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The Santa Eulalia Mining District occupies the central portion of the basin and range fault bounded Sierra Santa Eulalia, some 20 km to the east of Chihuahua City, the capital of Chihuahua state in Mexico. It is approximately 100 km to the north-west of Naica, also in Chihuahua, and nearly 700 km to the north-west of the Fresnillo mine in Zacatecas (#Location - West Camp: 28° 36' 25"N, 105° 52' 23"W).
For a brief overview of the distribution and character of the deposits in the carbonate replacement and related vein Pb-Zn-Ag belt in Mexico and the western United States, and links to the deposits of that belt, see the Regional Setting section of the Fresnillo record.
Santa Eulalia Deposits
The Santa Eulalia district mineralisation was first discovered by the Spanish in 1591, although there was no appreciable production until 1703. Prior to 1925 mining was focused on the high grade oxidised orebodies that outcropped. Since that date however, operations have exploited the massive sulphide and skarns that underlie the oxide mineralisation. Mining has continued to the present, but at a reduced rate compared to that achieved from the late 19th to mid 20th century (Megaw, 1990).
Published production and reserve figures include the following:
Reserves, 1989 - 8.4 Mt @ 115 g/t Ag (AME, 1994).
Production, 1703 to 1790 - 4.0 Mt @ 684 g/t Ag (Megaw, 1990).
Production, 1791 to 1880 - 0.7 Mt @ 770 g/t Ag (Megaw, 1990).
Production, 1881 to 1924 - 7.5 Mt @ 500 g/t Ag, 11.9% Pb (Megaw, 1990).
Production, 1924 to 1986 - 26.4 Mt @ 227 g/t Ag, 8.1% Pb, 7.3% Zn, 1.5% Sn*, 0.3% Cu* (Megaw, 1990).
Total metal production, 1703 to 1986 - 12 280 t Ag, 2 775 Mt Pb, 1.91 Mt Zn (Megaw, 1990).
* These grades only apply to those areas where these metals are locally found.
The Santa Eulalia Mining District is divided into the West and the East Camp. The West Camp, otherwise known as the Main Mineral zone, lies on the western flank of the range and is characterised by massive sulphide manto and chimney orebodies, with local iron-calcic skarns. It has accounted for approximately 80% of the Ag, Pb and Zn production of the district, and a minor proportion of the Cu and Au.
The East Camp, also known as the San Antonio Mine area, lies on the eastern fringe of the range, and is characterised by symmetrically zoned, intrusion cored, Zn-Pb skarns with peripheral massive sulphide bodies. It has produced Pb, Zn and Ag, as well as all of the districts Sn, V and REE. The 2.5 km wide intervening zone, known as the Middle Camp, is generally considered barren (Megaw, 1990).
The Santa Eulalia district lies within the basin and range terrane on the western edge of the Mexican Thrust Belt, near the eastern boundary of the Sierra Madre Occidental volcanic plateau (Megaw, et al., 1988). The Sierra Santa Eulalia, the 'basin and range' range in which the Santa Eulalia district lies, consists dominantly of lower Cretaceous limestone which overlies evaporites. These sediments occur within a broad anticlinal structure which trends NNW-SSE with gentle east and west dips (Megaw, 1990).
The limestones outcrop throughout the northern portion of the range and are the principal hosts to mineralisation. In the central section of the range they are masked by early Tertiary volcanics and volcani-clastics. This Tertiary cover thickens to the south until it is cut by the ring-fracture zone of the mid-Tertiary Santo Domingo caldera. The southern portion of the range consists almost entirely of caldera related volcanics which are essentially barren (Megaw, 1990).
The stratigraphic succession is as follows (Megaw, 1990, Preston, 1926), from the base:
• Cuchillo Formation, 285 m thick - which comprises,
- A lower anhydrite unit interbedded with black calcareous shale and limestone which is of Aptian (119 to 113 Ma) age;
- An upper unit of black, fossiliferous limestone and calcareous shale of early Albian (113 to 97.5 Ma) age.
• Benigno Formation, 105 m thick - composed of limestone, with an Albian dolerite sill, known as the Lower Sill Diabase, at the top. Oligocene felsite sills cut this unit.
• Lágrima Formation, 510 m thick - largely intermixed limestone of Albian age and porphyritic intrusives, including both Oligocene felsite sills and the Eocene (37.5 Ma) Upper Sill Diabase. The limestones are thin bedded at both the top and the base, but are thick bedded in the middle.
• Finlay Formation, - of Albian age. I is subdivided into,
- Main Fossil Bed, 55 m thick - medium bedded fossiliferous limestone.
- Intermediate Beds, 240 m thick - mainly medium bedded, partly fossiliferous limestone.
- Upper Fossiliferous Beds, 75 m thick - fossiliferous medium to finely bedded limestone.
• Capping Series, 0 to 900 m thick, comprising:
- A lower fanglomerate with andesite and rhyolite tuffs, and ash flows of Palaeocene to Eocene age, and
- An upper sequence of ash flow tuffs and basalts which were erupted from the Santo Domingo Caldera to the south. These have been dated at between 31.7±0.7 and 27.5±0.6 Ma, and as such are Oligocene.
Within the mineralised section of the district the host sediments are cut by a series of felsite dykes which are regarded as being closely related to mineralisation. In the West Camp these dykes, which have been dated at 26.6 Ma, underlie the mineralisation, while in the East Camp similar rocks form cores to the San Antonio mine skarns. These felsites are fine grained and banded, containing sparse quartz and K-feldspar phenocrysts. Where they outcrop in the San Antonio Graben they are highly altered, geochemically anomalous and form an en echelon pattern. Pervasive alteration to a felted mass of very fine grained quartz and K-feldspar is common underground, while sphalerite, galena, pyrite and arsenopyrite are locally found within the bands of the felsite, especially near contacts. The West Camp mineralisation apparently bottoms on, and surrounds a series of these felsite dykes, although the majority of the mineralisation has no direct spatial connection with the dykes. Where mineralisation abuts the felsites there are pre-, syn- and post-ore relationships, implying that the felsites were emplaced contemporaneously with the mineralisation (Megaw, 1990).
The East Camp skarns are all developed around the felsite dykes, again implying a close relationship. Physically and chemically similar felsites are also found, apparently intimately associated with the mineralisation at both Naica and Velardeña (Megaw, 1990).
The West Camp felsites thicken and coalesce to the south-east, suggesting a source in that direction, while those of the East Camp similarly point to a source to the south-west. The common source, as supported by magnetic data, may be between the two camps (Megaw, 1990).
Two groups of intermediate dykes crop out in the northern and southern parts of the West Camp. These appear to be broadly contemporaneous with the ore but not spatially or chemically related. A 37.8 Ma quartz-monzonite (adamellite) stock underlies the West Camp, while 37.5 Ma dolerite sills occur throughout both camps. Both are indicated to have been pre-mineralisation (Megaw, 1990).
The Sierra Santa Eulalia has a strong NNW structural grain which is reflected in the structures of several stages and in the pattern of mineralisation. This direction is parallel to the axis of Laramide folding, the major ore zones and the 'basin and range' faulting (Megaw, 1990).
The West Camp mineralisation occurs along six principal NNW trends which are marked at depth by laterally continuous, but vertically dis-continuous, pre-mineralisation fissures, whose structural continuity becomes progressively more obscure with height within the system. These NNW trends are cut by a number of WSW fissures which have also influenced the mineralisation (Megaw, 1990).
The East Camp mineralisation occurs within the ENE (20°) trending San Antonio Graben and related structures which are cut to the south by the Santo Domingo Caldera. These structures are normal faults that have influenced the emplacement of the San Antonio felsite dykes and associated skarn and replacement mineralisation. The west side of the graben is a single steep fault with between 250 and 400 m of displacement. The eastern side is formed by a scissor pair of steeply west dipping faults with 200 to 400 m of displacement. Most of the graben faulting appears to precede mineralisation (Megaw, 1990).
The limestones at Santa Eulalia are cut by innumerable fissures, which have been grouped into several systems on the basis of their relative age, dip and strike. Those striking near north-south (NNW to NNE) are generally pre-mineral, while the remainder are usually, but not invariably, post-mineral. Generally the near north-south structures are not fault planes, and are seldom very continuous, although they form continuous zones of smaller closely spaced fissures. The cross-fissures are as numerous as the earlier structures (Prescott, 1926).
The mineralisation in the Santa Eulalia district can be considered in two groups, corresponding to the West and East Camps respectively, as follows:
West Camp - The mineralisation in this camp has accounted for more than 80% of the production of the district and hence is regarded as being of greater significance. It is composed predominantly of a series of massive sulphide orebodies that exhibit systematic changes in morphology, mineralogy and structural controls, and in stratigraphic localisation upwards and outwards from the deep felsite sills. From the southern most and deepest mineralisation, to the shallower northern fringes, the orebody-structure sequence progresses from i). deep breccia bodies; to, ii). sill-contact mantos; to, iii). fissure related mantos; to, iv). tabular chimneys, to; v). elongate mantos (Megaw, 1990).
Specific carbonate units contain the bulk of the manto ore, although no consistent characteristics of these carbonates have been identified to explain the clustering (Megaw, 1990). The lower, or Main Fossil Bed of the Finlay Formation is very favourable to the formation of manto ore throughout its 70 m of thickness, as is the Upper Fossiliferous Bed of the same formation. The Intermediate Beds are less favourable to the development of mantos, although many of the chimneys are found within them (Prescott, 1926).
The majority of the ore has come from the transgressive pipe-like chimneys and the shallow dipping, generally concordant mantos. Other than the geometric relationships to the stratigraphy and their shape there is apparently little difference between chimneys and mantos. While the mantos are generally concordant they frequently jump from one bed to the next higher. The chimneys vary considerably in size and are extremely irregular in shape. A large chimney may have a width of perhaps 30 m, by a length of three times that dimension, while extending over a distance of 300 m "vertically" through the sequence. The mantos range from say 5 to 60 m in width parallel to the host bed, and up to 5 m in thickness, but may extend over a length of 4 km (Prescott, 1926).
According to Prescott (1926), it is possible to trace every manto, without a break, back to a chimney, and that every orebody then known could be traced back to a chimney. He claims that it is not always easy to trace a manto over its extent as there are blind fingers leading off the manto at various points along its path. He also indicates that the cross-sectional area of a manto will decrease from the intersection with the chimney to a point where it tapers and becomes too small to economically exploit. He gives examples of a manto with a cross-sectional area of 400 sq. m where it branches off a chimney, having declined to 10 sq. m at a distance of 2 km from the chimney. A manto may branch at some distance from the chimney, although this generally does not result in an increase in the overall cross-sectional area of mineralisation. The path of the manto is apparently always upwards, overall (Prescott, 1926).
Three types of ore are recognised in the West Camp mantos and chimneys. These are:
• The earliest, a complex and rare silicate gangue carrying a silver bearing iron sulphide and insignificant amounts of sphalerite and galena. This style is of limited extent and commercial importance (Prescott, 1926). Distinctive calc-iron-silicate skarn bodies composed of manganoan-fayalite, manganoan-hedenbergite, chlorite, ilvaite, quartz and rhodochrosite with sulphides, lie within or above some stage 2 massive sulphide bodies. Most are found near the base of the Finlay Formation (Megaw, 1990). These are presumed to be the stage 1 ores of Prescott (1926);
• The main silver-lead-zinc ore at Santa Eulalia (Prescott, 1926). This assemblage comprises the main massive sulphide mineralisation and is composed of a fine to coarse grained mixture of pyrrhotite, pyrite, sphalerite and galena with minor chalcopyrite. Pyrrhotite is dominant at depth, with pyrite becoming more important in the upper levels. Gangue minerals include fluorite, quartz and carbonates (Megaw, 1990);
• The latest phase, composed chiefly of galena or its oxidation products. It is relatively free of zinc and only contains minor amounts of iron. This phase is only of geological importance with a total production to 1926 of 10 000 t (Prescott, 1926).
East Camp - The mineralisation in this camp comprises a tabular calc-silicate skarn chimney that envelopes the en echelon San Antonio felsite dykes. The skarn is symmetrically zoned on either side of the dyke from a narrow dyke contact assemblage of chlorite, epidote and purple fluorite; to a 2 to 40 m wide zone of grossular-andradite, hedenbergite, actinolite, cummingtonite and colourless fluorite. Pyrrhotite, pyrite, sphalerite and galena dominantly occur interstitially to, or along fractures in, the calc-silicates, although some sphalerite is intergrown with the fibrous silicates. Pods and mantos of massive sulphide are found locally along the contact of the calc-silicates and limestone. Pyrite increases upwards at the expense of pyrrhotite, while sphalerite and chalcopyrite are relatively more abundant compared to the West Camp (Megaw, 1990).
Evidence for replacement is abundant in both camps. Fossils have been mineralised while chert nodules and lenses within the orebody are not. Thin shaly beds have been described passing through the massive sulphides un-replaced and continuing in the limestone on either side. Evidence of 'open space' filling is less common. Open vugs, often lined with spectacular crystals of ore sulphides and gangue minerals are common throughout the district, although these appear to be local voids within otherwise massive, non-crustiform ores. Banded ores are common, but crustified ores are relatively rare (Megaw, 1990).
Both camps are surrounded by non-overlapping alteration halos zoned with respect to the mineralisation centres. The proximal alteration zone consists of argentiferous manganese oxide impregnations of limestone and fracture fillings of volcanics. This material is obvious in the mine area as strong black staining on the volcanics over a wide area, and has been mined for smelter flux. A discontinuous halo of fluorite mineralisation lies outside of and distal to this zone, surrounded in turn by the sporadic development of jasperoid (Megaw, 1990).
The ore bearing limestones outcropped and now have large mine openings on them at surface. Some thin veins cutting the volcanics in the same area were followed down to orebodies in the limestone (Excursion Guide notes, Megaw, 1990).
For detail consult the reference(s) listed below.
The most recent source geological information used to prepare this summary was dated: 1994.
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.
Lueth V W, Megaw P K M, Pingitore N E, Goodell C 2000 - Systematic variation in Galena solid-solution compositions at Santa Eulalia, Chihuahua, Mexico: in Econ. Geol. v95 pp 1673-1687|
Megaw, P.K.M., Ruiz, J. and Titley, S.R., 1988 - High-temperature, carbonate-hosted Ag-Pb-Zn(Cu) deposits of Northern Mexico: in Econ. Geol. v.83, pp. 1856-1885.|
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