The use of partial extraction geochemistry for copper exploration in northern Chile

Abstract

Future discoveries of copper resources in the richly endowed porphyry copper belts of northern Chile are likely to occur in areas covered by post-mineral sedimentary and volcanic rock. Most of northern Chile is within a region of interior drainage that has existed since at least mid-Tertiary time when many of the porphyry copper deposits were being uplifted, eroded and supergene enriched. This setting, combined with accumulation of metal-enriched material eroded from the tops of porphyry copper deposits, has produced regions with high geochemical background and geochemically complex post-mineral cover that includes alluvium, lake and playa sediments, evaporites, aeolian material, ignimbrites and volcanic ash. In addition, the climate changed from semi-arid to hyper-arid in mid-Miocene time, creating a surface environment that is saline and slightly alkaline. The geochemical expression of the Chimborazo porphyry copper deposit in northern Chile was studied in detail using the following wet chemical extraction methods: 4-acid (HF-based), aqua regia, bulk cyanide leach, hot hydroxylamine hydrochloride, cold hydroxylamine hydrochloride, enzyme leach and de-ionized water. Limitations affecting several partial extraction methods include poor data quality, lack of pH control, re-adsorption, and the formation of colloids in the leachate. Experimentation with leach time and buffering capacity improved the robustness of some of the methods. Mechanical dispersion is the dominant secondary process in this region although chemical dispersion does occur laterally on a regional scale and vertically on a local scale. Vertical chemical dispersion is associated with dilatancy pumping of metalliferous groundwater along fractures. Supercedency, a process whereby the geochemical fingerprint of transported material is inherited from the underlying material, may have also played a role in the strong signature seen at Chimborazo.

Soil geochemical survey over concealed kimberlites in the Attawapiskat area in northern Canada

“…Bajc 1998;Cameron et al 2004;Kelley et al 2004Kelley et al , 2006. The results of the AA5 leach suggest that it is effective in showing the anomalies of kimberlite pathfinder elements, including REE, Y, and Ni.…”

“…Bajc 1998; Kelley et al 2004;Cameron et al 2004). Partial leaches would allow the dissolution of elements adsorbed or bound in a specific target phase(s) while not dissolving the entire soil.…”

Transported cover in northwestern Victoria, Australia — An impediment to geochemical exploration for gold

“…Exceptions are studies such as those by Lintern (2007), Sergeev and Gray (2001), and Tonui et al (2003). Partial extraction analyses of soils are commonly used in an attempt to detect subtle geochemical dispersion signatures (Bajc, 1998;Kelley et al, 2003;Mann et al, 1998;Williams and Gunn, 2002;Xueqiu, 1998). Partial geochemical extractions are specifically designed to leach mobile, dispersed, loosely-bound elements (Kelley et al, 2003).…”

“…Partial extraction analyses of soils are commonly used in an attempt to detect subtle geochemical dispersion signatures (Bajc, 1998;Kelley et al, 2003;Mann et al, 1998;Williams and Gunn, 2002;Xueqiu, 1998). Partial geochemical extractions are specifically designed to leach mobile, dispersed, loosely-bound elements (Kelley et al, 2003). Because the more strongly bound matrix concentrations of these elements are commonly only weakly digested by these leaches, high geochemical contrast (and subtle signatures) can be observed (Noble and Stanley, 2009).…”

Microwave-assisted continuous leaching on-line with inductively coupled plasma mass spectrometry for exploration and environmental geochemistry

“…Kelly et al, 2003). They therefore need sensitive techniques to detect elemental anomalies in soil samples, which have migrated from those blind deposits (Goldberg, 1998;Xueqiu, 1998).…”