Gas-transported elements as an exploration technique under post-mineral cover: Atlántida deposit and surroundings

Abstract

Soil gas sampling technique by means of passive collectors devices (OreHound®) was carried out over the Atlántida porphyry deposit, located in the Atacama Region of northern Chile. Atlántida is a porphyry Cu-Au-Mo deposit with adjacent development of skarn, in the contact between intrusives and limestones, and it is almost completely covered by post-mineral pediment gravels, ranging from 5-80 meters in thickness. Because of the wide combination of preexistent analysis (38 drillcores, surface mapping, surface geochemistry and geophysical data) plus the known of extent and its location in an arid climate, Atlántida is a suitable place to test the usefulness of gas sampling through collectors and figure if it is effective at reducing an exploration area to the scale of the deposit. Furthermore, the combination of high seismicity and hyper-aridity makes this region ideal for employing the anomalies in prospecting for concealed mineral deposits (Cameron et al., 2002). A total of 234 collectors were installed over Atlántida and surroundings and, after three months in the field, were retrieved and analyzed by ALS Chemex by their Super Trace aqua regia technique (ME-MS41L). From basic statistics of the results, it was determined that there is clear capture of signal to present real time processes, evidenced by lower and higher concentrations than the blanks. To localize contrast anomalies, univariate and multivariate methods are used in this study in order to define background and anomalous concentrations. Rabbit ear anomaly patterns were obtained in two WE profiles sections above Atlántida. Semi-anular anomaly patterns were obtained in the factor analysis interpolation maps and in the raw data ordinary cokriging interpolation maps above Atlántida. In particular, the elements Cu, Mn, Pb, Rb and Sr showed background values over the concealed deposit and positive anomalous values adjacent to it. On the other side, Zn showed a positive contrast anomaly over the deposit with background concentrations around it. Such patterns are believe to represent present oxidation processes and are most likely explained in terms of chemical dispersion mechanisms. Another factor shaping the surficial gas anomalies found in this study is the existence of preferential pathways (faults and/or fractures) through which gases move from underground upward to the surface.