Chap. 7] GRAVITATIONAL METHODS 73 



sition, depth of crystallization, porosity, depth of burial, moisture, and 

 so on, is desirable. 



1. Densities of igneous rocks generally increase with a decrease in Si02 

 content. For instance, the average density of granite is about 2.65, that of 

 gabbro, 3.00; the density of quartz porphyry is 2.63, that of diabase, 2.95; 

 the density of rhyolite is 2.50, that of basalt, 2.90. Holocrystalline igneous 

 rocks solidified at greater depth generally have a higher density than 

 effusive igneous rocks of the same chemical composition. Hence, the 

 igneous rocks older than Tertiary are heavier than Tertiary and younger 

 igneous rocks, as is seen from the tabulations in paragraph 2 following. 

 The density of igneous rocks decreases with an increase in the amount of 

 amorphous material. Igneous magmas solidified as volcanic glasses are 

 generally lighter than magmas with more crystalline matter. For instance, 

 the average density of basalt is 2.90; that of basaltic glass, 2.81; the 

 density of rhyolite is 2.50, but that of rhyolite glass is 2.26. 



2. Mean densities of ore bodies as a function of mineral composition. 

 Although the density of most commercial minerals is high, its influence 

 upon the mean density of an ore body is not always so great as may be 

 expected because of irregular distribution or lack of concentration through- 

 out the ore body. Table 5, which gives average quantities of commercial 

 ore per 100 tons of mined material, illustrates the small contribution to 

 the mean density of an ore body which may be expected from even the 

 heavy minerals. 



Table 5 



AVERAGE QUANTITIES OF COMMERCIAL ORE PER 100 TONS OF 



MINED MATERIAL 



Iron 25-45 tons Copper 1-2 J tons 



Manganese 10-25 " Tin 0.5-1 



Chromium 10-25 " Nickel 0.75-2.5 



Zinc 6-12 " Mercury 0.7-1 



Lead 5-12 " Silver 0.020-0.075 



Gold 0.0008-0.0015 



Source: F. Beyschlag, P. Kruach, and J. H. L. Vogt, Die Lagerataetten der niitzbaren Mineralien und 

 GeaUine, Vol. 1, p. 216 (1914). 



On the other hand, an association with (usually noncommercial) minerals 

 may increase the mean density of an ore body; for example, if silver or 

 zinc is associated with lead; copper and gold with pyrite; nickel with 

 pyrrhotite, and so on. If the quantities of minerals present are well 

 enough known, it is a simple matter to compute mean densities. For an 

 ore body containing 75 per cent quartz and 25 per cent galena, the re- 

 sultant density is 0.75 X 2.6 + 0.25 X 7.5, or 3.82. The tests in Table 6 

 were run on ore samples containing quartz as gangue and containing 



