Stephen E. Kesler

Analysis of fluoride in rocks and an application to exploration

Abstract The fluoride ∗ content of rocks can be measured using a selective ion electrode. Analyses were made using a sodium hydroxide fusion and ammonium citrate buffer and measurements can be compared to standard or can be made by standard addition using the Grans Plot method. The Grans Plot method yields fluoride values in agreement with known values for standard rocks. Values obtained by these two methods vary sympathetically. Furthermore, whole rock fluoride values vary sympathetically with water-leachable fluoride values, which permits use of simpler, less expensive methods for field exploration purposes. Application of these procedures to an area of tin mineralization indicates that easily distinguishable anomalous fluoride values for water leach and total fluoride in rocks are associated with areas of tin mineralization.

Tectonic Significance of Intrusive Rocks in the Maya Mountains, British Honduras

The Maya Mountains horst of British Honduras, is collinear with, and probably geologically related to, the Cayman Ridge in the Caribbean Sea. It contains one batholith of highly differentiated granite (Mountain Pine Ridge batholith) with Rb/Sr values of 8 to 16, and two modal muscovite granodiorite batholiths (Hummingbird and Cockscomb) with Rb/Sr values of 0.5 to 3.6. The Mountain Pine Ridge batholith is a true granite and is modally and chemically distinct from the dominantly granodioritic post-Jurassic intrusive rocks of the Caribbean and Central America. The Hummingbird and Cockscomb batholiths, although identical to the post-Jurassic rocks in relative quartz–plagioclase–alkali feldspar abundance, contain modal muscovite, which, along with the high K 2 O and SiO 2 contents of the batholiths, distinguishes them from the post-Jurassic rocks. The absence, from recent dredge hauls, of granitic material similar to British Honduran granitic and granodioritic rocks precludes the use of these submarine granitic exposures as definite evidence for pre-Jurassic crust in the submarine part of the Caribbean region.

Early Cretaceous basement rocks in Hispaniola

A medium-grained, foliated amphibolite found in the Cordillera Central of the Dominican Republic near the Haitian border yields concordant K-Ar ages of 123 ± 2 m.y. for hornblende and plagioclase. The rock exhibits a low Rb/Sr ratio (0.0217) and present-day 87Sr/86Sr ratio (0.7024) but differs in chemical composition from metamorphosed mafic rocks of the Duarte Formation. A previously reported K-Ar age of 127 ± 6 m.y., obtained on a hornblendite body from the Duarte Formation, agrees closely with the amphibolite age. The hornblendite also differs in composition from the metamorphosed mafic rocks of the Duarte Formation and from the amphibolite. The 123- to 127- m.y. ages are interpreted to represent a minimum date for a metamorphic event that affected the basement complex of Hispaniola. An unpublished K-Ar date for the Bermeja complex in Puerto Rico falls within this range, but older ages have been obtained from rocks in Cuba. A date of about 120 m.y. indicated by one interpretation of a previously reported conformable lead isotopic composition for galena from the Pueblo Viejo gold deposit in the upper part of the Los Ranches Formation volcanic rocks in the Dominican Republic also agrees closely with this 123- to 127-m.y. range; together with other data, it suggests that the extrusion of Los Ranchos volcanic rocks and metamorphism of the Duarte Formation could be genetically related.

Early island-arc intrusive activity, Cordillera Central, Dominican Republic

Reconnaissance studies of early island-arc intrusions in the Cordillera Central of the Dominican Republic demonstrate that these rocks are mainly hornblende tonalite with lesser amounts of hornblende diorite, quartz diorite, granodiorite and quartz monzonite. Two plutons (El Bao, Medina) are petrographically and chemically homogeneous, whereas two others (El Rio and Loma de Cabrera) are compositionally heterogeneous. Samples from these intrusions range in SiO2 from 49 to 70% with most rocks in the 59 to 62% range. K2O ranges from 0.24 to 3% and averages 1.2%. Cu, Zn, Co, Ni, V and possibly Cr decrease with increasing SiO2. Rb/Sr values for the intrusions are low but variable. Present-day 87Sr/86Sr values range from 0.7031 to 0.7045 for the El Bao and Loma de Cabrera batholiths and 0.7033 to 0.7091 for the Medina stock. These data do not generate isochrons. The Cordillera Central tonalite intrusions are the most abundant plutonic rock type in the Greater Antilles, although small, younger granodiorite and quartz monzonite stocks are present. The Cordillera Central intrusions are lower in SiO2, K2O, Rb, and Sr than the average composition of the Sierra Nevada batholith, but they are similar to the tonalites and trondjhemites from the western margin of the Sierra Nevada batholith. The low Rb/Sr ratios and low initital Sr87/Sr86 ratios for the Cordillera Central intrusions combined with the high liquidus temperatures required for the generation of tonalite magmas strongly favor a subcrustal source for these magmas in an island-arc setting.

An evaluation of the use of halogen and water abundances in efforts to distinguish mineralized and barren intrusive rocks

Abstract Analysis of the chlorine, fluorine and water content of approximately 200 samples from a total of fourteen mineralized and ten barren intrusive rocks from the Caribbean and Central America indicates that abundances of these constituents fail to distinguish mineralized rocks from barren rocks. Variations in background abundances arise from the increase in halogen content of potassium-rich rocks and from the depletion of halogens in altered and porphyritic rocks. A particularly well developed potassium-fluorine covariance is observed in tin-bearing granites, but such a covariance cannot be used to distinguish intrusive rocks associated with porphyry copper mineralization.

Compositional evolution of intrusive rocks in the eastern Greater Antilles island arc

Major-element chemical compositions of the Utuado, Morovis, San Lorenzo, Rio Blanco, Rio Vivi, Torrecilla, and Guyon intrusions in Puerto Rico and the nearby Virgin Islands batholith indicate that there was a decrease in K 2 O in the intrusive magmas over most of the last 40 m.y. of eastern Greater Antilles island-arc evolution (80 to 40 m.y. B.P.) with a possible slight increase after 40 m.y. B.P. The average rate of change of K 2 O content was about −0.06% K 2 O/m.y. This compositional evolution is similar to that observed in the intrusive rocks of the south-western United States. By analogy to interpretations made for that area, the observed compositional changes could have been caused by gradual flattening of the angle of subduction of the Caribbean plate beneath the south side of the eastern Greater Antilles until 40 m.y. B.P. and subsequent initiation of subduction along a new trench to the north.

Amphibolites of the Cartersville District, Georgia

The Cartersville district, which is bisected by the boundary between the Valley and Ridge and Piedmont provinces, contains thick lenses of thinly bedded, greenschist facies amphibolite in the southeastern (Piedmont) part that appear to correlate with lenses of carbonate-rich sedimentary rock in the Lower Cambrian Rome Formation to the northwest. Chemical analyses of the Cartersville amphibolites suggest that the magnitude and trend of their compositional variation can be best explained if the amphibolites were derived from a sediment containing minerals typical of the Lower Cambrian sedimentary environment in this area. This result casts doubt on the commonly assumed igneous parentage for the widespread amphibolites of the southeastern Appalachians.