(Ruby-sapphire) - Chromian mica-tourmaline rocks from Westland, New Zealand

Abstract

Boulders of the assemblage ruby-sapphire corundum, chromian muscovite, margarite, tourmaline (± chromian chlorite, Zn-Mn chromite and Mn-Ti magnetite) occur in glacial moraine and rivers of north Westland, South Island of New Zealand. The location, Cr-rich composition of the boulders and the presence of rare serpentinite rinds indicate that they are derived from ultramafic rocks (Pounamu Ultramafics) that occur within Alpine Schist of the Southern Alps. The largest sample is progressively zoned outwards from a corundum-margarite core, through an intermediate zone of Cr-muscovite, to an outer zone of Cr-chlorite that is in contact with serpentinite. Most finds consist of erosion-resistant corundum-rich cores. In the corundum, Cr2O3 content ranges from 0-5 to 13%, with red coloration becoming more intense with increasing Cr. In addition to the dominant Cr3+ ⇌At3+ substitution, those of (Fe, V)3+ ⇌ Cr3+ and (Ti4+ + Fe2+) ⇌ 2Cr3+ result in spectacular colour zoning from colourless to deep ruby red-carmine and pale blue to dark blue-violet. Corundum has grown by replacement of the micaceous matrix that consists of chromian Muscovite (0 · 10-4 · 10% Cr2O3) and chromian margarite (0·46-1·20% Cr2O3). Both micas contain a significant paragonite component (up to 21·5% in muscovite and up to 40·8% in margarite). Late phase muscovite is Ba rich with up to 4·77% BaO, and margarite has up to 0·66% SrO. Tourmaline occurs as veins, vein outgrowths and larger poikilitic crystals that replace the mica matrix. Chromium content ranges between 0·82 and 3·6% Cr2O3. High bulk rock Al (up to 78% Al2O3), K, Ca, Cr and Na, and low Si (14-5-23-1% SiO2), suggest that the corundum-Cr-silicate rocks are the products of extreme metasomatic alteration of quartzofeldspathic schist enclaves in serpentinite. Isocon analysis indicates that conversion of the schist to the micaceous matrix of the corundum neks involves conservation of Ca, Al, K, volatiles and Sr, a mass loss of ∼59% and a volume reduction of ∼69% consequent on removal oj 70-80% Si and all other elements (most >80%), with enrichment of between 900 and 1800% Cr. The formation of corundum from the mica matrix involved a further mass-volume reduction and decrements in Si, Ca, K, volatiles and Sr from reaction sites. Concentric mineral zonation in single rock samples and zoning-replacement in minerals, e.g. Cr in corundum and chromite, Ti, Fe2+ in corundum, Ba in muscovite, Sr in margarite, and Mn and Zn in chromite and magnetite, imply element redistribution during metasomatism. Experimental reaction between quartzofeldspathic schist and serpentinite at 450°C and 2 kbar produced reaction sequences containing newly formed Ca-plagioclase-phlogopitic micachlorite and muscomte-chlorite that in terms of composition are analogous with the observed (corundum-margarite)-muscovite-chlorite zonation. The temperature of metamorphism of garnet zone rocks (450 ±20°C) that contain the corundum-Crsilicate rocks is well below that of the breakdown of muscovite and margarite to form corundum and indicates the importance of fluid composition, particularly the cation-hydrogen variables aCa2+/H+, aK+/H+ and aSiO2. Introduction of boron into the schist (from serpentinite), and boron released from the breakdown of original tourmaline in the schist, resulted in tourmaline veining and reaction of the mica matrix to form tourmaline that invoved both a mass and volume increase and addition of Fe, Mg together with B.