The Hydrothermal Evolution of the Alvo Acai Cu (Au, Mo) Skarn Deposit, Carajas Province, Brazil

Author(s)

Fernandes, Kamila G. ; Xavier, Roberto P. ; Moreto, Carolina P. N. ; Melo, Gustavo H. C. ; Boyce, Adrian J.

• Document type: Journal article
• Source: ECONOMIC GEOLOGY; v. 118, n. 2, p. 25-pg., 2023-03-01.

Abstract

The Alvo Acai Cu (Au, Mo) skarn deposit forms part of several poorly explored copper deposits in the western sector of the Carajas province in Brazil. Here, a 2.86 Ga metasyenogranitic basement and a 2.71 Ga quartzite of the Liberdade Group are crosscut by diabase and granitic pegmatite dikes in the deposit area. The sequence of hydrothermal alteration at Alvo Aca & iacute; is marked by (1) early pervasive calcic-sodic alteration (hastingsite-albite) and (2) potassic iron alteration (biotite-almandine-grunerite) overprinted by (3) silicification (quartz), (4) prograde (grossular-hedenbergite), and (5) retrograde (actinolite-biotite-epidote) skarn stages. Late per-vasive chloritization along with epidote-calcite veinlets crosscut the previous alteration zones. The main stage of copper mineralization (I) is spatially and temporally related to the retrograde skarn alteration for which three chalcopyrite-bearing mineral assemblages are distinguished on the basis of textural relationships and mineral associations: (1) actinolite-chalcopyrite-pyrrhotite-pyrite-magnetite-molybdenite, (2) biotite-chalcopy-rite-pyrite-magnetite, and (3) epidote-pyrite-chalcopyrite-sphalerite. Minor chalcopyrite occurs along rare late -stage epidote-calcite veinlets (mineralization II). The evolution of a single hot H2O-NaCl-CaCl2-dominated magmatic fluid of moderate salinity (22.8-28.6 wt % NaCl + CaCl2 equiv) toward a cooler H2O-NaCl fluid, with likely variable amounts of FeCl2, MgCl2, and KCl of low to moderate salinity (0.1-33.2 wt % NaCl equiv). Fluid evolution as a result of progressive crystallization of the granitic pegmatite was likely the trigger for mineraliza-tion I as supported by calculated delta(OH2O)-O-18 values from retrograde quartz. The chalcopyrite and pyrite delta S-34 values (-1.5, -1.1, and -0.7 parts per thousand) point to a magmatic origin for the sulfur, which was most probably leached from sur-rounding igneous host rocks. The evolution of the Alvo Acai deposit encompasses the development of the first copper skarn mineralization recognized in the Carajas province. (AU)

• FAPESP's process: 16/13162-7 - Geochemical and isotopic evolution of Neoarchean and Paleoproterozoic copper systems of the Carajás Province, PA
• Grantee: Roberto Perez Xavier
• Support Opportunities: Regular Research Grants
• 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