EXPLORATION ’92 — Hillsborough to explore El Oro camp

Toronto-based junior Hillsborough Resources (TSE) is currently evaluating the possibility of reopening the historic El Oro gold camp northwest of Mexico City. The El Oro gold district produced more than 175 million grams gold (5.6 million oz.) and 1.6 billion grams silver (52 million oz.) prior to cessation of production in 1927.

Hillsborough is earning an interest in the 280-sq.-km property from publicly owned Industrias Luismin S.A. de C.V. of Mexico City. The El Oro mining district is 115 km northwest of Mexico City and falls within the north-northwest (N-NW) trending, 40 km wide by 600 km long, Mexican silver belt.

According to historical documents, gold-silver production in the area was mainly from two clusters of N-NW striking veins: the Tlalpujahua and El Oro clusters.

The western Tlalpujahua cluster is dominantly silver-bearing with subsidiary gold. Since it outcropped, the minerals were easily exploited by the Spanish colonists. Most production from this cluster took place during the 1700s and early 1800s. The area was actively explored up to the 1950s, but has seen only limited production since 1810. The principal veins in this cluster are the Borda and Coronas.

The El Oro cluster was the main source of gold production from 1896 to 1927. Minor intermittent production continued until 1959. The vein cluster is capped by up to 200 metres of Tertiary volcanics and, except for a small outcrop area, is completely blind. Most of the veins were discovered by underground drifting from known workings. The two main veins in this cluster are the Rafael and Verde.

Hillsborough plans to focus its exploration efforts on the Rafael and Verde veins in the El Oro cluster.

Both veins are oxide veins, consisting of quartz-carbonate and chlorite material. The veins are localized within N-NW trending faults that cut a succession of shallowly dipping, metamorphosed, Jurassic-aged sediments that are now termed phyllites. They are capped by a near-horizontal succession of unmineralized Tertiary-aged volcanics.

The Rafael vein is three kilometres long, averages 30 metres in width, and has yielded 11.9 million tonnes grading 10.7 grams gold per tonne and 115 grams silver.

The Verde vein is two kilometres long, averages 12 metres in width, and has yielded 6.3 million tonnes grading 12 grams gold and 160 grams silver. Old mine records indicate that these veins were selectively mined, and that much of the vein material left behind grades 3-5 grams gold or higher. Stope backfill material in several areas has yielded values of up to five grams gold and 54 grams silver.

Initial calculations suggest that the in situ vein remnants and stope fill contain more than 14 million tonnes grading 2-6 grams gold and 75-150 grams silver.

Several quartz-sulphide veins are also present on the property. These veins, subparallel to the Rafael and Verde veins, contain minor galena and pyrite. The sulphide veins tend to be narrower but higher in grade than the oxide veins. At this time, Hillsborough is concentrating on developing the oxide veins since they are amenable to heap-leach processing.

The property also contains an additional one million tonnes of tailings estimated to contain three grams gold per tonne.

Hillsborough plans to begin a $1-1.5 million exploration program on the property before year-end. Hillsborough’s Vice-President of Exploration, Robert Hewton, says that the aim of the program is to prove up ore that can be mined quickly and cheaply.

The first phase of exploration will be a mercury sampling program designed to delineate extensions of the Verde vein and to look for new veins east of the existing mine workings under the volcanic cap rocks.

The second phase will be to drill 3-4 sections on the southern, higher-grade sections of the Rafael vein to determine the grade of in situ vein material and stope backfill. Drilling will also be done on the southern extension of the Verde vein and other new targets delineated by the mercury sampling. The last phase of the program will be to collect bulk samples of stope fill from the Rafael vein and carry out leach tests on the tailings. Hillsborough is funding the exploration program from its working capital. If the first phase of the program is successful, Hillsborough hopes to use its engineering and mining skills to rehabilitate the extensive underground workings and study the feasibility of mining the deposit.