Fluid evolution in a volcanic-hosted epithermal carbonate–base metal–gold vein system: Alto de la Blenda, Farallón Negro, Argentina

Abstract

Alto de la Blenda is an ~6.6 Ma intermediate-sulphidation epithermal vein system in the Farallón Negro Volcanic Complex, which also hosts the 7.1 Ma porphyry-Cu-Au deposit of Bajo de la Alumbrera. The epithermal vein system is characterised by large extent and continuity (2km x 400m open to depth x 6m maximum width) and an average gold grade of ~ 8 g/t. The vein is best developed within an intrusion of a finegrained equigranular monzonite, interpreted as the central conduit of a stratovolcano whose extrusive activity ended prior to porphyry-Cu-Au emplacement at Bajo de la Alumbrera, which is in turn cut by minor epithermal veins. The Alto de la Blenda vein consists predominantly of variably Mn-rich carbonates and quartz, with a few percent of pyrite , sphalerite, galena and other sulphide and sulphosalt minerals. Four phases of vein opening, hydrothermal mineralisation and repeated brecciation can be correlated between different vein segments. Stages This is the Green Open Access final manuscript to the paper originally published in Mineralium Deposita (2016) 51:873-902, made available by researchcollection.ethz.ch 2 2 and contain the greatest fraction of sulphide and gold. They are separated by the emplacement of a polymictic breccia containing clasts of quartz feldspar porphyry as well as basement rocks. Fluid inclusions in quartz related to stages 2 to 4 are liquid-rich with 2-4 wt% NaCl(eq). They homogenise between 160 and 300°C, with very consistent values within each assemblage. Vapour inclusions are practically absent in the epithermal vein. Quartz fragments in the polymictic breccia contain inclusions of intermediate to vapour-like density and similar low salinity (~ 3 wt% NaCleq), besides rare brine inclusions containing halite. LA-ICP-MS analyses of epithermal inclusions indicate high concentrations of K, Fe, As, Sb, Cs, and Pb that significantly vary within and through subsequent vein stages. Careful consideration of detection limits for individual inclusions show high gold concentrations of ~0.5 to 3 ppm dissolved in the ore fluid, which contains variably high sulfur concentrations in excess over Fe and other chalcophile metals. Compositional variations are interpreted to reflect cooling and contraction of lower-density magmatic fluids at depth, like those preserved in porphyry clasts that were mechanically transported up by the polymictic breccia. Ore mineral precipitation from the magmatic fluid occurred by further cooling and possibly minor mixing with surface-derived water, leading to sulphide saturation, de-sulphidation of the magmatic fluid and consequent gold precipitation. The absence of flash boiling and/or reduction by carbonaceous host rocks has led to relatively modest but constant gold grades in the carbonatebasemetalgold veins of Alto de la Blenda.

LA-ICP-MS analysis of fluid inclusions: contamination effects challenging micro-analysis of elements close to their detection limit

“…[29][30][31][32] Gold can also be present at µg•g -1 levels in natural fluids but it is generally close to the detection limit. 3,[33][34][35][36] Even though Au is expected to be enriched in S-rich vapor 3 37 , the reduced amount of an analyte in low-density vapor inclusions produces shorter signals, which are subject to larger uncertainties regarding representative quantification by sequential sampling of the signal 18,20 . Counting statistics is the major source of uncertainty for quantification of elements close to the analytical detection limit, requiring Poisson statistics 18,38,39 rather than a Gaussian approach 11 .…”

“…Moreover, a potential bias may result in overestimation of average element concentrations in fluid inclusion assemblages, where only some inclusions provide detectable signals even if the bulk composition of all inclusions is expected to be similar. This issue has been addressed in recent critical discussions 36 , including the possibility of summation of multiple transient signals to avoid such a bias in justified cases 35 .…”

Indium-bearing paragenesis from the Nueva Esperanza and Restauradora veins, Capillitas mine, Argentina

“…Indium, among other chalcophile elements, might be sourced from a low-to intermediate-salinity fluid (e.g., Heinrich 2005), coming from an associated porphyry intrusion in the Farallón Negro Volcanic Complex (e.g., Sillitoe 1973Sillitoe , 1996Márquez-Zavalía and Heinrich 2016, and references therein).…”

Potential for Porphyry Copper Deposits in Northern Tōhoku (or the Exploration Potential for Base and Precious Metal Deposits in Japan 2020)

“…), compare well with some of the classic Neogene intermediate‐sulfidation epithermal vein systems of the world. These include veins that occur in districts with economic porphyry copper deposits, such as those in the Philippines (Mankayan district, both the Victoria veins and adjacent Far Southeast porphyry deposit; Chang et al, ; Baguio district; Cooke et al, ), Romania (Apuseni Mountains; Wallier et al, ), or Argentina (Farallón Negro Volcanic Complex, with the Farallón Negro veins and Bajo de la Alumbrera porphyry deposit; Márquez‐Zavalía & Heinrich, ). However, the world‐class intermediate‐sulfidation deposits of Mexico so far have not been found to be associated with economic porphyry copper mineralization (White et al, ).…”