Epigenetic vein-style ores are deposited from hydrothermal fluids. These fluids have a wide range of origins from being high temperature and magmatic or metamorphic to lower temperature and sedimentary. Many deposits are the result of more than one fluid and hence there is no universally accepted genetic classification system for these ores. An essentially non-genetic system, based upon metal or mineral associations, is used as it is the least controversial.
Tin-tungsten and base metal (copper, lead and zinc) vein deposits found in close spatial association with granite batholiths are a world-wide phenomenon. The mineralization is zoned, with high temperature assemblages lying close to, or within, the apical portions of the batholith (especially within granite cupolas) and lower temperature assemblages lying at increasing distances from the granite. The highest temperature assemblages are tin-tungsten-rich; these pass out into copper-rich, lead-zinc-rich and finally into fluorite, baryte or carbonate-rich zones. (Kelly and Turneaure, 1970; Groves and McCarthy 1978; Kelly and Rye, 1979; Smits, 1980; Evans (ed.), 1982)
These deposits are associated with the Cornubian batholith. This batholith contains about sixty emanative centres, each of which has a series of associated concentric mineral zones. Most of the mineralization is carried in veins; these are the approximately east-west-oriented 'normal lodes', 'caunter lodes' which are subparallel to them, and the later, approximately north-south-oriented, 'cross-courses'. The earliest mineralization is found within greisen-bordered veins, often in sheeted vein networks or stockworks. The veins comprise quartz plus wolframite and cassiterite with scheelite, lollingite, arsenopyrite and haematite within a tourmaline, chlorite, feldspar and muscovite gangue. Topaz, fluorite and tourmaline are present within the greisen borders.
The hydrothermal veins (lodes) carry three main assemblages:
A northwest-southeast vein system cuts a Devonian quartz-feldspar porphyry and Silurian argillites and slates, or follows the porphyry-metasediment junction. The massive sulphide veins carry minor quartz. The mineralogy of the veins shows differences between those cutting the porphyry, which have hexagonal pyrrhotite, troilite and galena with bismuth-rich inclusions, and those cutting the metasediments, which have monoclinic pyrrhotite, inclusion-rich sphalerite and galena with silver-rich inclusions.
Arsenopyrite occurs in a wide range of assemblages and is paragenetically early. Amongst the important arsenopyrite-bearing assemblages are tin-tungsten, gold and copper-lead-zinc. Optically it is difficult to distinguish lollingite from arsenopyrite.
Tetrahedrite group minerals are common and important silver carriers in many copper, or copper-lead-zinc ores.
These rare deposits occur as multi-stage veins with early quartz-haematite-uranium mineralization followed by silver minerals associated with carbonate gangue, and finally sulphides with silver and bismuth sulphosalts within fluorite, baryte, calcite or quartz. This complex assemblage of minerals is often called the five metal association.
Their origin is problematical, as the assemblages show both acid and basic affinities, and models for their genesis have invoked contributions from both granitic and basic fluids. The association is petrographically easy to recognize by its large number of phases (up to fifty different species have been recorded) and by their complex intergrowths. However, detailed mineralogy requires time, patience and extensive microanalysis. Many related minerals are present in this association and they have similar optical properties. This is especially true for the ruby silvers (pyrargyrite, proustite-pearceite) the iron-cobalt-nickel arsenides and sulpharsenides and bismuth sulphosalts. (Halls and Stumpfl, 1969; Berry (ed.), 1971; Petruk et al., 1971a,b,c)
Significant amounts of gold are produced as a by-product from base metal ores, notably those of copper. Deposits where gold is the primary economic metal are widespread and found in a variety of geological environments and have been classified by Boyle (1979) into nine major associations. These include gold placers, gold within quartz-pebble conglomerates, gold-bearing skarns and a number of gold-bearing vein associations.
The vein deposits have wide-ranging host lithologies and are found in a variety of geological environments, ranging from Archaean greenstone belts to Tertiary volcanic sequences. The auriferous and non-auriferous opaque mineralogy, gangue phases and associated wallrock alteration differ between the categories and, indeed, are different between individual deposits of the same class. However, some mineralogical generalizations are possible.
The most important gold minerals are native gold, gold-silver alloys, electrum, gold tellurides including calaverite, gold-silver tellurides including krennerite, sylvanite, petzite and auriferous hessite. Other gold minerals, including maldonite and aurostibite, are of local importance. Commonly associated opaque minerals are pyrite, pyrrhotite, arsenopyrite, chalcopyrite, sphalerite and galena. Less common are molybdenite, stibnite, tetrahedrite-tennantite, scheelite, cinnabar, realgar and tellurium and selenium-bearing minerals.
Gangue minerals include quartz, carbonate, chlorites, white micas, tourmaline, fluorite and baryte. Associated wallrock alteration is variable, often extensive, and includes silicification, propylitic and potassic varieties. Especial care should be taken in the preparation of gold-bearing material and in the subsequent optical identification of the opaque assemblages. Native gold is frequently found as small, less than 1µm to 10µm diameter inclusions in pyrite, arsenopyrite and base metal sulphides, often along their fractures, or as thin rims to these minerals. This fine grain size and the softness of gold dictate that an excellent polish is needed if gold is not to be overlooked.
In addition, its optical properties (which are very similar to brass, often used as polished thin section holders) and its tendency to pluck and smear require that extreme care is taken to avoid contamination of the sample. The golden-yellow colour, high reflectance and softness of gold make it simple to identify, even when it occurs as small grains. Colour and reflectance vary with fineness, low fineness gold is pale-coloured and has higher reflectance than pure gold, whereas cuprian gold is pinker. However, the same properties that allow easy identification of gold make the optical identification of the associated opaque minerals difficult, changing their perceived surface colours dramatically and lowering the reflectance apparent to the eye. This is especially true when other phases form small inclusions in gold; it often requires microprobe analysis, even to identify common phases. (Pyke and Middleton, 1971; Boyle, 1979; Twemlow, 1984; Radtke, 1985; Burrows and Spooner, 1986; Harris, 1986; Hayba et al., 1986; Saunders and May, 1986; Wood et al., 1986)
These can be mineralogically simple comprising quartz-stibnite veins within elastic rocks or stibnite, galena and arsenopyrite replacement ores within carbonates. Other antimony deposits are more complex; they include quartz, antimony and tungsten associations containing stibnite, scheelite and native gold in a quartz and carbonate gangue; and quartz, antimony and gold associations of stibnite, berthierite, native gold, tetrahedrite, pyrite, kermesite and antimony ochres. Commonly, the primary ores are oxidized to antimony ochres, including stibiconite and cervantite. (Ozerova et al., 1971)
Mercury and antimony sulphides are found in modern hot spring deposits where they infill vugs in siliceous sinter. Mineralogy is simple, comprising cinnabar, metacinnabar and stibnite. Economic mercury deposits are rare (Almaden in Spain is the best known); they also have a simple mineralogy of cinnabar, metacinnabar, pyrite, marcasite, plus lesser amounts of stibnite, sphalerite and pyrrhotite. The deposits form at low temperatures and so are accompanied by dolomite, baryte, chalcedony and quartz as gangue phases.