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Guanajuato District, Bolanitos Operation - Bolanitos, Lucero, Asuncion, San Miguel, Cebada, Golondrinas
Guanajuato, Mexico
Main commodities: Ag Au

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The Guanajuato mining district is located in the state of Guanajuato, some 400 km NW of Mexico City, Mexico.

Mining has been carried on in the district continuously for over 400 years to yield to 1990 a total of 34 850 tonnes of Ag and 175 tonnes of Au. Production in 1990 was 35 000 tonnes of ore per month @ 150 g/t Ag, 1.5 g/t Au.

Three extensive NW-SE trending mineralised fault systems are known in the district, cutting the 850 m thick Tertiary volcanics and clastics of the Sierra Madre Occidental, up to but not including the Oligocene member.   These volcanics overlie a Mesozoic sequence of black mudstones with local micritic limestone bands.

The Guanajuato district is underlain by a volcano-sedimentary sequence of Mesozoic to Cenozoic age rocks (see the stratigraphy in the Bolañitos description below) that are cut by three main NW-SE trending vein systems, which are, from west to east, the La Luz, Veta Madre and La Sierra systems. These systems are known over a strike length of 10 to 25 km and are generally silver-rich with silver to gold ratios from 72:1 to 214:1. The most extensive of these, Veta Madre, persists over a strike length of 25 km and widths of up to 90 m, with associated mineralised breccias. It dips at 35 to 55°SW with measured displacement of around 1200 and 1700 m near the Las Torres and La Valenciana mines respectively. Veins are made up of bands of amethyst, chalcedony and clear quartz, interlayered with base and precious metal sulphides and sulphosalts, with ore occurring as a series of shoots within the vein.   Breccias are developed in flexures in the vein systems and consist of silicified fragments of all host lithologies, quartz and vein ore in a quartz and chalcedony matrix. Ore minerals are predominantly silver sulphides and sulphosalts, electrum, galena, sphalerite, chalcopyrite and pyrite.

The district lies within what is known as the "Mexican 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 ~350 km to the NW. 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.

Pre-mineralisation deformation during the ~80 to 40 Ma Laramide orogeny in the broader "Silver Trend" produced WNW trending pre-mineral folds and thrusts in the Jurassic Esperanza Formation (see below) as observed in the Cebada mine on the Veta Madre vein system in the Guanajuato district. Early post-Laramide extension at ~30 Ma was oriented north-south to NNE, and controlled many vein deposits in the trend (e.g., Fresnillo, Zacatecas, La Guitarra). Guanajuato is interpreted to lie on a NNW trending terrane boundary that was reactivated as a sinistral transtensional fault zone that was exploited by early-stage intermediate sulphidation mineralisation. Subsequent regional extension at ~28 Ma that progressed from the ENE to WNW and produced basin and range type deformation and block faulting, and is associated with a second phase of mineralisation in the Guanajuato district. Ore shoots along the Veta Madre vein system were controlled by anti-clockwise jogs along the main structure and at intersections with WNW and NE fault zones during early-stage mineralisation, tending to produce relatively steep ore shoots plunging to the south along the Veta Madre fault system.

During the second phase of mineralisation, listric block faulting and tilting displaced sections of the Veta Madre vein to produce new vein systems such as La Luz, which appear to have formed as extensional arrays between reactivated WNW fault zones that acted as dextral transtensional structures. These second phase vein systems tend to form sub-horizontal ore zones, either reflecting fluid mixing zones, or structural controls due to changes in dip of the fault surface. This overprinting of two events results in some deposits having ore shoots with more than one orientation, with vertical gaps in ore grade.

The distribution of a peripheral three-quarter ring of rhyolite domes intruding bounding faults, the location of the Oligocene volcanic formations ponded within this ring, mega-breccia and a topographic rim, are taken together to support the presence of a caldera in the Guanajuato district (Randall et al., 1994). Following caldera formation, normal faulting, combined with hydrothermal activity at ~27 Ma (Buchanan, 1980), resulted in many of the silver-gold deposits found in the district. The largest of the three major mineralised fault systems within the Guanajuato Veta Madre follows a NNW trending fault system, whilst the other two systems are subparallel to it. Mineralisation occurs within these systems principally on normal faults oriented parallel to the main trend.

Bolañitos - (#Location: 21° 4' 15"N, 101° 19' 56"W).

One of the more significant current operations in the district is at Bolañitos that comprises four operating silver-gold mines, namely Bolañitos, Lucero, Asuncion and San Miguel, and two inactive mines (as of 2022) at Cebada and Golondrinas. Recent exploration and mining relavent to the Bolañitos Project deposits began in 1968 when the Mexican company Fresnillo acquired title over ground including the old Negociación Minera Santa Lucía mine, which became the Cebada mine. Following exploration work, production began at the Cebada mine, and between 1976 and 1995 it produced 1.277 Mt @ 4.04 g/t Au and 372 g/t Ag. In 2003, operations at the pre-existing Torres, Sirena, Peregrina and Apolo mines ceased, whilst the Bolañitos, Golondrinas, Asunción and Cebada mines stayed in production on a break-even basis. In 2007, the Canadian company Endeavour Silver Corp. consolidated holdings in the district as the Bolañitos Project, which included the Cebada, Bolañitos, Golondrinas and Asunción mines, as well as a number of closed mines, purchased from Peñoles and Minas de la Luz, the Mexican owner and operator at the time respectively. Endeavour continued low level production until discovering the first new orebody at Lucero in 2008. The mines were developed using mechanised, underground and ramp access, while the plant capacity was expanded by more than 300% to 1600 tpd capacity. Endeavour subsequently discovered ten further new orebodies, eight of which have been mined.

The Bolañitos operation is located in the west central portion of the state of Guanajuato, near the town of La Luz, ~12 km to the NW of the state capital, Guanajuato. It lies among a series of low mountains which are part of the Sierra Madre Occidental, near it northeastern magin with the Mesa Central, or 'Altiplano'. It is also situated along the southern and eastern flanks of the Sierra Madre Occidental geological province, a north-northwesterly trending Tertiary linear volcanic belt that is ~1200 km long and 200 to 300 km wide. That belt is composed of basaltic to rhyolitic flows and tuffs with related intrusive bodies. Volcanic activity responsible for the bulk of the upper sections of this volcanic group ended by the late Oligocene, although there was some eruptive activity as recently as 23 Ma in the early Miocene. The volcanism was initially associated with eastward subduction of the Farallon Plate, producing lava and tuff thicknesses on the order of 1 km. Subsequent extension related to the opening of the Gulf of California evolved into Basin and Range block faulting, uplift, erosion and the present-day geomorphology of the belt. Strata within the belt occupy a broad antiform, longitudinally transected by regional scale faults.

The Bolañitos mines are located in the eastern part of the Guanajuato mining district, in the southeastern portion of the Sierra de Guanajuato range, which is a 100 x 20 km anticlinal structure. Bolañitos is centred on the regional La Luz vein system on the northeast side of this structure, where typical primary bedding dips at 10 to 20°NNE. Economic mineralisation within the mine area is known to extend over as much as 250 m vertically from altitudes of 2300 to 2050 m, with the exception of the main La Luz vein that extends 400 m vertically from 2300 to 1900 m elevation.

The stratigraphic sequence within the district is as follows, from the base:
JURASSIC - marine sedimentary rocks, weakly to moderately metamorphosed and intensely deformed by shortening, specifically the:
Esperanza Formation - >600 m of carbonaceous and calcareous shale interbedded with arenite, limestone and andesitic to basaltic lava flows, all weakly metamorphosed to phyllites, slates and marble.
La Luz Formation, which unconformably overlies the Esperanza Formation and is >1000 m in thickness. It is predominantly composed of interbedded clastic sedimentary rocks and massive to pillow tholeiitic basalts dated at 108.4 ±2 Ma, with local rhyolite tuffs and agglomerates. Some volcanogenic massive sulphide occurrences are known in this unit. The La Palma diorite and La Pelon tonalite, which form the upper part of the Guanajuato arc are intruded into the unit. Pervasive propylitic alteration is common.
CENOZOIC - which unconformably overlies the Mesozoic basement and comprises continental sedimentary rocks, which generally occupy topographic lows> They include subaerial volcanic rocks, which are principally exposed in the ranges and higher plateaux. The Cenozoic cover has only undergone extensional deformation and in some places are gently tilted. They correspond to a period of tectonism accompanied by volcanism and intrusive magmatic activity.
Guanajuato Formation - 1500 to 2000 m of Eocene to Oligocene, conglomerate consists of pebbles to boulders of quartz, limestone, granite and andesite belonging to older rock units, all cemented by a clay matrix, with some interlayers of sandstone. Beds of volcanic arenites and andesitic lavas occur at the base of the conglomerate.
Loseros Formation - an Oligocene volcanic sequence, interpreted to have been deposited within, and adjacent to a caldera. The tuffs of the formation are a well-bedded, green to cream-red volcanic arenite that vary from 10 to 52 m in thickness, interpreted to be a surge deposit at the base of the Cubo caldera.
Bufa Formation - ~360 m of sanidine-bearing rhyolite-ignimbrite with biotite as a mafic phase, and is often massive, but ia also locally bedded. Moderate welding and extensive and pervasive silicification make it resistive, forming prominent cliffs east of the city of Guanajuato. It occasionally contains large lithic clasts of a range of lithologies, many of pre-volcanic basement. At Bolañitos, it is divided into three units: a lower breccia, overlain by dense, red rhyolite porphyry, that is, in turn, followed by a massive to bedded ignimbrite. The resistant rhyolite has been dated at 7 ±3 Ma (K-Ar) in the middle Oligocene.
Calderones Formation - a wide variety of volcanic rocks, including low- to medium-grade ignimbrites, deposits of pyroclastic flows, pyroclastic surge layers related to phreatomagmatic activity, airfall ash-rich tuffs, minor Plinian pumice layers, lahars, debris flows, reworked tuffaceous layers deposited in water, tuff-breccias and mega-breccias. The unit has a characteristic green to greenish blue colour due to ubiquitous chlorite alteration, whilst veins and dykes commonly have a propylitic selvage. At Bolañitos, it is >300 m thick, and has a basal contact marked by a megabreccia composed of large clasts of the Esperanza, La Luz and Guanajuato Formations that are often 5 to 10 m across, and it is interpreted to be the upper caldera-filling facies above the surge deposit and the Bufa ignimbrites.
Cedros Formation - 100 to 640 m of grey to black andesitic lava flows with interlayered red beds and andesitic to dacitic tuffs. This unit overlies the Cedros Formation, is widespread and is entirely post-caldera.
Chichíndaro Formation - a 100 to 250 m sequence of rhyolite is a sequence of domes and lava flows interbedded with poorly sorted volcanic breccias and tuffs. The domes and flows have a characteristic fluidal porphyritic texture, with the rhyolites dated at 32 ±1, 30.8 ±0.8 and 30.1 ±0.8 Ma (Gross, 1975; Nieto- Samaniego et al., 1996). It is closely related to the hypabyssal Peregrina intrusion. Locally the rhyolite domes contain disseminated tin and vapor-phase cavity-filling topaz distributed along the flow foliation. It is the youngest volcanic unit in the district.
Comanja Granite - of Eocene age, dated at 53 ±3 and 51±1 Ma (K-Ar; Zimmermann et al., 1990), occurring as a batholith, apparently emplaced along the axis of the Sierra de Guanajuato. The Bufa formation is the youngest unit cut by this granite.
El Capulin Formation - unconsolidated tuffaceous sandstone and conglomerate overlain by vesicular basalt, all of Quaternary age.

Only the Esperanza and La Luz Formations are found in the Bolañitos mine area, with mineralisation primarily hosted within the La Luz Formation, and dissipating at the contact with the Esperanza Formation. The most productive veins at Bolañitos strike parallel to the NW-SE striking, 35 to 55°SW dipping Veta Madre system. Mineralisation is directly related to faulting and occurs as open-space fillings in fracture zones or impregnations in locally porous wall rock. It favours dilatant zones in areas where fractures branch or change orientation, which may, in turn, be driven by wall rock competency and/or relative hardness of individual strata. Veins that formed in relatively open spaces are the most prospective. There are 21 veins within the Bolañitos mine area that are included in the mineral resource estimate (Endeavour Silver Corp. 2020). These mineralised veins are known to occur from an elevation of 2300 m down to an elevation of 1900 m.

Hydrothermal alteration within the district is closely related to, and occurs as halos surrounding, fractures, veins and brecciated zones. These halos range from a few centimetres to metres in width and can be divided into: i). propyllitic, ii). argillic, iii). phyllic, and iv). silicification from diatal to proximal. Broader wall rock alteration at Bolañitos in general is not significant at the main levels of mineralisation, although breccia zones within and near the primary structures do show the typical characteristics of low sulphidation epithermal vein type alteration. Alteration encountered within the structures occurs as halos of phyllic and silicic alteration, whilst argillic and propyllitic alteration are seen at higher levels above the mineralised interval at elevations of >2300 m above sea level. Propyllitic alteration is the most extensive and the strongest near fractures, especially at the intersections of veins. It consists of epidote, chlorite, clays and calcite. Phyllic alteration is not as pervasive, and is generally encountered within, or in immediate contact with, the vein, and typically comprises an assemblage of pyrite, illite and sericite with occasional kaolinite and montmorillonite. Argillic alteration consists of kaolinite, montmorillonite and halloysite. Silicification is restricted to veins and breccia zones, and typically extends only a few centimetres into the wall rock.

Mineralised veins at Bolañitos are banded and brecciated, with silver primarily occurring in dark sulphide-rich bands, while little mineralisation is found within the wall rocks. The major metallic minerals include pyrite, argentite, electrum and ruby silver, as well as some galena and sphalerite, generally deeper in the veins. Mineralisation is usually associated with phyllic alteration and silicification which forms haloes around the mineralised structures. The vein textures are attributed to brittle fracturing-healing cycles during and/or after faulting. Economic concentrations of precious metals are present in 'shoots' distributed vertically and laterally, separated non-mineralised segments of the veins. Overall, the style of mineralisation is pinch-and-swell with some flexures resulting in pinch-outs and others generating wide sigmoidal breccia zones. The deposits are classified as a low sulphidation systems with pyrite but no arsenopyrite. The silver-rich veins contain quartz, adularia, pyrite, acanthite, naumannite and native gold. Native silver is widespread in small amounts, although much of it is assumed to be supergene. Silver sulphosalts (pyrargyrite and polybasite) are commonly found at depth.

Remaining Ore Reserves and Mineral Resources at 31 December, 2020, were as follows (Endeavour Silver Corp. 2022):
Proved + Probable Ore Reserves - 312.8 Mt @ 49 g/t Ag, 2.53 g/t Au for 15 t of contained silver; 0.8 t of contained gold.
  cut-off grades for the different sections vary from 173 to 181 g/t Ag equiv.
Measured + Indicated Mineral Resources - 0.468 Mt @ 159 g/t Ag, 2.27 g/t Au for 74 t of contained silver; 2.7 t of contained gold.
Inferred Mineral Resources - 0.625 Mt @ 120 g/t Ag, 2.52 g/t Au for 75 t of contained silver; 1.6 t of contained gold.
NOTE: Ore Reserves are exclusive of and additional to Mineral Resources. These estimates are contained within the Bolañitos, Lucero, Asuncion and San Miguel mines.
TOTAL contained silver = 164 tonnes; gold = 3.5 t.

Metallurgical recoveries were 84.7% silver and 88.9% gold.

The information in this summary is drawn from "Endeavour Silver Corp., 2022 - Updated Mineral Resource and Reserve Estimates for the Bolañitos Project, Guanajuato State, Mexico; an NI 43-101 Technical Report prepared for Endeavour Silver Corp., 170p."

The most recent source geological information used to prepare this decription was dated: 2021.     Record last updated: 15/5/2022
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
   Selected References:
Mango H, Zantop H, Oreskes N  1991 - A fluid inclusion and isotope study of the Rayas Ag-Au-Cu-Pb-Zn mine, Guanajuato, Mexico: in    Econ. Geol.   v86 pp 1554-1561

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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