Essays #3 and #4, the last two essays on the monograph by Cameron et al. (1949), are now posted on the PIG page. As I noted in the Overview (Essay #1), this monograph is available for purchase in PDF from the bookstore of the Society of Economic Geologists.
Essay #3 , Fracture-Fillings and Replacement Bodies, reviews their presentation and discussion on the occurrences of fracture-fillings and replacement bodies. Their presentation seeks to make a clear distinction between those fracture fillings that have the compositions of inner pegmatite zones, and fracture fillings whose compositions are not those of a pegmatite zone. They regard the former as part of the primary development of zones from pegmatite-forming melt, and the latter as subsolidus hydrothermal veins. Read the section in the monograph, and my discourse on it, and you will see that this distinction gets tricky when they talk about hydrothermal quartz veins that emanate from primary quartz cores. Replacement bodies are trickier yet. They consider that reaction of early-formed minerals with more fractionated residual melt is a possibility for some replacements, but mostly they regard replacement as a hydrothermal phenomenon that follows the development of the primary stage of magmatic zones. Most of the examples and illustrations of replacement bodies are taken from work by R.H. Jahns in New Mexico.
Essay #4, Origin of Pegmatite Units, is their brief interpretation of the mountain of facts they have accumulated. In that conclusion, they state that pegmatite zones develop by the fractional crystallization of pegmatite-forming melt (and other terms used with some ambiguity) from the walls to the center of a pegmatite body in an essentially closed system. Most fracture-fillings and replacement bodies are regarded as hydrothermal in origin and follow the development of the primary zones because fracture fillings and replacement bodies are superimposed on solid rock. Their discussion contains an assessment of three working hypotheses for the origins of pegmatite units. Among the various models that they test with their data, one that is paramount today is conspicuously missing.
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