November 2008

Economic Geology

Homestake, South Dakota (Part 3)

By R. J. Cathro

“The Homestake gold mine is the largest producer from a single deposit outside South Africa and the largest deposit of its type in the world. Owing to the tremendous size of the gold system, the geologic model of this largely stratabound deposit is known worldwide as Homestake-type.” 

(Caddey, Bachman, Campbell, Reid, & Otto, 1992)

Great mines sometimes fade away without a final geological synopsis and tri­bute. Fortunately, the Homestake avoided that fate when the mine geology staff produced an excellent summary of more than a century of detailed study:

“The Homestake gold deposit is hosted within quartz-veined, sulphide-rich segments of an Early Proterozoic, carbonate-facies iron-formation in a sequence of originally calcareous, pelitic to semipelitic, and quartzose rocks. Strata that contain the Homestake deposit were completely deformed by a series of tight isoclinal and sheath fold events, and synchronous, extensive ductile and ductile-brittle shearing. Mine area rocks have been subjected to upper greenschist-lower amphibolite facies metamorphism. … Intrusion of granite similar to the 1.72 Ga Harney Peak Granite in an area northeast of the mine postdated regional prograde metamorphism, and appears to have been contemporaneous with later stages of brittle deformation.

Gold mineralization took place almost exclusively in the Homestake Formation, an iron-formation consisting of siderite and (or) grunerite schist in productive sections of the Homestake mine. … Nine ore ledges, or plunging fold structures, have produced gold. … Ore ledges are synclinal fold forms composed of a series of subordinate anticlines and synclines. …Overall, ore bodies constitute less than 3 per cent, by volume, of the Homestake Formation in the mine area.

At least three stages of mineral alteration took place, two of which were synmetamorphic and predated gold mineralization and a third synchronous with gold mineralization. A 1.84 Ga age provides a good approximation of regional dynamothermal metamorphism and major regional ductile deformation. We favor a post-peak metamorphic age (<1.84 Ga) and epigenetic origin for the Homestake gold deposit.” (Caddey et al., 1992).

It is interesting to note how much the description of the host rocks and structural controls of the deposit changed since two geological summaries were published in 1968 by Koschmann and Bergendahl, and by Slaughter. For example, iron formation was described as ‘sideroplesite (magnesium-rich siderite) schist’ in the earlier reports except when it had been metamorphosed to the garnet facies, where it was called ‘cummingtonite schist’ (cummingtonite is an amphibole). Also, the term ‘ledge’, which was used prominently to describe the synclinal structures that host most of the ore in the 1992 report, was used more in passing or not at all in 1968.

The Homestake Formation may have been 20 to 30 metres thick prior to folding and metamorphism, although it presently ranges from zero to 125 metres (in the fold hinges). All the gold was produced from nine ore ledges within synclinal fold forms composed of a series of subordinate anticlines and synclines. Interconnecting anticlinal fold forms were structurally less complex and generally barren, although they contained locally anomalous gold. The ledges plunge at dips of up to 45 degrees. The principal oreshoots, which occur as pencil- or pipe-shaped, chloritized quartz replacements parallel to the plunging ledges, have been interpreted as zones of increased permeability.

Gold and silver were the only metals that were recovered, although minor amounts of copper, lead and zinc were present. Most gold was anhedral, very fine grained (much less than 1 millimetre), occurred as disseminated blebs and flakes, and was spatially associated with a quartz-chlorite-pyrrhotite-arsenopyrite assemblage. Visible gold was very rare. Gold grades diminished abruptly at ore boundaries from greater than 6.2 grams per tonne to less than 0.2 grams per tonne within 0.5 metres. Higher concentrations were often found near the margins of one type of quartz vein. Gold fineness was variously measured between 838 and 899. The average Homestake gold/silver ratio is thought to be about 5:1 but comparisons are complicated because recoveries of silver were lower than those of gold, and were less precise.

Orebodies were sulphide-rich (5 to 7 per cent of the ore) and dominated by pyrrhotite, although arsenopyrite and minor pyrite and chalcopyrite were also present. Pyrrhotite was usually present in iron formation as streaks, blebs, and layers generally less than three millimetres thick and one centimetre long. Arsenopyrite was an important indicator of gold and was at least a minor constituent in parts of all orebodies. Its content (by volume) varied locally from a trace to more than 15 per cent. Pyrrhotite/arsenopyrite ratios ranged between 1:2 and 10:1 and generally increased from eastern to western ledges and with depth. In decreasing order of abundance, the principal gangue minerals associated with ore were chlorite, siderite, grunerite (another amphibole), quartz, biotite, garnet, and minor amounts of ankerite, muscovite, albite and graphite.

Stable isotope studies (Rye and Rye, 1974) showed that the sulphur in the deposit was derived from sea-water sulphate and had a sedimentary origin. It appears to have migrated into dilatant zones but didn’t cross formational boundaries. Fluid inclusions are consistent with a metamorphic origin and are different from those associated with Tertiary mineralization elsewhere in the district. The isotopic and geologic data suggest that the gold and other constituents of the Homestake deposit were indigenous to the Homestake Formation, were probably of syngenetic ‘exhalative’ origin, and were likely concentrated during metamorphism.

A lead isotope study (Rye, Doe, & Delevaux, 1974) of galena from quartz veins associated with mineralization from the Homestake Mine showed that it was derived from 2.5-gigayear source materials during a metamorphic and intrusive event dated at 1.6 gigayears. Galena in younger (Tertiary) vein deposits in the district was derived largely from Palaeozoic host rocks.

Caddey et al. (1992) concluded that the Homestake deposit formed as a result of four principal factors: 1) favourable stratigraphy; 2) development of a progressive tectonic system with dilational shears; 3) an intensive thermal and fluid system; and 4) a gold source. The spatially continuous nature of the Homestake Formation and increased permeability produced by shearing provided an efficient fluid path along the plunging iron formation. The nearby Cutting Stock provided a regional ‘hot spot’ below the mine that served as a focus of thermal activity. Fluids generated during metamorphism, although thought to contain no precious metals, produced significant district-scale metasomatic alteration and local graphite remobilization and defined zones of high fluid transport. Gold-bearing fluids derived from deep within the tectono-stratigraphic section were mobilized and deposited in the Homestake Formation.

Mine production before 1936 came from the Main and Caledonia Ledges using shrinkage and timbered stoping methods without backfilling. Those were replaced by a drawhole (uncontrolled block-caving) system until previously mined areas began caving to surface. Mechanized cut-and-fill and vertical crater retreat mining were then introduced using mill tailings for backfill (Caddey et al., 1992). As rock and ground water temperatures grew steadily hotter with depth, working conditions became increasingly uncomfortable. Following the closure of the mine in 2002 and routine surface reclamation and underground rehabilitation, the mine was sold for a nominal sum to The National Science Foundation and the State of South Dakota, who are preparing to establish a Deep Underground Science and Engineering Laboratory (DUSEL, 2008).

Except where cited, most of the information on the Homestake Mine has been derived from Caddey et al., (1992) with some input from Koschmann and Bergendahl (1968), and Slaughter (1968).
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