Summary:
Mineralization within the Cosalá mining district is related to granodioritic or granitic intrusions of the Sinaloa Batholith, a composite gabbroic to granodioritic complex that induced strong contact metamorphism in adjacent sedimentary and volcano-sedimentary units. Exposures of the sedimentary rocks and associated mineralization are small and surrounded by Tertiary volcanic rocks (Armbrust and Chlumsky, 2006).
Three types of intrusions are present in the San Rafael-El Cajón area. Medium- to coarse-grained granodiorite, which is part of the district-wide batholith, crops out in the western part of the project area and was also intersected at the bottom of a number of PRG drill holes in the El Cajón area. There are also large, local intrusions of diorite, often occurring as sills, that are interpreted to be related to the emplacement of the batholith. Andesitic dikes and sills, which are sometimes weakly magnetic, are also present.
The property-wide dioritic intrusions are often weakly magnetic and generally only weakly altered, although the dioritic intrusion(s) spatially associated with the El Cajón mineralization exhibit a pervasive skarn alteration assemblage consisting of albite, tourmaline, scapolite, epidote, calcite, titanite (sphene) and minor quartz. Similarly, the diorite at Zone 120 occurs as conformable sills or gently cross cutting dikes. These are generally lightly altered but are spatially related to mineralization. Though pervasively altered, the diorite contains only trace amounts of pyrite and chalcopyrite. The skarn-altered diorite was often logged by earlier geologists as quartz monzonite.
The skarn alteration in the vicinity of San Rafael intermittently covers a broad area of at least 20 km². Paragenetically, from earlier to later stage, typical skarn minerals are garnet (especially andradite and grossularite), pyroxene, wollastonite and potassium feldspar, followed by calcite, chlorite, epidote, quartz, sericite and pyrite. Calcite and chlorite abundances increase near the mineralized zones. A quartz-sericite-pyrite assemblage is associated with the dominant, massive-textured, sulphide replacement mineralization at San Rafael. A similar mineral assemblage is observed at El Cajón where the mineralization is developed in similar host rock, the Cretaceous limestone, but is more subtle and fine grained in Zone 120 where the mineralization is developed in preferential layers or beds in the volc-arenite. Skarn is developed preferentially in beds with higher carbonate content and is medium- to coarse-grained with fine-grained green garnets, epidote, calcite and chlorite with some silicification.
Three principal zones of sulphide mineralization have been identified within a broad area of skarn alteration in the vicinity of San Rafael and nearby El Cajón. The San Rafael Main Zone consists of masses of sulphide grains that occur as replacements at an unconformable contact between what is believed to be Tertiary dacite tuff and Cretaceous limestone. This surface exhibits significant development of karst with occasional caverns up to 150 m² in plan. Although it can be difficult to determine the host rock when total sulphide content is 90 to 100%, most of the massive sulphide replacement mineralization appears to be hosted in the rubble of the karst horizon or within the dacite tuff. It contains silver, lead and zinc mineralization with lesser gold and copper.
The main minerals are pyrite, pyrrhotite, sphalerite and galena with minor marcasite, chalcopyrite and magnetite. This mineralization in the San Rafael Main Zone is often associated with quartz-sericite-pyrite alteration that has been interpreted as more distal skarn alteration. It has also been suggested that the San Rafael Main Zone displays many similarities to volcanogenic massive sulphide deposits, such as those found in the Guerrero Terrane in central Mexico.
The Main Zone sulphide body is discrete, tabular, and lies along the shallow-dipping dacite tuff-limestone contact where it has been referred to as “massive-sulphide mineralization” in previous reports. The zinc, lead and silver minerals include sphalerite and galena. The contacts of all elemental zones generally overlap within the massive sulphide, but mineral-shell boundaries and their internal grade distribution are not necessarily coincident.
The Main Zone mineralization as currently defined has a 1,000 m strike length, is 15 to 20 m thick, and extends down dip continuously for 300 m and discontinuously for up to 600 m. The Main Zone deposit strikes 320° and dips variably between 10° and 30° towards the southwest.
The Main Zone sulphide mineralization has been oxidized to a variable depth below surface, usually less than 30 m, though in the northeast portion of the deposit oxidation can extend down dip for as much as 200 m.
The silver-gold Upper Zone lies within the Tertiary volcanic rocks approximately 50 to 100 m above the Main Zone sulphide replacement mineralization of the San Rafael deposit. The mineralized horizon can be up to 15 m thick but often is approximately 5 m thick. The Upper Zone is composed of irregular, sub-horizontal layers sub-parallel to the Main Zone. Mineralization consists of sulphides, however, sulphide content is much less than in the Main Zone. Weak base-metal mineralization with silver also occurs.
Zone 120 occurs not as a single horizon, but as multiple bedding- and intrusive-contact-related mineralized horizons. As currently defined, the Zone 120 mineralization occurs within a rock volume that is approximately 600 m long, 250 m wide and extends to a depth of about 350 m below the surface. It strikes in a direction of 330°, and below the massive sulphide, the bedding-related mineralization dips steeply to the northeast at approximately 50°. The Zone 120 mineralization is interpreted to occur along near-vertical contacts between diorite and skarn-altered lime rich volcarenites in the lower parts, and in quartz-sericite-pyrite-altered volc-arenites in the upper portions. Mineralization is associated with generally 2 to 10% sulphides and is more irregular in shape and more variable in mineral character than the San Rafael Main Zone. It consists of silver-copper-gold mineralization in the form of chalcopyrite and tetrahedrite with minor pyrite, galena, sphalerite, arsenopyrite, chalcocite, jalpaite, native silver, copper and bismuth.
El Cajón mineralization consists of mantos and chimneys developed in skarn and recrystallized limestone. El Cajón and Zone 120 mineralization are similar in character consisting of veinlets and replacements of chalcopyrite and tetrahedrite-tennantite. The El Cajón deposit is roughly oval shaped extending 550 m east-west and 400 m north-south, with the mineralization aligned along the general 330° strike and 20° northeast dip of the limestone country rock. It varies in depth from approximately 20 m below surface to a depth of approximately 250 m.
Zone 120 in the eastern portion of the San Rafael deposit contains silver-copper mineralization within garnet-pyroxene-calcite skarn. The strong metasomatic alteration and the close spatial relationship with a large dioritic intrusion suggest that Zone 120 represents a proximal skarn deposit. Silver-lead-zinc mineralization, in the form of massive sulphide replacements in the Main Zone and, to a lesser extent, in the Upper Zone is associated with quartz-sericite-pyrite alteration. This alteration and mineralization type is believed to be a more distal phase of the skarn system. El Cajón is a proximal silver-copper skarn related to an adjacent nearly cylindrical diorite intrusive body. Mineralization at El Cajón is replacement type and occurs as horizons in recrystallized limestone which are connected by mineralized zones localized by steeply dipping contacts, faults and fractures.