Geochronology and geochemistry of Ediacaran volcanic rocks of the Tighardine ore deposit formation (western High Atlas, Morocco).

Abstract

The study performed on the host rocks of the Tighardine deposit allows proposing a new lithostratigraphic succession, mainly constituted of three volcanic and volcano–sedimentary units and dolomitic bed and lenses. Volcanic rocks consist mainly of basalt, andesite, and less common dacites. They are characterized by a FeO/MgO ratio range of 0.8–10.0, TiO2 contents ranging between 0.6 and 3.2 wt%, and high values of Zr/Y (7.1 to 9.9), Nb/Y (0.5 to 1.1), and Nb/La (0.7 to 1.1) ratios, providing good evidence of intracontinental tholeiitic character with a subalkaline affinity. They have no Nb anomaly and high Ti/Yb ratios (> 4000), suggesting their derivation from an enriched source and emplacement in an extensional continental setting. Fractional crystallization is the main differentiation mechanism of these rocks, and crustal contamination may have contributed to their final geochemical signature. U/Pb dating these volcanic rocks revealed ages of 603.5 ± 3.3 and 596.1 ± 3.3 Ma. These early Ediacaran ages chronologically correlate to the age of the Lower Ouarzazate Supergroup located in the Precambrian area of the Moroccan Anti-Atlas. The Tighardine formations underwent superimposed polyphase metamorphism. Mineralogical investigations show evidence of local intense thermal metamorphism and hydrothermalism evidenced by andalusite and cordierite in pelites; diopside, tremolite, and actinolite in dolomite; and by actinolite in volcanic rocks. Late brown biotite may overgrow early metamorphic minerals in all facies. Hydrothermal transformation is particularly confined in the crossings of N–S and N70 to E–W faults. The local thermal metamorphism and hydrothermalism result from a hypothetical granitic intrusion at depth. The early disseminated polymetallic mineralization (Cu–Pb–Zn) is confined in the Ediacaran volcanic and volcanosedimentary formation of Tighardine. This volcanic activity contributes to the genesis of the Tighardine ore deposit. The economic ore deposit is related to the recrystallization and brecciation of this early-disseminated polymetallic mineralization, during the Variscan event or later.