Structural, mineralogical, and paleoflow velocity constraints on Hercynian tin mineralization: the Achmmach prospect of the Moroccan Central Massif
Résumé
The Achmmach tin mineralization (NE of the
Moroccan Central Massif) is associated with tourmaline-rich
alteration halos, veins, and faults hosted in sandstones and
metapelites of the Upper Visean-Namurian. These deposits
are reported to be late Hercynian in age and related to the
emplacement of late-orogenic granite not outcropping in the
studied area. Structural and paragenetic studies of the
Achmmach tin deposit were conducted in order to establish
a general model of the mineralization. From field constraints,
the late Hercynian phase is marked by a transition from
transpression to extension with deformation conditions evolving
from ductile to brittle environments. The transpression
(horizontal shortening direction roughly trending E-W) is coeval
with the emplacement of the first tourmaline halos along
several conjugated trends (N070, N020, and N120).
Thereafter, a tourmaline-rich breccia formed in response to
the fracturing of early tourmaline-altered rocks.
Subsequently, during the extensional phase, these structures
were reactivated as normal faults and breccias, allowing the
formation of the main tin mineralization (cassiterite) associated
with a wide variety of sulfides (arsenopyrite, chalcopyrite,
sphalerite, galena, pyrrhotite, bismuthinite, pyrite, and stannite). This evolution ends with fluorite and carbonate deposition.
The hydrothermal fluid flow velocity, calculated by
applying statistical measures on the tourmaline growth bands,
varies with the lithology. Values are lower in metapelites and
higher in breccia. In the general evolution model proposed
here, tourmaline alteration makes the rock more competent,
allowing for brittle fracturing and generation of open space
where the main Sn mineralization was precipitated.