Las Cruces is a volcanogenic massive sulphide deposit located in the easternmost Iberian Pyrite Belt. The deposit is situated in the Guadalquivir Basin, beneath Miocene sediments, within a zone of major block compartmentalization controlled by large ENE-WSW Alpine faults. The deposit is composed of a large lens of massive sulphides, dominated by pyrite, with smaller amounts of sphalerite, galena, chalcopyrite and tetrahedrite-tennantite (Blake, 2008; Knight, 2000). The deposit was isolated from the surface by a layer of sedimentary breccia and sandstone (up to 20 m thick) and overlain by c.a. 150 m of Messinian marl. This changed the conditions of what was a standard VMS deposit. The interaction between water circulating through the aquifers and the gossan cap led to the formation of a set of unusual rocks: the red rock dominated by siderite with traces of anglesite-cerussite and jarosite, and the black rock composed of iron sulphides, galena and calcite with abundant accessory phases including silver, mercury and lead sulphides as well as sulphosalts, cinnabar and gold (Blake 2008; Menor-Salván, 2011; Yesares et al. 2014; Tornos et al. 2017). There are also several evidences of alteration processes, like the intense argillic alteration beneath the gossan, or the pervasive silicification in the red and black rocks. While the argillic alteration has been dated in 10-11 Ma and is interpreted as related to the supergene alteration, the silification processes are controlled by Alpine faults and therefore have affected the secondary minerals (Tornos et al. 2017). Maybe the most interesting feature of the deposit is the presence of biomorphs in the black rock (Tornos et al. 2014) along with microbial communities still alive nowadays (Tornos et al., 2019), and related with the secondary minerals. In this work, a study of these biomorphs is presented with special attention to textures and fabric.

Peer reviewed