Sediment-hosted gold deposits represent a significant portion of the world’s gold resources. They 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. Yet the role played by these features on ore formation and the distribution of gold remains a long-standing debate. Here, we attempt to clarify this question via an integrated structural, mineralogical, geochemical, and modeling study of the Shahuindo deposit in northern Peru, representative of an epithermal gold deposit contained in a sedimentary basin. The Shahuindo deposit is hosted within Lower Cretaceous fluvio-deltaic carbon-bearing sandstone, siltstone, and black shale of the Marañón fold-and-thrust belt, where intrusions of Miocene age are also exposed. The 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. The defined gold reserves (59 tons; t) are located in the supergene zone in the form of native gold grains. However, a primary mineralization, underneath the oxidized zone, occurs in the form of invisible gold in arsenian pyrite and arsenopyrite. Here, four subsequent pyrite generations were identified—namely, pyI, pyII, pyIII, and pyIV. PyI has mean Au concentrations of 0.3 ppm, contains arsenic that is not detectable, and is enriched in V, Co, Ni, Zn, Ag, and Pb compared to the other pyrite generations. This trace element distribution suggests a diagenetic origin in an anoxic to euxinic sedimentary basin for pyI. Pyrite II and pyIV have comparable mean Au (1.1 and 0.7 ppm, respectively) and As (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. The pyIII generation is the major gold event in the primary mineralization, with pyrite reaching 110 ppm Au (mean ~7 ppm) and 5.6 wt % As (mean ~1.8 wt %), while coeval arsenopyrite attains 460 ppm Au. Pyrite III is also enriched in other trace elements such as Se, Ge, Mo, In, Ga, and Bi compared to the other pyrite generations, which is indicative of a magmatic source. Bulk analyses of the surrounding unmineralized rocks show only parts per billion levels of Au and less than 25 ppm As. These 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. Conversely, fluids exsolved from a pluton crystallizing at depth likely provided the great part of the gold endowment. Equilibrium thermodynamics simulations, using geochemical constraints established in this study, demonstrate that interaction between Au-As-S-Fe–bearing fluids and organic carbon-bearing rocks strongly enhanced the fluid ability to transport gold by maximizing its solubility as Au^I hydrosulfide complexes via a combined increase of pH and aqueous sulfide concentration. This finding challenges the traditional qualitative view of organic matter acting exclusively as a reducing agent for Au^I that should promote gold deposition in its native state (Au⁰) rather than enhance its solubility in the fluid. Our results have significant implications for the exploration of carbonaceous sedimentary environments. Such settings may provide a very effective mechanism for focusing gold transport. Subsequent scavenging of Au^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. Our integrated study underlines the important potential of sedimentary C_org-bearing rocks in the formation and distribution of gold and associated metal resources.

中文翻译：

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有机碳和砷对沉积物金矿形成的综合影响：以秘鲁 Shahuindo 浅成热液矿床为例

沉积物金矿床占世界黄金资源的很大一部分。它们的特点是普遍存在有机碳（C _org;或其变质产物石墨）以及看不见的含金砷黄铁矿的系统赋存。然而，这些特征对矿石形成和黄金分布所起的作用仍然是一个长期存在的争论。在这里，我们试图通过对秘鲁北部 Shahuindo 矿床（沉积盆地中的浅成热液金矿床的代表）进行综合结构、矿物学、地球化学和建模研究来澄清这个问题。 Shahuindo 矿床赋存于 Marañón 褶皱冲断带的下白垩统河流三角洲含碳砂岩、粉砂岩和黑色页岩内，其中还暴露出中新世时期的侵入岩。金矿体的就位受到结构（逆冲断层、横向断层）和岩性（侵入接触面、渗透层、砂岩背斜枢纽）特征的限制。确定的黄金储量（59吨；t）以天然金粒的形式位于表生带。然而，氧化带下方的初级矿化以砷黄铁矿和毒砂中看不见的金的形式出现。在这里，确定了随后的四代黄铁矿，即 pyI、pyII、pyIII 和 pyIV。 PyI 的平均金浓度为 0.3 ppm，含有检测不到的砷，与其他黄铁矿世代相比，富含 V、Co、Ni、Zn、Ag 和 Pb。这种微量元素分布表明 pI 的成岩起源于缺氧至缺氧沉积盆地。黄铁矿 II 和 pyIV 具有相当的平均 Au（分别为 1.1 和 0.7 ppm）和 As（分别为 2.4 和 2.9 wt%）浓度，并在从较低（磁黄铁矿、黄铜矿、闪锌矿）到较高（硫黄铁矿、闪锌矿、辉铜矿）的条件下沉淀）分别进行硫化。 pyIII 生成是原生矿化中的主要金事件，黄铁矿达到 110 ppm Au（平均约为 7 ppm）和 5.6 wt% As（平均约为 1.8 wt%），而同时代的毒砂达到 460 ppm Au。与其他黄铁矿世代相比，黄铁矿 III 还富含其他微量元素，如 Se、Ge、Mo、In、Ga 和 Bi，这表明其为岩浆来源。对周围未矿化岩石的大量分析表明，金的含量仅为十亿分之几，砷的含量低于 25 ppm。这些数据与质量平衡考虑相结合，表明沉积岩不可能是金的唯一来源，因为它们只能为矿床贡献一小部分砷和硫（和铁）。相反，从深部结晶的岩体中溶出的流体可能提供了大部分黄金资源。使用本研究中建立的地球化学约束进行的平衡热力学模拟表明，含 Au-As-S-Fe 的流体与含有机碳的岩石之间的相互作用通过最大化其作为 Au ^I的溶解度，强烈增强了流体传输金的能力。通过 pH 值和硫化物水溶液浓度的联合增加，形成氢硫化物络合物。这一发现挑战了传统的定性观点，即有机物仅充当 Au ^I的还原剂，应促进金在其自然状态 (Au ⁰ ) 的沉积，而不是增强其在流体中的溶解度。我们的结果对碳质沉积环境的勘探具有重要意义。这样的设置可以为集中黄金运输提供非常有效的机制。随后，砷黄铁矿在可渗透的构造和岩性圈闭中的沉淀促进了以化学结合形式从溶液中清除Au ^{I ，这些圈闭由更多的不可渗透单元结合，类似于石油系统中发生的情况。我们的综合研究强调了含碳}_有机质沉积岩在金及伴生金属资源形成和分布中的重要潜力。