Summary:
All of the prospects at Calcatreu, including the Vein 49, are epithermal gold or gold-silver bearing vein occurrences of the low sulphidation type. Lindgren (1933) classified a number of precious metals, base metal, mercury, and stibnite deposits as epithermal deposits. He also suggests that these deposits are formed by the discharge of hydrothermal fluids from magmatic sources at low temperatures (< 200° C). Sillitoe (1987) states that it is now generally accepted that precious metal deposits forming from meteoric waters with temperatures between 200 to 300°C are classified as epithermal deposits.
The known economic precious metal mineralisation at Vein 49 is best described as a low sulphidation banded quartz-adularia epithermal system.
Mineralisation at Vein 49 is assigned to the low sulfidation type, based on the presence of colloform to crustiform banding of quartz and adularia, the general lack of sulphides, the fine-grained nature of the sulphides present, and the alteration mineral species and their distributions.
The mineralisation is hosted in altered (silicified and clay-altered) brecciated host rock. The host rock is interpreted to be porphyritic andesite lava. The highest grades of mineralisation appear to be related to the zones of multiple brecciation and polyphase quartz-carbonate clay healing. Mineralisation trends (in the form of vein quartz) associated with the host units for Vein 49, Belen and Nelson are graphically.
Generally, the mineralised system is very low in sulphide content with minor pyrite and lesser galena and sphalerite. No significant concentrations of minerals containing copper, arsenic or bismuth are reported in the system. The mineralisation has been oxidised to an average depth of approximately 75 m. The surrounding host rocks have been oxidised to an average depth of approximately 30 m.
Gold occurs as electrum and as free gold. Grades average about 3 g/t Au with Vein 49 generally being higher and Nelson being lower. Individual assays were recorded as high as 60 g/t Au over widths of 1 m to 5 m. Mineralisation is largely restricted to quartz and calcite veins and stockwork. The zones of veining can be up to 20 m wide. Surrounding the quartz veining is a zone of argillic (altered) andesite. These rocks generally show low gold content, usually less than 0.5 g/t Au but locally in excess of 1.0 g/t Au, often when accompanied by silicification or quartz veinlets. The silver to gold ratio is approximately 10:1.
The depth of oxidation from surface varies from 29 m to 120 m and is strongly influenced by the erosional surface. The shallowest depths to primary mineralisation occur in the valley separating the Nelson area from the Vein 49 ridge.
The mineralised zone shows illite alteration and strong weathering near surface. The main clays associated with the mineralisation are illite and smectite. Kaolinite is very well developed in the hanging wall of the main Vein 49 structure and is interpreted to be the result of supergene weathering.
Petrological examination (Corbett, A., 1998) of a typical sample from Vein 49 revealed an intensely and finely polyphase-banded, fine-grained, quartz-pyrite filled structure showing illite alteration and strong weathering near surface.
In summary, the Calcatreu mineralisation controls show geological and mineralisation characteristics, typical of low sulphidation epithermal deposits. Vein systems at 11 prospects have been delineated within the project area. The Vein 49/Nelson system, which has a strike length of over two kilometres and widths of up to 20 m, has been the most intensely investigated of these and is the significant discovery at Calcatreu.
The Vein 49 and Nelson mineralisation has been the most intensely investigated of these and is the most significant discovery at Calcatreu. The Vein 49/Nelson mineralisation has been identified over a strike length of 2.5 km, to a maximum vertical depth of 370 m. Mineralisation widths vary from two metres to 20 m, averaging at 10 m width for the main mineralisation zones identified by drilling and outcrop trench mapping and sampling. For the Castro Sur prospect, mineralisation has been interpreted over a strike length of 1.7 km, down to a maximum vertical depth of 300 m. Mineralisation true width typically varies from one metre to 10 m thick.