Natural fracking and the genesis of five-element veins

Abstract

Hydrothermal Ag-Co-Ni-Bi-As (five-element vein type) ore deposits show very conspicuous textures of the native elements silver, bismuth, and arsenic indicating formation from a rapid, far-from-equilibrium process. Such textures include up to dm-large tree-and wirelike aggregates overgrown by Co-Ni-Fe arsenides and mostly carbonates. Despite the historical and contemporary importance of five-element vein type deposits as sources of silver, bismuth, and cobalt, and despite of spectacular museum specimens, their process of formation is not yet understood and has been a matter of debate since centuries. We propose, based on observations from a number of classical European five-element vein deposits and carbon isotope analyses, that Bnatural fracking,^i.e., liberation of hydrocarbons or hydrocarbon-bearing fluids during break up of rocks in the vicinity of an active hydrothermal system and mixing between these hydrocarbons (e.g., methane and/or methane-bearing fluids) and a metal-rich hydrothermal fluid is responsible for ore precipitation and the formation of the unusual ore textures and assemblages. Thermodynamic and isotope mixing calculations show that the textural, chemical, and isotopic features of the investigated deposits can entirely be explained by this mechanism.

Factors Controlling Hydrothermal Nickel and Cobalt Mineralization—Some Suggestions from Historical Ore Deposits in Italy

“…The SW Sardinian and Usseglio vein systems closely conform to recent geochemical models [6,8]. The textural relationships observed in the arsenide phases, such as the depositional sequence of mono-to diarsenides (e.g., nickelite to rammelsbergite, safflorite and löllingite), the stabilization of skutterudite and the stability of native Bi, are compatible with the conditions outlined in the pH-fO2-T °C diagrams in [8].…”

“…The complex textural relationships among arsenide phases, sulfarsenides and native Bi in the SW Sardinian and Usseglio veins are consistent with observations from other five elements systems and can be explained by fluctuations in oxygen fugacity within a 4-7 pH range, as modeled in [8]. According to these Authors, a reducing fluid for deposition of the arsenide assemblages is required, which is often documented by the occurrence of carbon components, including hydrocarbons like methane, e.g., [6,7]. Although the presence of hydrocarbons has not been documented so far in the studied systems, evidences of interactions between hydrothermal fluids and carbonaceous material are given by the occurrence of black shale fragments in the brecciated ores (SW Sardinia) and by the markedly negative δ 13 C signature of the carbonate gangue (SW Sardinia and Usseglio).…”

“…In the relevant literature carbonates occur as essential gangue components in the five-element vein-type deposits and correlate with fluid inclusions carrying a wealth of carbon components including hydrocarbons like methane e.g., [6,7]. According to [6], the sources of methane may be various, although the most common sources may be carbonaceous schists and shales, which are often occurring in the sequences hosting the ore deposits. No data are, for the moment, available about possible fluid inclusion-hosted hydrocarbon components in the ore deposits presented here.…”

“…An additional indication is provided by the isotopic signatures of the ore-related carbonate gangue. The tendency towards progressively negative δ 13 C signatures is considered as reflecting a role of organic carbon deriving from interaction of the fluids with carbonaceous shales and/or from the presence of hydrocarbon components in the migrating ore brines (e.g., [1,6,7]). The same applies also to the carbonate gangue from Usseglio, that shows even more negative δ 13 C signatures than most of the SW Sardinian carbonates (Figure 16a,b).…”

“…The available, limited yet precious experimental data on Ni and Co in hydrothermal conditions, e.g., [2][3][4][5], have highlighted a different behavior of these metals in solution and the need of further research for unraveling poorly understood aspects. However, recent investigations on natural ore deposits and related modeling, e.g., [6][7][8], have provided clues about the components of the mineralizing fluids as well as on reaction mechanisms effective for metal deposition. Yet speculations still exist on various issues, including the possible mechanisms of selective enrichment of Co versus Ni and the sources of Co and Ni, which may be multiple and diverse, or exotic, even in fluid-related ore deposits hosted in mafic-ultramafic rocks, as discussed in [9].…”