Mineralogy and ore fluid chemistry of the Roc Blanc Ag deposit, Jebilet Hercynian massif, Morocco
Abstract
The Roc Blanc Ag deposit is located about 20 km north of Marrakesh city (Morocco) in the Jebilet Hercynian
massif. The ore bodies consist of N-S to NE-SW quartz (±carbonates) veins hosted by the Sarhlef
marine sediments. These series, deposited in a Devonian-Carboniferous rift basin context, were
deformed during the Hercynian orogeny, and submitted to low-grade regional metamorphism.
Two major stages of fluid circulation and metal deposition are distinguished on the basis of mineralogical
and paleo-fluid studies carried out on quartz and dolomite (microthermometry, Raman spectroscopy,
LA-ICP-MS on individual inclusions, and O, H stable isotope data): (i) an early Fe-As stage,
characterized by the circulation of metamorphic aqueous-carbonic fluids, under P-T conditions lower
than 200 MPa ± 20 MPa and 400 C respectively, along N-S structures; (ii) the ore stage, characterized by
the circulation of a Na-Mg-K ± Ca high salinity brine, poor in gas but rich in metals such as Fe, Sr, Ba, Zn,
Pb, ± Cu (salinity ranging from 19.6 wt% to likely more than 30 wt% NaCl equiv.) and the deposition of a
sphalerite/dolomite-calcite assemblage; such a fluid likely evolved to a Na-K-(Ca-Mg)-Ag brine, with
significant Pb and Sb concentrations and lower Sr, Ba and Zn concentrations than in the preceding fluid
(salinity up to 19.4 wt% NaCl equiv.). The Ag content of the second mineralizing brine ranges from
0.9 mmol/kg to 9.4 mmol/kg solution (100 ppme1000 ppm), whereas the base metal brine is generally
Ag poor (up to 1.3 mmol/kg solution: 140 ppm). Dilution of the Ag brine by low salinity fluids (<6 wt%
NaCl equiv., and Th from 130 to 230 C) seems to be the main driving mechanism for the Ag ore
deposition at Roc Blanc, with a possible involvement of cooling and reduction reactions in black schists.
Base metal and Ag fluids may have circulated at average temperatures around 200 ± 30 C or slightly
higher and under hydrostatic pressures, along dominant E-W structures. The ore forming model proposed
for the Roc Blanc deposit is: (i) the penetration of sedimentary brines coming from the adjacent
basins into the basement (i.e. Hercynian formations), where they extracted Ag probably from abundant
mafic rocks; ii) the ore deposition in structural traps below the post Hercynian unconformity thanks to
brine mixing with low salinity fluids. The fluid circulation probably is related to the Atlasic rifting coeval
with the Atlantic Triassic opening. Such a model contrasts with the previous one relating the Roc Blanc to
the Hercynian granitic intrusions in the Jebilet. Ag deposition occurred during reworking of the early
structures associated with the Hercynian orogenic events and metamorphic fluid circulation which led to
the early Fe-As uneconomic stages forming the main N-S quartz veins. Similarities between The Roc
Blanc Ag deposit and the major Ag deposits from Anti-Atlas south of Morocco strongly suggest that they
resulted from a unique and large fluid circulation event and a major period of metal deposition.