Origin and geochemistry of the Chapada Cu-Au deposit, Goias, Brazil; a metamorphosed wall-rock porphyry copper deposit

Abstract

Chapada is a large (134 X 10 6 metric tons), low-grade (0.44% Cu, 0.35 g/metric tons Au), disseminated chalcopyrite-pyrite deposit within a geologically complex Pan-African mobile belt in central Goias state, Brazil. Mineralization is hosted largely by subhorizontal biotite schist, with lesser amphibolite, muscovite schist, and feldspathic quartzite. A foliated diorite is separated from the deposit by a shallow-diping 40-m-thick mylonite zone. Discontinuous lenses of mica-amphibole schists within the deposit are considered hydrothermally altered equivalents of this diorite which have intruded geochemically related wall rocks such as the locally porphyritic amphibolite seen in the deposit.Previous assignment of a mid-Proterozoic age to Chapada must be revised in view of our Rb/Sr whole-rock isochron, which yields a Pan-African age of 561 + or - 9 m.y. for the host schists. Rare earth element patterns of Chapada rocks have steep negative slopes with no Eu anomalies. Petrochemical data and an 87 Sr/ 86 Sr initial (I) value of 0.70414 + or - 0.00005 indicate Chapada host schists and diorite are similar to peraluminous calc-alkaline andesites of magmatic-arc affiliation. Mineralization occurred prior to upper amphibolite-grade regional metamorphism at 532 + or - 1 m.y., as indicated by an Rb/Sr mineral isochron as well as by sulfides paralleling foliation, sulfide inclusions in metamorphic minerals, and the presence of gahnite and zincian staurolite. Estimated conditions of peak metamorphism were: 650 degrees + or - 20 degrees C, 7.5 + or - 1 kb, log f (sub O 2 ) = -13 to -14, log f (sub S 2 ) = -0.5 to 0.5, and a (sub H 2 O) > 0.85. Mineral zonations in and around Chapada include a central magnetite-rich zone, a pyritic shell, and peripheral pyrrhotite and sphene. These mineral zonations and X Fe of biotites reflect a gross zonation in oxygen and sulfur fugacities from high within the deposit to relatively low in peripheral rocks. Pyrite and chalcopyrite have delta 34 S values of -0.4 to +0.6 per mil. K 2 O is enriched and CaO and Na 2 O are depleted in the host schists relative to the unaltered diorite. Within the deposit, several major element abundances describe a symmetrical pattern about the center of the deposit.The tonnage, grade, and ore mineralogy of the Chapada deposit are comparable to those reported from porphyry copper deposits of island-arc settings. Much of the gangue mineralogy at Chapada is also typical of porphyry deposits and has survived metamorphism, whereas the more aluminous original gangue minerals are now represented by such metamorphic minerals as kyanite and staurolite. These similarities, combined with the textural, zoning, and geochemical characteristics of the host rocks, suggest that Chapada is a metamorphosed porphyry copper deposit, which hosts mineralization in both wall rocks and intrusive rocks.