Exotic Cu-mineralization in Triassic red beds from Navas de San Juan (Jaén, Spain)

Abstract

The Navas de San Juan copper deposit is located at the southeast edge of the Iberian massif (Spain). It occurs within Triassic continental red beds that represent an undeformed, horizontal to sub-horizontal sequence, overlying unconformably folded Paleozoic metasedimentary rocks and, locally, granitic rocks. The red beds consist essentially of siliciclastic detrital materials including sandstone, silt, and clay, with some thin layers of micritic limestones and edaphic carbonates. This sequence was formed in a sedimentary framework typical of a low sinuosity fluvial system of braided type. The copper mineralization is hosted by a fine-grained sandstone (arkose) with abundant plant debris and detrital grains cemented by dolomite, which was deposited in a secondary fluvial channel. The copper mineralization includes chrysocolla, azurite and malachite with a stratabound morphology. Chrysocolla occurs unevenly in the sandy levels of the mineralized horizon replacing feldspar grains, coating detrital grains and dolomitic cement, and filling pores and cavities. Copper carbonates appear essentially inside or near the organic matter-rich zones filling cavities and veinlets. They also replace more or less limonitized plant debris preserving the original cellular structure of organic material. Azurite and malachite occur in different layers or zones without any evidence of reaction between them. The δC-values of azurite and malachite are indicative of a carbon source derived from the decomposition of C plant debris. The δ O values of the solutions in equilibrium with copper carbonates are consistent with solutions of meteoric origin. The differences between the values for azurite and malachite would reflect variations of the climatic conditions, probably related to seasonal changes in the hydrological parameters and arid degree of the system. Chrysocolla formation was due to the neutralization of copper solutions during the hydrolysis and replacement of the feldspars, with additional precipitation of chrysocolla in interstitial spaces, cavities and microfractures. Copper carbonates derived from the interaction between the neutralized copper solutions and the CO released by the decomposition of organic matter under oxidizing conditions. Based on geological, mineralogical and petrographic criteria, in conjunction with isotopic data, the Navas de San Juan deposit is interpreted to represent an exotic copper mineralization. The Cu-bearing vein mineralizations located in the Variscan basement are inferred as the possible copper source.