Leadville District Mineralisation - Leadville, Black Cloud
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See the record entitled Leadville, Aspen and Gilman Districts for geological setting, structure and mineralisation overview.
The Leadville group of silver, lead and zinc mines are part of a larger district spread over an area of approximately 240 x 100 km within central Colorado which embraces a series of Pb-Zn-Ag deposits. All of these deposits lie within the broad, north-east trending Colorado Mineral Belt in the state of Colorado, USA. The Leadville mines are found within 20 km to the south-west of the Climax porphyry Mo mine and 170 km north-east of the Creede Ag/Pb/Zn/Au vein system of the San Juan Volcanic Field.
The Black Cloud mine is the only remaining operation at Leadville (1996).
Published production and reserve figures for the district are as follows (Bryant & Beaty, 1989):
Leadville - 23.8 Mt @ 3% Zn, 4.2% Pb, 0.2% Cu, 320 g/t Ag, 3.65 g/t Au (Production to 1989),
Black Cloud - 0.8 Mt @ 8.1% Zn, 3.9% Pb, 0.2% Cu, 68 g/t Ag, 2.4 g/t Au (Reserve, 1989),
Mining in the immediate Leadville district commenced with the discovery of placer gold in 1860. This was followed by the location of lode gold in 1868. In 1874 the heavy brown material that had hampered placer gold operations was identified as argentiferous cerussite, and by 1876 several lodes had been delineated. In 1877 the town of Leadville was founded and by 1880 twelve smelters were in production treating 0.14 Mt of ore to yield 30 t of Ag and 30 000 t Pb per annum, corresponding to a recovered grade of around 220 g/t Ag and 21% Pb (Tweto, 1968).
Ore from the so called bonanza period was oxidised and mostly from depths of less than 150 m. By the late 1880s, many mines had reached the sulphide mineralisation, resulting in a decline in production. This decline was reversed by the discovery of gold in the Breece Hill area in 1893. These gold ores, but also the growing recovery of zinc sulphides from 1899 onwards, sustained the district until the depression of 1907 when production declined drastically. In 1909 the walls of some of the mines were discovered to be composed of zinc carbonates, leading to a period of production from 1910 until 1925 when this was the principal ore type exploited. Production then declined again until 1935 with the organisation of the Resurrection Mining Company which found ore veins beneath the old manto ore stopes in the eastern part of the district. A substantial production of complex sulphide ores from these veins continued for around 20 years. Exploration after World War II resulted in the discovery of a down faulted block of manto ore which was brought into production in the early 1950s, but was abandoned in 1957 due to rising costs. Late in 1965 ASARCO re-opened the Irene Mine in the same down faulted block (Tweto, 1968).
In 1971 the Black Cloud mine at Leadville was opened in the southern section of the district, and has produced 3 Mt of ore averaging 8.0% Zn, 3.9% Pb, 81.2 g/t Ag, 2.7 g/t Au up until 1989 (Thompson & Beaty, 1989). It is owned by ASARCO [53%] and Newmont Gold Company [43%]. Production has continued to the present, with 0.202 Mt of ore @ 7.7% Zn, 2.4% Pb, 58 g/t Ag, 1.4 g/t Au being extracted in 1994, yielding 13 971 t Zn, 4355 t Pb, 11.85 t Ag and 0.286 t Au (AME, 1995). The mine has a 505 m deep shaft and a flotation mill at the shaft collar. Concentrates are railed to smelters at East Helena in Idaho and El Paso in Texas (Thompson & Beaty, 1989).
Total production from Leadville between 1859 and 1963 has been 92 t Au, 7466 t Ag, 0.049 Mt Cu, 1.01 Mt Pb, 0.73 Mt Zn (Tweto, 1968). This has come from around 30 Mt of ore. In addition there has been 0.97 Mt of metallurgical Mn-Fe ore with 14 to 45% Mn, and 2.5 Mt of manganiferous fluxing ore produced, as well as fluxing magnetite-hematite ore (Tweto, 1968).
Mineralisation and Alteration
The bulk of the mineable ore at Leadville occurs as massive sulphide mantos, with lesser ore found within skarns and vein systems. The skarns were developed peripheral to and within the Breece Hill stock, surrounded by the vein and then the manto mineralisation (Thompson & Beaty, 1989). There is apparently a distinct difference between the vein and manto mineralisation, both in form and mineralogy (de Voto, 1983).
Mantos - The mantos at Leadville were referred to as 'contacts' in the early days of mining because they were found at the contact between porphyry sills and underlying carbonates (Tweto 1968). However, although mineralisation is found at these contacts, numerous examples are available of the massive sulphide manto orebodies within the Leadville Dolomite being cut by porphyry sills and dykes of the oldest Pando and next oldest Johnson Gulch porphyries. This is taken to imply that at least some of the manto ore predates the emplacement of Laramide porphyries (de Voto, 1983).
Multiple 'contacts', or mantos, were commonly present, with up to 11 in highly mineralised and well intruded areas. Where mineralisation was intense, all carbonates between the consecutive porphyry sills were mineralised, forming a single orebody with internal porphyry waste. In the Carbonate Hill area, in the north-west of the field, for examples, there are around 5 'contacts', each with a 1.5 to 15 m thick manto, spread over a vertical interval of 140 m. The uppermost was by far the most extensive. Where mineralisation was intense in this area the resultant composite orebody was up to 60 m thick (Tweto 1968).
The largest mantos and best defined orebodies in Leadville were those of Iron Hill and Rock Hill in the south-western part of the field. These were long, somewhat irregular, branching and anastomosing shoots trending north-east down dip, and spaced at 60 to 150 m apart in the strike direction. They lay at the first contact, below the Pando Porphyry in the northern part of Iron Hill, and at the second contact, below the Johnson Gulch Porphyry in the southern section. The larger ore shoots were 330 to 1000 m long, 60 to 160 m wide and 15 to 35 m thick. In places they were underlain by other stratigraphically lower orebodies and by fissure vein and stockwork deposits (Tweto, 1968).
Manto deposits in the eastern part of the field are far more numerous than in the western section, although they are also generally smaller. This is principally due to the greater abundance and discordance of porphyry dykes and the higher density of faults. Apparently, many of the mantos have vein roots, and branch out along bedding planes, from the central vein into successive favourable beds (Tweto, 1968).
The mantos at Leadville are 'massive sulphides' composed of pyrite, marcasite, marmatite, galena, tetrahedrite, pyrrhotite, electrum and a variety of trace minerals (Thompson & Beaty, 1989). Most of the ores are almost free of copper, although they may locally contain chalcopyrite (Tweto, 1968). The ore is typically 60 to 100% sulphide, with a gangue of silica, siderite, dolomite and late barite. Some mantos lack layering while other exhibit intricate banding. The banding parallels either bedding of the host rocks, or fracturing (Thompson & Beaty, 1989). Banding is also represented by curved and concentric rhythmic layering, as well as a clotty structure (Tweto, 1968). The bands and structures are marked by differing mineralogies, size variations of mineral constituents, or textural variations, particularly of the iron sulphides (Thompson & Beaty, 1989).
Pyrite is invariably the earliest sulphide, succeeded by overlapping sphalerite and galena, with the latter persisting after the sphalerite. Minor ore minerals such as chalcopyrite and argentite followed the galena. Widespread replacement of dolomite by mangano-siderite preceded the sulphide stage. Quartz principally accompanied the pyrite stage. Minor gangue minerals such as barite, rhodochrosite and dolomite are paragentically late. In addition all of the ore minerals may also be found as drusy crystals in vugs (Tweto, 1968).
In general the mantos have sharp contacts with the enclosing dolostones, although the wallrocks may exhibit a narrow zone of recrystallised white dolomite <2 cm thick at the contact. In addition, adjacent to ore, there may be weak to strong dissolution around dolomite grain boundaries to form material composed of porous, weakly consolidated dolomite sand. Locally small dissolution tubes extend outwards for up to 5 m from the manto margin. These tubes are termed 'birds-eye' textures and appear as small, <1 cm, white spots in darker dolostone with quartz-sulphide in their centres, thus forming a reflective central 'pupil' to the 'birds-eye'.
Veins - The veins at Leadville consist predominantly of quartz-pyrite-gold within and adjacent to the Breece Hill stock, and as quartz-pyrite-base metals peripheral to the gold bearing veins. According to Thompson & Beaty, (1989) both types of vein are associated with mantos where they cut favourable carbonate beds, ie. they represent feeders for the mantos. However de Voto (1983) suggest that the veins are younger than the mantos, and cites previous authors that have recorded such veins cross-cutting and post-dating mantos.
The bulk of the veins are mixed sulphide ores, differing from the mantos in that they contain appreciable Cu, Au and Ag. Most are highly pyritic and have a quartz or jasperoid gangue. Some are almost pure pyrite. Chalcopyrite accompanies the pyrite, either as veinlets cutting the pyrite or as interstitial fillings between the pyrite grains. Sphalerite and galena are also found within the veins, either as intergrowths or as distinct seams. Silver is present as argentite, with lesser pyrargyrite and argentiferous tetrahedrite (freibergite). In some silver rich veins, argentite occurs as fine intergrowths with bismuthinite and galena. Gold is present as native gold (Tweto, 1968).
The vein ores are in part fissure fillings, but more commonly have replaced the fractured rocks in and along faults and fissures. They characteristically widen in dolomites and certain quartzite and grit beds and narrow in shale and porphyries. They are also generally more siliceous in quartzites, grits and porphyries and contain more sphalerite and galena where they cut carbonates (Tweto, 1968).
The veins of the district apparently have a near north strike and are individually short. The Winnie-Luema and Fortune veins, the longest and most productive in the district have been traced for 1200 and 1500 m respectively. The former was stoped more or less continuously for a length of about 1000 m, to a depth of about 150 m, and was as much as 12 m thick. Branching veins and nearby 'replacement' bodies greatly enhanced the tonnage of Au-Ag-Cu-Pb ore mined. The Fortune vein was originally worked, largely for its bordering replacement bodies within the Leadville and Dyer Dolomites above the Parting Quartzite. Fissure and associated 'replacement' deposits on the Fortune and intersecting NE trending veins in the sedimentary section beneath the Parting Quartzite were the basis of the Resurrection operation. These deposits yielded 2 Mt of complex sulphide milling ore (Tweto, 1968).
Although smaller, many other veins in the eastern part of the field were very productive. In some places on Breece hill, swarms of intersecting small veins constituted stockworks of pyritic gold ore (Tweto, 1968).
Skarns - The skarns occur around the perimeter of the Breece Hill stock, as well as forming large blocks within the stock. They are magnetite rich. Calc-silicate minerals have been converted to serpentine by retrograde processes, while the magnetite has been replaced by sulphides in many places (Thompson & Beaty, 1989). The ores in this zone are characterised by magnetite-specularite-siderite, and are generally only economic when enriched by paragenetically younger Au-pyrite-chalcopyrite and sphalerite mineralisation (Tweto, 1968).
Primary Metal Zoning - Ag:Au ratio contours can be drawn, more or less concentrically surrounding the Breece Hill porphyry, increasing outwards, ie. with increasing distance from the stock, silver increases relative to gold. Any asymmetry is explained as a reflection of the dip on bedding and faults. The Mn content of the ore related carbonates also increases away from the Breece Hill plug (Thompson & Beaty, 1989).
Alteration - The porphyry wall rocks exhibit zonal alteration that may extend for up to 100 m from ore. The alteration ranges from an outer propylitic assemblage of chlorite-epidote-calcite, through weak argillic to intermediate argillic kaolinite-smectite-sericite-sulphides-dolomite to proximal phyllic with quartz-sericite-sulphides-siderite. The sulphides occur in veinlets, as well as pseudomorphs replacing feldspar phenocrysts. Trace element dispersion halos and zonal alteration of porphyries are apparently a good guide to mineralisation (Thompson & Beaty, 1989).
Secondary Ores - The secondary ores include both the oxidised or 'carbonate' mineralisation, and the enriched supergene sulphide ores. In glaciated zones the secondary zones are only thin, or are absent, although in most other parts they may extend to depths of as much as 120 to 180 m, and locally up to 270 m. The transition between the oxidised and sulphide zones may either be sharp, or have a broad overlap over a vertical range of as much as 180 m (Tweto, 1968).
The supergene enriched sulphide zone is far better developed in the veins than in the mantos. It is characterised by chalcocite and minor accompanying bornite and covellite. Silver is significantly enriched in the supergene zone, while above average Au is also found in the enriched zone (Tweto, 1968).
The most productive 'carbonate' mineralisation was represented by the argentiferous lead carbonate ores mined in the early stages of the fields exploitation. The Pb in these ores was present principally as cerussite, although some was in other forms. Silver accompanied the Pb in widely varying concentrations, principally as halides, but was also present in Pb poor bodies. Residual galena, was greatly enriched in Ag, sometimes with thousands of g/t (Tweto, 1968).
The oxidised 'carbonate' lead ores had four forms, namely: i). 'Sand carbonate' ore, composed of granular, crumbly cerussite, which was either white or stained brown or black by iron and manganese; ii). 'Hard carbonate' ore, which was far more widespread and made up of a ferruginous jasperoid containing microscopic crystals of cerussite and silver minerals; iii). a variety with either Pb-Ag or Ag alone occurring as a pasty to soupy mixture of iron sulphates and hydroxides; and iv). a siliceous ore that generally contained either Ag, Pb or both, and included some particularly rich Ag ores. These consisted essentially of jasperoid coated and seamed by silver halides. Some such ore was a grey jasperoid coated by translucent embolite, although much was yellow-brown or brown-black due to staining by Fe and Mn in both the jasperoid and coating. Lead-silver ores of these kinds were generally bordered by, and graded into Fe-Mn flux ores or Fe-Mn jasperoids (Tweto, 1968).
Oxidised zinc ores in part accompanied and bordered the oxidised lead-silver orebodies, but were in general at lower depths, while some were isolated in dolomite considerably below the oxide sulphide boundaries in other ores. The principal mineral in these ores were smithsonite, although calamine (hemimorphite), chalcophanite, hetaerolite and zinciferous clays, as well as more rare species were also present. The zinc minerals replaced dolomite or mangano-siderite in places. More commonly the zinc minerals were accompanied by Fe and Mn oxides, or by dark jasperoids which coloured the ore brown or black. Many large zinc carbonate bodies contained 30 to 40% Zn, and for many years 18 to 20 % Zn was the cut-off grade (Tweto, 1968).
Secondary ores in the veins were characterised by cerussite, native gold and in places by native copper, as well as iron oxides. Bismuth orebodies, all of which were oxidised, were characterised by bismuthinite (Tweto, 1968).
Fluid inclusion studies indicate a temperature of formation in the quartz-pyrite-gold veins of 469 to 422°C, while the mantos have yielded temperatures of 410 to 310°C. Fluid salinities are <5.2 equivalent weight % NaCl. Dating of apatite and zircon in the altered porphyries, presumed to be related to mineralisation, gave ages of 33.4±5.1 Ma (Thompson & Beaty, 1989).
For detail consult the reference(s) listed below.
The most recent source geological information used to prepare this summary was dated: 1996.
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
Symons D T A, Lewchuk M T, Taylor C D, Harris M J 2000 - Age of the Sherman-type Zn-Pb-Ag deposits, Mosquito Range, Colorado: in Econ. Geol. v95 pp 1489-1504|
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