@article{fdi:010089511,
  title = {{C}ombined effect of organic carbon and arsenic on the formation of sediment-hosted gold deposits : a case study of the {S}hahuindo epithermal deposit, {P}eru},
  author = {{V}allance, {J}. and {G}aldos, {R}. and {B}alboa, {M}. and {B}erna, {B}. and {C}abrera, {O}. and {H}uisa, {F}. and {B}aya, {C}. and van de {V}yver, {C}. and {V}iveen, {W}. and {B}eziat, {D}. and {S}alvi, {S}. and {B}russet, {S}. and {B}aby, {P}atrice and {P}okrovski, {G}. {S}.},
  editor = {},
  language = {{ENG}},
  abstract = {{S}ediment -hosted gold deposits represent a significant portion of the world's gold resources. {T}hey are characterized by the ubiquitous presence of organic carbon ({C}-org; or its metamorphosed product, graphite) and the systematic occurrence of invisible gold -bearing arsenian pyrite. {Y}et the role played by these features on ore formation and the distribution of gold remains a long-standing debate. {H}ere, we attempt to clarify this question via an integrated structural, mineralogical, geochemical, and modeling study of the {S}hahuindo deposit in northern {P}eru, representative of an epithermal gold deposit contained in a sedimentary basin. {T}he {S}hahuindo deposit is hosted within {L}ower {C}retaceous fluvio-deltaic carbon -bearing sandstone, siltstone, and black shale of the {M}aranon fold -and -thrust belt, where intrusions of {M}iocene age are also exposed. {T}he emplacement of the auriferous orebodies is constrained by structural (thrust faults, transverse faults) as well as lithological (intrusion contacts, permeable layers, anticlinal hinge in sandstone) features. {T}he defined gold reserves (59 tons; t) are located in the supergene zone in the form of native gold grains. {H}owever, a primary mineralization, underneath the oxidized zone, occurs in the form of invisible gold in arsenian pyrite and arsenopyrite. {H}ere, four subsequent pyrite generations were identified-namely, py{I}, py{II}, py{III}, and py{IV}. {P}y{I} has mean {A}u concentrations of 0.3 ppm, contains arsenic that is not detectable, and is enriched in {V}, {C}o, {N}i, {Z}n, {A}g, and {P}b compared to the other pyrite generations. {T}his trace element distribution suggests a diagenetic origin in an anoxic to euxinic sedimentary basin for py{I}. {P}yrite {II} and py{IV} have comparable mean {A}u (1.1 and 0.7 ppm, respectively) and {A}s (2.4 and 2.9 wt %, respectively) concentrations and precipitated under conditions evolving from lower (pyrrhotite, chalcopyrite, sphalerite) to higher (enargite, digenite, chalcocite) sulfidation, respectively. {T}he py{III} generation is the major gold event in the primary mineralization, with pyrite reaching 110 ppm {A}u (mean similar to 7 ppm) and 5.6 wt % {A}s (mean similar to 1.8 wt %), while coeval arsenopyrite attains 460 ppm {A}u. {P}yrite {III} is also enriched in other trace elements such as {S}e, {G}e, {M}o, {I}n, {G}a, and {B}i compared to the other pyrite generations, which is indicative of a magmatic source. {B}ulk analyses of the surrounding unmineralized rocks show only parts per billion levels of {A}u and less than 25 ppm {A}s. {T}hese data, combined with mass balance considerations, demonstrate that the sedimentary rocks could not be the sole source of gold, as they could only contribute a minor portion of arsenic and sulfur (and iron) to the deposit. {C}onversely, fluids exsolved from a pluton crystallizing at depth likely provided the great part of the gold endowment. {E}quilibrium thermodynamics simulations, using geochemical constraints established in this study, demonstrate that interaction between {A}u-{A}s-{S}-{F}e-bearing fluids and organic carbon -bearing rocks strongly enhanced the fluid ability to transport gold by maximizing its solubility as {A}u-{I} hydrosulfide complexes via a combined increase of p{H} and aqueous sulfide concentration. {T}his finding challenges the traditional qualitative view of organic matter acting exclusively as a reducing agent for {A}u-{I} that should promote gold deposition in its native state ({A}u-0) rather than enhance its solubility in the fluid. {O}ur results have significant implications for the exploration of carbonaceous sedimentary environments. {S}uch settings may provide a very effective mechanism for focusing gold transport. {S}ubsequent scavenging of {A}u{I} from solution in a chemically bound form is promoted by the precipitation of arsenian pyrite in permeable structural and lithologic traps, bound by more impermeable units, similar to what occurs in petroleum systems. {O}ur integrated study underlines the important potential of sedimentary {C}-org-bearing rocks in the formation and distribution of gold and associated metal resources.},
  keywords = {{PEROU}},
  booktitle = {},
  journal = {{E}conomic {G}eology},
  volume = {119},
  numero = {1},
  pages = {85--112},
  ISSN = {0361-0128},
  year = {2024},
  DOI = {10.5382/econgeo.5040},
  URL = {https://www.documentation.ird.fr/hor/fdi:010089511},
}