The early hydrothermal pyrites in phyllic altered rocks contain significantly higher Co and Ni than that of late hydrothermal pyrite in quartz veins from epithermal Au deposit.
Dear Liu,
On the basis of a sound evaluation of literature's data, the Co/Ni ratio in pyrite proves a more reliable indicator in ore genesis than previously retained.
Indeed, this is particularly true for volcanogenic pyritic ores which are constantly characterized by Co/Ni values (>5 but more often >10) definitely higher than those attained by sedimentary or hydrothermal pyrites. Moreover, once stated that an appropriate sampling has been performed, such geochemical studies, extended to as many deposits as possible from a same area, may provide relevant insights into the metallogenic history of the given area. Co-Ni bearing micro-inclusions, solid solutions, peripheral replacement by base metal sulphides, and rounded crystal edges are commonly observed features in hydrothermal pyrites, revealed by rim-core zonation and for the different types of pyrite based on in-situ laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) and electron probe microanalysis (EPMA, BSE images). You should consider that you may have Co bearing micro-inclusions within pyrite lattice such as Alloclasite ((Co, Fe)AsS) (a member of the arsenopyrite group) and Carrollite, (CuCo2S4), a sulfide of copper and cobalt.
On the basis of early formed hydrothermal pyrites Co/Ni ratios in a metallogenic model: a primary emplacement of ore deposits, followed by later deposition of smaller, prevalently replacement deposits through hydrothermal partial mobilization of the older ones. Last but not least, such differences indicate complex chemical evolution of the ore-forming fluids, and the overall geology and pyrite data may suggest that the ore-forming fluids were closely linked to magmatic activity in the given ore deposit.
Best wishes,
Shahrokh.
Dear Dr. Liu,
Studying the Co/Ni ratios is a classical topic and was among others investigated by myself (a list of Co/Ni ratios in metapsammopelites and “Kieslager-type deposits” (massive sulfides) has been published in DILL, H.G. (1985) Die Vererzung am Westrand der Böhmischen Masse. - Metallogenese in einer ensialischen Orogenzone. - Geologisches Jahrbuch, D 73: 3-461). The Co/Ni ratios in pyrite and pyrrhotite tend to increase along with increasing T and metamorphic grade, although it is not a real geothermometer. Ni and Co are also strongly affected by the primary depositional environment (degree of oxidation). Ni-Co sulfides and arsenides are quite common as dissemination in metamorphic rocks. Your Co/Ni ratio is higher than 1 and simply the result of the metamorphic story and physical-chemical regime of the host rocks. I suspect that your Co/Ni ratios can successfully be used to distinguish different physical-chemical regimes from each other. There are quite a lot of Ni-Co minerals encountered in metamorphic rocks apart from pyrrhotite and pyrite which are only of subeconomic grade but of significance for the determination of the origin of mineralizations of the so-called “Five-element association” (U-Ag-Bi-Co-Ni) (pentlandite, bravoite, millerite, Ag pentlandite, pentlandite, violarite, Ni siegenite, Co siegenite, polydymite, gersdorffite, cobaltite). They have been found in a super-deep drill hole down to 9000 m depth.
You should not only focus on Ni and Co but also place some emphasis on the Ag contents and the S/As ratios of these minerals .
With kind regards
H.G.Dill
Hi Dr Zhankun Liu,
I think high Ni and Co in your pyrite may reflect that it precipitaed from a magmatic fluid causing phyllic alteration. Although Ni and Co contents of pyrite in gold-bearing Qtz veins are lower than the pyrite in the altered rocks, their Co/Ni ratios may be similar - if so, it suggests that these pyrite may have formed from similar hydrothermal systems.
Regard,
Xue-Ming Yang
Dear Dr Liu,
both Ni and Co are transition elements that are present in ultramafic and mafic magmatic rocks. However Ni is representative of hydrothermal fluid with high interaction fluid/mafic rocks.
Mainly :
- the first hydrothermal fluid are low ration fluid /rock and the rocks are fresh not altered and contain the initial metal stock.
- the late fluids are higher ratio fluid/rock and the rocks are altered and depleted on Ni and Co.
best regards
Prof. A. WAFIK
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