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The Naica carbonate replacement silver and base metal ore deposits are located in the south-central part of the state of Chihuahua in Mexico, some 110 km to the south-east of the capital, Chihuahua City. It is approximately 100 km to the south-east of the Santa Eulalia mine, also in Chihuahua, and some 600 km to the north-west of the Fresnillo mine in Zacatecas (#Location: 27° 50' 43"N, 105° 30' 21"W).
The earliest mining claim in the Naica district dates from 1794, although the first surface discoveries and workings of note were after 1828, but only during the wet seasons. The first large scale mining was started in the 1890's by the Compañia Minera de Naica SA, but were suspended in 1911 due to water problems at the mine and political instability. Work re-commenced in 1924 with several companies extracting oxide ore over the period until 1951. In that year the Compañia Fresnillo SA de CV bought the claims and started a new mine with electric pumps and sulphide flotation circuits with a 400 tpd capacity. The company commenced operations in 1952, increased capacity and bought the surrounding properties, until in 1956 they were the only operator in the district, with a total monthly throughput of 60 000 t of ore (Querol & Vallejo, 1990). The mine came under the control of Industrias Peñoles in 1964 as Minera Maple SA de CV., and remained in production until October 2015 when it was decided to suspend activities indefinitely.
Published production and reserve figures include:
Reserve, 1991 - 4.5 Mt @ 230 g/t Ag, 1.5% Pb, 1% Zn (AME, 1994).
Proven Reserve, 1989 - 3.6 Mt @ 147 g/t Ag, 4.1% Pb, 3.2% Zn, 0.4% Cu, 0.17 g/t Au (Querol & Vallejo, 1990).
Probable Reserve, 1989 - 0.7 Mt @ 169 g/t Ag, 4.7% Pb, 4.0% Zn, 0.5% Cu, 0.17 g/t Au (Querol & Vallejo, 1990).
Production, pre-1951 - 1 Mt @ 400 g/t Ag, 16% Pb (mostly oxide, Querol & Vallejo, 1990).
Production, 1952 to 1989 - 18.6 Mt @ 173 g/t Ag, 5.3% Pb, 4.4% Zn, 0.4% Cu, 0.35 g/t Au (mostly sulphide, Querol and Vallejo, 1990).
The head grade at Naica in 1989 was 149 g/t Ag, 0.2 g/t Au, 4.2% Pb, 3.4% Zn, 0.3% Cu, 0.1% WO3;
Metallurgical recoveries were Ag - 93%; Au - 36%; Pb - 95%; Zn - 88%; Cu - 73% and WO3 - 28% (Querol & Vallejo, 1990).
Milled ore in 2014 - 0.711 Mt @ 81 g/t Ag, 2.7% Pb, 2.2% Zn (Peñoles annual report, 2014).
Remaining Ore Reserves, December, 2013 - 13.841 Mt @ 98.40 g/t Ag, 2.31% Pb, 6.52% Zn, 0.09% Cu, 0.03 g/t Au (Peñoles annual report, 2014).
The mine is an underground operation with a flotation based mill which had an annual capacity of 1.08 Mt of ore in 1994. In 2014 the installed mill capacity was 0.95 Mt per annum (Peñoles annual report, 2014).
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.
The Naica district is located within the north-west trending Basin and Range province of northern Mexico. It is on the western edge of the Mexican Thrust Belt and the eastern margin of the Sierra Madre Occidental volcanic plateau, near the terrane boundary (Megaw, et al., 1988). Locally, the mine is located on the north-eastern flank of a domed anticlinal structure that is 12 km long by 7 km wide, and is locally folded, faulted and deeply eroded. The rocks of the structure form the Sierra de Naica, which is composed of three main ranges, the Sierra de la Mina; the Sierra de Enmedio; and the Sierra del Monarco (Querol & Vallejo, 1990).
The rocks of the Sierra de Naica are predominantly sediments of Albian (113 to 97.5 Ma) to Cenomanian (97.5 to 91 Ma) age. These overlie the Aptian (119 to 113 Ma) rocks of the Cuchillo Formation which are absent in the mine area, but widespread in the surrounding area. The stratigraphy in the mine area is as follows, from the base (Querol & Vallejo, 1990):
• Cuchillo Formation - an evaporitic section that is the oldest member recognised to date;
• Aurora Formation, >1000 m thick - medium to thick-bedded limestone. The lower contact has not been observed. At surface it is composed of oolitic calcarenites with fine to coarse grained interclasts and bioclasts, bioclastic micrites and abundant chert. In the mine area the lower part of the unit has been thoroughly recrystallised, and is represented by three facies, namely a white marble, grey marble and limestone. The degree of recrystallisation is apparently independent of lithology. This unit is regarded as being early to late Albian in age.
• Benevides Formation, 30 to 60 m thick - a sequence of calcareous shale with disseminated pyrite and abundant macro-fossils. An age of middle to late Albian is suggested on fossil evidence.
• Loma de Plata Formation, around 450 m thick - subdivided into,
- a lower, 30 to 40 m thick member of 60 to 80 cm stratified micrite, and,
- a upper member of thick, almost massive limestone.
The upper and lower contacts are sharp and obvious. This unit is considered to be late Albian.
• Del Rio Formation, around 20 to 30 m thick - shales and shaly siltstone that overlie the Loma de Plata Formation, and are regarded as being of late Albian to Cenomanian age.
• Buda Formation, - a grey to tan coloured, thickly stratified limestone. It has a similar distribution to the Del Rio Formation, and has had its top sections removed by erosion.
• Igneous Rocks - The only igneous rocks recognised in the district are sills and dykes of felsite, variously described as alaskite or albitite. In thin section they are seen to be a fine grained rock composed of Na-K feldspar and quartz. Accessory minerals may include pyrite, calcite, fluorite and traces of chalcopyrite. Within the mine it is represented by a fine grained, light grey to tan rock with a conchoidal fracture. It has a banding which is parallel to the host rock and is normally altered to a skarn. In outcrop it has a reddish colouration with Leisegang rings. Radiometric dating indicates and age of 26 Ma.
• Alluvial cover and talus.
The dominant structure of the Naica District is a domal antiform, centred above a magnetic anomaly. The elevated magnetic response has been interpreted to represent an intrusive mass, at a depth of around 2000 m (Querol & Vallejo, 1990).
Mineralisation at Naica is found on the eastern flank of the dome, in the core of a subsidiary dome that forms a structural nose plunging to the north-east. Pre-ore fracturing and faulting corresponds to a north-west fracture set which hosts the majority of the sills and mantos. A second set of fracturing dipping at 60° both to the south-east and north-west, forms a graben striking to the north-east. Displacement on these faults is of the order of several tens of metres. This set displaces the mantos but controls the distribution of the chimneys in the upper section of the mine. As such it is thought to be coeval with the ore emplacement (Querol & Vallejo, 1990).
A third, post-ore set strikes north-west, with examples such as the south-west dipping Gibraltar Fault with a 50 m vertical offset; the Naica Fault which has a 200 m vertical displacement; and the Montana Fault which dips to the north-east.
The ore deposits of Naica are characterised by moderate to large tonnage bodies that have replaced limestones with sulphides of Pb, Zn and some Cu. These occur as massive sulphides, or are disseminated in skarns, with minor amounts of Bi, W, Au and Mo. The ore mineralogy comprises the following, in the order of apparent deposition: ilvaite, arsenopyrite, mackinawite, pyrite, molybdenite, chalcopyrite, pyrrhotite, tetrahedrite, bismuthinite, tetradymite, kobellite, polybasite, galena, bornite, sphalerite, covellite, marcasite, hematite, goethite, magnetite, chalcocite, rutile, gold and pyrargyrite. The gangue mineralogy consists of vesuvianite, manganiferous-hedenbergite, garnet of variable composition (from grossularite-spessartine in the mantos to andradite-spessartine in the chimneys), wollastonite, scheelite, adularia, quartz, fluorite, dolomite, calcite and anhydrite. All of these silicates were deposited prior to the sulphides, while fluorite, calcite, dolomite and anhydrite are contemporaneous with the sulphide formation and later (Querol & Vallejo, 1990).
Ore is present in two principal forms, according to their geometry and mineralogy. These are:
• Mantos - which are tabular skarn/silicate bodies containing disseminated sulphides, and usually having cores of felsite. Some are conformable with bedding while others are cross-cutting. The mantos usually strike in a north-west direction, with variable dips from vertical, which follow fractures, to near horizontal where they parallel bedding. More than 17 mantos are known, most of which are inter-connected. The individual mantos vary from a few cm's in thickness, up to 25 m and have widths of as much as 500 m. The mantos are ubiquitous, having been tested to depths of 800 m below the surface. Individual mantos may contain up to 3 Mt of ore. The mineralogy of the skarns include garnet, vesuvianite, wollastonite, hedenbergite, quartz, calcite, fluorite, molybdenite, pyrite, pyrrhotite and marcasite. The ore mineralogy includes galena, sphalerite, chalcopyrite and scheelite. Silver is found as microscopic inclusions of kobellite [a Bi-Pb sulphosalts] (Querol & Vallejo, 1990). Based on their spatial relationship with felsite sills and dykes, the mantos are divided into:
Endoskarn mantos, which are usually coarser grained than the exoskarns, are characterised by the ubiquitous presence of scheelite and molybdenite, and have an Ag:Pb ratio that is usually greater than the exoskarns. They are emplaced within the felsite bodies, commonly not interstitially, although in some cases the felsite may be completely replaced (Querol & Vallejo, 1990).
Exoskarn mantos, which are composed of fine grained silicate-carbonate minerals with rare scheelite and molybdenite and Ag:Pb ratios similar to that of the chimneys (see below). They are spatially associated with the outer parts of the endoskarns and terminate laterally in silicified limestone. While they are generally smaller in size than the Endoskarns, they are commonly conformable with the limestone bedding (Querol & Vallejo, 1990).
The most abundant sulphide developments are found along the contacts of the skarn where it replaces the adjacent marble, although some mineralisation is also present within the skarn itself. However, although the sulphide mineralisation within the skarn, and the felsite that forms a core to the skarn, is not as intense as in the outer marble replacement sulphide zones, the higher content of silver renders them economic. Scheelite and molybdenite are always contained within the endoskarns and felsite, while Pb and Zn are more abundant in the exoskarns (Querol & Vallejo, 1990).
• Chimneys - are local, transgressive, elliptical, pipe or tube-like, cylindrical bodies which are composed principally of massive sulphides and scarce silicates. They have plunges of >45° and cross-sectional areas of up to 4000 sq. m (Querol & Vallejo, 1990).
Sixty chimneys have been located at Naica with diameters ranging from 3 to 70 m. Most chimneys end in the oxidised zone, although one, the Torino-Tehuacán has a 4 sq. m outcrop. At depth most chimneys end in mantos, although some end in veins. In their upper levels most are controlled by a NE-SW fracture system. The largest known chimney is the Torino-Tehuacán with a vertical extent of more than 800 m and tonnage of 3.3 Mt in the sulphide zone (Querol & Vallejo, 1990).
The chimneys at Naica have been divide into two types, namely: i). the Sulphide-silicate, and ii). the Massive sulphide chimneys. Most massive sulphide chimneys grade with depth into sulphide-silicate chimneys. Some small chimneys on the margins of the mineralised system are composed of massive sulphides (pyrite-galena-sphalerite), with no trace of silicates, and grade at their top and bottom extremities into small veins (Querol & Vallejo, 1990).
The average Ag:Pb ratio in the Naica orebodies is influenced by the orebody types, as follows: i). Endoskarn mantos = 62.5; ii). Exoskarn mantos = 31.4; iii). Sulphide-silicate chimneys = 30.7; and iv). Massive sulphide chimneys = 25.5 (Querol & Vallejo, 1990).
Fluid inclusion studies indicate at least three fluid types accompanied mineralisation, with varying temperatures of from 119 to 490°C, and a range of salinities and compositions. Some temperatures were apparently as high as 565°C. The fluid inclusions contain halite, KCl, fluorite, sylvite and anhydrite, all of which have been interpreted as having been derived from the evaporite beds that lie below the host sequence. Magnetic data, supported by the high temperatures, imply a magmatic body at a depth of 2 km. The structural position of the mantos apparently indicates that they preceded the chimneys and were formed at the maximum energy of the geothermal event that was related to the formation of the mineralised system. The hydrothermal event is indicated to have taken place during the Oligocene, in the late stages of magmatic intrusion in the district. It was coincident with the last phase of volcanism in the Sierra Madre Occidental, which is represented close-by to the west (Querol & Vallejo, 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.
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.|
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 takes no responsibility what-so-ever for inaccurate or out of date data, information or interpretations.
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