The Productora Prospect in North-Central Chile: An Example of an Intrusion-Related, Candelaria Type Fe-Cu-Au Hydrothermal System

The Productora prospect is situated 15 km SSW of the town of Vallenar in the Third Region, north-central Chile. It lies within the "Chilean Iron Belt" close to the north-trending Atacama Fault system. The belt contains a variety of Fe oxide ±Cu ±Au-bearing deposits including Candelaria (366 million tonnes averaging 1.08 % Cu, 0.26 g/t Au) which is situated in the Punta del Cobre region approximately 150 km north of Productora. The age, structural setting, alteration assemblages and styles of mineralisation in the Productora area resemble those present at Candelaria, but Productora differs in its spatial association with intrusive rocks and its lack of skarn assemblages.

Productora includes a north-trending zone of hydrothermal alteration (Fe oxide-albite-K spar-tourmaline-sericite-silica) that exceeds 8 km in strike length and 3 km in width. This zone contains 13 small former mines, 8 of which were worked for magnetite and the remaining 5 for Cu-Au. In addition, there are over 80 shallow pits and surface occurrences containing some Fe oxide ±Cu ±Au ±U +REE ± apatite mineralisation.

The mineralisation and alteration are structurally and stratigraphically controlled. They are preferentially developed in more permeable tuffaceous units of a presumed Cretaceous package but also follow NW-striking and, to a lesser extent, N-trending structures related to the Atacama Fault system.

Hydrothermal alteration is mainly centred around two Early Cretaceous (c. 130 Ma) felsic intrusions, the Cachiyuyito and El Molle stocks. Six proximal to distal alteration zones are recognised. Zone I alteration assemblages in the stocks comprise pervasive albite with veins of actinolite, magnetite and epidote. The immediately adjacent country rocks are overprinted by Zone II alteration dominated by albite, chlorite, actinolite, magnetite, apatite and coarse calcite veins. More distal still is Zone III with widespread K-spar and tourmaline, as well as magnetite, hematite, secondary biotite and silica. This passes out to Zone IV in which the volcaniclastics are strongly replaced by albite and silica ± epidote and hematite. Zones V and VI represent the uppermost and lowest temperature alteration. The former is largely preserved in an elongate, 1.5km-wide down-faulted block and is characterised by massive silica, specular hematite, sericite and dumortierite, whilst the outermost Zone VI has pervasive propylitic and albitic alteration with rare jasper veins.

Three styles of Fe-oxide dominant mineralisation are identified, each of which tends to occur within a specific alteration zone. The most proximal and common of these is magnetite ± apatite veins and replacements hosted by Zone II alteration assemblages. This style seldom contains significant Cu or Au and is represented by the Mariposa, La Bandera, La Chulula, La Negrita and the El Molle magnetite mines.

The second and most economically important style of mineralisation is typified by the Cu-Au ± REE + U ± Mo ± Co mineralisation at the Productora and Santa Innes mines. It is more commonly associated with Zone III alteration (K-spar-silica and tourmaline) but in the case of the Monseratt Mine and Remolina pit it is hosted by Zone II mineral assemblages. Hypogene mineralisation includes magnetite, chalcopyrite, pyrite and native gold. Surface leaching has resulted in the development of irregular leached caps containing dioxide and phosphate mineralisation at surface and secondary Cu enrichment locally at depth. Both the hypogene and secondary Cu mineralisation is Au-rich (100 to 9700 ppb) and they contain anomalous REE's (50 to 560 ppm La). Trace quantities of the U oxide mineral, torbernite, are also present. Sulphur isotope ratios for chalcopyrite, pyrite and chalcocite collected from Style 2-type mineralisation show 5 S values ranging from -8.2 to +1.2 which are lower than those recorded in the Punta del Cobre region further north.