An unusual Ni-Sb-Ag-Au association of ullmannite, allargentum, Au-rich silver and Au-bearing dyscrasite from Oselské pásmo “silver” Lode of Kutná Hora Pb-Zn-Ag ore district (Czech Republic)

Abstract

A rare and unusual Ni-Sb-Ag-Au association of ullmannite, NiSbS, associated with allargentum, Ag 6 Sb, Au-rich silver, Au 9.7 Ag 85.0 Sb 2.9 , and Au-bearing dyscrasite was found in medieval mine dump material of the Oselské pásmo "silver" lode of the historic Kutná Hora Pb-Zn-Ag ore district. Ullmannite is the first and only Ni sulphide found in the base--metal paragenesis of otherwise exclusively Ag-(Cu)-(Pb)-Sb sulphosalts. No other nickel mineral is known from this base-metal ore district. The mineral was identified in two polished sections of one sample as anhedral grains up to 50 µm across enclosed in allargentum. The average chemical composition of ullmannite Ni 26.76-27.17, Sb 57.80-59.17, S 15.14-15.28 wt. % corresponds to the empirical formula Ni 0.98 Sb 1.01 As 0.01 S 1.00 on the basis of Ni + Sb + S = 3 apfu, i.e. it is close to the ideal formula with only trace contents of As. Au-rich silver with 15.52-16.34 wt. % of gold was found in association with freibergite and pyrargyrite as anhedral inclusions up to c. 5 µm across. The chemical composition corresponds to Au 9.7 Ag 85.0 Sb 2.9 on the basis of 100 apfu. One more gold-containing phase was determined: Au-bearing dyscrasite with ~0.7 wt. % of Au. The sample also produced an example of Ag-rich "bonanza" with stephanite, acanthite, pyrargyrite and miargyrite. The likely source of nickel and gold were the serpentinized ultrabasic bodies, cut by the "silver" lodes in the South of the ore district. The serpentinites contain 2000 ppm of Ni and low but stable contents of Au. Penetration of hydrothermal fluids could have caused the mobilization of nickel and gold in serpentinites and in earlier ore mineralization (arsenopyrite). This process resulted in the breakdown of earlier silver minerals while allargentum, dyscrasite and stephanite crystallized. The discovery of Ni-Sb-Ag-Au association in Kutná Hora ore district sheds new light on the elemental variability of this hydrothermal vein-type deposit of Variscan age.

The Gersdorffite-Bismuthinite-Native Gold Association and the Skarn-Porphyry Mineralization in the Kamariza Mining District, Lavrion, Greece

“…At the neighboring Imiter Ag-Hg deposit, Morocco, Essaraj et al [38] distinguished two mineralizing events: The first is related to magmatic intrusions and comprises uneconomic Co-Ni-As ores that were superimposed by an economic, Ag-rich mineralization, deposited by basinal brines, which leached the metals from the basement rocks. Similarly, the likely source of Ni and Au in the Ni-Sb-Ag-Au association, as exemplified by the presence of ullmannite, allargentum, Au-rich silver, and Au-bearing dyscrasite, from the Kutná Hora Pb-Zn-Ag ore district (Czech Republic), was the serpentinized ultramafic bodies, which were cut by the "silver" lodes in the south of the ore district [39]. The scenario of remobilization and leaching of metals from previous mineralization and/or host rocks, including the metabasite boudins found in the Lavrio schists, through the late involvement of non-magmatic fluid in the ore system, was recently favoured by Scheffer et al [40], for at least the Pb-Zn-Ag vein-style mineralization in the Lavrion district.…”

Ag-Pb-Sb Sulfosalts and Se-rich Mineralization of Anthony of Padua Mine near Poličany—Model Example of the Mineralization of Silver Lodes in the Historic Kutná Hora Ag-Pb Ore District, Czech Republic

“…Therefore, mineralogical research of this part of the ore district had to rely only on occasional finds of primary mineralization on medieval mine dumps, mostly localized in areas built up by medieval town [8], but still partly preserved south of the historic centre of the town. Known silver minerals from southern lodes of the ore district include acanthite [9,10], allargentum [9,11], diaphorite [12,13], freibergite [13], freieslebenite [14], miargyrite [13,15], owyheeite [16], proustite [13], pyragyrite [13], pyrostilpnite [17], stephanite [11], silver [9],Au-rich silver and Au-bearing dyscrasite [11].No selenium minerals have been known, no selenium contents above detection limit determined.…”

<strong>The Ni-Bi-Au association at Kamariza and &lsquo;km-3&rsquo;, Lavrion ore district, Greece</strong>

“…However, at Bou Azzer, where no Bi-bearing minerals were reported, acidic magmatic fluids under moderately reducing conditions at high fluid/rock ratios leached the elements Co, Ni, Fe, and As from serpentinites [26]. Similarly, the likely source of nickel and gold in the Ni-Sb-Ag-Au association with ullmannite, allargentum, Au-rich silver and Au-bearing dyscrasite from the Kutná Hora Pb-Zn-Ag ore district (Czech Republic), was the serpentinized ultrabasic bodies, cut by the "silver" lodes in the south of the ore district [28]. The scenario of remobilization/leaching of metals from previous mineralization and/or host rocks, through the late involvement of nonmagmatic, surface-derived fluids in the ore system was recently favored by Scheffer et al [29], and already discussed by Leleu et al [30] and Morin and Photiades [31] for the formation of the Pb-Zn-Ag breccia-style mineralization.…”