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The Fresnillo mine is located in Zacatecas State, Mexico. It lies on the Mexican Plateau between the Sierra Madre Occidental and Sierra Madre Oriental, to the west of the Mexican Thrust Belt.
Fresnillo falls within a ~1000 km long carbonate replacement and related vein Pb-Zn-Ag belt that runs in a NW-SE direction through Mexico, and includes the silver deposits of Santa Eulalia, Naica, Parral, Santa Barbara, San Francisco del Oro and Bismark, all in Chihuahua; Ojuela and Platosa in Durango; Sierra Mojada in Coahuila, Real de Angeles, Concepcion del Oro and San Martin in Zacatecas; Guanajuato District in Guanajuato; and Taxco in Guerrero.
This mineral belt is characterised by metals-rich Tertiary magmatic bodies intruding structurally prepared carbonate host rocks, either limestone or dolomite, of Jurassic or Cretaceous age. The deposits within the belt include high temperature replacement tabular 'manto', fault intersection controlled 'chimneys' or 'pods' of massive to semi-massive sulphides, as well as transgressive veins. Many of these deposits are remote from intrusions, although those more proximal have skarn alteration.
In Mexico, this belt follows the interface between the Tertiary Sierra Nevada Occidental volcanic plateau and the Laramide Mexican thrust belt and lie within, or on the margins of that major fold-thrust zone (Megaw et al., 1988).
It is interpreted to be the result of three deformation pulses and the onset of the Tertiary magmatic event which created favourable ground preparation and a magmatic source for mineralising fluids, as follows (Megaw et al. 1988, Megaw 2002):
• D1 - an initial extensional regime related to the breakup of Pangea at 166 Ma in the Mid Jurassic resulted for the formation of deep-seated north-south to NNW trending structures and rift basins;
• D2 - NE-SW directed compression during the Cretaceous to early Tertiary Laramide Orogeny deformed the Mesozoic sedimentary rocks and produced a series of roughly parallel north-northwest trending folds and faults;
• D3a - Mid- to Late-Tertiary extension reactivated and reopened earlier faults, including the structures bounding the Coahuila Platform, and developed further NW-SE oriented ground preparation.;
• Mid-Tertiary magmatism - the D3a extension event was accompanied by widespread magmatism, controlled by the newly reopened faults, which allowed intrusion at shallow levels into the structurally prepared Mesozoic carbonate sequence. Most of the carbonate replacement deposit in Mexico were formed during this event, with the largest deposits forming over deep-seated large-scale fault zones.
The most significant of these deposits within the belt are hosted in Jurassic to Cretaceous carbonate successions of the Chihuahua and Sierra Madre terranes, which together with the Coahuila and Maya terranes are underlain by Palaeozoic or possibly older continental crust (Ruiz et al., 1988). Mineralisation throughout the belt predominantly occurs in a limestone-dominant transgressive sedimentary succession deposited during Jurassic-Cretaceous flooding of pre-middleTriassic basement terranes (Haenggi, 2002). Exceptions include Real de Angeles which is hosted by a Cretaceous clastic sandstone, above the main carbonate section, and the deposits of the Parral, San Francisco del Oro and Santa Bárbara districts which are predominantly hosted by the Cretaceous Parral Formation shales. The basement was block-faulted during the Middle Jurassic extension, resulting in a series of linear basins, including the NW-elongate Chihuahua Trough, which controlled sedimentation (Megaw et al., 1996). NE-SW directed Laramide compression during basin inversion between ~84 and ~43 Ma stacked the host sequences into east-dipping thrust sheets (Muehlberger, 1993), accompanied by gentle to tight folding and extensive axial faulting and fracturing (Hewitt, 1966; Handschy and Dyer, 1987). This deformation is interpreted to have created structurally enhanced permeability that subsequently influenced the location of mid-Tertiary intrusions that ranged from diorite through to rhyolite compositions. Related hydrothermal fluids also followed these structural zones, depositing a range of skarn, carbonate replacement and vein deposits (Clark and de la Fuente L., 1978; Megaw et al., 1988). Megaw et al. (1988) notes that mineralisation is commonly hosted in limestone layers capped by impermeable strata, interpreted to have confined fluid flow and mineral precipitation to distinct zones. In northern Mexico, Pb isotope data from deposits suggests mixing of reworked Pb from the basement and magmatic Pb from the Cenozoic mantle linked to rapid spreading at the East Pacific Rise prior to the opening of the Gulf of California (McDowell and Keizer, 1977; Cumming et al., 1979; Megaw et al., 1996; Nieto-Samaniego et al., 1999). Mineralisation throughout the region is bracketed between 57.9 and 25.9 Ma, coinciding with the main period of the Tertiary Sierra Madre Occidental volcanic activity, with maxima between 35 and 29 Ma (Megaw et al., 1988; Albinson et al., 2001; Camprubi et al., 2006; Velador et al., 2010). Altered and/or mineralised volcanics and intrusives of Tertiary age are also found in almost all of the Mexican districts, with mineralisation apparently occurring during a restricted period from 47 to 26 Ma (Megaw et al., 1988).
This belt continues into the United States where it swings to trend NE-SW for a further >1000 km. Many of the significant orebodies in this part of the belt are also hosted by carbonate rich sequences, although these are generally of lower to middle Palaeozoic age. Again similar mineralisation also occurs within some other lithologies within the same belt. These Palaeozoic carbonate successions overlie Proterozoic metamorphics and are in turn overlain by Permian to Mesozoic clastic sedimentary rocks, which may include some carbonates, and Tertiary to Quaternary volcanics. Ore deposits such as Taylor in southern Arizona and those at Leadville, Aspen and Gilman in Colorado, and at Tintic and Park City in Utah are predominantly hosted by Palaeozoic carbonates. However, deposits such as those at Central City, which are vein occurrences in Proterozoic metamorphics, and Creede which also represents vein systems, but this time within mid Tertiary caldera complex volcanics, are found within the same mineral belt as Leadville, Aspen and Gilman, and have the same mineral assemblage. The Creede vein systems are very similar to the Cripple Creek Au deposits in both form, age and host rock.
Taylor is located in southern Arizona at the transition from the Mexican to US sections of the overall 2200 km long belt. Leadville, Aspen, Gilman and Park City lie within the NE-trending Colorado Mineral Belt that is defined by an ~500 x 25 to 50 km belt of plutons. Mineralisation in this belt is estimated to be between 50 and 30 Ma in age, with Central City being nearer 60 Ma (Megaw et al., 1988). The ore vein system at Creede is dated at around 25 Ma. The 28 to 23 Ma Henderson Urad and 33 to 24 Ma Climax porphyry molybdenum deposits also occur within the Colorado Mineral Belt.
At ~40 Ma igneous intrusions responsible for the mineralisation in the Park City deposits and the Bingham Canyon porphyry Cu-Au-Mo-Ag deposit were emplaced in the the east-west aligned 'Uinta trend'. Tintic is located ~90 km to the SSW of Bingham Canyon in Utah as part of a north-south cluster of deposits.
Fresnillo is some 130 km to the north-west of the Real de Angeles silver mine and around 100 km to the south-east of the San Martin district ore deposits, both of which are also in Zacatecas. It is approximately 600 km to the south-east of the Naica mine in Chihuahua.
Underground mining has been conducted at Fresnillo on nearly 150 veins of which more than 30 have been major ore producers. Several manto and chimney deposits (as described blelow) have also contributed to the district production. The main district is sub-divided into three zones, namely:
i). the south-eastern portion, which includes the more recently discovered Ag rich veins of Santa Elena and Santa Niño;
ii). the central stockwork zone, approximately 2 km to the north-west, which encompasses numerous veins striking in a variety of directions; and
iii). the north-west or Fortuna portion, a further 1 km to the WNW, which is characterised by deep mantos, chimneys and disseminated sulphide deposits as well as a small porphyritic quartz-monzonite stock (Ruvalcaba-Ruiz & Thompson, 1988).
The central area was the site of the first reported ore discovery in the first half of the 16 th century, with production commencing in 1554, exploiting silver oxides in the near surface stockwork. The main mines were developed between 1717 and 1754 by small miners, although operations were arrested in 1757 by flooding in the lower levels. The government of Zacatecas worked the mines from 1830 to 1833, although a lack of capital forced the sale to a British backed company in 1835. This company, the Compañia Zacatecano-Méxicana, carried on operations from 1836, until 1872, by which time the mines had reached 425 m with ore having been extracted from some 35 shafts. In 1900 a New York based owner bought the tailings and built a leaching plant on site. In 1910 he organised the Fresnillo Company of New York that built a 400 tpd cyanide leaching plant. Later, in 1919, the Fresnillo Company leased the properties to the Mexican Corporation, which had British capital. They worked the area from 1921 to 1943, increasing the capacity of the cyanide leach plant to 3000 tpd (Ruvalcaba-Ruiz & Thompson, 1988; Gemmel, et al., 1988; Querol & Palacios, 1990).
Treatment of the sulphide ore by flotation was initiated in 1926. In 1929 the Fresnillo Company of New York and the Mexican Corporation formed the Fresnillo Company as a joint venture. In 1961 that company was 'Mexicanised' through the purchase of 60% by the Peñoles Group to form the Compañia Fresnillo SA. By the late 1960's however, the operations of the Fresnillo mine had become marginal, and as a result geological, geochemical and geophysical investigations were initiated to locate additional, higher grade reserves. In 1975 the Santa Niño vein was discovered with an initial intersection at a depth of approximately 300 m of 3 m @ 1087 g/t Ag, 1.62 g/t Au, 0.4% Pb, 0.7% Zn (Ruvalcaba-Ruiz & Thompson, 1988; Gemmel, et al., 1988; Querol & Palacios, 1990).
Published production and reserve figures are as follows:
Production 1333 and 1903 - 2480 t Ag (Querol & Palacios, 1990).
Historic Production from 1921, Oxide ore - 13.0 Mt @ 190 g/t Ag, 0.3 g/t Au (Querol & Palacios, 1990).
Production, Sulphide ore, 1926 to 1981 - 24.5 Mt @ 298 g/t Ag, 0.6 g/t Au, 0.3% Cu, 2.9% Pb, 3.8% Zn (Querol & Palacios, 1990).
Production, Sulphide ore, 1982 to 1987 - 2.06 Mt @ 510 g/t Ag, 0.32 g/t Au, 0.04% Cu, 0.6% Pb, 1.3% Zn (Querol & Palacios, 1990).
Production, Sulphide ore, 1988 - 0.45 Mt @ 724 g/t Ag, 0.38 g/t Au, 0.02% Cu, 0.3% Pb, 0.6% Zn (Querol & Palacios, 1990).
Production, 1900-1988 - 10 000 t Ag, 19 t Au, 70 000 t Cu, 700 000 t Zn+Pb (Ruvalcaba-Ruiz & Thompson, 1988).
Reserves, 1993 - 3.1 Mt @ 263 g/t Ag, 0.3 g/t Au. 0.2% Cu, 2.5% Pb, 2.4% Zn
= 815 t Ag, 0.93 t Au, 6200 t Cu, 152 000 t Zn+Pb (AME, 1994).
Reserves in 2020 (Fresnillo website) - 4643 t of silver and 15 t of gold in ore averaging 234 g/t Ag, 0.76 g/t Au.
During the early periods of production the Ag:Au ratio was close to 670, although this has declined to around 525 in the ore worked in the twentieth century (Ruvalcaba-Ruiz & Thompson, 1988).
The Fresnillo mine is (1994) one of the three main operations of the Cia Fresnillo SA de CV which is owned by Industrias Penoles SA de CV (60%) and the Cyprus AMAX Minerals Co (40%). The other two are the large Naica deposits in Chihuahua and the smaller El Monte in Hidalgo. The plant capacities of the three mines are: Fresnillo - 0.7 mtpa; Naica - 1.08 mtpa; and El Monte 0.27 mtpa. In 1991 the three mines together produced some 2.1 Mt of ore which yielded 540.16 t Ag, 32 487 contained tonnes of Pb and 27 266 contained tonnes of Zn. In 1992 a total of 2.74 Mt of ore were extracted from these mines to produce 704.3 t Ag, [Zn & Pb production not quoted]. In 1994 the three Fresnillo mines together constituted the worlds largest silver producer (AME, 1995).
Fresnillo is an underground mine with three shafts, a total of 1059 employees and a daily mining rate of 1800 t of ore (AME, 1995).
The geology of the Fresnillo district is broadly composed of a 1 km thick suite of Palaeocene to Oligocene (63 to 32 Ma), dominantly andesitic volcanics which overlie deformed Mesozoic marine sediments and submarine mafic volcanics. The Mesozoic sediments include shales, sandstones, greywackes, carbonatic rocks and limestones. Regionally this sequence is cut by major Laramide plutons and related stocks and dykes, and by minor Eocene to Oligocene silicic to intermediate stocks. These are all overlain by a further 1 km thick sequence composed of Oligocene (32 to 19 Ma) rhyolitic ash flow tuff sheets. Regionally, precious metals are concentrated in the Cretaceous sediments and volcanics, and in both the lower and upper volcanic complexes, while the base metals are preferentially concentrated in the Cretaceous sediments and the lower volcanics (Ruvalcaba-Ruiz & Thompson, 1988; Gemmel, et al., 1988).
In the Fresnillo district the host succession comprises the following (Ruvalcaba-Ruiz & Thompson, 1988; Gemmel, et al., 1988; Lang, et al., 1988; Querol & Palacios, 1990), from the base:
• Proaño Group, which is of early Cretaceous age and has been divided into,
Valdecañas Formation, 700 m thick - also known as the 'lower greywacke', a rhythmically alternating sequence composed dominantly of grey-greenish, thinly bedded, immature sandstone or greywacke, with grey to black thinly bedded carbonaceous and calcareous shales and discontinuous intercalated beds of micrite.
Plateros Formation, up to 650 m thick - which can be further sub-divided into a lower 300 m thick sequence of calcareous and carbonaceous shales, greywackes and micritic limestone, overlain by 350 m of alternating greywackes, and carbonaceous and calcareous shales.
• Chilitos Formation, 200 m thick - green to dark green andesitic flow breccias and lavas, locally intercalated with probable air-fall tuffs, shales, greywackes, lenses of marls and micritic limestone. Fossil evidence indicates a lower Cretaceous age.
• Fortuna Limestone, 500 m thick - thinly and medium bedded dark grey carbonate rock, with a transitional upper contact into the overlying unit. Overall it consists of stratified limestone made up of 10 to 40 cm thick beds, with some thin-bedded chert and calcareous shales. This unit is also known regionally as the Cuesta del Cura Formation.
• Cerro Gordo Limestone, 300 m thick - medium to thickly bedded grey carbonate, similar in composition to the preceding unit.
• Fresnillo Formation, 400 m thick - continental conglomerate with angular to sub-angular clasts, that grades upwards into arkosic sediment and felsic tuff. It may have been deposited prior to the thrusting of the Chilitos Formation as there are no volcanic clasts in the conglomerate. Probably of late Palaeocene to Eocene age.
• Early Tertiary Volcanics - which are only rarely preserved, but comprise limited exposures of andesitic tuff.
• Middle Tertiary Volcanics, 500 m thick - a sequence of predominantly rhyolitic ash-flow tuffs/ignimbrites and lavas with quartz and sanidine phenocrysts. Massive silicification is common to the west of the district although veining and faulting are generally absent. K-Ar dates from an ignimbrites near the base of the sequence returned an age of 38.4±0.8 Ma. Felsic volcanics higher in the succession have been dated at 27 to 28 Ma.
• Middle Tertiary Intrusives - an almost cylindrical 80 x 45 m porphyritic stock of quartz-monzonite (adamellite) has been encountered in the deeper levels of the mine, below 1000 m depth. At the surface it has protracted to a narrow dyke. K-Ar dating yields ages of around 31.6 to 32.4 Ma. The stock contains up to 20% strongly resorbed quartz phenocrysts and in the mine is strongly propylitised and argillised. Phenocrysts of plagioclase and amphibole comprise up to a further 20% of the rock, distributed through a fine groundmass. In addition there are andesite dykes cutting the Valdecañas Formation, and rhyolite dykes related to the compositionally similar overlying ash-flow tuffs.
• Late Tertiary Volcanics, 30 m thick - minor isolated olivine basalt flows. A similar accumulation elsewhere in Zacatecas was found to be 10.97±0.25 Ma.
Holocene alluvium and caliche, generally from 1 to 20 m thick.
The structure of the Fresnillo district is controlled by two phases of deformation. A broad anticlinorium that plunges gently towards the south-east, reveals crumpled sediments of the Proaño Group which resulted from the first phase, a period of folding and thrust faulting during the late Cretaceous. The second phase during the middle to late Tertiary comprised block faulting which caused a southwards tilting of the Cretaceous and Tertiary sediments and volcanics. The most prominent influence of the latter in the mine area is the Fresnillo Fault which juxtaposes the Fresnillo Formation and overlying Tertiary felsic volcanics with the upper greywacke unit of the Chilitos Formation. The throw on this fault is of the order of 300 m. Extensive development of minor tensional faults and fractures followed and coincided with hydrothermal activity, resulting in the vein mineralisation in the ore zones (Gemmel, et al., 1988).
According to Albinson (1988), at the time of the formation of the Fresnillo ore deposit, the palaeosurface was less than 500 m above the top of mineralisation in the Santa Niño vein. In contrast, at the Sombrerete deposit, near San Martin, some 100 km to the north-west, the palaeosurface appears to have been more than 1000 m above the top of the mineralisation.
Three styles of mineralisation are evident at Fresnillo. These are as follows:
• Mantos and Chimneys, which are found in the north-western section of the mine, around the Fortuna Shaft, 2 km to the north-west of the General Shaft. Two major stratabound manto deposits exist in the basal units of the Valdecañas Formation, occurring in the calcareous members of the both the middle member shale and lower greywacke. They are known as the 'Lower' and 'Upper' mantos and are 12 m and 5 m thick respectively. Other smaller mantos are found in different parts of the mine (Ruvalcaba-Ruiz & Thompson, 1988; Querol & Palacios, 1990).
In some sections of the mine mantos are spatially related to a series of veins and develop along favourable strata no more than 25 m from the veins and up to 100 m along the veins, with one exception which persists for 400 m along bedding, and has a thickness of 8 m. Production from this latter manto has been more than 1 mt of ore with 65 g/t Ag, 2.2% Pb and 3.3% Zn. In other zone the mantos are spatially related to a quartz-monzonite stock. Other mantos again are localised at the crests of folds, and may be up to 500 m from the margin of the stock (Ruvalcaba-Ruiz & Thompson, 1988; Querol & Palacios, 1990).
These mantos are associated with a gangue of axinite1 and quartz, with lesser, chlorite, calcite, hedenbergite, epidote, siderite, carbonaceous material and a clay assemblage replacing primary detrital minerals. A skarn assemblage of axinite, hedenbergite and almandine-grossularite, with tremolite-actinolite, is found within the intruded Cretaceous lower Proaño Group, adjacent to the quartz-monzonite stock. Within the mantos these silicate minerals have been partially replaced by marmatite, pyrrhotite, pyrite, chalcopyrite, galena, sphalerite, tetrahedrite, argentite2, marcasite, arsenopyrite, magnetite, matildite and pavonite. The sulphides may be both disseminated and 'stratified' (Ruvalcaba-Ruiz & Thompson, 1988; Querol & Palacios, 1990).
The Chimneys at Fresnillo dip steeply and are cylindrical shaped sulphide replacement bodies that cross-cut the stratigraphic units. They are only found around the margins of the quartz-monzonite (adamellite) stock and contain the same silicate minerals as the mantos, with later sulphide replacement. Texturally they appear to be vertical extensions of the mantos, with strong brecciation at the top and slightly higher (+20 ppm) Ag. Mine production from the mantos and chimneys between 1963 and 1985 has been 2.12 mt @ 67 g/t Ag, 2.6% Pb, 5% Zn and 0.12% Cu (Ruvalcaba-Ruiz & Thompson, 1988; Querol & Palacios, 1990).
Manto and chimney bodies predate, or are contemporaneous with base metal veins carrying a similar assemblage, while the silver rich veins, as described below, are younger (Ruvalcaba-Ruiz & Thompson, 1988).
• Stockwork Vein Zones, which predominate in the central Cerro Proaño section of the mine, adjacent to the General Shaft. Numerous veins are present in this area. Most dip steeply to the south, but as they approach the surface, they branch and flatten. In the Cerro Proa–o section, there is a concentration of such veins with random strikes, forming a stockwork zone at the top of seven veins that together, have the form of a vertical funnel. This is known as the Glory Hole Stockwork and covers a surface area of 700 x 200 m. Individual stockworks have a width of 10 to 150 m and are developed along the major veins for as much as 300 m. In spite of the past thorough mining the area had proven open pit reserves of 7 mt @ 85 g/t Ag and 0.23 g/t Au in 1990, mainly of oxides (native Ag, cerargyrite and native gold) with some remaining sulphides and sulpho-salts (Ruvalcaba-Ruiz & Thompson, 1988; Querol & Palacios, 1990).
Veins between the Fortuna Shaft and the Cerro Proaño section have enhanced base metal values at depth, although the silver content increases at the expense of the base metals above the 385 m level. The deeper base metal levels are similar to those found in the mantos as is the mineralogy. As such it is possible to recognise two vein varieties at Fresnillo. One style is characterised by the relatively shallow silver rich veins with only low base metal levels. These trend nearly east-west. At a greater depth in the central section they are underlain by the second style, which strike north-westerly and have higher base metal grades, while similar veins are found to the south-east (as described below) in the vicinity of, and including, the Santa Elena, Santa Niño veins. Limited mine exposures suggest that the silver rich veins cut those with base metal fillings (Ruvalcaba-Ruiz & Thompson, 1988).
• Larger Veins - These have historically been the main ore source at Fresnillo. In the central area, over 100 such veins are know. Their widths average 1.5 m, although the ore shoots within them may be up to 1100 m long and have a vertical extent of up to 950 m (Querol & Palacios, 1990).
The most important veins in the district are the Santa Elena, Santa Niño and San Ricardo veins which are 500, 1000 and 1500 m respectively to the south-east of the General Shaft. The first two veins strike at 76 to 85°, with a southerly dip of between 76 and 63°E. The average thickness of the Santa Niño vein is 2.8 m with an average grade of 776 g/t Ag, while the Santa Elana vein is 2.2 m thick and averages 407 g/t Ag. The San Ricardo vein strikes at 280° and dips at 70°N. Several minor, near parallel veins are present between the major structures (Ruvalcaba-Ruiz & Thompson, 1988).
As described above, the veins may be subdivided into: i). a silver rich or 'light sulphide' variety which average 735 g/t Ag per 1% Pb; and ii). the base metal, or 'heavy sulphide' veins that average 30 g/t Ag per 1% Pb. Some of the veins contain 'heavy sulphides' at depth and 'light sulphides' nearer the surface, with the change taking place at the 425 m below surface level. The silver rich section of the veins have a vertical range of less than 400 m, although the base metal veins extend over a vertical range of as much as 900 m (Ruvalcaba-Ruiz & Thompson, 1988; Querol & Palacios, 1990).
Ore minerals in the silver rich, 'light sulphide' veins include pyrargyrite, with varying amounts of acanthite1, proustite, miargyrite, polybasite, galena, sphalerite, chalcopyrite. The gangue is composed chiefly of quartz, pyrite, calcite, sericite, kaolinite, montmorillonite, chlorite and adularia. The ore shoots generally average 770 g/t Ag with less than 2% Pb+Zn (Ruvalcaba-Ruiz & Thompson, 1988; Querol & Palacios, 1990).
The Pb-Zn, or 'heavy sulphide' veins are composed of pyrite, sphalerite, galena, pyrrhotite and arsenopyrite. Silver is present as polybasite, freibergite, pyrargyrite and bismuth sulpho-salts in the lower levels, but is also contained in galena. Gangue minerals are axinite, hedenbergite and quartz. Silver values are higher at the top and bottom of the veins Querol & Palacios, 1990).
In detail the Santa Niño vein trends at 70°, with local variations of up to 20°, and dips of 60 to 80°SE. It does not outcrop, although trenches at its projected intersection with the surface exposed a small stockwork of veins and veinlets of calcite and quartz. It has a strike length of around 4 km and is known over a vertical extent of nearly 500 m. Vein widths vary from 0.1 m to more than 4 m, averaging 2.85 m. It has reserves and production of 3.2 Mt @ 703 g/t Ag, 0.4 g/t Au, 0.03% Cu, 0.72% Zn and 0.34% Pb between 215 and 425 m below surface, and a further 2 Mt of probable ore between the 425 and 695 m levels (Gemmel, et al., 1988; Querol & Palacios, 1990).
The host rocks to the Santo Niño vein are the 'upper greywacke' of the Plateros Formation, the Chilitos Formation and the overlying Fresnillo Formation. The vein follows a fault with slickensides. In general, argillites and greywackes of the Plateros Formation occur in the footwall, while volcanics and interbedded lenses of greywacke and argillites of the Chilitos Formation are found in the hangingwall. The contact between the two formations is generally sub-parallel to the vein, is cross-cut by the vein, although in places the vein also follows the contact. In sections the vein cuts the faulted contact between the Chilitos and Fresnillo Formations which are juxtaposed along section of the Fresnillo Fault. Where cut by the vein, the Fresnillo Fault is offset by 4 m (Gemmel, et al., 1988).
The Santo Niño vein pinches and swells along strike, varying from a single vein, to a multiple split or horsetail feature. The main vein can be divided into three structural zones, all of which are believed to be related to oblique deformation with dominant dextral strike-slip displacement. The western structural zone is characterised by a linear strike with pinching and swelling, while the central section is characterised by loops formed by the vein splitting and rejoining, thus enclosing ovoid blocks of country rock. The eastern structural zone is composed of right stepping en echelon segments. Four separate 'vein stratigraphic stages' record the multiple fissure openings that provided a conduit for mineralising media. Stages I and II represent separate and distinct breccias. Stage III contains well developed crustiform banding, while stage IV is a massive flooding of coarsely crystalline calcite (Gemmel, et al., 1988).
The ore minerals of the Santo Ni o vein are a complex intergrowth of fine grained base metal sulphides and silver-bearing sulpho-salts. In order of abundance these minerals are pyrite, sphalerite, galena, pyrargyrite, polybasite, chalcopyrite, arsenopyrite, tetrahedrite, argentite/acanthite, stephanite, marcasite, proustite, antimonpearceite, pyrrhotite and selenian polybasite. Oxidised fractures contain limonite, hematite, Mn-oxide, malachite, azurite and native silver. Gangue minerals include quartz, chalcedony, calcite, chlorite and clay. The sphalerite has a strong oscillatory and banded colour zoning related to its Fe content. The silver content of the veins is contained as silver-bearing sulpho-salts and sulphides and not as argentiferous galena. Mineral deposition is divided into the following paragenetic groupings: i). early pyrite and arsenopyrite; ii). base metal sulphides, pyrite and arsenopyrite; iii). Ag-bearing sulpho-salts and sulphides; and iv). supergene sulphides and oxides (Gemmel, et al., 1988).
Hydrothermal alteration products adjacent to the Santa Elena and Santo Niño veins form irregular and narrow zones within the host rocks. Often there is no apparent alteration selvage to the narrow veinlets that split from the main veins. The alteration selvages to the main veins exhibit the following alteration styles:
i). Propylitisation - mainly represented by calcite, epidote and chlorite with variable pyrite, montmorillonite and kaolinite. Where no other alteration style is present this style is adjacent to the vein, although otherwise it is furthest from the vein, separated by up to 4 m. It is especially well developed in andesitic hosts;
ii). Argillic alteration - principally consisting of kaolinite and montmorillonite with variable pyrite, minor halloysite, sericite, chlorite and epidote. This style affects andesitic rocks and the greywackes and shales of the Plateros Formation;
iii). Phyllic alteration - which essentially comprises sericite with pyrite and minor illite, kaolinite and montmorillonite. Phyllic and argillic alteration affects some of the Tertiary volcanics;
iv). K-silicate alteration - principally occurring as adularia which is found within the veins and adjacent wall rocks. In general the K-silicate alteration flanks the vein, passing outwards into an argillic zone and then propylitisation (Gemmel, et al., 1988).
In the Fortuna Shaft area, the quartz-monzonite (adamellite) stock is found with the associated upper and lower mantos. Where chimney or manto deposits are found near or at the intrusive contact, the plagioclase in the quartz-monzonite is intensely epidotised with the development of secondary albite. Ferromagnesian minerals have been completely replaced by chlorite, magnetite and minor clay-minerals. Disseminated pyrite occurs within the altered quartz-monzonite. Shales of the Valdecañas Formation, where in contact with the stock, exhibit intense silicification with occasional calc-silicate-calcite-pyrite veinlets. Relict sedimentary structures/bedding only become discernible 10 to 15 m from the contact. Outside of the silicified zone the rock is replaced by banded hedenbergite and axinite which alternate with sulphides. Greywackes beyond the silicified zone have irregular zones of almandine-grossularite which have been replaced by later chlorite and actinolite (Gemmel, et al., 1988).
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.
Albinson F, T 1988 - Geologic reconstruction of paleosurfaces in the Sombrerete, Colorado, and Fresnillo districts, Zacatecas State, Mexico: in Econ. Geol. v83 pp 1647-1667|
Gemmell J B, Simmons S F, Zantop H 1988 - The Santo Nino Silver-Lead-Zinc vein, Fresnillo district, Zacatecas, Mexico: Part I. Structure, vein stratigraphy, and mineralogy: in Econ. Geol. v83 pp 1597-1618|
Lang B, Steinitz G, Sawkins F J, Simmons S F 1988 - K-Ar age studies in the Fresnillo Silver district, Zacatecas, Mexico: in Econ. Geol. v83 pp 1642-1646|
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.|
Ruvalcaba-Ruiz, D.C. and Thompson, T., 1988 - Ore deposits at the Fresnillo Mine, Zacatecas, Mexico: in Econ. Geol. v.83, pp. 1583-1596.|
Simmons S F, Gemmell J B, Sawkins F J 1988 - The Santo Nino Silver-Lead-Zinc vein, Fresnillo district, Zacatecas, Mexico: Part II. Physical and chemical nature of ore-forming solutions: in Econ. Geol. v83 pp 1619-1641|
Velador J M, Heizler M T and Campbell A R, 2010 - Timing of Magmatic Activity and Mineralization and Evidence of a Long-Lived Hydrothermal System in the Fresnillo Silver District, Mexico: Constraints from 40Ar/39Ar Geochronology : in Econ. Geol. v.105 pp. 1335-1349|
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