Integrated study of supergene copper deposits from Atacama Desert, Northern Chile: coupled petro-geochemical approach and U-Pb LA-ICP-MS in situ dating
Résumé
Supergene copper mineralization (SCM) are nowadays the economic viability of many porphyry copper deposits worldwide. These mineralization are derived from supergene processes, defined by Ransome (1912) as sulfide oxidation and leaching of ore deposits in the weathering environment, and any attendant secondary sulfide enrichment. For supergene copper mineralization to form, favorable tectonics, climate and geomorphologic conditions are required. Tectonics control the uplift needed to induce groundwater lowering and leaching of sulphides from a porphyry copper deposit. Climate controls copper leaching in the supergene environment and groundwater circulation towards the locus where supergene copper-bearing minerals precipitate. Two types of SCM have been recognized: 1) in-situ SCM, which are products of descending aqueous solutions and 2) exotic SCM, which are the products of lateral migration of supergene copper solutions from a parental porphyry copper deposit (Sillitoe, 2005).
In the Atacama Desert, such deposits seem to take place during specific Tertiary climatic periods and relief formation. But many uncertainties remain regarding the genesis and the exact timing for their formation. In this study, a coupled approach combining a petro-geochemical study and LA-ICP-MS U-Pb dating were applied to four mining copper deposits (e.g. Mina Sur, Damiana, El Cobre, Zaldivar) from hyperarid Atacama Desert of Northern Chile. Textural features are the same in all the deposits with chrysocolla as the abundant mineral, followed by black chrysocolla, pseudomalachite and minor atacamite and copper wad. Their geochemical compositions (i.e. major, traces and rare Earth elements) also show homogeneous results suggesting similar process in their genesis. U-Pb dating were performed on black chrysocolla, chrysocolla and pseudomalachite from all the deposits. Apart from Mina Sur deposit, all the mines mentioned above showed high common lead content. To try to extract in these deposit an U-Pb age, complementary analyses to quantify accurately common lead concentration are ongoing, using MC-ICPMS. At Mina Sur, U-Pb dating performed on pseudomalachite bands yields a crystallisation age of 18.4 ± 1.0 Ma. For the black chrysocolla clasts, the 206Pb/238U apparent ages are ranging from 19.7 ± 5.0 Ma down to 6.1 ± 0.3 Ma, a spreading that we interpret as the result of uranium and lead mobility linked to fluid circulation following crystallization. Isotopic analyses, i.e. Cu and O isotopes, are in progress to better constrain the source and nature of these fluids. This study demonstrates, for the first time, that supergene copper mineralization presents a chronological potential and can be dated, at least in some case, by the U-Th-Pb method. Furthermore, the age obtained on pseudomalachite indicates that Mina Sur deposition took place as early as 19 Ma, a result that is in agreement with geological constraints in the mining district and the supergene ages already known in the Atacama Desert. These promising results represent a new tool to understand the physico-chemical, climatic and geological conditions that prevailed during the formation of supergene copper deposits and a proxy for their prospection around the world and maybe date climatic variation.