Mineralogy, Host Rock Alteration, and Fracture Pattern at Mine Hill and Copper Mine Point, Virgin Gorda, British West Indies

Ratte, Charles

Mine Hill and Copper Mine Point occupy the southeastern tip of Virgin Gorda, which is situated 70 miles ENE of Puerto Rico. The host rock is quartz diorite and shows varying degrees of alteration with the development of clay minerals, sericite and epidote in the feldspars, and chlorite, biotite and epidote developed at the expense of hornblende. The alteration associated with the sulfide mineralization involves the development of quartz-K-spar veins. There are two prominent fracture sets. One set strikes N-S with dips ranging from 60° R through vertical to 70° W. Slikensided surfaces and brecciation are associated with the N-S set of fractures. The second set of fractures strike essentially E-W and dip 60°-80° S. Sulfide mineralization is most prolific in the N-S set, of fractures, occurring in association with quartz-K-spar fissure veins 1/2" to 2" wide, and in massive quartz, pods 4' to 5.' wide located along a faulted aplite dike near the top of Mine Hill. E-W fractures are tight, show no introduction of quartz and K-spar, and carry little indication that sulfide' mineralization was extensive. Primary sulfide minerals are sparse. These include pyrite, chalcopyrite and molybdenite. Copper oxides are quite abundant in the fissure veins on the east slope of Mine Hill. Spotty occurrences of molybdenum oxide (ferrimolybdite?) were recognized as coatings on primary molybdenite. Limonite is ubiquitous. Sulfide minerals are associated With the quartz-K-spar fissure veins. It was obvious that the greater the concentration of fractures the' greater the concentration of valuable minerals. The area was mapped on this basis, with five fracture-concentration zones established. I. 0 to 5 fractures /10' II. 5- to 10 fractures /10' III. 10 to 25 fractures /10' IV. 10 to 25 fractures /10' W/pronounced evidence of shearing V. Zone of major shearing and alteration.

Mineralogy, Host Rock Alteration, and Fracture Pattern at Mine Hill and Copper Mine Point, Virgin Gorda, British West Indies

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