Abstract
Alaskan-type complexes commonly contain primary platinum-group element (PGE) alloys and lack base-metal sulfides in their dunite and chromite-bearing rocks. They could therefore host PGE deposits with rare sulfide mineralization. A detailed scanning electron microscope investigation on dunites from the Xiadong Alaskan-type complex in the southern Central Asian Orogenic Belt revealed: various occurrences of platinum-group minerals (PGMs) that are dominated by inclusions in chromite grains containing abundant Ru, Os, S and a small amount of Pd and Te, indicating that they mainly formed prior to or simultaneously with the crystallization of the host minerals; A few Os–Ir–Rurich phases with iridium/platinum-group element (IPGE) alloy, anduoite (Ru,Ir,Ni)(As,S)2−x and irarsite (IrAsS) were observed in chromite fractures, and as laurite (RuS2) in clinopyroxene, which was likely related to late-stage hydrothermal alteration. The rocks in the Xiadong complex display large PGE variations with ∑PGE of 0.38–112 ppb. The dunite has the highest PGE concentrations (8.69–112 ppb), which is consistent with the presence of PGMs. Hornblende clinopyroxenite, hornblendite and hornblende gabbro were all depleted in PGEs, indicating that PGMs were likely already present at an early phase of magma and were mostly collected afterward in dunites during magma differentiation. Compared with the regional mafic–ultramafic intrusions in Eastern Tianshan, the Xiadong complex show overall higher average PGE concentration. This is consistent with the positive PGE anomalies revealed by regional geochemical surveys. The Xiadong complex, therefore, has potential for PGE exploration.
Highlights
Platinum-group element (PGE) mineralization in Alaskan-type complexes is invariably present in dunite units [1,2], occasionally in the vein-type settings associated with bodies of magnetite-rich clinopyroxenite in the dunite or peridotite core, in pegmatitic and micaceous rocks [3]
In order to investigate potential PGE and metal sulfide mineralization in the Xiadong complex, we present the observations of platinium-group minerals (PGMs) and sulfides as well as PGE whole-rock geochemistry of Xiadong intrusive rocks
PGMs were only found in dunite sample
Summary
Platinum-group element (PGE) mineralization in Alaskan-type complexes is invariably present in dunite units [1,2], occasionally in the vein-type settings associated with bodies of magnetite-rich clinopyroxenite in the dunite or peridotite core, in pegmatitic and micaceous rocks [3]. Platinum-rich minerals, mainly Pt–Fe alloys are thought to form in the early stages of magmatic evolution and are associated with chromite formation [6]. As the Alaskan-type complexes originated from highly oxidizing and low-sulfur magmatism [8], they commonly occur with the presence of PGE alloys and lack base-metal sulfides [8]. Sulfide mineralization coupled with chromite and/or PGE mineralization was explored in a few Alaskan-type complexes (e.g., Turnagain, British Columbia; and Duke Island, Alaska) [9,10]. Further investigation into the occurrences and geochemical histories of chromites, platinium-group minerals (PGMs) and sulfides is required in these Alaskan-type complexes
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