A petrological, mineralogical and lead isotope study of copper in sandstones and related sediments
Copper-bearing sulphides in sandstones are only one category in a range of sediment-hosted stratiform deposits of copper, lead and zinc that do not have obvious volcanic associations. This work investigates the late Proterozoic Pernatty Lagoon copper deposits in South Australia and selected late Palaeozoic and Mesozoic Sandstone-Copper deposits of the Southwestern United States.
The Stuart Shelf in South Australia is made up of a flat-lying series of Proterozoic (Adelaidean) rocks that envelop the older Carpenterian Gawler Craton to the west, and terminate on the Torrens Fault to the east. The sedimentary units are correlated into folded Adelaidean rocks east beyond the fault. There is a ridge of older sediments at the centre of the Stuart Shelf called the Pernatty Culmination. The structure was in existence during the Proterozoic and has controlled the deposition of later sediments, therefore later sedimentary units lap onto the Pernatty Culmination. Disconformable contacts are important features because major sedimentary hiatuses separate a number of the Stuart Shelf units.
The Pernatty Lagoon copper deposits lie on the Pernatty Culmination where 'basement' is the quartz arenitic Pandurra Formation. Disconformably above the Pandurra Formation is the Tapley Hill Formation. The Tapley Hill Formation does not cover the Pernatty Culmination, consequently the next unit, the Tent Hill Formation (specifically the Whyalla Sandstone) lies disconformably on the Pandurra Formation. Those parts of the Tapley Hill Formation on the Pernatty Culmination are a laminated dolomitic rock with some oolitic, brecciated and more massive stromatolitic parts and show evidence of being deposited in shallow water estuarine conditions. 30 to 40 kilometres to the southwest and west of the mine the Tapley Hill Formation is a black organic-bearing dolomitic shale that is a deeper water facies. The Whyalla Sandstone is a quartz to subfelsarenite with cross bedding, ripple markings and features that in all suggest a mixed fluvial-aeolian origin. Deltaic sands are suggested as a sedimentary model.
The Cattle Grid ore body, and the small East and West Lagoon ore bodies are hosted by the disrupted and brecciated upper surface of the Pandurra Formation and to a lesser extent by the Whyalla Sandstone. In the Cattle Grid ore body, the largest and most economically significant ore body, the brecciation is particularly well developed and is an important factor in understanding the ore body. Textural evidence in the ores and sediments suggests the lower beds of the Whyalla Sandstone were present when the sulphides were introduced.
A detailed study of the mineralogy of the Cattle Grid ore body is reported. The ore body is zoned with a core of chalcopyrite with sphalerite and pyrite rimmed by bornite which in turn rimmed by chalcocite. The major metal in the ores is copper, but there are substantial amounts of zinc and lead. Minor and trace metals conform to the zoning and can be related to specific sulphide phases or textures. Cobalt is an important minor element. as is bismuth. Silver and gold are present in trace amounts. as arsenic.
An important feature of the mineral assemblages is that they have been redissolved and redeposited through a number of cycles, thus obliterating many of the early textures and assemblages. The first minerals deposited were carrollite-arseniferous pyrite spheroids that have been mostly redissolved. Whether or not other copper-bearing minerals were deposited with the early assemblages cannot be determined. A second generation of minerals include marcasite, pyrite, sphalerite and a second generation of carrollite. The first chalcopyrite recognised is slightly later than the second generation of minerals, but this may be due to remobilization. The first events are complex as they clearly represent a prolonged period of deposition, resolution and redeposition. They are now preserved because the sulphides have completely filled openings in the arenites which have excluded all subsequent waters.
The dominant mineral assemblages are bornite with chalcocite and chalcocite with bornite. The chalcocite zone contains appreciable quantities of bismuth. Replacement textures of chalcopyrite replacing bornite, bornite replacing chalcopyrite and chalcocite replacing bornite are seen. but there is no evidence for the contention that the ore body is the result of massive replacement of pyrite by copper-bearing minerals. A significant set of textures is those from direct deposition. The colloform textures are common in the ore bodies and represent low temperature direct deposition of copper minerals under variable redox conditions. They contain sulphur-rich bornites that are in the main without exsolution laths of chalcopyrite. Sulphur-rich bornites are confined to the colloform textures, and 'replacement' bornite is compositionally 'normal'. Gold and silver are enriched in the phases involved in the colloform banding. Previous studies have shown that the East and West Lagoon ore bodies have mineralogical and textural features in common, but the small ore bodies do not have the early arseniferous-pyrite-carrollite, marcasite-pyrite, pyrite-sphalerite assemblages. Framboidal pyrite was common in the small ore bodies, but is absent in the Cattle Grid ore body, except in thin shale bands that are present day aquifers. As such they can be shown to be a later texture.
The evidence cited for metal sulphide recycling has profound implications for sulphur isotope studies, because the bulk of the mineralogy is not primary. The Pernatty Lagoon studies to date have not taken this factor into account, so are of questionable validity. There is a need to repeat the exercise.
The sandstone-copper deposits studied in detail were the Radium King Mine in Utah and the Nacimiento Mine in New Mexico. Prospects in the Scholle district New Mexico and in the Sacramento Mountains in southeastern New Mexico were also studied. In addition, reference is made to the Lisbon Valley copper deposits in Utah. The Radium King Mine is hosted by the Triassic Shinarump Member of the Chinle Formation. The Chinle sands are in channels that cut into the Upper Member of the Moenkopi Formation. The Nacimiento Mine is hosted by the Triassic Agua Zarca Sandstone that is a channel cut into the Permian Cutler Sandstone. The Cutler Sandstone is the name given to the Abo Formation in northern New Mexico. The Scholle district is the type locality for the Abo Formation and contains a series of arkose-filled channels that cut into the Bursum Limestone. Mineralization at Scholle is confined to the lower arkose-filled channels. The geology in the Sacramento Mountains is very similar to that at the Scholle district, but the ore deposits contain appreciable lead. The Lisbon Valley deposits are hosted by the Cretaceous Dakota Sandstone, but show evidence of remobilization near and around the Lisbon Valley Fault.
This study has concentrated on the relationships between coal and sulphides but a revision of the sulphide textures and mineralogy has been done where necessary. The sulphides in the sandstone-copper deposits show evidence of 'recycling' as with the Pernatty Lagoon sulphides, which implies that care must be taken with sampling for sulphur isotope studies. The sulphides in the sandstone-copper deposits replace wood structures in delicate detail. Wood that is not replaced proceeds through the chemical and morphological changes of coalification. Most coal is at subbituminous to bituminous rank, and the low rank coal has relic brown coal textures. Semifusinites, fusinites and rank fusinites are found in the carbonaceous matter, showing that the precursor material to the present coal had a variable history. Studies of silicification of wood assist in understanding 'suIphidization' of wood in that a delicate balance of redox levels is necessary to decay organics, yet still preserve sulphide. Sulphide pseudomorphism of wood structures is evidence that the metal in the sulphides (copper, iron, lead, zinc and perhaps cobalt) entered the depositional sites early in the history of the rock units, after sedimentation and before diagenesis.
A lead isotope study of sulphides, host rocks and potential source rocks from Pernatty Lagoon in South Australia has been undertaken. Measurements of the lead isotopic ratios of sulphides from the Nacimiento Mine and Scholle district have been done also.
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The Pernatty Lagoon deposits owe their existence initially to the ingress of a relatively well evolved basinal brine that introduced the metal and possibly the sulphur into the sediments. Subsequently the sulphides have been 'recycled' by pore waters/groundwaters to give final configuration that conforms to patterns of porosity and permeability in the rocks. The U.S. Sandstone-Copper deposits in the Permian may be related to an early, relatively poorly evolved brine, but have been recycled through time. The first metals deposited were early in the rock history, after sedimentation, but before diagenesis. The Nacimiento Mine possibly derived its metals from the underlying Permian sediments, again before diagenesis. Remobilization of copper in the host sediments has made structure an important ore control, and faults combine with permeability to give the final patterns of ore occurrences.
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