Carbonate alteration of ophiolitic rocks in the Arabian–Nubian Shield of Egypt: sources and compositions of the carbonating fluid and implications for the formation of Au deposits

Abstract

Ultramafic portions of ophiolitic fragments in the Arabian-Nubian Shield (ANS) show pervasive carbonate alteration forming various degrees of carbonated serpentinites and listvenitic rocks. Despite the extent of the alteration, little is known about the processes that caused it, the source of the CO 2 , or the conditions of alteration. This study investigates the mineralogy, stable (O, C) and radiogenic (Sr) ) low salinity fluid, with trapping conditions of 270°C to 300°C and 0.7 to 1.1 kbar. The serpentinites are enriched in Au, As, S and other fluid-mobile elements relative to primitive and depleted mantle. The extensively carbonated Atg-serpentinites contain significantly lower concentrations of these elements than the Lz-serpentinites suggesting that they were depleted during carbonate alteration. Fluid inclusion and stable isotope compositions of Au deposits in the CED are similar to those from the carbonate veins investigated in the study and we suggest that carbonation of ANS ophiolitic rocks due to influx of mantle-derived CO 2 -bearing fluids caused break down of Au-bearing minerals such as pentlandite, releasing Au and S to the hydrothermal fluids that later formed the Audeposits. This is the first time that Au has been observed to be remobilized from rocks during the lizardite-antigorite transition.

Origin of Talc and Fe-Ti-V Mineralization in the Kletno Deposit (the Śnieżnik Massif, SW Poland)

“…The similar oxygen isotope composition of dolomite veins from Kletno and a comparable serpentinite-hosted dolomite vein from Nasławice in theŚlęża Ophiolite (δ 18 O = 13.0% ) also points to the basite/ultrabasite carbonation as a mechanism of dolomite veins from Kletno formation ( Figure 11B). In addition, the isotopic compositions of carbonated serpentinites of the CED shows a mixing trend between the depleted-mantle and sedimentary carbonates [66], thus consistent with our interpretation of rocks from the Kletno deposit origin, in which CO 2 -bearing fluids causing carbonation originated from the carbonate sediments due to the influx of the mantle-derived gabbroic magma intrusion. Although REE and trace element patterns of hornblendites and the mineralized dolomite vein differ from those of the paragneiss and mica schist ( Figure 9A), similarities in their Fe and Ti contents suggest some genetic links.…”

“…In the Kletno deposit, the Si-rich fluids interaction with serpentinites, leading to the talc schist formation, probably took place at low-pressure mid-amphibolite facies conditions, which can be inferred from the occurrence of talc-tremolite-anthophyllite paragenesis in the examined talc schist. According to Boskabadi et al [66], and references therein, the talc-tremolite-anthophyllite assemblage is stable at the low pressure (<0.2 GPa) mid-amphibolite facies, although it can also be stable due to a higher SiO 2 -H 2 O activity in the fluid, related with its carbonate-poor nature. The latter explanation is unlike in the case of the Kletno deposit, where talc schist occurs in the intimate proximity of dolomite veins and carbonate-bearing serpentinite and hornblendites, and not far from marble bodies.…”

“…The dolomite veins from Kletno (δ 18 O: 12.8% to 13.9% , and δ 13 C: −7.7% to −7.2% ) show isotopic similarities to listvenitic rocks from the Central Eastern Desert (CED) of Egypt (carbonates δ 18 O: 6.4% to 10.5% , and δ 13 C: −8.1% to −6.8% ) [66]. These listvenitic rocks have been interpreted as a product of ophiolite carbonation due to the influx of a mantle−derived CO 2 −bearing fluids [66].…”

“…The dolomite veins from Kletno (δ 18 O: 12.8% to 13.9% , and δ 13 C: −7.7% to −7.2% ) show isotopic similarities to listvenitic rocks from the Central Eastern Desert (CED) of Egypt (carbonates δ 18 O: 6.4% to 10.5% , and δ 13 C: −8.1% to −6.8% ) [66]. These listvenitic rocks have been interpreted as a product of ophiolite carbonation due to the influx of a mantle−derived CO 2 −bearing fluids [66]. The similar oxygen isotope composition of dolomite veins from Kletno and a comparable serpentinite-hosted dolomite vein from Nasławice in theŚlęża Ophiolite (δ 18 O = 13.0% ) also points to the basite/ultrabasite carbonation as a mechanism of dolomite veins from Kletno formation ( Figure 11B).…”

“…A nephrite from the Złoty Stok deposit, formed in the way of dolomitic marble replacement under the influence of granite-derived fluids, is presented after Gil et al [31]; (B) The oxygen (δ 18 O) vs. carbon (δ 13 C) isotope composition of carbonates. The barren and mineralized marbles, and skarns from the Złoty Stok deposit, are presented after Mikulski and Speczik[63], the Panzhihua Fe-Ti-V deposit (SW China) after Ganino et al[64], and carbonate veins within serpentinites of the Central Eastern Desert (Egypt) after Boskabadi et al[66].…”