Copper–arsenic decoupling in an active geothermal system: A link between pyrite and fluid composition

Abstract

Please cite this article as: Tardani, D., Reich, M., Deditius, A.P., Chryssoulis, S., Sánchez-Alfaro, P., Wrage, J., Roberts, M.P., Copper-arsenic decoupling in an active geothermal system: a link between pyrite and fluid composition, Geochimica et Cosmochimica Acta (2017), doi: http://dx.doi.org/10. 1016/j.gca.2017.01.044 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. and propylitic (650 to 1000 m) alteration zones of the geothermal system. We show that 43 the concentrations of precious metals (e.g., Au, Ag), metalloids (e.g., As, Sb, Se, Te), 44 and base and heavy metals (e.g., Cu, Co, Ni, Pb) in pyrite at the TGS are significant. Final (revised) version submitted to Geochimica 45Among the elements analyzed, As and Cu are the most abundant with concentrations 46 that vary from sub-ppm levels to a few wt. % (i.e., up to ~5 wt. % As, ~1.5 wt. % Cu). 47Detailed wavelength-dispersive spectrometry (WDS) X-ray maps and SIMS depth vs.

Distinguishing Ore Deposit Type and Barren Sedimentary Pyrite Using Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry Trace Element Data and Statistical Analysis of Large Data Sets

“…Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) allows for the determination of the trace element content of individual minerals. These data are useful because different ore deposit types have different fluid sources, metal sources, and depositional mechanisms, all of which can significantly affect the trace element content of the minerals that precipitate from them (Gregory et al, 2014;Tardani et al, 2017). Furthermore, these trace elements can be preserved in their mineral hosts during successive hydrothermal and metamorphic events.…”

The Arsenic Fault-Pathfinder: A Complementary Tool to Improve Structural Models in Mining

“…Particularly, for porphyry systems, the geochemical behavior of arsenic has been recently investigated, showing that arsenic is geochemically decoupled from copper (Cu). In these hydrothermal systems [19,20], decoupling between copper and arsenic is due to changes in the composition of the hydrothermal fluid [20,21]. Copper and As selective geochemical partitioning [19][20][21][22] is considered the result of a fluid-phase separation process (or boiling), in which a single-phase ore-fluid is separated into a low-density vapor and denser brine phase ( Figure 1).…”

“…In these hydrothermal systems [19,20], decoupling between copper and arsenic is due to changes in the composition of the hydrothermal fluid [20,21]. Copper and As selective geochemical partitioning [19][20][21][22] is considered the result of a fluid-phase separation process (or boiling), in which a single-phase ore-fluid is separated into a low-density vapor and denser brine phase ( Figure 1). In this fluid separation, arsenic is preferentially concentrated into the low-density-low-salinity vapor phase, while copper is partitioned preferentially into the high-density saline brine fluid phase [23,24].…”

“…This fluid-phase separation takes place at the brittle-to-ductile transition (BDT) depths for porphyry systems (~1-3 km) [24,25], which is defined as the boundary-zone between the lithostatically pressured up-flow of hot magmatic fluids and the hydrostatically pressured convection of cooler meteoric fluids [25]. However, only the As-rich vapor phase is considered to be able to migrate upwards in the system through the permeable fracture network [21] developed in the upper crust ( Figure 1). Interestingly, the As-rich vapor fluid phase migration can trigger fluid overpressure conditions during its ascent, resulting in fracturing in the shallowest parts of the hydrothermal system [21], and this process can be accompanied by a preferential precipitation of As-bearing minerals in the superficial fracture network (Figure 1).…”

“…However, only the As-rich vapor phase is considered to be able to migrate upwards in the system through the permeable fracture network [21] developed in the upper crust ( Figure 1). Interestingly, the As-rich vapor fluid phase migration can trigger fluid overpressure conditions during its ascent, resulting in fracturing in the shallowest parts of the hydrothermal system [21], and this process can be accompanied by a preferential precipitation of As-bearing minerals in the superficial fracture network (Figure 1). The copper-arsenic geochemical decoupling implies that Cu-bearing sulfides preferentially precipitate in the deeper part of the porphyry system, while As-bearing sulfides would precipitate in shallower parts, frequently filling the permeable fracture network (Figure 1), accompanied by a minor proportion of Cu-bearing sulfides [19][20][21].…”