Tertiary metallogeny of precious metals in NW-Argentina

The Tertiary metallogeny of NW-Argentina is of utmost significance. Economically important deposits such as, porphyry copper-gold (Bajo de la Alumbrera, Agua Rica, Organullo district), stock-work and vein-type silver-tin and silver (Pirquitas, El Quevar, Cerro Redondo, La Concordia) and vein type selenides (plus gold, silver, uranium, possibly PGE`s) (Sierras de Umango with Cerro Cacho, Famatina, Cacheuta) were formed, amongst others. Most of the precious metal-bearing mineralizations are possibly controlled by tectonics related to the uplift of the Puna block, and are genetically related to volcanics and plutonic rocks. It is the purpose of this project to investigate in detail the economic geology of four selected districts: with precious metals in NW-Argentina (Pirquitas, La Concordia, Cerro Atajo, Sierras de Famatina, Cacheuta and Umango; compare Fig. 1 of the "formless" application), to develop genetic models for each of them, and compare their interrelationships. This modelling should be based on standard methods of geological mapping with special emphasis devoted to the structural history, and finally applying more sophisticated methods such as, fluid inclusion characteristics and stable isotopes (C,O) to shed some light on the conditions of formation, -the fluid evolution and the age(s) of associated mineralization/wall rock alteration. The structural studies combined with 40Ar/ 39Ar age dating may allow to demonstrate the relationship between formation of mineralized structures and plate movement like Farallon/South America - interaction. Furthermore, these structural studies may have some predictive power for structurally controlled mineralizations. Special attention will be directed to a very detailed study of particular (ore)mineral associations, the properties of individal species and mineral groups (such as the sulfosalts) as well as the submission of proposals for new mineral species to the IMA CNMMN. The overall aim of this project is to gain a better understanding (1) why and in which way mineral deposits had been formed in this part of Argentina, and (2) to which extent the expected results of this project can be used for continuouing prospection and exploration of still unknown mineral deposits.

The Andean cycle is significant for the formation of vein-type and other deposits in NW-Argentina. The following deposits can be related with certainty to this cycle. They are clearly bound to faults and are always accompanied by typcical wall rock alteration. The genesis of the deposits is linked to the wide-spread Tertiary volcanism. The differences in the mineralogies can be explained by different compositions of the upper crust. The selenium province of the Sierra de Umango (La Rioja) is dominated by copper selenides. Brodtkorbite which is a new Cu-Hg-Se mineral is a rare constituent in one of the deposits. Significant concentrations of Au, Ag, Pd and Pt occur in several depostits which carry Hg, Cd, Tl, Co, Ni, As and Sb as trace elements. The proof of a regional Pd anomaly is new. Average grades of 13 ppm Pd, 9 ppm Pt and upt to 17 ppm Au are typical for these deposits. The selenium ores are associated with wall rock alteration (formation of hematite) of the hosting amphibolites. The selenides were precipitated from Se-rich oxidizing fluids within the stability field of hematite at temperatures below 112C. The silver-tin deposit Pirquitas (Jujuy) is the southernmost representative of the Ag-Sn formation of adjacent Bolivia. The deposit is composed of veins and a stock-work type mineralization. The highest metal concentrations occur in the hinge zones of anticlines. The silver content of the ores shows a strong mineralogical zoning. The amount of hocartite and complex Bi-bearing sulfosalts increases with depth. A zinc-dominated ore types shows an enrichment in indium, which is bound to the minerals sphalerite, sakuraiite and petrukite. Apart from cassiterite significant grades of tin are associated with the occurrence of suredaite and "As-franckeite", two new mineral species, which were observed in the "Oploca" vein system. A silver-rich ore type contains native gold which has never been recognized in this deposit type. The polymetallic and precious metals containing epithermal deposit of Capillitas (Catamarca) shows a significant anomaly of germanium. The occurrence of germanium in association with lead, zinc and rhodochrosite can be related to the silver-germanium mineral argyrodite and a new phase belonging to the polybasite group. In copper dominated veins of the district, germanium is present in the form of three new mineral species (putzite, ctamarcaite, "Ge-stannoidite"). Thallium (in bornite-rich ores), Te, V and Sn minerals are characteristic associates of the ore at Capillitas. The epithermal vein-type deposits belong to the high- and low suphidation types, and are associated with wide-spread wall rock alteration. The copper-gold deposits of Cerro Ataja (Catamarca) are characterized by a vein-type and a disseminated mineralization. The mineralogy of the veins is dominated by fahlore of varying chemistry, sulfosalts and native gold while high grades of gold accompanied by hübnerite occur in the disseminated part of the deposit. The fault-controlled vein system of the La Concordia district (Salta) is again typified by a complex polymetallic mineralization with Bi-bearing sulfosalts and native gold. The types of wall rock alteration are indicative of a porphyry copper mineralization with some gold at depth. This porphyry mineralization has been recently confirmed by exploration drilling.