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Leadville, Aspen, Gilman Districts |
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Colorado, USA |
| Main commodities:
Zn Pb Au Ag Cu
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Super Porphyry Cu and Au
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The Leadville, Aspen and Gilman groups 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 Gilman and Aspen mines are approximately 40 km to the NNW and 45 km to the west of Leadville respectively. The following description will concentrate on the Leadville mines with lesser detail on Aspen and Gilman. The Black Cloud mine was the only remaining operation at Leadville in 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),
0.608 Mt @ 8.1% Zn, 3.2% Pb, 47 g/t Ag, 2.4 g/t Au (Reserve, 1994, AME, 1995),
Gilman - 11.7 Mt @ 8.5% Zn, 1.5% Pb, 0.7% Cu, 228 g/t Ag, 1.7 g/t Au (Production to 1989),
Aspen - 2.3 Mt @ 2.1% Zn, 5.9% Pb, 16.4% Ba, 1500 g/t Ag (Production+Reserve, 1989).
Geology
The stratigraphic succession in the greater Leadville district is as follows, from the base:
Middle Proterozoic basement, developed on the southern margin of the Archaean Wyoming Craton, comprising:
Un-named metamorphics, 1790 to 1660 Ma - comprising interlayered quartzo-feldspathic gneiss and amphibolite, and interlayered biotite schist, gneiss and migmatite. These rocks are typically metamorphosed to amphibolite grade and are complexly deformed (Bryant & Beaty, 1989).
Rout Plutonic Suite, 1780 to 1650 Ma - comprising coarse grained, gneissic, locally porphyritic granitoid which is largely concordant with the enclosing gneisses, although some phases are discordant and only weakly foliated. These granites appear to be largely syn-tectonic and are chiefly granodiorite to quartz-monzonite in composition, but include peridotite to granite (Tweto, 1968; Bryant & Beaty, 1989).
Berthoud Plutonic Suite (Silver Plume Granite), 1400 Ma - an equigranular to porphyritic biotite-muscovite granite. This suite occurs as a myriad dykes, stocks and irregular discordant plutons (Tweto, 1968; Bryant & Beaty, 1989).
Pikes Peak Batholith, 1000 Ma - biotite and hornblende-biotite potassic granite, with subsidiary plutons of syenite, adamellite, alkali-diorite and related rocks (Tweto, 1968; Bryant & Beaty, 1989).
Unconformity,
Cambrian, comprising,
Sawatch Quartzite, 28 to 35 m thick - medium bedded, fine grained, white quartzite, that is pinkish near the top and grey to tan at the base. Thin beds of quartz-pebble conglomerate are found at the base locally (Tweto, 1968; Bryant & Beaty, 1989).
Peerless Formation, 18 to 24 m thick - thin bedded maroon, buff and green dolomite and dolomitic shale, grading downwards into brown glauconitic sandstone. It is generally grey to black in mine workings (Tweto, 1968). Locally includes light greenish-grey shale (Bryant & Beaty, 1989).
Ordovician,
Manitou Dolomite, 27 to 36 m thick - medium bedded white to grey crystalline dolomite characterised by associated white chert bands (Tweto, 1968).
Unconformity,
Devonian to Lower Carboniferous (lower Mississippian),
Chaffe Formation, sub-divided into,
• Parting Quartzite Member, 7.5 to 14 m thick - white to tan, cross-bedded, coarse grained quartzite and quartz-pebble conglomerate. In places there is a 1.5 to 4.5 m thick bed of buff, green and maroon shale at the base (Tweto, 1968), and local greyish-yellow dolomite beds (Bryant & Beaty, 1989).
• Dyer Dolomite Member, 20 to 32 m thick - thin to thick bedded, light grey to black, fine grained dolomite with a characteristic hackly fracture and minor limestone (Tweto, 1968; Bryant & Beaty, 1989). This member is of late Devonian age (Bryant & Beaty, 1989).
Local Unconformity,
• Gilman Sandstone, 1.5 to 7.5 m thick - sandy dolomite with chert fragments, grading downward into coarse grained, yellow-grey sandstone (Tweto, 1968), with some sedimentary breccia. It is of lower Carboniferous in age (Bryant & Beaty, 1989).
Lower Carboniferous (lower Mississippian),
Leadville Dolomite, 0 to 60 m thick - massive dark grey to blue-black dolomite and limestone with irregular cherty zones. It is normally fine grained, although widely altered to a coarsely crystalline variety (Tweto, 1968). This unit has been divided into a lower Red Cliff Member which is a grey to dark grey, thin to thick bedded dolomite; and the upper Castle Butte Member which is a bluish-grey thick bedded limestone (Bryant & Beaty, 1989).
Unconformity,
Upper Carboniferous (Pennsylvanian),
Molas Formation, 0 to 12 m thick - structureless red and yellow siltstone and mudstone containing abundant chert fragments (Tweto, 1968), or regolithic clay (Bryant & Beaty, 1989). It is a 'pockety' unit that has been interpreted to represent a product of erosion, largely representing the slightly reworked, insoluble residue from the chemical weathering of the Leadville Dolomite in pre late Carboniferous time (Tweto, 1968).
Belden Formation, 45 to 120 m thick - sandy dolomite with chert fragments, grading downward into coarse grained yellow-grey sandstone. It thickens south-eastwards across the district and is of middle Pennsylvanian age (Tweto, 1968). It also includes grey to black shale in sections, with dark grey to black limestone and dolomite, and local beds of white evaporite (Bryant & Beaty, 1989).
Minturn Formation, 300 m are preserved at Leadville, of the 3000 m in the district, comprising a lower 150 m of even bedded, black to white, coarse grained micaceous quartzite, grey to black shale with some intercalated limestone and dolomite; and an upper section of lenticular feldspathic sandstones, grits and conglomerate (Tweto, 1968). It is predominantly grey, but may be red in the upper sections and at the base (Bryant & Beaty, 1989).
Unconformity,
Permian to Mesozoic, sequence may locally overlie the preceding succession. These are composed of sandstones with lesser siltstone and shale, with minor grey limestone. The sandstones are red, purple, grey and green, and range from 0 to 2000 m thick, composed of thinner Permian, Triassic and Jurassic fluvial and aeolian deposits overlain by thicker Cretaceous marine deposits (Bryant & Beaty, 1989).
Unconformity,
Late Cretaceous to Early Tertiary, igneous rocks, comprise a series of multiple intrusions and associated extrusives. The earliest intrusives have been dated at between 70 and 40 Ma and are largely monzonites and granodiorites. Another suite, which includes the largest batholiths, are dated at around 40±5 Ma, ie. Eocene to Oligocene. Post 40 Ma intrusives are composed mainly of leucocratic, alkali-feldspar bearing rhyolite-granite porphyry. The last major episode is represented by the 28 to 22 Ma, Oligocene, volcanic fields, with multiple ash flow tuffs and intrusives of the San Juan Mountains caldera complexes (Bryant & Beaty, 1989). These younger volcanics are predominantly ash flow tuffs which vary in composition from silicic rhyolite to both silicic and mafic dacite (Bethke & Lipman, 1989).
The ore deposits of the region have been related to these various stages of porphyroid activity, as follows: i). The older 70 to 40 Ma bodies - Central City; ii). The post 40 Ma plutons - Climax and Henderson Mo, and the Leadville and Alma Ag/Pb/Zn mineralisation; 3). The post 28 Ma volcanics - the Creede and other San Juan Mountains Ag/Pb/Zn/Au mineralisation.
All of the Palaeozoic sediments of the district are extensively dissected by late Cretaceous and Tertiary intrusive porphyries which are largely present as sills or slightly discordant sheets. They also occur as dykes, plugs and irregular compound bodies. The Precambrian is also cut by less extensive sills and plugs.
In the immediate Leadville district the following porphyroids have been recognised, from the oldest:
• Pando Porphyry - a fine grained, light grey to white quartz-latite porphyry that is generally sericitised (Tweto, 1968). It was emplaced along the unconformity at the top of the Leadville Dolomite (Thompson & Beaty, 1989), and has been dated at 70 Ma (Tweto, 1968), and 72 Ma (Beaty, etal., 1989).
• Lincoln Porphyry - bluish to greenish-grey quartz-monzonite porphyry with abundant 15 to 25 mm K-feldspar and 5 to 15 mm quartz phenocrysts (Tweto, 1968). Recent dating suggests that this porphyry is older than thought at the time of Tweto (1968), who regarded it to be younger than the Johnsons Gulch Porphyry. It has subsequently yielded a date of 65.6 Ma, 10 km from Leadville (Tweto, 1968; Thompson & Beaty, 1989).
• Sacramento Porphyry - grey to greenish-grey quartz-monzonite porphyry, in which plagioclase is the most important phenocryst (Tweto, 1968). Age dating yields 43.9 Ma (Thompson & Beaty, 1989).
• Evans Gulch Porphyry - grey-green, very fine, porphyritic, almost equigranular, quartz-monzonite porphyry (Tweto, 1968).
• Johnsons Gulch Porphyry - grey quartz-monzonite porphyry with scattered 25 mm pink K-feldspar phenocrysts (Tweto, 1968). This is the youngest of the interpreted pre-ore porphyries, and is most notably represented by a stock beneath Breece Hill which cuts the Evans Gulch Porphyry and has been dated at 43.1 Ma (Thompson & Beaty, 1989).
• Younger Porphyries, which include at least 6 varieties found in the Breece Hill, Ball Mountain areas, the most significant of which are quartz-monzonite and quartz-latite porphyries, some with banding and abundant inclusions (Tweto, 1968).
• Little Union Quartz-latite - brownish-grey quartz-latite with conspicuous biotite phenocrysts.
• Rhyolite Porphyry - slightly porphyritic white intrusive rhyolite (Tweto, 1968).
• Rhyolite and Rhyolitic Explosion Breccia - flow and chill banded, dense intrusive rhyolite and vitrophyre, grading into explosion breccias composed largely of exotic rock fragments (Tweto, 1968).
At Leadville, the Breece Hill 'stock' is not technically a stock. It is apparently a composite of many different intrusions in a tenuous matrix of sediments. Early large intrusions were predominantly concordant, so that bands of sediment continued through the 'stock'. In reality it is a pile of thick sills which locally are fused to form a 'Christmas tree' laccolith. The pile has as a root a thick elliptical dyke which was developed along a fault zone. This dyke was cut in turn by pipe like bodies and breccias of the younger porphyries known at Leadville (Tweto, 1968).
Tertiary - Pliocene,
Dry Union Formation, 0 to 180 m thick - brown pebbly sand, silt and interbedded sand and gravel (Tweto, 1968).
Unconformity,
Quaternary - Pleistocene,
Ancient Glacial Drift, 0 to 45 m thick - till (Tweto, 1968).
Malta Gravel, 0 to 100 m thick - crudely bedded cobble gravel (Tweto, 1968).
Younger Glacials, 0 to 100 m thick - drifts from six glacial episodes, composed of till and outwash gravels (Tweto, 1968).
Structure
According to Tweto (1968) the structure of the Leadville district is simple in its broad features, but complex in detail. The Palaeozoic sediments are located on the eastern flank of the Sawatch anticline, dipping homoclinally eastward at about 15°. The tilted slabs of sediments and included, usually sill-like porphyries, are broken by a series of generally north-trending faults, most of which have an up-throw to the east. This results in the east dipping rocks being repeated to the east of each fault, such that the conformable manto orebodies are repeatedly outcropping and at workable depths for a down dip east-west interval of around 6.5 km (Tweto 1968).
The structural complications at Leadville are the result of the interplay of faulting and intrusion and the complex history of the faulting. With the exception of a few early faults, the majority are interpreted to be of Laramide age, ie., late Cretaceous to mid Tertiary. Many of the faults are believed to have undergone repeated movement as successive porphyry bodies were emplaced. The porphyry bodies have apparently both expanded the sequence and assimilated it. Many of the faults were reactivated later during a period of block faulting, related to the Rio Grande Rift, which continued into the Pleistocene (Tweto 1968).
Many faults change character along strike, with constant dip-slip structures being normal in one section and reverse in others. This has been explained as a reflection of differing thicknesses of sills emplaced along the strike of the fault. In many such examples, the displacement on a fault as measured in the stratigraphic displacement below a sill can be normal, and reverse above. In many cases also, porphyry bodies have occupied and obliterated faults after movement. Similarly in some cases, post-ore porphyroids have cut and destroyed conformable massive sulphide manto orebodies (Tweto 1968).
The Leadville district is located in an area of complex porphyry activity at the intersection of two major fault systems. To the south, the dominant fault direction is represented by the 340° [NNW] trending Mosquito Fault which dips west and has a displacement of near 1500 m. North of the district the principal fault trend is at 15° [NNE], again with a throw of 1000 to 1500 m. To the north the NNW trend dies out, although to the south the NNE set continues. Both fault sets dip to the west, so that the total stratigraphic displacement across the combined system is of the order of 3000 m. A third, but lesser set parallels the Colorado Mineral Belt tectonic direction, with a NE-SW trend (Tweto 1968). This third trend appears to control the elongation of many of the manto 'runs', as illustrated on the accompanying plans. It has been noted however that although some of the manto 'runs' follow faults, the majority are developed within joints (de Voto, 1983).
Mineralisation and Alteration
Mineralisation in the greater Leadville district takes a number of forms, including: i). early skarn ore, which is magnetite rich and surrounds some intrusive bodies; ii). subsequent, generally concordant massive sulphide mantos; iii). transgressive vein systems. Many intermediate ore styles are also evident, including pipes, stockworks and irregular bodies. Surface exposure of the mantos and veins at Leadville, and subsequent physical transport of weathered material led to the development of iv). placer mineralisation, although most of this ore did not survive the Cenozoic glaciation (Tweto, 1968; Thompson & Beaty, 1989).
In the bulk of the literature the mineralisation is taken to be related to the porphyry systems of the district. It is well established that many of the deposits occur close to Laramide porphyry bodies. However, de Voto (1983) has argued that while the skarn mineralisation, which is more Cu and Au rich, is related to the intrusives, the main massive sulphide mantos are pre-intrusive, late Carboniferous 'Mississippi Valley type' deposits which have been complicated and remobilised by the porphyry emplacement. While his argument is not irrefutable, many of his observations appear to be valuable.
De Voto (1983), notes that the carbonate hosted manto deposits occur within the same package of rocks and share many characteristics in common over an area of the order of 10 000 km2 in central Colorado. The host rocks are almost exclusively those lower Palaeozoic carbonate rocks which occur most closely below the extensive and well developed late Mississippian (middle Carboniferous) unconformity that represents a palaeo-karst-solution erosion surface. The carbonate units that occur most closely below this surface, namely the Castle Butte and Redcliff Members of the Leadville Dolomite, contain the largest and most significant ore deposits. He notes however, that ore is also found in the underlying Dyer and Manitou dolomites and even a dolomitic band in the underlying Sawatch Quartzite. He observed however, that these units are mineralised where they occur adjacent to major karst valleys in the unconformity surface, or where the overlying carbonates are anomalously thin, or have been removed at the unconformity. None the less the most significant deposits are within the Leadville and Dyer dolomites, within 60 m of the unconformity surface.
It has been further suggested that ore textures in outcrop, in old workings, and as described in the literature, imply that the mineralisation has been emplaced in caverns, collapse chimneys, sink-holes, solution enlarged vertical walled joints and palaeo-surface soil/rubble profiles. The 'massive sulphide' manto orebodies in these features are found within dolomite breccias, disaggregated dolomite sand and grey clay with minor black chert breccia fragments. The dolomite breccias are composed of generally angular to sub-rounded pebble to boulder sized clasts of either a single colour and rock type or of several. These are suspended in a dolomite sand matrix, or they may occur as a moderately packed, clast-supported breccia. The disaggregated dolomite sand filling 'cavities' in the dolomite host is sometimes banded. In many examples significant fragments of black shale and grey clay from the overlying Belden and Molas Formations are also found within the infilled 'cavities' (de Voto, 1983).
Orebodies in the upper portion of the Leadville Dolomite commonly coincide with areas of local or regional 'solution-erosion' thinning of the Leadville strata. These areas of thinning correspond to a thickening of the overlying Molas Formation which has been interpreted by a number of authors as a regolithic residuum of chemically weathered Leadville Dolomite (de Voto, 1983).
Most of the significant deposits are hosted within dolomitised rocks, rather than limestones, although some modest deposits are found in un-dolomitised limestones. The Manitou and Dyer units have been extensively dolomitised on a regional scale throughout central Colorado. The Redcliff and Castle Butte Members however, have been selectively dolomitised in specific zones which are elongated in a NNW direction, parallel to one of the major fault directions of the district. Dolomitisation in these units appears to be regional and not spatially related to any intrusive centre, and is not found in carbonates above the Mississippian unconformity (de Voto, 1983). He also cites numerous lines of evidence supporting the diagenetic, pre-mineralisation, origin of dolomitisation within the Leadville Dolomite.
In addition to dolomitisation, the host sequence has also been subjected to silicification to form jasperoids. These jasperoids exhibit a diverse range of morphology, crystallinity and possibly origins, varying from light, to dark grey to rusty brown and from crypto-crystalline to finely crystalline. It occurs as a replacement of carbonate rocks, carbonate breccia fragments and matrix in breccias, and to a limited extent in the diagenetic dolomitised 'zebra rocks'. The 'zebra rocks' comprise alternating laminations of dark grey micro-crystalline and light grey, coarsely crystalline dolomite. The distribution of jasperoid is more widespread than the known mineralisation, although some deposits are found within close proximity of the jasperoids. It has been suggested that diagenetic silicification of carbonates may have precluded the mineralising processes. Consequently silicification and mineralisation at economic grades are generally mutually exclusive (de Voto, 1983).
The mineralogy of the manto Pb-Zn-Ag deposits differs from district to district throughout central Colorado, although there are some common characteristics. The high pyrite deposits, such as those at Leadville and Gilman, have a paragenetic sequence, from first to last, of siderite -> pyrite -> high-Fe sphalerite -> galena -> tetrahedrite -> minor late barite and dolomite. Sometimes minor quartz and calcite are also found in these deposits. Minor chalcopyrite, accompanied by silver minerals, occurs at Leadville, while major chalcopyrite mineralisation is found late with tetrahedrite and pyrite at Gilman. Siderite is absent at some locations. Except for some pre-sulphide quartz at Leadville related to a local intrusive, silica is 'surprisingly' low in these deposits. The average metal contents of the sulphides that have been mined historically are 10 to 14% Pb+Zn and 30 to 120 g/t Ag. In some deposits such as at Aspen, there is appreciable early barite, low Fe sphalerite, galena and silver as tetrahedrite, tennantite and/or acanthite. These generally have much lower pyrite and only minor quartz within the mantos (de Voto, 1983).
For detail on the mineralisation at each district see the records: Leadville District - Mineralisation, Aspen District - Mineralisation and Gilman District - Mineralisation.
For further detail consult the reference(s) listed below.
The most recent source geological information used to prepare this decription 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.
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Selected References:
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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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Titley S R, 1996 - Alteration of mineralized carbonate rocks in the epicrustal environment: in Porphyry Related Copper and Gold Deposits of the Asia Pacific Region, Conf Proc, Cairns, 12-13 Aug, 1996, AMF, Adelaide, pp 3.1 - 3.10
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Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge. It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published. While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
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