The hypogene evolution of the Spence porphyry copper deposit, northern Chile

Abstract

Porphyry Copper Deposits (PCDs) form from upper crustal H2O saturated magma systems and occur along ancient and active convergent margins. As the world’s major source of Cu and Mo, a significant source of Au, Re and minor amounts of Pd, Ag, Se, Te, Zn, Bi and Pb, understanding where these systems form is imperative to sustaining supply of these resources. The relationship between PCDs and large upper crustal intermediate to felsic plutons is well documented, however the proportion of such plutons that host PCDs is relatively small. Conversely, the relationship between PCDs and overlying magmatic systems such as volcanoes is less well understood, although it offers valuable insights into the behaviour of porphyry‐forming magmas in the upper crust and how best to explore for PCDs that are concealed at depth. In this respect PCDs may bridge the gap between the plutonic and volcanic environment and thus offer valuable scientific insights into this relatively understudied transition zone. The Palaeocene age Spence PCD in northern Chile has been mined for two decades and represents an ideal natural laboratory where the unique geological niche that PCDs occupy may be studied in detail. In this thesis, field observations and extensive study of drill‐core are combined with geochronology and geochemistry to conduct a thorough investigation into how this world‐class deposit formed during the hypogene stage of its evolution. Firstly, the deposit’s geological model is interrogated, and a revised model is proposed that better reflects the geological complexity of the deposit. In this model observable cross‐cutting relationships and features of the primary magmatic mineralogy are emphasised as they can be studied whether the rock is altered or fresh, whereas alteration represents a major challenge to whole‐rock geochemical study. The revised geological model was then implemented to map the deposit at the subsurface level through integrating drill‐core observations with 3D models of the deposit. Through this exercise a relative chronology of magmatic and hydrothermal events was proposed that was based upon unambiguous crosscutting relationships observed in drill‐core. This relative chronology demonstrated that at least three magmatic hydrothermal events had occurred during the evolution of the deposit, and that these events occurred both before and after major magmatic breccia forming events. Furthermore, this new map revealed new levels of geological complexity including previously unidentified structures and geological contacts that explained the distribution of Cu and Mo assay grades better than the previous model. The relative chronology was integrated with high‐precision geochronology to reveal a history of ore-forming magmatism lasting more than 600 kyrs that gradually migrated from the south to the north following the long axis of the deposit. the geochronology demonstrated that rather than three magmatic hydrothermal events, at least 6 had occurred across the deposit during its protracted lifespan. The geochronology was further utilised to constrain the evolution of key geochemical signatures that are indicative of PCD‐related magmatism both at Spence and within a regional dataset of plutons outcropping proximal to Spence. The results illustrated that these geochemical signatures evolved over different timescales ranging from >10 Myrs to <1Myr reflecting the range of geological processes that were occurring. Finally, a regional investigation was conducted along the Antofagasta Calama Lineament which coincides with Spence. It was revealed that magmatism at Spence and PCD‐forming magmatism more broadly had migrated steadily following the structural trend of the ACL. It is argued that this structure facilitated mineralisation in the past and may have done so during several metallogenic events.

Date of Award	26 Nov 2020
Original language	English
Awarding Institution	University of Bristol
Sponsors	BHP
Supervisor	Frances Cooper (Supervisor), Jon Blundy (Supervisor) & simon tapster (Supervisor)