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The Diagenetic-Hydrothermal Metallogenic Model: An Update for Unconformity-Type Uranium and Stratiform Copper Deposits

by David H. Quirt1 and Jan Hoeve1

Quirt, D. and Hoeve, J. (1986): The dia9enetic-hydrothermal metallogenetic model: an update for unconfonnity-type uranium and stratifonn copper deposits; 1n Sunmary of Investigations 1986, Saskatchewan Geological Survey; Saskatchewan Energy and Mines, Miscellaneous Report 86=4.

Under a five-year program, studies of clay mineral host rock alteration around individual Athabasca unconformity-type uranium deposits were complemented by regional studies of clay mineral distribution and diagenesis in the Athabasca Group rocks. Objectives were twofold: firstly, to gain an understanding of the interrelationships between diagenesis, host rock alteration and mineralization and, secondly, to identify diagnostic ore fluid criteria that could be applied in exploration programs to drillhole evaluation.

During the past decade and a half, northern Saskatchewan has emerged as one of the most important uranium districts of the world, following the discovery of spectacular unconformity-type deposits in the Athabasca Basin. Initial discoveries were made at Rabbit Lake and Cluff Lake during the late sixties. Three mines are currently in production: Rabbit Lake (Collins Bay B deposit), Cluff Lake (Claude and Dominique-Peter deposits) and Key Lake (Gaertner deposit). Two deposits have been mined out, the Rabbit Lake deposit and Cluff Lake D zone. At about the same time as in Saskatchewan, unconformity-type uranium mineralization was also discovered in the Northern Territories of Australia. Deposits in the two districts share many characteristics, geological setting, age, host rock alteration, mineralogy and geochemistry, as illustrated by J abiluka, Ranger and Nabarlek (Ferguson et al., 1980; Ewers et al., 1983; Dahlka mp and Adams, 1981 ).

Despite their economic significance and a decade of exploration and research, the unconformity-type uranium deposits still constitute a little-understood class of mineralization. In Saskatchewan, early genetic theories assumed an erosional origin for the mineralization, based on the intimate spatial association of ore with the sub-Athabasca paleoweathering profile (Knipping, 1974; Langford, 1974; McMillan, 1977, 1978). Later it became clear that diagenesis of the Athabasca Group red beds had played an important part in the mineralization process (Pagel, l 975a, b; Hoeve and Sibbald, 1976; 1978; Hoeve et al., 1980), while most recently a diagenetic-hydrothermal metallogenetic model was proposed in which ore formation and host rock alteration are directly linked to stages of burial diagenesis and basin evolution (Hoeve and Quirt, 1984). This diagenetic-hydrothermal model calls

1Mineral Resources Division, Resources Sector, Saskatchewan Research Council, Saskatoon.

upon fluid interaction between the Athabasca Group and crystalline basement under conditions of deep burial and highgrade diagenesis during an advanced stage of basin evolution.

A stable isotope study (0, H, C) (Hoeve et al., 1986) of authigenic minerals in Athabasca Group sandstones and of host rock alteration assemblages associated with the deposits has better characterized the fluids involved in the ore-forming process and has tested the diagenetic hydrothermal concept of fluid interaction of sandstone and basement. Included in this study were the Rabbit Lake, Collins Bay, Key Lake, Midwest Lake and McClean lake deposits, which together represent a range of unconformity-type mineralization styles.

The isotopic evidence presented in Hoeve et al. (l 986) points to the involvement of two distinct fluids in the hydrothermal process. One is an oxidizing fluid, identified as formation water of the Athabasca Group. The other is a reducing, hydrocarbon-bearing fluid thought to have been generated in the metamorphic basement by reaction of formation waters of the Athabasca Group with graphite contained in metapelitic rocks.

Isotopic and geological evidence indicates that mixing of the two fluids took place above the unconformity at the Midwest deposit and below the unconformity at the Key Lake and Rabbit Lake deposits. This is significant considering that, at Midwest, virtually all ore is above the unconformity, whereas, at Rabbit Lake and Key Lake, most ore is located below the unconformity. For deposits of the Midwest type, mixing of reducing fluids emerging from basement rocks with oxidizing formation waters of the Athabasca Group created a very high redox gradient and a very low pH gradient in the basal sandstone. It appears that two reaction fronts were formed: firstly, a redox front controlling precipitation of ore and secondly, a pH front (or a pK front, if potassium was picked up from the basement by the reducing fluids) controlling the conversion of diagenetic kaolinite into illite.

The isotopic evidence presented in Hoeve et al. ( 1986) supports the diagenetic-hydrothermal metallogenetic model, presented earlier (Hoeve and Sibbald, 1976, 1978; Hoeve et al., 1980; Hoeve and Quirt, 1984), involving fluid interaction between the Athabasca Group and the crystalline basement, under conditions of deep burial and high-grade diagenesis, during an advanced stage of basin

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evolution. Since the basin floor consists of high-grade metamorphic rocks of inherently low permeability, all such interaction must have been restricted to the vicinity of the unconformity, thus accounting for the observed spatial association of ore deposits with the paleoweathering profile. The model envisages that oxidizing formation waters penetrated below the unconformity, via fault zones, and reacted with graphitic basement rocks to yield reducing fluids containing hydrocarbon and carbon dioxide.

Mineralization is attributed to mixing between the newly generated reducing fluids and oxidizing, metal-bearing formation waters of the Athabasca Group. Depending on characteristics of he plumbing system (where fluid mixing took place), ore was formed just above, at, or just below the unconformity.

On a basin-wide scale, Hoeve and Quirt (1984) documented an intimate relationship between episodes of tectonic reactivation, diabase magmatism, stages of diagenesis and episodes of hydrothermal activity as represented by the first and second stages of mineralization at 1350 to 1250 Ma and 1100 to 1050 Ma, respectively. They proposed that periodic hydrothermal activity at the unconformity may have been regulated by free convection in the basal Athabasca Group aquifer (Manitou Falls Formation), induced by increased heat flow across the basin floor during pulses of tectonism and diabase magmatic activity. They further proposed that positive thermal anomalies at the unconformity, evident as zones of quartz dissolution (Hoeve and Quirt, 1984), directly reflected the presence of graphite-bearing rocks in the basin floor.

The thermal conductivity of graphite is orders of magnitude higher than that of silicates (Clark, 1966; graphite: 400-1000 x 10-3 cal/cm sec0 c; feldspar: l O x 10-3 cal/cm sec°C), making it likely that rocks with even small admixtures of graphite would exhibit greatly increased thermal conductivity, particularly in directions parallel to metamorphic foliation. Tilsley ( 1980), in a somewhat different context, pointed out that thermal gradients in graphite-rich rocks may be as low as l to 2°C/km. Thus, for a steeply dipping sheet of graphitic rock extending l to 2 km below the Athabasca Group unconformity and for a geothermal gradient estimated at 35°C/km (Pagel, l 975a, b), the temperature at the unconformity may well have been in the order of 30 to 5o0 c higher than that of adjacent non-graphitic gneisses. In a situation of increasing heat flow, the significance of thermal anomalies at the unconformity may then have been to lock developing convective cells on graphitic metapelite layers subcropping at the basin floor. The extremely elongate shape of many unconformity-type orebodies (several kilometres long and tens of metres wide and a few tens of metres high) suggests that convective cells in the Athabasca Group aquifer developed into couplets of

longitudinal rolls straddling traces of graphitic layers at the unconformity.

Dual flow systems such as envisaged for the unconformity-type deposits may have been driven by a combination of large-scale free convection in the Athabasca aquifer and by small-scale forced convection in the basement (Hoeve and Quirt, 1984). The former would be controlled by the overall heat flow through the basin floor, whereas the latter would depend on the localized extra heat flow channelled through zones of graphitic metapelite.

Fluid mixing between oxidizing and reducing fluids implies hydrodynamic control on ore formation. Isotopic and mineralogical evidence for example, suggests that fluid mixing in ore-forming systems led to the establishment of a distinct redox front. Once established, the front may have remained stationary or migrated. The importance of a stationary redox front in the mineralization process can be inf erred from the absence of a zone of secondary hematite in barren alteration haloes and from the fact that all other characteristics of barren haloes appear to be identical to those of ore-bearing haloes, including quartz dissolution, residual clay enrichment, clay alteration and superimposed bleaching (Hoeve and Quirt, 1984).

Thus, within the context of the diagenetic­hydrothermal model, the significance of convection is that it provides for focussed fluid flow, whereas the significance of a stationary redox front is that it provides for focussed precipitation. Since the model is characterized by continuous replenishment of ore constituents, carried by oxidizing formation waters of the Athabasca Group, and by continuous generation of hydrocarbon reductants in the basement, very high grades of mineralization may accumulate over time. The 50 to 100 Ma duration of the intervals of each of the two stages of mineralization in the Athabasca Basin may point to the persistence of favourable and stable hydrodynamic conditions for very long periods of time.

Although at the moment ore-bearing and barren haloes cannot be isotopically distinguished, the oO-cSD relationships between hydrothermal chlorites and illites (Hoeve et al., 1986) indicate that isotopic analysis may provide information on the question of whether fluid mixing took place below or above the unconformity. In the former case, mineralization may be expected below the unconformity, and drilling for "root" zones in the metamorphic basement may be a worthwhile strategy. In the latter case of mixing above the unconformity, no basement "roots" are to be expected.

The diagenetic-hydrothermal model is not restricted just to unconformity-type uranium deposits. The origin of stratiform copper deposits has been a longstanding controversy between proponents of syngenetic and epigenetic

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mechanisms. In recent years, there has been a movement towards the epigenetic camp with the growing realization that ore metals were introduced after sedimentation in the course of burial diagenesis (Brown, 197 l; Bartholome et al., 1973; Bartholome, l 974; Rentzsch, l 974). The early- or late-diagenetic timing of mineralization, however, remains highly uncertain, as does the source of metals. Copper may have been leached from associated red beds during burial and diagenesis or introduced from deep-seated lavas, either during burial metamorphism (Livnat, l 983), or directly during magmatic activity (Annels, l 984). Similar uncertainties surround the transport of ore constituents and the regime of fluid flow.

Stratiform copper deposits in many regions of the world and unconformity-type uranium deposits in the Athabasca Basin share a common geological setting, characterized by thick sections of fluviatile red-bed sandstones and conglomerates sandwiched between a metamorphic basement and a shale-rich marine unit. A potential for mineralization exists not only at the unconformity, but also at the top of the red bed aquifer, if the overlying marine shale is chemically reduced and rich in organics. The stratiform uranium deposits of the Proterozoic Franceville Basin in Gabon and the Permian Lodeve Basin in France provide examples. General aspects of the model can be applied to stratiform copper deposits, and we have proposed (Hoeve and Quirt, l 986) that Proterozoic stratiform copper deposits of the Central African and Michigan Copper Districts, and Permian Kupfershiefer deposits in Central Europe, are examples of diagenetic-hydrothermal mineralization at the top of a red-bed aquifer.

In a broader perspective, ore formation can be considered as an inherent aspect of diagenesis and basin evolution. This has opened the prospect of developing a unified geological model for a spectrum of sediment-hosted, stratiform and unconformity-associated metal deposits, whose unifying feature is an origin that is intimately tied to red-bed diagenesis.

Depending on whether the process of basin evolution and diagenesis goes to completion or is arrested at intermediate stages, a variety of deposits may be generated, ranging from those formed at a low temperature in a shallow environment shortly after burial, to those that formed under conditions of deep burial and high-grade diagenesis during an advanced stage of basin evolution. We have proposed (Hoeve and Quirt, 1986) that giant stratiform copper deposits relate to red-bed copper deposits as unconformity-type uranium deposits relate to sandstone-type uranium deposits. The uranium deposits represent end products and transient intermediate products, respectively, of a process of basin evolution and red bed diagenesis that has gone to completion in the Athabasca Basin. Similar relationships may exist between syngenetic to early

diagenetic sandstone lead deposits (Largentiere; Yava) and epigenetic unconformity-related lead deposits (Maubach and Mechernich, Eifel, Germany; Sibley Basin, Ontario; Artillery Lake, Northwest Territories; Otish-Mitassini Basin, Quebec).

The apparently time bound character of Gabon and Lodeve - type stratiform uranium deposits, unconformity-type uranium deposits and stratiform copper deposits (Jacobson, 1975; Robertson et al., 1978; Toens and Andrews-Speed, 1984) suggests that crustal and tectonic conditions allowing the process of basin evolution, red-bed diagenesis and ore formation to proceed to completion have rarely been met in the earth's history. Unconformity - type uranium deposits are primarily restricted to the Middle Proterozoic, whereas stratiform uranium deposits such as in Gabon and Lodeve are known from the Middle Proterozoic and Permian. Stratiform copper deposits appear in the geologic record in early Proterozoic times, but have their main development during the Middle Proterozoic at ca. l ODO Ma, with a second major phase during the Permian. The Middle Proterozoic and the Permian were both periods when the land areas on earth were assembled into super continents, creating conditions conducive to the formation of extensive intracratonic basins, to widespread fluviatile terrestrial sedimentation and to the formation of red-beds.

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