Diatremes : Characteristic Textural Features and Mineralization

Douglas J.Kirwin

Ivanhoe Mines Ltd., Vancouver

Abstract

Diatremes are large scale shallow level volcanic events produced by the explosive interaction of ascending magmas and meteoric water. They are often multiphase pipe-shaped bodies which may be associated with precious and base metal ore deposits. Diatremes have three principal components; a vent (the pipe itself), an annular tuff apron (pyroclastic ejecta around the pipe), and a maar (epiclastic and carbonaceous sediments deposited in the crater void at the top of the pipe). Each of these features has recognizable diagnostic textures providing the exploration geologist with clues as to the levels of preservation and erosion of a diatreme complex. The vent breccias comprise hetrolithic rounded to subrounded clasts ranging in size from millimeters to metres in diameter set in a rock flour matrix. The clast to matrix ratio is typically less than 1:1 and fluidized bedded textures are common, especially in the upper zones of the vent. The lower sections of some diatreme pipes tend to have more equigranular clasts in contrast to upper levels which often contain large to very large slumped wall rock blocks. Tuff aprons are comprised of loosely consolidated pyroclastics derived from explosion of the vent. Characteristic textures are layers of accretionary lapilli and current bedding derived from base surge. The size and distribution of tuff apron varies according to the magnitude of the phraetomagmatic event and the prevailing wind directions at the time. Large tuff aprons can also contain rounded clasts of subvolcanic porphyries derived from deep down in the vent. Maars are complex features with well-bedded carbonaceous sediments and intercalated epiclastic units displaying rapid and frequent compositional variation. Subject to the force of the phraetomagmatic explosion, the vertical extent of the maar infill may be in excess of 100 metres. There is often an irregular transition zone upwards from the top of the vent to the basal part of the maar. Carbonised wood fragments are sometimes observed at these levels.

Most diatreme complexes are multiphase events formed during several explosive pulses related to the intrusion of porphyritic subvolcanic domes and dykes; all of which probably took place in a relatively short period of time. Epithermal precious metal mineralization and in particular, high sulphidation gold-copper system related to volatile–charged dacitic dome complexes are often associated with diatreme volcanism and sometimes have a deeper level porphyry copper component. Widespread zones of enargite, barite and gold are typical mineral assemblages found above, overprinting or adjacent to porphyry copper mineralization. The more alkalic volcanic settings seem to generate low sulphidation epithermal deposits with gold mineralization. In general mineralization tends to be concentrated near or at the margins of diatreme vents. This is because of greater permeability in fracture zones at the margins as opposed to much less permeable areas within the vent that have high proportions of impermeable rock-flour matrix. Conversely there are examples where the bulk of the mineralization is disseminated with the vents. It is important to note that in many instances, late stage base and precious metal bearing vein systems overprint the diatremes. Careful observation of clasts when mapping a diatreme complex can reveal important clues with respect to mineralization. Sulphide-bearing clasts in the tuff apron or maar may indicate a highly mineralized vent breccia, while mineralized quartz-veined clasts in a vent breccia might be derived from an underlying porphyry copper system.

Because of their unconsolidated nature and shallow thickness, tuff aprons are easily eroded or poorly preserved. Key recognition features are accretionary lapilli, base surge bedding, and sometimes mineralized or subvolcanic clasts. Tuff aprons can be very difficult to distinguish in the field from non-diatreme related volcaniclastic piles and often the only recourse is their 3D geometry in relation to a known maar or vent.. Maars may be identified by the presence of well-bedded carbonaceous strata intercalated with chaotic epiclastic units that frequently have mineralized clasts. Detailed mapping is required to place them in context with the overall vent geometry and in many cases this may only be discernable in drill core. Vents can primarily be identified by breccias exhibiting fluidization textures. Hetrolithic breccias with rounded to subrounded clasts set in a normally grey colored rock flour matrix tend to be very distinctive in many diatremes. The margins of diatreme vents are diagnostically very sharp displaying steep-sided wall rock contacts. The upper parts of vents (transitional with lower maar facies) may possess house-sized wall rock blocks that have slumped or spalled from the vent margin. Large blocks of tuff apron or maar sediments with disorientated bedding may also be present, creating much confusion during core logging. The lower parts of vents tend to be much more massive and regular in terms of clasts distribution, in that the clasts appear smaller, more rounded and have less compositional variation, within a highly fluidized silicified matrix. Where only the basal part of a diatreme is exposed in the field, it may be very difficult to distinguish from a hypabyssal pipe that did not have a phraetomagmatic component. Useful but not infallible guides are the degree of fluidization of the breccia, (and hence a low clast to matrix ratio) and steeply dipping sharp wall rock contacts. It is noted that kimberlite pipes are diatremes, which are not discussed in this presentation, other than to say they possess all the textural features outlined above, and are initiated at lower crustal levels.

Diatreme complexes can host very significant precious and base metal ore deposits, eg. gold and silver at Cripple Creek in Colorado and Rosia Montana in Romania, and copper at El Teniente in Chile. Diatremes are excellent