A Carbonaceous Sedimentary Source-Rock Model for Carlin-Type and Orogenic Gold Deposits

Abstract

This paper presents evidence and arguments that carbonaceous sedimentary rocks were a source for Au and As in sediment-hosted orogenic and Carlin-type gold deposits and develops a corresponding genetic model. In this two-stage basin-scale model, gold and arsenic are introduced early into black shale and turbidite basins during sedimentation and diagenesis (stage 1) and concentrated to ore grades by later hydrothermal, structural, or magmatic processes (stage 2). In reduced continental margin basin settings, organic matter, sedimented under anoxic to euxinic conditions, immobilizes and concentrates gold, arsenic, and a range of trace elements (particularly V, Ni, Se, Ag, Zn, Mo, Cu, U) present in marine bottom waters, into fine-grained black mudstone and siltstone of slope and basin facies. During early diagenesis, gold and certain other trace elements (Ni, Se, Te, Ag, Mo, Cu, ±PGE) are preferentially partitioned into arsenian pyrite that grows in the muds. These processes produce regionally extensive black shale and turbidite sequences enriched in syngenetic gold and arsenic, commonly from 5 to 100 ppb Au and 10 to 200 ppm As. Rare organic-and sulfide-rich metalliferous black shales may contain up to 1 to 2 ppm Au and over 1,000 ppm As, present as refractory gold in arsenian pyrite and nanoparticles of free gold. During late diagenesis and early metamorphism (stage 2) the diagenetic arsenian pyrite is recrystallized to form coarser grained pyrite generations, and the organic matter is cooked to bitumen. Under higher grade metamorphism (lower greenschist facies and above) arsenian pyrite in carbonaceous shales is converted to pyrrhotite. These processes release gold, arsenic, sulfur and other elements (Sb, Te, Cu, Zn, Mo, Bi, Tl, and Pb) from the source rocks to become concentrated by hydrothermal processes, locally to produce gold ores, in structural sites such as fold hinge zones, shear or breccia zones within or above the black shale sequence. LA-ICP-MS analyses of diagenetic pyrite in carbonaceous sediments, both associated and not associated with gold deposits, suggests that invisible gold contents of greater than 250 ppb in diagenetic pyrite, are indicative of carbonaceous shale source rocks with the potential to produce economic gold deposits. Application of this sedimentary source-rock model enables a systematic exploration approach for sediment-hosted gold deposits, based on the distribution, composition and structure of carbonaceous shale sequences and their contained diagenetic pyrite.

Sulfur radical species form gold deposits on Earth

“…S10), typical of arc magmas (2,3,29,31). In iron-rich metamorphic environments typical of orogenic gold deposits (7,8,10,(32)(33)(34)(35), the low solubility of pyrite at <500°C is the major limiting factor for S − 3 ; therefore, in such settings, S − 3 is capable of concentrating Au only at hot stages of metamorphism, between 500°C and 700°C, where S − 3 concentrations are high enough to yield Au-enriched fluids within amphibolite facies (Fig. 3B).…”

“…This gold may be subsequently remobilized by later fluids and transported as AuðHSÞ − 2 to deposition sites hosted in lower-degree metamorphic (mostly greenschist facies) or sedimentary rocks such as Carlin-type deposits. Although the models of formation of this important type of gold deposit are far from being unanimous, suggesting either deep (>10 km) metamorphic (8) or magmatic (38) sources for gold, both types of source are consistent with the enhanced Au mobilization and concentration by sulfur radical species in S-rich high-temperature fluids generated either by prograde metamorphism of Au-pyrite-bearing rocks or magma degassing at depth accompanied by vapor-liquid unmixing, and subsequent transport of Au to the deposition site at lower temperatures (<300°C) predominantly as AuðHSÞ − 2 .…”

“…the close association of gold with carboniferous rocks in some types of deposits (7,8,32). For comparison, Au solubility in the form of traditional sulfide or chloride complexes is far less sensitive to variations in oxygen fugacity (Fig.…”

“…gold | sulfur | ore deposit | hydrothermal fluid | trisulfur ion T he formation of gold deposits on Earth requires aqueous fluids that extract gold from minerals and magmas and transport and precipitate the metal as economic concentrations in ores that are three to six orders of magnitude larger than the Au mean content (∼0.001 ppm) of common crustal and mantle rocks (1)(2)(3)(4)(5)(6)(7)(8)(9). However, natural data on gold contents in fluids are very scarce due to difficulties of direct access to deep geothermal fluid samples, rarity of representative fluid inclusions trapped in minerals, and analytical limitations for this chemically most inert metal (1,4,9,10).…”

“…In between these two contrasting hydrothermal solution compositions lies a vast domain of geological fluids, which are commonly generated by magma degassing at depth (2,3,9) or prograde metamorphism of sedimentary rocks at high temperatures (7,8,10). These fluids are characterized by variable salt content, slightly acidic to neutral pH, and the presence of both oxidized (sulfate and sulfur dioxide) and reduced (H 2 S) sulfur forms in a wide temperature range (∼300-700°C).…”