Guanacevi - Santa Cruz, El Curso Sur, Milache, Santa Cruz Sur and Porvenir Norte

Durango, Mexico

Main commodities: Ag Au
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The Guanaceví low-sulphidation epithermal, quartz carbonate, fracture-fill vein silver deposits are located in northwestern Durango in Mexico, ~3.6 km west of the town of Guanaceví, 260 km NW of the capital city of Durango, and ~980 km NW of Mexico City (#Location: 25° 55' 1"N, 105° 59' 15"W).

Mining in the Guanaceví district extends back to at least 1535 when the mines were first worked by the Spanish. However by the mid-seventeenth century silver production collapsed when mercury, necessary to the refining process, was diverted to the silver mines of PotosĂ­ in present day Bolivia. By the start of the 18th century, Guanaceví had again become an important mining center in the Nueva Vizcaya province. The peasant uprisings of 1810 to 1821 were disastrous to the Mexican mining industry with both the insurgents’ soldiers and royalist troops all but destroying the mining production in Mexico, including the Guanaceví district. The vast majority of production came prior to the 1910 Mexican Revolution with the GuanacevĂ­ mining district being known for its high silver grades, as well as being one of the top five silver mining districts in Mexico. Production has been sporadic since the 1910 Revolution. During the 1920s, the Mexican company Peñoles purchased several mines including the Santa Cruz mine in the Guanaceví district, and undertook mining operations through the 1930s until production ceased in 1942. Subsequently, attempts were made to recommence operations with little success. A cyanide leach plant was completed by a private company in 1992 and processed the old tailings from the flotation plant until around 2003. The titles were acquired by Endeavour Silver in 2004. Shortly after the acquisition closed, management discovered the first high-grade orebody and production commenced in 2005 and continues to the present (2022) with annual production between 2005 and 2020 that varied from 102 000 to 435 000 tonnes of ore treated per annum at grades that varied from 230 to 450 g/t Ag. Historic official production records indicate that ~15 000 tonnes of silver and silver equivalents had been mined from the district prior to 2003, making it one of the top five silver mining districts in Mexico (Endeavour Silver Corp., 2022).

The Guanaceví district lies on the eastern flanks of the Sierra Madre Occidental physiographic province where it transitions to the Mesa Central, or 'Altiplano', at an elevation of ~2700 m asl. The regional geology is dominated by volcanic piles that form a large rhyolitic ignimbrite and tuff complex. It lies within what is known as the "Silver Trend or Belt", a broad NW-SE zone up to 200 km wide that includes many other significant silver deposits such as Frenillo which is ~475 km to the SE.

For a brief overview of the distribution and character of the deposits in carbonate replacement and related epithermal and other vein Ag and Pb-Zn-Ag of the "Silver Trend" in Mexico and the western United States, and links to other deposits of the belt, see the Regional Setting section of the Fresnillo record.

The principal stratigraphic units in the Guanaceví district are as follows, from the base:
Guanaceví Formation - the oldest unit in the district, an Upper Jurassic or Lower Cretaceous, polymictic basal conglomerate composed of angular to sub-angular fragments of quartz and metamorphic rocks set in a sandy to clayey matrix within sericitic and siliceous cement. The unit is at least 450 m thickness, although the lower contact has not been observed. In most areas, the upper contact is structural, occurring as high-angle normal faults, but locally in the San Pedro area (3 km to the north), there is an abrupt upper contact with fairly fresh, dark colored andesitic flows of the Lower Volcanic Sequence that appear conformable to the underlying Guanaceví Formation. The Guanaceví Formation is structurally exposed as a horst, occupying the central portion of the northwest trending Guanaceví erosional window, flanked by sets of NW striking normal faults that offset the overlying Upper and Lower Volcanic Sequences down to the SW and NE on corresponding sides of the window. Mineralisation within the horst is hosted by the conglomerate, both as dilatational high-angle fracture-filled structures and, in the San Pedro area, as manto-like replacement bodies below the upper contact of the conglomerate with overlying andesitic units of the Lower Volcanic Sequence.
Lower Volcanic Sequence - composed of andesitic rocks and associated sedimentary units divided into a loosely defined package of flows and volcaniclastic sediments correlated with Eocene volcanism that occurs throughout the Sierra Madre of Mexico. In the Porvenir and Santa Cruz mine workings the andesite occurs as a pale green to nearly black volcanic flow ranging from aphyric to plagioclase-hornblende phyric. Plagioclase is the common phenocryst type with crystals ranging from 1 to 2, up to 10 mm. Hornblende phenocrysts are 1 to 4 mm across. In porphyritic andesites, feldspar phenocryst abundance approaches 5%, and hornblende abundance is generally less than 3%. The Lower Volcanic Sequence appears to be a coarsening-upward series of volcaniclastic sediments capped by an andesite flow. The sedimentary lithologies are siltstones overlain by sandstone with minor intercalations of conformable conglomerate beds. The siltstone-sandstone sequence becomes dominated by conglomeratic beds at the top of the volcaniclastic package. The cumulative thickness of the siltstone-sandstone beds is up to 120 m, whilst conglomerate beds vary from a few centimetres to 150 m thick at the top of the package and differ from the conglomerates of the Guanaceví Formation in that the clasts are mainly andesite of varying textural types.
Upper Volcanic Sequence - rhyolite crystal-lapilli tuff units inferred to be of Oligocene age that unconformably overlie the andesites, and are generally structurally disrupted and altered by oxidation and silicification. The rhyolite is strongly argillically altered with silicification overprinting argillic alteration in the immediate hanging wall of quartz veins and other silicified structures. The rhyolite commonly contains rounded quartz 'eyes' up to 4 mm in diameter, whilst the matrix consists of adularia, kaolinite and quartz. Local concentrations of biotite crystals up to 2 mm across are common. The rhyolite has variable textures from thin-bedded ash flows to coarse lapilli tuffs with lithic clasts of andesite or rhyolite up to 50 cm in diameter. The latter commonly exhibit alteration rims indicating high temperatures and fluids in the volcanic environment. The thickness of the rhyolite tuff assemblage has not been measured, but appears to exceed 300 m. Geochemical signatures suggest the lower portion of the rhyolites are magmatically linked to the underlying andesites. The similarity of REE patterns of the rhyolite crystal-lapilli tuff and the andesitic rock units suggests a common source for the two volcanic packages, although there is some doubt as to the actual ages of the Upper and Lower volcanic units.

The Guanaceví erosional window is interpreted to be the result of crustal uplift, apparently centred about 3 km west of Guanaceví. With some exceptions, fracture-filling vein mineralisation is localised on the flanks of the uplift centre, suggesting a relationship between uplift and mineralisation. Three principal trends of high-angle normal faults characterise the region, are as follows: i). NW faults dominant structural trend of the district, with a significant NNE trending, likely conjugate, set. This generation of structures hosts most of the mineralisation in the district and are taken to indicate an early NE-SW extensional event; ii). NE faults which postdate the mineralised structures and are interpreted to represent NW-SE directed extension; and iii). East-west faults, which are the youngest and imply extension rotated to north-south. Uplift has been interpreted to have occurred at the onset of Upper Volcanic Sequence eruptions in the Oligocene, accompanied by NE-SW extension, and was coeval with mineralisation.

The Santa Cruz vein, the principal host to silver and gold mineralisation in the Guanaceví operation, occurs on the west side of the district wide horst cored by the Guanaceví Formation. It is part of a major fault system that trends NW-SE at ~135° with dips of 45 to 70°SW and juxtaposes the Guanaceví Formation in the footwall to the NE, against andesite and/or rhyolite in the hanging wall to the SW. From the Santa Cruz Sur to Milache sections, it extends over a length of 6.5 km and averages ~3 m in width. Mineralisation however, is not continuous but occurs in steeply NW-raking shoots with streike lengths of up to 200 m. Broader and higher-grade mineralised ore shoots tend to occur along flexures in the Santa Cruz vein structure, where sigmoidal loops are developed, both along strike and down dip. The vein in the Deep Santa Cruz section, for instance, splays into two, three or four separate mineralised strands with the intervening wall rocks also often well mineralised, producing mining widths locally of up to 20 m. These sigmoidal loops tend to develop with some regularity along strike and all of the ore shoots at the Santa Cruz mine have a plunge of ~60°NW. A shallow NW plunging striation, raking at 15 to 30° occurs on a number of fault planes within the Santa Cruz structure, consistent with an observed sinistral translation seen on minor faults that produce small offsets of the Santa Cruz vein. Around the periphery of ore zones in particular, the vein develops imbricate structures, either as imbricate lenses shallowly oblique to the principal Santa Cruz trend or as vein segments offset by similarly trending minor faults. The trend of these imbrications is generally slightly more westerly than the main Santa Cruz vein/fault trend and more steeply dipping. Veining is also often influenced by cross-cutting north-south structures, which rarely appear to offset the main fault, but produce minor jogs. These north-south structures are often associated with the development of manganese oxide concentrations and elevated silver grades.

A second, but less continuous vein, sub-parallel to the Santa Cruz vein, is economically significant in the Porvenir Dos section (1.8 km NW of the Santa Cruz Mine), the northern portion of deep North Porvenir, in the Milache section on the northwestern extremity of the vein system, and finally in the Santa Cruz Sur section on the southeastern extremity of the system. It is referred to in these areas as the 'Footwall vein'.

The sedimentary and volcanic rocks are hydrothermally altered, with propylitic chlorite alteration the most widespread, evident for up to 150 m from the veins. Narrower bands of potassic and argillic alteration, predominantly composed of kaolinite and adularia, are typically up to 25 m thick in the hanging wall, with silicification adjacent to the veins. Phyllic alteration is absent in the district.

The Santa Cruz vein is silver-rich, with lesser gold, lead and zinc. Mineralisation has averaged 500 g/t Ag and 1 g/t Au over a 3 m true width. The principal minerals are argentite-acanthite, limited gold, galena, sphalerite, pyrite and manganese oxides. Gangue minerals include barite, rhodonite, rhodochrosite, calcite, fluorite and quartz. The mineralisation from surface to Level 6 in the Santa Cruz mine is mainly oxidised, with a transition zone of oxides to sulphides occurring between Levels 6 to 8, although some sulphide ore has survived to be above Level 6. Mineralisation was emplaced by a series of episodic hydrothermal events that generated finely banded textures. Higher-grade mineralisation in the district is commonly associated with multiple phases of banding and brecciation. The first phase, deposited white quartz, white calcite and pyrite as stockwork veining, often with horse-tail structural bifurcation both in the horizontal and vertical planes to form imbricate pods. The second phase produced semi translucent quartz with argentite, scarce gold and oxides of manganese (2%) and rare lead and zinc sulphide (4%), the latter occurring particularly in the lower part of the hydrothermal system. The second phase was accompanied by barite, rhodonite, rhodochrosite, fluorite and calcite. This second phase comprises multiple pulses of mineralisation reflected by bands of massive, banded or brecciated quartz. Massive and massive to banded quartz are commonly associated with carbonate which is predominantly manganoan calcite and calcitic rhodochrosite. Rhodonite is much less abundant than carbonates but is not uncommon.

Remaining Ore Reserves and Mineral Resources at 31 December, 2020, were as follows (Endeavour Silver Corp. 2022):
Proved + Probable Ore Reserves - 1.089 Mt @ 343 g/t Ag, 0.93 g/t Au for 373 t of contained silver; 1.01 t of contained gold.
  cut-off grades for the different sections vary from 237 to 280 g/t Ag equiv.
Measured + Indicated Mineral Resources - 0.661 Mt @ 369 g/t Ag, 0.83 g/t Au for 343 t of contained silver; 0.55 t of contained gold.
Inferred Mineral Resources - 1.527 Mt @ 440 g/t Ag, 1.03 g/t Au for 672 t of contained silver; 1.66 t of contained gold.
NOTE: Ore Reserves are exclusive of and additional to Mineral Resources. These estimates are contained within the El Curso Sur, Milache, Santa Cruz, Santa Cruz Sur and Porvenir Norte sections of the Santa Cruz vein system.
TOTAL contained silver = 1282 tonnes.

Metallurgical recoveries were 84.6% silver and 85.7% gold.

The information in this summary is drawn from "Endeavour Silver Corp., 2022 - Updated Mineral Resource and Reserve Estimates for the Guanaceví Project, Durango State, Mexico; an NI 43-101 Technical Report prepared for Endeavour Silver Corp., 146p"

The most recent source geological information used to prepare this summary was dated: 2021.    
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd.   Unauthorised copying, reproduction, storage or dissemination prohibited.

  References & Additional Information

Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge.   It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published.   While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants:   i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and   ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.

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