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.

Late Cretaceous mantle peridotite of the Birjand ophiolite (eastern Iran) contains variably serpentinized and carbonated/listvenitized rocks. Transformation from harzburgite protolith to final listvenite (quartz + magnesite/± dolomite + relict Cr-spinel) reflects successive fluid-driven reactions, the products of which are preserved in outcrop. Transformation of harzburgite to listvenite starts with lizardite serpentinization, followed by contemporaneous carbonation and antigorite serpentinization, antigorite-talc-magnesite alteration, finally producing listvenite where alteration is most pervasive. The spectrum of listvenitic assemblages includes silicacarbonate, carbonate and silica listvenites with the latter (also known as birbirite) being the youngest, based on crosscutting relationships. The petrological observations and mineral assemblages suggest hydrothermal fluids responsible for the lizardite serpentinization had low aCO 2 , oxygen and sulfur fugacities, distinct from those causing antigorite serpentinization and carbonation/listvenitization, which had higher aCO 2 , aSiO 2 , and oxygen and sulfur fugacities. The carbonate and silica listvenite end-members indicate variations in aSiO 2 and aCO 2 of the percolating hydrothermal fluids, most likely driven by local variations in pH and temperature. Beyond the addition of H 2 O, serpentinization did not significantly redistribute major elements. Progressive infiltration of CO 2 -rich fluids and consequent carbonation segregated Mg into carbonate and Si into silica listvenites. Trace element mobility resulted in different enrichments of fluid-mobile, high field strength, and light rare earth elements in listvenites, indicating a "listvenite mobility sequence". The δ 13 C, δ 18 O and 87 Sr/ 86 Sr values of magnesite and dolomite in carbonated lithologies and veins point to sedimentary carbonate as the main C source. Fluid-mobile element (e.g., As and Sb) patterns in carbonated lithologies are consistent with contribution of subducted sediments in a forearc setting, suggesting sedimentderived fluids. Such fluids were produced by expulsion of pore fluids and release of structurally bound fluid from carbonate-bearing sediments in the Sistan Suture Zone (SsSZ) accretionary complex at shallow parts of mantle wedge. The CO 2 -bearing fluids migrated up along the slab-mantle interface and circulated through the suture zone faults to be sequestered in mantle peridotites with marked element mobility signatures.

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Nickel isotope fractionation patterns in continental ultramafic environments generally show a depletion of δ60Ni in weathered rocks and an enrichment in bedrock samples. The present study focuses on stable Ni isotope fractionation patterns in carbonate-rich, ultramafic ophiolite samples with concomitant fluids at an active serpentinization site in southwestern Turkey, with a comparison to results from an inactive serpentinization site in the Eastern Desert of Egypt with carbonate-rich samples. All solid phase data from the inactive serpentinization area are consistent with previously reported values from serpentinites, whereas the solid precipitates in the active area (SW Turkey) give values slightly heavier than previously reported data. However, the Ni isotopic signatures in the active serpentinization system likely reflect the scavenging of light Ni by iron oxide and carbonate precipitation, as has been previously demonstrated in laboratory coprecipitation experiments. It is also possible that the active system results resemble previous laboratory experimental results that show a relatively strong initial fractionation between fluids and solids, which then diminishes with time due to aging of the precipitates.

Trace element composition of sulfides and O, C, Sr and S isotopic data are assessed to constrain the evolution and potential fluid and metal sources of the Um Garayat gold deposit. Ore microscopy and BSE investigations of quartz veins show blocky arsenopyrite and pyrite replaced in part by pyrrhotite, chalcopyrite, sphalerite, galena, and gersdorffite. Free-milling gold occurs commonly in close association with the late sulfides, and along fractures in pyrite. On the other hand, recrystallized pyrite is disseminated in host metavolcaniclastic/metasedimentary rocks that commonly contain carbonaceous material. In-situ LA-ICP-MS analysis of sulfides show the recrystallized pyrite enriched in most trace elements, while blocky pyrite contains only some traces of arsenic. Detected concentrations of gold (up to 17 ppm) were only reported in arsenopyrite disseminated in quartz veins. The δ 34 S values of blocky pyrite and pyrrhotite in quartz veins define a narrow range (1.6 to 3.7 ‰), suggesting a homogenous sulfur source which is consistent with the dominantly mafic host rocks. The recrystallized pyrite has a distinctive sulfur isotope composition (δ 34 S-9.3 to-10.6 ‰), which is rather comparable to diagenetic sulfides. Hydrothermal carbonate in quartz veins and wallrock have nearly constant values of δ 18 O (10.5 to 11.9 ‰) and δ 13 C (-4.2 to-5.5‰). Based on constraints from mineral assemblages and chlorite thermometry, data of six samples indicate that carbonate precipitation occurred at ~280°C from a homogenous hydrothermal fluid with δ 18 OH2O 4.4±0.7‰ and δ 13 C =-3.7 ± 0.8 ‰. Strontium isotope values of two samples (87 Sr/ 86 Sr = 0.7024 and 0.7025) are similar to the initial 87 Sr/ 86 Sr ratios of island arc metabasalts (~710 Ma) in the South Eastern Desert. The generally homogenous sulfur, C, O, Sr isotope data are suggestive of metamorphogenic fluids, likely produced from dominantly mafic volcanic rocks at the greenschist-amphibolite facies transition.

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<p>Neoproterozoic ophiolites in the Eastern Desert (ED) of Egypt are pervasively carbonated and listvenitized. Two types of carbonation are recognized: 1) intergrown magnesite (and to lesser extent dolomite) with serpentine and talc that in cases form pure carbonate veins, and 2) cryptocrystalline magnesite veins filling the fractures crosscutting other ophiolitic host rocks. Few studies address the conditions of carbonate alteration of ultramafic rocks, especially the temperature of altering fluids. We employ clumped isotope thermometry on natural dolomite and magnesite from 17 variably carbonated ophiolitic rocks and veins in the ED. Five samples of antigorite-bearing serpentinite, talc-carbonate, and associated carbonate veins yield wide range temperatures of magnesite and dolomite between 213 to 426°C (285±73°C). These temperatures are comparable with previous fluid inclusion thermometry carried out on some of the vein samples (homogenization temperature between 225 to 383°C; Boskabadi et al. 2017). Ten samples of fully quartz-carbonate altered peridotites (i.e. listvenites) record even a wider range of clumped isotope carbonation temperatures between 90 and 452°C (227±112°C). In contrast, two samples of late-stage veins of cryptocrystalline magnesite record lower temperatures of 19 and 28°C. While the constraints on the pressure of carbonation are lacking, the wide range of temperatures for the carbonates in antigorite-bearing serpentinite, talc-carbonate, and listvenite lithologies suggest that carbonation probably occurred at variable depths, whereas the low temperature of cryptocrystalline magnesite veins points to conditions nearer the surface most likely associated with post-obduction processes. Therefore, different sources of carbon and CO<sub>2</sub>-bearing fluids should have been responsible for the formation of high- and low-temperature carbonates in the region.</p><p> </p><p>  Boskabadi et al. 2017. International Geology Review 59, 391–419.</p>