بنك الاسئلة EMR 2010 2ND SEMESTER 443

I) Write shortly on three of the following: (15 Marks)

1)      Stream Sediments Sampling Survey

Stream sediment surveys are very useful for mineral exploration because of greater dispersion in the stream environment.  Greater dispersion means greater ability to detect an ore body from a greater distance.  A drainage basin is an area with a network of streams like the branches of a tree:  smaller streams join together leading into larger and larger streams.  It is assumed that the values will decrease downstream from the source, so following the “path” of increasing values upstream. may lead to mineralization

2)      Potassic and Argillic alteration Types.

Potassic:   (Biotite, K-feldspar, Adularia)  Potassic alteration is a relatively high temperature type of alteration which results from potassium enrichment.  This style of alteration can form before complete crystallization of a magma, as evidenced by the typically sinuous, and rather discontinuous vein patterns.  Potassic alteration can occur in deeper plutonic environments, where orthoclase will be formed, or in shallow, volcanic environments where adularia is formed

Argillic:  (Clay Minerals)  Argillic alteration is that which introduces any one of a wide variety of clay minerals, including kaolinite, smectite and illite.  Argillic alteration is generally a low temperature event, and some may occur in atmospheric conditions.  The earliest signs of argillic alteration includes the bleaching out of feldspars.
A special subcategory of argillic alteration is “advanced argillic”.  This consists of kaolinite + quartz + hematite + limonite.  feldspars leached and altered to sericite.  The presence of this assemblage suggests low pH (highly acidic) conditions.  At higher temperatures, the mineral pyrophyllite (white mica) forms in place of kaolinite.

3)      Zoned vein Deposits.

-Zoned vein deposits are deposits which form along fractures and faults as open-space fillings or replacements.  They are generally polymetallic.  Many have been mined for copper, lead and zinc, although substantial gold and silver credits occur locally.  These deposits generally fall in the category of low tonnage, high grade types of deposits.  There are two broad categories: 1) vein deposits associated with porphyry base metal deposits, and 2) vein deposits not associated with porphyry base metal deposits. 

-Zoned vein deposits which are associated with porphyry base metal deposits appear to form at lower temperatures during a later mineralization event.  These veins are characterized by a strong sense of zoning from high temperature minerals in proximal (closer to the pluton) portions of the veins, to low temperature minerals in distal (far away) portions of the veins.   Proximal portions of the veins are copper-rich and contain sulfide minerals with high metal:sulfur ratios.  Distal portions of the same veins are lead-zinc-rich and contain sulfide minerals with lower metal:sulfur ratios.  At Butte, Montana, alteration halos adjacent to the veins change dramatically along the length of the vein and with increasing distance from the central porphyry copper-molybdenum deposit.  Proximal portions of the veins are characterized by advanced argillic alteration adjacent to the vein which is superceded outwards by sericitic alteration.  Distal portions of the veins are characterized by propylitic alteration adjacent to the vein which gives rise to fresh unaltered rock further away from the vein.  Zoned vein deposits which are not associated with porphyry base metal deposits are characterized by having moderate, more uniform temperatures over a larger area.  Zoning in these types of vein deposits is usually a function changes in the fugacity of sulfur along the length of the vein

4)      Placer Minerals

n  Two types of minerals form placers:  1) minerals which are more resistant to chemical and mechanical erosion (called “resistant” minerals), and 2) minerals which have high specific gravities (called “heavy minerals”).  There are three categories of resistant minerals, including those which are relatively inert (non-reactive), those which are malleable (tend to bend rather than break), and those which have greater hardness :

n  Inert Minerals: 

n  Inert Oxide Minerals:   

n  Chromite, Magnetite, Ilmenite

n  Inert Silicate Minerals: Zircon

n  Maleable Minerals:             

n  Gold

n  Hard Minerals:                         

n  Diamond     & Corundum

5)      Structural controls on mineralization

Nearly all hydrothermal deposits exhibit some degree of structural control on mineralization.  Structures (fractures, faults or folds) which form prior to a mineralizing event are referred to as “pre-mineral”.  Geologists are keenly interested in pre-mineral structures because these structures influence the localization of ore by hydrothermal fluids utilizing these pathways.  By mapping these structures and projecting the geometry in the subsurface, new ore deposits may be discovered. Structures which form after a mineralizing event, and hence may be responsible for offset or removal of mineralized zones, are referred to as “post-mineral”. Fractures and fault zones provide excellent pathways for hydrothermal fluids to circulate through. Open-space filling has long been recognized as the primary method of vein formation. The formation of breccia and gouge due to the grinding action of the rocks adjacent to the fault plane increases the ‘structural porosity’, which in turn increases the permeability.  Under certain conditions, breccia or gouge may itself provide the host for mineralization

II) Choose: (3 Marks)

1)      Soil samples collected from the (“A” horizon – “B” horizon – “C” horizon) can  detect a mineral deposit from a greater distance. 

2)      The (grade – tonnage - halo) of the ore is the average concentration determined from numerous assays of drill samples.

3)      The (positive – possible – probable) ore is exposed and sampled on four sides.