Fluid evolution of the Humedo porphyry-related gold deposit, southern Ecuador: Evidence from the boron isotope and chemical variations of tourmaline

Abstract

Coexisting porphyry Cu-Au and low- to intermediate-sulfidation epithermal Au-Ag mineral systems are well developed in the Oligocene–Miocene volcanic and porphyry rocks in the Cangrejos–Zaruma district, Ecuador, northern Andes. The Humedo deposit, with a defined gold resource of 8 t @ 5 g/t, is a recently discovered Miocene porphyry–epithermal gold deposit in the southwest Cangrejos–Zaruma district. Here the textures, geochemistry, and B isotopes of tourmaline are investigated to understand the fluid evolution of the Humedo gold deposit. Four types of tourmaline (TurA–D) associated with three hydrothermal stages have been identified at Humedo: TurA in the matrix of stage II magmatic hydrothermal breccias, TurB in pervasive phyllic altered and silicified rocks of stage III, and TurC and TurD aggregates hosted in the extensively silicified rock and/or hydrothermal breccias of stage IV. All Humedo tourmaline belongs to the alkali group, with schorlitic–dravitic compositions. The TurA is characterized by the highest Fe, Nb, W, and Sn contents, the heaviest B isotopes (δ11B = 1.4–5.6 ‰), isovalent Al–Fe substitution, and low Eu/Eu* values, suggesting that oxidized magmatic fluids dominated during stage II. The TurB grains hosted by metamorphic and intrusive host rocks have different contents of Sc, Zn, V, and Ni, indicating a local impact on compositions from the host rocks. The low values of δ11B (−9.7 to −2.2 ‰), the correlation of δ11B with Co, and the Fe–Mg substitution of TurB suggest that fluids of stage III were reducing and possibly influenced by interaction with graphite-bearing meta ultramafic–mafic rocks of the Raspas Formation and/or Palenque Unit. The TurC–D is marked by the highest Mg contents of all groups, combined with the positive correlations of δ11B with Mg and Ba, suggesting the existence of external waters that interacted with shallow metasedimentary or volcanic sedimentary rocks during stage IV. The geochemistry and isotope characteristics of tourmaline thus record changes in the ore-forming fluid compositions (magmatic vs. external) and nature (oxidized vs. reduced) caused by fluid-rock interaction with regional country rocks in the Humedo porphyry–epithermal system.