Extreme enrichment of Arsenic and Antimony during alteration of serpentinised peridotites to form listvenite-like dolomite-quartz rocks and Ni-Cr-rich jasper and quartzites in the Highland Border Complex of Scotland

Alteration of serpentinised peridotites of Highland Border Complex in Scotland took place in two steps. Listvenite-like dolomite-quartz rocks formed by addition of CaO, Sr and CO 2 at constant MgO and SiO 2 involving a mass increase of ca. 140 %. Stage-two involved the dissolution of dolomite evinced by the abundant pores and rhombohedral-shaped grains of quartz to form Cr- and Ni-rich jasper and quartzites. Formation of the jasper-quartzites involve a mass reduction of ca. 80%. The listvenite-like and jasper-quartzite rocks have enrichment in the fluid mobile elements Ba, Sr, Cs, As and Sb. The As is present in the Aluminium-Phosphate-Sulphate group of minerals formed during alteration of Cr-spinel. Cr-spinel also alters to porous hematite and ferrihydrite with patches containing up to 5.5 wt% As 2 O 3 . Enrichment of As, related to alteration of chromite, is previously unknown from natural rocks, but strongly resembles efficient methods used for remediation of this toxic element. Formation of quartzite and jasper from peridotite and their common presence as pebbles both in the Devonian Old Red conglomerates, in the Highland Border Complex and in Devonian Basins in the Scandinavian Caledonides, highlights their importance and potential for provenance- and tectono-stratigraphic correlations. Thematic collection: This article is part of the Ophiolites, melanges and blueschists collection available at: https://www.lyellcollection.org/topic/collections/ophiolites-melanges-and-blueschists Supplementary material: https://doi.org/10.6084/m9.figshare.c.6764598