@article{fdi:010061824,
  title = {{F}ormation and deformation of pyrite and implications for gold mineralization in the {E}l {C}allao district, {V}enezuela},
  author = {{V}elasquez, {G}. and {B}eziat, {D}. and {S}alvi, {S}. and {S}iebenaller, {L}uc and {B}orisova, {A}. {Y}. and {P}okrovski, {G}. {S}. and de {P}arseval, {P}.},
  editor = {},
  language = {{ENG}},
  abstract = {{T}he {E}l {C}allao mining district is the most important gold-producing region in {V}enezuela. {I}t is hosted in the {P}aleoproterozoic {G}uasipati-{E}l {C}allao greenstone belt, which forms part of the {G}uayana craton, the {V}enezuelan extension of the {G}uiana {S}hield of {S}outh {A}merica. {I}t consists of volcanic and volcanosedimentary sequences that were affected by several deformation events, particularly localized shear zones. {T}he {C}olombia mine, the largest active mine in the district, produces 4 tonnes (t) (128,600 oz) of gold annually with reserves estimated at 740 t (24 {M}oz) and grades of up to 60 g/t. {G}old mineralization is concentrated within a vein network in the {C}olombia corridor, a shear fracture-hosted mesh of interconnected quartz-ankerite-albite veins enclosing fragments of altered metabasaltic host rocks. {G}old occurs mostly in the metabasaltic fragments and is spatially associated with pyrite, in which it occurs as invisible gold, micron-sized native gold inclusions, and filling fractures. {B}ased on optical and scanning electron microscopy-backscattered electron observations, two types of pyrite are recognized: a simple-zoned pyrite and a less common oscillatory-zoned pyrite. {B}oth types consist of a mineral inclusion-rich core and a clearer rim; however, in oscillatory-zoned pyrite, the latter is composed of complex rhythmic overgrowths of alternating {A}s-rich and {A}s-poor bands. {L}aser ablation-inductively coupled plasma-mass spectrometry ({LA}-{ICP}-{MS}) analysis and elemental mapping reveal the presence of invisible gold in all generations of pyrite. {T}he highest concentrations (5-23 ppm {A}u) are found in oscillatory-zoned pyrite rims, which correlate with the highest {A}s concentrations (16,000-23,000 ppm). {I}n {A}s-poor bands, {A}u (up to 1.5 ppm) and {A}s (300-6,000 ppm) concentrations decrease by about an order of magnitude. {C}opper, {B}i, {T}e, {S}b, {P}b, and {A}g always occur with invisible gold, particularly in pyrite cores, suggesting that at least part of the gold occurs in sulfosalt nanoparticles of these metals and metalloids. {V}isible native gold grains occur as small inclusions throughout core and rim of both pyrite types, as well as in fractures within it. {I}n both occurrences, chalcopyrite, sphalerite, tellurobismuthite, ankerite, albite, and chlorite accompany native gold, and gold fineness ranges between 900 and 930. {A}t an early stage of vein mesh formation, pyrite formed in the metabasaltic fragments at the expense of ankerite, which, in turn, resulted from alteration of {F}e-{T}i oxides. {G}old, together with other chalcoplaile elements, was incorporated within the structure of pyrite, most likely by destabilization of metal sulfide complexes during ankerite replacement. {S}ubsequent cyclic reactivations of the shear zone caused development of pressure shadows around pyrite, generating local and repeated decreases in pressure, which triggered local boiling of the hydrothermal fluid, as evidenced by the presence of primary fluid inclusions containing immiscible liquid-rich and vapor-rich aqueous-carbonic fluids. {T}his process was responsible for a number of physical-chemical changes in the liquid, all of which contributed to the formation of the {A}s- and {A}u-rich overgrowths in pyrite: (1) removal of {H}2{O} into the vapor phase, inducing saturation of dissolved metals in the remaining liquid; (2) an increase in p{H} due to partition of {H}2{S} and {CO}2 into the vapor, thus decreasing the solubility of sulfide minerals; and (3) an adiabatic decrease in temperature, lowering the solubility of {A}s and {A}u in the liquid. {W}aning of this process restored precipitation of {A}s-poor pyrite, until the onset of a new cycle. {B}ecause pressure drops are more significant adjacent to open spaces, oscillatory-zoned pyrite probably crystallized near newly formed veins whereas simple-zoned pyrite formed away from them. {P}reviously formed pyrite underwent fracturing during reactivation of the deformation, especially through the brittle deformation events that postdated shearing, resulting in local pulverization of pyrite. {T}his newly created porosity facilitated fluid circulation and remobilization of structurally bound gold, as well as of other chalcophile elements ({A}g, {C}u, {B}i, {T}e, {P}b, and {S}b), which reprecipitated together with pyrite in the form of native gold, sulfides, and tellurides, either as small inclusions or as larger grains within fractures. {T}his remobilization process facilitates the exceptionally high gold tenor found in the deposit, where the {C}olombia corridor is intersected by the {S}anta {M}aria fault.},
  keywords = {{VENEZUELA}},
  booktitle = {},
  journal = {{E}conomic {G}eology},
  volume = {109},
  numero = {2},
  pages = {457--486},
  ISSN = {0361-0128},
  year = {2014},
  DOI = {10.2113/econgeo.109.2.457},
  URL = {https://www.documentation.ird.fr/hor/fdi:010061824},
}