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

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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].

Detailed 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

Mineralogical, petrological and fluid inclusion study of the Brehov ore deposit (Eastern Slovakia)

Levente Molnár¹, Ferenc Molnár¹, Pavel Bačo²

¹Department of Mineralogy, Eötvös Loránd University, Budapest, Hungary

²Štátny geologický ústav D. Štúra, Werferova, Košice

We have analyzed drill core (VSB-2 and VSB-2a) and surface samples from the buried base metal ore deposit near Brehov. The samples were divided into four lithological groups: rhyodacite, hydrothermal breccias, pyroclastics, diorite. The rhyodacitic samples are heavily altered to illite-sericite, while kaolinite is the product of steam heated acid-sulfate alteration in pyroclasics. The ore forming minerals occur in stockwork and veins (sphalerite, galena, pyrite, hematite). The gangue minerals are quartz together with barite in pyroclastics and rhyodacite and with calcite in diorite. Results of fluid inclusion study which was carried out on quartz samples show that boiling occurred at different depths of the same paleohydrological system, and thus the 10 wt% NaCl boiling curve can be fitted to the location of samples from various depths. This data shows that we can count about 200 meters of erosion in relation to the paleogroundwater table. Compared to the Tokaj Mts., the age of the volcanic rocks and mineralization fits into the trend observed in the Tokaj Mts., where hydrothermal centres exposed on the surface are progressively younger towards south. Fluid inclusion data for the Brehov deposit show similar salinity as fluids in the transitional zone between the porphyry and epithermal mineralization in the Zlatá Baňa stratovolcano. According to these results, a base metal sulfide rich low sulphidation type epithermal mineralisation is present near Brehov and the hydrothermal system can be connected to the subvolcanic diorite intrusions of the area.

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