First in situ U-Pb dating of supergene copper mineralization: case study from Atacama desert, Chile

International audienceDirect dating of copper mineralization remains challenging, yet an important objective for both metallogenic studies and prospecting purposes. Here, we present a first attempt of U-Pb dating of copper-rich minerals from the Mina Sur exotic deposit from the Chuquicamata copper mine (Chile).The Mina Sur exotic copper deposit is located in northern Chile, within the Chuquicamata mining district, about 6 km south of the world class Chuquicamata porphyry copper deposit. It was discovered in 1957 from drilling in sub-alluvial channels and copper extraction began in 1969. The copper mineralization consists mainly of chrysocolla, Mn-rich chrysocolla (also known as copper-pitch or black chrysocolla), pseudomalachite bedding and atacamite emplaced mainly as cement in gravels but also filling fractures in the bedrock. Geomorphological, mineralogical and structural studies (Mortimer et al., 1977; Munchmeyer, 1996) have shown that the Mina Sur deposit is derived from the lateral migration of copper-rich solutions from the Chuquicamata porphyry deposit. The latter was formed by the 33-31 Ma old hydrothermal alteration (Ar/Ar dating of K-feldspar and sericite; Reynolds et al., 1998) of the 35-33 Ma old porphyry (U/Pb dating on zircon; Ballard et al., 2001). However, the age of the Mina Sur exotic deposit is unknown.A suite of Mn-rich chrysocolla clasts surrounded by pseudomalachite layers have been characterized and dated by in-situ U-Th-Pb LA-ICP-MS analyses. The data obtained on pseudomalachite define a concordant date of 19.45 0.21 Ma. In the Mn-rich chrysocolla clasts, the data yield concordant to sub-concordant dates between 12 and 7-6 Ma. These dates are therefore not consistent with the date obtained on the pseudomalachite layers that coat them. This could reflect either Pb loss linked to late fluids that affected specifically these clasts or an analytical bias due to the fact that there is no chrysocolla standard available for in-situ U-Pb dating. Assuming a recent lead loss, the chrysocolla data yield an intercept date of 23 ± 15 Ma, which is compatible with the younger U-Pb date obtained on the pseudomalachite.This preliminary study demonstrates, for the first time, that supergene copper mineralization can be dated by the U-Th-Pb method. Furthermore, the age obtained on pseudomalachite indicate that Mina Sur deposition took place at ca. 20 Ma, about 10 Ma after the unroofing and hydrothermal alteration at Chuquicamata, a result that is consistent with the supergene ages already known supergene in the Atacama Desert

Integrated study of supergene copper deposits from Atacama Desert, Northern Chile: coupled petro-geochemical approach and U-Pb LA-ICP-MS in situ dating

International audienceSupergene 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

First U-Pb LA-ICP-MS in situ dating of supergene copper mineralization: Case study in Chuquicamata mining district, Atacama Desert, Chile

International audienc

First in situ U-Pb dating of supergene copper mineralization: case study from Atacama Desert, Chile

International audienceDirect dating of copper mineralization remains challenging, yet an important objective for both metallogenic studies and prospectingpurposes. Here we present a first attempt of U-Pb dating of copper-rich minerals from the Mina Sur exotic deposit from theChuquicamata copper mine (Chile). The Mina Sur exotic copper deposit is located in northern Chile, within the Chuquicamatamining district about 6 km south of the world class Chuquicamata porphyry copper deposit. It was discovered in 1957, fromdrilling in sub-alluvial channels and copper extraction began in 1969 (Mortimer et al., 1977). The copper mineralization consistsmainly of chrysocolla, Mn-rich chrysocolla (also known as copper-pitch or black chrysocolla), and pseudomalachite bedding andatacamite emplaced mainly as cement in gravels but also filling fractures in the bedrock. Geomorphological, mineralogical andstructural studies, have shown that the Mina Sur deposit is derived from the lateral migration of copper-rich solutions from theChuquicamata porphyry deposit. The latter was formed by the 33-31 Ma old hydrothermal alteration (Ar-Ar dating of K-feldsparand sericite) of the 35-33 Ma old porphyry (U-Pb dating). However, the age of the Mina Sur exotic deposit is unknown. A suite ofMn-rich chrysocolla clasts surrounded by pseudomalachite layers have been characterized and dated by in-situ U-Th-Pb LAICP-MS analyses. The data obtained on pseudomalachite define a concordant date of 19.45 ± 0.21 Ma. In the Mn-rich chrysocollaclasts, the data yield concordant to sub-concordant dates between 12 and 7-6 Ma. These dates are therefore not consistent withthe date obtained on the pseudomalachite layers that coat them. This could reflect either Pb loss linked to late fluids that affectedspecifically these clasts or to an analytical bias due to the fact that there is no chrysocolla standard available for in-situ U-Pbdating. Assuming a recent lead loss, the chrysocolla data yield an upper intercept date of 23 ± 15 Ma, which is compatible withthe younger U-Pb date obtained on the pseudomalachite. This preliminary study demonstrates, for the first time, that supergenecopper mineralization can be dated by the U-Th-Pb method. Furthermore, the age obtained on pseudomalachite indicate that MinaSur deposition took place at ca. 20 Ma, about 10 Ma after the unroofing and hydrothermal alteration at Chuquicamata, a result thatis consistent with the supergene ages already known in the Atacama Desert

Rodrigo Riquelme

International audienceSince the second half of the 20 th century, exotic copper mineralization represents a prime target for many of the mining exploration companies operating in the hyperarid Atacama Desert, in northern Chile. Although there is evidence that the emplacement of such deposits took place during specific Tertiary climatic periods and relief formation, many uncertainties remain regarding the exact timing for their deposition and/or the genetic link between the exotic deposits and the primary porphyry copper deposits. We present a first attempt of U-Pb dating of copper-rich minerals from the Mina Sur exotic deposit from the Chuquicamata mining district. A suite of Mn-rich black chrysocolla clasts surrounded by pseudomalachite layers has been characterized and dated in context using both nanosecond and femtosecond in-situ laser ablation ICP-MS analyses. U-Pb dating performed on pseudomalachite beds yields a crystallisation age of 18.4 ± 1.5 Ma. For the Mn-rich chrysocollas clasts, the 206 Pb/ 238 U apparent ages range from 17.5 ± 0.2 Ma down to 5.8 ± 0.1 Ma, a spreading interpreted as the result of U and/or Pb mobility linked to fluid circulation following crystallization. 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 at ca. 19 Ma, about 12 Ma after the unroofing and hydrothermal alteration of the Chuquicamata deposit, a result that is consistent with the supergene ages already known in the Atacama Desert. These promising results represent a new tool for the understanding of the physico-chemical, climatic and geological conditions that prevailing during the formation of exotic deposits and a proxy for their exploration in Chile and elsewhere in the World

First U-Pb LA-ICP-MS in situ dating of supergene copper mineralization: case study in the Chuquicamata mining district, Chile

International audienc

Dating technics applied to supergene copper deposits

International audienceConstraining the timing of geological events is a key issue in many Earth Sciences related studies. Over the last 15 years, technicalimprovement associated with the development of data modeling provided new perspectives due to greater analytical precisions,the ability to measure low contents and better interpretation of complex histories by combining different dating technics. In theAtacama Desert, supergene processes formed world-class ore deposits that are mined because of their high Cu-contents. Thesupergene mineralization are mainly found closely to porphyry-Cu deposits or in a distal location as exotic Cu-deposits. The mainproblem with most of these exotic deposits is that they are often buried under sediments and therefore hard to find. The formationof such deposits is proposed to be related to specific climatic periods and relief conditions. However, many uncertainties remainregarding the emplacement ages of these deposits. These data are of prime importance for the exploration of new deposits andto constrain the paleoenvironmental conditions needed for their formation. Therefore, we have carried out a study integratingthermochronology (fission tracks and (U-Th)/He) and geochronology (LA-ICP-MS U-Pb). We applied thermochronology onporphyry-Cu deposits and modeled the data set to constrain their exhumation history. Our results, compared to previouslypublished supergene oxide mineralization dating, indicate that supergene mineralization occurred ~10 Ma after the main periodof porphyry-Cu exhumation during a stage of slow landscape denudation. This appears to be a required condition for efficientsupergene Cu-enrichment in arid context. However, this study rose the question about the accuracy of supergene Cu-enrichmentdating previously defined using K-Ar and Ar-Ar dating on associated minerals (not directly on the mineralization itself). Therefore,we attempted to date directly Cu ores (pseudomalachite and Mn-rich chrysocolla from Chuquicamata) using the in-situ U-PbLA-ICP-MS method. The data obtained define a concordant date at about 20 Ma, coherent with supergene ages already knownin the Atacama Desert, and demonstrating, for the first time, that supergene Cu mineralization can be dated by the U-Pb method.Our studies demonstrate that the combination of different dating technics and modelling is a powerful tool that can contribute topropose a metallogenical model for the formation of supergene exotic Cu mineralization