G. P. Merrill— Free Gold in Granite. 309 



Art. XXX VI. — An Occurrence of Free Gold in Granite • 

 by George P. Merrill. 



The specimen described below was received at the National 

 Museum at the close of the New Orleans Exposition 

 (1884-85). It was labelled as simply u gold ore, Sonora, 

 Mexico," and appeared on casual inspection to be an ordinary 

 mica granite with small disseminated particles of free gold 

 scattered over the surface. It was seen at once to be some- 

 thing worthy of investigation, although at the time it was 

 assumed to be doubtless a case of local impregnation of the 

 rock forming the wall of a vein. The sample was laid aside 

 and quite forgotten until the' fall of 1895, when the writer's 

 attention was once more called to it while preparing for the 

 Atlanta Exposition a series of specimens illustrating the occur- 

 rence of gold and silver in nature. It has now been the 

 subject of more careful study, with the results as given below : 



As above noted, the rock * appears on casual inspection to 

 be an ordinary black- mica granite considerably weathered, 

 somewhat brownish and quite friable. Quartz, feldspar and 

 black mica are all conspicuous and easily determined, the feld- 

 spars occurring in crystals some 5-10 mm in diameter. The 

 striking feature of the rock is, however, an abundant sprinkling 

 of small flecks of native gold, not merely over the surface, but 

 throughout the entire mass of the specimen, which measures 

 some 125 mm X 80 mm , by 40 mm in greatest thickness. 



Under a pocket lens these flecks are so abundant that in 

 extreme cases a half dozen or more are comprised within a 

 space of 10 mm square. They are very small, rarely exceeding 

 a millimeter in diameter, and are disseminated throughout the 

 rock, not only as interstitial flecks among the mica scales, but 

 apparently enclosed in both the feldspar . .id quartz granules. 

 To finally determine this point, a number of thin sections were 

 prepared and submitted to microscopic examination, with 

 results in every way confirmatory. 



The rock is, as stated, apparently a normal black mica-gran- 

 ite, the feldspar having a pronounced microperthite structure, 

 and being in most cases badly muddied through kaolinization. 

 The mica has undergone a considerable amount of chloritic 

 alteration. Quartz occurs in the customary interstitial gran- 

 ular forms ; fluidal cavities are neither large nor abundant. 

 Magnetite is a common accessory and more rarely apatite and 

 sphene. No sulphides of any kind were detected. The gold 

 occurs associated with the mica or again wholly imbedded in 



* Specimen 64987 U. S. National Museum Collections. 



