Fluid evolution of the Humedo porphyry-related gold deposit, southern Ecuador: Evidence from the boron isotope and chemical variations of tourmaline
Fluid evolution of the Humedo porphyry-related gold deposit, southern Ecuador: Evidence from the boron isotope and chemical variations of tourmaline
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厄瓜多尔南部 Humedo 斑岩相关金矿床的流体演化：来自硼同位素和电气石化学变化的证据
DOI：
10.1016/j.oregeorev.2020.103894
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发表时间：
2021
影响因子：
3.3
通讯作者：
Lai Jianqing
中科院分区：
文献类型：
--
作者：
Wang Rongchao;Liu Zhankun;Hollings Pete;Zhou Xin;Guo Yongchao;Li Bin;Yu Miao;Yang Bin;Mao Xiancheng;Lai Jianqing
厄瓜多尔北部安第斯山脉沧热霍斯-扎鲁马地区渐新世-中新世火山岩和斑岩中发育着斑岩共存的铜-金和低-中硫化浅成热液金-银矿物系统。Humedo金矿床是新近在沧热霍斯-扎鲁马地区西南部发现的一个新发现的中新世斑岩-浅成热液金矿床，确定金矿资源量为8g/t@5g/t。本文研究了电气石的结构、地球化学和B同位素，以了解呼梅多金矿床的流体演化。在Humedo发现了与3个热液阶段有关的4种电气石(Tura-D)：II期岩浆热液角砾岩基质中的TurA、III期Turbin普遍存在的叶状蚀变硅化岩和IV期广泛硅化岩石和/或热液角砾岩中的TurC和TurD集合体。吐拉岩石具有最高的Fe、Nb、W、Sn含量，最重的B同位素(δ11B=11.4~5.6‰)，Al-Fe等价替代，Eu/Eu*值低，表明第二阶段以氧化岩浆流体为主。变质岩和侵入岩中的TurB颗粒具有不同的Sc、Zn、V和Ni含量，表明局部影响了寄主岩石的成分。δ11B的低值(−9.7~−2.2‰)、δ11B与Co的相关性以及TurB的铁镁替代表明，Ⅲ期流体正在减少，并可能受到拉斯帕斯组和/或帕伦克单元中含石墨的超镁铁质岩石相互作用的影响。电气石的镁含量是所有岩类中最高的，δ11B与镁、钡呈正相关，表明在IV期存在与浅层变质沉积或火山沉积岩相互作用的外部水。电气石的地球化学和同位素特征记录了胡梅多斑岩-浅成热液系统中流体-岩石相互作用引起的成矿流体成分(岩浆与外部)和性质(氧化与还原)的变化。
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: TurAin the matrix of stage II magmatic hydrothermal breccias, TurBin pervasive phyllic altered and silicified rocks of stage III, and TurCand TurDaggregates 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 TurAis 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 TurBgrains 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 ofδ11B (−9.7 to −2.2 ‰), the correlation ofδ11B with Co, and the Fe–Mg substitution of TurBsuggest 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–Dis marked by the highest Mg contents of all groups, combined with the positive correlations ofδ11B 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.