Formation and deformation of pyrite and implications for gold mineralization at the El Callao Mining District, Venezuelaстатья
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Аннотация:The El Callao Mining District is the most important gold producing region in
Venezuela. The district is hosted in the Paleoproterozoic Guasipati-El Callao
Greenstone Belt, which is part of the Guayana Craton, the Venezuelan extension of the
Guiana Shield of South America. It consists of volcanic and volcano-sedimentary
sequences that have undergone several deformation events, particularly, localized shear
zones. The Colombia mine, the largest active exploitation in the district, produces 4 tons
of gold annually with reserves estimated at 740 tons and grades of up to 60g/t. Here,
mineralization is concentrated within a vein network, known as the Colombia corridor,
a shear-fracture hosted mesh of interconnected quartz-ankerite-albite veins enclosing
fragments of altered basaltic host rocks. Gold is found almost exclusively in the basalt
fragments, closely associated to pyrite in which it occurs as invisible gold, as micron24
sized inclusions, and filling fractures.
The nature, texture and chemical composition of pyrite have been investigated in
detail by a variety of analytical techniques. Based on optical and SEM-BSE
observations we distinguished two types of pyrite, a simple-zoned (PySZ) and, less
common, a rhythmic-zoned pyrite (PyRZ). Both types consist of a mineral inclusion-rich
core and a clearer rim but, in PyRZ, the latter is composed of complex rhythmic
overgrowths of alternating As-rich and As-poor bands. LA-ICP-MS data and images
reveal the presence of invisible gold in all pyrite generations. The highest
concentrations (5 to 23 ppm, n = 5) are found in PyRZ rims, correlated with the highest
As concentrations (16,000 to 23,000 ppm, n = 5). In As-poor bands, gold (< 0.01 to 1.5
ppm, n = 76) and arsenic (300 to 6,000 ppm, n = 76) concentrations decrease by about 1
order of magnitude. Cu, Bi, Te, Sb, Pb and Ag are always detected with invisible gold,
particularly in pyrite cores, suggesting that, like these metals, at least part of the gold is present as sulfosalt nano-particles. Visible gold grains occur as small inclusions
throughout core and rim of both pyrite types, as well as in fractures within this sulfide.
In both occurrences, chalcopyrite, sphalerite, tellurobismuthite, ankerite, albite, and
chlorite accompany gold and gold fineness ranges between about 900 and 930.
We suggest the following scenario for pyrite formation and gold mineralization.
At an early stage of vein mesh formation, pyrite formed in the basalt fragments at the
expenses of ankerite. Invisible gold, together with other chalcophile elements,
precipitated within the structure of pyrite, most likely by destabilization of metal-sulfide
complexes during ankerite replacement. Subsequent cyclic reactivations of the shear
zone caused development of pressure shadows around pyrite, generating confined,
repeated decreases in pressure, which triggered local boiling of the hydrothermal fluid.
This process was responsible for a number of physical chemical changes in the fluid, all
of which contributed to the formation of the As- and Au-rich overgrowths in pyrite.
These were 1) removal of H2O into the vapor phase, inducing saturation of dissolved
metals in the liquid; 2) an increase in pH due to partition of H2S and CO2 into the vapor,
thus decreasing the solubility of sulfide minerals; and 3) an adiabatic decrease in
temperature, lowering the solubility of As and Au. Waning of this process restored
precipitation of regular As-poor pyrite, until the onset of a new cycle. Since pressure
drops are more significant adjacent to fluid drains, PyRZ probably crystallized near
newly formed veins whereas PySZ formed away from them.
Previously formed pyrite underwent fracturing during reactivation of the
deformation, and even more so through the brittle deformation events that postdated
shearing, resulting in local pulverization of pyrite. This new porosity facilitated
remobilization of invisible gold, as well as other chalcophile elements (Ag, Cu, Bi, Te,
Pb and Sb), which reprecipitated in pyrite in the form of free gold and sulfides, either as
small inclusions or as larger grains, within fractures. This process explains the highest
gold concentrations found in the mine, where the Colombia corridor is intersected by
the Santa Maria fault.