Antimony quartz and antimony-gold quartz veins from northern Portugal

Abstract

In northern Portugal, there are two main areas containing antimony, lead-antimony and antimony-gold deposits. The antimony and lead-antimony quartz breccia veins from the Bragança district are mainly hosted by Silurian phyllites. The antimonygold quartz breccia veins from the Dú rico-Beirã region are mainly hosted by the Cambrian schist-metagraywacke complex and also Ordovician phyllites and quartzites. These veins are up to 200 m long, but their thickness ranges from a few centimeters to 3.6 m.In each deposit, some minerals show multiple paragenetic settings, suggesting that mineral precipitation is a polyphase process. Arsenopyrite, pyrite and pyrrhotite are the earliest sulfides to form. The earliest generation of euhedral gold is earlier than or contemporaneous with arsenopyrite and earlier than pyrrhotite and pyrite. Berthierite and stibnite are the most important antimony-bearing minerals. Stibnite is most abundant. Later anhedral gold grains are present within fractures between arsenopyrite and stibnite and in stibnite, which is partially replaced by the gold. Gold inclusions rarely occur in quartz. Carbonates precipitated in the last hypogene stage.The drastic fluid cooling during the mineralizing event was an important cause of mineralization. However, acidification of fluids might have been the main mechanism for stibnite precipitation, as it would have destabilized Sb(OH) 3 . Gold associated with arsenopyrite, pyrite and quartz was probably carried as a bisulfide complex by the early aqueous carbonic fluid with 2 wt% equiv. NaCl at 290-340°C, while the late generation of gold associated with berthierite and stibnite was transported as a bisulfide complex by the H 2 O-NaCl (2 wt% equiv.) fluid at 128-225°C.Variscan folding, long deep faults, shear zones, and deformation in the Bragança district and Dú rico-Beirã region would have provided anomalous crustal heat flow to sustain the extensive and long-lived hydrothermal activity. The metamorphic fluids derived from the country metasediments and heated meteoric water, which also leached metals from these metasediments, were responsible for the origin of the mineralized veins. The lead isotopic data for stibnite indicate a homogeneous source of lead of crustal origin, from a dominant metasedimentary source, for the studied antimony, lead-antimony and antimony-gold quartz veins. This is consistent with related European mineralizations.Previously, hydrothermal alteration halos around Archaean orogenic gold deposits have been documented at the metre to 100 m scale centred around the gold lode. The preferred model for this spatial relationship and the available fluid inclusion and stable isotope data was a one fluid model with gold deposition largely caused by fluid-wallrock reactions and phase separation. Camp-scale studies in the St. Ives gold camp and at the Kanowna Belle and Wallaby deposits in Western Australia have documented camp-scale hydrothermal alteration cells which are asymetric to gold.Several hundred metres to kilometre scale magnetite-pyrite or ...

Metallogenesis at the Terramonte Pb–Zn–Ag quartz vein, Portugal: Geological, mineralogical and geochemical evidences

“…4d) suggests that Sb substitutes for As as found by Cathelineau et al (1988), Couto et al (1990) and Neiva et al (2008). Both As and Sb likely replace S in pyrite as the inverse correlations of As vs. S and Sb vs. S occur (Fig.…”

“…However, the first stage is smaller at the Terramonte Pb-Zn-Ag quartz vein, whereas at the Ribeiro da Igreja Sb-Au deposit carbonates occur in the third stage and there is a fourth Sb-stage, indicating that their late hydrothermal fluids are richer in Sb than those from Terramonte. The Pb isotopic compositions of stibnite from the Sb-Au deposits suggest a homogeneous crustal source of lead from the metasedimentary sequences (Neiva et al, 2008).…”

“…The Pb-Zn-Ag quartz vein from Terramonte is located in the Dúrico-Beirã region, at southeast of Sb-Au quartz veins from this region which were studied by Couto et al (1990) and Neiva et al (2008) (Fig. 2a).…”

As–Sb–Bi–Au mineralization in the Baogutu gold deposit, Xinjiang, NW China

“…Native arsenic has also been discovered in brecciated marbles in Carinthia, Austria, where the mineralization predominantly consists of native arsenic and realgar, but no gold mineralization is present (G d and Zamann, 2000). Native antimony commonly occurs in antimony deposits and Sb-enriched gold deposits (Normand et al, 1996;Kontak et al, 1996;Bellot et al, 2003;Neiva et al, 2008). Compared to native arsenic and antimony, native bismuth is more commonly seen in various gold or non-gold deposits (Skirrow and Walshe, 2002;T rm nen and Koski, 2005;Oberthur and Weiser, 2008).…”

“…However, gold mineralization is more commonly associated with stibnite rather than native antimony in antimony-gold deposits (Kontak et al, 1996;Neiva et al, 2008), and with As-rich pyrite or arsenopyrite rather than native arsenic in As-rich gold deposits (Cline, 2001;Tomkins et al, 2004). The photomicrographs provided by Tomkins et al (2004) demonstrated that native arsenic was later than native gold in paragenesis.…”