Using integrated in-situ sulfide trace element geochemistry and sulfur isotopes to trace ore-forming fluids: Example from the Mina Justa IOCG deposit (southern Perú)

Abstract

The Cretaceous Mina Justa iron oxide copper-gold (IOCG) deposit in southern Perú is an important deposit in the Central Andean IOCG mineralization belt. At Mina Justa, Stage I alteration is represented by albite-actinolite, followed by Stage II K-feldspar-magnetite alteration. Stage III alteration is composed mainly of prismatic actinolite, and Stage IV of specular hematite. In-situ SIMS sulfur isotope and LA-ICP-MS trace element analyses were conducted on the major sulfide phases, i.e., pyrite from the magnetite-pyrite-K-feldspar alteration (Stage V) and chalcopyrite from the Cu mineralization (Stage VI). Results show that the δ34S values of Stage V pyrite range from −0.5 to +6.4‰, indicating that the sulfur is mainly magmatic and has possible late external fluid incursion. Co/Ni (0.1 to 209) and Se/S (0.3 to 1.4 × 10−4) ratios show clear coupling with the δ34S values, which is also indicative of external fluid incursion during Stage V pyrite formation. Two distinct fluid sources were identified for Stage V: One shows magmatic affinity with low δ34S (<+2.5‰) and Co/Ni (<1) but high Se/S (1.2 to 1.4 × 10−4), and the other shows non-magmatic fluid (e.g., basinal brine) affinity with high δ34S (>+2.5‰, up to +6.4‰) and Co/Ni (>10, up to 209) but low Se/S (as low as 0.3 × 10−4). The Stage VI chalcopyrite grains that did not replace Stage V pyrite have δ34S values clustering around +1.0‰, suggestive of a magmatic-like origin. Given the low homogenization temperature of fluid inclusions (∼88 to 220 °C, mode ∼130 °C) and Ca-rich nature of the fluids in Stage VI, the ore-forming materials may have derived from the andesitic host rocks, although derivation via mixing with magmatic fluids may have also been possible. For the Stage VI chalcopyrite grains that replaced Stage V pyrite, both their δ34S values and trace element patterns show inheritance of the replaced pyrite. In summary, Stage V ore-forming fluids was likely magmatic initially, with external fluid incursion occurring later in this stage; whereas the fluids of Cu mineralization stage may have come mainly from the external fluids that leached the andesitic wall rocks, during which the external fluids had inherited certain sulfur isotope and trace element signatures of the pyrite they replaced.