A Review of the Geology and Mineralisation of the Alumbrera Porphyry Copper-Gold Deposit, Northwestern Argentina.

The Bajo de la Alumbrera porphyry copper-gold deposit is located within the northern Sierras Pampeanas in the eastern Andes Mountains of northwestern Argentina. It formed in a tectonically favourable location within a major arc-oblique wrench fault system, the Tucuman Transfer Zone. Initial andesitic volcanism deposited on crystalline Lower Palaeozoic basement, and subsequently emplaced dacitic subvolcanic stocks are directly related to eastward subduction of the Nazca oceanic plate beneath the western continental margin of South America. Structural preparation and shallowing of the angle of subduction of the Nazca plate -related to the arc-normal Juan Fernandez Ridge on that plate - probably aided the ascent of calc-alkaline oceanic arc-related magma into the Tucuman Transfer Zone.

The commencement of volcanism was approximately coincident with the shallowing of the angle of subduction beneath the northern Sierras Pampeanas. Intrusion of the mineralised porphyries was contemporaneous with the development of thick-skinned shortening which produced uplift of the Sierras de Aconquija basement block to the southeast at between 10 and 5 Ma. Structural degradation of the crystalline basement brought about by the development of a broad asthenospheric wedge above the descending Nazca plate, aided the development of block uplift caused by generally east-west shortening at that time.

Ar-Ar dating of the various mineralised intrusive phases indicates intrusive activity lasted for approximately 270 000 years. The upper age of mineralisation is interpreted to be at the onset of feldspar destructive alteration (phyllic-argillic styles) at 6.75 ± 0.09 Ma. The presence of an apparently older, unmineralised phase (the Los Amarillos Porphyry - LAP) adjacent to the mineralised porphyries suggests that the evolution from unmineralised to mineralised magma may have been quite rapid. However, due to the highly altered nature of the LAP, no clear genetic relationships have been established between these porphyries.

Mineralisation was focussed on the intrusive centre and the surrounding andesitic host rocks. Reconstruction of the original geometry of the deposit indicates that the cluster of porphyry stocks and dykes that define the intrusive centre formed a sub-circular body with a diameter of around 500 m, while the overall dimensions of the mineralised system (at greater than 0.15% Cu) was approximately 800 x 800 m. The vertical dimension of the mineralisation is less easily measured, but was probably in the range of 800 to 1000 m. Approximately 3.36 million tonnes of copper and 409 tonnes of gold were deposited within this volume.

Fluid inclusion studies indicate that deposition of mineralisation seems to have been strongly controlled by cooling of the mineralising fluids, with sulphide phases being formed as the fluid cooled below a 400 to 360°C temperature threshold.

Mineralisation was accompanied by the near simultaneous formation of quartz-magnetite and potassic alteration in the porphyries and potassic alteration of the adjacent andesites. An unmineralised propylitic halo developed in the andesites beyond the limit of potassic alteration. Feldspar destructive (phyllic ± argillic) alteration postdated the mineralisation and probably formed in response to degradation of the thermal plume and consequent induction of increasingly acidic phreatic water into convection cells adjacent to the intrusives. Feldspar destructive alteration is usually accompanied by decreasing grade, suggesting that at Alumbrera this alteration stage remobilised and removed mineralisation.

Structural rotation of the northern Sierras Pampeanas during the Upper Miocene to Lower Pliocene resulted in the reactivation of earlier structures in a transpressional regime that caused dismemberment of the porphyry mineralisation at Alumbrera. Strong alteration of the earliest of the major faults suggests that structural disruption occurred during the development of feldspar destructive alteration, and may have been responsible for the termination of this alteration event. Subsequent movement resulted in displacement of the ore blocks across both normal and reverse faults.

Active uplift of the Sierras Pampeanas and consequent rapid erosion prevented the development of significant secondary oxide or supergene mineralisation at Alumbrera.