Laserfiche WebLink
2.1 Volcanic Breccias . <br />Volcanic breccias occupy much of the volume of the diatreme and represent a diverse class of <br />rocks. They are best described as poorly sorted agglomerates of heterolithic rock fragments <br />supported by a rock flour ± crystalline matrix. Rock fra<*ments (clasts) range in size from <br />microscopic to several meters in diameter. Clasts are typically equidimensional and subrounded, <br />with the degree of rounding indicative of transport distance within the diatreme. The breccias <br />have compositions that vary according to their clast types, but have a bulk composition that is <br />generally phonolitic (see descriptions in Lindgren and Ransome, 1906, p. 97-100). Clast types <br />include phonolites of various textures (porphyritic, aphanitic and trachytic), coarsely crystalline <br />syenites and subordinate populations of mafic alkaline rocks. Precambrian rock fragments are <br />common, and become progressively more abundant towazds the contact with the surrounding <br />Precambrian rocks`. The breccia matrix varies from indurated rock flour to crystalline, with <br />crystalline matrices becoming more abundant at depth. At all levels of exposure, the breccia <br />matrix is pervaded by xenocrystic feldspaz fragments. Bulk compositions of the matrix vary <br />substantially according to alteration types, but can be generally described as phonolitic. <br />All known exposures of breccia have been hydrothermally altered to varying degrees. The • <br />volcanic breccias have high permeabilities and porosities relative to other rock types, making <br />them particularly susceptible to alteration. The original textures and compositions of the <br />breccias have been largely obscured by multiple episodes of hydrotherma] alteration. Alteration <br />types are diverse (as discussed below), ranging from high-T, biotite-stable assemblages to ]ow-T <br />adularia flooding. The breccias have been variably sulfidized and carbonated, with sulfides and <br />carbonate minerals replacing mafic igneous phases, occupying void spaces and flooding the <br />matrix in some cases. An increase in carbonation is seen at depth, as discussed below. <br />I Finely pulverized rock fragments <br />Z The contact between the volcanic breccias and surrounding Precambrian rock is typicall}~ gradational; Phonolitic <br />diatremal breccia grades outward into Precambrian breccia, Precambrian crackle breccia and eventually to <br />moderately fractured, but unbrecciated Precambrian rock. The contact zone is typically 10-100 meters wide. <br /> <br />Crrpp/c Crcck d~ rinor Gold mining Co. Sl~rphrrd Afi!lrr, /nr. <br />1VJ/AFlONDIP-ORII'EIl WJ6llggonUnnrFwrne..ibr 3 Afnrr/i ?~()~ <br />