Summary:
Deposit Types
The Santana Project is in southeastern Sonora within the prolific Upper Cretaceous–Paleocene Laramide magmatic–hydrothermal metallo–tectonic event. The largest known deposits of the Laramide event are porphyry copper systems and their associated breccias, skarns, and vein deposits; however, the mineralization at Santana represents a style that has not been reported to date for this region of northwestern Mexico. Porphyry-style mineralization at Santana has been ruled out because of the absence of typical A/B veins and potassic alteration of porphyry systems, and by the presence of ironmanganese carbonate and specular hematite that is typically not present in porphyry systems.
At Santana, gold is hosted by breccias and intra-mineral dykes and stocks. The sericite-stable nature of the alteration and the type of quartz observed in the area indicate mesothermal temperatures close to 300°C, and emplacement levels of at least 500 m below the paleosurface.
The mineralization at the Project represents classic magmatic-hydrothermal breccias, the product of overpressured fluids exsolved from an underlaying magma chamber. The mineralization and the hosting breccias have a clear intrusive-related genetic affinity, associated to calc-alkaline-oxidized dioritic and quartz-dioritic intrusions.
Under this genetic model, felsic porphyritic dykes and hydrothermal fluids were emplaced within the upper carapace of cogenetic larger intrusive bodies and in the contemporaneous Tarahumara volcanic rocks. The magmatic-hydrothermal breccias were formed because of cooling, degassing, and emplacement of late-felsic dykes and stocks. Once the porphyritic dykes and stocks cooled down to about 300°C the sericite-chlorite-iron oxides, carbonate stable fluids formed pervasive and veinletcontrolled alteration and mineralization within the intra-mineral dikes and in the matrix of the breccias and formed some replacement and disseminated mineralization along permeable volcanic units. A second pulse of hydrothermal fluids introduce most of the sulphides and probably the gold observed in the deposit, sulfides are mostly represented by pyrite with minor amounts of pyrrhotite, marcasite, sphalerite, galena, and chalcopyrite.
The mineralization style and genetic model have significant implications for exploration activities. Since these deposits are not considered typical porphyry systems, no geometries and zoning typical of porphyries should be expected, including deep potassic or the zoning from sericitic to potassic mineralization zones. The bulk of the known mineralization at Santana is present in elliptical – sub vertical breccia bodies.
At Santana the host rocks of the breccias are cogenetic, pre-mineral quartz diorites larger intrusions and volcanic rocks of the Tarahumara Formation. It is very likely that the breccia (hidden from surface) can be very weak, and include stockwork, veins, geochemical anomalies, and narrow breccia dykes. Erosion has exposed some of the mineralized breccia and QFP units, but there are likely more blind breccias on and surrounding the Santana property area. Gold occurs mainly within the breccia and in the intra-mineral QFP. Some mineralization can be found on the margins of the breccia in fault-veins, stockwork veinlets, and on the contacts of different volcanic units.
The sizes of known intrusive-related magmatic hydrothermal breccias in various mining districts in the cordilleras of the Americas vary from 10 to 10s of millions of tonnes grading up to 1 g/t Au, representing 300,000 to over 1-million-ounce gold deposits that can form important mining districts with clusters of deposits.
Mineralization
Mineralization at the Project occurs within breccias that have a jigsaw-type texture. These breccias typically comprise angular elongated fragments that have a preferential sub-vertical orientation. Review of core and outcrop indicates that these fragments did not undergo large displacements or rotations, which left open spaces between them that were subsequently infilled by gold-bearing hydrothermal minerals. The breccias are principally clast-supported and monomictic and are found in pipe-like bodies. The presence of gold mineralization is directly related to the areas dominated by the breccia intervals.
Mineralization at the Project is of the intrusive-related gold (Au-Cu-Ag-W) type and is associated with calc–alkaline-oxidized large intrusive centres, but the mineralization is not reduced as in the Alaskan type, being likely formed in the back-arc environment. Gold is hosted by hydrothermal breccias and their causative inter-mineral dykes and stocks. The sericite-stable nature of the alteration and the type of quartz observed in the area indicate mesothermal levels of emplacement (~300°C) below possible eroded, epizonal levels, and around high-temperature sheeted-vein-controlled intrusive mineralization. The size of the known intrusive related deposits associated with inter-mineral dykes, stockwork, replacements, and breccias can vary from ten tonnes to tens of millions of tonnes, grading up to around 1 gram per tonne of gold (g/t Au), representing 300,000 to over 1 million troy ounce (oz) gold deposits, and can form important mining districts with clusters of deposits.
The volcanic units of the Tarahumara formation are characterized by a pervasive chloritic alteration giving the rock a light- to dark-green colour. Less intense chloritic alteration also affects most of the intrusive units and is restricted to partial or full alteration of the mafic minerals, such as biotite or hornblende. More intense alteration associated with mineralization is linked to the emplacement of hydrothermal breccias, which are, in general, sub-vertical pipe-like structures a few hundreds of metres in diameter. They can be readily identified as topographic highs and have leached hilltops, such as those seen at Benjamin, Nicho, and Nicho Norte. The breccias have small alteration halos of sericitic alteration, silicification, occasionally tourmaline, and abundant pyrite. The breccia itself is usually of the jigsaw type, with a matrix composed of quartz, pyrite, and calcite in about equal proportions. The breccia clasts are strongly silicified and sometimes have sericitic alteration. The volcanic rock is weakly magnetic where chloritic alteration prevails. Locally, where sericite is more abundant, no magnetite is present, but it could have been removed.
The Santana Project hosts several small prospects and numerous coloured anomalies defined by ironoxide staining and pervasive sericite alteration. Historical production from the Project area was limited to structurally controlled zones of gold mineralization, in places accompanied by copper mineralization.
The four principal mineralized areas include Nicho, Nicho Norte, Divisadero, and Benjamin. The Nicho Norte and Divisadero zones are geologically similar and occur within a 600 m by 600 m area. The Nicho and Benjamin zones are more associated with mineralization that is related to the Tarahumara Formation.