Geochemical evidence for magmatic fluids in porphyry copper mineralization

Abstract

The Koloula Igneous Complex comprises 26 different intrusive phases that have been divided into two major magmatic episodes — cycle 1 intrusions (≧4.5 Ma) and cycle 2 intrusions (2.4 to 1.5 Ma). The cycle 2 intrusions are further divided into the Inamumu Zoned Pluton (IZP) which is composed of 6 concentrically disposed quartz diorite and tonalite units; and several satellite intrusions. The IZP is host to porphyry-copper mineralization, whereas the cycle 1, and cycle 2 satellite intrusions are barren. Presently exposed mineralization in the IZP (A system) represents the deeply eroded core of a porphyry copper system, where widely-spaced veinlet alteration envelopes (≦ 1 mm thick) are separated by large volumes of unaltered rock.

Compositional trends in biotites and amphiboles from both individual grains and throughout the differentiation series of the IZP, indicate fluctuating but generally increasing $f_{O_2 }$ existed through the sequence from early magmatic→ late magmatic→ early hydrothermal conditions. In amphiboles, compositional domains (Mg-rich) that are indicative of high $f_{O_2 }$ are correlated with episodes of fluid exsolution, independent evidence of which is provided by multiple generations of fluid inclusions in quartz phenocrysts. These high $f_{O_2 }$ domains in amphiboles have higher Si, Mn, and Ca contents, but are depleted in Fe, Ti, Na, K, and Cl relative to the less “oxidizing” domains. The latter elements are those that are known from veinlet alteration assemblages and fluid-inclusion evidence to have been preferentially partitioned into the co-existing fluid phase (≡“late magmatic” hydrothermal solution).

By contrast, amphiboles from barren rock types that are slightly older than, and of the same age as the IZP, exhibit restricted compositional ranges, and are more Fe-rich. Some individual grains and two cycle 2 satellite intrusions indicate Fe-enrichment during progressive crystallization. Siliceous deuteric amphiboles are commonly as Mg-rich as the high $f_{O_2 }$ amphibole domains from the IZP, but are easily distinguished from them by their lack of smooth compositional trends versus Si, and by their highly variable Mg and Fe contents.

Biotites from the IZP also indicate progressive oxidation, whereas biotites from the barren rock types show either little compositional variation or progressive Fe-enrichment. Biotites from the barren intrusions are richer in Cl, Li and Rb and poorer in Ba than those of the mineralizing intrusions. fHF was very low (∼ 0.003 bars) in both barren and mineralizing intrusions. During progressive differentiation, Rb content decreased and Ba content increased in IZP biotites, which is atypical, yet explicable owing to the former presence of a competing fluid phase during biotite crystallization.

Because $f_{O_2 }$ is a function of degree of fluid exsolution, then in igneous systems with sufficient Cu, Cl, and ultimately S, progressively higher $f_{O_2 }$ should potentially lead to more mineralized intrusions. Higher $f_{O_2 }$ is reflected by steeper ΣFe versus Si gradients in amphibole domains. Indeed, such a graph for amphiboles from 5 igneous complexes, indicates that two economically mineralized units produced steeper Σ Fe∶Si than those from weakly mineralized intrusions. Steep Σ Fe∶Si trends that do not continue to amphibole domains more siliceous than Si=7.3 (atoms per 23 oxygens) are unlikely to have resulted from subsolidus crystallization and these intrusions are unlikely to be strongly mineralized.