A new finding of Cu-Au alloy in association with rodingite minerals in the Kaa-Khem ophiolitic belt,Tuva |
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| Authors: | V V Murzin Ch K Oydup D A Varlamov |
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| Institution: | (1) Western Transportation Institute, Montana State University, Bozeman, MT 59715, USA;(2) State Key Laboratory of Powder Metallurgy, Central South University, Changsha, Hunan, 410083, People’s Republic of China;(3) American Water Chemicals Inc, Tampa, FL 33619, USA |
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| Abstract: | A new Cu-Au alloy occurrence is located at the southeastern flank of the Malye Kopty massif of ultramafic rocks in the Vendian-Early
Cambrian Kaa-Khem ophiolitic belt. Lithic clasts with Cu-Au alloy segregations (up to 15 mm in size) intergrown with other
minerals were found in alluvium of the Kara-Oss Creek valley, which extends along the fault zone crosscutting ultramafic rocks.
Cu-Au alloy occupies the main volume of clasts and fills the network of veinlets in grained aggregates consisting of andradite
(2–18% grossular component) and diopside (X
Fe = 0.01–0.05). Cu-Au alloy contains small ingrowths of andradite (up to 43% grossular component), diopside (X
Fe = 0.14–0.19), chlorite (penninite), chalcocite that contains up to 1.5 wt % Au, Cu-bearing greenockite (6.07–13.67 wt % Cu,
0.48–1.56 wt % Zn, and 0.76–1.06 wt % Au), and magnetite. The chemical composition of Cu-Au alloy is nonuniform. The central
parts of large Cu-Au alloy segregations consist of Ag-bearing tetraauricupride (AuCu) blocks (3.2–6.4 wt % Ag). They contain
veinlet-shaped AuCu zones with 13.3–14.5 wt % Ag. The AuCu blocks are cemented by late Cu-Au alloy, whose composition is close
to auricupride (AuCu3). Taking into account the limits of component miscibility in the Au-Ag-Cu system, the temperature of the Cu-Au alloy formation
was estimated at 350–600°C. This temperature corresponds to the formation conditions of garnet-pyroxene rodingite mineral
assemblage (Plyusnina et al., 1993). The studied Cu-Au alloy samples from the Malye Kopty massif are very similar to Cu-Au
alloy minerals hosted in the Alpine-type ultramafic rocks of the Karabash massif in the southern Urals. This similarity is
confirmed by identical chemical compositions of pyroxene, garnet, and chlorite, and similar PT conditions of their formation. The data show that primary ore mineralization of gold-rodingite type occurs in the Kaa-Khem
ophiolitic belt. |
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