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And Fe,Mg-Chlorite: a Genetic Linkage to W, (Cu, Mo) Mineralization in the Magmatic-Hydrothermal System at Borralha, Northern Portugal

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And Fe,Mg-Chlorite: a Genetic Linkage to W, (Cu, Mo) Mineralization in the Magmatic-Hydrothermal System at Borralha, Northern Portugal Mineralogical Magazine, May 2018, Vol. 82(S1), pp. S259–S279 Fe-, Fe,Mn- and Fe,Mg-chlorite: a genetic linkage to W, (Cu, Mo) mineralization in the magmatic-hydrothermal system at Borralha, northern Portugal 1,* 1 2 I. BOBOS ,F.NORONHA AND A. MATEUS 1 Instituto de Ciências da Terra – Polo Porto, Departamento de Geociências, Ambiente e Ordenamento do Território, Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre 687, 4169-007 Porto, Portugal 2 Departamento de Geologia e IDL, Faculdade de Ciências, Universidade de Lisboa, C6, Campo Grande, 1746-016 Lisboa, Portugal [Received 23 January 2017; Accepted 22 December 2017; Associate Editor: Krister Sundblad] ABSTRACT A genetic linkage between W, (Cu, Mo)-mineralization and chlorite minerals, and the discrimination of different mineralization events in the magmatic-hydrothermal system of Borralha, northern Portugal, is discussed on the basis of textural relationships, crystal chemistry and stable isotopic data obtained from chlorite. Chlorite minerals were identified in assemblages with quartz, feldspars, tungstates and sulfides. X-ray diffraction studies of selected chlorite minerals shows a trioctahedral structural type. Electron probe micro-analyses identified four different compositions and associations: (1) Fe,Mn-chlorite with scheelite I; (2) Fe-chlorite with wolframite + scheelite II ± sulfide; (3) Fe,Mg-chlorite with molybdenite + bismuthinite; 3+ 2+ and (4) Mg,Fe-chlorite with chalcopyrite. The composition of Fe-chlorite (Al3.01Fe0.25Fe7.95Mn0.26 3+ 2+ Mg0.19)11.66(Si5.44Al2.56)8O20(OH)8 corresponds to daphnite and Fe,Mn-chlorite (Al2.69Fe0.02 Fe7.54 Mn1.08Mg0.62)11.89(Si5.31Al2.68)4O20(OH)8 to a mixed composition between daphnite and amesite. 3+ 2+ The Fe,Mg-chlorite (Al2.89Fe0.24Fe6.42Mn0.21Mg2.08)11.84 (Si5.31Al2.79)8F0.31O20(OH)8 corresponds to 3+ 2+ ripidolite and Mg,Fe-chlorite (Al2.63Fe0.37Fe1.72Mn0.01Mg6.40Ca0.26)11.39(Si6.02Al1.98)8O20(OH)8 to pych- nochlorite. Chlorite geothermometry estimates a temperature for Fe,Mn-chlorite (scheelite I) from 400°C to 500°C, for Fe-chlorite (Mn-rich wolframite + scheelite II ± sulfide) from 250 to 350°C, for Fe,Mg-chlorite (Mo- mineralization) from 200°C to 250°C and for Mg,Fe-chlorite at ∼150°C. Oxygen isotopes (V-SMOW) yielded values of +3.8 (1σ) (Fe-chlorite), +6.91 (1σ) (Fe,Mn-chlorite) and +1.5 (1σ) (Fe,Mg-chlorite). The δ18 ∼ σ σ calculated OF of Fe- and Fe,Mg-chlorite is +3.75 (1 ) and +1.45 (1 ) for the mineralizing fluid, whereas for Fe,Mn-chlorite it is +8.17 (1σ). The δ18O data obtained from quartz in W- and Mo- mineralization yielded values of +12.6 and +11.4 (1σ), whereas for adularia δ18O is about +10 (1σ). These estimates allow us to conclude that the Fe,Mn-chlorite crystallized from a magmatic-hydrothermal fluid, whereas the Fe- and Fe,Mg-chlorite quartz and adularia resulted from a mixed contribution between meteoric and magmatic-hydrothermal fluid. K EYWORDS: chlorite, tungstates and sulfides, crystal *E-mail: [email protected] chemistry, geothermometry, oxygen isotopes, https://doi.org/10.1180/minmag.2017.081.104 Borralha, Portugal. This paper is part of a special issue entitled ‘Critical-metal mineralogy and ore genesis’. The Applied Mineralogy Group of the Mineralogical Society and the IMA Commission on Ore Mineralogy have contributed to the costs of Open Access publication for this paper. © The Mineralogical Society 2018. This is an Open Access article, distributed under the terms of the Creative Commons Attribution licence (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted re-use, distribution, and reproduction in any medium, provided the original work is properly cited. I. BOBOS ET AL. Introduction Ag vein-type ore deposit (Inoue et al., 2010) has been established. Beside the relative enrichment of THE metallogenic processes intimately associated Fe or Mg in the octahedral sheet in chlorite, the with a variety of late- to post-magmatic events are systematic presence of measurable amounts of Mn sometimes poorly understood in detail due to a has been tested as a new pathfinder for chlorite wide compositional diversity of the fluids involved developed during an ore-forming processes. For in metal transport and deposition, where the late instance, Fe,Mg,Mn-chlorite has been identified in silicate phases may be an indicator of a distinct Pb-Zn, Bi, Ag ore veins from the Toyoha stage of mineralization. One of the most common geothermal system, Japan (Inoue et al., 2010), late silicate phases is chlorite. It is frequently found and Fe,Mn,Mg-chlorite was recognized in the Zn- either in hydrothermal alteration haloes that host Pb, Ag vein-type ore epithermal system in NE Chile ores or accompanying the ore-forming phases. The (Chinchilla et al., 2016). chlorite represents an alteration product following The main goal of the present contribution is to the breakdown of different primary minerals as the characterize the optical, structural and crystal chem- H+ or Mg2+,Fe2+-metasomatism proceeds. These istry of chlorite minerals associated with tungstate reactions are usually related to propylitic alteration and sulfide mineralization from the W, (Cu,Mo) ore (i.e. actinolite + epidote + chlorite or epidote + deposit of Borralha (Portugal). The relationships chlorite + calcite or chlorite + calcite + hematite between the development of chlorite minerals and assemblages). The reactions are quite common different ore-forming stages as well as discrimination and well developed in a wide range of ore-forming of different mineralization events based on stable systems, including iron-oxide copper gold, por- isotopic studies of chlorite minerals are addressed. phyry Cu, Cu-Mo or Cu-Au, epithermal Au-Ag, and retrograde alteration stages of skarn-type deposits (e.g. Lowell and Guilbert, 1970; Geology Gustafson and Hunt, 1975; Meinert, 1992; Sillitoe, 2000, 2010; Cooke et al., 2014). The The Variscan orogeny is an obduction-collision chemical composition of chlorites from the distal orogenic event (e.g. Ribeiro et al. 1990, 2007; fringes of hydrothermal envelopes is distinct from Matte, 1991), where three main deformation phases that characteristic of metamorphism. (D1, D2 and D3) have been documented in The wide range of crystal chemistry and structure northern Portugal (Ribeiro, 1974; Noronha et al., displayed by the chlorite minerals group may also 1981). The maximum crustal thickening was be used as a tool to determine the direction towards achieved by the end of D1/D2 phases, whereas the mineralized area, to estimate the temperature the D3 phase (intra-Westphalian age) relates to the under which the ore process took place (e.g. final stages of the continental collision process. Cathelineau and Nieva, 1985; Walshe, 1986; Most of the granite intrusions and the associated Inoue et al., 2010) or to estimate the heat flux thermal metamorphic peak are coeval with the D3 from a magmatic-hydrothermal centre (Sillitoe, phase (e.g. Ferreira et al., 1987; Dias and Ribeiro, 2010). Recently, the relationships ‘chlorite vs. 1995; Noronha et al., 2000; Ribeiro et al., 2007). metal assemblages’ were revisited and explored to The syn- to late-orogenic magmatic activity use chlorite as a proxy for detecting porphyry ore generated voluminous granitoid batholiths, often deposits (Wilkinson et al., 2015). with contrasting compositions, and the emplace- Dispersion or remobilization of metals in long- ment was, in most cases, controlled by major lived magmatic hydrothermal systems occurs via Variscan structures. circulation of compositionally diverse fluids under The W, (Cu,Mo) deposit of Borralha is located in a relatively large range of temperatures, generating the northwestern Iberian Peninsula near the bound- useful environmental markers when ore-forming ary between the Central Iberian Zone and the minerals are deposited with other phases such as Galicia Trás-os-Montes subzone (Fig. 1), where chlorite. When precipitated directly from the different synorogenic Variscan granites intrude hydrothermal fluid, chlorite displays a chemical Palaeozoic metasedimentary rocks to form two- composition that reflects many of the constraints mica peraluminous (syn-D3) granites and biotite- imposed by the fluid chemistry during mineraliza- rich (syn-D3) granites. The ore deposit occurs at the tion. For example, a genetic linkage between Mg, contact between metasedimentary formations Fe-chlorite or Fe,Mg-chlorite and Cu-Au mineral- (Silurian?) and the syn-D3 porphyritic Borralha ization (Dora and Randive, 2015) or a Pb-Zn, Bi, biotite granite (∼315 Ma) and a syn-D3 two-mica S260 CHLORITE LINKAGE TO W-, (Cu, Mo) MINERALIZATION FIG. 1. Geological map of the northern part of Portugal showing the location of the W, (Cu,Mo) ore deposit of Borralha, Gerês Mountains (after Ferreira et al., 1987). granite (Fig. 2) assumed to be related to a concealed The Borralha ore deposit is an important post-D3 granite body (i.e. the Gerês granite). tungsten deposit of Portugal second only to Petrogenetic and petrophysical studies of the Panasqueira. It was mined from 1903 to 1985 for post-D3 Gerês granite show that the magma rose wolframite, scheelite and argentiferous chalco- relatively high in the crust and settled at upper pyrite. The mineralization occurs in quartz veins crustal levels (6–7 km), generating a temperature and in the two breccia pipes known as Santa Helena gradient which triggered contact metamorphism at and Venise. The Santa Helena breccia occurs in 200 MPa and 500–600°C which was sufficient to outcrop (Fig. 2), whereas the Venise breccia was promote a vigorous hydrothermal convection cell identified underground during mining. Rock frag- (Noronha and Ribeiro, 1983; Noronha et al., 2000). ments of variable size, cemented with mineralized S261 I. BOBOS ET AL. FIG. 2. Geological map of the Borralha region (after Noronha, 1983). coarse-grained quartz aggregates, characterize both evolution of the post-D3 Gerês granite: (1) Mo, breccias.
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