Abstract
The sulphide ores from the Julcani mining district (Peru) display many
retrograde reactions that may be attributed to solid-state processes accompanying
cooling. Fahlores
[˜(Cu,Ag)10(Zn,Fe)2(Sb,As)4S13]
from the Herminia mine exhibit pronounced downstream enrichments in molar
Ag/(Ag+Cu) ratios that are strongly correlated with the abundance of
PbS-AgSbS2-AgBiS2 phases. These
correlations, discontinuous core to rim Sb/(Sb+As) enrichments in bournonites, and
prominent reaction textures involving fahlores, bournonites and galenas provide
strong evidence that the fahlores in these ores have been enriched in Ag by the
Ag–Cu exchange reaction
which occurred during cooling following mineralization of a
PbS-AgSbS2-AgBiS2 galena and has been
documented elsewhere. Secondary
PbS-AgSbS2-AgBiS2 minerals
aramayoite, bismuthian diaphorite
[Pb2Ag3(Bi,Sb)3S8],
and diaphorite were produced from primary galenas with cooling of ores with high
Pb/Cu and Bi/Sb; pyrargyrite formed from the breakdown of the
Ag10Zn2Sb4S13
component in the most Ag-rich fahlores, as an exsolution product of galena, and
from replacement of aramayoite and galena with the evolution of semimetal
sulphides. Based on mineral compositions, phase equilibria, a thermochemical
database for sulphides and sulphosalts, and the reintegrated composition for
primary grains of Ag-rich
PbS-AgSbS2-AgBiS2 phases, we estimate
that the primary temperature of hydrothermal mineralization was >320±10°C, that
these reactions ceased to affect fahlore Ag/(Ag+Cu) ratios and Bi/(Bi+Sb) ratios
of aramayoite and miargyrite after cooling through 220±10°C. Galenas, however,
appear to have continued to adjust their compositions to reflect even lower
temperatures by continued exsolution of semimetals and production a diverse suite
of sulphosalts that occur in fine intergrowths with galena.
Thermodynamic modeling of phase equilibria in the system Na2O – CaO – Al2O3
