"Rock of the Month #200, posted for February 2018" ---

Galena

- to quote from the recent discussion of the Madan mining district in Bulgaria - "is one of the most familiar ore minerals, noted for its cubic crystal forms, sub-metallic grey lustre, and serious heft (density: specific gravity 7.5). It is soft (Mohs 2.5) with a grey-black streak". Three samples are depicted and discussed this month.

The first two samples are from the Viburnum Trend, a famed belt of lead deposits in the midcontinental United States. Galena is a very widespread ore mineral, with wide distribution in: base and precious metal veins (with other metals such as copper and zinc, silver and gold); volcanogenic massive sulphides; sedex (sedimentary exhalative) deposits; and Mississippi Valley Type ores, the latter hosted in carbonate sedimentary sequences. Minor amounts also occur in uranium deposits and other settings.

Mississippi Valley Type (MVT) deposits

are hosted in carbonate sedimentary sequences of relatively shallow-water continental shelf sediments. They are associated with dolomitization of limestones, and multiple stages and styles of brecciation. Sphalerite (ZnS) and galena (PbS) are the classic ore minerals in fresh ore. These sulphides may be carried along by ice in glaciated regions, thus becoming indicator minerals for exploration in such terrain. One research group that focused on MVT ores and their genesis was that led by Frank Beales at the University of Toronto. Well-known Canadian MVT deposits include Pine Point, Polaris, Nanisivik and Daniel's Harbour.

The Viburnum Trend

The Viburnum Trend in southeast Missouri is a discrete mineralized belt, as detailed below. It lies well east of the famed Tri-State region of west Missouri, northeast Oklahoma and southeast Kansas (see below). Mines on the Viburnum Trend include Magmont and Magmont West, Buick (Moloc) and Sweetwater, West Fork, Fletcher and Brushy Creek.

Sample 2522 (Fig. 1) - Coarse crystalline galena from the storied Magmont mine. The Magmont ores are of Devonian-Carboniferous age, circa 360 Ma. Magmont Mine area glauconites yield an Rb-Sr isochron age date of 359±22 Ma with initial Sr isotope ratio of 0.7234±0.0273 (2σ). The glauconite age is much younger than the Cambrian host rocks, and probably represents the time of ore formation in S.E. Missouri (Stein and Kish, 1985).

Sample 3095 (Fig. 2) - Very coarse galena showing textbook perfect cubic cleavage, from an unspecified mine. Irwin Kennedy (pers.commun., 2013) questioned the Viburnum Trend association for this sample, suggesting that it could be from elsewhere in the midwest: maybe the Picher mine (Oklahoma) or Baxter Springs (Kansas). He further suggests (pers.commun., 2018), not at all facetiously... "After many years of statistical analysis, I know there's a 60% chance it's from Kansas, 39% chance it's from Oklahoma and 1% chance it's from West Missouri". A brief note on this region, the "Tri-State district", follows this section. This said, Darryl Macfarlane of Grenville Minerals is confident of his source, and - though it is not absolutely verifiable - plumps 99% for the Sweetwater mine, in the Viburnum Trend.

The origin of MVT deposits involves the migration of fluids in sedimentary basins, with metal transport by chloride-rich brines and bacterial reduction of metal-bearing complexes. MVT deposits appear at the interface between the organic-geochemical environment of the "oil window" and more typical hydrothermal systems. Organic matter and sulphur may appear in such materials as anhydrite and bitumens (Anderson and Beales, 1984). Dolomites are the "favored host rocks" for low-temperature Pb-Zn ores, perhaps because of their origins in evaporitic settings. Evaporite strata were present in large breccia-hosted ore deposits in southeast Missouri: gypsum and anhydrite are preserved as inclusions in host dolomites (Beales and Hardy, 1980). Tectonic uplift after the Alleghanian orogeny created the topography required to drive brines out of the basins and onto adjacent domes where the deposits were formed (Garven et al, 1993).

The southeast Missouri lead district, including the Viburnum Trend, has seen over three centuries of exploration and mining since French explorers noted riverbank lead showings circa 1700 (Wharton et al., 1975; Ohle, 1996). The St. Joseph Lead Company was formed in 1864 to exploit ores at Bonne Terre in St. Francois county. Ore reserves in the Lead Belt were much depleted by the end of World War II. Exploration continued and the initial discovery near Viburnum was made by St. Joe, drilling in September 1955, with initial production from St. Joe's Viburnum No.27 mine which opened in mid-1960. The Viburnum Trend then rose to be the world's largest Pb producer, with over 15% of global recorded production. Competition between exploration companies following the 1955 discovery of the district led to secrecy, and little published data, a situation remedied by publication of a field guide to the Viburnum Trend (Wharton et al., 1975). There are five main centres of mineralization in an area of 2300 km². There are two major lead belts in the region. The Old Lead Belt lies around Bonne Terre, Desloge, Leadwood and Fredericktown. The New Lead Belt (Viburnum Trend) extends south for about 40 miles (64 km) from the town of Viburnum (Ohle, 1990).

Bedrock Cambrian and Ordovician sediments (Lamotte sandstone at the base, then a thick series of dolomites) lie unconformably upon Precambrian basement. The largest deposits are in the Upper Cambrian Bonneterre Formation, which is about 90 m thick at the Magmont mine. Collapse breccias are very important ore carriers in the Viburnum Trend, but nonexistent in the Old Lead Belt. The Viburnum Trend mineralization was more intense and less controlled by host rock types (19 MT of metal in 400 MT of rock, cf. 9 MT in 300 MT of rock in the Old Lead Belt). The gangue minerals are dolomite, calcite, quartz, dickite and bitumen. Ore and gangue minerals alike were introduced into the host sequence, the upper Cambrian Bonneterre Formation, which in the vicinity of ore deposits consists of dolomite and shaly dolomite. The detailed paragenesis (sequence of deposition) is complex (Hagni, 1986, 2000). Some metals are recovered as byproducts, including Ag, Cd, Ga and Ge. The content of silver in pyrite depends on the crystal habit of the iron sulphide host. Unusually, Ag is more concentrated in sphalerite (and the Zn concentrate) than in galena and the Pb concentrate (Hagni, 2000).

MVT paragenesis may include multiple habits of galena, e.g., early-late: cubes, cubo-octahedrons, and octahedrons (Heyl, 1983). A study of some 400 ore samples from the No.7 orebody in the Sweetwater mine described a spectrum of galena crystal habits ranging from pure octahedral to pure cubic forms. Most of the 10 recognized types can be recognized by hand specimen observations (Sun et al., 1993a,b). At the Sweetwater mine, galena with complex arborescent habits may display up to 4 sequential textural generations (Layne et al., 1989). Sulphide textures may depend also on adjacent gangue, as at Magmont, with its "saw kerf galena" (Marikos, 1989). Bitumen occurs in the Bonneterre dolomite. At the Sweetwater mine there is an association of Cu and Pb sulphides (chalcopyrite and galena) with anthraxolite (hydrocarbon). The occurrence of dendritic galena in anthraxolite establishes a direct link between late-stage mineralization and hydrocarbons. A similar link may occur with mineralization at Magmont (Niewendorp and Clendenin, 1993).

If evaporitic sulphates are a plausible. proximal source of sulphur, then Pb isotopes indicate that sources of lead in the Viburnum Trend could include both oil-field brines and the Precambrian (Grenville, 1200-1000 Ma) basement. SIMS (ion microprobe) analyses indicate that Pb in early octahedral galena is less radiogenic than Pb in late cubic galena (Crocetti et al., 1988). Rapid changes in Pb isotope ratios in growth zones in galena, on a scale of 250 microns, suggest discontinuous inputs of Pb from one or more sources (Deloule et al., 1986). SIMS analysis also reveals similar variations in S isotopes (McKibben and Eldridge, 1995). A fluid inclusion study of the Bonneterre Formation, seeking sources of basinal brines and epigenetic dolomite, found no discernible temperature gradient over an area >25,000 km². Multiple basinal fluids appear to have interacted, coeval with dolomitization and Pb-Zn ore formation (Shelton et al., 1992). While galena is our focus here, sphalerite is another major ore mineral. One form, "schalenblende" was originally a mixture of the two ZnS polymorphs, sphalerite and wurtzite, as found at the West Fork mine. The wurtzite later inverted to sphalerite (Mavrogenes et al., 1992). At the Fletcher mine the accessory sphalerite is generally found with higher-grade galena, in open space fillings with secondary dolomite (Wharton et al., 1975).

Tri-State District

The Tri-State district of the midwest and southern USA was a major lead and zinc producer from 1850 to 1950, with over 4,000 mines beneath a 2,000 square-mile (5,200 km²) area of southwest Missouri, southeast Kansas and northeast Oklahoma. Caves in the MVT deposits have yielded many fine specimens (Brockie et al., 1968; Bicknell et al., 2017). Hydrothermal fluids on the Viburnum Trend are enriched in potassium, whereas fluids of the Tri-State deposits have low K contents and characterize deposits where the fluids migrated through large volumes of carbonate rock (Viets and Leach, 1990). The wider Ozark region includes the Viburnum Trend, the Old Lead Belt and the Tri-State districts, plus other mineralized regions. The various MVT districts may have formed locally in a large, interconnected hydrothermal system (Plumlee et al., 1994).

Keno Hill, Yukon

Keno Hill and Galena Hill lie roughly 60 km northeast of Mayo and 350 km north of Whitehorse. The Keno Hill Ag mining district produced more the 217 million oz of silver, 1913-1989, from ore carrying bonanza average grades of 1389 ppm Ag plus 5.6% Pb and 3.1% Zn. Recent exploration has included the Bellekeno, Onek and Lucky Queen mines. The ore at Lucky Queen consists of native Ag with disseminated Ag sulphosalts, plus galena and sphalerite in quartzite (Anon, 2009). Substantial Ag-Pb ores were discovered at Keno Hill in July 1919 and ore shipments began in the winter of 1920-1921. It was soon noted that freibergite is common, and one of the chief Ag minerals, while siderite is the most abundant gangue phase (Cockfield, 1924). By 1930, 14% of Canadian Ag production was sent downriver from Mayo on the shallow-draught sternwheeler Keno. Low silver prices and high transport costs ended operations in 1932, but later mining continued until the closure of the last operation, United Keno Hill, in 1989. The "Silver Trail" of the south-central Yukon is now a touristic attraction (Lundberg, 2002).

The Keno Hill camp is an example of Ag- Pb- Zn veins in a clastic metasedimentary terrane, and as such can be compared to the Kokanee Range in BC, Coeur d'Alene, Idaho, Freiberg and the Harz Mountains, Germany and Pribram, Czechoslovakia. Such veins exhibit distinct metal ratios, and contain galena, sphalerite, Ag minerals and sulphosalts, with siderite, quartz or calcite gangue. The veins are typically enclosed by spatially restricted phyllic hydrothermal alteration. The veins are typically late features in the evolution of an orogen, often near a crustal-scale fault, and are not genetically related to felsic intrusions. Mineralization is precipitated from dilute to saline hydrothermal fluids at 250-600℃, at an average depth of 6 km. Pb is mostly locally derived from upper crustal rocks (Beaudoin and Sangster, 1992).

Robert Boyle investigated the Keno Hill camp over a number of years. He noted that thick-bedded quartzites and greenstones were the most favourable host rocks, and that dilational zones associated with three styles of "vein faults" hosted lodes (Boyle, 1957). Carmichael (1957) provides additional observations on faulting in the local mines. Breccias in the mineralized structures are cemented by siderite and quartz,and in some zones by galena and sphalerite. There are at least three stages of hypogene (primary) mineralization, plus a complex suite of secondary minerals in the supergene zone: "both silver and lead have been enriched in the oxidized zones; zinc has been markedly leached (ibid., p.59). Boyle described native silver in deeply oxidized zones, and the native metal was even found in ice in the permafrost zone at local mines (Boyle, 1960; Cathro, 2006).

In his monograph on the area (Boyle, 1965) he notes that siderite and ankerite are the main hypogene carbonates. Ore minerals include arsenopyrite, chalcopyrite, freibergite, galena, sphalerite, and pyrargyrite. Galena may be invaded and partially replaced by tongues of anglesite (ibid., p.130). The lead sulphide (ibid., pp.127-133) is said to contain more silver when massive than coarsely crystalline, and so schistose and microcrystalline galena may be more Ag-enriched. Ag, together with Sb and Bi, is known to be soluble in Pb sulphide, so that these elements may enter galena in solid solution during crystallization. Upon cooling, a proportion of these elements may exsolve from the host galena to form small inclusions. Most polished sections reveal tiny inclusions in galena, silver minerals such as freibergite and pyrargyrite. The author eventually published a book on the natural occurrence of silver (Boyle, 1968) and, later, other volumes on gold and uranium.

The Keno Hill district displays zoned mineralization, spatially associated with a Cretaceous granitic pluton intruding quartzite. The vein system is largely restricted to highly fractured, graphitic Keno Hill Quartzite of Carboniferous (Mississippian) age. The quartzite acted as a district-scale aquifer for ore fluids, and erosion has revealed a vein system 40 km long (Lynch et al., 1990). 7 mineralogical zones are recognized. Tetrahedrite is distributed along 25 km of the system, and is the main Ag ore mineral: Ag/Cu and Fe/Zn ratios are highest at the outer edges of the system, where freibergite dominates over tetrahedrite (Lynch, 1989).

Sample 2273 is from the Keno Hill silver mines near Mayo, Yukon Territory, northwest Canada. These are vein-type ores, as distinct from the MVT deposits. Lorne Burden (pers.commun., 2018) notes that the silver-rich galena ores from the Keno Hill mines display curved crystal faces in hand specimen. I have no assay data for the sample in Fig. 4, but the appearance of the crystals is consistent with this interesting empirical observation. The explanation is presumably based on the crystal chemistry of galena: the silver atom is much smaller than the lead atom, for which the silver atom would substitute in argentiferous galena (see also the note on Santa Eulalia, below).

Figures 3-4.
Top: The Farquhar+++ collection of lead isotopic samples were stored in glass jars. Here is this month's third sample, with the original label, beside two of Robert Boyle's publications on the Keno Hill district.
Below: This is sample 455 of the Farquhar collection. The labelled provenance is the "Makeno mine, Mayo", collected by E.D. Riddle in 1952 and obtained via R.W. Boyle of the Geological Survey of Canada *. GCW sample 2273, rescued from oblivion in 2002. The piece is 403.56 grams, 6x4x3 cm in size. Boyle also provided Farquhar with the latter's sample 457 (GCW 2235), a galena ore he collected in 1951 from the B.B. Group at Indian Mountain Lake, north of the East Arm of Great Slave Lake, N.W.T.

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