ALTERATION, MINERALIZATION AND FLUID EVOLUTION OF VEGAS

JOSEFINA PONS; FRANCHINI MARTA; LAWRENCE MEINERT

Lima, Perú

Congreso; XIII Latin American Geological Congress & XIV Peruvian Geological Congress,; 2008

Sociedad Geológica del Perú

The mineralized skarns in the Vegas Peladas district are similar to other Fe skarn deposits worldwide (Einaudi et al., 1981; Meinert, 1984; Meinert et al., 2005) in the following features: (1) the presence of prograde exoskarns rich in andradite-grossularite garnet, diopside-hedenbergite pyroxene, and retrograde amphibole, epidote, quartz, chlorite and calcite; (2) magnetite, hematite and mushketovite are the main iron ore minerals and the most important orebodies are associated with retrograde mineral assemblages; (3) sulfides, mainly pyrite, are scarce and occur in late stages and distal zones; (4) skarns are zoned in space and time; (5) the distribution of alteration and mineralization during early, prograde stages was strongly influenced by the hosts composition (both igneous and sedimentary) whereas late, retrograde alteration was more strongly controlled by structure; and (6) sodic metasomatism (scapolite and albite) is intense in the skarn associated with the granite but less common in the skarn associated with diorite. In contrast, the presence of orthoclase as alteration product of the diorite is not common in Fe skarn descriptions (Einaudi et al., 1981; Meinert, 1984). The alteration mineralogy of the skarns and the iron enrichment toward garnet crystal rims and at deposit scale, toward outer zones, suggest an increase in high oxygen fugacity and iron precipitation as temperature decreased. This is supported by the increase of hematite relative to magnetite from inner to outer zones. Both deposits have high salinity fluids capable of transporting large amounts of Fe in solution and in both skarns, most iron ore is associated with retrograde alteration, indicating that a temperature decrease was the main cause of iron precipitation. Previous work assigned a Palaeogene age to the Fe skarns of southern Mendoza. New dating (Franchini et al., 2007) and the present study demonstrate that the iron skarns are associated with the least differentiated plutons, dikes and sills of the voluminous and ubiquitous magmatism of the Upper Miocene. These Neogene igneous rocks derived from primitive magmas originated in the mantle, with little or no crustal contamination; their emplacement was structurally controlled. Thus, these plutons at the intersection of the main lineaments, thrusts, and fold cores in poorly explored areas, can host iron mineralization in associated skarn. The abundance of primary amphibole and magnetite in these igneous rocks and in the skarn gives them a strong magnetic response which is important from a prospecting point of view. The distribution of alteration assemblages and distal vein and veinlets with Fe oxides and retrograde minerals can be used as an exploration guide in regions with poor exposure or without outcrops. The skarn mineralogy also can be used to distinguish these deposits from the IOGC-manto deposits that are also present in this segment of the Andes Cordillera.