Summary:
The La Guitarra Property contains more than one hundred epithermal veins that are hosted by tuffs, breccias, granite, and metasedimentary rocks of the Guerrero Terrane. The veins trend northwest to east-west and are described as intermediate sulphidation epithermal veins containing silver, gold, and some lead and zinc. Individual veins pinch and swell and vary in width from tens of centimetres to more than twenty metres, whereas ore shoots contained within veins have widths usually between one to four metres. Intersection of northwest to east-west veins with northeast and north-south faults and fractures have been suggested as main controls for ore shoot localization.
Deposit Types
The vein deposits at Guitarra have physical, chemical, and mineralogical characteristics of a low to intermediate sulphidation epithermal type deposit. These characteristics fit the precious metal vein model proposed by Buchanan (1981).
Epithermal deposits form at shallow depths in volcanic-hydrothermal and geothermal environments. The genesis of these deposits is complex due to the involvement of fluids with various origins. Camprubí et al. (2006) propose that magmatic, crustal meteoric and surficial meteoric fluids were all involved in the formation of epithermal veins at Guitarra. This was based on gas chemistry data from a fluid inclusion study using oxygen and hydrogen stable isotope data.
Property Mineralization
Mineralization at the project is classified as a low-to-intermediate sulphidation epithermal system. Hundreds of veins are present in a belt 4 km wide and 15 km long. An erosional window through younger basalts forms this northwest-southeast striking belt. Vein widths of economically interesting silver and gold mineralization vary from less than 1 metre to +20 metres in width.
Gangue mineralogy consists of banded quartz, amethyst quartz, coliform chalcedony, finegrained crystalline quartz, calcite, fluorite, pyrite, marcasite, barite, anhydrite, illite–smectite, adularia, and alunite. Primary ore mineralogy consists of proustite–pyrargyrite, electrum, acanthite, polybasite, sphalerite, galena, and chalcopyrite. Oxidized copper and lead minerals are present in small quantities in the near-surface environment.
The Guitarra mine veins can be grouped into three mineralization stages: Stage I, is a basemetal rich event, while Stages 2 and 3 deposited most of the precious-metal assemblages (Camprubí et al., 2006). Stage II is the most important in terms of economically interesting silver and gold deposition.
The main textures observed in the veins are coarse to fine banding, colloform, bladed quartz, and breccia textures. Fine dark bands containing sulphides and bladed quartz textures after calcite have been observed to correlate with higher silver and gold concentrations. Banding and bladed textures are commonly associated with boiling and the deposition of precious metals in an epithermal environment. The breccias usually contain angular quartz clasts that range in size from a few millimetres to tens of centimetres and are supported by a silicified matrix or cemented by quartz and ±marcasite.
West District Vein Systems
The West District veins are hosted in the Miocene volcanics, the Eocene intrusives, and the metasediments. Surface mapping to date has delineated over 15 km of mineralized veins and breccias. Production from 1992 to 2013 was derived primarily from northeast striking veins at the Guitarra mine and from 2014 to 2018 at the Coloso mine.
Comales Nazareno System
The Comales Nazareno system is located in the northwest portion of the property. Its veins are part of the 290-320° vein set, and they outcrop for approximately 3.7 km. The system contains the Comales, Nazareno, Nazareno del Alto, and other vein splays. Host rocks are the Miocene volcanics, possibly extending into the metasediments at depth.
Coloso System
The Coloso vein system is located east of the Comales-Nazareno system and is part of the 290- 320° vein set. The length of the system is over 2 km based on mapping and drilling, however, in all likely hood it is a continuation of the Guitarra mine trend to the northwest. The known vertical extent of mineralization is over 400 metres and is open at depth. The principal veins are Jessica, which dips to the southwest and Joya Larga, which dips to the northeast. Vein mineralization is hosted in the Miocene volcanics, with intercepts in the metasediments being reported in deeper drilling. Several vein splays have been recognized in the system including the Jessica footwall, Jessica hanging wall and Joya Larga hanging wall. Velador et al. report the vein splays are narrower than the main veins and average 1 metre in width. There is a possibility for additional splays and mineralization remains open at depth. The projected intersection of the Jessica-Joya Larga veins is an attractive target.
Guitarra Mine System
The Guitarra mine vein system consists of ten veins with strikes ranging from 290-314°. These veins dip at angles between 72° and 82° to the southwest. The system has been explored over a length of 3.5 km. The known vertical extent of mineralization from surface to the deepest diamond drill-hole intersection is 700 metres. Underground mapping shows the presence of antithetic vein structures striking both east-west and north-south. These veins have been found to host economically interesting mineralization.
East District Veins
The East District contains both metasediment and Miocene volcanic hosted veins.
Metasediment Hosted Veins
These veins strike both northwest and east-west. A large number of historical mines exploited silver and gold mineralization in the metasediment-hosted veins, including Mina de Agua, Animas, Los Locos, Quebradillas, Candelaria de Zayas, and Magdalena, to name a few.
Vein widths vary from tens of centimetres to over 20 metres, with the Santa Ana-Mina de Agua vein averaging +7 metres of silver mineralization with lesser amounts of gold. The combined strike length of metasediment veins mapped to date totals over 25.3 km.
Volcanic Hosted Veins
Mapping thus far has delineated 13 km of veins hosted in Miocene andesites. Historical mines include Animas, Pursima, and El Rincón. The volcanic-hosted veins extend into the metasediments both along strike and at depth. These vein systems strike both northwest and east-west, with dips predominately to the southwest and south, respectively. The combined strike length of volcanic hosted veins mapped to date is 13 km.