The Alvo Açaí Deposit: an example of a copper skarn in the Carajás Domain

Author(s)

Kamila Gomes Fernandes

• Document type: Master's Dissertation
• Press: Campinas, SP.
• Institution: Universidade Estadual de Campinas (UNICAMP). Instituto de Geociências
• Defense date: 2020-02-20

Examining board members

Roberto Perez Xavier; Lena Virginia Soares Monteiro; Vinicius Tieppo Meira

• Advisor: Roberto Perez Xavier; Carolina Penteado Natividade Moreto

Abstract

The Alvo Açaí copper deposit is located within neoarchean metasedimentary rocks of the Liberdade Group and associated mafic and felsic plutonic intrusions in the northwestern sector of the Carajás Domain. The metasedimentary lithotypes include hastingsite-bearing quartzite and almandine-grunerite-biotite schists commonly crosscut by diabase, syenogranite, and granitic pegmatite dikes. The onset of the hydrothermal system is marked by calcic-sodic alteration (hornblende-albite) followed by prograde (hedenbergite-andradite) and retrograde (actinolite-biotite-epidote) skarn stages. Pervasive chloritization and late epidote-calcite veinlets are also observed. The copper mineralization is exclusively related to the continuous development of the retrograde skarn stage. Three distinct ore assemblages are distinguished on the basis of their textural relationship with the retrograde mineral associations: (I) chalcopyrite-pyrite-pyrrhotite-magnetite-actinolite; (II) chalcopyrite-pyrite-magnetite-biotite; and (III) pyrite-chalcopyrite-sphalerite-epidote. Ore assemblage I occurs in veins, veinlets, breccias, massive ore zones and disseminated whereas ore assemblages II and III only occur disseminated. The mixing of, at least, two fluids was the main controlling mechanism of ore precipitation in the Alvo Açaí as depicted from fluid inclusion data: (1) moderate to high salinity (21 to 53% NaCleq) H2O-NaCl-CaCl2 fluid with temperatures ranging between 175 and 487 °C, probably exsolved from the crystallizing granitic pegmatite; and (2) low to moderate salinity (up to 33% NaCleq) H2O-NaCl (-KCl) fluid with temperatures lower than 267 °C, which is interpreted as externally-derived fluids of uncertain origin. ?18OH2O values (n = 3) estimated from homogenization temperature of fluid inclusions from the hedenbergite-andradite quartz vein vary between +6.27 to + 7.27¿. Chalcopyrite (n=2) and pyrite (n = 1) ?34S values (-1.5 to -0.7 ¿) plot within the range expected for sulfur of magmatic origin. Microthermometric data combined with ?18OH2O and ?34S values corroborates the hypothesis that the mineralization stage was favored by continuous mixing between magmatic fluids and meteoric or basinal waters. The Alvo Açaí shows Ca-Na alteration as commonly described for iron oxide-copper-gold (IOCG) deposits in the Carajás Domain, however the occurrence of skarn alteration with mineralization associated had not been reported yet in the Carajás Domain. The presence of a skarn-type mineral assemblage and textures as well as a spatial relationship with causative intrusions allows us to classify the Alvo Açaí as part of the subgroup of Cu (-Au) skarns that have been described for IOCG provinces worldwide in more proximal magmatic-hydrothermal settings. The presence of alkaline-metasomatism and skarn alteration at the Alvo Açaí not only demonstrates the different ramifications of IOCG provinces, but also demonstrates the economic potential of the northwestern sector of the Carajás Domain for other types of iron-oxide related deposits, such as skarns (AU)

• FAPESP's process: 18/00066-5 - Potential for clastic-dominated and skarn copper-deposits in the Carajás domain (PA): the example of the açaí and angelica deposits
• Grantee: Kamila Gomes Fernandes
• Support Opportunities: Scholarships in Brazil - Master