Rob and all,
Yes at Yerrington it is suggested that the Na-Ca-alteration is associated
with locally derived triassic-jurasic sequence brines - 2/3 of the
radiogenic Sr from the alteration assemblage has this signiture! Also in
the deeper portions of the system episodic sequences of magmatic fluid
dominated k-alteration is overprinted by Na-Ca-alteration. The Na-Ca
assemblage has inferred maximum temperatures of around 400oC and is not
associated spatially with sulphide mineralisation unlike the k-alteration.
It is also suggested that at Yerrington the cooler out-flow zones of these
Na-Ca-fluids develope intermediate pthe propylitic alteration and shallow
level hem-mt alteration.
I guess that you would also want to see an increase on some scale of the
quantity of Na +- Ca in the rock as those essential minerals that you list
can all be produced during closed system cooling in a typical diorite!!!
See :
Dilles, JH & Einaudi, MT; 1992. Wall rock alteration and hydrothermal flow
paths about the Ann-Mason porphyry copper deposit, Nevada - A 6 km vertical
reconstruction: Econmic Geology, v 87, p1963-2001.
Carten, RB; 1987. Sodium-calcium metasomatism: Chemical, temporal and
spatial relationships at the Yerrington, Nevada, porphyry copper deposit:
Economic Geology, v81, p1495-1519.
Cheers
James
At 18:48 10/01/01 +0000, you wrote:
>I made a summary, from various sources, a while back and it is:
>
>Essential albite-oligoclase, epidote, actinolite
>Major chlorite, apatite, diopside, magnetite
>minor hematite, calcite, quartz, rutile, titanite, scapolite
>
>As well as in porphyry systems and those enigmatic Chilean iron deposits,
similar alkali metasomatism is reported on a regional scale in arc
volcanics (GSA Bull 110, p. 326, Geology 23, p.913), and is attributed to
isotopically heavy, mod to highly saline waters. Sound like marine or
lacustrine waters developing in enclosed basins.
>
>Could such basins have existed above the Chilean deposits, and porphyry
systems like Yerington? Is the presence of this alteration (saline
foprmation waters) a coincidence, or does it play a fundamental role in the
mineralising system?
>
>I would like to know because it is easy to muddle the above alteration
with propylitisation, which also can be regionally developed in contact
metamorphic, burial metamorphic or geothermal environments.
>
>Rob
>
>----------------------------------------------
>Dr Robert CR Willan,
>Magmatic-Hydrothermal Processes
>Geological Sciences Division,
>British Antarctic Survey,
>Cambridge CB3 0ET, UK
>Tel: 01223 221420
>FAX: 01223 362616
>Email: [log in to unmask]
>-----------------------------------------
>
>
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