Fluid–rock interactions and the role of late Hercynian aplite intrusion in the genesis of the Castromil gold deposit, northern Portugal

Abstract

Castromil (northern Portugal) is one of several important orogenic gold deposits located within the “Central Iberian” geotectonic zone of northwest Iberia. The deposit occurs at the margin of a Variscan, syn- to late-D3 biotite granite, and is spatially associated with a small tourmaline aplite body that intrudes the granite at its contact with a secondary anticline of Palaeozoic arenaceous and argillaceous metasediments of the Valongo Belt. Identification of the ore fluids and their pathways, and the reconstruction of the P– T– X conditions during mineralisation were obtained by combining the geometric characteristics of veins and microstructures together with a detailed study of the inclusion fluids. Several stages of fluid percolation following contact metamorphism can be recognised. At each stage, the contact zone, characterised by intrusive aplites, related faults and fractures, appears to have focused the hydrothermal flow and acted as a structural conduit for deeper-sourced hydrothermal fluids. The earliest fluid stage (Stage I) is characterised by aqueous-carbonic fluids dominated by CO 2 and CH 4 that were probably generated by high-temperature fluid–rock interaction (400–500 °C) with graphitic schists interbedded with the metasediments. These fluids were responsible for significant alteration (greisenisation) of the aplite and its host granite, and the formation of silicified, flat lying structures that can be traced along the strike length of the deposit. At temperatures between 400 and 500 °C, fluid pressure ranges from 230 to 300 MPa, which is equivalent to a depth of 10±1.5 km. The second stage of mineralisation (Stage II: As-ore stage) is also characterised by aqueous-carbonic fluids and represents the main phase of quartz–arsenopyrite–pyrite deposition. The third stage of mineralisation (Stage III: Au-ore stage) was accompanied by intense microfracturing of the preexisting quartz veins and the preferential deposition of gold along microfractures in the sulphides. The introduction of gold corresponds to the percolation and mixing of two distinctive aqueous fluids of contrasting salinity at relatively low temperatures (150–275 °C). Based on compositional and temperature data, it is suggested that during the main phase of uplift, shallow waters penetrated deep into the basement, allowing gold to be leached from potential source rocks (most probably the Palaeozoic metasediments) and deposited in structural and geochemical traps formed during earlier stages of the hydrothermal system. The decrease in pressure during the As-ore stage corresponds to a significant tectonic uplift (around 5–6 km), and probably marks the transition from lithostatic to hydrostatic pressure conditions. Furthermore, if uplift had already been initiated during aplite emplacement, the prevailing sub-isothermal high-temperature conditions provide an explanation for the presence of decrepitated aqueous-carbonic inclusions in metamorphic quartz lenses and veins in the surrounding metasediments. To conclude, localised heat flows linked to late Hercynian magmatism at deeper structural levels appears to be the main cause of fluid circulation at Castromil. Evidence suggests that contact zones related to faulting along a secondary anticline of the Valongo Belt controlled both aplite intrusion and subsequent long-lived hydrothermal fluid circulation. The proposed genetic model differs from orogenic gold deposit models in emphasising the role of late stage aqueous fluids in the development of economic grade (10–15 g/t) gold ores.