Sediment-Hosted Copper deposits (SHCD) occur within sedimentary basins, generally at the contact between subaerial red-bed sequences and overlying marine or lacustrine shales, siltstones, sandstones, or carbonate rocks (Kirkham, 1989), consisting of relatively thin (generally <30 m and commonly < 3 m) sulfide-bearing zones from moderate-temperature (50°-150°C), moderate to high salinity (from 7 to + 30 eq. wt % NaCl), oxidized fluids (hematite-buffered) at oxidation-reduction interfaces (REDOX fronts). These deposits appear in a variety of sedimentary rocks with a variety of reductants. The configuration of the mineralized zone ranges from sheet like, with extensive horizontal dimensions, to tabular or roll-front geometries, with limited horizontal dimensions (Kirkham, 1989).
Approximate synonymous for SHCD are Diagenetic Sedimentary Copper, Red-Bed Copper, Stratiform Copper, Kupferschiefer type, Sandstone Copper, Shale-Hosted Copper, Reduced facies Copper, Sabkha Copper, and Revett Copper.
Sediment Hosted Copper Deposits are valuable because:
- They are the second most important source of copper worldwide after Porphyry Copper deposits.
- The world most importance source of cobalt (60%)
- Important producer of by-product silver
- SHCD contains consistent grades and lateral continuity along bedding (1.2 to 5% Cu), and
- SHCD may contain large resources of by-product Au, U, platinum-group, and rare-earth elements.
Some of the most common features of the SHCD are:
- They are stratabound.
- Restricted to a narrow range of layers within a sedimentary sequence but do not necessarily follow sedimentary bedding.
- Epigenetic and diagenetic, that is, they are formed after the host sediment is deposited, but in most cases, prior to lithification of the host.
- They form independently of igneous processes.
- Uniform lateral continuity of mineralization along bedding.
- A major thickness of permeable, coarse-grained, red-bed clastic sediments in the immediate footwall of the cupriferous zone.
- Temporal and spatial association of host rocks with strata formed in warm arid climates (evaporitic units, red-beds).
- Deposition of Cu from aqueous, chloride-rich solutions upon crossing the redox front between footwall red beds and reduced, sulphide-bearing grey beds of the host strata
- Associated with rift basins filled with continental red-beds+-bimodal volcanic strata.
- Deposits are normally closely associated with evaporite sequences which provide an oxidizing environment, source for large sulfur volumes, and high salinity brines for metal transport.
- Mineralization normally is associated with the presence of reduced, high-organic sedimentary units above and / or lateral to oxidized red-bed sequences. Oxidized units are hematite-bearing with the notable absence of iron sulphides. Oxidized units normally contain a high permeability prior to and often after mineralization.
- Zoning, where recognized, is commonly from hematite-rich, sulfide barren zones to chalcocite-bornite-chalcopyrite-pyrite zones. It is interpreted as the basin-ward precipitation of ore fluids.
Genetic implications for SHCD are:
- There must be an oxidized source rock.
- There must be a brine source to mobilize copper.
- There must be a source of reduced fluid to precipitate copper and form a deposit.
- There have to be conditions favourable for fluid mixing.
Although there are multiple occurrences of SHCD worldwide, the supergiant deposits are present in only three basins: the Paleoproterozoic Kodaro-Udokan basin of Siberia, the Neoproterozoic Katangan basin (Central African Copperbelt), and the Permian Zechstein basin of northern Europe (Hitzman et al, 2010).
In the picture below is one of the multiple outcrops of chrysocolla-rich sandstone at Cuatro Ciénegas, Coahuila, Mexico. These Cu-Ag bearing mantos sit above red-beds units. (Photo extracted from the Brigadier Gold Limited´s website).
- Hitzman, M.W., Selley, D., and Bull, S., 2010, Formation of Sedimentary Rock-Hosted Stratiform Copper Deposits Through Earth History: Economic Geology, v.105, pp 627-639.
- Kirkham, R. V., 1989, Distribution, settings, and genesis of sediment-hosted stratiform copper deposits: Geological Association of Canada Special Paper36, p 3–38.