Jorge Manuel Relvas Abstracts

Modern and ancient VMS magmatic-hydrothermal systems

 

Jorge Manuel Relvas

Geology Department, Faculty of Sciences, University of Lisbon, Lisbon, Portugal

(jrelvas@fc.ul.pt)

 

There is overwhelming evidence in typical VMS systems that evolved seawater was the main ore fluid component and that ore metals and sulphur were leached from their footwall sequence. These processes account for quartz–chlorite–sericite–pyrite alteration assemblages that formed from low- to moderate-salinity fluids with δ18O < 5‰. The few deposits that are most likely to have a significant magmatic–hydrothermal component are those that deviate significantly from this norm. Studies of well-established magmatic–hydrothermal deposits such as porphyry Cu, high-sulphidation epithermal and skarn/carbonate replacement Sn deposits indicate that these deposits formed from volatile-rich magma bodies, in quite variable geotectonic settings. Over the last decades, tectonic and chemical controls on magmatic–hydrothermal ore systems have been increasingly recognised. However, criteria used either to indicate, or to exclude a magmatic–hydrothermal component in VMS systems are generally equivocal. An assessment of existing evidence suggests that a minority of VMS deposits have significant magmatic–hydrothermal contributions. These can be split into two groups: (1) Cu-Au-rich deposits characterised by advanced argillic alteration assemblages and sulphur isotope evidence of disproportionation of magmatic SO2, and (2) Sn-rich deposits in which the Sn is associated with high-temperature assemblages, cassiterite has magmatic-like trace element signatures and the Sn-bearing ore fluids were 18O-enriched. Of these, the first group is reasonably known, whereas only one example of the second group – the Neves Corvo deposit – is documented yet.

 

 

Metallogenesis at the Neves Corvo deposit, Iberian Pyrite Belt

 

 

Jorge Manuel Relvas

Geology Department, Faculty of Sciences, University of Lisbon, Lisbon, Portugal

jrelvas@fc.ul.pt

 

The Iberian Pyrite Belt (IBP) stands out amongst volcanic-hosted massive sulfide provinces as truly giant, perhaps unique in its overall content of zinc and copper, high density and huge size of its deposits (> 85 deposits; total tonnage > 1,700 Mt; 7 giant deposits > 100 Mt). Massive sulfide deposition occurred during the waning stages of felsic volcanism within a thin volcanic-sedimentary complex during Upper Devonian times. The IPB metallogenesis combine many distinctive characteristics of VMS deposits worldwide with some features akin to SEDEX deposits. Crustal thinning and magma underplating provided long-lasting high heat flow and high regional geothermal gradients, which promoted leaching reactions and, hence, the generation of metal-rich solutions in deep-seated hydrothermal aquifers. A major component of metals directly supplied by magmatic sources seems unlikely for the generality of the deposits, given the similar and regionally homogeneous radiogenic isotopic signatures and metal ratios showed by the ores and their footwall successions, combined with the shallow emplacement and dry nature of the felsic magmatism.

Neves Corvo is the richest and one of the largest deposits known to date in the IPB. The geochemistry of the Neves Corvo ores contrasts with that of typical IPB deposits. The primary copper grades and the copper ratio significantly deviate from the IPB standards. In addition, the total tin metal content (in excess of 0,3 Mt), the tin grades attained by the stringer and massive cassiterite ores (up to 60 % SnO2), and the copper-tin metal association in the massive sulphide ores are truly unique features among the IPB deposits known. A magmatic-hydrothernal model is proposed for the genesis of this particular deposit. Pb, Os and Nd isotopic signatures preserved in the stringer and massive cassiterite ores, imply external sources, which compare to those of granite-affiliated cassiterite. Alteration mineralogy and geochemistry, coupled with oxygen and hydrogen isotopes indicate that the Neves Corvo ore fluids were hotter and more acidic than typical IPB ore-forming fluids, but the overall low-sulfidation characteristics of the deposit are beyond debate. Nevertheless, unlike in typical porphyry-copper and epithermal systems, magmatic fluids associated to tin-rich granitic plutons are reduced and contain H2S as the dominant vapor species. These circumstances imply that any magmatic fluid contribution to the Neves Corvo ore-forming system is not expected to require assignment of its mineralogical record to predominant high-sulfidation characteristics.