Geologic descriptionBoulder Creek drains across the north contact of the Late Cretaceous (78.8 +/- 2.9 my; Hudson and Arth, 1983. p. 769) Cape Mountain biotite granite where it intrudes Mississippian marble and limestone (Sainsbury, 1972). Detrital cassiterite is present throughout the steep and boulder-clogged headwater reaches but the placer deposits are best developed at lower elevations (100 to 250 foot surface elevations) where the stream gradient decreases and the drainage enters onto the coastal lowland. The upstream portions of this placer in Boulder Creek valley characteristically have a well defined, narrow (less than 80 feet wide) pay streak with an average tin content of one pound tin per cubic yard (based on 22 churn-drill holes; Mulligan, 1996, p. 19). The lower part of the placer, where the drainage enters onto the coastal lowland, is more dispersed, both laterally and vertically. Here pay can be 30 to 40 feet thick and 1,200 feet or more in width. The tin content of this more dispersed, lower part of the placer is less; 54 churn-drill holes indicate average grades of 0.5 pounds (or less) tin per cubic yard. This lower grade part of the deposit does not extend downstream below about 100 feet surface elevation. Beach deposits with shell fragments were encountered in some churn-drill holes from lower parts of the placer deposit (at about 80-100 feet elevation). Reworking of alluvial deposits by beach processes may be responsible for the more dispersed character of this part of the placer (Mulligan and Thorne, 1959, p. 47-48). Some of the placer concentrates are radioactive. Seven samples had eU contents of 0.003 to 0.021 %. This radioactivity is approximately proportional to the amount of monazite and xenotime that is present. Most of the radioactivity may be due to thorium but traces of uranium were identified in the samples (Mulligan, 1966, p. 62). |
