Subsurface and Office Representation in Mining Geology 1029 



examined and mapped whether outcrops are present or not. Areas where 

 the bedrock is covered should be clearly indicated and accounted for, so 

 that clear distinction between observation and interpretation and examined 

 or unexamined areas will result. Inferred contacts are never represented 

 on detail maps even for short distances. A factual compositional repre- 

 sentation of over-burden usually makes apparent the location of subsoil 

 contacts (veins, dikes, lithology, and general features) ; contact lines are 

 undesirable if no outcrops are present. 



Soil, gravel, mine dumps, tailings, and mine or slope fill are repre- 

 sented texturally and compositionally. Soil representation should blend 

 with outcrop representation along the boundary area, because in the field 

 the demarcation between outcrop and overburden is usually indefinite and 

 gradational. 



Fine soil is represented in gray; if fragments or clastic materials are 

 observed in the soil, relative amounts, composition, and lithology of such 

 fragments should be qualitatively and quantitatively indicated by super- 

 imposed color dots (clastic symbol). Representation symbols are shown 

 on the accompanying legend chart. The compositional colors are those 

 used to represent outcrop lithology; soil containing kaolinized granite, 

 vein quartz, and limonite is indicated by gray with independently super- 

 imposed green, orange, red, and purple dots. An air brush may be used 

 on office maps to apply fine-textured gray background as well as the coarse- 

 colored stipples. 



Man-made deposits (dumps, mine fill, and tailings) are potential ore 

 reserves and are represented with the same care of geology in place. With 

 increased metal prices, lower handling and processing costs, and increased 

 metallurgical recovery, man-made deposits, especially if old, are* now 

 profitably exploited. Much production of Butte copper is from old "gob" 

 (mine fill) and tailings. The "gob" is mapped with the same meticulous 

 care used in maping virgin ground. The amount and type of vein material 

 and ore-fragment attitude are indicated on geologic note sheets. The 

 same colorimetric legend is utilized for mapping man-made deposits as for 

 unbroken rock. The representation is so stippled as to indicate fragment 

 composition and degree of angularity. Detail compositional side notes 

 are supplementary and essential. 



Detail representation of man-made deposits facilitates geologic deter- 

 mination of inaccessible ore deposits and mine excavations. Careful map- 

 ping of mine dumps will give indications of the length of inaccessible mine 

 tunnels, lithology, distance to veins, type of ore, etc. 



Stratigraphic Representation — Stratigraphy is usually not mapped in 

 the field by mining geologists. The stratigraphic "contact" units are de- 

 termined and indicated on office maps by superimposing very light pastel 

 colors over the lithologic representation. It is sometimes difficult to de- 

 termine the lithologic factors localizing ore deposits; empirical strati- 

 graphic controls are in some cases determined by stratigraphic representa- 



