Cover sheet Cover sheetTitle Gold and trace element zonation in pyrite using a laser imaging technique: Implications for the timing of gold in orogenic and Carlin-style sediment-hosted deposits

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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.

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Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) analyses of 1,407 sedimentary (diagenetic and syngenetic) pyrites from 45 carbonaceous shale and unconsolidated sulfidic sediment samples, ranging in age from Paleoarchean to present day, show a considerable range of trace element compositions. Arsenic, Ni, Pb, Cu, and Co are among the most abundant trace elements, with medians ranging from 100 to 1,000 ppm. Less abundant elements Mo, Sb, Zn, and Se have median ranges of 10 to 100 ppm, and Ag, Bi, Te, Cd, and Au have median ranges of 0.01 to 10 ppm. Our dataset reveals three main groups of trace elements that are incorporated into pyrite in different ways. Group 1 elements (As, Ni, Co, Sb, Se, and Mo) are contained uniformly throughout the pyrite and may be held within the pyrite crystal structure or as nanoinclusions evenly distributed within pyrite. Group 2 elements (Bi, Pb, Ag, Au, Te, and Cu) generally occur uniformly at low concentrations and may be incorporated into the pyrite structure but are highly variable at high concentrations, where they may also occur as microinclusions. Group 3 elements (Zn and Cd) tend to have highly variable abundances and generally occur in pyrite as microinclusions of sphalerite.Factor analyses of the dataset identified five factors that account for 65.4% of the variance in pyrite trace element concentrations. Factor 1 includes Pb, Bi, Au, and Te, and explains 18.1% of the variance, possibly due to As(II) (Qian et al., 2013) or As(III) substituting for Fe in pyrite, which induces the uptake of these elements. Factor 2 includes Co, Ni, and As and accounts for 13.6% of the variance, possibly due to the presence of As(-I) substituting for S(-II) in pyrite, which, in turn, promotes the uptake of Ni and Co. Factor 3 includes Zn and Cd and explains 12.3% of the variance and is due to the presence of sphalerite inclusions. Factor 4 includes Se, Ag, and Sb and explains 11.0% of the variance, which is believed to reflect coeval input of these elements into the oceans during periods of increased oxygenation. Factor 5 includes Mn, Cu, and Mo and explains 10.4% of the variance. It is likely that this behavior is due to these elements being delivered together to the sediments by adsorbing to Mn (hydro)oxides, which are released when the Mn (hydro)oxides dissolve in reducing bottom waters or pore waters.Variations in pyrite texture do not show consistent compositional patterns between different samples, though within the same sample later formed pyrite tends to have lower trace element abundance. Many trace elements associated with mafic extrusions/circulation of fluids through mafic rocks (Ni, Co) are more enriched in Archean sedimentary pyrite at times when mafic volcanism/circulation of fluids through mafic rocks was more active. Similarly, some trace elements tend to be more enriched in Phanerozoic pyrite due to increasing levels of atmospheric oxidation.

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