Diversity of platinum-group mineral assemblages in banded and podiform chromitite from the Kraubath ultramafic massif, Austria: evidence for an ophiolitic transition zone?

Abstract

We report highly unusual platinum-group mineral (PGM) assemblages from geologically distinct chromitites (banded and podiform) of the Kraubath massif, the largest dismembered mantle relict in the Eastern Alps. The banded chromitite has a pronounced enrichment of Pt and Pd relative to the more refractory platinum-group elements (PGEs) of the IPGE group (Os, Ir, Ru), similar to crustal sections of ophiolites. On the contrary, the podiform chromitite displays a negatively sloping chondrite-normalised PGE pattern typical of ophiolitic podiform chromitite. The chemical composition of chromite varies from Cr# 73-77 in the banded type to 81-86 in the podiform chromitite. Thirteen different PGMs and one gold-rich mineral are first observed in the banded chromitite. The dominant PGM is sperrylite (53% of all PGMs), which occurs in polyphase assemblages with an unnamed Pt-base metal (BM) alloy and Pd-rich minerals such as stibiopalladinite, mayakite, mertieite II, unnamed Pd-Rh-As and Pd(Pt)-(As,Sb) minerals. This banded type also contains PGE sulphides (about 7%) represented by a wide compositional range of the laurite-erlichmanite series and irarsite (8%). Os-Ir alloy, geversite, an unnamed Pt-PdBi-Cu phase and tetrauricupride are present in minor amounts. By contrast, the podiform chromitite, which yielded 21 different PGMs, is dominated by laurite (43% of all PGMs) which occurs in complex polyphase assemblages with PGE alloys (Ir-Os, Os-Ir, Pt-Fe), PGE sulphides (kashinite, bowieite, cuproiridsite, cuprorhodsite, unnamed (Fe,Cu)(Ir,Rh) 2 S 4 , braggite, unnamed BM-Ir and BM-Rh sulphides) and Pd telluride (keithconnite). A variety of PGE sulpharsenides (33%) including irarsite, hollingworthite, platarsite, ruarsite and a number of intermediate species have been identified, whereas sperrylite and stibiopalladinite are subordinate (2%). The occurrence of such a wide variety of PGMs from only two, 2.5-kg chromitite samples is highly unusual for an ophiolitic environment. Our novel sample treatment allowed to identify primary PGM assemblages containing all six PGEs in both laurite-dominated podiform chromitite as well as in uncommon sperrylite-dominated banded chromitite. We suggest that the geologically, geochemically and mineralogically distinct banded chromitite from Kraubath characterises the transition zone of an ophiolite, closely above the mantle section hosting podiform chromitite, rather than being representative of the crustal cumulate pile.

Chromian Spinel Composition and Platinum-Group Element Mineralogy of Chromitites From the Milia Area, Pindos Ophiolite Complex, Greece

“…The majority of ophiolitic chromitites worldwide is characterized by the coexistence of alloys and sulfides of laurite type. Such occurrences include the Vourinos chromitites, Greece and the Tiébaghi ophiolite, New Caledonia (Augé 1985(Augé , 1988, Thetford Mines, Quebec, Canada (Corrivaux & Laflamme 1990), Samar ophiolite, Philippines (Nakagawa & Franco 1997), Oman ophiolite (Ahmed & Arai 2003), Kraubath ultramafic massif, Austria (Malitch et al 2003), Ortaca ultramafic massif, Turkey (Uysal et al 2005), Mayarí-Baracoa ophiolitic belt, eastern Cuba (Gervilla et al 2005) and the Kop ultramafic complex, northeastern Turkey (Uysal et al 2007).…”

Solid Inclusions in Au-nuggets, genesis and derivation from alkaline rocks of the Guli Massif, Northern Siberia

“…with varying Au:Ag ratios) with oriented exsolution lamellae of tetra-auricupride and an outer pure gold rim, quite characteristic of Guli nuggets [3], [4]. Cu and [7]. The maximum Cu-content detected in the nuggets in this study is 4wt.% in Au-Ag-Cu composite grains and up to 3.26wt.% in single grains.…”

“…The Guli placer-forming sediments are related to the Ingarinda, Saby, Gule and Selinga rivers, and their tributaries. Recent and Upper Quaternary alluvial, fluviatile and terrace beds are productive for platinum group minerals (PGM) and native gold [2][3][4][5][6][7]. The precious metal nuggets are particularly located between unconsolidated sediments and bedrock.…”

“…The precious metal nuggets are particularly located between unconsolidated sediments and bedrock. It is suggested that the placer deposits were formed during unroofing and disintegration of rocks of the Guli massif after Early Cretaceous times [5][6][7].…”

Investigation of Platinum-Group Minerals (PGM) from Othrys Chromitites (Greece) Using Superpanning Concentrates

“…Moreover, application of PGE contents should always be used with caution, as they do not always reflect the PGM paragenesis in the host formations [27]. Previous studies of concentrates from Pindos and Vourinos chromitites reported a larger number of PGM grains, and a greater range of compositions, including some species that had not been detected from in situ observations, as well as a greater variation of their grain size relative to conventional studies in polished sections [13,14,17].…”