Summary:
Silver and gold mineralization at Martha is hosted in Jurassic-aged felsic volcanic rocks of the Chon Aike Formation of the Bahia Laura volcanic complex (BLVC). Host rocks are relatively shallowly dipping ignimbrites, locally inter-bedded with thin sections of sedimentary strata.
Martha mineralization is epithermal, intermediate sulfidation in style, hosted in quartz veins, veinlets and vein breccias cutting the margins of the Jurassic-aged caldera (Primero de Abril Caldera; Paéz et al, 2015). Vein widths vary significantly on surface from a few centimeters up to several meters. Banded textures are common in the wider veins. Adularia is a common gangue mineral in the quartz veins.
Silver and gold mineralization at Martha, along with lesser amounts of base metals, occurs within quartz rich veins, veinlets, breccias and stockworks. The style of mineralization is interpreted to epithermal in original. Epithermal silver and gold deposits, according to Sillitoe and Hedenquist (2003), occur as “both vein and bulk-tonnage styles may be broadly grouped into high-sulfidation (HS), intermediate-sulfidation (IS), and low-sulfidation (LS) types based on the sulfidation states of their hypogene sulfide assemblages”.
Martha Mineralized Zones
Silver and gold mineralization at Martha is located within a series of quartz-rich, locally banded and brecciated, veins and veinlets. The style of mineralization has been interpreted to be intermediate-sulfidation in character (Paéz et al, 2015 and Sillitoe, 2005). The main trend of the mineralized systems is WNW and EW and dip steeply to moderately to the S, SW. On surface, mineralized structures can be several meters wide but often are much less than a meter in true width but may expand in width in the subsurface.
Base and precious metal-bearing minerals at Martha (Paéz et al, 2015 are galena, sphalerite, chalcopyrite, polybasite [(Ag,Cu)6(Sb,As)2S7][Ag9CuS4], silver-bearing tetrahedrite, pyrargyrite (Ag3SbS3), freieslebenite (AgPbSbS3), acanthite and native silver. Pyrite and arsenopyrite are common. Gold-bearing minerals have not been identified.
Dating of base and precious mineralization is often performed by dating spatially-related minerals. At Martha, Sernageomin (2011), for Paéz’s thesis (Paéz, 2011), dated vein-hosted adularia (a low-temperature variety of orthoclase with the chemical formula KAlSi3O8) at 156.5 mya (million years ago). The host rock was dated at 157.6 mya. Alteration assemblages at Martha have been studied by several workers including the exploration staff at Coeur (Birak et al, 2007) and can be generally grouped into major sets: propylitic, silico-argillic, argillic and silicic. Cedillo Frey (2009) documented the following alteration observations at the Martha Oeste vein, which is situated on strike with, and west of, the Martha vein (translated and paraphrased from original Spanish by the Qualified Person).
“Crystallization of sericite, quartz and adularia occurred from pH neutral, hydrothermal fluids ranging in temperature from 200 ° C to 250 °C. This alteration developed two main zones:
a) An outer zone, extending 1 to 5 meters from the vein with sericite as replacement of potassium feldspar, plagioclase, biotite and phengite (muscovite-like mineral). This alteration increased towards the vein forming muscovite and illite.
b) An Inner zone, extending less than 1 meter from the vein, with massive silicification.
In both zones, adularia can be found but is most abundant in the inner zone.”
The Martha property veins are clustered into several discrete groups with the majority located in the main Martha area.
Veins in the Martha cluster are (from west to east):
- Martha West, Martha, R4, Del Medio System (containing Francisca, Catalina and Belen), Nordeste, Isabel, Futuro, Martha East, Esperanza.
- The new discovery made north of the Martha vein by Hunt (Martha West), is located in this cluster.
- The Wendy vein, another mineralized vein that has received some exploration drilling, occurs about 8 km SW of the Martha cluster.
Veins in the Northern cluster are (from west to east): Isabel Oeste, Betty West, Betty Sur, Martha Norte, Ivana and Betty East.
Veins and targets in the Ana cluster are: Tesoro (sulfidic breccia pipe); Leonor; Angela; Alejandra; Teresa: Beatriz.
In 2017, an extension of the mineralization in the Martha vein was made with mapping and percussion drilling. The extension, “Martha West”, sits approximately 50 meters to the south-southwest of the Martha portal. This small extension is approximately 35 meters by 45 meters (north to south by east to west) in size to-date and extends 20 meters below surface. At its western end, the mineralized structure is narrower than on the west. The mineralized structure is open at depth and appears to plunge to the ESE.