Experimental study of the formation of chalcopyrite and bornite via the sulfidation of hematite: Mineral replacements with a large volume increase

Abstract

Chalcopyrite (CuFeS 2 ) and bornite (Cu 5 FeS 4 ) are the most abundant Cu-bearing minerals in hydrothermal Cu deposits, forming under a wide range of conditions from moderate-temperature sedimentary exhalative deposits to high-temperature porphyry Cu and skarn deposits. We report the hydrothermal synthesis of both chalcopyrite and bornite at 200-300 °C under hydrothermal conditions. Both minerals formed via the sulfidation of hematite in solutions containing Cu(I) (as a chloride complex) and hydrosulfide, at pH near the pK a of H 2 S(aq) over the whole temperature range. Polycrystalline chalcopyrite formed first, followed by bornite.Assuming that Fe behaves conservatively, the transformation of hematite to chalcopyrite involves a large increase in volume (~290%). The reaction proceeds both via direct replacement of the existing hematite and via overgrowth around the grain. Chemical exchanges between bulk solution and hematite are enabled by a network of micrometer-size pores. However, in some cases the chalcopyrite overgrowth develops large grain sizes with few apparent pores and in these cases fluid transport may have been via a network of fractures. Similarly to the replacement of hematite by chalcopyrite, bornite forms via the replacement of chalcopyrite. The reaction has a large positive volume (~230%), and proceeds both via chalcopyrite replacement and via overgrowth.This study shows that replacement reactions can proceed via coupled dissolution-reprecipitation even where there is a large volume increase between parent and product mineral. This study also provides further evidence about the controls of reaction pathways onto the final mineral assemblage. In this case, the host initial fluid was undersaturated with respect to Fe-bearing minerals. Upon slow release of Fe at the surface of hematite, a mineral assemblage of chalcocite, bornite, and finally chalcopyrite is expected. However, in practice chalcocite did not nucleate on the surface of hematite. Rather relatively slow nucleation of bornite enabled high concentrations of Fe to build up near the dissolving hematite, so that chalcopyrite (high-sulfidation experiments) or chalcopyrite+pyrite (low sulfidation) crystallized first.

Textural and compositional complexities resulting from coupled dissolution–reprecipitation reactions in geomaterials

“…In U-free experiments, chalcopyrite replaced hematite directly (Zhao et al 2014a). In contrast, the presence of U…”

“…7c). Li et al (2015) and Zhao et al (2014a) observed that the replacement of hematite (Fe 2 O 3 ) by chalcopyrite (CuFeS 2 ; V m ~ +196%; Table 1) results in both a porous chalcopyrite that replaces the hematite in a pseudomorphic manner, and a chalcopyrite overgrowth that grows from solution. Hence, it is clear that such a large volume increase is accommodated through the transport of species away from the replacement interface rather than via reaction-induced fracturing as has been observed in other systems exhibiting a volume increase (Jamtveit et al 2009;Plümper et al 2012;Putnis et al 2007b).…”

“…78% pseudomorphic pentlandite + 3H + + 1.875O 2(aq) ⟶ violarite + 1.5Ni2+ + 0.75Fe 2 O 3 + 1.5H 2 O S -16.63% pseudomorphic (Xia et al 2009a) cryptoperthite ⟶ 0.54Ab 96 Or 04 + 0.46Or 87 Ab 17 (patch perthite) K-Na-Si-O -0.58% pseudomorphic (Norberg et al 2013) aragonite ⟶ calcite Ca-C-O 8.12% pseudomorphic with minor overgrowth (Perdikouri et al 2013) 54% ICDR with significant overgrowth (Zhao et al 2014b) 2pyrrhotite + 14H 2 S (aq) + 7O 2(aq) ⟶ 16pyrite + 14H 2 O Fe 15.70% pseudomorphic with minor overgrowth (Qian et al 2011) 2zoisite + rutile + quartz ⟶ 3anorthite + titanite + H 2 O Ti 18.78% pseudomorphic (Cruz-Uribe et al 2014; Kapp et al 2009) 2forsterite + 3H 2 O ⟶ lizardite + brucite Si 47.83% replacement with fracture generation (Kelemen and Hirth 2012; Plümper et al 2012) magnetite + 6H 2 S (aq) + O 2(aq) ⟶ 3pyrite + 6H 2 O Fe 59.80% replacement and overgrowth(Qian et al 2010) 3% ICDR with significant overgrowth(Zhao et al 2014a) …”

Characterization of porosity in sulfide ore minerals: A USANS/SANS study

“…It is likely that the Cu-Fe sulfide minerals at Olympic Dam originally formed by the replacement of hematite, and have subsequently been remobilized (Haynes et al 1995). Recently Zhao et al (2014a) was able to form chalcopyrite and bornite by the replacement of hematite under hydrothermal conditions. Again it is a surprising feature that these nominally massive sulfide ore minerals have such a relatively high porosity.…”

“…In all cases the product mineral showed evidence of reaction generated porosity. The ore-forming reactions that have been studied include the replacement of pentlandite by violarite (Tenailleau et al 2006;Xia et al 2009Xia et al , 2008Xia et al , 2007, pyrrhotite by marcasite and pyrite (Qian et al 2011;Xia et al 2010Xia et al , 2007, hematite by chalcopyrite (Zhao et al 2014b), chalcopyrite by bornite (Zhao et al 2014a), magnetite by pyrite (Qian et al 2010) and by arsenian-pyrite (Qian et al 2013), calaverite by gold (Zhao et al 2009, and sylvanite and krennerite by gold-silver alloy (Xu et al 2013;Zhao et al 2013). In all these replacement reactions, porosity was generated in the daughter minerals regardless whether the reaction involves volume contraction or expansion.…”