The unusual Las Cruces copper mineralization: is the enrichment an actual supergene system?
Miguélez N.G. 1, Tornos F. 1, Velasco F. 2, Videira J.C.3
1Instituto Geológico y Minero de España, Madrid, Spain; 2Universidad del País Vasco, Bilbao, Spain; 3 Cobre Las Cruces s.A., Gerena, Sevilla, Spain
n.miguelez@igme.es
Las Cruces Cu deposit is a volcanogenic massive sulphide (VMS) with an unusual and well preserved large zone of secondary copper enrichment. It is situated at the SW edge of Iberian Pyrite Belt (IPB), 15 km NW of Seville (Spain), beneath 150m of Tertiary sediments belonging to the Guadalquivir Basin. What makes different Las Cruces from other deposits in the world is the presence of a paragenesis more typical of hydrothermal systems than of supergene ones.
The main commodity is copper (16 Mt @ 7%Cu) with Au and Ag credits in the uppermost gossan and sub-economic Zn, Cu and Pb in the primary mineralization. Cobre Las Cruces exploits the mine since mid 2 009, with an initial investment of 1 180 m$ and expected production of 1 Mt of Cu cathode during 15 years. The opening of the mine has involved important technical and environmental challenges, using hydrometallurgic methods and keeping aquifer safe with dual extractive-reinjection wells rings treating the water through inverse osmosis.
The primary mineralization shares many features with the VMS deposits of the southern IPB, being located into an anoxic shale of late Devonian age overlying a dacitic dome complex; the geologic setting is very similar to that of the Aznalcóllar-Los Frailes deposits located ca. 10 km W [1]. The massive sulphide lens is composed by dominant pyrite with sphalerite, chalcopyrite, galena, quartz and chlorite with traces of cassiterite and barite. A large stockwork zone developed on its base which is hosted by dacite and affected by sericitization and chloritization. The massive sulphide lens is up to 100m thick and strikes broadly east-west over a proven distance of l km [2]. It dips ca. 35ºN and has been dissected by the erosional surface.
The uppermost part of massive sulphide and stockwork has been affected by secondary Cu-enrichment processes, which were described as a supergene system that worked after tilting and before being covered by the Miocene sediments [3].
D
etailed
mapping shows the existence of the well defined subhorizontal and
discontinuous alteration zones that from top to bottom include: an Au
and Ag enriched complex “gossan” up to 20 m thick where two main
lithologies are present. A “Red Gossan” that is made up of
massive haematites with accessory siderite and goethite, and a “Black
Rock” which is fine grained galena and pyrite with carbonates,
anglesite, greigite-marcasite, and pyrrhotite [2]. Accesory phases
include apatite, pyromorphite, cinnabar, nontronite, rutile and
cassiterite [4].The gossan were erosioned and fragments of it occur
as isolated clasts in the lowermost sediments. These rocks
discontinuously cap up to 5 m thick unit of brechoidal fine grained
silica which distribution seems to be controlled by north-south
faults. The transition to the cementation zone is composed by 2-3 m
of sand-like barren pyrite. The Cu-sulphides enrichment zone occurs
as massive replacement and veins of chalcocite and covellite.
Locally, these veins show a core made up of pyrite, chalcopyrite,
bornite and quartz and/or calcite; tennantite-tetrahedrite and
enargite can be locally common. Vertically, the size and amount of
veins diminish but the extent of the supergene alteration is
controlled mainly by N-S faults.
Sketch showing the relationship between primary mineralization and secondary enrichment alteration at Las Cruces deposit
This alteration is dramatically different to the subaerial alteration that took place in most of the outcropping VMS deposits of the IPB, where these are made up of a goethite cap overlying a thin jarosite layer and a small or absent cementation zone with covellite and chalcocite.
The petrographic study under microscope seems to point that the copper enrichment is related to a late low temperature hydrothermal event. Thus lithogeochemistry and stable isotope studies [5] were run for a better understanding of this large enrichment in copper. The data of the enrichment zone samples is closer to previous data related to hydrothermal events than to supergene events.
The discordant and subhorizontal morphology of the secondary mineralization indicate that the Cu-enrichment processes occurred after folding during the Variscan orogeny and all the related magmatic-hydrothermal activity. Thus, this is not related with seafloor oxidation, as it was described before [2], what occurs in many VMS after the cessation of the hydrothermal activity. The deposit was affected by denudated during regional uplift and later subaerial alteration similar to that of the other deposits of the IPB. Crosscutting relationships and the relative chronology suggest that the supergene alteration has affected the ore deposit for a relatively long period; even there are evidences that point to the ongoing activity of the system nowadays.
What makes Las Cruces different is the latest alteration and evolution below the marine sediments of the Guadalquivir basin. The largest alteration took place below these sediments in a highly confined environment, and due to the interaction with the Niebla-Posadas aquifer, which keeps on going nowadays.
The uncommon assemblage is dramatically different from that of supergene environments and the presence of a silica cap overlying quartz-rich veins with covellite, chalcocite and enargite is more common in high sulphidization epithermal environments [6].
The model that we propose implies reaction of groundwater, with cyclic changes between calcium bicarbonate and sodium chloride, with the massive sulphides at different fluid-rock ratios. This produced abnormal high heat rates; in fact, present day water has temperatures as high as 40ºC [4]. Preliminary isotope data suggest major biogenic activity with extremophilic microbes being responsible of widespread CO2 generation and sulphate reduction.
References:
1. Conde, C., Tornos, F. and Doyle, M. (2007) Geology and lithogeochemistry of the unique Las Cruces VMS deposit, Iberian Pyrite Belt, Digging Deeper. Proceedings of the 9th Biennial SGA Meeting Dublin, IAEG., pp. 1101-1104
2. Knight, F. C. (2000) The mineralogy, geochemistry and genesis of the secondary sulphide mineralisation of the Las Cruces, Spain, PhD thesis, University of Cardiff, pp. 434
3. Moreno, C., Capitan, M. A., Doyle, M., Nieto, J. M., Ruiz, F. and Saez, R. (2003) Edad mínima del gossan de Las Cruces: implicaciones sobre la edad del inicio de los ecosistemas extremos en la Faja Pirítica Ibérica, Geogaceta 33, pp. 67-70
4. Blake, C. (2008) The mineralogical characterisation and interpretation of a precious metal-bearing fossil gossan, Las Cruces, Spain, PhD thesis, Department of Earth, Ocean and Planetary Sciences, University of Cardiff, pp. 330
5. Miguélez, N.G., Mathur, R., Tornos, F., Velasco, F. and Videira, J.C. (2011) Cu isotope geochemistry in the unusual Las Cruces supergene copper deposit, Goldschmidt conference abstracts, pp. 1467
6. Sillitoe, R. H., Hannington, M.D. and Thompson, J.F.H. (1996) High sulfidation deposits in the volcanogenic massive sulfide environment, Economic Geology 91, pp. 204-212