Intrusion-Related Gold Deposits: new insights from gravity and hydrothermal integrated 3D modeling applied to the Tighza gold mineralization (Central Morocco)

Khalifa Eldursi (1, 2) , Yannick Branquet (1, 2) , Laurent Guillou-Frottier (1, 3, 2) , Guillaume Martelet (1, 3) , Philippe Calcagno (3)

1 ISTO - Institut des Sciences de la Terre d'Orléans - UMR7327
2 Métallogénie - UMR7327
3 BRGM - Bureau de Recherches Géologiques et Minières (BRGM)

The Tighza (or Jebel Aouam) district is one of the most important polymetallic districts in Morocco. It belongs to the Variscan Belt of Central Meseta, and includes W-Au, Pb-Zn-Ag, and Sb-Ba mineralization types that are spatially related to late-Carboniferous granitic stocks. One of the proposed hypotheses suggests that these granitic stocks are connected to a large intrusive body lying beneath them and that W-Au mineralization is directly related to this magmatism during a 287-285 Ma time span. A more recent model argues for a disconnection between the older barren outcropping magmatic stocks and a younger hidden magmatic complex responsible for the W-Au mineralization. Independently of the magmatic scenario, the W-Au mineralization is consensually recognized as of intrusion-related gold deposit (IRGD) type, W-rich. In addition to discrepancies between magmatic sceneries, the IRGD model does not account for published older age corresponding to a high-temperature hydrothermal event at ca. 291 Ma. Our study is based on gravity data inversion and hydro-thermal modeling, and aims to test this model of IRGD and its related magmatic geometries, with respect to subsurface geometries, favorable physical conditions for deposition and time record of hydrothermal processes. Combined inversion of geology and gravity data suggests that an intrusive body is rooted mainly at the Tighza fault in the north and that it spreads horizontally toward the south during a trans-tensional event (D2). Based on the numerical results, two types of mineralization can be distinguished: 1) the “Pre-Main” type appears during the emplacement of the magmatic body, and 2) the “Main” type appears during magma crystallization and the cooling phase. The time-lag between the two mineralization types depends on the cooling rate of magma. Although our numerical model of thermally-driven fluid flow around the Tighza pluton is simplified, as it does not take into account the chemical and deformation contributions, it provides evidence for abandoning the time-lag as an argument for invalidating the existence of a genetic link between older mineralizations and/or hydrothermal events spatially related to younger parent plutons.

Mots clés: hydrothermal fluid flow; rock alteration index; intrusion-related gold deposits; combined inversion; Jebel Aouam; 3D geological and gravity modeling

Domaines: Géologie appliquée; Géophysique [physics.geo-ph]

Khalifa Eldursi, Yannick Branquet, Laurent Guillou-Frottier, Guillaume Martelet, Philippe Calcagno. Intrusion-Related Gold Deposits: new insights from gravity and hydrothermal integrated 3D modeling applied to the Tighza gold mineralization (Central Morocco). Journal of African Earth Sciences, 2018, 140, pp.199-211. ⟨10.1016/j.jafrearsci.2018.01.011⟩. ⟨insu-01690381⟩