Hi Des
A good review of this type of alteration is presented for the Yerrington
system in Dilles & Enaudi, 1992 (Economic Geology). From my experience at
Yerrington mine the NA-Ca alteration is rpesent as epidote + act + Na-plag
pods and veinlets - however I had the same problem with this as you - how
do you identify Na-Ca alteration as opposed to late stage magmatic
minerals? Marcos Enaudi said he felt that you have to see the alteration
of ksp to albite to be really sure you are dealling with externally derived
water Na-Ca-type alteration. I guess some people here may have other ideas???
Hope this helps
James
At 13:24 10/01/01 -0000, you wrote:
>Hello all,
>
>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).
>
>Has anybody encountered such alteration and do you have any ideas for
>the
>field identification of this alteration, especially in tonalites and
>volcaniclastic rocks.
>
>Many thanks in advance,
>
>Des Leech
>
>
>-----------------------------------
>Desmond Leech
>
>Centre for Earth and Environmental Science Research,
>School of Earth Sciences and Geography,
>Kingston University,
>Penrhyn Road,
>Surrey KT1 2EE.
>
>PH. (0)20 85472000 ext 2215
>FAX (0)20 85477497
>EMAIL [log in to unmask]
>
>
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