Summary:
The MDN properties all lie within the Deseado Massif, a region with a geological setting known to be amenable to gold and silver deposition.
Calandrias Silicified structures and vein breccias host gold mineralisation in Calandrias Sur and Calandrias Norte. High-grade gold and silver veins (greater than 3 g/t Au and 50 g/t Ag), veinlets, stringers, and breccias are closely related to subaerial rhyolite flow domes that were emplaced at the junctions of northwest- and northeast-trending fractures.
The veins are composed of dense brown chalcedony at the surface, partly hydrothermally brecciated. This chalcedony is locally crustiform, and cavities within the veins are filled with massive kaolinite. High levels of arsenic, antimony, and mercury are present, which - combined with the presence of chalcedony and late vug-filling kaolinite - suggests they are shallowly exposed, possibly near the tops of ore shoots (Sillitoe, 2009b). The veins are irregularly developed but locally are up to 1 m in width. Pyrite and marcasite are found in both brecciated and non-brecciated vein material.
At Calandrias Sur, the Ag:Au ratios are in the range of 10 to 130, averaging 25, while at the Calandria Norte vein, the ratios are in the range < 1 to 45, averaging 6. Both deposits have relatively low base-metal contents. Mineralisation in the two deposits is not restricted to quartz veins but also occurs as disseminations in wall rock and chalcedonic hydrothermal breccia filling.
Calandrias Sur
The bulk of the mineralisation is centred in the core of the Calandrias Sur rhyolite dome. It consists of thick zones (up to 90 m) of disseminated sulphides, stockworks, and subordinate breccias. The highest gold and silver grades correlate with increased silicification intensity and stockwork veinlets' density.
Calandrias Sur is on a northwest-trending rhyolite dome that hosts gold and silver mineralisation cut by steeply dipping structures. The mineralisation was initially thought to be almost entirely within the dome's limits, but later mapping, sampling, and drilling confirmed it continues into the surrounding volcaniclastic rocks. In all cases, the mineralisation has poor expression at the surface.
Mineralisation at Calandrias Sur has three main types of breccias:
• Monomictic breccia with fine-grained angular fragments and brown chalcedony cement without sulphides (grades are between 0.1 g/t Au and <10 g/t Ag).
• Monomictic breccia with sub-angular to rounded fragments. Microcrystalline quartz to chalcedony cement. Low in sulphides (<2%). (grades of the breccia are between 0.2 g/t Au and 0.6 g/t Au and 5 g/t Ag to 15 g/t Ag).
• Monomictic breccia with pervasively silicified monomictic clasts, sulphide-rich (<20% volume), (grades between 1g/t Au and 40 g/t Au and 30 g/t Ag to 400 g/t Ag).
Calandrias Norte
The Calandrias Norte rhyolite dome, located about 600 m north of Calandrias Sur, hosts northeast-trending veins and vein/breccia mineralisation entirely within the circular rhyolite dome. This vein and vein/breccia mineralisation lies within a broader area of weak stockwork to disseminated mineralisation. High-grade gold-silver mineralisation starts 50 to 60 m below the surface. At least three distinct vein systems are found within Calandrias Norte: the Calandria Norte vein, the Tongoril and the Morena in the northwest. At the discovery outcrop of the Calandria Norte Vein, where the mineralisation was initially identified by trenching, the vein-breccia is up to 0.7 m wide with multiple parallel veins; the vein breccia strikes northeast and dips steeply to the northwest. The vein-breccia is filled with brown chalcedony and microcrystalline quartz with black sulphides. This vein has been modelled along a 500 m strike with widths typically between 0.4 and 0.8 m. Assay values returned between 2.3 g/t Au and 20.2 g/t Au, and 8 g/t Ag and 55 g/t Ag at the surface (Rodriguez et al., 2009).
A wide range of sulphide textures are present (massive, colloform-crustiform) but do not correlate with gold mineralisation. An exception is marcasite needles or skeletal marcasite, ubiquitous in La Calandria Vein Zone high-grade gold zones. There is usually a sharp change in the vein textures along a single structure in the section. Gold and silver can also be highly erratic within a single cross-section.
Martinetas
The Martinetas region contains multiple mineralised structures occurring as large “vein swarms” with minor intervening stockwork. and by more discrete localisation of continuous veins. The area has been extensively explored for more than 25 years, and for the most part, all known large deposits in the project area have been mined.
Mineralisation styles vary between sheeted and discrete quartz veins. The analysis of fractures, faults and host rocks of the vein systems shows that the main factors controlling the style and orientation of the mineralisation are the lithology, deformation style and fault kinematics (De Martino, 2021).
The Coyote and Cerro Oro deposits comprise a series of narrow, sub-parallel, anastomosing quartz veins varying in width from tens of centimetres to several metres, typically averaging 1 m or less in thickness. Au-Ag mineralisation is variable within the veins, with some minor stockwork mineralisation extending into the host volcanics.
Five resource areas have been delineated. The main resource is at the Coyote and Cerro Oro deposits, comprising a series of narrow, sub-parallel, anastomosing quartz veins varying in width from tens of centimetres to several metres, typically averaging 1 m or less in thickness. Au/Ag mineralisation is variable within the veins, with some minor stockwork mineralisation extending into the host volcanic lithology. Conceptually, near-surface oxidised stockwork precious metal mineralisation might provide a low-grade, conventional, heap-leachable resource.
Paloma Trend
The following description of the Sulfuro Vein within the Palom trend mineralisation has been taken from Fernadez et al. (2020).
The Sulfuro Vein corridor, located in the southern part of the La Paloma district, constitutes a 1000 m long and up to 600 m wide block occupied by several segments of mineralised lodes between 0.3 and 6 m thick. To the north, this trend turns to a WNW and an ENE orientation that confers an arcuate shape. In the Sulfuro vein, the mineralisation has been defined for 850 m along strike and to a depth of 250 m, with an average width of 3 m.
Deposit Types
Las Calandrias Area
The Deseado Massif hosts numerous low- to intermediate-sulphidation, epithermal, precious metal quartz vein and vein-breccia deposits, of which Las Calandrias Sur and Las Calandrias Norte deposits are examples. Hydrothermal activity and formation of these deposits closely followed episodes of the Jurassic magmatism and extension in the Deseado Massif (Ristorcelli et al., 2018).
Martinetas and La Paloma Trend Area
The gold-silver deposits of the Deseado Massif are hosted in silicic volcanic and volcanic sedimentary Jurassic rocks related to arc or back-arc settings in Andean or extra-Andean settings.