and telluride minerals (tellurium) are important host phases for critical minerals (table 3). The most common telluride minerals in orogenic gold deposits include calaverite (AuTe₂), sylvanite (AgAuTe₄), hessite (Ag₂Te), and petzite (Ag₃AuTe₂; table 3). Pyrite may contain critical metals such as arsenic, cobalt, and nickel, but the metals are likely found as minor to trace components within the crystals (Taylor and others, 2021). Gangue (the valueless rock or mineral aggregates in an ore) mineralogy is dominated by quartz and various carbonate minerals and lesser amounts of mica (such as muscovite, chlorite, and fuchsite).

Formation of Coeur d’Alene-type Mineral Systems

Coeur d’Alene-type mineral systems form in a manner similar to orogenic mineral systems, but there are some key differences: Coeur d’Alene-type mineral systems have different source rocks and more saline fluids, have chloride as the major ligand, and comprise a different suite of metals. Major commodities within Coeur d’Alene-type polymetallic sulfide deposits are silver, lead, zinc, and copper; minor commodities in these deposits include gold and byproduct cadmium, cobalt, antimony, and barite. Of the major and minor commodities, antimony, barite, cobalt, and zinc are considered critical minerals. Additional critical minerals that may be enriched in these polymetallic sulfide deposits include arsenic, bismuth, gallium, germanium, indium, and manganese. Antimony and uranium deposits occur in other parts of the system; antimony is a critical mineral (Hofstra and Kreiner, 2020).

Coeur d’Alene-type systems have also been called “mesothermal base and precious metal systems” (Leach and others, 1988), “polymetallic sulfide and antimony deposits” (Long, 1998a), or “silver-lead-zinc veins in clastic metasedimentary rock terranes” (Beaudoin and Sangster, 1992). In the United States, these types of deposits are largely in deformed and metamorphosed rocks of the Proterozoic Belt Basin along the Lewis and Clark line (a fault zone that extends from near Wallace, Idaho [not shown in fig. 1], to east of Helena, Montana; Wallace and others, 1990) in Idaho and Montana (fig. 1). Zinc-, lead-, silver-, and gold-rich ores are in different districts and are suggested to have formed during the Proterozoic or Late Cretaceous–early Tertiary (Zartman and Stacey, 1971; Leach and others, 1998; Ramos and Rosenberg, 2012). Metamorphism accompanying orogenesis provides the energy required to form Coeur d’Alene-type deposits. Fluids, ligands, and metals are all sourced from siliciclastic sedimentary rocks undergoing metamorphism in a similar manner to what is outlined for orogenic mineral systems (refer to the “Formation of Orogenic Mineral Systems” section).

Oxygen isotope studies of vein quartz from the Coeur d’Alene region indicate a mix of meteoric and metamorphic fluids derived from rocks of the Belt Basin were involved in ore deposition (Constantopoulos and Larson, 1991; Constantopoulos, 1994). In addition to oxygen isotopes, lead isotopes of sulfides and strontium isotopes of vein carbonate minerals further suggest that strontium and lead were derived from rocks of the Belt Basin and pre-existing lead-zinc mineralization, either in the Proterozoic or Mesozoic (Rosenberg and Larson, 1996; Leach and others, 1998; Fleck and others, 2002). Another study of lead, strontium, and neodymium isotopes in carbonate minerals suggested that these elements were also derived from Archean basement rocks (Ramos and Rosenberg, 2012). Ore deposition occurred between 250 and 350 degrees Celsius (°C), based upon fluid inclusion analysis (Leach and others, 1988). Fluid mixing of deeply sourced and meteoric fluids is an important process leading to mineralization (Beaudoin and Sangster, 1992). However, the correlation between productive veins and specific stratigraphy highlights the importance of local fluid–rock interactions and favorable rheology to mineralization; base metal veins are more commonly located within quartzite, whereas silver-rich veins are within silicified siltstone of the Proterozoic Revett Formation (Mauk and White, 2004), and quartz-stibnite veins are in the Proterozoic Prichard Formation for deposits in the Coeur d’Alene region (Hofstra and others, 2013).

Although bisulfide complexes are the most important ligand for complexing gold within orogenic gold deposits, fluid inclusion analyses by Hofstra and others (2013) indicate that antimony and silver-rich veins within the Coeur d’Alene region contain low concentrations of hydrogen sulfide (H₂S). In contrast, the moderate to high salinity indicates that chloride complexes were important ligands transporting metals during formation of silver and antimony veins. Chloride complexes are also inferred to be important in the Coeur d’Alene base metal veins, which has been demonstrated for zinc and lead mobility during metamorphism (Hammerli and others, 2015).

The source of metals in Coeur d’Alene-type polymetallic veins is considered the Belt Basin. Sedimentary rocks, potentially the pyrite-bearing rocks of the lower part of the Prichard Formation, which contains sedex lead-zinc deposits in the Canadian part of the Belt Basin, likely released lead, zinc, silver, and antimony during metamorphism. This model is supported by lead isotope studies showing that Coeur d’Alene ores have Proterozoic lead signatures similar to those signatures of the Sullivan lead-zinc sedex deposit in Canada (Fleck and others, 2002). The silver-rich veins are hosted in the Revett Formation, which regionally contains sedimentary rock-hosted copper deposits (Boleneus and others, 2005), including the Snowstorm deposit at the east end of the district. The correlation between copper, silver, and antimony in these veins suggests that they formed as ascending fluids that contained antimony, silver, and variable lead and zinc mixed with copper-bearing fluids derived from the Revett Formation (table 4).

The shear zone veins of the Coeur d’Alene region are located within the western part of the Lewis and Clark line, which extends from Coeur d'Alene, Idaho, to Helena, Montana (Wallace and others, 1990). This fault zone contains high-angle faults of variable kinematics that have been intermittently active since the middle Proterozoic with major strike-slip movement in the Late Cretaceous (Bennett and Venkatakrishnan, 1982; Wallace and others, 1990). Individual faults within this fault zone may stretch for as many as 250 kilometers along strike. Ore-bearing veins are preferentially located within fractures and faults that intersect large folds within brittle host rocks; the ore province itself is located within the most deformed part of the Lewis and Clark line.

For Coeur d’Alene-type mineral systems, phyllic alteration composed of sericite, quartz, and pyrite is common for the wall rocks adjacent to veins (Beaudoin and Sangster, 1992). In the Coeur d’Alene region, however, alteration may be broader and is typified by bleaching of the rock through destruction of feldspar and hematite and the addition of siderite and sulfides (Fryklund, 1964; Mauk and Strand, 2002).

Mining and Beneficiation of Ore Deposits and Materials

Underground mining techniques have historically been the most common method of mining orogenic gold deposits, but open-pit mining is not uncommon. The grade-tonnage model of Bliss (1986) indicates that 50 percent of deposits have gold grades of at least 16 grams per tonne (g/t) and ore tonnages of at least 30,000 tonnes, which are indicative of the high-grade low-tonnage nature of these deposits. Because of the continued increase in gold prices, open-pit bulk mining of lower grade, hydrothermal alteration zones that would have previously been considered waste material has become more common since the Bliss (1986) grade-tonnage model was created. Inclusion of lower grade alteration zones has led to changes when applying the grade-tonnage model of Bliss (1986) to the current (2025) economic environment (Davies and others, 2020).

Mining techniques for Coeur d’Alene-type systems are similar to techniques for orogenic mineral systems because both are vein deposits. Underground mining methods predominate, but open-pit mining is occasionally used. Underground mining by stoping is common, but the room-and-pillar mining method is also considered a viable method. Using tailings as backfill is a common practice.

Beneficiation of ore materials is done through initial crushing and milling, gravity separation, flotation, and cyanide extraction with carbon leaching (Adams, 2016). Coarse-grained native gold from orogenic gold deposits has historically been concentrated through gravity separation techniques because of its high specific gravity that permits easy separation from other associated gangue and ore minerals in veins. The gravity separation method allows the remaining material, including critical minerals, to be collected in piles that are free of any processing chemicals. Another standard practice that is especially efficient for recovery of fine gold particles, low grade ore, or gold that is inadequately separated from the rock is cyanide extraction and carbon leaching. Cyanide extraction and carbon leaching can be done by itself after crushing and milling or after an additional step of concentrating ore minerals through froth flotation. Refractory gold, such as gold tellurides, requires oxidation, flotation, and leaching to free the gold at high recovery rates (Zhang and others, 2010). Roasting followed by cyanide leaching has been the preferred method of gold recovery from telluride ores (Ellis and Deschênes, 2016). Preleaching of tellurium from gold telluride ore prior to removal of gold has been investigated as a way to collect tellurium and to avoid any detrimental effects that this critical mineral has on the recovery of gold (Yang and others, 2021). Mercury amalgamation was an early method used for gold recovery but is rarely used today because of health and environmental hazards related to mercury.

Beneficiation of Coeur d’Alene-type ores begins with milling (crushing and grinding) and is followed by flotation or gravity separation that produces the metal concentrates (McMahon and others, 1974). Flotation reagents include sodium cyanide, zinc sulfate, lime, copper sulfate, xanthate, and methyl isobutyl carbonyl, which are common for processing polymetallic mineralization. Sintering of concentrates may be done depending on the processing methodology. Concentrates are then shipped to a smelter.

Geochemistry of Orogenic Gold Deposits

Samples with gold values above 0.5 parts per million (ppm; 0.5 g/t or 0.16 ounces per short ton) cutoff (n=67) are plotted relative to average upper crustal abundance in figure 5. The median, minimum, and maximum concentrations of each element are listed in table 5.

Analyzed samples contain high concentrations of antimony, arsenic, cadmium, cobalt, copper, iron, molybdenum, silver, tellurium, tungsten, and zinc. Gold concentrations are plotted against the concentrations of some select critical minerals (arsenic, bismuth, cobalt, antimony, tellurium, and tungsten) for all samples in figure 6. Correlation coefficients for critical minerals with gold are reported in table 6. A much stronger correlation between gold and tellurium may be found for individual deposits, especially those deposits hosted by local intrusions within metasedimentary rock-dominated orogens (Goldfarb and others, 2005, 2017). For example, gold mined at the Kensington deposit, Alaska (not shown in fig. 1), is predominantly hosted in telluride minerals and would therefore have a very strong correlation between the two metals; no ore geochemistry for the Kensington deposit is available for this study, but analysis by Heinchon (2019) indicated a significant correlation between gold and tellurium recording a coefficient of determination (R²) value of 0.82.

Geochemistry of Coeur d’Alene-type Deposits

The median, minimum, and maximum concentrations of each element in silver-rich Coeur d’Alene-type ore samples with a cut off value of 250 ppm silver (n=44) are listed in table 7 and are plotted relative to average upper crustal abundance in figure 7. The median, minimum, and maximum concentrations of each element in zinc-rich Coeur d’Alene-type ore samples with a cut off value of 3 weight percent zinc (n=36) are listed in table 8 and plotted relative to average upper crustal abundance in figure 8.

Major ore minerals in Coeur d’Alene-type deposits include sphalerite, galena, tetrahedrite, stibnite, chalcopyrite, pyrite, and arsenopyrite (table 3). Other important minerals include boulangerite (Pb₅Sb₄S₁₁) and scheelite. Gersdorffite (NiAsS) is uncommon except for at the Silver Summit mine where it was a large enough component to be considered cobalt and nickel ore (Fryklund, 1964). Pyrrhotite (Fe_1-XS) from the Coeur d’Alene region contains per mille levels of cobalt and nickel (Fryklund and Harner, 1955). Common gangue mineralogy includes quartz, siderite, dolomite, and calcite. Antimony has been produced in the Coeur d’Alene region, and antimony contents are typically higher in silver-rich ores and in quartz-stibnite veins (Leach and others, 1998).

Notable enrichments of critical minerals compared to upper crustal abundance for silver-rich ore (silver concentration greater than 250 ppm) include antimony, arsenic, bismuth, indium, and manganese (table 7). Critical mineral enrichments relative to upper crustal abundance for zinc-rich ores (zinc concentration greater than 3 weight percent) include antimony, arsenic, bismuth, cobalt, germanium, indium, manganese, selenium, and zinc (table 8). Zinc is a primary commodity mined in Coeur d’Alene-type ores (Long, 1998a, b) but is still considered a critical mineral (USGS, 2022a). Tungsten is elevated in a few samples that can be either silver- or zinc-rich but is more commonly found at levels near or below the detection limit.

Of the enriched critical minerals in Coeur d’Alene ore, only a few have median concentrations in the analyzed ore greater than 50 ppm. In silver-rich ore (table 7), the highest median values for the enriched critical minerals include antimony (median value of 1,635 ppm), arsenic (median value of 154.5 ppm), manganese (median value of 1.02 weight percent), and zinc (median value of 1.62 weight percent). In zinc-rich ore (table 8), the highest median values for the enriched critical minerals include antimony (median value of 323.5 ppm), manganese (median value of 0.41 weight percent), and zinc (median value of 18.5 weight percent). The median value for cobalt in zinc-rich ores analyzed in this study is 27.95 ppm; however, cobalt was stockpiled as a byproduct at the Bunker Hill zinc smelter (Davis and Buck, 1958). The concentration of the major commodities silver, lead, and zinc are plotted against the concentrations of select critical minerals (antimony, arsenic, bismuth, cobalt, indium, manganese, selenium, tungsten) in figures 9–11, respectively, and the major commodities are plotted against each other in figure 12. Chemical data important for silver-rich ores are plotted in figure 13 for comparison with mineral chemistry data for tetrahedrite in figure 14.

Using a limited dataset, Long (1998a) showed that there is neither a significant correlation among silver, lead, or zinc grades for lead-zinc veins, nor among silver, copper, and lead grades for silver veins. The additional dataset analyzed in this report supports the assertions made by Long (1998a). The strongest positive correlation coefficients with silver in Coeur d’Alene-type deposits (table 4) are for gold (0.80), copper (0.63), and palladium (0.63); the strongest positive correlation coefficients with zinc are sulfur (0.90), cadmium (0.77), and indium (0.46); the strongest positive correlation coefficient with lead is sulfur (0.27); and the strongest positive correlation coefficients with antimony are copper (0.75), gold (0.60), and silver (0.49).

Tetrahedrite ([Cu,Fe,Zn,Ag]₁₂[Sb,As]₄S₁₃) is the main ore mineral for silver (table 3). Although the chemical formula for the tetrahedrite series of minerals contains copper, iron, zinc, antimony, and arsenic, there is no demonstrated correlation with silver and iron or zinc in ore. There is a demonstrated positive correlation between silver and copper (0.63), and arsenic and antimony in ore (fig. 13) even though tetrahedrite crystals display negative correlations between silver and copper (−0.987) and arsenic and antimony (−0.958; fig. 14) because of elemental substitutions in the tetrahedrite crystal structure (for example, Biagioni and others, 2020). Fryklund (1964) hypothesized that significant bismuth and mercury may be found within tetrahedrite ore, although the correlation coefficient between bismuth and silver is low and mercury was not analyzed.

Abundant carbonate phases, including siderite, dolomite, and calcite compose much of the gangue in Coeur d’Alene-type deposits. Calculated correlation coefficients between carbon and iron (0.77), carbon and manganese (0.80), and iron and manganese (0.67) indicate that siderite (FeCO₃) is the main residence of iron and manganese (table 4). Critical minerals that may be minor constituents in sphalerite include gallium, germanium, indium, and manganese (Cook and others, 2009). The weak negative correlation between zinc and manganese (−0.34) in ore indicates that manganese does not reside in sphalerite. The low correlation coefficients between zinc and gallium (0.19) and zinc and germanium (0.20), together with the low median concentrations of gallium (6 ppm) and germanium (2 ppm) in zinc ore, suggest that these critical minerals would unlikely be economic to recover.