Geophysical and geochemical prospecting for the La Melada deposit, San Luis, Argentina

The La Melada deposit occurs within the mafic-ultramafic belt of the Sierra de
San Luis and contains Ni, Cr, Co, Cu and PGE mineralization. The deposit comprises
two main bodies of lenticular form and a group of dikes, and covers 30 km2. The main
bodies are located within metamorphic rocks with NNE-SSW orientation. The deposit
is of hydrothermal origin, and linked to low-grade metamorphism.
Studies of the La Melada deposit gave the following conclusions: (1) major- and
minor-element chemical abundances indicate that rocks follow a tholeiitic trend to
oceanic ridge basalt or island-arc rocks; (2) ore is characterized by disseminated
pyrrhotite, chalcopyrite, magnetite, and minor pyrite and chromite. Mylonites have
magnetite and scarce pyrrhotite, hematite and goethite.
The La Melada area is characterized by a negative regional magnetic anomaly of
about the 150 nT thought to have originated in the Ordovician. Local positive magnetic
anomalies in mylonitic zones resulted from magnetite remobilization during the
Devonian. High magnetic susceptibilities occur in the contact zone between the
mylonites and the eastern body. They coincide with Cu, Zn, Ag, Pb and U anomalies.
Two deposit types are identified by their geochemical halos: (1) Ni, Cr, Co and
PGE mineralization, and, (2) veins containing Cu, Zn, Ag, Pb and U.
Increasing basicity towards the southern end of the bodies was determined by the
Ni/Co discriminant index, and correlates with MgO distribution. The economic
potential index S/Ni is high in both bodies, especially in the intrusions of the south
sector. Aprospective Ni-Cr-Co and PGE target was located in intrusions of the southern
sector using geophysical and geochemical prospecting