Intertdonal Geology Reoieur, Vol. M, 2002, p. 7X-764,. Copyright 0 2002 by V. H. Winston & Son, Inc. All rights reserved.

Petroleum-Rich Fluid Inclusions in Fluorite, Purisima Mine, Coahuila,

E. GONZALEZ-PARTIDA, A. CARRILLO-CI-IAVEZ,~

Centro de Geociencias, Universidad National Auto’nomn de M&ico (UNAM), Campus Juriquilla AP 1.742, Juriquilla, Qro., M&ico 76230

J. 0. W. GRIMMER, AND J. PIRONON CREGU; Domaine Scient$que Victor Grignard; Entrge #I$ Fuculte’ des Sciences, Universite’ Henri-Poincurb Nancy I; R.P. 23;

54501 Vundoeuvre-Le’s Nancy Cedex, France

Abstract

Mexico has been a world leader in fluorite production. This study demonstrates a genetic link between petroleum basinal fluids and fluorite deposits. Fluid inclusions in fluorite from the La Purisima mine, northeast Mexico, were analyzed using UV-fluorescence, petrographic, and micro- thermometric lechniques. Three different populations of inclusions were identified: (1) petroleum- rich (L, -I- V); (2) aqueous-rich (L, + V), and (3) immiscible liquids (L, + L, + V), where L, is liquid hydrocarbon, LZ is a brine, and V is vapor phase. The L2 + V inclusions are primary, whereas the L, + V and the L, + L, + V are pseudo-secondary. Some or the L, + V petroleum-rich inclusions show necking down and leakage. Frequently, these inclusions contain a solid phase composed of heavy hydrocarbons (solid bitumens?). At low temperature, the petroleum-rich L, + V inclusions were unaffected by freezin,, v and il was only possible to measure ice melting temperature (Tm = -102°C) in one three-phase, petroleum-rich inclusion (L, -t L, + V), which corresponds to a salinily of 1415 wt% NaCl equivalent. UV-fluorescence micro-thermometry indicates the presence of lighl: and heavy hydrocarbons in the oily inclusions. During heating runs, these oil-bearing inclusions have homogenization temperatures between 50” and 150°C. The aqueous-rich L, -I- V inclusions range in salinity from 5.7 to 18.13 wt. % NaCl equivalent with homogenization temperature ranging from 75’ to 155°C. These inclusions show white fluorescence under UV, indicative of a brine.

The salinity and homogenization temperature of all lhe inclusions are similar LO [hose of Missis- sippi Valley-type (MVT) d elosits and petroleum basins. In this study, we interpreted [he Upper 1 Jurassic rock sequence of the Chihuahua-Sabinas basin I;o be the source of the oil-trapped inclu- sions, and very likely, of some of the fluorine. The alkaline magmatism of the Trans-Pecos Province was the source of mineralizing fluids, and hence, of some of the fluorine too. This event remobilized the petroleum brine and hydrocarbons trapped as fluid inclusions within fluorite.

Introduction

STUDIES AND ANALYSES of oilfield brines during the past decades have shown a close genetic relationship with the mineralizing fluids of the Mississippi Valley- type deposits (MVT). The presence of oil within fluo- rite (F) from MVT deposits is quite common (Twen- hofel, 1947; Weller et al., 1952; Hall and Friedman, 1963; Perhac and Heinrich, 1964; Barton, 1967; Haynes and Kesler 1987; Sverjensky, 1989; Touray,

lEmai1 aclrlresses for Drs. Gonzlilez-Partida and Cawillo- Cl1iivez: egp@geociencias.unam.mx; amhiente@geoscien-

1989; Kesler ancl Van der Pluijm, 1990; Rankin et al., 1990). Also common in MVT deposi,ts is the pres- ence of organic matter and oil within fluid inclusions (Hagemann and Hollerbach, 1985; McLimans, 1987; Bodnar, 1990; Goldstein, 1990; Karlsen at al., 1993; Goldstein and Reynolds, 1994)).

For several decades Mexico was the world leader in fluorite produc-tion. In the Muzquiz area (Fig. 1) alone, there are eight mining districts (San NIiguel- La Mariposa, El Tule, Las Cabras, Buenavista, La Encanrada, San Vicente, Sierra de1 Carmen, and La Linda-Aguachile) with more than 10 millions tons of fluorite in reserves with grades between 40 and

cias.unam.mx

0020-6814/02/613/755-10 $10.00 755

756 GONZ/fLEZ-PARTIDA ET AL.

ENCANTADA le Ita Sn Jorge TULE

BUENAVISTA SN VICENTE

LAS CABR 2 S LA MARIPOSA

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FIG. 1. Regional distribution of the two fluorite belts (dashed line pattern) in Mexico (eastern: Ylhpda-Parral-

Navidad-Inde-Bolafios-Guadalcazar-Las Cuevas-El Realito-Taxco; western: Hansonburg-Pica Entero-Encantada- Paila-Tamaulipas). The areas depicted with the dotted pattern are the main petroleum basins in Mexico (Cantarel and Active Luna are currently the most productive fields). The Chihuahua Basin coincides with the eastern fluorite belt, and the Sabinas basin with the western belt. Insert: Distribution of principal fluorite deposits in northern Mexico (Pica Ete- reo-Encantada area, Coahuila).

90% of Ca F, (Van Alstine et al., 1962; Fuentes and Gonzalez, 1990). The La Purisima mine is located approximately 180 km northeast of Muzquiz, Coahuila, within the Encantada-Buenavista mining District, northern Mexico (Fig. 1). In this district, strataform fluorite mineralization occurs as exten- sive deposits at the western portion of the Encantada and Buenavista ranges. This mining district is underlain by Jurassic-Cretaceous sedimentary rocks (Chiuahua-Sabinas basin) that generated petroleum during the Late Tertiary, and by Tertiary igneous intrusive rocks (Cuevas-Leree, 1984;

Limon-Gonzalez, 1986; Gonzalez-Garcia and Holguin-Quidones, 1992). This spatial relationship between the .fluorite deposits and the igneous activ- ity has been the target of several investigations. McAnulty (1974) h s owed a direct relationship between the fluorite and Tertiary rhyolitic domes, sills, and dikes.

This study reports the results from fluid-inclu- sion, petrographic, microthermometric, and UV-flu- orescence analyses. The genetic role of the petroleum in the formation of these deposits is dis- cussed as well.

FLUID INCLUSIONS

BURRO-PEYOTES

FIc;. 2. A. General geologic map of lhe La Purisima mine area and surroundings. The “Drilling exploration zone”

(dashetl line pattern) consists of shale oC [he Del Rio Formation; the area symbolized by vertical lines and dots represents alltaline-talc-alkaline intrusions; the area symbolized with a pattern of crosses represents alkaline intrusions; the area of small asterisks represenls 11x5 I-Ierradura dike, a talc-alkaline inlrusion. B. General E-W structural cross-section of

the area, indicating the location of the fluorite hodies and the main sedimentary formalions (see text for description); the stra$q$ic c&m-m (upper right) in&ales the approximate thickness of the sedimentary formations. C. Regional close- up of the Trans-Pecos alkaline magmatic province. D. General outline of the main Jurassic-Cretaceous paleogeographic

elements in Ihe region.

Mesozoic Geologic ad Tec,ronic Framework of Norl;hern Mexico

Fluorite deposits occur in two belts in north-cen- tral Mexico (Ruiz et al., 1985; Fig. 1). The distribu- tion of the Tertiary fluorite province in Coahuila is related to two Mexican oil basins: (1) Sabinas and (2) Chihuahua (Figs. 1 and 2). Tbe genesis of the oil in these two basins is related to geologic evolution

from Late Jurassic to Early Tertiary time. During the Late Jurassic, the tectonic framework began to be defined, with the formation of shallow depressions, positive elements, and a marine transgression. The distribution of shallow basins and positive elements defined the sedimentary and structural trend of Jurassic and Cretaceous formations in the Sabinas Basin. The Coahuila Peninsula, a positive element, was covered by sea water until the Late Aptian- Albian (Early Cretaceous). Meanwhile, more than

GONZdLEZ-PARTIDA ET AL.

1200 m of evaporates, carbonates, shale, and sand- stone were cleposited in the Sabinas basin.

The Cretaceous was characterized by the devel- opment of a extensive platform with a well-devel- oped reef on its margin (Cupid0 Formation), with a thickness of up to 880 rn. During the Late Creta- ceous, the sea water continued its transgression over the positive elements. Two large platforms were developed and separated by a deeper basin. The Aurora Formation, a 600 m ,thick calcareous reef, was deposited over the La Peiia Formation, 20-60 m of shale and calcareous shale covering the Cupid0 Formation. The Wichita Group (90 m of shale and limestone), which acts as host rock for the fluorite deposits, includes the Georgetown, Del Rio, and Buda formations. By the end of the Cretaceous the terrigenous material increased and dominated the carbonates. The terrigenous material was derived from the erosion of the new highlands produced by the early stages of the Laramide orogeny. The Ter- tiary started with intense magmatic activity and extensional tectonics. Oligocene alkaline magmas in the area of Texas and Northern Mexico are docu- mented, and according to Clark et al. (1982), alka- line magmatism has been reported in northern Mexico along with temporally related extensional faulting (30 to 39 Ma) related to the Basin and Range province. Price and Henry (1984) suggested that the overlap between extensional faulting and alkaline magmatism (30-32 Ma) coincides with a regional change in magmatic composition (from talc-alkaline to alkaline). The Trans-Pecos alkaline province, composed of alkaline mafic and silicic rocks, ranges in age between 39 and 32 Ma, and the intrusions alld dikes have a general NNE trend. This structural trend is perpendicular to the orientation of the S, deformational tensor of the early stage of the developlnent of the Basin and Range province (Price et al., 1990). The Cenozoic history of the Trans Pecos province can be summarized as follows: (1) Laramiclic folding and intrusions; (2) abundant magmatic activity caused by subduction; and (3) intracontinental extensional tectonics forming the Basin and Range province.

On the o-ther hand, the petroliferous potential has been widely reported (Gonza&z, 1976; Cuevas- Leree, 1984; Limon-Gonzal6z, 1986; Gonzalez and

Q uifiones, 1992), indicating that area contains 0.05% of the total petroleum in Mexico. Geochemi- cal studies (Gonz&lez and Quifiones, 1992) indicate that the oil was generated mostly by calcareous shale and carbonates from the Upper Jurassic, and

in a lesser proportion by Tertiary and Cretaceus rocks. The organic-matter content in the Upper Jurassic rocks range from 0.5 to >l% (La Casita, Pefia, and Pimienta formations).

Local Geology and Fluorite Mineralization in Northem Mexico

In the NNW part of the state of Coahuila, there are outcrops of Lower and Upper Cretaceous calcar- eous sedimentary rocks folded into anticlines and synclines with a SE-NW trend. Folds are inter- rupted by normal faulting and Tertiary igneous intrusions. These igneous rocks (rhyolites) are genetically related to Be, U, and F ore deposits (compositional similari ties). The Cenozoic his tory of the Trans-Pecos province has three main stages: (1) Laramide folding and faulting; (2) magmatism asso- ciated with subduction processes; and (3) exten- sional processes associated with the Basin and Range province. During the Cenozoic, there was no overlap between folding and magmatism, with Lara- mide folding starting in the Early Cretaceous, with a peak during Paleocene-Eocene time. Magmatic activity followed this deformation event.

The host rock for the fluorite mineralization is the Lower Cretaceous reef limestone of the George- town Formation. The overlapping Upper Cretaceous shales of the Del Rio Formation act as a seal.

The Encantada District forms part of the eastern belt (Muzquiz-Las Cuevas), which is the most pro- ductive (highest grade). The ore in the eastern belt occurs mainly as stratiform beds, veins, and stock- works hosted by Cretaceous carbonates, in contrast with fluorite deposits of western Texas and New Mexico, which are hosted by Middle to Upper Ter- tiary rocks and are genetically related to the alka- line igneous rocks of the Trans-Pecos magmatic province (Clark et al., 1982). The Chihuahua-Sabi- nas basin of northern Mexico (non-productive) over- laps with the eastern fluorite belt. At the Purisima mine, mineralization occurs in a 1 m thick concor- dant bed with abundant gangue calcite. Kesler (1977) reported other fluorite ore structures such as chimneys and veins with calcite, silica, and gypsum.

McAnulty (1974) related the mineralization of fhe Encantada District to rhyolitic domes, dikes, and sills that crosscut the Cretaceous rocks. Kesler (1977) proposed that the Ca needed for the fluorite formation was derived from the host limestones, and .the F was derived from a proximal unexposed mag- matic source. This magmatic source also could have

FLUID INCLUSIONS 759

contributed the water, and possibly sulfur and heat, for [he mineralizing sys tern.

I?luid Iuclusion Microthermometric and UV-Fluorescence Observations

Methodology

A LINKAM (Model THMSG-600) stage, mounted on a pelrographic microscope with 12.5 x 2.5 eye- pieces and a 100x objective, has been used to record Tm (final ice melting temperature) and Th (homoge- nization temperature) of two-phase aqueous and petroleum inclusions and of three-phase oil + water inclusions. The stage was calibrated using a syn- tbetic reference. Technical details on sampling, sample preparation, analytical criteria, and repro- ducibility can be found in Martinez-Ibarra (1999). For temperatures of ice fusion (T,) Tr = + 0.1” C; for homogenization temperatures (TJ T,, = rt 5” C.

The fluorite of the La Encantada mine contains many inclusions (with diameters ranging from 1 to 100 mm); some are dark and decrepitated. Aqueous and petroleum inclusions are frequently associated in planes. Th e 1 argest inclusions are petroleum inclusions. The liquid oil phase contains numerous solid particles.

Fluid inclusions were first studied petrographi- tally and classified based on their properties. On this basis, microthermometric measurements were done at freezing and high temperatures. Low-tem- perature measurements involved cooling samples to -200” C; observations on the freezing of the inclu- sions were made and mehing temperatures (Tm) of the inclusions were determined. Inclusions from La Encantada are very fragile, and many decrepitate during freezing experiments. The salinity (NaCl equivalent) concentration of the inclusion fluids was calculated after Bodnar (1993). High-temperature measurements involved the recording of the temper- ature of homogenization (Th) of the inclusions.

Photoluminescence (UVF) occurs when photons are excited instead of electrons. This phenomenon includes fluorescence and phosphorescence, refer- ring to immediately and retarded light emission, respectively. The UVF observations were done with a petrographic microscope equipped with a mercury lamp. The incident light from the mercury lamp causes the photoluminescence of the material. Dur- ing these observations, a ultraviolet (UV) filter was used because of the fluorescence of the hydrocar- bons (HC) under UV light. The emitted light spec- trum can be quantitatively measured, or

qualitatively determined by direct observation of the color 0 the fluorescence light. The hydrocarbon composition, and its fluorescent light, depends on several factors such as the genesis, thermal history, biologic degradation, and migration history. Fur- thermore, fluid inclusions containing oil generally exhibit fluorescence under UV light due to the pres- ence of aromatic HC and ni-trogen, sulfur, and oxy- gen compounds. The aromatic HC (heavy HC) fluoresce yellow and orange under UV light, whereas, the aliphatic WC (light HC) are a luminant blue.

Results

Fluid inclusions within fluorite from the La Purisima mine are abundant and occur as three types, according to the criteria proposed by Shep- herd et al. (1985): (1) hydrocarbon rich (IL, + V); (2) liquid-rich (L2 + V), and (3) immiscible liquids (L, + L, + V), where L, is liquid hydrocarbon, LZ is a brine, and V is vapor phase. The liquid-rich LZ t V inclusions are primaly inclusions located on crystal- lographic planes. The L, + LZ + V are pseudo-sec- ondary inclusions located on the border of fluorite crystals and not in fractures. These petroleum-rich inclusions are three to four times larger that the non- petroleum inclusions, exhibit a brownish-yellow liq- uid phase, and occasionally, a dark solid phase, which likely represents heavy hyclrocarbons (aro- matics). The solid phase is generally locatecl at the edge of the inclusion. Petroleum-rich L, + V inclu- sions have a variable 1iquid:vapor ratio, and occa- sionally 100% L, inclusions (100% liquid hydrocarbon) are observed. The petroleum-rich inclusions also exhibit other pseudo-secondary characteristics (shape, distribution, and size). Most of these inclusions (L, -I- V) show a rounded shape and are called “Pacman.” But some of these inclu- sions have an elongatecl shape (called “elongate”).

Other characteristics observed in these inclu- sions are necking down and leakage. Figure 3 shows the shape and distribution of the fluid inclusions within fluorite and their behavior under UV-fluores- cence techniques. Figure 4 shows the histograms with the Tm and Th of the petroleum-rich and aque- ous-rich fluid inclusions.

At low temperature, the hydrocarbon-rich inclu- sions with the brownish-yellow liquid phase (L, t V) were unaffected by freezing, a physical property of petroleum-rich inclusions composed of light hydro- carbons. In fact, the UV-fluorescence analyses indi- cate the presence of a mixture of light and heavy

760 GONZ,dLEZ-PARTIDA ET AL.

,; :’ ‘:

FIG. 3. Black and white microphotograph showing: (A) aqueous fluid inclusions under natutal light; (A’) same field under UV-fluorescence; under the microscope the different coloration (blue, corresponding to 45-50 API grade) indi- cates that the liquid is a brine; (B) three-phase petroleum fluid inclusions under natural lighl; (B’) same field under UV- fluorescence (corresponding to lo-15 API grade). The grey tone bar is the taken from Bodnar (1990) and represents Lhe relaiion between the API grade (numbers) and fluorescence grey tone of the hydrocarbons under UV light.

hydrocarbons. The Th of all three ‘types of petroleum fluid inclusions (Pacman, elongated, and three- phase) have a range from 50” to 15O”C, with the main population at 100°C. Pure petroleum fluid inclusions (100% petroleum, L,+V) did not show Tm; in the three-phase inclusions it was only possi- ble to measure a Tm of 10.2”C, which corresponds to salinity of 14.15 wt% NaCl equivalent.

The aqueous-rich L, + V inclusions have two groups of different salinities (Tm = -10 and Tm = -5°C). The entire range of Tm for the aqueous .fluid inclusions is from Tm = -3.5 to Tm = -14.5 “C, cor- responding to salinities ranging from 5.7 to 18.13 wt% NaCl equivalent. The eutectic temperature (Te)

is in all aqueous fluid inclusions - -2l”C, and cor- responds to an H20-NaCl mixture. The correspond- ing salinities range between 7.86 and 18.13 wt% NaCl equivalent. The Th histogram of the aqueous fluid inclusions is broad (Fig. 4B), but shows only one mode cenlerecl at 120°C.

Homogenization temperatures of aqueous inclu- sions are generally higher than those of petroleum fluid inclusions, ranging from Th = 75 to Th = 155”C, and one isolated value of Th = 170°C. Pres- sure correction is very small for the aqueous fluid inclusions, and under UV observations the inclu- sions show a white color, characteristic of brine without hydrocarbons.

FLUID INCLUSIONS 761

16 , I

12

IO

ii8 6

4

2

0

40 60 80 100 120 140 160

-15 -14 -13 -12-11 -10 -9 -8 -7 -6 -5 -4 -3 0 25 50 75 100 125 150 175 Tm ("C) Th ('C)

30 B

25 -

20

ii 15

10

5

0

70 80 90 100 -I

110 120 130 140 150 160 170 180 Th('C)

4 2- D o-

-2- $3 -4 - -6 : -8

-10 -12 i

-16!

FIG. 4. Low- and high-temperature data. A. Th (homogeniz&on temperature) of all three types of petroleum fluid inclusions ranging from 50” to 15O”C, with a genera1 peak at 100°C. B. Th histogram of the aqueous fluid inclusions is broad, but uni-modal, with. a peak at 120°C. C. The aqueous fluid inclusions present two groups of different melting temperatures (Tm = -10 and Tm = -5°C). Th e corresponding salinity range is from 7.8 to 18.1 wt% N&l equivalent. D. This graph shows the relation between Tm and Th, indicating ihat the higher-salinity fluid inclusions have lower Th.

Discussion

Fluid inclusions within fluorite from the Buenavista-Encantada mineralized districl: were analyzed by Kesler (1977), who reported Th ranging from 170” to 340°C. Al: a contact zone between the fluorite ore body and intrusive rocks, Kesler reported Th between 380” and 430” C (Cerro El Pilote), and salinity ranging from 8.5 to 18.2 (12.8 average) wt.% equivalent NaCl. Kesler related the mineralizing process with boiling fluids derived from a magmatic source.

Hannah and Stein (1990) observed that the mag- matic and hydrothermal processes related to the for- mation of fluoride deposits are: (1) tectonic framework; (2) chemical and mineral source; (3) assimilation; (4) contamination and thermal gradi- ent; (5) magma composition; (6) volatile content; (7) temperature; (8) depth; (9) volume of fluids; and (10) time. Webster et al. (1987) suggested that fluoride and chloride play important roles in the transport of MO, Cu, Pb, and Zn.

Recent studies on fluoride deposits associated with extrusive and intrusive igneous rocks have shown that fluorite precipitation is the result of the mixing of carbonate-rich water with hydrothermal, magma-derived, fluoride-rich fluids (Burt and Sheri- dan, 1980). In contrast, Ruiz et al. (1985) suggested that, in addition to the mixing of carbonate water and fluoride-rich fluids, a drop in brine temperature, a rise in pH, and reaction with the host rocks also are needed. Deloule (1982) d emons trated that solubility of fluorite decreases as pH increases. According to this idea, the cooling of a mineralizing brine is the strongest evidence for fluorite precipitation.

Various ideas have been proposed for the fluo-

rite precipitation: (1) cooling of mineralizing fluicIs; (2) dilution of mineralizing fluids with meteoric waters; (3) mixing of two brines with different com- positions; and (4) pH increase in the original acid fluids (Kesler 1977; Richardson and Holland, 1979; Burt and Sheridan, 1980; Deloule, 1982; Ruiz et al., 1985). Richardson and Holland (1979) suggested that the cooling of mineralizing fluids

762 GONZALEZ-PARTIDA ET AL.

is the most eficient mechanism for fluorite pre- cipitation.

For the mineralizing process at the Encantada- Buenavista fluorite district, Kesler (1977) proposed that the Ca came from the host limestones and the fluoride was derived from a magmatic source. McAnulty (1974) related the rhyolitic volcanic rocks (dikes and sills) intruding the Cretaceous limes tones to the fluorite genesis.

Roedder et al. (1968) reported oil in fluid inclu- sions at the neighboring Hansonburg fluorite district (same Basin and Range province), located north of the Encantada-Buenavista district. The salinity of the Hansonburg district fluid inclusions ranged between 10 and 17 wt% NaCl equivalent, and have homogenization temperatures ranging between 140 and 205°C; suspended organic matter was present at times within the fluid inclusions, probably mobi- lized from sediments.

The Th and salinity measured in hydrocarbon- rich inclusions and non-hydrocarbon inclusions in the present study (La Purisima mine) (50” to 155°C and 5.7 to 18.13 wt% NaCl) are similar to those reported on fluid inclusions from Mississippi Val- ley-type deposits and petroliferous sedimentary basins. As a matter of fact, past studies on petro- leum brines have shown a close genetic relationship with the Mississippi Valley-type deposits, also showing that the involved water is “formational” water (Twenhofel, 1947; Weller at al., 1952; Perhac and I-Ieinrich, 1964; Barton, 1967). Both Hall and Friedman (1.963), and Rankin et al. (1990) have shown that organic matter and oil (asphaltenes) are commonly present wi,thin fluid inclusions from fluo- rite deposits. They also have shown the simil.arity in composition between the fluid inclusions of fluorite deposits and inclusions from pe troliferous sedimen- tary basins and Mississippi Valley-type deposits.

Conclusions

We consider the oil in the fluid inclusions from La Purisima mine to be derived from the Upper Jurassic oily shales of the Chihuahua-Sabinas basin. The oil in this basin was formed during the Late Cre- taceous and until the Early Tertiary (Paleocene- Eocene). Cuevas-Leree (1984), Limon-Gonzalez (1986), and Gonzalez-Garcia and Holguin-Quifiones (1992) have proposed that the oil generated in the Chiuhuahua-Sabinas basin went out of ,the “petro- leum window,” because of the absence of oil traps.

On the other hand, the alkaline magmatic Trans- Pecos province could be the source and mobilizer of fluoride, and mobilizer of petroleum brine. The corn-

bination of these factors with Ca derived from the host limestones resulted in the precipitation of fluo- rite ore bodies of the Encantada-Buenavista district, and in the formation of hydrocarbon-rich and non- hydrocarbon &id inclusions within the fluorite.

Acknowledgments

This research was funded in part by grant UNAM-PAPIIT # IN100900. The authors wish to thank to Dr. Marcos Zentilli, Dalhousie University; Dr. Dan Kontak, Nova Scotia Department of Natural Resources, and Dr. Nick Wilson, Geological Survey of Canada, Calgary, for their valuable comments and suggestions. We also wish to thank to Ings. Rodriguez, Victoria, and Vaca for assistance during field work. Finally, we wish to thank Sara Solis for assislance in preparing the figures.

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