_Geochemistry, Mineralogy, and 

Geochronology of the 

U.S. Virgin IslandS" 

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Geochemistry, Mineralogy, and 

Geochronology of the 

U.S. Virgin Islands 

By HENRY V. ALMINAS and EUGENE E. FOORD 

U.S. Geological Survey 

ROBERT E. TUCKER 

U.S. Army 

U.S. GEOLOGICAL SURVEY BULLETIN 2057 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1994 
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary 

U.S. GEOLOGICAL SURVEY 

Gordon P. Eaton, Director 

For Sale by U.S. Geological Survey, Map Distribution 

Box 25286, MS 306, Federal Center 

Denver, CO 80225 

Any use of trade, product, or firm names in this publication is for descriptive purposes only and 

does not imply endorsement by the U.S. Government 

Library of Congress Cataloging-in-Publication Data 

Alminas, Henry V., 1938-

Geochemistry, mineralogy, and geochronology of the U.S. Virgin Islands I by Henry V. 

Alminas and Eugene E. Foord [and] Robert E. Tucker. 

p. em. - (U.S. Geological Survey bulletin; 2057) 

Includes bibliographical references. 

Supt. of Docs. no.: I 19.3:2057 

1. Geochemistry-Virgin Islands of the United States. 2. Minerals-Virgin Islands of 

the United States. I. Foord, Eugene E. II. Tucker, Robert E. (Robert Edward), 

1952- . III. Title. IV. Series. 

QE75.B9 no. 2057 

[QE514] 

557.3 2-<lc20 93-7254 

[551.97297'22] CIP 
CONTENTS 

Abstract.......................................................................................................................... 1 

Geographic Setting......................................................................................................... 1 

Geology ...... _. ...... ........... ................ ........... ..................... .. . . . . . ......... ... . . . .. . .. ...... ... .. . . . . ........ 2 

Geologic Age.................................................................................................................. 7 

Sample Collection .. . . . . . . . .. . . .......... .. .. .... .......... ............. .. .. . . . .. ................................. .. . .. . . . .. 8 

Sample Preparation ............ .. . . .. ... . . . . ................ .... ... . . . . ........ ... . . . ............. .... ... . . .. . . . . . . ... . . .. . 8 

Analytical Techniques.................................................................................................... 8 

Geochemistry . . . . . . . .. ................ ......... ............ .. . . . . .. . . . . . . ............ .. .. . . . .. . . . .. . . .. . . . .. . ... . . . . ........ .. 9 

General................................................................................................................. 9 

St. Croix ........................................................................ ...................... ................. 12 

St. John................................................................................................................. 16 

St. Thomas............................................................................................................ 18 

Mineralogy . ............. .. . .. .............. ............. ........... ................ .. ... . . . . ............... ... .. ....... ....... 21 

Native Elements and Alloys................................................................................. 21 

Halides.................................................................................................................. 31 

Sulfides................................................................................................................. 31 

Sulfates, Phosphates, and Carbonates .................................................................. 31 

Silicates ............ .................................................................................................... 32 

Laboratory Crystal Synthesis............................................................................... 32 

Discussion :··..................................................................... ..... ........................................... 32 

Summary........................................................................................................................ 35 

References ....................................... ................................................... ............................ 35 

FIGURES 

1. Index map of the U.S. Virgin Islands and vicinity................................................................................................ 2 

2-4. Simplified geologic maps of: 

2. St. Croix........................................................................................................................................................ 3 

3. St. John ...................................................................................... ................................................................... 5 

4. St. Thomas.................................................................................................................................................... 6 

5-7. Sample preparation flow sheets for: 

5. Soil and stream-sediment samples................................................................................................................ 8 

6. Heavy mineral concentrates of soils and stream-sediments......................................................................... 9 

7. Outcrop samples . . . . . ............ ... . . . ......... ............. ... .... . . . . .. . .. . . . . . . . .................. .................. ........ ... . ... . .... . ..... . .. . . . . . . 9 

8. Graphs of a comparison of tin concentrations in six sample media from St. Croix.............................................. 11 

9. Map of tin and lead distribution in the nonmagnetic fraction of heavy min~ral concentrates and in 

magnetites on St. Croix .................................................. ; ........................................................... ~.......................... 12 

10. Map of gold distribution in B-horizon soils and rocks on St. Croix..................................................................... 13 

11. Graphs showing association relationships for five geochemical anomalies on St. Croix..................................... 14 

12. Map of tin and lead distribution in the nonmagnetic fraction of heavy mineral concentrates and in 

magnetites on St. John........................................................................................................................................... 15 

13. Map of gold distribution in B-horizon soils and rocks on St. John....................................................................... 16 

14. Graphs showing association relationships for four geochemical anomalies on St. John...................................... 17 

15. Map of tin and lead distribution in the nonmagnetic fraction of heavy mineral concentrates and magnetites 

on St. Thomas........................................................................................................................................................ 18 

16. Map of gold distribution in B-horizon soils and rocks on St. Thomas.................................................................. 19 

17. Graphs showing association relationships for four geochemical anomalies on St. Thomas................................. 20 

III 
IV CONTENTS 

18. Photomicrograph of native tin/lead nugget (SC514) coated by a complex, fine-grained mixture of 

Pb-Sn oxides, hydroxides, oxychlorides, with fine blebs of native Pb and Sn ..................................................... 22 

19. Photomicrograph showing detail of the eutectoid core portion and rind of the grain depicted in 

figure 18 (SC514).................................................................................................................................................. 23 

20. Scanning electron microscope photomicrograph showing a close-up view of the eutectoid intergrowth of 

native Sn and Pb in the core portion of the SC514 nugget ............ ......................... .. .......... .......... ........................ 23 

21. Backscatter electron photomicrograph of a portion of a Pb/Sn nugget from the graben area on St. Croix.......... 24 

22. Compositional map of grain pictured in figure 21 showing the mineralogical complexity common 

to the Sn-Pb nuggets.............................................................................................................................................. 24 

23-25. Electron dot photomicrographs of: 

23. Pb distribution in grain shown in figure 21 .................................................................................................. 24 

24. Cl distribution in grain shown in figure 21. .................................................... .. ......... ............................... .... 24 

25. Sn distribution in grain shown in figure 21 .................................................................................................. 24 

26-28. Photomicrographs of: 

26. Eutectic intergrowth of romarchite/cassiterite and Pb oxides, hydroxides, carbonates, and 

oxychlorides with a blade-like native tin fragment cross-cutting the eutectic texture in a 

grain from St. John...................................................................................................................................... 25 

27. Nugget from St. Croix showing several euhedral crystals ofCu6Sn5 in a complex, layered 

matrix of romarchite/cassiterite and Pb-Sn oxides, and oxychlorides ........................................................ 25 

28. Cu6Sn5 crystals in nugget shown in figure 27 .............................................................................................. 26 

29. Electron dot photomicrograph of Sn in the eutectoid portion of nugget (SC514) shown in figures 18 and 19 .... 26 

30. Electron dot photomicrograph of Pb in the eutectoid portion of nugget (SC514) shown in figures 18 and 19 .... 26 

31. Backscatter electron photomicrograph of a portion of a mineralogically complex Cu nugget............................. 27 

32-36. Electron dot photomicrographs of: 

32. Cu in the portion of nugget (SC514PA) shown in figure 31 ........................................................................ 27 

33. Pb in the portion of the nugget (SC514PA) shown in figure 31 ...................... ............................................. 27 

34. S in the portion of the nugget (SC514PA) shown in figure 31 ..................................................................... 27 

35. Sn in the portion of the nugget (SC514PA) shown in 31 ........................ .................................. .......... ......... 28 

36. Cl in the portion of the nugget (SC514PA) shown in figure 31 ................................................................... 28 

37. Mineral distribution map of the portion of the nugget (SC514P A) shown in figure 31 .. ..................................... 28 

38. Backscatter electron photomicrograph of a portion of Cu nugget (SC514PA) showing a nickel-bearing 

Cu5.

6Sn(?) alloy intergrown with native Cu and associated with chalcocite, cotunnite, sphalerite and 

other minerals........................................................................................................................................................ 29 

39. Electron dot photomicrograph map ofCu in the part of nugget (SC514PA) shown in figure 38......................... 29 

40. Electron dot photomicrograph map of Sn in the part of nugget (SC514P A) shown in figure 38 ......................... 29 

41. Photomicrograph showing anhedral, lenticular masses of tin oxide within a magnetite/maghemite grain.......... 29 

42-45. Scanning electron microscope photomicrographs of: 

42. Discrete segregations of fine-grained tin oxide crystals in a magnetite/maghemite grain........................... 30 

43. Sn chloride/oxychloride and Sn-Pb chloride/oxychloride filling a vug in a rock sample of the 

Caledonia Formation ................................................................................................................................... 30 

44. The Sn-Pb chloride/oxychloride shown in figure 43 .................................................................. .................. 30 

45. Romarchite (SnO) in a rock sample............................................................................................................. 30 

46. Synthetic minerals grown at room temperature and one atmospheric pressure.................................................... 33 

47. Block diagram illustrating the conceptual model of precious- and base-metal mineralization in the 

St. Croix graben, U.S. Virgin Islands.................................................................................................................... 34 
CONTENTS v 

TABLES 

1. X-ray fluorescence and six-step emission spectrographic analyses of olivine gabbro, gabbro, and diorite from 

St. Croix, U.S. Virgin Islands................................................................................................................................ 4 

2. A listing of rock samples from St. Thomas and St. John used for K/Ar age dating. ............................................ 7 

3. Analytical techniques............................................................................................................................................. I 0 

4. Proportional distribution of Sn, Pb, Au, and Ag in the U.S. Virgin Islands.......................................................... 12 

5. Minerals and compounds found to occur in the weathering environment in the U.S. Virgin Islands.................. 21 

6. Microprobe analyses of native Sn and Pb nuggets occurring in eutectic or eutectoid intergrowths from the 

U.S. Virgin Islands . . . ......... ................ .... . ... .. . .. . . . .. .... .. .. .. . ... . . . . . . ......... ........... .. ..... ... . . . .. ............ .......... ........... ........ .. 22 

7. Microprobe analyses of native Cu, chalcocite, and cuprite in copper nugget 84SC514PA from 

St. Croix, U.S. Virgin Islands................................................................................................................................ 28 

8. Microprobe analyses of Cu6Sn5 (11-bronze) from three sample sites in the U.S. Virgin Islands. ......................... 28 

9. Microprobe analyses ofCu5.

6Sn(?) phase from copper nugget from sample site 84SC514PA from 

St. Croix, U.S. Virgin Islands................................................................................................................................ 31 

GEOCHEMISTRY, MINERALOGY, AND 

GEOCHRONOLOGY OF THE U.S. VIRGIN ISLANDS 

By H.V. Alminas, E.E. Foord and R.E. Tucker 

ABSTRACT 

This geochemical and mineralogic study of the U.S. 

Virgin Islands indicates the presence of a previously undocumented Sn-Pb and precious metals metallogenic province 

in the northeastern portion of the Greater Antilles Island arc. 

These islands consist of island-arc related Cretaceous and 

Tertiary volcanic and volcaniclastic rocks, carbonates, and 

near-surface intrusives. 

The mineralized areas on the three main islands (St. 

Thomas, St. John, and St. Croix) are closely related to major 

faults and transect all rock types. They are characterized by 

widespread alteration (silicification and sericitization) and 

an identical geochemical association consisting of Au, Ag, 

Te, Sn, Pb, Cu, Zn, Ni, Cr, Sb, As, Bi, Mo, Cl, and Ba. This 

association is exceptionally chlorine-rich and is strongly 

deficient in W, F, and S. The mineralization probably took 

place in middle and late Miocene (33-20 Ma). 

The base-metal mineralogy in the secondary environment within the mineralized areas is highly unusual and 

complex and is dominated by halides and native metals. The 

native metals identified include Au, Ag, Sn, Pb, Cu, Te, Bi 

and a Cu-Sn alloy. Several incompletely characterized Sn 

and Sn-Pb chlorides or oxychlorides, in addition to cotunnite 

(PbCl) and paratacamite (Cu2(0HhCl) are widespread. The 

rare mineral romarchite (SnO) is more widespread than cassiterite, which is usually poorly crystallized. The native metals and the chlorides/oxychlorides are generally found 

intergrown in a complex manner within individual grains. It 

is fairly clear, however, that the mineral progression in the 

case of tin is from Sn chloride/oxychloride to romarchite to 

cassiterite within the weathering environment. 

Synthesis studies in which Pb- and So-rich saline solutions were introduced into a CaCOrrich environment at 25° 

C and 1 atmosphere, produced a Pb-Sn mineralogy nearly 

identical, both morphologically and compositionally, to that 

found in outcrop samples from the U.S. Virgin Islands. 

The unusual mineralogy and the laboratory studies 

indicate that the tin-, lead-, and precious metals-rich 

hydrothermal solutions might have been injected into calcareous ooze in a shallow submarine environment. The 

immediate vicinity of hot solution introduction might have 

been substantially enriched in NaCl as a result of convection cells in the sea water. All metals, aside from gold, were 

then precipitated in the form of chlorides and some native 

metals. Subsequently, organic acids related to vegetation 

might have served as electron donors to convert some of 

the chlorides to native metals. 

The metallogenic province seen in the U.S. Virgin 

Islands very probably extends into the geologically and 

structurally similar British Virgin Islands located immediately to the north. 

GEOGRAPHIC SETTING 

The U.S. Virgin Islands are part of the Greater Antilles 

Island arc and are located to the east and southeast of Puerto 

Rico (fig. 1). They consist of three major islands (St. 

Thomas, St. John, and St. Croix) and some 50 smaller islands 

that are located primarily along the coasts of St. Thomas and 

St. John. 

St. Thomas, the northwesternmost island, is located 64 

km east of Puerto Rico and has an area of approximately 91 

km2. St. John is located immediately to the east of St. Thomas and is approximately 54 km2 . St. Croix is 216 km2 and 

is located some 64 km to the south of St. Thomas. 

St. Thomas and St. John are characterized by mountainous topography, with very irregular coast lines and numerous small islands adjacent to the coastlines. The topography 

on St. Croix is more subdued, its coastlines are regular and 

only two small islands occur along the northeastern coast. 

The climate is maritime tropical with average annual 

rainfall at higher elevations in the 127-152 em range and that 

at lower elevations in the 51-76 em range. No well defined 

wet or dry season exists. Temperatures generally average 

29° C with only a 4-5° C variation through the year. 
2 GEOCHEMISTRY. MINERALOGY. AND GEOCHRONOLOGY OF THE U.S. VIRGIN ISLANDS 

Caribbean Sea 

0 200 

I ,· •' I 'I I 

0 100 

Figure 1. Regional setting of the U.S. Virgin Islands. 

I 

200 

The streams on these islands are intermittent with generally steep gradients and short courses before emptying into 

the ocean. 

GEOLOGY 

In a regional context, the U.S. Virgin Islands are in the 

northeastern portion of the Greater Antilles Island arc. The 

Puerto Rico trench is to the north, the Annageda trench to the 

east and the Muertas trough to the southeast. Schell and Tarr 

400 

I I 

600 KILOMETERS 

I 

300 MILES 

( 1978) postulated that in this northeastern comer of the Caribbean plate, the portion of the plate above the critical melting 

depth is carried along with the rest of the American plate 

beneath the edge of the Caribbean plate. The stresses exerted 

on the American plate, as it is bent and twisted, probably 

cause fracturing into segments and blocks and the reactivation of existing zones of weakness, such as oceanic fracture 

zones that originated near the Mid-Atlantic Ridge during initial rifting. This block-like segmentation and interaction of 

blocks may explain why the Puerto Rico trench appears block 

faulted on the surface but yields focal mechanism solutions 
GEOLOGY 3 

Kj Judith 

0 1 2 3 4 KILOMETERS 

I' I I I I I I 

I 

0 1 2 MILES 

EXPLANATION From Whetten (1966) 

Alluvium (Holocene) Mount Eagle Group ---- Contact 

(Upper Cretaceous) 

Kingshill Marl (upper Miocene) 0 Judith Fancy Formation 

_ ___;u:;..__ Normal fault D 

Jealousy Formation (lower Eocene 

to middle Miocene) 

Fountain Gabbro (Late Cretaceous) 

Southgate Diorite (Late Cretaceous) 

~ Cane Valley Formation 

0 Allandale Formation 

~ Caledonia Formation 

--,~-,- Thrust fault 

--1--- Anticline 

-----"1--- Syncline 

Figure 2. Simplified geologic map of St. Croix. 

suggestive of horizontal thrusting and normal faulting. Further, Schell and Tarr suggested that the presence of the postulated east-to-west moving underriding slab offers a 

plausible explanation for such anomalous data as intermediate-depth seismicity, focal mechanism solutions, and gravity 

anomalies. Also, that the east-to west movement does not 

actively supply crustal material to the asthenosphere; thus, no 

volcanism occurs on the Puerto Rico-Virgin Island block. 

The geology of St. Croix was mapped by J.T. Whetten 

in 1959-1961. A simplified geologic map of St. Croix is 

shown in figure 2. Whetten (1962, 1966) indicated that St. 

Croix is underlain by strongly folded Upper Cretaceous and 

gently folded Tertiary sedimentary rocks, and igneous intrusions with contact metamorphic aureoles, Late Cretaceous or 

early Tertiary in age. 

The oldest exposed rocks on St. Croix are those of the 

Mount Eagle Group. The oldest formation in this group is the 

Caledonia Formation that is a 3,000- to 7 ,000-m turbidite 

sequence of volcanic sedimentary rocks of Campanian or 

older age. The Caledonia Formation grades laterally and to 

the south into the Allandale Formation and vertically upward 

into the Cane Valley Formation. The upper part of the Caledonia contains the East End Member. The Judith Fancy Formation overlies the Caledonia, Allandale, and Cane Valley 

Formations. All these rocks contain tuffaceous material or 

fragments of altered glass and of crystalline material deposited originally as volcanic ash. Whetten noted that the rocks 

are probably of marine origin because the sediments show 

signs of reworking by currents. 

After deposition of the Upper Cretaceous sedimentary 

rocks, the region was folded and faulted and intruded by 

igneous rocks. 

Two stocks (Fountain Gabbro and Southgate Diorite) 

were intruded along the axial planes of younger folds (fig. 2). 

The sedimentary rocks adjacent to these intrusions were contact metamorphosed to a pyroxene hornfels facies. In addition, numerous small dikes of highly variable composition 

occur throughout St. Croix (table 1). Lamprophyres are common in the East End Range and basaltic-andesite porphyry 

dikes occur in the Northside Range. Highly felsic rhyolite 

dikes occur at several locations (see table I). Most of the 

faults on the island are normal. A graben in the center of the 

island is filled by approximately 2,500 m of Tertiary sedimentary rocks. Only the upper part of this is exposed. The 

rocks consist of montmorillonitic mudstone of the Jealousy 

Formation. The overlying Kingshill Marl, of late Miocene 

age, is a fossil coral reef. A late Miocene age is indicated for 

this unit by a recent study (Gill, 1990). 
4 GEOCHEMISTRY, MINERALOGY, AND GEOCHRONOLOGY OF THE U.S. VIRGIN ISLANDS 

Table 1. X-ray fluorescence and six-step emission spectrographic analyses of olivine gabbro, gabbro, and diorite from 

St. Croix, U.S. Virgin Islands 

[All iron expressed as Fe203. LOI, loss on ignition; N, not detected at value shown; L, detected, but below value shown] 

Sample no .... 1 2 5R2 14 34 36 37 42 43 44 

Si02············· 49.9 43.8 51.5 45.8 53.8 53.9 47.9 45.8 44.7 58.4 

Al203··········· 20.4 16.9 19.9 18.5 15.8 17.3 14.0 14.3 13.5 17.3 

Fe203··········· 9.97 13.3 8.93 12.8 9.42 6.96 10.6 14.1 15.5 6.07 

MgO ............. 4.55 7.79 4.35 5.91 4.40 5.50 9.42 7.82 8.09 2.43 

CaO .............. 9.32 13.0 10.0 12.2 7.24 9.65 12.2 10.8 12.3 6.68 

Na20 ............ 3.23 1.68 3.19 2.27 3.22 3.29 2.31 1.99 1.57 3.57 

K20 .............. 0.34 0.61 0.70 0.44 1.98 1.25 0.85 0.78 0.34 1.79 

Ti02············· 0.65 1.08 0.57 0.81 1.03 0.39 1.04 0.88 0.98 0.52 

P205 ............ 0.12 1.05 0.16 0.10 0.37 0.44 0.35 0.15 0.16 0.23 

MnO ............. 0.21 0.21 0.14 0.22 0.15 0.21 0.27 0.20 0.24 0.20 

LOI (900°C) . 1.51 1.09 0.97 1.30 2.23 0.99 1.11 3.69 3.00 2.59 

Total ...... 100.20 100.51 100.41 100.35 99.64 99.88 100.05 100.51 100.38 99.78 

B ............... N10 L10 10 L10 10 10 N10 N10 15 20 

Ba .............. 150 200 500 150 2,000 700 500 500 150 1,000 

Co ............. 30 50 50 30 30 50 50 70 70 15 

Cr .............. 10 200 50 50 50 200 700 500 300 20 

Cu ............. 30 300 50 30 50 30 20 70 100 30 

Ga ............. 20 30 30 30 30 30 20 30 20 30 

Ni .............. L5 50 15 5 10 50 70 50 50 L5 

Pb .............. 15 15 15 10 15 20 15 10 10 15 

Sc .............. 20 30 30 30 20 20 30 30 50 15 

Sr ............... 700 1,000 500 300 500 700 500 500 700 700 

v ............... 200 300 150 200 200 150 200 200 300 150 

y ............... 20 30 20 15 20 20 30 20 20 20 

Zr .............. 30 70 50 15 150 50 70 30 30 70 

SAMPLE DESCRIPTIONS 

1. Gabbro collected above Davis Beach at lat. 17°45'30", long. 64°50'17". 

2. Olivine gabbro collected at road junction above Davis Beach at lat. 17°45'34", long. 64°49'16". 

5R2. Gabbro sample collected in quarry located at lat. 17°44'19", long. 64°48'53". 

14. Olivine gabbro collected in a road crust above Davis Beach at lat. 12°45'38", long. 64°49'56". 

34. Gabroic diorite collected at Spring Garden at lat. 17°44'07", long. 64°41'58". 

36. Gabbro collected on Marie Hill at lat. 17°44'33", long. 64°39'46". 

37. Olivine gabbro collected at top of Marie Hill at lat. 17°44'37", long. 64°39'55". 

42. Olivine gabbro dike collected at the Grapetree Resort at lat. 17°44'48", long. 64°36'41". 

43. Olivine gabbro-gabbro dike collected at GrapetreeResort at lat. 17°44'57", long. 64°35'25". 

44. Diorite dike collected at Grapetree Resort at lat. 17°44'57", long. 64°35'25". 

The geology of St. Thomas and St. John has been 

studied by many authors including Cleve (1881), Earle 

(1924), Meyerhoff (1926), and more recently by Donnelly 

(1959, 1966). The rock unit descriptions and structural 

relationships of the rocks have been taken from Donnelly 

(1959, 1966). The rocks in the study area are island-arc 

related Cretaceous and Tertiary volcanic and volcaniclastic 

rocks, carbonates and near-surface intrusives. Simplified 
GEOLOGY 5 

64°50' 64°45' 64°40' 

18°25' ,--------------------------.----------------------,---.---------, 

G'eatThatch lsi~ <:3 ~ 

18°22'30" Congo ------__Qf!E~BRITA/N Kd UNITED sJ-.- - ~~~- ATES -----

Kt Kd ---

Ktc Cay 

Jrrb 

Kt ~Lovango 

~~ K~ay Grass Mingo j.,. "\ 

Cay Cay ~ 

Cruz Bay 

~ 

I 

4 KILOMETERS 

I 

2 MILES 

f! 

Kd 

Kd 

~ 

18°17'30"c__ ___________________ ____J_ ____________________ __._ ___ _____J 

EXPLANATION 

Alluvium (Holocene}-Gravel and sand 

Dioritic rocks (Upper Cretaceous(?) or 

Lower Tertiary(?)) 

Hans Lollik Formation (Upper Cretaceous)-

Augite andesite volcanic breccia and tuff 

Tutu Formation (Lower Cretaceous)-

Volcanic wacke 

Congo Cay Limestone Member 

Outer Brass Limestone (Lower Cretaceous)-

Thin-bedded, siliceous limestone 

Figure 3. Simplified geologic map of St. John. 

geologic maps of St. John and St. Thomas are shown in 

figures 3 and 4. 

According to Donnelly (1959, 1966), the oldest rocks in 

the study area occur along the southern coasts of both 

islands. These rocks consist of keratophyres, spilites, and 

radiolarites, collectively called the Water Island Formation. 

The keratophyres are felsic extrusive or shallow intrusive 

rocks presumably of submarine origin. These rocks contain 

predominantly albite and quartz. The spilite is a mafic rock 

Louisenhoj Formation (Lower Cretaceous)-

Augite andesite volcanic breccia and tuff, 

with minor conglomerate; unconformable 

with Water Island Formation 

k£'K/ 0j Water Island Formation (Lower Cretaceous)-

Keratophyre flows, flow breccias, and tuff; 

radiolarian cherts; spilite flows 

---- Contact 

-~~- Fault-Dotted where covered. Arrows indicate 

relative movement 

* Mountain 

of submarine origin. Within the Water Island Formation the 

spilite is recognized by its slightly greenish outcrops. Chlorite and albite are the dominant minerals present. Remnant 

phenocrysts of diopside and augite are common. Amygdules 

are abundant and generally contain epidote, calcite, or 

prehnite. 

Radiolarian tuffs occur predominantly near the top of 

the Water Island Formation and contain two genera of 

spumellines that resemble Thecosphaera and Flustrella 
6 GEOCHEMISTRY, MINERALOGY, AND GEOCHRONOLOGY OF THE U.S. VIRGIN ISLANDS 

Kt 14 

Kl Ko rf Outer Brass 

~Island 

0J-KI 

Kl 

Salt Cay~~ Qa 

Oa ~ 

~ Savana 

~Island 

0 

I I 

Kw-t" 

Kw~Saba Island 

4 KILOMETERS 1

1 I 

2 MILES Buck --.IIJt Kd 

Island (J)'JJ 

Kw 

18°16' L__L_ _____________ l__ ____________ ___L _____________ __L_ ___ _ 

EXPLANATION 

[ : 0(1 J Alluvium (Holocene)-Gravel and sand 

5J Dikes and plugs (Upper Cretaceous(?) 

or Lower Tertiary(?))-Quartzandesine porphyry; andesinehornblende porphyry a Dioritic rocks (Upper Cretaceous(?) or 

0 Lower Tertiary(?)) 

~ Hans Lollik Formation (UpperCretaceous)-

Augite andesite volcanic breccia and tuff 

Tutu Formation (Lower Cretaceous)-

Volcanic wacke 

Outer Brass Limestone (Lower Cretaceous)-

Thin-bedded, siliceous limestone 

Figure 4. Simplified geologic map of St. Thomas. 

similar to Early Cretaceous (144-97.5 Ma) forms in eastern 

Mexico (Donnelly, 1966). 

The next oldest rocks are those of the Louisenhoj Formation unconformably overlying the Water Island Formation. The former unit is a sequence of pyroclastic to 

epiclastic augite andesites, which are believed to have originated from a nearby vent. 

Overlying the Louisenhoj Formation is the Outer Brass 

Limestone. This unit is about 200 m thick, consisting of 

about 90 percent thinly bedded siliceous limestones and 

about 10 percent interbedded crystal tuffs. These tuffs are 

but a couple of meters thick. 

The Tutu Formation overlies the Outer Brass Limestone. This unit consists of interbedded sequences of fine- to 

Louisenhoj Formation (Lower Cretaceous)-

Augite andesite volcanic breccia and tuff, 

with minor conglomerate; unconformable 

with Water Island Formation 

Quartz keratophyre dikes and plugs 

(Lower Cretaceous) 

Water Island Formation (Lower Cretaceous)-

Keratophyre flows, flow breccias, and tuff; 

radiolarian cherts; spilite flows 

---- Contact u ---=---=::;~;- Fault-Dotted where covered. Arrows indicate 

relative movement. U, upthrown side; 

D, down thrown side 

coarse-grained volcanic wackes that appear to be reworked 

Louisenhoj andesites. Some 2000 m of this unit are exposed 

in the American Virgin Islands. Sedimentary textures in the 

Tutu Formation suggest a high energy environment of deposition (Donnelly, 1966). The Tutu Formation on Grass Cay, 

Mingo Cay, and Lovango Cay and at Mary Point (St. John) 

is now a gametiferous skarn. Within the Tutu Formation, the 

Coki Point Megabreccia lithofacies of Donnelly (1959) is 

composed of large limestone blocks up to 75 em in length, 

porphyritic andesite boulders, and thin bedded wacke (Donnelly, 1966). The breccia beds are generally 10 m in 

thickness. The breccia crops out at Sunsi Bay and Tutu Bay 

on St. Thomas and possibly at Mary Point on St. John. The 

limestone blocks contain a neritic fauna that includes 
GEOLOGIC AGE 7 

Table 2. K-Ar age dates of rock samples from the Water Island and Louisenhoj Formations on St. 

Thomas and St. John Islands 

[Sample number coding: ST, St. Thomas; SJ, St. John] 

Sample Formation Rock 

no. type 

ST115R Water Island rhyolite 

ST120R Water Island rhyolite 

SJ048R Water Island rhyolite 

SJ685R Louisenhoj rhyolite 

SJ747R1 Louisenhoj andesite 

SJ748R Water Island andesite 

SJ765R1 Water Island rhyolite 

gastropods, echinoids, and corals (Donnelly, 1966) The 

presence of Caprinuloidea puts the origin in Albian time 

(113-97.5 Ma). 

In the U.S. Virgin Islands, the Congo Cay Limestone 

Member (of Donnelly, 1959) of the Tutu Formation is a light 

gray recrystallized limestone found only on Congo Cay. 

The Hans Lollik Formation crops out over the entire 

island of Hans Lollik. This formation consist of augite 

andesite, pyroclastic to epiclasic in origin. The Hans Lollik 

Formation is very similar in texture and composition to the 

Louisenhoj Formation. 

Hydrothermal alteration is widespread along the southern shores of both St. Thomas and St. John and on many of 

the small islands along their coasts. Sericitic alteration is 

widespread throughout these two northern islands. Silicification is common and is especially well-developed on Red 

Point (St. Thomas) and Saba Island. 

GEOLOGIC AGE 

The rocks of St. Thomas and St. John have been 

mapped as a layered sequence. The Water Island Formation 

is the oldest; the rocks are younger to the north. The age 

given to the Water Island Formation is based on the age 

determined for the overlying Lower Cretaceous Coki Point 

Megabreccia lithofacies of Donnelly (1959) of the Tutu Formation that contains an Albian (97.5-113 Ma) fauna. 

Two rocks from the Water Island Formation were dated 

by DonnelJy (1966). One rock gave an age of 58-62±5 Ma 

but was presumed to have lost argon, and was discounted. A 

sample of hydrothermally altered rock gave an age of 

106-110±10 Ma (Donnelly, 1966). This date agreed with the 

fossil evidence of Albian age in the overlying Coki Point 

Megabreccia lithofacies of the Tutu Formation and has been 

used as a minimum age for the Water Island Formation. 

Latitude Longitude Age 

(In megannums) 

18°20'08" 64°56'39" 40.7 ± 1.1 

18°19'40" 64°55'52" 65.8 ± 2.9 

18°19'39" 64°45'57" 50.2 ± 5.7 

18°20'30" 64°43'27" 37.7 ± 1.1 

18°20'55" 64°46'25" 42.1 ± 0.6 

18°21'31" 64°42'05" 38.6 ± 0.8 

18°19'23" 64°43'52" 30.9 ± 0.3 

Five felsic rocks from the Water Island Formation on 

St. Thomas and St. John and two rocks of the Louisenhoj 

Formation from St. John were dated using the K-Ar whole 

rock technique (Tucker, 1987). The dating was done by 

Michael Diggles of the U.S. Geological Survey. The felsic 

rocks range in age from 65.8±2.9 to 30.9±0.3 Ma (table 2). 

Samples of the Louisenhoj Formation range in age from 

42.1±0.6 to 37.7±1.1 Ma. 

The rhyolite at site SJ685R lies between the mineralized zone on Bordeaux Mountain and a sulfide mineralized 

zone observed in drill cuttings from a nearby well. The 

absence of extensive alteration suggests that the rhyolite at 

site SJ685R was emplaced after the mineralization process, 

and thus represents a minimum age of mineralization in the 

immediate vicinity. Sample ST115R is only slightly altered 

along hairline fractures. Most of the rocks in the vicinity of 

sample site ST115R show signs of thermal alteration and 

mineralization. This sample of a small rhyolite plug may 

indicate a minimum age of mineralization (41 Ma) in the 

immediate vicinity. 

Intrusive rocks on Puerto Rico range in age from 126 to 

some 37 Ma (Cox and others, 1977) with copper metallization being spatially related to quartz diorite and quartz 

diorite porphyry, which was determined to be about 40 Ma 

(Cox and others, 1977). This age is coincident with the minimum age of mineralization postulated for St. John, and possibly St. Thomas, and probably indicates a regional 

mineralization process. 

In the British Virgin Islands, hornblende and biotite 

from the granodiorite on Virgin Gorda were dated at 34.2 

and 36.3 Ma respectively (Cox and others, 1977). One sample of the Narrows pluton (of Donnelly, 1966) was dated at 

24.3±0.3 Ma (Kesler and Sutter, 1979). These ages are consistent with the rocks dated during this study (table 2). 

A continuous band of garnet-epidote skarn crops out on 

the Cays and extends to Mary Point on St. John. 
8 GEOCHEMISTRY, MINERALOGY, AND GEOCHRONOLOGY OF THE U.S. VIRGIN ISLANDS 

Development of the garnet-epidote skarn is more pronounced in the southern exposures and decreases northward. 

This skarn metamorphism and marbleization (marmorization) continue on to Tortola (Helsley, I960). The presence of 

a skarn mineral assemblage and marble in the sedimentary 

rocks reflects a thermal metamorphism. The Congo Cay 

Limestone Member of Donnelly (1959) of the Tutu Formation has the textural appearance of having been thermally 

recrystallized. On Tortola (British Virgin Islands), the contact between the skarn and marble and the Congo Cay Limestone Member is apparently gradual (Helsley, I960). 

Geological observations and the geochemical data indicate 

that metamorphism occurred before periods of significant 

mineralization. 

On Mingo Cay, a 1.5 m wide iron oxide dike cuts the 

marmorized rocks. This dike shows no indication of having 

been deformed or altered, indicating dike emplacement 

probably occurred after uplift of the sedimentary beds. 

A small gabbroic body occurs in a basin south of Mary 

Point. A recent road cut near the top of the hill within the 

basin exposed a fault contact between the Tutu Formation 

and the gabbro. No evidence could be found that would indicate that the gabbro crystallized in place. Examination of the 

gabbro by SEM (scanning electron microscope) revealed the 

presence of minute veinlets of Agi and barite. This mineralogical assemblage is found throughout St. Thomas and St. 

John in association with hydrothermal mineralization. These 

observations indicate that the gabbro was in place before the 

mineralization occurred. 

SAMPLE COLLECTION 

Samples collected for this study include rock, soil and 

stream-sediments. Stream-sediment samples were collected 

primarily from third-order streams. Soil samples were collected within a 20 m radius of the stream-sediment sites and 

in intervening areas when additional sampling detail was 

required. Outcrop samples were collected at stream-sediment and soil sites, when available, and in areas of special 

interest. A total of605 stream-sediments, 632 soils, and 8I7 

outcrop samples were collected on all three islands. 

Stream-sediment samples were collected as a composite of material over a 2-5m length of the stream. Two samples were collected at each site: a 450 gm sample for sieving 

and a 4.5 kg sample for heavy mineral separation. Soil samples were generally collected at a 20-30 em depth, generally 

within the B horizon. But, commonly, no distinct A-B horizon differentiation could be seen. In some instances bedrock 

was encountered before a distinct B horizon. 

Rock samples were collected as composited 450 gm 

chip samples. Larger quantities of sample material were collected at visibly altered or mineralized outcrops. 

Discard 

>0.177 mm fraction 

450 g soil or stream-sediment 

sample 

Si~ve 80 mesh 

(0.177 mm) 

Split 1 for analysis: 

by spectrograph, and 

by atomic absorption 

for Au 

Save 

<0.177 mm fraction 

I 

Split 

Split 2 

I 

Water leach 

I 

Analysis of leach 

water by ion 

chromatograph 

Figure 5. Sample preparation flow sheet for soil and streamsediment samples. 

SAMPLE PREPARATION 

All sample types were heated for 6 hours at I20° C in 

the U.S.G.S. Denver laboratories in accordance with the U.S. 

Department of Agriculture regulations. 

Figure 5 shows the sample preparation flow sheet for 

the 450 gm stream-sediment and soil samples; figure 6 

shows a flow sheet for the 4.5 kg stream-sediment and soil 

samples; and figure 7 shows a flow sheet for the rock 

samples. 

The oxalic acid leachates were prepared by bringing 5 

gm of the bromoform light fraction to a boil in 20 ml of 5% 

oxalic acid and boi1ing for I 0 minutes. The hot solution 

was filtered through a no. 41 Whatman filter, and the filtrate was brought to dryness on a hot plate. The resultant 

material was heated for 4.5 hrs. at 4500 C in a muffle furnace to convert the oxalates to oxides. The oxides were 

then homogenized by mortar and pestle prior to analysis 

(Alminas and Mosier, I975). 

The water-leach extractions were performed by placing 

I gm of soil in a test tube with I 0 rnl of deionized water. The 

water and soil were thoroughly mixed and the test tube was 

placed on its side. The test tubes were remixed every second 

day. The samples were centrifuged after 8 days and the water 

decanted into a clean test tube for analysis. 

ANALYTICAL TECHNIQUES 

The U.S. Virgin Island samples, collected for the purpose of this study, were analyzed by a number of analytical 

techniques. Table 3 lists the analytical techniques used for 
GEOCHEMISTRY 9 

4.5 kg soil or 

stream-sediment 

I 

~ 

Panning 

Discard light Save heavy 

fraction fraction 

I 

Sieve 18 mesh 

(0.98 mm) 

Save >0.98 mm fraction 

for reference 

Magnetic fraction 

I 

Split 

Save for mineral 

determinations by: 

optical methods, SEM, 

microprobe, X-ray, and 

for future reference 

Hand ,-- grind Heavy 

I fraction 

Analysis ~~h ~ spectrogr~ 

Magnetic separation 

S.G. Frantz Co. LB-1 

isodynamic separator 

0.6 amp., side slope=15 

forward slope = 20 

<0.98 fraction 

I 

Hand magnet 

separation 

Nonmagnetic 

fraction 

I 

Bromoform 

separation 

I 

Light 

fraction 

I 

Oxalic 

acid 

leaching 

I 

Dry leachate 

analyzed by 

spectrograph 

------~------------~ Nonmagnetic fraction 

I 

Split 

Split 1 saved for mineral 

determinations by: SEM, Xray, optical methods, and for 

reference samples. 

Split 2 

I 

Hand 

grind 

I 

Magnetic fraction 

Han~ grind 

I 

Analyze by 

spectrograph 

Analyze by 

spectrograph 

Figure 6. Sample preparation flow sheet for heavy mineral concentrates of soil and stream-sediment samples. 

450 gm 

rock sample 

Split 1 for: 

optical inspection 

slabbing for SEM 

reference sample 

I 

Split 2 

Jaw crusher 

vertical grinder 

to <100 mesh 

(<0.149 mm) 

I 

Analysis by: 

spectrograph and 

atomic absorption 

for Au 

Figure 7. Sample preparation flow sheet for outcrop samples. 

studying the different sample types. As can be seen, all the 

bulk sample types (rocks, <80 mesh soils, <80 mesh streamsediments) were analyzed by the six-step D.C.-arc semiquantitative emission spectrographic method (Grimes and 

Marranzino, 1968) for thirty-one elements. In addition, they 

were analyzed for Au by flame (rocks) or flameless (soils 

and stream-sediments) atomic absorption methods. The 

three heavy mineral concentrate fractions and the oxalic acid 

leachates were analyzed by a modified six-step D.C.-arc 

semiquantitative emission spectrographic method. All of 

the resultant data has been published in two U.S. Geological 

Survey Open-File reports by Hopkins and others (1986) and 

McHugh and others (1989). 

A Cambridge scanning electron microscope with 

energy dispersive spectrometry (Tracor Northern) capability 

was used to examine numerous rock slabs, bulk rocks, and 

processed samples. Electron microprobe analyses were done 

on an ARL SEMQ (scanning electon microscope quantitative) instrument using both natural and synthetic standards. 

Operating conditions were 15 or 20 Kv, 20 second count 

times on peaks, 4 second count times on background positions, spot size 1 to 5 microns. X-ray diffraction studies 

were done using Chas. Supper 114.6 mm Gandolfi cameras, 

and Pillips generator at 40 K v, 30 Ma, CuKa radiation. 

Major element analyses were performed using a Philips 

PW/600 simultaneous x-ray spectrometer that has a Rh endwindow x-ray tube operating at 35 Kv and 60 Ma. Calibration was done using glass disks of 32 reference samples. A 

deJongh model was used for matrix corrections. The Abbey 

compilation (Abbey, 1980) of the best values for major 

oxides was used for all calibrations. A more detailed discussion of the procedure may be found in Taggart and others 

(1987). 

GEOCHEMISTRY 

GENERAL 

This geochemical study of the U.S. Virgin Islands 

has delineated a previously unknown precious-metals and 

lead-tin metallogenic province in the northeastern portion of the Greater Antilles island arc. The mineralization 

detected on these islands (St. Thomas, St. John, and St. 

Croix) is characterized by an identical geochemical association consisting of Au, Ag, Te, Sn, Pb, Cu, Zn, Ni, Cr, Sb, 

As, Mo, Cl, and Ba. Although the geochemical association is common to all three islands, the inter-element 

ratios and overall anomaly intensities vary· from island to 

island and from anomaly to anomaly. 

The nonmagnetic fraction of the heavy mineral concentrates was the most effective sample medium in detecting the 

base-metal portion of the mineralization (fig. 8); whereas, 

low-level (ppb range) gold determinations of <80 mesh 

materials were effective in delineating the precious metals 

aspect. Water leaching of soils and the oxalic acid leaching 

of the bromoform lights fraction were useful in determining 

the mobility of elements in the secondary environment. 

The REM (relative element magnitude) (VanTrump 

and Alminas, 1978) program was used in comparing the 

associations and mineralization intensities between the 
10 GEOCHEMISTRY, MINERALOGY, AND GEOCHRONOLOGY OF THE U.S. VIRGIN ISLANDS 

Table 3. Analytical techniques used in the study of the U.S. Virgin Islands samples 

Sample type Analytical technique 

Outcrop source Six-step D.C.-arc semiquantitative 

emission spectrographic method 

Flame atomic absorption method 

for Au 

< 80 mesh soils Six-step D.C.-arc semiquantitative 

emission spectrographic method 

Flameless atomic absorption method 

for Au 

< 80 mesh source Six-step D.C.-arc semiquantitative 

emission spectrographic method 

Flameless atomic absorption method 

for Au 

Nonmagnetic Modified six-step D.C.-arc semifraction quantitative emission spectrographic method 

Magnetic fraction ----------------Do----------------------

Magnetite fraction ----------------Do----------------------

Oxalic acid ----------------Do----------------------

leachates 

Major element X-ray fluorescence method 

analyses 

Source or remarks 

Grimes and Marranzino, 1968; 

Meier, 1980. 

Meier, 1980. 

Grimes and Marranzino, 1968. 

Meier, 1980. 

Grimes and Marranzino, 1968. 

Meier, 1980. 

Grimes and Marranzino, 1968. 

------------------Do-----------------

------------------Do-----------------

Alminas and Mosier, 1976; 

Grimes and Marranzino, 1968. 

Taggart and others, 1987. 

islands as well as between the major anomalies within an 

island. The REM program is designed to rank the magnitudes of individual element anomalies within a multielement geochemical anomaly. Calculations are performed 

on a cell basis rather than on individual sample sites. These 

calculations are performed on the basis of two parameters 

for each of the elements within the selected elemental suite. 

The first of these, the intensity factor, is derived by dividing the mean of all anomalous values of a given element 

within the defined area by its corresponding threshold 

value. The second, the area factor, is derived by dividing 

the number of sample locations within the defined area that 

contain anomalous levels of this element by the total number of sample locations within this area. The intensity and 

area factors are multiplied for each element and the product 

is the individual element magnitude (EM). All individual 

element magnitudes are added to give the total anomaly 

magnitude of the cell. In addition, each individual element 

magnitude is divided by the anomaly magnitude of the cell 

and the quotient is expressed in percent. This is the relative 

element magnitude (REM). 

As per the data listed by Hopkins and others (1986) and 

McHugh and others (1989), the base-metal mineralization is 

most intense on St. Croix, especially with respect to Sn, Pb, 

Ni, Cr, and Sb. Silver, in the nonmagnetic fraction, is also 

most widespread on this island. The evidence for gold mineralization on St. Croix in rock and soil samples is the weakest of the three islands. St. John is intermediate in base-metal 

and gold mineralization and is characterized by the highest 

concentrations of copper and barium. St. Thomas shows the 
100-J 

15-----

CfJ 

UJ 

....J 

a.. 

~ 

<( 

CfJ 10- LL 

0 

1-

z 

w 

u 

c:: 

UJ 

a.. 5-

z 

....J 

~ 96 -0 a.. 

~ r----- <( 

CfJ 

LL 

0 5-

Iz 

w 

u 

c:: 

UJ 

a.. 

ROCKS 

n=220 

TIN CONTENT IN PARTS PER MILLION 

I J I I 

SOILS 

<80 MESH 

n=312 

z _.J 0 L.D 0 0 T""""" .....-- N M 

CfJ 

UJ 

....J 

90 

~ 10 

<( 

CfJ 

LL 

0 

1-

z 

w 

u 

a: 

w 

a.. 

TIN CONTENT IN PARTS PER MILLION 

OXALIC ACID 

LEACHATE 

n=236 

z _.J a a a a a a a NML.Dr-.~~~ 

TIN CONTENT IN PARTS PER MILLION 

GEOCHEMISTRY 

38 

15 

CfJ 

UJ 

....J 

a.. 

~ 

<( 

CfJ 10 LL 

0 

1-

z 

UJ 

u 

c:: 

w 

a.. 

CfJ 

UJ 

....J 

a.. 

~ 80 CfJ 

LL 

0 5 1-

z 

w 

u 

c:: 

w 

a.. 

z ---' 

00 ,b w 

....J 

~ 10 

<( 

CfJ 

LL 

0 

1-

z 

w 

u 

c:: 

UJ 

a.. 

z ---' 

a a a a a a a a a a a a a a 

N M ~ r-. ~ ~ ~ g ~ ~ § § ~ ~ 

TIN CONTENT IN PARTS PER MILLION 

MAGNETIC 

FRACTION 

a a a a a o a o a a a o N M L.D r-. 0 L.D 0 D 0 0 0 0 ..--- ..--- N M L.D r-. ~ ~ 

TIN CONTENT IN PARTS PER MILLION 

NON-MAGNETIC 

FRACTION 

TIN CONTENT IN PARTS PER MILLION 

1\ 

11 

Figure 8. Histograms comparing tin concentrations in six sample media from St. Croix. L indicates that the metal was detected but at a 

concentration below the lowest indicated value. N indicates that the metal was not detected. 
12 GEOCHEMISTRY, MINERALOGY, AND GEOCHRONOLOGY OF THE U.S. VIRGIN ISLANDS 

Table4. Proportional distribution of tin, lead, gold, and silver in the U.S. Virgin Islands 

Locality Sn (NM-1) Pb (NM-1) Au (rock) Au (soil) Ag (rock) 

St. Croix 54 42 9 25 1 

St. John 34 26 27 33 45 

St. Thomas 12 26 64 42 54 

EXPLANATION 

ITIIIIJ] ~2000 ppm Sn 

~ ~lOOOppmPb • 

~2000 ppm Sn and 

~1000 ppm Pb 

~200 ppm Sn in 

magnetite 

Geographical Localities 

A. Whites Bay E. Christiansted 

B. Canegarden Bay F. Salt River Bay 

C. East Point G. Mount Victory 

D. SevenHills H. Frederiksted 

c 

STCROIX 

Figure 9. Tin and lead distribution in the nonmagnetic fractions of heavy mineral concentrates and in magnetites on St. Croix. 

least evidence for base-metal mineralization, but evidence 

from rock and soil samples rank this island highest in gold 

mineralization (table 4). 

The base-metal mineralogy in the secondary environment of these islands is unique. Tin occurs primarily in 

native form, as a tin or tin-lead chloride, or in the form of 

romarchite (SnO), and poorly crystallized cassiterite. Lead 

occurs in the native form, as cerussite, as tin-lead chloride(s), 

as lead chloride(s), and, rarely, as galena. Copper occurs in 

the native form, as a copper-tin alloy, as paratacamite 

(Cu2(0H)JCI), as one of the oxides, and rarely as chalcopyrite. Nickel occurs primarily in the form of gersdorffite 

(NiAsS). 

Gold occurs primarily in native form, as electrum, 

and as gold and gold-silver tellurides. Silver occurs primarily in the form of silver sulfide, silver iodide, silver chloride, and as silver and silver-gold tellurides. Commonly, 

the silver sulfide contains areas of disseminated ultrafine 

gold particles. The mineralogy will be discussed in detail 

later in this report. 

ST. CROIX 

The most extensive and intense tin and lead anomalies 

occur on the island of St. Croix (fig. 9). Here tin is detected 

in the nonmagnetic fraction at a 20 ppm level at 78 percent 

of the concentrate sample sites and at a concentration of 

~2000 ppm at 33 percent of the sites. Lead is found at concentration of ~ 1000 ppm at 31 percent of the sites. Tin in 

magnetite occurs at a concentration of ~200 ppm at 34 

percent of these sites. 

The most extensive base-metal anomalies on this island 

occur (west to east) in the Mt. Victory area, along the westem graben wall (Whites Bay to Salt River Bay), along the 
GEOCHEMISTRY 13 

EXPLANATION 

ITIIIIIIJ ~3 ppb Au in soil 

~ ~5 ppb Au in soil 

ED ~10 ppb Au in soil 

• ~50 ppb Au in outcrop 

h. ~500 ppb Ag in outcrop 

D ~500 ppb Ag in nonmagnetic fraction 

of heavy mineral concentrates 

Mount Victory area Geographical Localities 

A. Whites Bay E. Christiansted 

B. Canegarden Bay F. Salt River Bay 

C. East Point G. Mount Victory 

D. Seven Hills H. Frederiksted 

c 

Figure 10. Gold distribution in B-horizon soils and rocks on St. Croix. 

eastern graben wall (Canegarden Bay to Christiansted), in 

the Seven Hills area and at East Point (figs. 9 and 10). 

Although the gold anomalies on St. Croix are less 

intense than those on St. Thomas and St. John, gold in soils 

occurs at the 3 ppb level at 43 percent of the sites and at the 

10 ppb level at 8 percent of the sites. These gold anomalies 

delineate essentially the same areas on this island as do the 

base-metal anomalies. 

The Mt. Victory anomalous area, in the northwestern 

comer of St. Croix (fig. 10), occurs in an area underlain by 

rocks of the Caledonia, the Allandale, the Cane Valley, and 

Judith Fancy Formations. The structural setting in this area 

is complex. The Annaly syncline occurs in the north and the 

Prosperity thrust in the south (fig. 2). As a whole, the area is 

characterized by extreme shearing. Small quartz and epidote 

veins are common and a number of small, unmapped intrusives occur throughout the area. Hydrothermal alteration is 

evident although strongly masked by pervasive weathering. 

The base- and precious-metals anomalies in this area 

are nearly coextensive. The Mt. Victory anomaly has the 

highest overall values on the island with an EM value of 

20,213. The prorated association relationship (REM) is 

shown in figure llA. 

The Mt. Victory area has a soil-gold EM value of 92, 

and thus ranks third within this island. On an inter-area 

basis, this area ranks first in lead (EM=7258) and copper 

(EM= 1451) contents. 

The Whites Bay-Salt River Bay anomaly, which 

inludes both base- and precious-metals, occurs in the westcentral portion of the island and extends in a northeasterly 

direction from Whites Bay, in the southwest, to Salt River 

Bay in the northeast (fig. 9). This anomaly extends along the 

western wall of the St. Croix graben and is underlain by 

rocks of the Judith Fancy Formation, the Jealousy Formation, and the graben-filling Kingshill Limestone. This area is 

also structurally complex. In addition to the northeast-trending graben wall one major anticline and two synclines occur 

at right angles to the graben in the northern portion of the 

anomaly. The Coble syncline and a parallel unmapped structure, farther to the north, are characterized by intense gold 

anomalies in the soils (figs. 2 and 1 0). 

A zonational aspect between the base- and preciousmetals anomalies is evident at the Whites Bay-Salt River 

Bay anomaly. The gold-rich areas occur centrally (figs. 2 

and 10) and the base metal anomalies form a halo around 

them (figs. 9 and 10). This is the second most intense basemetal anomaly on the island with an EM value of 13,518. 

The prorated association relationship for this area is shown 

in figure liB. 

The soil-gold anomaly value for this area ranks first 

within this island with an EM value of 423. On an inter-area 
14 GEOCHEMISTRY, MINERALOGY, AND GEOCHRONOLOGY OF THE U.S. VIRGIN ISLANDS 

35 

30 

gJ 25 :::l 

~ 20 

~ 

~ 15 

10 

A 

35 

30 

gJ 25 :::l 

~ 20 

~ 

L.U 

0:: 15 

10 

B 

35 

30 

gJ 25 :::l 

~ 20 

~ 

~ 15 

10 

46.6 

Sn Pb Cu Ba Ni Zn Ag As 

ELEMENT 

72.9 

Bi Mo Sb 

0.3 

Sn Pb Cu Ba Ni Zn Ag As Bi Mo Sb 

ELEMENT 

45.5 47.6 

0.1 0.1 

Sn Pb Cu Ba Ni Zn Ag As Bi Mo Sb 

C ELEMENT 

35 

30 

gJ 25 :::l 

~ 20 

~ 

~ 15 

10 

D 

35 

30 

gJ 25 :::l 

~ 20 

~ 

L.U 15 a: 

10 

E 

87.6 

0.3 

Sn Pb Cu Ba Ni Zn Ag As Bi Mo Sb 

ELEMENT 

0.3 

Sn Pb Cu Ba Ni Zn Ag As Bi Mo Sb 

ELEMENT 

Figure 11. Association relationships for five geochemical anomalies on St. Croix. A, Mt. Victory area; B. Whites Bay-Salt River Bay; 

C, Canegarden Bay-Christiansted; D, Seven Hi1ls area; E, East End. 
GEOCHEMISTRY 15 

EXPLANATION 

ITllJJII] ~1000 ppm Sn 

~ ~1000 ppm Ph • 

STJOHN 

~1 000 ppm Sn and 

~1000 ppm Ph 

~100 ppm Sn in 

magnetite 

Geographical Localities 

A. Great Cruz Bay E. Johns Folly Bay I. Mary Point 

B. Maria Bluff F. Base Hill J. Hawksnest Bay 

C. Dittlif Point G. Bordeaux Mtn. K. Battery Gut 

D. White Cliffs H. Marney Peak L. Cocoloha Cay Point 

(/ 

Figure 12. Tin and lead distribution in the nonmagnetic fraction of the heavy mineral concentrates and in magnetites on St. John. 

basis, this anomaly also ranks first in tin (EM=9849) 

content. 

The Canegarden Bay-Christiansted anomalous area 

occurs in the central portion of the island (figs. 9 and 1 0) 

and extends from Canegarden Bay in the south to Christiansted in the north. This anomaly occurs over the eastem graben wall and is underlain by rocks of the Judith 

Fancy Formation to the east and the Kingshill Limestone 

to the west. The Kingshill Limestone is somewhat silicified and epidotized in the northeastern portion of the 

anomaly and tends to form hills. The gold anomaly is 

more intense in the northern portion of the area and the 

base-metals occur more in the south. This is the third most 

intense base-metal anomaly on the island with an EM 

value of 12,446. The prorated association relationship for 

this area is shown in figure 11 C. This soil-gold EM value 

here is 76 and thus this area ranks third on St. Croix with 

respect to gold content. 

The Seven Hills anomalous area is located in the 

eastern end of the island and is centered on the Seven 

Hills Range. The base-metal portion of the anomaly is 

continuous and occurs in the central portion of the area. 

The gold anomaly is quite fragmented and tends to form 

a halo in the outer portions and is underlain exclusively by 

rocks of the Caledonia Formation. This area is characterized by two northwest-trending anticlines and synclines 

(fig. 2). The base-metal anomaly ranks fourth in magnitude on St. Croix with an EM value of 6253. The prorated association relationship for this area is shown in 

figure liD. The soil-gold anomaly intensity is at some 

variance with that of the base metals and ranks second for 

the island with an EM value of 189. 

The East Point anomaly occurs at the extreme eastern 

tip of St. Croix. This area is characterized by a centrally 

located soil-gold anomaly with an intermittent, outer basemetal halo. The area is underlain by rocks of the Caledonia 

Formation exclusively and no major structures have been 

mapped here. This is the-least pronounced of the five basemetal anomalies on St. Croix, with an EM value of 4838. The 

prorated association relationship for this area is shown in figure liE. The soil-gold anomaly, here, is also the lowest on 

the island with an EM value of 52. 
16 GEOCHEMISTRY, MINERALOGY, AND GEOCHRONOLOGY OF THE U.S. VIRGIN ISLANDS 

·IJ 

c STJOHN 

EXPLANATION 

Geographical Localities 

• ;::SO ppb Au in rock A. Great Cruz Bay G. Bordeaux Mtn. 

B. Maria Bluff H. Marney Peak 

A ;::500 ppb Ag in rock C. Dittlif Point I. Mary Point 

D. White Cliffs J. Hawksnest Bay 

OJIII]] ;::3 Pt;~h Au in soil 

~ ;::5 ppb Au in soil 

- ;::lOppb.Auinsoil D ;::500 ppb Ag in nonmagnetic fraction 

of heavy mineral concentrates 

E. Johns Folly Bay K. Battery Gut 

F. Base Hill L. Cocoloba Cay Point 

Figure 13. Gold distribution in B-horizon soils and rocks on StJohn. 

ST.JOHN 

St. John ranks second in the intensity and extent of 

base-metal anomalies among the three islands. Here, tin is 

detected in the nonmagnetic fraction at a 20 ppm level at 52 

percent of the concentrate sample sites and at a concentration 

of~2000 ppm at 19 percent of the sites (fig. 12). Tin in magnetite occurs at a concentration of ~200 ppm at 12 percent of 

these sites. 

The most extensive base-metal anomalies on this 

island occur (west to east) in the Hawksnest Bay-Dittlif 

Point area, the Battery Gut-Mamey Peak area, The Mary 

Point-Base Hill area, and the Bordeaux Mountain area 

(fig. 12). 

The soil-gold anomalies on St. John are more intense 

than those on St. Croix. Gold in soils occurs at the ~3 ppb 

level at 45.6 percent of the sites and at the ~10 ppb level at 

18.5 percent of the sites. These gold anomalies occur in generally the same areas as the base-metal anomalies (compare 

figures 12 and 13). 

The Hawksnest Bay-Dittlif Point anomalous area 

occurs near the western end of the island (fig. 12), and is 

underlain predominantly by rocks of the Louisenhoj Formation. A major northeast-trending fault cuts the central portion 

of this area. Pronounced hydrothermal alteration is visible on 

Dittlif Point and the first peninsula to the east. In addition, 

this peninsula is characterized by the presence of abundant 

sulfides in the outcrops. 

The base-metal and soil-gold anomalies in this area are 

nearly coextensive. Of the four calculated areas on St. John, 

this area has the lowest base-metal EM value at 8411. The 

prorated association relationship is shown in figure 14A. The 

EM value for the soil-gold anomaly for this area is 342, 

which is the second lowest for this island, although it would 

rank just below that of the western graben wall anomaly on 

St. Croix. 

The Battery Gut-Mamey Peak anomaly extends along 

a major northeast trending fault in the central portion of the 

island. This area is underlain in equal parts by rocks of the 

Louisenhoj Formation to the north and the Water Island Formation to the south of the fault. The extensive zone of hydrothermal alteration occurring along this zone is frequently 

masked by pervasive weathering. The base-metal anomaly 

here is somewhat more areally restricted than the soil-gold 

anomaly, but it still has an EM value of 13,913. The prorated 

association relationship for this area is shown in figure 148. 

The soil-gold anomaly in this area has the lowest EM value 

for this island with a value of228. This EM value would rank 

this area second highest as compared to the anomalies on St. 

Croix. 
GEOCHEMISTRY 17 

73.1 40.5 39.7 

40 40 

35 35 

30 30 

(/) 25 (/) 

Q) Q) 25 ::J ::J 

~ 20 ~ 20 

~ ~ 

w 15 w 15 c: c::: 

10 10 

0.2 0.1 1.1 0.4 

Sn Pb Cu Ba Ni Zn Ag As Bi Mo Sb Sn Pb Cu Ba Ni Zn Ag As Bi Mo Sb 

A ELEMENT c ELEMENT 

39.9 44.3 

40 40 

35 35 

30 30 

(/) 

25 (/) Q) Q) 25 ::J ::J 

~ 20 ~ 20 ~ ~ 

w w c::: 15 c::: 15 

10 10 

6.8 6.6 

0.4 

Sn Pb Cu Ba Ni Zn Ag As Bi Mo Sb Sn Pb Cu Ba Ni Zn Ag As Bi Mo Sb 

B ELEMENT D ELEMENT 

Figure 14. Association relationships for four geochemical anomalies on St. John. A, Hawksnest-Ditliff Point; B, Battery Gut-Mamey 

Peak; C, Mary Point-Base Hill; D, Bordeaux Mountain. 

The Mary Point-Base Hill base-metal ano~aly incorporates the largest area on St. John, which is also true for the 

soil-gold anomaly occurring here. This area is complex both 

geologically and structurally; the area is underlain by rocks 

of the Louisenhoj Formation, the Outer Brass Limestone, 

rocks of the Tutu Formation and dioritic rocks. The area is 

cut by major northwest- and northeast-trending faultssome of which show a spatial relationship to the geochemical patterns. Pronounced hydrothermal alteration is visible 

here, though it is frequently obscured by the pervasive 

weathering. The area incorporates the most intense basemetal anomaly on St. John with an EM value of 17,388. The 

prorated association relationship is shown in figure 14C. The 

soil-gold anomaly here has an EM value of 397 and is thus 

the second most intense on the island. On an inter-area basis, 

this area has by far the highest EM values for tin, lead, and 

silver. 

The Bordeaux Mountain area is located in the eastcentral portion of the island. Here, the base-metal and 

soil-gold anomalies center on the gossan-capped Bordeaux Mountain. This area is underlain exclusively by 

rocks of the Water Island Formation with little or no structural features noted in this area The Bordeaux Mountain 

gossan is at least 7 m thick and reveals no replacement 

texture. The base-metal and soil-gold anomalies are 

essentially coextensive. This is the second most intense 

base-metal anomaly on St. John with an EM value of 

16,884. The prorated association relationship here is 
I8 GEOCHEMISTRY, MINERALOGY, AND GEOCHRONOLOGY OF TilE U.S. VIRGIN ISLANDS 

STTHOMAS 

EXPLANATION 

ITIIIIDJ :;:::1 00 ppm Sn 

~ ;::lOOppmPb • 

A. Fortuna Bay 

B. Black Point 

C. Red Point 

D. Charlotte-Amalie 

E. Kings Wharf 

Geographical Localities 

F. Havensight Point 

G. Stailey Bay 

H. Water Point 

I. Mingo Cay 

J. Thatch Cay 

:;:::1 00 ppm Sn and 

:;:::100 ppm Ph 

;::tOO ppm Sn in 

magnetite 

K. Wintberg Peak 

L. Magens Bay 

M. Mail Bay 

N. Jersey Bay 

P. Long Point 

Figure 15. Tin and lead distribution in the nonmagnetic fraction of heavy mineral concentrates and magnetites on St Thomas. 

shown in figure I4D. The soil-gold anomaly here ranks 

first in intensity on St. John with an EM value of 468. 

In addition to the above delineated areas, three smaller 

anomalies occur along the southern St. John coast. The westernmost of these occurs at Maria Bluff-Great Cruz Bay. This 

area is characterized by gold and silver anomalies. Here, a 

sample of a strongly altered siliceous intrusive contained 5.5 

ppm gold and abundant lead-rich jarosite. 

The White Cliffs area, to the east of Dittlif Point, consists primarily of a fine-grained hydrothermal quartz with 

minor amounts of alunite and is capped by a thin siliceous 

gossan. The quartz contains low levels of gold and silver as 

well as copper in the form of chalcopyrite. Turquoise veins 

occur along minor fractures and the cliffs are coated with 

native sulfur and paratacamite. 

The last minor anomaly occurs at Johns Folly Bay, 

along the east coast of St. John. This area is characterized by 

strong hydrothermal alteration. 

An east-west trending string of small islands occurs to 

the north of and between St. John and St. Thomas. These 

islands are named (west to east) Thatch Cay, Grass Cay, 

Mingo Cay, Lovango Cay, Congo Cay, and Whistling Cay 

(figs. 3 and 4). This string of islands represents a skarn zone 

extending into Mary Point on St. John. Fine-grained marble 

and gametiferous rocks are common between Mingo Cay 

(on St. Thomas) and Mary Point. This area is characterized 

by magnetite veins, copper-bearing barite and calcite veins, 

and low levels of gold and silver. 

ST. THOMAS 

Lead and tin anomalies are least pronounced on the 

island of St. Thomas (fig. I5). Here tin is detected in the 

nonmagnetic fraction at a 20 ppm level at only 55 percent of 

the concentrate sample sites and at a concentration of 2000 

ppm at 13 percent of these sites. Lead is found at a concentration of 1000 ppm at I9.6 percent of the sites. Tin in magnetites occurs at a concentration of 200 ppm at II percent of 

these sites. 

The most extensive base-metal anomalies on this island 

occur (west to east) in the Mail Bay-Black Point area (fig. 

I5), the Magens Bay-Kings Wharf area, the Havensight 

Point-Stalley Bay area and the Wintberg Peak-Water Point 

area (fig. 15). 

In contrast with tin and lead, the soil-gold anomalies on 

St. Thomas are more intense than those on St. John or St. 

Croix. Gold in soils occurs at the 3 ppb level at 55 percent of 

the sample sites and at the 10 ppb level at 13 percent of the 

sites. These gold anomalies delineate essentially the same 
GEOCHEMISTRY 19 

' 

STTHOMAS 

t 

0 

0 

ITIIDilJ ~3 ppb Au in soil 

~ ~5 ppb Au in soil 

me ~10 ppb Au in soil 

EXPLANATION 

• ~50 ppb Au in rock 

IJ. ~500 ppb Ag in rock 

D ~500 ppb Ag in nonmagnetic fraction 

of heavy mineral concentrates 

A. Fortuna Bay 

Geographical Localities 

F. Havensight Point K. Wintberg Peak 

B. Black Point G. Stailey Bay 

C. Red Point H. Water Point 

D. Charlotte-Amalie I. Mingo Cay 

E. Kings Wharf J. Thatch Cay 

p 

L. Magens Bay 

M. Mail Bay 

N. Jersey Bay 

P. Long Point 

Figure 16. Gold distribution in B-horizon soils and rocks on St Thomas. 

areas on this island as do the base-metal anomalies, but they 

tend to be more extensive (fig. 16). 

The Mail Bay-Black Point anomaly occurs to the west 

of Charlotte-Amalie. This area is underlain exclusively by 

rocks of the Louisenhoj Formation and no major structure 

was indicated here. The Harry S. Truman airport area, immediately to the southeast of this anomaly, was strongly altered 

and metallized. This airport area is characterized by malachite pods containing disseminated iodargyrite (silver 

iodide) and a small, iron-rich and extremely siliceous breccia 

pipe at Red Point. This material was being rapidly removed 

to be used as fill in the extension of the airport runway. The 

base-metal anomaly in this area is more extensive than the 

soil-gold anomaly. This area has the highest overall basemetal anomaly intensity on St. Thomas with an EM value of 

12,928. The prorated association relationship here is shown 

in figure 17 A. 

The soil-gold anomaly for this area has an EM value of 

73, which is the lowest for the island. On an inter-area basis, 

this anomaly ranks the highest in lead (EM=8395). 

The Magens Bay-Kings Wharf anomaly is located in 

the vicinity of and to the north of Charlotte-Amalie. This 

area is underlain predominantly by rocks of the Louisenhoj 

Formation and, to a lesser extent, by the Outer Brass Limestone and rocks of the Tutu Formation in the north. No major 

structures are indicated in this area. Hydrothermal alteration 

is visible along the north shore. Generally, however, outcrop 

exposures over most of this area are poor. 

This is the second most intense base-metal anomaly on 

St. Thomas with an EM value of 9858. The prorated association relationship for this area is shown in figure 17 B. The 

soil-gold anomaly ranks second on St. Thomas with an EM 

value of96. 

The Havensight Point-Stalley Bay area is located in the 

eastern part of St. Thomas along the southern shore line (figs. 

15, 16). This is a geologically and structurally complex area. 

The area is underlain predominantly by rocks of the Water 

Island Formation intruded by a number of quartz-andesine 

porphyry dikes. Numerous major north- and northwest-trending faults cut the area. This area is characterized by the presence of quartz porphyry intrusives, pronounced and 

widespread hydrothermal alteration, and the presence of 

numerous epidote, quartz, and iron oxide veins. Some of these 

quartz veins are precious-metal rich, and one of these is 0.6 

m (2ft) thick with values as high as 20 ppm Au and several 

hundred parts per million silver. One epidote vein contains 
20 GEOCHEMISTRY, MINERALOGY, AND GEOCHRONOLOGY OF TilE U.S. VIRGIN ISLANDS 

35 

30 

~ 25 :::J 

~ 20 

~ 

~ 15 

10 

A 

35 

30 

~ 25 :::J 

~ 20 ~ 

w 

c::: 15 

10 

B 

64.9 

0.2 

Sn Pb Cu Ba Ni Zn Ag As Bi Mo Sb 

ELEMENT 

45".6 46.5 

0.4 0.1 0.1 0.3 

Sn Pb Cu Ba Ni Zn Ag As Bi Mo Sb 

ELEMENT 

~ 25 :::J 

~ 20 

~ 

w 

a: 15 

10 

c 

35 

30 

~ 25 :::J 

~ 20 

2: 

LlJ 

c::: 15 

10 

D 

57.2 

0.1 

Sn Pb Cu Ba Ni Zn Ag As Bi Mo Sb 

ELEMENT 

46.3 

Sn Pb Cu Ba Ni Zn Ag As 

ELEMENT 

Bi Mo Sb 

Figure 17. Association relationships for four geochemical anomalies on St. Thomas. A, Mail Bay-Black Point; B, Magens Bay-Kings 

Wharf; C, Havensight Point-Stalley Bay; D, Wintbcrg Peak-Water Point. 

>200 ppm tellurium. Gossany material is abundant immediately to the southeast on Long Point. Very coarse iron oxide 

box work occurs in Jersey Bay, to the east of Stalley Bay. The 

base-metal anomaly in this area ranks fourth in intensity on 

St. Thomas with an EM value of 5909. The prorated association relationship is shown in figure 17 C. The soil-gold anomaly is substantially more extensive and continuous than the 

base-metal anomaly. This area ranks second on St. Thomas 

in soil-gold content with an EM value of 1247. 

The Wintberg Peak-Water Point area is located along 

the northeastern St. Thomas shore. This area is underlain by 

a wide range of rock types. In the southern part of the area, 

rocks of the Water Island Formation are cut by numerous 

northeast-trending dikes of quartz-andesine porphyry. South 

to north, the area is underlain by rocks of the Louisenhoj Formation, the Outer Brass Limestone, and the Tutu Formation. 

The area is bounded to the west by a major northwest-trending fault. Pronounced hydrothermal alteration is visible 

along the coastal portion of this area. Iron introduction is 

very visible-as the name Redhook Point (north of Water 

Point; fig. 4) implies. The base-metal anomaly is substantially less extensive and is located centrally within the soilgold anomaly here (figs. 15, 16). This is the least intense 

base-metal anomaly on St. Thomas with an EM value of only 

3099. The prorated association relationship for this area is 

shown in figure 17 D. The soil-gold anomaly here is more 

intense than any other area on all three islands with an EM 

value of 1414. 
MINERALOOY 21 

MINERALOGY 

Until this study of the U.S. Virgin Islands, the presence of mineralization, the extent of this mineralization, and 

the mineralogy associated with it were totally unknown. Preliminary results were given by Foord and others (1988). 

Because of the complex elemental associations, the existing 

mineral assemblages are diverse. In addition, because of the 

depositional (hydrothermal and weathering) complexity, 

much additional work is necessary, particularly on the 

hydroxides, oxychlorides, oxide-hydroxides, and related 

compounds of lead and tin. However, a hitherto unrecognized mineral assemblage has been documented by this 

work. Table 5 lists the minerals and compounds identified in 

this study. Several aspects of the total mineralogy are particularly noteworthy. 

NATIVE ELEMENTS AND ALLOYS 

With regard to occurrence of the native elements, all are 

straightforward in terms of occurrence except for native tin 

and native lead. Relatively few worldwide occurrences of 

these two metals have been reported singly or combined (as 

alloys or intergrowths of the two metals) ([Sn] Lawrence, 

1951; Silman, 1954; Aleksandrov, 1955; Turovsky, 1956; 

Hosking, 197 4; Uwadiale and Hall, 1984; [Pb] N ovgorodova 

and others, 1981; Krylova, 1975; Kucha, 1981; [Sn and Pb] 

Filimonova and others, 1981 ). Recently, what appear to be 

eutectic intergrowths and solid-solutions oflead and tin have 

been reported by Tomson and others (1989) and Okrugin and 

others (1981). Of all of the native metals found by us, tin and 

lead are the most common. Both of these metals, and others 

(such as native iron and aluminum), have been reported 

mainly from secondary and weathering environments as well 

as organic-rich environments and much less frequently from 

primary and/or hydrothermal sources. Some of the references given above document high temperature and pressure 

primary origins for native elements, for example, in rhyolite, 

andesite, syenite, and granite. We are well aware of the possibility that some of the native tin and lead found in the Virgin Islands may be man-made (from pewter); but for manmade materials, the mode of geologic occurrence, the trace 

element geochemistry, and the textures of samples differ 

markedly from what we conclude to be naturally occurring 

material. The existence of these metals here, as in other 

worldwide locations, strongly favor a natural origin for the 

two metals in all or nearly all of the occurrences. 

Native tin and native lead frequently occur together as 

eutectoid intergrowths that are mantled by native tin that in 

tum is coated by a complex fine-grained mixture of lead 

and/or tin oxides, hydroxides, carbonates, oxychlorides, 

and other unidentified compounds (figs. 18, 19). Figure 20 

shows a closeup view of the tin-lead eutectic intergrowth. 

Table 5. List of minerals and compounds found to occur in 

the weathering environment in the U.S. Virgin Islands. 

Native elements 

Gold 

Silver 

Copper 

Tin 

Lead 

Tellurium 

Bismuth(?) 

Sulphur 

Oxides and 

hydroxides 

Chromite 

Cassiterite 

Romarchite 

Cuprite 

Tenorite 

Litharge 

Minium 

Magnetite 

Maghemite 

Hematite 

Ilmenite 

Rutile 

Goethite 

Lepidocrocite 

Alloys 

Electrum 

eu6sn5 

eu5.

6sn(?) 

Halides and halogenides 

Tellurides 

Au-Te 

Au-Ag-Te 

Ag-Te(?) 

Bi-Te(?) 

Sulfides 

Chalcocite 

Sphalerite 

Acantite 

Galena 

Pyrite 

Marcasite 

Chalcopyrite 

Arsenopyrite 

Gersdotffite 

Sn-Cl-OH/Sn-Cl-OH-H20/Sn-Cl-H2o 

Sn-Pb-Cl-OH/Sn-Pb-Cl-OH-H20tsn-Pb-Cl-H20 

Pb-Cl-OH/Pb-Cl-OH-H20!Pb-Cl-H20 

Cotwmite 

Paratacarnite 

Phosgenite 

Chlorargyrite 

Bromargyrite 

lodargyrite 

Sylvite 

Fe-Cl/Fe-Cl-OH/Fe-Cl-OH-H2o bismoclite-daubreeite 

Sulfates, phosphates, 

carbonates, and vanadates 

Barite 

Cerussite 

Hydrocerussite 

Jarosite-plumbojarosite 

Apatite 

Turquoise 

Monazite 

Pyromorphite 

Mouramite(?) 

Bismutite 

Alumite 

Malachite 

Silicates 

(excludes common 

rock-fonning minerals) 

Epidote 

Ni-silicate 

Y-silicate 

Wollastonite 

Gamet group (chiefly 

andradite-grossular) 

Phrenite 
22 GEOCHEMISTRY, MINERALOGY, AND GEOCHRONOLOGY OF THE U.S. VIRGIN ISLANDS 

Figure 18. Photomicrograph of native tin-lead nugget (SC514) coated by a rind 

consisting of a complex, fine-grained mixture of lead plus tin oxides, hydroxides, 

and oxychlorides with fine blebs of native lead and tin. The core portion consists of 

a eutectoid intergrowth of native tin and lead. The vugs are filled with tin, tin-lead, 

or lead oxychlorides. Field of view = 9x4mm. 

Table 6. Microprobe analyses of native tin and native lead nuggets occuring in 

eutectic or eutectoid intergrowths from the U.S. Virgin Islands, in weight percent 

Nugget type ......... Native Sn 

Sample no .......... SC514A 

Number of 

analyses ............ . 

Sn ...................... .. 

Pb ...................... .. 

Total ............ .. 

3 

99.6 

0.3 

99.9 

SJ601A 

0.0 

100.6 

100.6 

Nuggets and masses of native lead with or without coatings 

of secondary oxides, carbonates, hydroxides, oxychlorides, 

and other unidentified compounds are uncommon, and 

comparable masses of native tin are rare. The majority of 

"native-metal appearing" nuggets and grains are actually 

mixtures of both metals plus a complex assemblage of leadtin oxides, oxychlorides, carbonates, hydroxides, and other 

unidentified compounds. Only rarely are native tin and 

native lead found individually. Nuggets of all three types 

(Pb-Sn, Pb, Sn) are as much as 5 mm in maximum dimension. In many cases, the eutectic-like intergrowths are 

composed of native lead or tin (but not both) and cerussite, 

NativePb 

SC514 

6.0 

94.6 

100.6 

SJ601 

2 

0.2 

99.7 

99.9 

29 

2 

0.0 

101.0 

101.0 

cotunnite, and other as yet unidentified oxychlorides and 

hydroxides (see figs. 21-25). The eutectic texture is also 

present within mixtures of lead-tin oxides, hydroxides, carbonates, oxychlorides, and other compounds (fig. 26). 

Euhedral crystals of Cu6Sn5 frequently occur within this 

complex mixture (figs. 27 and 28). Nuggets and masses 

may be panned or concentrated by heavy liquids from 

stream and colluvial sediments. Figures 18 and 19 show 

one of these zoned nuggets (SC514 ), and figures 29 and 30 

are element distribution maps for tin and lead for an area 

within the eutectic portion of the same sample. Microprobe 

analyses for native tin and lead are given in table 6. 
MINERALOGY 23 

c (l) ""'0 ""'0 +-' I- cu +-' c 0 (l) (l) u (l) 0: > (l) > +-' > +-' 

cu cu 

z +-' 

cu z 

z 

Figure 19. Photomicrograph showing detail of the eutectoid core 

portion and rind of the grain shown in figure 18, sample (SC514). 

Field of view= 2 x lJ.lm. 

Figure 20. Scanning electron microscope (SEM) photomicrograph showing a closeup view of the eutectoid intergrowth of native 

tin and lead in the core portion of the SC514 nugget (seen in fig. 

19). Scale bar= lO J.lm. 
24 GEOCHEMISTRY, MINERALOGY, AND GEOCHRONOLOGY OF THE U.S. VIRGIN ISLANDS 

Figure 21. Backscatter electron (BSE) photomicrograph of a portion of a lead-tin nugget (sample Box lOC) from the graben area on 

St Croix. This picture is of the same area shown in the map in figure 22. Scale bar= 5 J..Lm. 

ffiPb 

(/Pb-0-CI 

Figure 22. Compositional map of grain seen in figure 21 showing the mineralogic complexity common to the lead-tin nuggets. 

The various portions of the grain are identified by elemental content 

and not by chemical formula. Scale bar= 5 J..Lm. 

Figure 25 (facing column). EDPM map of tin in grain (sample 

Box 1 OC) shown in figure 21. Scale bar = 5 J..Lm. 

Figure 23. Element distribution photomicrograph (EDPM) map 

of lead in grain (sample Box lOC) shown in figure 21. Scale bar= 

5 J..Lm. 

Figure 24. EDPM of chlorine in grain (sample Box 10C) shown 

in figure 21. Scale bar= 5 j.lm. 

1::;: ;'-:: c :'';-> ; ·· .."'' .. ~-: . '. 
Lead: oxides 

hydroxides -------?:t---- -­

carbonates 

oxychlorides 

MINERALOGY 25 

t...:;..----- Romarchitecassiterite 

Figure 26. Photomicrograph of a mineral grain from St. John showing a eutectoid intergrowth of romarchite-cassiterite (dark brown 

portions) and lead oxides, hydroxides, carbonates, and oxychlorides (white portion). Metallic blade-like native tin can be seen crosscutting the eutectoid texture. Field of view = 2xl mm. 

Romarchite ::-1-'--+""~!'!" 

cassiterite rich 

bands 

Vug filled with 

romarchite 

(SnO) crystals 

Bands rich in lead-tin 

oxides, hydroxides, 

carbonates, and 

oxychlorides 

Figure 27. Photomicrograph of a nugget (sample Box lOD) from St. Croix showing several white euhedral crystals of Cu6Sn5 in 

a complex, layered matrix of romarchite-cassiterite (dark brown bands) and lead-tin oxides, hydroxides, carbonates, and oxychlorides (buff). The large oblong vug is filled with euhedral crystals of romarchite (SnO). Field of view = 2x 1.5 mm. 
26 GEOCHEMISTRY, MINERALOOY, AND GEOCHRONOLOGY OF THE U.S. VIRGIN ISLANDS 

Complex lead-tin mineralogy consisting of 

oxides, hydroxides, carbonates, and 

oxychlorides 

Romarchitecass ite rrte 

rich portion 

of matrix 

Cn6Sn5 crystals. 

Crystal cores filled 

with matrix material 

Figure 28. Photomicrograph showing a closeup of two of the Cu6Sn5 crystals (white) in the nugget (sample Box 100) shown in figure 

27. The crystals occur in a matrix (yellow) of cerussite, romarchite and lead-tin oxides, hydroxides. carbonates and oxychlorides. Field of 

view = 0.6 x 0.4 mm. 

Figure 29. EDPM map of tin (light phase) in the eutectoid portion of the nugget (sample SC514) shown in figures 18 and 19. 

Scale bar = 5 Jlm. 

Figure 30. EDPM map of lead (light phase) in the eutectoid portion of nugget (sample SC514) shown in figures 18 and 19. Scale 

bar= 10 Jlm. 
MINERALOGY 27 

Figure 31. BES photomicrograph of a portion of a mineralogically complex copper nugget (sample SC514PA). A mineralogic 

map of this portion of the nugget is shown in figure 37; whereas, 

EDPM maps of individual elements, within this area, are shown in 

figures 32 through 36. Scale bar = 10 ~m. 

Figure 32. EDPM map of copper (light area) in a part of the nugget (sample SC514PA) shown in figure 31. Scale bar= 10 ~m. 

Energy dispersive spectrometry (EDS) analyses of 

numerous other samples of native lead and tin show that the 

individual metals are usually quite pure. Small bits of the 

harder tin are included within the mass of softer native lead 

because they were forced into the lead during sample preparation. In addition, alumina (Al203), also present within 

native lead as tiny specks, were probably emplaced during 

the final polishing. Microprobe analyses of the alumina contained within native lead in sample SJ601A and within 

cerussite in sample SJ105S are nearly identical to those for 

synthetic alumina. 

Actual artifacts (piping and containers from 18th and 

19th century stills, obtained from a local resident), consisting of elemental lead and tin plus lead, were examined. The 

lead portion of the two types of artifacts contains alumina 

spheres that are not found in the natural material. The 

Figure 33. EDPM map of lead (light area) in the part of the nugget (sample SC514PA) shown in figure 31. Scale bar= 10 ~m. 

Figure 34. EDPM map of sulfur (light area) in the part of the nugget (sample SC514PA) shown in figure 31. Scale bar= 10 ~m. 

alumina spheres are from the use of a gibbsite flux. In addition, the material from the artifacts typically contains 100 

ppm or more silver while the natural material does not. 

Nuggets and spheres of native copper are also relatively common. They are 0.5 to I mm across, or similar 

in size to the nuggets and masses of native tin-lead intergrowths. An example of these nuggets is shown in figures 

3I to 37 (sample SC5I4PA). Copper nuggets and spheres 

are sometimes mineralogically complex, such as sample 

SC514PA, and then contain copper, zinc, and lead sulfides, copper-tin alloys, copper oxides, and other intergrown minerals. 

Coatings of cotunnite, paratacamite, tenorite, cuprite, 

and other minerals are present on some spheres. Spheres of 

tenorite and cuprite, as much as I mm or more across, are 

presumably after native copper. A microprobe analysis of 
28 GEOCHEMISTRY, MINERALOGY, AND GEOCHRONOLOGY OF THE U.S. VIRGIN ISLANDS 

Figure 35. EDPM map of tin (light area) in the part of the nugget 

(sample SC514PA) shown in figure 31. Scale bar= 10 ~m. 

Figure 36. EDPM map of chlorine (light area) in the part of the 

nugget (sample SC514PA) shown in figure 31. Scale bar= 10 ~m. 

Native copper 

Pb-Sn-chloride 

Figure 37. Mineral distribution map of the part of the nugget 

(sample SC514PA) shown in figure 31. 

Table 7. Microprobe analyses of native copper, chalcocite, 

and cuprite grains in one nugget of native copper (sample 

SC514PA) from the U.S. Virgin Islands, in weight percent 

Native Chalcocite Cuprite 

Mineral ........ copper (Cu2S) (CuO) 

(Cu) 

Number of 

analyses ...... 11 2 

Cu ........ 90.4 79.4 79.2 (ideal 79.9) 

Sn ........ 8.4 0.0 0.1 

Ni ........ 0.3 

Zn ........ 0.5 0.0 0.0 

s ........ 20.2 0.0 

Pb ........ 0.0 0.1 0.0 

Total 99.6 99.7 79.3 

Table 8. Microprobe analyses of Cu6 Sn5 (n-bronze) nuggets 

from three sample sites in the U.S. Virgin Islands, in weight 

percent 

Sample ..... lOC lOD SJ738-2 Theoretical 

Grain16 11-bronze 

Number of 

analyses ..... 9 3 3 

Sn ..... 61.5 61.1 61.1 60.9 

Ni ..... 0.0 0.3 0.6 0.0 

Cu ..... 38.4 38.2 38.2 39.1 

Total ..... 99.9 99.6 99.9 100.0 

native copper is given in table 7 showing a tin content much 

higher than that of associated chalcocite and cuprite. 

Analysis of another sample of native copper in sample 

SC514I yielded (in wt. percent) copper 98.9, zinc 0.9, 

total=99.8. Tin, nickel, lead, sulfur, silicon, chlorine, and 

aluminum were looked for but were not detected. 

Native gold and electrum are widespread throughout 

the U.S. Virgin Islands. Perhaps the best occurrence of 

native gold, along with associated Ag-Bi-Te mineralization, 

is in a 1 m thick quartz vein that strikes N. 70° W. and dips 

15° N. on the south side of St. Thomas adjacent to Charlotte 

Amalie Harbor. Here, particles of gold as broad as 1 mm are 

in vuggy bull quartz. The grade of samples from this quartz 

vein ranged from 0.2 to 0.75 oz/ton gold. Alloys include 

electrum and Cu6Sn5 (11-bronze). This latter alloy has been 

found at Panasqueira, Portugal, in oxidized tin ores (Clark, 

1972) and in gold-platinum group concentrates in southeast 

Borneo (Stumpfl and Clark, 1965). This alloy is widespread 

on all the Virgin Islands. Microprobe analyses are given in 

table 8 for material from three samples sites. 
MINERALOGY 29 

Figure 38. BSE photomicrograph of a portion of a copper nugget 

(sample SC514PA) showing nickel-bearing Cu5.6Sn alloy intergrown with native copper and associated chalcocite, sphalerite, and 

cotunnite. Scale bar = 5 jlm. 

Figure 39. EDPM map of copper in the part of nugget (sample 

SC514PA) shown in figure 38. Scale bar= 5 IJ.m. 

Individual crystals of the 11-bronze are generally euhedral, and skeletal and range from 2-400 Jlm in maximum 

dimension. They are hosted by a mixture of fine-grained, 

white, lead-tin oxides, hydroxides, carbonates, and oxychlorides (fig. 27). Figure 28 shows two skeletal crystals in a 

matrix of cerussite, romarchite, and cassiterite. Both 11-

bronze and native tin can also occur together in a matrix of 

cerussite and other lead minerals. Confirmation of the 11-

bronze is afforded by X-ray diffraction data closely matching synthetic and natural material (Clark, 1972). 

Figure 40. EDPM map of tin in the part of the nugget (sample 

SC514PA) shown in figure 38. Scale bar= 5jlm. 

Figure 41. Photomicrograph showing anhedral, lenticular masses 

of white tin oxide (of the first type) within a magnetite-maghemite 

grain (cream-buff) from sample site 84SJ601 on St. John. Field of 

view = 0.3x 0.2 mm. 

We also have found very sparse amounts of what appears 

to be Ni-bearing Cu5.

6Sn intergrown with native copper (figs. 

38-44), and associated with chalcocite, cotunnite, sphalerite, 

and other as yet unidentified minerals. The mineral is not 

homogeneous but shows an exsolution texture (fig. 38). 

Microprobe analyses of this phase are given in table 9. The 

analytical totals are approximately 6 weight percent low, and 

no other elements heavier than sodium were detected. 

X-ray diffraction studies of this latter phase were not 

possible because of the exceedingly small grain size. 
30 GEOCHEMISTRY, MINERALOOY, AND GEOCHRONOLOGY OF THE U.S. VIRGIN ISLANDS 

Figure 42. SEM photomicrograph of discrete segregations of 

subhedral to euhedral fine-grained tin oxide crystals in another 

grain of magnetite-maghemite from sample site 84SJ601 on St. 

John. An example of the first mode of occurrence of tin oxide can 

also be seen in this picture. Scale bar= 20 ~m. 

Figure 43. SEM photomicrograph of tin chloride-oxychloride 

(spherical) and tin-lead chloride and/or oxychloride (octahedral) 

filling a vug in a rock sample of the Caledonia Formation collected 

at sample site 84SC225 on St. Croix. Scale bar = 20 ~m. 

Various tellurides of gold and silver as well as bismuth 

have been identified by SEM methods. No X-ray diffraction 

studies of the tellurides were possible because of the small 

grain size. 

Metallic oxides of tin, tin-lead, lead, lead-tin, copper, 

iron and chromium oxides have been identified (table 3). 

We wish to draw particular attention to the occurrence of 

black tin oxide-romarchite (SnO). Romarchite has been 

found on all the Virgin Islands, both in outcrop samples as 

well as in soil and stream-sediment concentrates. The only 

other reported occurrence of romarchite is as an alteration 

product, along with hydroromarchite, Sn302(0Hh (Howie 

and Moser, 1973), of tin pannikins that were immersed in 

Figure 44. SEM photomicrograph showing a closeup of the 

tin-lead chloride and/or oxychloride occurring at sample site 

84SC225. Scale bar = 10 ~m. 

Figure 45. SEM photomicrograph of a romarchite (SnO) 

crystal in a rock sample collected at sample site 84SJ515 on St. 

Croix. Scale bar = 1 ~m. 

fresh water (Organ and Mandarino, 1971). X-ray lines for 

romarchite from the U.S. Virgin Islands are shifted slightly 

to lower d-spacings reflecting the substitution of about 

10-20 molecular percent isostructural PbO. 

As can be seen in figures 9, 12, and 15, elevated concentrations of tin are commonly present in magnetic iron 

oxides on all three of the U.S. Virgin islands. On the basis 

of SEM EDS studies of grain mounts and polished sections 

of intergrown magnetite and maghemite grains variably 

altered to hydrous iron oxides (goethite and/or lepidocrocite), it appears that the tin is present as discrete inclusions of 

tin oxide as well as in limited solid solution in magnetite. 

Figure 41 shows discrete inclusions of tin oxide in a 
MINERALOGY 31 

Table 9. Microprobe analyses of of Cu5.

6Sn(?) phase from copper nugget from sample site 84SC514PA from St. Croix, U.S. 

Virgin Islands, in weight percent 

[Other elements looked for and not detected: S, Si, Cl, Zn, and Al. Electron dispersive spectrometry analysis shows minor Co to be present as well] 

Type of material .......... Light-gray 

Number of analyses ..... 

Sn ................. . 

Ni .................. . 

Cu ................. . 

Pb ................. . 

Total ........ . 

matrix 

8 

24.1 

1.6 

67.5 

0.3 

93.5 

Dark exsolution 

laths 

3 

20.6 

1.2 

72.3 

0.2 

94.3 

composite magnetite-maghemite grain. The tin oxide also 

occurs in the form of microveinlets that are tens of microns 

thick and as much as several millimeters in length within the 

mixed magnetite and maghemite grains. Figure 42 shows 

two modes of occurrence of tin oxide in the iron oxide 

grains. The first type (figs. 41 and 42) is represented by 

anhedral and lenticular masses of tin oxide. The second type 

(fig. 42) is represented by very discrete segregations of subhedral to euhedral fine-grained tin oxide crystals. The second type is nearly pure Sn02; whereas, the first type contains 

appreciable iron. The small grain size precluded X-ray diffraction identification. 

SEM-EDS studies also showed the occurrence of a discrete lead mineral within iron oxide grains collected at the 

same sites as the tin oxide-bearing ones. X-ray diffraction 

study data for the lead mineral could not be matched with 

any known compound. Emission spectrographic analysis of 

the hand-magnetic fraction indicate that the lead and tin 

occur together on all three islands. The sample shown in figures 41 and 42 (84SJ601) is characterized by a >2000 ppm 

tin content and I 000 ppm lead. 

The regional correlation between the tin contents in the 

nonmagnetic fraction of heavy mineral concentrates (including native metals and halides), the tin contents in the handmagnetic fraction (including cassiterite in magnetite/ 

maghemite) and the precious metals in the whole rock, soil, 

and stream sediment samples provides conclusive evidence 

that the unusual tin and lead mineral assemblage is not of 

man-made origin. 

HALIDES 

Of particular interest and significance are the occurrences of halides and halogenides. Hosking (1970), 

described tin chloride and tin oxychloride alteration products 

from tin ingots in seawater. Abhurite, a tin hydroxychloride, 

was described by Matzko and others (1985). At least two 

new hydroxide chlorides or hydrated chlorides of tin and tin/ 

lead have been found in the U.S. Virgin Islands. All these are 

white and are fine-grained. A nugget of native tin-lead contains a cavity filled with two tin oxychloride or hydroxychloride minerals. Figures 43 to 45 show the morphology of 

some of these tin-lead minerals. These compare very well 

with some of the phases grown in the synthetic experiments 

(figs. 46A-46F). X-ray diffraction data were obtained on 

both natural and synthetic materials, but the intimate admixture of the various hydrated tin-lead oxides, hydroxides, oxychlorides, and other compounds prevented discrimination of 

individual minerals. Attempts were made to match diffraction data with known compounds; except for the minerals 

named in this section, matches were not found. One or more 

iron chloride minerals are present as well. Other rare chlorides of lead, copper, and bismuth, as well as cerargyrite, 

iodargyrite, and bromargyrite have also been identified. 

SULFIDES 

Sulfides are present as well, even though this area has 

deep tropical weathering that is not conducive to the survival 

of sulfides in the surface or near-surface environment. The 

mineralogy is simple for the most part, and drill hole cuttings 

or core would undoubtedly reveal the presence of additional 

species. Pyrite, galena, sphalerite, chalcocite, and acanthite 

predominate (table 3). A Ni-As-S mineral was identified by 

SEM EDS, and may be gersdorffite (NiAsS). This has not 

been confirmed by X-diffraction). A tin sulfide is present but 

the specific mineral or minerals have not yet been determined. 

SULFATES, PHOSPHATES, AND 

CARBONATES 

Several different sulfates, phosphates, and carbonates 

have been identified as well as one vanadate, probably mottramite. The most common species are barite, cerussite, and 

hydrocerussite. Hydrocerussite is relatively common along 

with cerussite. Cerussite is distinctive because of its blue 

luminescence under the electron beam. 

An induction coupled plasma-atomic emission spectroscopic (ICP-AES) analysis of blue-green turquoise from the 

White Cliffs on St. John showed: 14.4% AI, 7.8% Fe, 0.19% 

Na, 14.2% P, 4500 ppm As, 260 ppm Ba, 220 ppm Bi, 5.7% 

Cu, 100 ppm Ga, 1000 ppm Sc, and 7300 ppm Zn. This 

material is best characterized as a ferrian turquoise. X-ray 

diffraction studies confirmed the mineral to be turquoise. 
32 GEOCHEMISTRY, MINERALOOY, AND GEOCHRONOLOGY OF THE U.S. VIRGIN ISLANDS 

SILICATES 

Much more work needs to be done on the various silicates. The minerals listed in table 3 are those occurring in 

mineralized areas in addition to quartz, feldspars, and other 

rock-forming silicates. An occurrence of wairakite from St. 

Thomas was described by Donnelly (1962). 

This unique mineral assemblage points toward an 

unusual mineralization system. While we know a fair 

amount about the mineralogy and geochemistry of high-temperature tin deposits and occurrences, our knowledge oflowtemperature tin minerals and their geochemistry is woefully 

lacking. 

LABORATORY CRYSTAL SYNTHESIS 

A series of laboratory crystal synthesis experiments 

were conducted at room temperature and one atmosphere 

pressure in an attempt to replicate the formation of lead and 

tin chlorides and oxychlorides in an environment similar to 

that postulated to have existed within the St. Croix graben 

during mineral formation. Dilute solutions of tin and lead 

chlorides were injected into a sea-water saturated CaC03 

slurry at a slow rate. Subsequent to the exhaustion of the 

chloride reservoirs, the slurry was permitted to evaporate to 

dryness, and the resultant material was inspected using the 

SEM and X-ray diffraction techniques. Chlorides/oxychlorides of tin (fig. 46A), lead (fig. 46B), tin-lead (fig. 46C) and 

romarchite (fig. 46D) were identified. These synthetic minerals were compositionally and morphologically identical to 

those found in the U.S. Virgin Islands. It is believed, but 

could not be verified, that native tin was also present. Experiments using lower pH values produced coarse romarchite 

(fig. 46E) and abundant native tin (fig. 46F). 

DISCUSSION 

The precious- and base-metal anomalies found on the 

U.S. Virgin Islands cross nearly all the rock types exposed 

on the islands, including the Kingshill Marl of mid to late 

Miocene age on St. Croix (Lidz, 1988). A good spatial correlation between the geochemical patterns and major 

mapped structural features can be seen on all three islands, 

but they are especially clear on St. Croix and St. John. 

Hydrothermal alteration (predominantly silicification and 

sericitization) is widespread and is especially visible along 

the southern shores of St. John and St. Thomas. The alteration occurring in the interior parts of the islands tends to be 

masked by the pervasive and deep weathering and can only 

be seen in road cuts or at construction sites. Pyritization was 

widespread on the islands but fresh pyrite can only be found 

encapsulated in the more siliceous rock units. Other sulfides 

are rare, although a little galena, chalcopyrite, and sphalerite 

have been found in the heavy-mineral concentrates and rock 

samples. Epidotization on the islands was widespread and 

sometimes massive. Barite and gossan fragments are common in heavy-mineral concentrates. 

Gossan is present at several locations on the islands. 

The most massive gossan caps Bordeaux Mountain on St. 

John and is greater than 7 m in thickness. A thin gossan is 

also present on Cocoloba Point and a thin siliceous gossan 

caps the paratacamite coated White Cliffs (St. John), which 

consist of fine-grained hydrothermal quartz with thin turquoise veinlets. 

A precious-metals bearing skarn, characterized by marble, garnetiferous rocks, and iron-bearing, as well as copperbearing, quartz and barite veins, occurs on the islands to the 

north of and between St. Thomas and St. John, extending 

eastward to Mary Point on St. John. 

Gossan and/or massive limonitic boxwork also occurs 

on St. Thomas, especially in the Jersey Bay area. No gossan 

has been noted on St. Croix although gossan fragments as 

well as epidote and barite, are widespread in the concentrates 

of soil and stream sediments. 

Water-soluble chlorine in the soils of the islands consistently delineates halos over and around the base- and precious-metals anomalies. These chlorine-enriched areas do 

not show any correlation with nearness to shore-line, prevailing wind direction, or the highest areas on these islands. 

The highly unusual halide and native metal rich basemetal mineralogy found in the surficial materials as well as 

outcrops and the predominance of silver halides points to a 

genetic sequence that involves an extremely chlorine-rich 

solution phase. This conclusion is also supported by the survival of the tin and lead chlorides/oxychlorides only in settings of encapsulation, whether these be within native-metal 

grains or within outcrops of dense, siliceous units. 

At this stage of the study, the U.S. Virgin Island 

geochemical anomalies indicate a mineralization that is a 

distinct and unique mineral occurrence. The current erosional level intersects this mineralization system at its upper 

levels. It is postulated that this mineralization is probably 

related to subcropping felsic to intermediate intrusives of 

late Tertiary age. The metal-rich hydrothermal solutions 

were probably introduced into shallow, calcareous, oozerich, semi-restricted sea basins-as exemplified by the central St. Croix graben (fig. 47). Here, the graben boundary 

faults acted as the main hydrothermal solution pathways. 

Within the graben the more porous terrigenous alluvium 

zone, near the graben walls, would facilitate the further 

migration of these fluids and their gradual dissemination into 

the graben water. The immediate area of the graben was 

probably additionally enriched in sodium chloride as a result 

of sea water convection currents due to the thermal gradient 

resulting from the introduction of the warm hydrothermal 

solutions. Higher rates of evaporation might result within the 

shallow part of the graben as a result of the thermal gradient 

brought about by the introduction of the warm hydrothermal 
A. Spherical tin chloride-oxychloride. 

B. Large crystal of lead chloride-oxychloride. 

C. Octahedral crystal of tin-lead chloride-oxychloride. 

DISCUSSION 33 

D. Octahedral crystals of romarchite (SnO). 

E. Spherical romarchite (SnO) with enlargement to show individual crystals on the left. 

F. Feathery, skeletal crystal of native tin. 

FIGURE 46. SYNTHETIC MINERALS GROWN AT ROOM TEMPERATURE AND ONE ATMOSPHERE PRESSURE BY 

INJECTING CHLORIDES OF LEAD AND TIN INTO SEAWATER-SATURATED, POWDERED CaC03. 

[Shown at different scales.] 
34 GEOCHEMISTRY, MINERALOOY, AND GEOCHRONOLOGY OF THE U.S. VIRGIN ISLANDS 

EXPLANATION 

A Zone of Au deposition 

8 Zone of Sn and Pb oxides 

coprecipitation with iron oxide 

C Sn, Pb, and Sn-Pb chlorides 

oxychlorides and hydroxides 

t Higher evaporation rates 

[llllllllllil!Illll Sea water 

Figure 47. Block diagram illustrating the conceptual model of precious- and base-metal mineralization in the St. Croix graben, U.S. 

Virgin Islands. 

solutions as well as the relatively shallow water setting here. 

It is probable that the introduced metals were deposited primarily as halides. All evidence indicates the presence of 

hydrothermal solutions that were tungsten, fluorine, and sulfur poor but which were very rich in chlorine. Paucity of sulfur is indicated by the nearly total absence of nearly all basemetal sulfides-aside from pyrite, which is not part of this 

mineralization. In addition to occuring within the halides, 

oxides, and carbonates, the base-metals here also occur built 

into the layered magnetite/maghemite layers in substantial 

concentrations primarily adjacent to the eastern and western 

graben walls. A zonational aspect is evident relative to the 

graben walls: gold is concentrated in the immediate vicinity 

of the walls and of cross-trending structures; whereas, silver 

and base-metals are zoned outward from the graben boundary faults. The graben-filling Kingshill Limestone (or marl) 

has been silicified and epidotized to varying degrees and 

base-metal chlorides have been found cementing fragments 

of biota within the marl, but more commonly the base-metal 

chlorides occur within the grains of native tin and lead that 

are nearly ubiquitous in the soils within the graben. Gold is 

widespread near the graben walls generally in the form of 

subrnicron particles although some coarser particles of gold 

have been found in sediments from the most strongly silicified and epidotized portions for the graben. 

Simple laboratory syntheses, at room temperature and 

one atmosphere pressure, produced tin and lead and tin-lead 

chlorides, that are compositionally and morphologically 

identical to those found on the U.S. Virgin Islands. Romarchite and some possible native tin were also produced. 

Increasing the pH of the solution produced coarse, wellcrystallized romarchite and abundant native tin. 
REFERENCES 35 

Recent unpublished work by the authors has indicated 

that lead, tin, iron, magnesium, and other chlorides, oxychlorides, carbonates, hydroxides, and other compounds are 

associated with base- and precious-metal mineralization in 

the Bayhorse area in Idaho as well as the Duluth Complex 

and the granite-greenstone terrain in northeastern Minnesota. Total chlorine, determined using stream-sediment 

samples, form halos that are associated with most of the 

examined mineralized areas in Idaho. 

SUMMARY 

This geochemical study of the U.S. Virgin Islands demonstrates the presence of an extensive tin-lead and preciousmetals metallogenic province in the northeastern portion of 

the Greater Antilles island arc. The U.S. Virgin Islands 

hydrothermal systems are characterized by a near absence of 

tungsten, fluorine, and sulfur and a superabundance of chlorine. The abundance of chlorine is probably due, at least in 

part, to the submarine origin of this mineralization. These 

conditions produced a highly unusual base-metals mineralogy consisting, primarily of halides, complex halide-native 

metal intergrowths, and native metals. 

The authors believe that the currently available data on 

the U.S. Virgin Islands demonstrates this area to be one worthy of further study for economic as well as scientific reasons. 

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Novgorodova, M.I., Gorshkov, AI., Trubkin, N.V., Veretnikov, 

V.M, and Tsepin, AI., 1981, An exotic mineral association 

with uraninite: Mineralogicheskii Zhurnal, v. 3, p. 102-108. 

[In Russian]. 

Okrugin, A.V., Oleinikov, B.V., Zayakina, N.V., and Leskova, 

N.V., 1981, Native metals in trap rocks of the Siberian Platform: Zapiski Vsesoyuznogo Mineralogicheskogo Obshchestvo, v. 110, p. 186--204. [In Russian] 

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v. 10, p. 916. 

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Manuscript approved for publication January 29, 1993 

Graphics by Roger Highland 

Edited by Thomas Kohnen 

Photocompositon by Shelly A. Fields 

* U.S. ~ PRIN1'll(; OFFICE: 1994-673-046/86059 

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A.J., and Stewart, K.C., 1987, Analysis of geological materials 

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---1966, Geology of St. Croix, U.S. Virgin Islands: Geological 

Society of America Memoir 98, p. 177-239. 


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