ARMSTRONG: FORT FRASER MAP-AREA 3l

This type of alteration was presumably brought about by dilute
carbonate solutions rising along the fracture zones. The following
chemical reaction may represent in part the alteration of serpentine
to carbonate: |

3HiMegsSi01+3CO.=3MgCO;+2Si0.+2H,0

serpentine magnesite
As the carbonate is ankeritic and not pure magnesite, the iron and
calcium must be accounted for. The iron probably came from the
serpentine, as iron protoxide in many cases replaces a small part of
the magnesia in the serpentine. The calcium may have been absorbed
from rocks with which the solutions came in contact. The quartz
mixed with the ankeritic carbonate may be the silica released by the
reaction.

The carbonate-talec rocks are less abundant than the carbonate-
quartz-mariposite rocks. They have a greasy feel, are greenish buff,
and are composed of approximately 40 per cent ankeritic carbonate
and 60 per cent talc. Specks of magnetite, chromite, pyrite, and
chalcopyrite occur as accessory minerals. The high percentage of talc
imparts to these rocks a dense appearance. Angular fragments of
unreplaced serpentine are found in them.

This type of alteration was presumably brought about also by
dilute carbonate solutions. The chemical reaction might be expressed
as follows:

2HiMg;Sis0o+3CO2=HaMg:(SiOs)4+3MgCO;+3H,0

serpentine talc . magnesite
This reaction it will be noted differs from the first only in the
larger proportion of carbon dioxide to serpentine. This suggests that
the concentration of the carbonate solutions may have decided the
nature of its end products.

On the north slope of Mount Williams several outcrops of fibrous,
white tremolite schist mottled with green chlorite were observed.
Hess (1933, pp. 636-637) states that the mineral succession series—
hornblende, actinolite (tremolite), chlorite, talc, and carbonate—is
characteristic of steatitization and if the temperature is sufficiently
high at the start of the alteration, hornblende will be the first mineral
to form; otherwise any lower temperature mineral may be the first
to form. These tremolite schists may, therefore, have been formed
by the same solutions that caused steatitization, but at somewhat
higher temperatures. |