Yeguas Hill Area
In the Yeguas hill, which is located in the southern sector of the mining district of Andacollo, 6.5 km to the south-east of the homonymous town, a group of gold veins incipiently explored and exploited can be found. One of them is called “Desconfianza”, mine owned by Minera Cordillera del Viento S.A. There are also other important mines in the area such as Helena, El Porvenir, 17 de Octubre, La Victoria, Martha Ester, Salamanca and the veins of Yeguas Hill; among all of them, the mine Helena has been the most exploited one and is the best known.
Structure and mineralization of the main veins
This mine is situated in the south-west slope of Yeguas hill, in the confluence of the streams Atreuquito and Milla Michicó. There, three veins of lentiform habits can be found, fitted in volcanic rocks of the formation La Premia. (Llambías et al, 2007). They present directions ranging between N 74º and 66º E and inclinations of 70º to 75º to S. The strike reaches 300 meters and the power of the veins varies from 0.30 to 0.70 meters.
The mineralization is made up of galena, sphalerite, gold pyrite and chalcopyrite in quartz veinstone, with limonite, malachite and azurite among the oxidation minerals.
Chemical tests made on three mineralization representative samples showed the following results: Pb: 3,3-47,7% - Zn 5,5-30,5%; Cu: 0,7-1,80%, Au: 2-4,5 g/t ; Ag 100-300 g/t. 75 meters of drifts were made in the mine Helena. (Rovere et al, 2004).
Located immediately to the north of the vein Helena, it includes 6 properties of 6 hectares each one. Two of them are placed adjacent and parallel to the vein Helena, while the remaining ones are located in the direction N-S at 90º in relation to the first ones. The disposition of the properties East-West cover the main structure of the vein whereas those North-South cover less developed fractures of general course NE-SW. The veins are placed in the same wall than those in the environment, and just like its neighbors, the mineralization is represented by filling of gold pyrite, associated with galena and sphalerite, in quartz veinstone with abundant limonite. The mining works consist in a drift or main entry of about 25 / 30 meters (currently flooded) along the vein E-W, sub-vertical and with power ranging from 0,35 to 0,70 m. Trenches and cuts along veins that run in diagonal to the main one show occasionally some guides of quartz with pyrite of poor development and thickness.
Placed in volcanic rocks of the same geological entity, mainly consisting in rhyolite and compact volcanic tuffs with dacitic to rhyolitic composition and minor proportion of andesitic terms. This site contains sub-parallel quartz veins of 0.40 to 0.60 m. of power, as filling of gravitational fractures of strong dip. Gold is associated to quartz in oxidation areas and to pyrite and chalcopyrite in primary mineral areas. The gold content varies from 4 to 8 grams per ton.
Veins of Yeguas Hill
A group of gold pyrite veins is located in the slope E and SE of the Yeguas Hill with power of more than 1 meter in some cases and strikes of 200 to 300 meters or even more. This is the case of the vein called “Quebrada” which can be followed with some interruptions along 350 meters; its direction is N 80º E with an inclination of between 75º to 80º to the north. The mining works consist in several superficial openings done in oxidation areas and a couple of poorly developed drifts where the pyric filling is exposed. Together with the vein “Quebrada I” sub-parallel to the previous one, they were partially exploited obtaining a mineral with an average content of gold of 10 g/t (Stoll, 1957). A third vein called “Reina” is located in the lower slope of the hill approximately 300 meters south from the previously mentioned veins; its direction is also E-W, also inclined with strong dip to the north and its power rarely surpasses 0.30 m.
The vein called “Colorada” can be found in the southern part of the area. Its surface is rich in limonite and hematite, which explains its name. A sample of 1.20 meters gave a trial of 2 g/t. (Stoll, 1957). At the end of the drift, which is no longer than 25 meters, the vein reaches only 30 cm wide, containing plenty pyrite in clay filling. A sample of this part showed 10 g/t. Au.
Veins of the string Milla Michicó
Stoll (1957) mentioned a group of minor veins located at both sides of the string called Milla Michicó, downstream of its confluence with the stream Atreuquito. A four parallel vein system can be found there: the vein “Milla Michicó” to the north of the homonymous string and veins “Don Pedro”, “Salas” and “Schmidt” on the south bank. In general, they are veins of quartz with pyrite, galena and some sphalerite with direction N 70º E with power of 0,20 to 0,80 meters and grades ranging between 4 and 15 g/t Au.