(Au, Ag)Te₂Minerals from Epithermal Gold Deposits in Japan

Abstract

Chemical composition and mode of occurrences of (Au, Ag)Te2 minerals such as calaverite (AuTe2), sylvanite (AuAgTe4) and krennerite ((Au, Ag)Te2) in epithermal gold telluride ores from Suzaki, Kawazu and Teine are examined. In the ores from Suzaki, (Au, Ag)Te2 minerals occur in microbands of tellurides and fine quartz. The minerals in telluride bands change from krennerite, via calaverite‐native tellurium, to sylvanite, in the order of crystallization. A sample from Kawazu contains sylvanite and native tellurium with stutzite, hessite and tetradymite in the coarser gray quartz part. The Teine sample also contains sylvanite and native tellurium with barite and quartz. The peak patterns of XRD of calaverite, krennerite and sylvanite from Suzaki are almost identical to that of JCPDS 43–1472, JCPDS 8–20 and JCPDS 9–477, respectively. The Te, Au, Cu, and Ag contents of calaverite from Suzaki range from 56.4 to 57.9 wt.%, from 41.6 to 42.6 wt.%, from 0.28 to 0.45 wt.% and from 0.14 to 0.31 wt.%, respectively, corresponding to the formula Au0.97Ag0.01Cu0.02Te2. The Te, Au, Ag, and Cu contents of krennerite from Suzaki range from 59.6 to 61.4 wt.%, from 31.3 to 33.6 wt.%, from 4.91 to 6.13 wt.% and from 0.66 to 0.80 wt.%, respectively, corresponding to the formula Au0.71Ag0.22Cu0.05Te2 with Au and Ag ranging from 0.68 to 0.74 and from 0.20 to 0.25, respectively. The Te, Au, Ag, and Cu contents of sylvanite from Suzaki range from 61.5 to 63.4 wt.%, from 24.1 to 27.4 wt.%, from 10.0 to 12.5 wt.% and from 0.00 to 0.12 wt.%, respectively. The Te, Au, Ag, and Cu contents of sylvanite from Kawazu range from 62.7 to 63.3 wt.%, from 23.5 to 24.1 wt.%, from 12.0 to 12.5 wt.% and from 0.09 to 0.16 wt.%, respectively. The Te, Au, Ag, Cu and Fe contents of sylvanite from Teine range from 61.8 to 63.5 wt.%, from 23.6 to 24.7 wt.%, from 11.9 to 13.3 wt.%, from 0.01 to 1.65 wt.% and from 0.00 to 0.02 wt.%, respectively. The average formulae of sylvanite from Suzaki, Kawazu, and Teine are expressed as Au1.06Ag0.94Cu0.02Te4, Au1.00Ag0.95Cu0.02Te4 and Au1.01Ag0.95Cu0.06Te4, respectively. Judging from the mineral assemblages of these ores and other localities, Au–Te mineralization in the Japanese Islands can be divided into four types: native gold–calaverite at Date and Agawa, krennerite(−native tellurium) at Osore‐zan and Mutsu, sylvanite–native tellurium–hessite at Teine, Kawazu, Kobetsuzawa, and Kato, and polyminerallic assemblages at Suzaki and Kushikino. The pH–Eh diagram of aqueous tellurium species and tellurium minerals at 250°C indicates that (Au, Ag)Te2 minerals in epithermal gold telluride mineralization would have been formed under middle to low Eh and acidic (to intermediate) pH conditions. It is possible that dilute tellurium‐containing fluid would scavenge dilute gold.

Distribution of gold and silver and its relation with hypogene ore minerals in the Esperanza porphyry deposit, Antofagasta Region, Chile

“…Thus, the hessite is highly likely to have an ideal composition. The coexistence of electrum (19-32 at.% Ag) and hessite with the ideal composition is conformable with the phase relations of the Au-Ag-Te system inferred on the basis of natural mineral assemblages under epithermal conditions (Afifi et al, 1988;Shiga and Urashima, 1988;Komuro and Nakata, 2017;Chao et al, 2019). While this study did not determine the Ag concentrations of the Esperanza chalcopyrite, previous studies report that natural chalcopyrite contains significant amounts of Ag: up to 2,211 ppm in the Mantos Blancos (Chile) manto-type deposit (Reich et al, 2013), up to 1,180 ppm in the Metaliferi Mountains (Romania) porphyry deposit (Cioac a et al, 2014), and between 1,414 and 2,900 ppm in the Hilton (Australia) and up to 1.24 wt.% in the Um Samiuki (Russia) massive sulfide deposits (Harris et al, 1984;Shalaby et al, 2004).…”

“…Thus, the hessite is highly likely to have an ideal composition. The coexistence of electrum (19–32 at.% Ag) and hessite with the ideal composition is conformable with the phase relations of the Au‐Ag‐Te system inferred on the basis of natural mineral assemblages under epithermal conditions (Afifi et al ., 1988; Shiga and Urashima, 1988; Komuro and Nakata, 2017; Chao et al ., 2019).…”

Crystal structure, XANES and charge distribution investigation of krennerite and sylvanite: analysis of Au—Te and Te—Te bonds in Au1–x Ag x Te2 group minerals

“…The IMA (International Mineralogical Association) officially recognizes several valid mineral species: for three of them the ratio (Au,Ag)/Te is 1 2 : calaverite (AuTe 2 ), sylvanite [(Ag,Au) 2 Te 4 ], krennerite (Au 3 AgTe 8 ); for muthmannite (AgAuTe 2 ) and empressite (AgTe) it is 1; for montbrayite (Au 2 Te 3 ) it is 2 3 ; for petzite (Ag 3 AuTe 2 ) and hessite (Ag 2 Te) it is 2; stunzite, instead, is non-stoichiometric (Ag 10.96 Te 7 ). Despite their simple chemistry, the crystal structure of these minerals shows relatively complicated relations, which have been so far only partially elucidated (Tunell & Pauling, 1952;Cabri, 1965;Bachechi, 1971;Schneider & Schulz, 1993;Dye & Smyth, 2012;Komuro & Nakata, 2016;Bindi & Keutsch, 2018). Hereafter we focus on gold-silver tellurides.…”