Sierra Mojada District
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The Sierra Mojada deposits lie within a historical high grade silver, lead, zinc mining district discovered in 1879, and located ~2 km south of the towns of Sierra Mojada and La Esmeralda and 250 km North of the city of Torreon in Coahuila State, Mexico (#Location - 27° 16' 23"N, 103° 40' 16"W).
Mining at Sierra Mojada commenced in 1879 and continued until 1990. Total production from the district has been estimated at ~9 Mt of Ag-Zn-Pb-Cu ore. The Sierra Mojada mineralised system is known to extend for >6 km east-west and 1.5 km north-south and is divided into elongated parallel northern and southern zones that straddle the Sierra Mojada Fault Zone. The deposit was mined primarily underground and there are ~150 km of underground workings within the district. Sierra Mojada is part of a ~30 km long corridor of mineralisation that follows the Sierra Mojada/San Marco Fault Zone.
The geology of the district comprises a well preserved Cretaceous carbonate platform, representing a marine transgression, deposited over the Jurassic to Cretaceous 'San Marcos' Red Bed conglomerates. The principal zone of sulphide and oxide mineralisation is hosted along the east-west trending Sierra Mojada (San Marcos) fault which lies at the base of the Sierra Mojada Massif and cross cuts the Mesozoic rock package (Silver Bull Resources, 2018).
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.
Sierra Mojada lies within the Sierra Madre Oriental and is located at the northeastern margin of a WNW-ESE elongated tectono-stratigraphic terrane, the Coahuila Block, near the boundary with the Coahuila Fold and Thrust Belt, to the NE. The Triassic to Tertiary San Marcos Fault System, a WNW trending transcurrent fault zone with associated conjugate structures, extends through Sierra Mojada and is the informal boundary between these two terranes. This fault system parallels the regional Late Triassic to Middle Jurassic Mojave-Sonora Megashear that is ~150 km to the SSW. The latter has an apparent sinistral offset of ~800 km. The southern margin of the Coahuila Block is the sinistral Torreón-Monterrey lineament which has an offset of ~400 km. It parallels and lies between the San Marcos Fault System and the Mojave-Sonora Megashear. The San Marcos Fault System also forms the southern margin of the late Jurassic and Cretaceous Sabinas Basin which is mainly to the north of, but also laps onto the Coahuila Block. The northern margin of this basin and southern limit of the Coahuila-Texas Craton, ~250 km to the NNE, is marked by the La Babia (or Rio Bravo) fault, interpreted to constitute a shear couple with the San Marcos Fault (Stockhausen, 2013 and references cited therein).
The basement rocks in the section of the Coahuila Block hosting the Sierra Mojada district are understood to be Late Palaeozoic in age, composed of moderately metamorphosed flysch and unmetamorphosed andesitic volcanic rocks cut by Permian to Triassic granite and granodiorite intrusive units. These intrusives and volcanic rocks are interpreted to represent the roots of an island arc system produced SE of the NE-SW trending late Palaeozoic Ouachita-Marathon orogenic belt which marks the southeastern margin of the Laurentian craton. The older cratonic margin and Palaeozoic arc are normal to the Mesozoic structural trends. The Coahuila block was the source of siliciclastic detritus deposited during the Jurassic and Early Cretaceous in the Sabinas basin following regional deformation along the San Marcos fault system. The Sabinas basin is interpreted to have formed during the Jurassic opening of the Gulf of Mexico and contains over 6000 m of Jurassic to Cretaceous continental red bed, evaporitic and carbonate rocks. A post-rift marine transgression deposited extensive Middle Jurassic to Late Cretaceous clastic to carbonate rocks throughout the region. While the architecture of sedimentary basins in northeastern Mexico is structurally controlled, basin-bounding structures are regarded as being mostly inactive during deposition of the carbonate platform sequence. To the west of the Coahuila block, extensive, magmatism took place from the Late Cretaceous to the Middle Eocene in the Sierra Madre Occidental. These rocks graded from calc-alkaline in the west, to a more alkaline towards the Sierra Madre Oriental.
During the Jurassic through to the early Cretaceous, the San Marcos Fault System recorded three separate pulses of normal movement, down-dip and stepping basinward towards the north. In the Sierra Mojada district, the San Marcos faults strike at 282° and dips at 65°N. The northern most, and most recent step records a 100 m down-drop.
The San Marcos Fault System has also been the structural guide to younger Late Cretaceous to Pleistocene igneous activity along its length, including the Carmago volcanic field and Quatro Cienegas thermal area, 150 km to the SE and 100 km to the NW of Sierra Mojada respectively.
The San Marcos structures are displaced by a series of NE trending structures which are believed to be conjugate structures related to the San Marcos-La Babia shear couple. Throughout northern Mexico, NE structures are associated with mineralisation from depth and at Sierra Mojada these NE structures may be the original conduits for ingress of hydrothermal fluids in the district. They are typically normal and high angle, dipping at 90 to 65°, with down-drop to the SE.
The Late Cretaceous to Eocene Laramide event resulted in the reactivations of Early Mesozoic rift-related basement faults, with Cretaceous strata over the Coahuila Block only subjected to low intensity deformation. One of the reactivated structures was the San Marcos Fault System which was subjected to reverse displacement. The Laramide event also formed the Sierra Madre Oriental fold and thrust belt to the south of the Coahuila block, and the Coahuila Fold and Thrust Belt to the NE of the Coahuila block deforming the main Sabinas Basin sedimentary sequence. The Coahuila Fold and Thrust Belt has abundant isolated, NW-SE trending elongated, tight anticlines separated by broad valley-forming synclines. This structural style is related to the presence of Middle Jurassic evaporites which are exposed in the cores of the tight anticlines. This structural style suggests active salt diapirism during the Laramide orogeny (Stockhausen, 2013 and references cited therein).
This section of Mexico also underwent Basin and Range deformation from as early as 30 Ma with major pulses of extension between 24 and 12 Ma. The youngest structures in the district are related to this extensional event. They are normal high angle faults varying from 0 to 20° strike, with 90 to 55° dips, with down-drop to the east and west, forming a series of horst and graben structures. These structures have offsets of 5 to 25 m, and are important at Sierra Mojada as they are a major inheritor of remobilised supergene and oxide mineralisation and many of the historic workings trace these structures (Stockhausen, 2013 and references cited therein).
The Sabinas Basin sequence in the Sierra Mojada district represents a mostly Early Cretaceous transgression, beginning with subaerial red beds and near shore beach sandstones, followed by carbonate rocks deposited in shoal, lagoonal, shelf and platformal regimes. At Sierra Mojada, Lower to Mid-Cretaceous carbonate rocks are overlain by younger red bed and breccia units. The stratigraphy in the district may be summarised as follows, from oldest to youngest (after Stockhausen, 2012 and references cited therein; and Gryger, 2010):
La Casita Formation - which is extensively developed in the main Sabina Basin, but is absent over the Coahuila Block at Sierra Mojada. It comprises a thick sheet of Late Jurassic red, coarse and arkosic conglomerate and sandstone and extensive deltaic sandstone that grades up into the San Marcos Formation.
San Marcos Formation - which regionally is up to 1000 m thick, with the thickest sections to the north of the San Marcos fault, suggesting this structure was active during deposition and downthrow of the Sabinas Basin to the north of the Coahuila Block. In the Sierra Mojada district, it has a thickness of ~70 m. It consists of Lower Cretaceous alluvial rocks comprising conglomerates containing andesitic volcanic pebbles within a siliceous matrix and several metre thick siltstone interbeds.
La Mula Formation - which unconformably overlies the San Marcos Formation and is believed to represent a change from an alluvial depositional environment to a near shore beach environment. Locally in the Sierra Mojada district, it is known as the Sierra Mojada Sandstone, and crops out within an overturned sequence south of the towns of La Esmeralda and Sierra Mojada. It comprises up to 25 m of fine- to medium-grained, subrounded to rounded, well sorted quartz sandstone.
Cupido Formation - that is the lowest Mesozoic carbonate unit, and was deposited from the Barremian to Aptian throughout much of northeastern Mexico. Locally, in the Sierra Mojada district, it is ~90 m thick, and has a gradational contact with the underlying La Mula Formation. The basal portion of the unit includes medium grey colored skeletal grainstone and wackestone with local mudstones that have a moderate degree of bioturbation. These strata are interpreted to have been deposited in a restricted lagoonal to peritidal environment. The upper sections contain brown-grey packstones and grainstones with some oolitic lenses, suggesting deposition in a high energy shoal environment.
La Peña Formation - conformably overlies the Cupido Formation throughout northern Mexico representing a basin-wide transgression that marked the end of the Aptian. Locally at Sierra Mojada, it comprises a series of coarsening-upward cyclical limestone units that total ~60 m in thickness. The base of each cycle is typically a dark grey to black carbonaceous mudstone, whilst the tops of individual cycles are generally brownish-grey packstone or wackestone with coarser-grained strata, often containing large fossils. The upper section of the La Peña Formation is less fossiliferous and consists of 40 to 60 cm thick beds of light grey packstone and wackestone. The cyclical nature and relative abundance of argillaceous material suggests deposition in a lagoonal environment.
Aurora Formation - conformably overlies the La Peña Formation. It outcrops along the cliffs forming the southern boundary of the Sierra Mojada Valley. Geological mapping and drill sections suggest it has a thickness of ~500 m. In the Sierra Mojada, the Aurora shelf rim is exposed, progressing from platform margin to shelf rim and platform interior facies.
The basal section is the principal host rock to the hypogene sulphide and supergene oxide deposits at Sierra Mojadawhich. It mostly comprises grey to brown micritic mudstone and wackestone with some fine-grained fossil debris, grading upwards into distinctly more fossiliferous, medium grey wackestone and grainstone with discontinuous intervals containing lobate chert nodules and minor mudstone. These lower facies were deposited in an open marine platform to shallow slope environment.
The overlying Upper Aurora Formation, previously the Georgetown Formation, is composed of fossiliferous grainstone and wackestone similar to much of the underlying Aurora Formation. It is regionally a diagenetic dolostone and is locally referred to as the Peñoles Dolostone after the local open pit magnesite mine operated by Peñoles, known as Mina Dolomita. No known metallic mineralisation is associated with this unit apart from the magnesite.
Upper Conglomerate - which unconformably overlies the Cretaceous carbonate rocks and is of Tertiary age (Stockhausen, 2012). However, it has alternatively been interpreted as allochthonous San Marcos Formation red beds that have been thrust over the Cretaceous carbonates (Gryger, 2010) during Laramide contraction. It is composed of angular to subrounded, coarse to boulder sized granite, granodiorite, andesite and dacite clasts, with rare limestone clasts, within a fine grained quartz-rich matrix containing lesser detrital feldspar, hematite and calcite. It typically contains homogenous beds that are metres to tens of metres thick, and was probably formed within an alluvial fan and braided stream environment. The known maximum preserved thickness of the Upper Conglomerate unit in the Sierra Mojada area is 170 m. It is locally altered to a ferruginous breccia. Those that regard this unit to be an allochthonous sheet of San Marcos Formation point out that underground it show a consistent association with low angle faulting. It is the main host rock to high grade supergene silver-copper mineralisation.
Limestone Megabreccia - is the youngest stratigraphic unit observed at Sierra Mojada. It is a clast-supported breccia composed of variably weathered, angular to subrounded, pebble to boulder sized clasts of Aurora and Upper Aurora formation limestone in a matrix of calcite and minor quartz. It exhibits highly variable limestone clast orientations, and is characterised by abundant joints, but does not appear to be cut by faults. Unlike Quaternary alluvium in the district, the Limestone Megabreccia only contains limestone blocks, lacks well-rounded clasts, contains minor to no shaly to silty matrix material, and has a much higher resistance to weathering. It is likely this unit was formed by gravity sliding and landslides from uplifted Aurora and Upper Aurora Formation carbonate rocks to the south of the Sierra Mojada deposit.
Although no associated intrusive system has been found, elevated Cu, Mo, As and Hg in the area are taken to suggest a likely intrusive association. In addition, drilling has encountered felsite sills at depth, interleaved with metamorphosed dolostone, intense massive and stockwork silicification, and disseminated base metal sulphides. This and other direct and circumstantial evidence encountered in drilling at Sierra Mojada suggest intrusive rocks are present and were likely the thermal drivers of basin brine sourced mineralisation into a district wide metal zonation.
The Sierra Mojada district contains multiple occurrences of near-surface mineral concentrations principally hosted by Cretaceous carbonate strata, ranging from:
• Hypogene carbonate replacement polymetallic Fe-Zn-Pb-Cu-Ag sulphides and sulphosalts zones that are locally preserved, and associated with hydrothermal dolomite and silica, the mineralogy of which, with sulphur isotope data, suggest deposition from ~200°C hydrothermal fluids. Mineralisation comprises semimassive to massive sulphides, largely occurring as remnants within the lead zone manto mineralisation. Examples of this mineralisation include a series of east-west trending high angle structures hosting sulphide mineralisation that have been delineated on the western end of the district, below supergene zones. Two distinct lodes of high grade sulphide mineralisation that are 4 to 5 m wide have been tested, each extending for >150 m along strike. The northern of these is dominated by high grade Ag and Cu, up to 1300 g/t Ag and 3.3% Cu respectively, whilst the southern lode also has high grade >20% Zn (Silver Bull Resources release, March 2018).
The original sulphide assemblage comprised pyrite, galena, sphalerite, chalcopyrite, arsenopyrite and tennantite in a gangue of quartz, carbonates, barite, and likely some fluorite with minor celestine. It is estimated that up to 30% of the original mineralisation was gangue minerals. The hypogene sulphide mineralisation appears to have been introduced into reactive dolostone units and karst features in the Aurora and Upper Conglomerate Formations via the San Marcos and NE fault systems.
• Supergene 'non-sulphide zinc' deposits, dominantly composed of smithsonite and hemimorphite associated with local Mn-Fe oxides.
Supergene processes are interpreted to have involved the re-mobilisation and re-constitution of the hypogene sulphide mineralisation into manto-style bodies over a strike length of >3 km. Cyclical leaching over an extended period has apparently mobilised the lead, silver and zinc in the system and re-deposited them into the fracture and cavern (karst) systems developed along the Sierra Mojada fault zone and conjugate structures, part of the regional San Marcos Fault System. The different solubility of the metals has resulted in a segregated zoning of the mineralisation, creating zones that are silver, lead and zinc rich, as follows:
i). Shallow Silver Zone - a near surface silver oxide accumulation ±Zn and Pb, hosted along the Sierra Mojada fault system, comprising a coherent zone of mineralisation that is ~3.3 km long, up to 1 km wide and 100 to 300 m thick. It outcrops on the western end of the district and dips at ~10°E under colluvial cover. Mineralisation is hosted in breccias of the Upper Conglomerate unit, the ferruginous breccia alteration facies, and in reactive dolostone and limestone of the underlying Aurora Formation. Significantly, mineralisation is also controlled by the dense array of structures in the district. Due to these structural and lithologic controls, mineralisation is interpreted to have developed in four configurations:
a). Stratabound mantos, primarily in karst breccias within reactive dolostone units and porosity that resulted from volume reduction during dolomitisation;
b). High-grade (>100g/t Ag) veins, mainly in faults and chimneys related to the mixed structural architecture and intersections of low and high angle faults;
c). Unconformity controlled breccia mineralisation deposited along the contact between the Cretaceous carbonate sequence and the Upper Conglomerate. Mineralisation is interpreted to have been related to the Cretaceous-Tertiary weathering surface, although structures following the unconformity have led to low-angle displacement in many localities and possibly structural sites of deposition. However, as this contact is alternatively interpreted to be a thrust structure (as discussed above), this mineralisation would have been concentrated in a major reverse fault zone;
d). Disseminated replacement mineralisation between the mantos and structures.
ii). Base Metal (Zinc) Zone - which begins with the Lead Zone in the highest stratigraphic position, followed downward by the Red and White Zinc zones. These styles of mineralisation primarily occur as mantos, with each zone containing subordinate amounts of mineralisation related to their neighbouring mantos. All of the mantos dip at ~10°E, controlled by dolostone and subordinate limestone host rocks within the middle Aurora Formation. These mantos were developed from hypogene semimassive to massive sulphide pyrite-sphalerite-galena mineralisation as a result of oxidation and supergene enrichment. These zones may be characterised as follows:
- Silver-rich Pb carbonate mineralisation, little of which is in the current resource, although it sustained mining in the district for the first 20 years until its exhaustion in 1905. The manto, which is stratigraphically above the Red Zinc Zone, was historically known as the 'Snake', 'Manto' and 'Scraggly' beds of the middle Aurora Formation. The Lead zone was mined continuously over a 4 km strike length, 30 m width and up to 6 m thickness. Some 3.5 Mt of ore with an average grade of 15% Pb and 370 g/t Ag were mined, composed of cerrusite-anglesite, chlorargyrite and native silver.
- Red Zinc Zone mineralisation, which is principally stratabound, has a dominantly hemimorphite mineralogy that fills pores in residual and re-sedimented Fe oxides. It follows reactive dolomitic host rocks, and karst fill and dolomitisation breccia, that define units historically known as the 'Santa Getrudia, Hallazgo and North Encantada' horizons of the middle Aurora Formation. The more massive mineralisation is vuggy with zebra replacement textures as well as laminated cave floor deposition and soft-sediment deformation. Remnant pyrite, galena and sphalerite have been noted although the overall level of oxidation is strongly pervasive. This zone occurs as a continuous manto some 2.5 km long, up to 200 m wide and up to 160 m thick, but averaging ~80 m in thickness and ~130 m in width. It dips at ~10°E following the the local stratigraphy, and is located in the footwall of the south dipping Sierra Mojada fault.
Mineralisation comprises massive hemimorphite (Zn4Si2O7(OH)2•H2O), with subordinate amounts of smithsonite (ZnCO3) and minor hydrozincite (Zn5(CO3)2(OH)6). This manto mineralisation includes a strong mixture of Fe oxide with minor Mn oxide imparting the red colouration. Massive red zinc manto mineralisation is fringed by a halo of fault and fracture controlled red zinc, primarily within, but not restricted to the footwall. The lead oxide platternite (PbO2) is common. Mineralisation commonly contains ~20 to 30% Zn and ~55 g/t Ag. Other Red Zinc zones are noted in the district and one, the Yolanda, is currently (2018) being exploited on a small scale by a local mining cooperative.
- White Zinc Zone mineralisation, which underlies the Red Zinc Zone and occurs as a series of mantos, chimneys and filled structures. Mineralisation follows reactive limestone and dolomitic host rocks in karst fill and dolomitisation breccias, in what was historically known as the 'Trinidad' horizon in the lower Aurora Formation. Mineralisation includes classic karst cave-floor accumulations with soft sediment deformation. Mineralisation also has a very strong structural component, occupying multiple steeply dipping faults.
The zone comprises two coherent bodies, each of which is ~100 to 200 m wide and up to 70 m thick. They are separated by the NE trending Campamento fault, which has down-thrown the east relative to the west body. The thickest section of the Red Zinc Zone directly overlies the White Zinc Zone towards the centre of the linear deposit where the total thickness of zinc mineralisation is >200 m.
Mineralisation exhibits local dissolution features, including internal sediments interbanded with, and cemented by, smithsonite. The White Zinc Zone is primarily composed of smithsonite which imparts the white colour, with only very minor overprinting hemimorphite, and is slightly higher in zinc grade than the Red Zinc Zone. There is very little iron oxide and low levels of lead. Massive White Zinc Zone mineralisation grades ~25 to 40% Zn and ~3 g/t Ag.
Similarities in Pb isotopic compositions of smithsonite, hemimorphite and cerussite to Sierra Mojada galena support the interpretation that the non-sulphide zinc deposits originated from the hypogene polymetallic carbonate-replacement sulphide deposits, with flow of metal-bearing groundwater being controlled by local topography and structural features, principally the Sierra Mojada fault in an extensional terrane (Kyle, Ahn and Gilg, 2018).
As discussed above, there are varying views on the age of the Upper Conglomerate, which has implications for the deposition of the Shallow Silver Zone that straddles the contact between the Cretaceous carbonate rocks and the Upper Conglomerate. Gryger (2010) argues that the Upper Conglomerate is an allochthonous plate of Lower Cretaceous San Marcos Formation clastics which overrode the autochthonous Lower to Mid Cretaceous carbonate strata and immediately overlying colluvial unit during Laramide reactivation of the Sierra Mojada Fault as a SW vergent reverse/thrust fault. The upper plate is interpreted to also include overlying facies of the La Mula and Cupido formations. This would suggests regional-scale tectonic transport of this immature fluvial conglomerate and overlying carbonate rocks from a downdip depozone within the Sabinas Basin. Gryger (2010) also argues that kinematic indicators are consistent with a SW-NE axis of maximum compression for Paleogene Laramide shortening throughout the Coahuila Fold and Thrust Belt. The thrust fault bisects the principal ore zone within the Lower Aurora and upper La Peña formations. This relation constrains the minimum age of ore emplacement to the Paleogene and suggests hypogene mineralisation was genetically tied to the late stages of the Laramide Orogeny and implied magmatic activity.
Alternatively, Stockhausen (2012) mounts a detailed argument that the Upper Conglomerate is a distinct unit and not the San Marcos Formation. He shows that the two units have different colours, clast rock types, clast sizes, clast rounding, clast variability, sorting, matrix composition and clast support. This suggests that even if the two units are equivalent, they represent markedly different facies and the autochthonous plate would have been transported over an appreciable distance. However, there is evidence for at least some displacement along the base of the Upper Conglomerate as would be expected at such a rheology contrast. However, as Stockhausen (2012) interprets the Upper Conglomerate to be of Tertiary, mineralisation would still have been no older than Paleogene in age.
The district has been subjected to a complex history of hypogene sulphide mineralisation followed by oxidation and supergene alteration. Hydrothermal alteration follows a sequence of dolomitisation, and carbonate and silica alteration, followed by late carbonate, silica, argillic and iron oxide assemblages related to the supergene-oxidation events. Approximately 80% of the mineralisation of the district is hosted by dolostone, with the remainder in limestone (Dumala and Barry, 2018).
• Diagenetic dolomite which is widespread in northeastern Mexico, and is of importance as the process can increase the porosity of a unit by as much as 15 to 20% due to density difference between limestone an dolomite. East of the Sierra Mojada district, the carbonate section has been subjected to pervasive dolomitisation, apparently following NE-trending faults. The Aurora Formation is also pervasively dolomitised in the western part of the district, in the area of overturning near the Sierra Mojada village. This alteration is interpreted to have been the result of the introduction of relatively low temperature brines from adjacent evaporite-rich basins. It is not known to carry base or precious metal mineralisation but is believed to be part of the host rock preparation stage for later metals mineralisation (Dumala and Barry, 2018).
• Hydrothermal dolomite which occurs as irregular pods of totally altered dolomitised limestone surrounded by zones of partially diaigenetic dolomitised limestone throughout the Sierra Mojada district. These zones may be up to tens of metres thick and occur both along NE trending faults and along the upper contact of the carbonate section with overlying Upper Conglomerate. The Sierra Mojada sulphide bodies are found primarily, but not exclusively, within dolomitised units. Hydrothermal dolomite is interpreted to represent the influx of higher temperature hydrothermal fluids prior to and during hypogene sulphide deposition. At Sierra Mojada, hydrothermal dolomitisation is reflected by a fracture controlled, distinct tan to pink colouration evident throughout the district (Dumala and Barry, 2018).
• Silicification occurs in two separate pulses in the Sierra Mojada district (after Dumala and Barry, 2018);
i). an early pre-sulphide phase which is generally fine grained, affecting carbonate rocks throughout the district, especially those within or adjacent to fault zones which exhibit varying degrees of silicification and jasperoid development. Limestone clasts in tectonic, dolomite and karst breccias are frequently pervasively replaced by very fine-grained, light grey to dark blue, anhedral quartz reflecting this alteration stage.
ii). a late syn- to post-sulphide mineral phase. Early fine-grained silicified limestone is locally cut by later medium- to coarse-grained, subhedral quartz veins that follow faults and the basal contact of the Upper Conglomerate. This quartz is commonly associated with Pb, Ag, Zn, Cu and Fe sulphide and oxide minerals and is spatially associated with zones containing Fe- and Mg-rich replacive carbonate minerals and sulphides or their oxidized products. Typically there is a decrease in silica content outward from structures.
• Sericitisation, which is commonly found in the ferruginous breccia and within the Upper Conglomerate. Abundant sericite is locally found above NE-trending faults, and occurs high in the Upper Conglomerate (Dumala and Barry, 2018).
• Carbonate Alteration also occurs in two phases at Sierra Mojada (after Dumala and Barry, 2018);
i). an early pre-and syn-mineral phase which affects hydrothermal dolomite throughout the district, occurring as a later cross-cutting assemblage of ferroan to magnesian-rich replacement carbonate minerals. These are found along NE-trending faults and at the upper contact of the carbonate section. They comprise an assemblage of ankerite, siderite and magnesite which also locally cuts and replaces diagenetic dolomite and previously undolomitised limestone. These carbonate minerals are fine-grained, with a relatively similar grain size to earlier diagenetic dolomite. They have a pink to red colouration at surface, but are a pale grey colour where unoxidised. They may also may be enriched in Pb and Sr and commonly display abundant very fine-grained dendritic Mn oxide minerals. Fe and Mg-rich carbonates are intergrown with iron and base metal sulphides and barite indicating they were precipitated during the hypogene mineralising event. Red and pink carbonate minerals are also commonly intergrown with iron-oxide and zinc-oxide minerals.
ii). a late supergene phase, occurring as late calcite veinlets throughout the Sierra Mojada district, but most prevalently along the Sierra Mojada fault zone. These calcite veinlets are typically 1 to 20 cm thick and cut carbonate rocks, the ferruginous breccia and the Upper Conglomerate. The calcite in these veinlets is fine-grained, anhedral, and commonly intergrown with Zn, Pb and Fe oxide minerals and acanthite, and may contain barite inclusions. Coarse-grained normal to zincian calcite also locally replaces limestone, silicified limestone, dolostone, and Fe and Mg-rich replacive carbonate rocks, as well as the matrix of the ferruginous breccia adjacent to zones containing late calcite veinlets. Sericitised Upper Conglomerate rocks are crosscut by calcite veinlets indicating it postdated sericitisation. These calcite veinlets and replacive calcite zones are recently formed and are interpreted to be part of the supergene processes.
• Argillic Alteration is found throughout the district at the contact between Cretaceous carbonate rocks and the Upper Conglomerate, occurring as light grey and tan to tan-brown clay-rich zones. These zones are composed of kaolinite, illite and halloysite, accompanied by fine-grained quartz, limonite, hematite and calcite. Tan-brown intervals contain more abundant clay relative to the light grey coloured, fine-grained quartzose variety. Within the ferruginous breccia there is variable interstitial kaolinite and illite with minor halloysite surrounding quartz and carbonate rock clasts (Dumala and Barry, 2018).
• Ferruginous Breccia represents both a distinct alteration facies and a separate lithology, due to its direct association with mineralisation. It may comprise a mixture of Upper Conglomerate, Aurora Formation dolostone and limestone, karst breccia and limonite breccia. It contains clasts of medium to coarse grained, sub-rounded limonite after sulphide which carry elevated concentrations of Ag and Zn. The shape of these clasts suggests they are detrital rather than representing in situ sulphide precipitation. Also, the presence of both sulphide and oxide-rich clasts infer the ferruginous breccia formed after both the hydrothermal event responsible for sulphide precipitation and supergene modification of the sulphide replacement bodies.
The base of the ferruginous breccia is generally very irregular. Breccia fills fractures extending downward as much as 7 m into the carbonate sequence, and may enclose large, angular, cobble-sized limestone and replacive carbonate mineral clasts. In addition, the ferruginous breccia contains silicified carbonate clasts indicating the fine grained silicification event took place prior to karstification, reinforced by the presence of ferruginous breccia beneath fine-grained travertine in karst cavities within the limestone sequence. Consequently, the ferruginous breccia appears to represent both surficial deposits formed by chemical and mechanical weathering of carbonate rocks and karst-fill material.
Ferruginous breccia is frequently overlain by the Upper Conglomerate, and locally lenses of ferruginous breccia are interlayered with lenses of Upper Conglomerate suggesting these units formed synchronously. The ferruginous breccia has not been identified outside of the Sierra Mojada district, and is interpreted to represent surficial oxidation of exposed sulphide replacement bodies in the carbonate sequence as well as infill of karst cavities formed by both normal weathering and acid generated during sulphide oxidation (Dumala and Barry, 2018).
Published Mineral Resources (Silver Bull Resources Inc. website, viewed November, 2018) were:
Zinc Zone - at a 6% Zn cutoff
Measured + Indicated Resource - 10.03 Mt @ 11% Zn, 20.5 g/t Ag, 0.02% Cu, 0.34% Pb;
Inferred Resource - 0.01 Mt @ 13.9% Zn, 0.0 g/t Ag, 0.0% Cu, 0.02% Pb.
Silver Zone at a 50 g/t Ag cutoff
Measured + Indicated Resource - 19 Mt @ 1.7% Zn, 102.5 g/t Ag, 0.09% Cu, 0.41% Pb;
Inferred Resource - 0.10 Mt @ 0.1% Zn, 80.0 g/t Ag, 0.01% Cu, 0.0% Pb.
Combined resource at a lower cut-off as at 31 October, 2018
Measured + Indicated Resource - 70.4 Mt @ 3.4% Zn, 38.6 g/t Ag, 0.04% Cu, 0.3% Pb;
Inferred Resource - 0.1 Mt @ 6.4% Zn, 8.8 g/t Ag, 0.02% Cu, 0.2% Pb.
This summary is based on information from the Silver Bull Resources Inc. website (viewed November, 2018); Kyle, Ahn and Gilg, 2018 (see below); Stockhausen, T., 2012 - Upper Conglomerate and its importance to the Sierra Mojada Ag-Zn deposit system, Coahuila, Mexico; MSc thesis submitted to Colorado School of Mines, 121p.; Dumala, M. and Barry, T., 2018 - Technical report on the resources of the silver-zinc Sierra Mojada Project Coahuila, Mexico; an NI 43-101 Technical Report prepared by Archer Cathro and Associates Ltd. for Silver Bull Resources Inc., 190p.; and Gryger, M.S., 2010 - Geologic Framework of the Sierra Mojada Mining District, Coahuila, Mexico: An Integrative Study of a Mesozoic Platform-Basin Margin; MSc thesis submitted to the University of Texas, Austin, 399p.
The most recent source geological information used to prepare this summary was dated: 2018.
This description is a summary from published sources, the chief of which are listed below.
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Sierra Mojada District
Kyle, J.R., Ahn, H. and Gilg, H.A., 2018 - Nature and origin of the nonsulfide zinc deposits in the Sierra Mojada District, Coahuila, Mexico: constraints from regional geology, petrography, and isotope analyses: in Mineralium Deposita v.53, pp. 1095-1116.|
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