Santa Barbara, San Francisco del Oro, Parral, Granadena
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The Santa Bárbara, Parral & San Francisco del Oro Districts are clusters of mines that constitute three districts located within an area of approximately 250 km2 in the far southern extremity of Chihuahua, Mexico and together form the principal mining area within the state (Borbolla, 1990). Santa Bárbara and San Francisco del Oro are both approximately 18 to 20 km to the south-west of Parral. The three districts are approximately 190 km to the south of the Fresnillo mine and 90 km SSW of Naica (Grant & Ruiz, 1988).
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 first discovery of gold veins in the Santa Bárbara district was in 1536. However, in 1563 an exploration party passing the Santa Bárbara area recognised signs of mineralisation, resulting in the commencement of the first official workings which exploited Ag and Au from the Agua mine. This mine was still active in 1990. From 1567 until 1616 mining of shallow oxide ores made Santa Bárbara the most important centre in the region. In 1616 however, the areas importance declined due to wars with the Indians and a lack of security on the highways (Escuredo, et al., 1990; Silva & Gonzalez, 1990).
Mining commenced near Parral in 1631 after the discovery of the first significant veins at the La Negrita mine in 1629, (later to become the Prieta mine). Other 'silver deposits' had been reported from the Parral area as early as 1610, concealed by the 'thick wild bushes'. Further discoveries were made through the 1630's and 1640's, while the rich veins at San Francisco del Oro were first encountered in 1658, including those of the San Pedro mine which commenced operations in 1659. These were followed in 1673 by the Clarines mine which was still in production in 1990. These discoveries resulted in the city of Parral becoming an important economic centre (Escuredo, et al., 1990; Borbolla, 1990).
During that period, and for some time after, the deposits of the region were only exploited for silver, due to the cost of transport, metallurgical complexity and the fact that the interval mined was largely within the oxide zone which extended to depths of from 30 to more than 300 m below the surface. Until the middle of the 18th century, rich lead ores were smelted, while the lead poor ores were treated by amalgamation (Borbolla, 1990).
In the years from 1745 to 1750 the workings in the major mines reached the sulphide zone and were abandoned. The mines remained inactive for many years until technological advances in ore treatment and pumping resulted in a second period of exploitation which lasted until 1890. In 1901 a bonanza stockwork orebody was discovered at the Palmilla mine at Parral, which was subsequently worked for more than 25 years. In 1925 ASARCO purchased the La Prieta mine, also at Parral, and worked it until 1975 when declining Pb grades (from the earlier 8 to 10%, down to 2.5%), increasing Cu (which caused metallurgical problems) and low Ag prices forced the closure of the mine. From 1975 three mines in the Parral district have produced around 0.275 mt of ore per year, averaging 175 g/t Ag (Borbolla, 1990).
At San Francisco del Oro, following a period of inactivity after the decline of the mines in the late 18th century, the English company Marine Mines of Mexico worked a number of the properties from 1880 to 1908. In 1913 another British company, the San Francisco Mines Company of Mexico continued the exploitation of the area and by 1920 was using selective flotation, with increasing production, except for 1932 when economic conditions closed the mine (Escuredo, et al., 1990).
Operations at the Santa Bárbara mines had been hampered by the Mexican war of independence from 1810, but re-opened in 1833. By the end of the 19th and the beginning of the 20th century, organised companies with strong investments brought new exploration and exploitation methods to the area, resulting in a change from small to relatively large operations. With the application of cyanide methods in 1925-26 and flotation techniques, the sulphide zone encountered at that time could be economically exploited. This resulted in a substantial increase in the reserves of the area which are still being worked to the present (Silva & Gonzalez, 1990).
Published production and reserve figures for the three districts are as follows:
San Francisco del Oro
150 g/t Ag, 5% Pb, 8% Zn, 0.6% Cu, 0.5 g/t Au (Av. grade, Grant & Ruiz, 1988).
= 13 500 t Ag (District Prod. to 1980, Grant & Ruiz, 1988).
10.2 Mt @ 140 g/t Ag, 4.2% Pb, 6.5% Zn, 0.28 g/t Au (Res., 1982, USBM).
140 g/t Ag, 4.2% Pb, 6.5% Zn, 0.28 g/t Au (Av. Grade., 1990, Escudero, et al., 1990).
6.8 Mt @ 97 g/t Ag, 1.5% Pb, 2.3% Zn, 0.3% Cu (Res., 1991, AME, 1995).
5 Mt @ 190 g/t Ag, 0.9% Zn, 0.8% Pb, 0.06 g/t Au (Res., 1985, USBM).
10 Mt @ 180 g/t Ag, 4.5% Zn, 3.5% Pb, 1.5 g/t Au (Prod. to 1985, USBM).
16 to 20 Mt @ 400 g/t Ag (Prod., Veta Colorado Mine to 1929, Borbolla, 1990).
including, 0.01 Mt @ 1715 g/t Ag, 14.5 g/t Au (Borbolla, 1990).
4.1 Mt @ 175 g/t Ag (District Prod., 1975 to 1990, Borbolla, 1990).
5 Mt of ore (Prod., 1536 to 1890, Silva & Gonzalez, 1990).
5 Mt of ore (Prod., 1890 to 1925, Silva & Gonzalez, 1990).
15 Mt of ore (Prod., 1925 to 1973, Silva & Gonzalez, 1990).
14.3 Mt @ 109 g/t Ag, 0.34 g/t Au, 2.3% Pb, 4.2% Zn (Prod., 1974 to '88, Silva & Gonzalez, '90).
46 Mt @ 106 g/t Ag, 0.48 g/t Au, 1.99% Pb, 0.57% Cu, 3.75% Zn (Res., '89, Silva & Gonzalez, '90).
25.3 Mt @ 105 g/t Ag, 2.1% Pb, 3.7% Zn, 0.6% Cu (Res., 1991, RTZ, 1992).
The mines at San Francisco del Oro are owned and operated by Empresas Frisco SA de CV (1994). According to Grant & Luiz (1988), some five mines were operational at that time, annually producing around 0.9 Mt @ 150 g/t Ag, 0.5 g/t Au, 5% Pb, 8% Zn, 0.6% Cu and 5 to 12% fluorite. In 1992 the production from the underground mines was 0.909 Mt of ore which yielded 64.882 t Ag, 29 051 t Zn, 14 401 t Pb, 1702 t Cu and 0.203 t Au (AME, 1995).
The current Santa Bárbara operations, which were opened in 1921, comprised an underground mine with 1755 employees and a capacity of 1.8 Mt per annum. Production in 1993 amounted to 44 000 t Zn, 19 000 t Pb, 5400 t Cu, 110.0 t Ag and 0.22 t Au. The mine is owned and operated by Minerales Metalicos del Norte SA (AME, 1995).
The ore deposits at Parral, Santa Bárbara and San Francisco del Oro are located on the eastern margin of the Sierra Madre Occidental volcanic plateau, at the boundary with the Sierra Madre Oriental (Megaw, et al., 1988). The deposits occur predominantly as veins with minor stockworks and massive sulphides, hosted by the Cretaceous Parral Formation shales, and to a lesser extent by the overlying Tertiary andesite and rhyolite (Grant & Ruiz, 1988; Borbolla, 1990).
The stratigraphy of the region is as follows, from the base:
• Parral Formation, >1000 m thick - these are the oldest rocks outcropping in the district. They are composed of dark grey to black carbonaceous shale and calcareous shales/siltstone, with lesser beds and lenses of argillaceous limestone and limestone. Beds of shales are generally uniform and 15 to 20 cm thick, while the limestone lenses and beds are around 50 cm thick. Locally the shales contain nodules and bands of chert, as well as light brown calcite concretions and abundant calcite veinlets (Borbolla, 1990). In the San Francisco del Oro and Santa Bárbara districts, in contrast to the sequence around Parral, as described above, the Parral Formation is more compact, hard and dense, grey to black in colour, rich in carbonaceous material and the strata are thinner (1 to 4 cm), being predominantly calcareous shales with sporadic horizons of limestone which may be 10 cm thick. The shales and calcareous shales weather to red and orange colours, and in addition to clays, contain recrystallised calcite that fills small fractures. They are also usually well folded and fractured, with small scale drag folds. Within the region the unit generally dips at 25 to 30°W (Escudero, et al., 1990; Grant & Ruiz, 1988).
The age of the sequence is variously stated as being upper Jurassic to Cretaceous (Grant & Ruiz, 1988), to late Aptian to early Albian, ie. late lower Cretaceous (Borbolla, 1990; Escudero, et al., 1990). However stratigraphic columns accompanying these papers show the formation to be Albian to Cenomanian, followed by a depositional break (Borbolla, 1990), and Aptian to Eocene (Escudero, et al., 1990; Grant & Ruiz, 1988).
• Eocene, Escobedo Group, totalling around 665 m in thickness - composed predominantly of a volcanic series which form characteristic angular mountains with steep slopes and cliffs. The oldest members are believed to be of late Eocene age, while the upper sections are as young as Oligocene, with an age date of 34.92±0.75 Ma (Borbolla, 1990). To the north and north-east of Parral and in the Santa Bárbara district this Group is represented by andesitic lavas. In hand specimen they are light grey to dark blue in colour with clearly visible plagioclase crystals. In this area they lie unconformably on the Parral Formation which has been folded by the intervening Laramide event. Isotopic dating at Parral and Santa Bárbara return Oligocene ages of 34.92±0.75 Ma and 30.63±0.35 Ma respectively.
In the Parral district the Escobedo Group has been subdivided into the,
Lower Escobedo Group, averaging 263 m thick - which has two members, namely an older brown conglomerate with subangular fragments of calcareous shale which are well cemented, but poorly sorted. The clasts range from a few mms up to 10 to 15 cm across. This conglomerate is overlain by a series of sandy tuffs and greenish-grey intermediate agglomerates (Borbolla, 1990).
Middle Escobedo Group, 280 m thick - composed of intermediate rocks, basaltic andesites with rare sill like horizons up to 2 m thick. Some possibly intrusive porphyritic andesites have also been recognised (Borbolla, 1990).
Upper Escobedo Group, 112 m thick - which is more acid than the two underlying units. It is composed of andesitic tuffs and basaltic andesites at the base; rhyolitic agglomerates, vitrophyres and glass in its middle sections; and an upper suite of rhyolites, dacites and some glass (Borbolla, 1990).
• Red Fanglomerate, 0.5 to 10 m thick - which is only known locally in a small area in the north of the region. It is composed of diverse rock fragments that are 2 to 5 cm across, sometimes up to 10 cm, set in a matrix of sand or calcareous cement. The largest fragments comprise shale, andesite, rhyolite, rhyolitic tuff and volcanic glass, with lesser quartz-orthoclase-magnetite porphyry and limestone. It rests unconformably on the Parral Formation and is overlain by rhyolites (Escudero, et al., 1990).
• Oligocene to Early Miocene Acid Volcanics, including rhyolite, rhyodacite and small amounts of trachyte are widespread throughout the districts. Towards Parral there is a broad range of acid rocks, including rhyolites, rhyolitic tuffs, ignimbrites and rhyodacites as well as banded rhyolites. In this area the thickness of the unit is variable, generally being from 350 to 400 m thick. Their age is inferred by their proximity to the great 'flows' of the Sierra Madre Occidental which are of the age detailed above (Borbolla, 1990). The rhyolites occur as hard, compact, pinkish tuffs which may be altered to a light grey colour. Where less altered they are orange-red with quartz and sanidine phenocrysts in a glassy matrix. Where present the fanglomerate unconformably underlies the rhyolites (Escudero, et al., 1990).
• San Rafael Conglomerate, which fills the San Rafael Valley. The conglomerate is composed of rounded pebbles and cobbles of limestone, shale, rhyolite and vein material, all cemented by calcite. It covers the lower part of the rhyolite hills, is found beneath the basalt, and is cut by two basalt plugs (Escudero, et al., 1990).
• Miocene Basalts, 56 m thick - which are the youngest igneous rocks encountered, occurring as remnants of extensive flows which originally covered a large part of the area from San Francisco del Oro to Parral. They form wide, sub-horizontal mesas with abrupt cliffs. The basalt is hard and compact, varying from grey to black, and is aphanitic to fine grained with some vesicles, locally containing rare, altered olivine crystals. An age determination at Santa Bárbara returned a value of 6.87±1.99 Ma [Pliocene to Pleistocene]. In the San Francisco del Oro and Santa Bárbara districts, there are three basalt cones (Borbolla, 1990; Escudero, et al., 1990)
• Intrusives -Various intrusives are recorded from Santa Bárbara to Parral, varying in composition from granite, to quartz-monzonite, to diorite and monzonite. The Parral intrusive, near the La Prieta mine is an irregular body with dimensions of around 7 x 4 km, almost completely surrounded by the Parral Formation shales. Petrographically it is an quartz-monzonite with biotite and hornblende. More restricted exposures are known to the north-west of Parral. Dykes of diorite and quartz-monzonite are also found within the district, being more numerous in the vicinity of larger intrusives, sometimes reaching thicknesses of 30 m and lengths of 4 km (Borbolla, 1990).
Quaternary alluvium, is found along creeks and rivers in flooded areas and is composed of local rock fragments, the most abundant of which are shales (Escudero, et al., 1990).
The rocks of these three districts have apparently been affected by a period of compression during the Laramide orogeny of the late Cretaceous to Tertiary. This produced a number of folds which deformed the Parral Formation, particularly an asymmetric anticline which is located between the Santa Bárbara and San Francisco del Oro districts. This antiform has dips of 30°W on its south-western limb and 8°N on its north-eastern flank. Its axis trends 332°, and plunges to the north at 12°. In detail this structure is very complex with numerous drag folds and minor faults. This folding has been interpreted to have been the result of NE-SW directed compression (Escudero, et al., 1990; Borbolla, 1990; Silva & Gonzalez, 1990).
At the close of the Eocene the deformation changed to an extensional regime producing a series of faults. The faulting and fracturing in the districts can be sub-divided into:
i). Pre-mineralisation faults and fractures - these were formed in two stages, the first being two sets of fractures and shears, accompanying the folding, and having a parallel trend along the anticlinal axis. The second fracturing stage is the result of tensional deformation (Silva & Gonzalez, 1990). These faults are occupied by the sulphide veins bearing gold and silver and by siliceous alkaline dykes;
ii). Post-mineralisation faults - which comprise a number of different varieties, including: a) faults that are similar to the pre-mineral structures, but are a little later than the vein development and do not carry sulphides; b) post-vein faults that are almost perpendicular to the veins and are filled with calcite, fluorite & barite; c) a similar set, also perpendicular to the main sulphide veins, but occupied by basic dykes; d) faults at various orientations that are filled with clay gouge (Escudero, et al., 1990; Silva & Gonzalez, 1990).
Mineralisation & Alteration
The mineral deposits of the Parral, Santa Bárbara and San Francisco del Oro districts are epigenetic. Approximately 95% of the mineralised structures are veins with lesser stockwork and replacement bodies, the latter being mainly at Santa Bárbara. The deposits of the three districts are described separately below:
Parral - At Parral there are two types of veins, namely, fissure veins and fissure-filling veins.
The fissure veins are characterised by hydrothermal mineralisation occupying pre-existing shears that are interpreted to have been created by post-orogenic relaxation. They generally dip at 55 to 75° to either the E or W, have a north-south strike and may persist over lengths of 2 to 8 km. Thicknesses vary from 2 to 20 m. Mineralisation is basically sulphides of Pb and Zn with silver in a well brecciated and silicified matrix. They are also characterised by deep oxidation, commonly to more than 200 m below the surface, with a variable transition to the underlying un-oxidised sulphides. Rich ore shoots which alternate with barren zones is also a characteristic of these veins. The longitudinal extent of the rich shoots is variable, but may be from 400 to 1000 m, commonly persisting to depth (Borbolla, 1990).
The fissure-filling veins are associated with a secondary fracture system having NW and NE striking members, possibly related to the emplacement of the Parral intrusive. These veins are well defined, but of lesser length, and do not persist with depth. Their thickness rarely exceeds 2 m, and they are differentiated from the fissure veins by their strike direction, and shallow oxidation which is usually less than 15 to 50 m (Borbolla, 1990). There is a zonal distribution of commodities in the veins surrounding the intrusive. Close to the contact the veins are rich in silica and pyrite with Au, but are low in Pb-Zn. At a greater distance Pb & Zn are more abundant, while Au values disappear. Finally good Au and Ag values are associated with sulphides of Pb and Zn, as well as barite, calcite and silica (Borbolla, 1990).
Stockworks of veins are developed at the intersection of two or more veins, or where two veins are in close proximity (Borbolla, 1990).
Mineralisation is possibly of late Oligocene to early Miocene age, being younger than the Escobedo Volcanic Series which host some ore, but older than the acid flows (Borbolla, 1990).
The mineral assemblages encountered comprise: i). Sulphide minerals - argentite, pyrargyrite, argentiferous galena, galena and sphalerite; ii). Oxide minerals - embolite, bromyrite, pearceite, argentite, argentojarosite, cerussite, anglesite and smithsonite; iii). Gangue minerals - fluorite, barite, quartz, calcite, manganite, pyrolusite, hematite, goethite and smithsonite (Borbolla, 1990).
San Francisco del Oro - The mineralisation at San Francisco del Oro is principally composed of veins, and only rarely as replacement bodies (Escudero, et al., 1990).
The veins of the district are divided into three zones, namely the i). oxidised, ii). enriched and iii). primary zones. The oxidised interval varies from a depth of 50 to 170m, depending on the surface relief. The minerals found in this interval include gold, silver, anglesite, cerussite, zinc-sulphate, azurite, malachite, limonite, native copper, smithsonite and others. The principal minerals within the supergene enriched zone are bornite, covellite, chalcocite, chalcopyrite, sphalerite, marmatite, galena, anglesite and pyrite. In the underlying primary sulphide zone the ore is composed of sphalerite, galena, chalcopyrite, pyrite, arsenopyrite, gold, silver, quartz, fluorite, calcite, barite and high temperature silicates such as garnet and pyroxene. Silver is closely associated with galena, with no discrete silver phase having been identified (Escudero, et al., 1990).
The predominant strike of the veins is north-south, with dip variations to the north-west and north-east, from vertical to around 45°. The veins that were being exploited in 1990 were narrow, generally ranging from 0.1 to 1.5 m in the Frisco mine, to around 3 m at the Granadeña mine thicknesses, while in the Clarines mine they may reach 6 m in width. In general the veins are of uniform thickness, although locally two or more may branch off the main structure. The host rocks of the veins within the district are the grey to black carbonaceous shales, or lighter coloured calcareous facies. All become silicified in the lower levels. These beds are well formed, ranging from thin laminations, up to thicknesses of 6.5 m, with dips from the horizontal to 20°NW. Locally they are tightly folded with steeper dips (Escudero, et al., 1990).
In the Granadeña mine Pb-Zn-Ag veins are localised along small displacement faults that cut obliquely across the hinge zone of the broad asymmetric anticline. Single veins may be more or less continuous over lengths of up to 800 m. Veins average 1.5 m in thickness, but vary from several cm's to 3 m. Pb-Zn-Ag mineralisation is found over a vertical interval of more than 500 m. The ore was emplaced in several distinct stages that display cross-cutting relationships. The first stage ores contain massive sphalerite, galena and very minor chalcopyrite, while the second stage is composed of abundant calc-silicates and chalcopyrite with minor sphalerite and galena. Silver was deposited during both stages, although the bulk is associated with the early galena. Stages 3 and 4 contain quartz, calcite and fluorite and are not important ore phases (Grant & Ruiz, 1988).
Replacement bodies are found at the Frisco mine where they occur as disseminations associated with veining, or in small irregular pockets in the most calcareous shales (Escudero, et al., 1990). In the Granadeña mine two large "massive sulphide" bodies are found, one of which has dimensions of 200 x 70 x 200 m, containing 10 to 50% sulphides. The gangue is composed of axinite1, andradite, epidote and chlorite. Relict bedding is observable in these bodies, defined by silicate and sulphide layering. While veins account for 95% of the ore in the district as a whole, in the Granadeña mine, 20 to 30% is in replacement bodies (Grant & Ruiz, 1988). Sulphides in the replacement ores include sphalerite, arsenopyrite, galena, pyrite, marmatite, chalcopyrite and a little bornite. Pyrite is not as conspicuous as in the veins, but is intimately intergrown with arsenopyrite. Chalcopyrite is generally more visible than pyrite. Silver values are low, except where galena levels are locally high (Escudero, et al., 1990).
Wall rock alteration in the San Francisco del Oro district is characterised by silicification, which is more intense at depth than on the surface. In addition, actinolite, epidote and garnet are commonly developed in well silicified calcareous shales in some levels, locally forming banded creamy-grey chert like rock. At the Granadeña mine vein related alteration differs with both the stage of veining and with the host rock composition. Early sulphide rich veins have alteration envelopes composed of epidote, axinite, chlorite, minor andradite and quartz. Late calc-silicate veins have selvages of fine grained manganoan hedenbergite, andradite, axinite, monticellite2 and quartz.
Variations with the Parral Formation also influence the alteration assemblage. Where calcareous siltstones are cut by veins, calc-silicates are formed, while there are fewer calc-silicates where veins cut carbonaceous shales. Alteration does not generally extend far into the wall rock, commonly only forming 1 to 2 m selvages around stage 1 veins, with an inner zone of epidote, chlorite and axinite and an outer zone of fine grained quartz and recrystallised calcite. Wider envelopes are found around stage 2 veins, sometimes extending up to 25 m from the vein. Alteration within 2 to 3 m is pervasive and intense, with the rock texture and bedding being obliterated within 1 m of the vein. The innermost part of this zone consists of alternating 0.5 to 2 cm bands of ilvaite, manganoan hedenbergite, andradite and sulphides. These bands may be parallel to the vein, or occur as irregular, sinuous and lobate forms. Outwards from this inner zone the wall rock is pervasively recrystallised to an assemblage of axinite, hedenbergite, andradite and quartz, with or without monticellite and calcite.
The alteration enclosing the "massive sulphide" replacement orebodies is similar to that enveloping the stage 1 veins. From an inner sphalerite, galena and minor chalcopyrite zone these bodies grade outwards to envelopes rich in axinite, andradite, chlorite, epidote and quartz. The most distal zones of the replacement orebodies are characterised by pervasive silicification of the wall rocks and small sulphide filled veinlets (Grant & Ruiz, 1988).
Santa Bárbara - The ore veins of the Santa Bárbara district occupy fractures which cut both the shales of the Cretaceous Parral Formation and the overlying andesites of the Eocene Escobedo Group, and dip at 50 to 90°. The main veins outcrop and are persistent in both strike and depth. The may be up to 4 km long, vary in thickness from 0.5 to 25 m and continue to depths of 600 m below the surface (Silva & Gonzalez, 1990).
There are three mineralisation stages and assemblages in the district, namely: i). the earliest, which is characterised by galena and sphalerite; ii). the second which is rich in chalcopyrite and sphalerite in a quartz gangue, with some fluorite; and iii). the third stage which comprises fluorite, barite and calcite (Silva & Gonzalez, 1990).
The veins vary in character and orientation with lithology, due to the physical properties of the different rock types and their chemical response to mineralising and alteration processes. These veins are inclined and increase in length with depth, while declining in thickness. Two different vein styles have been classified, namely: i). Simple veins - which are persistent in both trend and depth, with nor branching or splintering; and ii). Complex veins - the most common and economically significant, which exhibit splinters, wedges of enclosed shale, and brecciation, apparently caused by movement on the host fractures. These movements also result in variations in the dip and trend of the veins. The splinters and wedges are commonly found in the hangingwall of the main veins which become wider and contain better ore grade. In several cases the fracture system, including the associated splinters and enclosed horses, reaches 25 m in thickness. In some instances these thickenings correspond to a change in strike of the veins (Silva & Gonzalez, 1990).
Quartz is the dominant gangue mineral, commonly accompanied by alteration silicates, such as garnet, epidote and chlorite. Calcite, fluorite, barite, pyrite and arsenopyrite are also present. Other lesser gangue minerals include diopside, hedenbergite, enstatite and orthoclase. The ore minerals comprise sphalerite, marmatite, galena and chalcopyrite, with traces of native gold. Argentite occurs as minute 2 to 18 µm inclusions in sphalerite, and from 1 to 32 microns in galena. Native silver is found near the surface (Silva & Gonzalez, 1990).
The mineralogy of the veins varies with depth, with four zones being recognised, as follows: i). Leached zone, near the surface; ii). Oxide zone; iii). Zone of secondary enrichment; and the iv). Primary sulphide zone. The latter is the most important economically. In the primary sulphide zone the predominant ore sulphide is brown sphalerite and associated black marmatite. Galena is of secondary importance and occurs with sphalerite, chalcopyrite, silver bearing phases and with tetrahedrite and tennantite. Chalcopyrite is of tertiary importance, occurring either as massive sulphides or as intergrowths with sphalerite. The proportion of massive chalcopyrite increases with depth. Pyrite is found in the vein structures, as metasomatic replacement of the vein walls, and as isolated crystals in rhyolitic dykes. Arsenopyrite, although not rare, occurs in association with pyrite and chalcopyrite. Magnetite and pyrrhotite are found to the south-west of the district (Silva & Gonzalez, 1990).
The paragenetic sequence comprises early silver-lead-zinc mineralisation, occurring as sphalerite, galena and silver forming massive sulphides, accompanied by quartz. This was followed by a second stage comprising copper-silver-gold in a gangue of quartz with pyroxene and garnet. Gold appears to be associated with all of the minerals, although it most commonly accompanies chalcopyrite. The last phase is represented by the calcite-fluorite-barite association (Silva & Gonzalez, 1990).
All of the veins in the district exhibit primary variations and zoning of the sulphide mineralogy. Sphalerite persists in the upper and intermediate levels, but increases with depth, corresponding to a decrease in the galena content. Chalcopyrite also increases with depth. Although the deepest veining is to the north-east, the sulphide zonation, including the pyrite and arsenopyrite, indicate a source from the north or north-west (Silva & Gonzalez, 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.
Grant G J, Ruiz J 1988 - The Pb-Zn-Cu-Ag deposits of the Granadena mine, San Francisco del Oro-Santa Barbara district, Chihuahua, Mexico: in Econ. Geol. v83 pp 1683-1702|
Hoffman D J 1981 - Structural factors in ore position, quality and cut-off grades in fissure veins at Santa Barbara, Chihuahua, Mexico: in Econ. Geol. v76 pp 1921-1928|
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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