Pyrites are known to form in low-temperature sedimentary to high-temperature magmatic environments. They also form from extensively wide variety of ore-bearing fluids. S isotope data often helps in unraveling their genesis. However, it may not give a true picture if any hydrothermal solution has incorporated pre-existing bacteriogenic S. Under such a scenario how to convincingly establish the hydrothermal origin of pyrite? Please suggest some classic literature.
You selected one of the strangest host rocks for U, calcareous rocks. There are only a few sites where U is enriched to (sub)economic grade such as in the USA
• Red hematite-bearing sandstone forms the footwall rocks („Entrada sandstone“)
• Primary source of U rests in the Fe-Mg silicates (e.g. chlorite, biotite)
• Todilto Limestone is representative of algal stromatolites
• Evaporative pumping of uraniferous ground water causes migration into the carbonaceous/ reducing environment of the Todilto Limestone.
As you might learn from that case history there is no room for pyrite to originate from hydrothermal solutions. The other non-metamorphosed sites are in Jordan and Israel where U is accumulated next to phosphorites in a calcareous host rocks.
From the point of view of U deposition there is no hint to describe a reducing environment in any way related to hydrothermal solutions.
A different approach can be taken by investigating the trace element content (Ni, Co, U, Se, As…) of pyrite and its S isotopes in combination with organic matter co-precipitated together with pyrite. The presence of Ni pyrite (bravoite), Se-bearing pyrite … speak in favor of supergene affiliation, whereas high As contents and high Co/Ni ratios would suggest a hydrothermal / high temperature derivation.
Further reading of the papers below will make you acquainted with pyrite in the various physico-chemical regimes:
DILL, H.G., KUS J., GOLDMANN S., SUÁREZ-RUIZ I., NEUMANN T., and KAUFHOLD S. (2019) The physical-chemical regime of a sulfide-bearing semi-graphite mineral assemblage in metabasic rocks (SE Germany) – A multidisciplinary study of the missing link between impsonite and graphite. International Journal of Coal Geology 214: (in print)
DILL H.G., WEHNER, H. and BLUM, N. (1993) The origin of sulfide accumulation in arenaceous rocks beneath carbonaceous horizons in fluvial depositions of late Paleozoic through Cenozoic age (SE-Germany).- Chemical Geology, 104: 159 – 173.
DILL, H. G. and KEMPER, E. (1990) Crystallographic and chemical variations during pyritization in the Upper Barremian and Lower Aptian dark claystones from the Lower Saxonian Basin (NW Germany). - Sedimentology 37: 427-443.
DILL, H. G., EBERHARD, E. and HARTMANN, B. (1997) Use of variations in unit cell length, reflectance and hardness determining the origin of Fe disulphides in sedimentary rocks.- Sedimentary Geology, 107: 281-301.
DILL, H.G. (1986) Metallogenesis of the Early Paleozoic Graptolite Shales from the Graefenthal Horst (Germany). - Economic Geology, 81: 889-903.
H.G.Dill
I do not think you are asking the right question really. Hydrothermal systems can circulate meteoric and magmatic water. Isotope systematics of Oxygen and Sulfur are the two I look at for determining the degree of magmatic influence in a copper mineralizing system. There will generally be a mixing signature. I assume you are "hunting" for a magmatic system. Perhaps the more relevant question is will uranium related processes either thermal or radiochemical influence the isotopic fingerprint. All I can do is guess that based on general principles thermal isotopic fractionation might confuse the interpretation of oxygen isotope signatures more than the heavy ones such as sulfur. This question hit the copper business some time back when folks asked if copper breccia pipes were magmatic or "karstic". The quick answer was that at first folks thought copper breccia pipes were karstic and not interesting, at least according to Dr. Richard Sillitoe, a prominent figure. Then he had to waffle and apparently finally recanted as folks kept finding big copper where it was not so clear what was going on. (Los Bronces, Los Solfatos etc.). I happen to be involved with just such a situation. It turns out the copper "breccia pipe" I am working with is right within a giant gondwana cataclastite zone in Chile. The cataclastite is apparently mis-identified and is now thought to be some 25,000 miles long and seems related to the big structural event. The cataclastite is a nice pathway for a magmatic venting. So...karst or magmakarst or breccia pipe? I enclose an example of my breccia pipe crosscutting what was called "El Volcan" unit...a giant cataclastite that is sometimes called "Choiyoi" so I gather. So how can you tell the karstic ones from the magmatic ones? Take a whole bunch. Then if you find sulfur with a strongly negative delta among your samples...go with the magmatic. Story.
Thank you Dr. Dill for your answer and the list of papers. At the outset, I would like to inform that in addition to the deposits/occurrences indicated there are two more deposits in India (Gogi and Tummalapalle), where calcareous sedimentary rocks form a part of the host rock /the dominant host rock.
Actually the issue is with the delta S value of pyrite occurring both as vein and as disseminations in one of such deposits. The delta S values (for both varieties) are dominantly negative (as low as -31 permil), which are apparently (in association with lithology) indicative of BSR. But the mode of occurrence as vein puts a constraint on such direct interpretation. It is likely that the pyrites occurring as vein incorporated remobilized bacteriogenic S during it formation. How to convincingly establish such remobilization by hydrothermal solution?
Regards,
Himadri Basu
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