Lineaments; ineffective guides to ore deposits

Abstract

For many years some geologists have been much impressed by real or imagined linear arrangements of various geologic features: drainage lines, faults, joints, volcanoes, ore deposits, and even anticlines. Such real or fancied lineaments have commonly been attributed to deep-seated wrench (strike-slip) faults. Some seem to regard the San Andreas fault as typical; others refer to zones as much as 100 miles wide as lineaments, but surely no one would suggest that a zone that wide would be a guide to ore. In this paper no zones wider than ten miles are considered lineaments.Some geologists consider lineaments to be the record of worldwide stress fields embracing the entire crust. This paper reviews some of the literature in an attempt to evaluate the significance of lineaments. I conclude that, although sea-floor lineaments-marked as they are by both magnetic and sounding evidence•are indeed important structures, the significance of lineaments in the far more beterogenous continental crust has been greatly exaggerated; the value of lineaments as guides to ore is virtually negligible. Most orebodies, even the greatest such as Butte or Bingham, are so small compared to, say, a ten-mile-wide zone, .that their discovery would not be significantly facilitated even though exploration is restricted to the zone. Most postulated ore zones are more than ten miles wide, and many that have been postulated can be shown to be nonexistent. Lineaments T•E suggestion that the earth's crust is •divided into segments by rectilinear faults dates back at least toElie de Beaumont's "somewhat fantastic pentagonal theory of the arrangement of mountain chains" (quotation from W. H. Hobbs, himself a supporter of the tetrahedral theory of the earth). Hobbs (1904, 1911, 1912) was much impressed by linear arrangements of various topographic and geologic features in New England. The lineaments he recognized are composite: the line of a topographic scarp may be continued by a straight stretch of stream, this by a geologic contact or a fall line.The most prominent lineaments he recognized trend about N 5 ø W, a second, less prominent, set is oriented N 50 ø E, a third is about N 45 ø W, with less well-defined sets trending N 35 ø W, N 44 ø W, N65 ø E, N75ø W, andN85 ø W. Clearly almost any fault trend drawn at random would lie close to one or another of these trends. The spacing ooe t•he nets Hobbs recognized ranged from 40 to 125 miles; their width varied as the scaled width of a pencil line varies with the scale of the map on which it is drawn. Hobbs conceded that lineaments are best recognized on more generalized small-scale maps; they tend to become obscure or zigzags on larger scale maps.On a much smaller scale, Hobbs found joints in the granite of Cape Ann to fall into sets striking N 20 ø to 25 øW, NtoN5 øW, aboutN15 ø E, N30 ø to 35 ø E, about N 45 ø to 50 ø W, due E, and N 44 ø E. He attached regional significance to the principal trends. It would be hard to find a New England lineament trend, however, that differs greatly from one of tho...

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