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Real de Angeles

Zacatecas, Mexico

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The Real de Angeles silver deposit is located in the State of Zacatecas, in central Mexico. It is some 61 km to the south-east of the City of Zacatecas, the capital of the state, and lies within the Pb-Zn-Ag belt that runs in a northwest-southeast direction through Mexico, and include the silver deposits of Parral, Santa Bárbara, San Francisco del Oro, Fresnillo, Zacatecas and Guanajuato (Pearson, etal., 1988, Pearson & Clark, 1990).

The "municipality" of real de Angeles was first recognised in 1586, although it is likely that mining had commenced prior to that date. The earliest record of mining activity was in 1705. Two mines were developed between 1760 and 1777. Mining activities were reactivated again from 1840 to 1860. A further period of mining took place between 1890 and 1910, this time producing a Pb-Ag concentrate. Mining activity ceased as a result of low silver prices, low grades and the Mexican Revolution. During the foregoing periods mining focussed mainly on silver rich oxidised ores in vein structures (Bravo, 1991; Pearson, etal., 1988).

In 1969, Explomin, SA de CV and Placer Mexicana SA, both of which were funded by Placer Development Ltd., initiated an evaluation study and diamond drilling program at Real de Angeles (Pearson, etal., 1988). In 1970 however, the Gamma CA Co contracted with the mine concessionaires to carry out exploration that included 29 diamond drill holes. This contract was cancelled in 1972, and Explomin, SA de CV took over the exploration rights in 1973. Exploration closed in September 1975 with a feasibility study (Bravo, 1991).

Surface mineralisation and alteration, and the remains of the old mine workings are said to have clearly indicated the presence of the mineralised zone. Four drilling campaigns were executed during the exploration program. The first entailed 20 holes in 1971 and early 1972. A further 58 holes in 1974-75 led to a preliminary economic estimate of reserves for the feasibility study. An additional four holes in late 1975 indicated an extension at depth to the south-west. This was followed up by Minera Real de Angeles in 1984, resulting in an increase in reserves in that area. By 1990 a total of 91 diamond drill holes for a total of 17 400 m of core had defined the reserves as stated below. The drill holes are spaced on a 50 m grid. Except for 9 vertical holes, all have an azimuth of 208° and dip at 45 to 75°. The average depth was 150 m, with some to 400 m (Bravo, 1991).

Reserve and production figures include:

Original pre-mine Reseserve, 1982 - 85 Mt @ 75 g/t Ag, 1% Pb, 0.92% Zn (Pearson, etal., 1988).
Oxide Reserve, 1982 - 8.5 Mt @ 76 to 83 g/t Ag, 0.8% Pb, 0.65% Zn, 0.035% Cd (Pearson, etal., 1988).
High Grade Reserve, 1982 - 13 mt, which includes,
 10.5 mt @ 128 g/t Ag, 1.3% Pb, 1.10% Zn (Pearson, etal., 1988).
Low Grade Reserve, 1982 - 65 mt, which includes,
  45.2 mt @ 65 g/t Ag, (Pearson, etal., 1988).

In 1979, Explomin, SA de CV was dissolved and Minera Real de Angeles SA de CV came into existence to develop the mine (Pearson, etal., 1988). Construction commenced in January 1980, following a decision to proceed in November 1979, and continued until July 1982 when production began (Bravo, 1991). Output in 1982 started at a rate of 10 000 tpd, but by 1990 had been expanded to 15 000 tpd, although the total ore+waste removed daily was around 128 000 t (Pearson & Clark, 1990). The mine closed in April 1992 for an indefinite period as a result of low Ag, Pb and Zn prices and high smelter charges. However, in late 1992 it re-opened again. In May 1993 Placer Dome sold its 49% holding in Minera Real de Angeles SA de CV to Empresas Frisco SA de CV (AME, 1995).

The mine is an open-cut operation, with an estimated final depth of 400 m, upper diameter of 1200 m, average wall slope of 45°, and 12 m high benches. In 1985 some 4.99 mt of ore and 14.1 mt of waste were extracted. The mill treated 4.90 mt of ore for 354 t Ag, 38 400 t Pb and 31 180 t Zn. Recoveries were Ag - 78%, Zn - 65% and Pb - 82%, with a smelter availability of 94% in 1984 (Bravo, 1991). When the mine re-opened in 1992, the pit walls had a slope of 62° and the benches were 15 m high. At the same time recoveries were Ag - 77.5%, Zn - 75% and Pb - 85%. In the year of 1991 6.025 mt of ore were mined at a head grade of 52.1 g/t Ag, 0.9% Zn, 0.9% Pb, while 5.44 mt was treated (AME, 1995).

Geology

The Real de Angeles deposit is located on the south-eastern margin of the Sierra Madre Occidental volcanic province. The latter is characterised by extensive Oligocene rhyodacitic ash-flow tuffs which are from 0.8 to 1 km thick. In most parts of the Sierra Madre Occidental, especially on its western margin, these ash-flow tuffs overlie the predominantly andesitic Lower Volcanic Series, which have, in places been intruded by batholiths and smaller intrusives. To the east however, in Zacatecas, near Real de Angeles, these andesites are usually absent, except for microdiorite laccolithic equivalents found some 75 km to the north-west. In their place a red-bed conglomerate is commonly present (Pearson & Clark, 1990).

The geological succession in Zacatecas below these younger volcanics is as follows, from the base (Pearson, etal., 1988):

Triassic,
* Coapas Formation equivalent - phyllite, schist and slate, with associated sandstone and limestone, and in some localities "greenstones".
Jurassic,
* Rodeo and Taray Formations -sericitised phyllites;
Unconformity,
* Nazas Formation - continental red-beds, predominantly conglomerate;
* Zuloaga Formation - interbedded compact limestones;
* La Caja Formation - a sequence of limestones and calcareous mudstones with interbedded chert;
Cretaceous - Lower,
* Taraises Formation - intercalated limestone and mudstone with disseminated pyrite and lenses of chert;
* Cupido Formation - interbedded limestones with nodules and lenses of chert, and intercalations of calcareous mudstone;
* La Pena Formation - bedded to laminated limestone containing chert nodules, with local interbedded mudstone and siltstone facies;
* Aurora Formation - composed predominantly of a resistant limestone which is interbedded to interlaminated with bands of chert;
* Cuesta del Cura Formation - a resistant limestone with a persistent and wavy stratification, and variable sized beds and lenses of chert.
Cretaceous - Upper,
* Indidura Formation - argillaceous limestone, intercalated with carbonaceous shale, with a variety of differing facies;
* Caracol Formation - interbedded to interlaminated sandstone, shale and siltstone with minor limestone, exhibiting some variation in lithology;
* Parras Formation - calcareous sandstone and black limestone with a transitional contact with the underlying Caracol Formation.
Unconformity.
Tertiary - Eocene to Oligocene,
* Red conglomerate - basal red conglomerate unit overlying the unconformity;
* Rhyolite Ash Flow Tuffs of the Sierra Madre Occidental;
Tertiary - Pliocene,
* Ahuichila Formation - sandstone, tuffaceous sandstone and conglomerate
Quaternary - alluvium and caliche deposits.

This sequence changed during the late Cretaceous from dominantly carbonates, to an upper clastic facies. The Cretaceous succession was folded by the late Cretaceous to Tertiary Laramide event and subsequently uplifted by the Basin and Range stage which was characterised by normal faulting. There-after erosion removed the volcanic cover and exposed the underlying mineralised terrane (Pearson, etal., 1988, Pearson & Clark, 1990).

Two periods of igneous intrusion are recognised in the district. The first is possibly Laramide in age and generated a number of granitic intrusives 25 km to the north-east, with associated Au, Ag, Cu, wollastonite, fluorite and radioactive mineralisation. The second is of Tertiary age and resulted in the emplacement of rhyolite dykes and small stocks some 15 km to the north of Real de Angeles with accompanying Au, Ag, Zn, Cu and phosphorite mineralisation (Pearson & Clark, 1990).

The Real de Angeles deposit is hosted entirely within the late Cretaceous Caracol Formation which locally comprises a repetition of sandstone and siltstone beds of varied thickness. The orebody occurs on a topographic dome on the northern slopes of a set of rolling hills, the Cerro Real de Angeles, which are capped by Cretaceous limestone. The deposit consists of a silicified stockwork sulphide orebody overlain by an oxidised clay capping (Pearson, etal., 1988).

The coarse and fine grained clastic rocks of the Caracol Formation vary from finely intercalated, with individual laminae of less than 5 mm in thickness, to interbedded sequences where individual beds might be 50 to 500 mm thick. Thin sections of the sandstones reveal angular to sub-angular grains. Quartz grains may be in contact, or be supported by a silty, carbonaceous matrix. Sericite and chlorite occur as very fine grained crystals in trace to minor amounts. Approximately 5% of the sandstone is composed of pore spaces which may be up to 500 µm across. The sandstone also contains 10 to 30% feldspar, with traces of epidote, zircon, apatite and sphene (Pearson, etal., 1988, Pearson & Clark, 1990).

The finer clastics consist mainly of siltstone, although mudstone and shale are also present. Very fine grained carbonaceous material is abundant in this facies and often occurs as stringers concordant with the sandstone and siltstone bedding and laminations. Carbon is also present in micro-stylolite structures and matrix material, and appears to have been re-mobilised into fractures and veinlets by later hydrothermal activity. It has been partially transformed to graphite (Pearson, etal., 1988).

Structure

The structural fabric of the Real de Angeles deposit is characterised by steeply dipping normal faults and abundant fracturing. There are three basic systems of faults in the deposit area. The first fault set ranges from pre-ore to syn-mineralisation and strike from 270° to 340°, with dips of 33° to 85°N. These faults are commonly filled by veins with a fluorite-base metal assemblage which have been the basis of much of the historic production. The second fault set appear to closely follow the ore stage. They strike at 340° to 20° and dip at 70° to 90°E, are mainly filled with calcite and commonly brecciate the host rock. The third fault set is post-ore, strikes from 315° to 45° and dips from 40° to 60° W. These are filled by oxides (Pearson, etal., 1988).

A combination of the three fault systems, and their attendant fracturing, defines the northern margin of the orebody, while to the south and south-east there is a gradual grade boundary to the ore, coinciding with a lithologic change from a sandstone-siltstone facies to one of predominantly mudstone and clays (Pearson, etal., 1988).

Mineralisation and Alteration

The Real de Angeles orebody has a downward tapering oval shape with surface dimensions of 400 x 450 m. It persists to a known depth of 400 m below the surface. The silicified rocks of the deposit have a greater resistance to erosion than the surrounding lithologies and as a consequence had developed a peculiar shaped 40 m high hill. The lateral boundaries of the orebody are sharply faulted. The sulphide content of the deposit diminishes rapidly towards the periphery where the veinlets are filled with calcite instead (Bravo, 1991).

The economic mineralisation generally occurs as disseminations and as stockwork, vein and massive sulphide aggregates. The stockwork is a swarm of veinlets that range from <1 mm to several cm¹s thick, preferentially developed within individual sandstone beds, commonly perpendicular to bedding. Disseminations occur as diminuative mineral grains scattered in the intergranular spaces of the sandstone and its groundmass, and as replacements. The veins occupy fault zones or areas of strong parallel fracturing which cross-cut the sediments. Some of the veins occur within the disseminated body, but mostly towards the southern margin. Massive sulphide aggregates occur mainly as 0.15 to 0.6 m thick ³beds² or lenses parallel to bedding planes (Bravo, 1991).

Veins occupying continuous through-going structures comprise only a small fraction of the total sulphide of the orebody. The majority is found as disseminations and as dilatant vein filled fractures which are believed to be related to extension of the sediments. Mono-lithologic sequences at Real de Angeles are seldom well mineralised. Veinlets are best developed in those sequences which comprise siltstone with interlaminated sandstone layers from 3 to 9 mm thick. Any increase in the sandstone thickness commonly coincides with a decrease in the dilatant fractures and a resultant drop in grade to 35 to 50 g/t Ag. Sandstone lenses with interlaminated siltstone, and those with slumped or deformed bedding, are the optimum locations for high grade ore in stockworks (Pearson & Clark, 1990).

The veining detailed above occurs in two main forms (Pearson & Clark, 1990), namely:

1). Dilatant fracture veinlets - these are the most abundant and economically significant. Within the Real de Angeles district, calcite filled dilatant fractures are common in the Caracol Formation, although within the orebody such fractures are wider, more numerous and contain sulphide minerals. Vein widths may be up to 50 mm thick, although the sulphide filled fractures average from 1 to 5 mm. In places the fractures containing the veins appear to have been widened, with the underlying laminae being forced up into the fracture. The fracturing which controls this veining is post-consolidation and both cross-cuts and follows bedding. The conformable veins commonly follow sandstone-siltstone contacts, usually at the upper boundaries of the siltstone. Ore deposition in these veins appears to have been more a replacement of favourable siltstone laminae, although brecciation is sometimes observed. The concordant veinlets are the most abundant and commonly interconnect the multitude of discordant veinlets.
2). Fluorite-base metal veins - which only constitute a small part of the total vein system, although they follow the trends of past mining. These veins are both concordant and transgressive. They follow continuous faults and fractures and are characterised by an abundance of fluorite and calcite gangue, as well as arsenopyrite. The main veins of this type trend at 270 to 337° and dip at 33 to 95°N.

Within the sulphide orebody there are zones carrying more than 100 g/t Ag, characterised by a stockwork of inter-connecting veins and veinlets of both types (Pearson & Clark, 1990).

The disseminated mineralisation generally occurs along bedding planes in siltstone laminae. This style is not as obvious as the fluorite-base metal veining, but accounts for a significant proportion of the available sulphide. The sulphides are disseminated as pore space fillings and as replacements of host rock grains and laminations. The mineral assemblage is similar to that found within the veins, with the general exception of arsenopyrite and adularia. Disseminations vary from 2 to 30% sulphides, averaging 5 to 10%. Disseminated sulphides averages 1 to 1.5% combined Pb+Zn, and 40 to 80 g/t Ag (Pearson & Clark, 1990).

The following alteration types are recognised at Real de Angeles, in order of paragenesis:

* Hornfels - composed primarily of chlorite, sericite and calcite as concordant bands in the fine grained lithofacies of the host rock. Fine grained chlorite is found as rhombohedral or hexagonal pseudomorphs with centres of chlorite, calcite or carbon (some of which is present as graphite). Carbon and graphite are intergrown with and replaced by sericite, all of which are hosts to scattered sulphide grains. In the deepest part of the orebody an amphibole of the tremolite-actinolite series has been found. While propylitic and ore stage sulphides overprint the hornfels, the presence of amphibole in association with a high temperature alteration assemblage implies the possibility of an intrusive at depth (Pearson & Clark, 1990).
* Silicification - is the primary alteration type at Real de Angeles, affecting all but the most impervious beds. Sandstone beds and lenses, interlaminated sandstone-siltstone and deformed/slumped sediments are commonly intensely silicified. This alteration is pre-ore and takes the form of matrix material cementing sand and siltstone grains, as quartz overgrowths, as sand grain replacements, and as milky quartz occupying dilatant fractures (Pearson & Clark, 1990).
* K-silicate - alteration takes the form of adularia and is restricted to within 5 mm of the walls of faults, fractures and veinlets of the ore forming stage. The most common form of this alteration is grain by grain replacement of quartz sand grains in sandy layers and is believed to be coeval with the formation of adularia within the veins themselves (Pearson & Clark, 1990).
* Propylitisation - alteration occurs as a halo around dilatant fracture veinlets and stockworks of both fluorite-base metal and dilatant sulphides throughout an interbedded /interlaminated section of siltstone-sandstone. This phase overlaps both the ore and post-ore stages. The associated mineral assemblage, in order of abundance, is chlorite, sericite, calcite, biotite and pyrite. The combination of these minerals is variable, ranging from a chlorite-sericite-calcite assemblage around dilatant fracture-veinlet mineralisation containing pyrrhotite-galena-sphalerite, to a sericite-calcite-pyrite combination in veins of dominantly arsenopyrite (Pearson & Clark, 1990).
* Argillisation - is present as a trace constituent in association with both vein types, becoming less abundant in shallow (<100 m) veins. An oxidised clay cap also surrounds and overlies the sulphide orebody. This is a combination of argillisation overprinted by oxidation and supergene enrichment. The pronounced silicification of the ore zone ends abruptly in contact with the argillic envelope. The clay cap consists of kaolinite and other brown clay minerals which occur in and along fractures in the sandstone matrix and appear to replace laminae of sandstone or siltstone (Pearson & Clark, 1990).

A total of 27 ore and gangue minerals have been recognised within the sulphide zone at Real de Angeles. The most common of these are: pyrrhotite [30 to 35%]; sphalerite [15%]; adularia [15%]; galena [10%]; quartz [5 to 10%]; calcite [5 to 10%]; fluorite [2%]; pyrite [2%]; and arsenopyrite [1%]. Other minerals present at levels of <0.5% include chalcopyrite, chalcocite, freibergite, stephanite, jamesonite, marcasite, argentite, etc., (Pearson & Clark, 1990).

Within the orebody the dilatant veins are entirely filled with sulphides accompanied by minor gangue. The ore and gangue minerals occur as intergrowths of varying grain size. The dominant dilatant fracture mineralogy is pyrrhotite-galena-sphalerite, while the fluorite-base metal veins are mainly fluorite-sphalerite-galena-calcite. Minor constituents are arsenopyrite, pyrite, discrete silver phases, jamesonite, quartz and adularia. Early ore stage mineralogy of arsenopyrite, pyrite and adularia is preserved as thin bands within the fluorite-base metal veins, enveloped by ore-stage to post ore-stage minerals (Pearson & Clark, 1990).

Pyrrhotite is the most abundant sulphide and is always associated with sphalerite and galena, except in the fluorite-base metal veining. Intergrown chalcopyrite and pyrrhotite inclusions are increasingly more common within sphalerite as depth increases. Silver occurs as discrete phases of microscopic size, and also apparently as a solid solution in galena. Both modes are of economic importance, although the galena contains 71% of the known silver. The silver phases identified includes freibergite which is the most abundant discrete silver mineral observed. Chalcocite contains silver which is recovered in the lead concentrate. Argentite and stephanite are present in trace amounts, diminishing with depth, as does freibergite. Freibergite, chalcocite, stephanite and argentite are coarsest near the surface where they are commonly intergrown with each other and with galena, sphalerite and chalcopyrite (Pearson & Clark, 1990).

The major gangue minerals are adularia, quartz, calcite and fluorite. Other non-ore minerals include alunite, sericite, chlorite, amphibole and dolomite. The majority of the gangue minerals are found within the fluorite-base metal veins, where fluorite dominates, with accessory chlorite and sericite. The dilatant veins show a variety of gangue mineralogies, related to a progressive development of fracturing and infilling. For example there are quartz only veins, quartz-adularia and adularia only clusters (Pearson & Clark, 1990).

Age dating of fluorite deposited towards the closing stages of mineralisation is indicated to be 45.2±1.1 Ma. The homogenisation temperatures for the ore-stage has been determined to be 388 to 268°C, with a mean of 290°C. It has been suggested that the ore is related to a breccia pipe environment (Pearson & Clark, 1990).

Geophysics

While the ore minerals are not magnetic and do not respond to electrical methods, the associated sulphide gangue mineralogy of pyrrhotite and pyrite do. The pyrrhotite results in a magnetic variation of more than 1000 nT over the ore zone. In addition induced polarisation indicated a clear susceptibility high (over 10 milliseconds) and low resistivity response (under 75 ohm-m).

For detail see 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.


  References & Additional Information
   Selected References:
Pearson M F, Clark, K F, Porter E W  1988 - Mineralogy, fluid characteristics, and Silver distribution at Real de Angeles, Zacatecas, Mexico: in    Econ. Geol.   v83 pp 1737-1759


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