MINERAL DEPOSITS AND THEIR GENESIS 495 Figure 10 A diagram to illustrate the environment of sedimentation of Zambian Copperbelt host rocks A marked zonation of copper and iron sulphides exists near the shoreline formed by the hilly basement rocks The diagram (1976) illustrates a syngenetic theory of origin for mineralization, which is not universally accepted Reproduced from Fleischer VD, Garlick WD, and Haldane R (1976) Geology of the Zambian copperbelt In: Wolf KH (ed.) Handbook of Stratabound and Stratiform Ore Deposits, vol 6, pp 223 350 New York: Elsevier Figure 11 Sketch illustrating the occurrence of carnotite roll front ore bodies in fluvial channel sandstones The direction of front migration in this case was from right to left Reproduced from Selley RC (2000) Applied Sedimentology, 2nd edn p 38 San Diego: Academic Press tonnes averaging about 2.7% Cu has been mined) and laboratory studies suggests that metal-bearing sulphate brines were introduced through tectonic thrust faulting and that copper sulphides were epigenetically precipitated by organic matter in the host rocks Neither theory has demonstrated the source of the metals in the seawater (for syngenesis) or in the deep-seated fluids (for epigenesis), or fully explained all the observed field relationships and laboratory data Knowledge of diagenetic processes is advancing and is illustrated by roll-front uranium deposits in Colorado, where carnotite has been precipitated from migrating pore waters in confined aquifers (Figure 11) The mottled lead sulphide sandstone ore bodies of the extensive Laisvall and related deposits of the Swedish Caledonides may also be due to migrating mineralized fluids The discordant masses of zinc and lead sulphide mineralization that are widespread in sedimentary carbonate sequences, such as the Mississippi Valley deposits of the USA, have clearly been deposited later than their host rocks The ore forming fluids were for many years attributed to igneous sources, for lack of a better option It has now been shown in some well-documented cases that oilfield brines migrating up-dip from deep sources are the ore forming fluids from which sulphides were precipitated From years of meticulous field observations backed up by laboratory studies (such as fluid inclusions, stable isotope and geochemical analysis) there has emerged an elegant and logical understanding that both oil and ore deposits are linked through such common controls as source, migration path, trap and seal The effect of this advance in knowledge about mineral deposits has been dramatic The philosophy of prospecting for new mineral fields of this type has switched, to put the case over-simply, from searching carbonate sequences near intrusive granites to studying fluid migration paths in sedimentary basins containing hydrocarbon source rocks and evaporites An understanding is developing of the thermochemical reactions (at elevated pressures and temperatures) between hydrocarbons and sulphate brines that can release aggressive rock-altering fluids and hydrogen sulphide, a powerful precipitant of base metals This example, and that of the Zambian Copperbelt, demonstrates that ores in ancient sediments are best studied, like the hydrocarbon deposits to which they may be linked, in the context of the entire geological history of the containing sedimentary basin