> The magnetite deposits of Chile have reported Ca-Na alteration
> envelopes surrounding the ore zones, which are reported as evidence
> for a metasomatic genesis to theses deposits. My problem is how to
> identify this type of alteration in the field, especially when it is
> reported in a fine to medium grained tonalitic to dioritic host rock.
> Unfortunately the host rocks contain appreciable amounts of primary
> Ca and
> Na minerals ( ie actinolite, diopside, albite).
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From what I remember (field observations), the actinolite crystals (dark
green in colour) can be really big (up to 8 cm), at least this is what I saw
in Cerro Iman, one of the Cretaceous iron deposits from northern Chile.
Besides, I don't think you will find actinolite in these rocks as a primary
phase. Another mineral you have to look for is scapolite (not primary
either).
Cheers:
Rob Oyarzun
P.S. Try also Jean Jacques Menard, a French geologist and a good friend of
mine, he did his PhD on these Chilean deposits:
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R. Oyarzun (PhD., C.Eng., MIMM)
Dept. de Cristalografia y Mineralogia
Facultad de Ciencias Geologicas
Universidad Complutense
28040 Madrid, SPAIN
Telephone: 34-91-394 48 78
Fax: 34-91-394 48 72
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