Geology and geochemical analysis of mineralizing fluids at the St. Cloud and U.S. Treasury Mines, Chloride Mining District, New Mexico

Journal of Geochemical Exploration, 42 ( 1991 ) 61-89

61

Elsevier Science Publishers B.V., Amsterdam

Geology and geochemical analysis of mineralizing fluids at the St. Cloud and U.S. Treasury mines, Chloride mining district, New Mexico D.I. Norman, R.W. Harrison and C. Behr Andres New Mexico Institute of Mining and Technology, Dept. of Geosciences, Socorro, NM 87801, USA (Received September 15, 1990; revised and accepted January 15, ! 99 i )

ABSTRACT Norman, D.I., Harrison, R.W. and Behr Andres, C., 1991. Geology and geochemical analysis of mineralizing fluids at. the St. Cloud and U.S. Treasury mines, Chloride mining district, New Mexico. In: S.E. Kesler (Editor), Fluid Inclusion Gas Analyses in Mineral Exploration. J. Geochem. Explor., 42:61-89. The St. Cloud-U.S. Treasury vein system is part of widespread, late Oligocene, epithermal mineralization that occurs along the eastern front of the Black Range, southwestern New Me~,ico, and occupies a normal fault related to the initial development of the Rio Grande rift. Stage l mineralization consists of galena, sphalerite, and chalcopyrite; and Stage 2 mineralization is bornite, sphalerite, chalcocite, digenite, betektinite, and complex copper-silver sulfides. Timing of mineralization is closely tied to an episode of regional volcanism that commenced at approximately 30 Ma and lasted until about 28 Ma. This episode consisted of two pulses of volcanism, initially with mafic to intermediate volcanism that was closely followed by rhyolite volcanism. Age dates and paleodepth considerations indicate that the two stages of mineralization are related to these two volcanic pulses, respectively. Fluid inclusion homogenization temperatures and salinities for both stages of mineralization overlap and range from 210 to 294~C, and from 0.0 to 3.4 eq.wt.% NaCI. Stage l inclusions have slightly higher T, and salinity values than Stage 2. Within these ranges of temperature and salinity for both stages of mineralization, three populations were identified by cluster analysis and cumulative frequency plots: high temperature, high salinity (A); high temperature, low salinity (B); and low temperature, low salinity (C). Gaseous species found in inclusion fluids analyzed by quadrupole mass spectrometry are CO2, C,H,, i i ' ) 0 5 * 1 2 , %.~i14,

etry measurements indicate that C,H, is principally propene, butene, and penl.ene. Stage 2 inclusion fluids contain relatively greater amounts of H2S, CO2 and total gases than Stage l inclusion fluids. Within each stage, the concentrations of all gases except H2S increase with elevation, whereas H2S shows an inverse relationship with elevation. Inclusion fluids from non-mineralized vein material have low concentrations of all gases. Concentrations of N2, COe and CH4 in inclusions are greatest in surface samples overlying and immediately adjacent to mineralized areas. Inclusion populations B and C for both stages of mineralization have low total-gas contents and are correlated with non-mineralized vein material; population A for both stages of mineralization have relatively higher concentrations of all gases, particularly H2S and CO2, and is correlated to sulfide mineralization. Analyses of channel samples from surface outcrops of the St. Cloud-U.S. Treasury vein indicate that a positive anomaly in CO2, N2 and N2/CO2 exists over mineralized shoots. These analyses could have located the blind ore bodies that were found at the St. Cloud mine.

0375-6742/91/$03.50 © 1991 Elsevier Science Publishers B.V. All rights reserved.

62

D.I. NORMAN ET AL.

The gas anomaly and greater concentrations of gases that occur in samples from upper mine levels is attributed to fluid boiling and trapping of vapor in inclusions. Calculations offs, from gas analysis agrees with that determined from the Fe content of sphalerite for both stages of mineralization. Gas analysis also indicates total gas equilibrium. From metal-solubility and pH calculations, it is interpreted that only population A fluids for both stages of mineralization had the capability of transporting significant quantities of copper and silver to form the observed ore deposits. Population C inclusion fluids are postulated to be from near-surface, steam-heated waters; and population B inclusion fluids are believed to have been derived from these steam-heated waters as they further warmed while mixing with population A fluids in the geothermal system. The most important mechanism for mineral deposition at the St. Cloud-U.S. Treasury deposits was fluid boiling. A simple boiling model that incorporates thermometric and mass-spectrometric data predicts both grade and mineral zoning in these deposits.

INTRODUCTION

The St. Cloud and U.S. Treasury mines exploited polymetallic, sulfide mineral deposits from structurally controlled ore shoots. These deposits have been the most productive of the numerous, epithermal vein systems in the Chloride mining district located along the eastern flank of the Black Range, Sierra County, New Mexico (Fig. l ). Production from these deposits was approximately two million ounces of silver, nine thousand ounces of gold, three thousand tons of copper, and several thousand tons each of lead, zinc, and highsilica smelter flux. At present time, both mines are inactive. Several kilometers of barren quartz-filled vein material exist between (and locally over) strongly mineralized ore shoots on the St. Cloud-U.S. Treasury system. Possible chemical controls on the location of mineralized shoots were investigated through studies on fluid inclusion microthermometry, fluid inclusion microanalysis, and mineralogy. Samples collected from mine workings, drill-core intercepts, and surface outcrops provided an excellent coverage of these deposits. The primary questions asked were: Could similar investigations aid in exploration for other mineral deposits within the Chloride mining district?; and, Could fluid inclusion analysis be of value in identifying similar mineralization elsewhere? Our conclusions are that the St. Cloud-U.S. Treasury deposits have distinct geochemical signatures and that the compositions of gaseous species in quid inclusions are good guides to such deposits, despite the complexity of their mineralizing systems. GEOLOGY OF THE CHLORIDE M I N I N G DISTRICT

Epithermal vein deposits Mineral deposits in the Chloride mining district occur as open-space, fissure-filling veins. Vein material consists dominantly of quartz, calcite, adularia, and minor fluorite gangue. Sulfide and native-metal mineralization

GEOLOGY AND GEOCHEMISTRY OF MINERALIZING FLUIDS. CHLORIDE MINING DISTRICT, NM

,..= O =

I-,,:

I...

0..)

m~

e~

t/l

I,.,,

),I:~V"IO q~

ol.

.v -

I

\

/

\

/ \

i

• ..,

1=

J~

/ \

,"I ...........

z ~

o=:

/ "~---:"::::) ..... T--" I

\.!

E.=

/

~o ,.,

¢=

L) L.

I I

"~i!!.;..

¢j

l ~_~ i~ •- m -~ . . . .

.I

LE:~

6~

64

D.I. NORMANET AL.

within these veins occur in distinct, structurally controlled shoots. Chloride district veins crop out over an area of greater than 175 km 2 (Fig. 2) and occupy high-angle, normal faults related to the initial development of the Rio Grande rift (Harrison, 1986 ). Reported age dates for vein adularia range from 26.2___ 1.2 Ma to 28.9+_ 1.1 Ma (Harrison, 1986), and indicate that vein formation was contemporary to immediately after a major episode of late Oligocene volcanism in the Black Range (Harrison, 1990). The Chloride mining district is divisible into northern and southern subdistricts based upon differing metal contents, mineralogy, Ag/Au ratios, and to a lesser extent on gangue mineralogy and alteration (Harrison, 1989a). The St. Cloud and U.S. Treasury mines are located in the southern subdistrict, characterized by two paragenetic stages of base-metal-rich, pyrite-poor sulfide mineralization (Fig. 3, Table 1 ). Stage 1 mineralization consists dominantly of galena, sphalerite, and chalcopyrite and is typically poorer in silver relative to Stage 2 mineralization. Chalcopyrite was deposited late in the paragenesis of Stage 1. It is more abundant in upper levels of individual systems and displays a district-wide zonation. Stage I mineralization is fine- to coarse-grained, and exists both in openspace fracture filling with euhedral quartz and in wide zones of disseminations within wall rock. Gold in Stage 1 occurs in arsenic-bearing sulfosalts associated with galena and in association with chalcopyrite; silver occurs in sulfosalts and sparsely in chalcopyrite, which contains only about 0.02-0.03 wt.% silver. Stage 2 mineralization consists of abundant sphalerite and copper-rich minerals such as bornite, chalcocite, digenite, covellite, with minor amounts TABLE I Characteristics of southern Chloride district sulfide stages

Metal content Mineralogy

Alteration Ag occurrence

Stage !

Stage 2

Pb, Zn, Cu, (Mo, Ag, As, Au) ga, sph, cpy (minor sulfosalts, native Au) sericitic, argillic, propylitic in sulfosalts, 0.020.03 wt.% in cpy

Cu, Zn, Ag, (Pb, Cd, Au) bn, cc, dig, cpy, Cu-Ag sulfides, sph, be, ga, (Cd-Co-N isulfides, native Au) k-spar, propylitic

Au occurrence

native, assoc, with As

ga =galena: sph =sphalerite: be=betekhtinite.

cpy=chalcopyrite,

lattice substitutions in Cu minerals: 0.5-0.7 wt.% in bn, 0.8 wt.% in cc, up to 84 wt.% in dig-be; in Cu-Ag sulfides native, in upper levels bn=bornite:

cc=chalcocite:

dig=digenite;

GEOLOGY AND GEOCHEMISTRY OF MINERALIZING FLUIDS. CHLORIDE MINING DISTRICT. NM

/

65

CA%'YON 00ME

'%..~.

-,,6m:~,,,+'ert " 7

( ~

.

,]j +....~SAWMILL PEAK

~

MINES (llRuorlne (2)Ulmed~mo

=

' '

I .

.(5)

/

'/'

(~Liiffle Gilmlie (mmmmm,,.' (9)RNd~ler

.

\

(ll~ llloCld~ll (lllBeullnou~l O2}No. F.r+

(,,Sllwg' Monumm~

(20)~dnJoh!

)"

t'r)

(4)Ke~t(l~

(14)McTovl.-h US)W. 5,r,. (m)Hwoler

z

q

~(/8) I j,

(9/76 •

,

7~'

t~,.~, :a+

"'~ "~

I,,I

Nono

(10] '?9*

/

/

'

i]

,+ o+ - -

i PEAK

COMPLEX

,

r,.

Fig. 2. Major epithermal vein systems of the Chloride mining district; St. Cloud ( 18 ) and U.S. Treasur~ ( 17 ) mines are located on the same structure in southeastern portion ofthe distri"t.

66

D.I. NORMAN ET AL.

STAGE I

STAGE 2

Goleno

Sphalerite Cholcopyrite

---

Bornite Cholcocile Digenite

Betekhtinite Quortz Adulorio

Colcite Rhodochrosite Borite Notive

Au

---

Fig. 3. Mineral paragenesis for veins in southeastern Chloride mining district.

of galena. The only known occurrences in North America of the rare mineral betekhtinite [ (Cu,Ag)~o.2.,.(Fe,Pb,Cu ) t +xS6] are in Stage 2 mineralization of the southern Chloride mining district, where it is a common component. Copper-silver sulfide (stromeyerite, jalpaite, and mckinstryite) also occur in varying amounts. Minerals of Stage 2 are complexly intergrown, frequently showing myrmeketic and exsolution textures. Grain size in Stage 2 varies from very fine-grained in shallow portions of deposits to medium-grained at depths of a hundred meters or more. Stage 2 mineralization is strongly associated with adularia alteration of wall rock. The silver content of Stage 2 mineralization is extremely high, with silver occurring as complex copper-silver sulfides and as lattice substitutions for copper in copper sulfides. Fire-assayed mineral separates indicate as much as 0.8 wt.% silver in chalcocite and 0.66 wt.% in bornite. As noted by Skinner (1966), the system Cu-Ag-S is characterized by extensive solid solutions, highly disordered cation sites, and unquenchable reaction rates. Most of the silver in the southern Chloride mining district probably precipitated as hightemperature, silver-rich, cubic bornite solid solution and as silver-iron-rich, hexagonal, closest-packed chalcocite; unmixing processes upon cooling resulted in observed silver-rich, tetrahedral bornite, silver-rich chalcocite, stromeyerite, mckinstryite, and jalpaite (Harrison, 1989a). Gold occurrences in Stage 2 mineralization are highly erratic and very finegrained. In general, the greatest gold concentrations are found in upper portions of individual deposits. At the U.S. Treasury mine, gold in Stage 2 mineralization averaged 4.2 ppm within the upper 60 m of the deposit.

Stratigraphy Mineral deposits of the Chloride mining district are hosted by Tertiary volcanic and volcaniclastic deposits of the Mogollon-Datil volcanic field. Detailed descriptions of Chloride area stratigraphy are given in Harrison ( 1986, 1990).

GEOLOGYANDGEOCHEMISTRYOF MINERALIZINGFLUIDS.CHLORIDEMININGDISTRICT.NM

67

Most of the mineral deposits of the Chloride district occur within the Eocene Rubio Peak Formation of Jicha ( i 954). This formation in the Chloride area consists of 800-1000 m of dominantly intermediate volcaniclastic and volcanic rocks that overlie fluvial red-bed sediments of the Permian Abo Formation with angular unconformity (Fig. 4). Within the lower portion of the Rubio Peak Formation, there are gravity slide blocks of Pennsylvanian limestone as much as 150 m thick (Maxwell and Heyl, 1976; Harrison, 1989b). The St. Cloud deposit has a hanging wall of limestone and a footwall of volpaleosurfoce rhyolite Iovas

N

ash flow luffs

basaltic andesite ,~4 ,--" Paymaster

~

J

of

""

Poverty Creek

Readjuster [

~ . . -~'" ~ ' " • '~-" ~ " "

Gold Hill I

•..~.,~ • "~

Silver Monumentl J

~

U.S. Treasury J Colossal I Ivant~o~.Emporia ]

:2""," . . . . . . ~ ~'bb ~,~'~':g~'.~'a

.

• ...

-

~

~

Kneeling Nun T~f

Rubio Peak Formabon . . . . . . lower

,~ ~

.6.':'('...'.::,. 1~ ~ ~

exotic limestone blccks

Apac he-Hogs ier ! Midnight I St Cloud Dreadnought G. Republic Fluorine

.

Rubio Peak ~ ,~ ~

Formation

I

300m

¢.':

~..~

~•.~-..' ' : ~.':;G

Formot,o' n

Fig. 4. Schematic diagram showing paleostratigraphic reconstruction of Chloride mining district at time of mineralization, and vertical extent of known rain,., a; deposits; deposits are placed adjacent to footwall lithology. (Note: full vertical extent of some deposits is not accurately known. )

5000'

~

~

.

IVIARGINOF •

US TREASURY

' ~ WRENCH FAULT . ~

GRAY EAGLE

•, - N 4 6 " w

.

.

.

~

/

w.e~cH FA

7000'

-'~----"---

"

S46oE._,. ". $75OE_..,

9000'

ST CL,OUO

a

Fig, 5, Longitudinal section to St. Cloud-U.S. Treasury vein system showing influence of Eocene wrench faults on mid-Tertiary mineral deposits ( hachured pattern ), wrench faults are vertical and strike north-northeast.

6200'

6400'

6600',

6800'.

7000',

7200'

I-'

z ,...]

z 0 ~o

O0

Ch

GEOLOGY AND GEOCHEMISTRY OF MINERALIZING FLUIDS, CHLORIDE MINING DISTRICT, NM

69

caniclastic rocks. The U.S. Treasury deposit has ~olca~cla~tic rocks in both hanging wall and footwall. Overlying the Rubio Peak Formation in the Chloride mining district is a thick sequence of silicic pyroclastic rocks and intermediate to silicic lava flows, with minor intercalated volcaniclastic sediments. These rocks are the product of two volcanic episodes; the first occurred during the late Eocene ( ~ 36-35 Ma) as voluminous pyroclastic eruption,s during development of the Emory caldera; the second episode occurred after a 5-m.y. hiatus in volcanic activity and began initially as multiple, effusive eruptions of intermediate lavas that blanketed the entire Black Range and were closely followed by vigorous, widespread eruptions of silicic lavas and ash-flow tuffs from scattered flowdome complexes. Overall, the second volcanic episode lasted from approximately 30 to 28 Ma. Numerous vents for this later volcaaism are recognized around the Chloride mining district, including the MoCcasin John Rhyolite flow-dome complex and Chloride Creek dome (outflow dated 4°Ar/39Ar at 28.95, McIntosh, 1989), Franks Mountain intrusive (dated by Rb-Sr at 27.6 _+1.2 Ma, Eggleston and Norman, 1987 ), Kline Mountain intrusive, and the Sheep Creek dome. Structure

Structural features in the Chloride mining district were the most important controls on locating sites of mineral deposition. A pre-mineralization, northnortheast-striking, right-lateral wrench fault system with a minimum of 3. l km of strike-slip movement developed vertical, deeply penetrating fractures (Harrison, 1989b,c). This wrench faulting is Eocene in age and is part of a late-stage deformation of the Laramide orogeny (Chapin and Cather, 198 l; Chapin, 1983; Harrison and Chapin, 1990). During the late Oligocene, high.angle, normal faults related to the initial development of the Rio Grande rift cross-cut the older wrench faults in the southern Chloride mining district. These normal faults were subsequently flooded with hydrothermal fluids and their intersections with wrench faults became the sites of strongly mineralized shoots (Harrison, 1988b). The mineralized shoots along the St. Cloud-U.S. Treasury vein system are excellent examples ofthis primary structural control exerted by the old wrench faults on ore deposition (Fig. 5 ). A secondary, but economically significant, structural control on mineralization at the St. Cloud deposit is related to the occurrence of exotic, gravity slide limestone blocks within the volcaniclastic deposits of the lower Rubio Peak Formation. Because of differing rock strengths and bedding characteristics between these two ~°ocktypes, faul~s commonly change dip angle when they pass from one lithology into another. In general, high-angle faults flatten when they enter the exotic limestone blocks and then steepen when they pass downward into the massive volcaniclastic deposits° The brecciation and open-

70

D.I. NORMAN ET AL.

space development that occur at these deflection sites localized heavy concentrations of sulfide mineralization and were termed "favorable zone" by Freeman and Harrison (1984).

Alteration Alteration in and around the Chloride mining district displays regional patterns that arc closely tied to lithology, paleostratigraphic depth, and proximity to rhyolitic intrusives and epithermal veins. The intermediate rock types of the lower Rubio Peak Formation are mildly to pervasively altered to a propylitic assemblage of epidote, calcite, chlorite, plus or minus pyrite. The overlying, more-silicic rocks show a mixture of mild propylitic and mild to intense argillic alteration. Poorly welded ash-flow tufts are typically altered to illite, kaolinite, and zeolites. Advanced argillic alteration surrounds most of the rhyolitic intrusions that exist in the Chloride area (Ericksen et al., 1970; Eggleston and Norman, 1986 ). Upper portions of the Abo Formation, that locally contains red-bed-type copper mineralization, are bleached and veined by calcite and epidote. Systematic vertical sampling through the Abo Formation reveals a 5-fold decrease in copper values from about 60 m depth to the top of the formation (22 to 4 ppm, respectively). In the southern Chloride mining district, alteration of wall rocks adjacent to epithermal veins varies with both lithology and stage of mineralization. Wall rocks of intermediate composition typically show potassic alteration and silicification. Where limestone occurs in the hanging wall of the St. Cloud deposit, thin ( 10-20 cm) selvages of tremolite and wollastonite with minor amounts of diopside, prehnite, quartz, and calcite are common adjacent to massive quartz veins. Stage 1 mineralization is closely associated with sericitic wall-rock alteration; Stage 2 mineralization is strongly associated with kfeldspar alteration (Harrison, 1989a). Disseminations of sphalerite, chalcopyrite and galena replace volcaniclastic rocks as much as 50 m away from some mineralized vein sections. ST. CLOUD-U.S. TREASURY DEPOSITS

The St. Cloud an.d U.S. Treasury deposits occur along a single fault that contains ep~thermal, quartz-calcite +_sulfide mineralization over a distance of 2300 m of strike length. Ore shoots at the St. Cloud deposit formed over a lateral extent of 550 m along a favorable zone defined by limestone in the hanging wall and volcaniclastic rocks in the footwall (Fig. 5 ). These shoots were also centered on intersections with older wrench faults. Except for two small areas of the St. Cloud mine, ore occurrences are essentially blind bodies that do not extend to the surface. Approximately 50% of the favorable zone

71

GEOLOGY AND GEOCHEMISTRY OF MINERALIZING FLUIDS. CHLORIDE MINING DISTRICT, NM

contained high-grade, silver-copper-zinc mineralization, and 90% of economic reserves at the St. Cloud deposit occurred within this structural trap. At the U.S. Treasury mine, steeply plunging mineral deposits developed along intersections between old wrench-fault strands and normal faults. Intermediate volcaniclastic rocks make up both walls of these deposits. Regional Stage 1 and Stage 2 mineralizations occur at both the St. Cloud and U.S. Treasury mines. At the St. Cloud mine, both stages occupy the same shoots with a general relationship of Stage 1 mineralization on the footwall, Stage 2 mineralization on the hanging wall, with a transitional interval of Stage 1 replacing Stage 2 in the middle. At the U.S. Treasury mine, the individual stages occupy discrete shoots, and where shoots do overlap, Stage 2 mineralization halos brecciated Stage 1 mineralization. At both mines, both stages of mineralization are notably lacking in iron, with only minor amounts of pyrite, hematite, and magnetite in the ores. Trace araounts of cadmium, arsenic, cobalt, nickel, titanium, chromium, molybdenum, tellurium and antimony are found in both stages. Microscopically, Stage _

=

4zoo

•. ..;.D'.'.... .222~2222j2222222222. ~ 2.

•

4tpo !~ '

~O~ + ': ;t!" -;,) ....=~.:~"

5700

.:. ..! -". 'L'

"~!":'::"~!. ..

. ..

ili!i!ii!iii::i:i:i ....

7000

. :~.:.V:i

i"<

d

~500

"

600(1

\

6000

D

[~

log (AI/Au)

AI/Au

1.5 to !.7

(32 lo 50)

1.7 to 1.9

(50 lo 80)

1.9 to 2.0

(80 lo 126)

2.1 lo 2.3

(126 to 200)

2.5 to 3.0

(2OO to !000)

Fig. 6. Contured A g / A u ratios from assays of exploration driil core, and from production assays (includirg mineralization fi'or, both stages). The heavy c~iagonal line represents the wrench lault around which the miner%ization is centered.

72

D.I. NORMAN ET AL.

l mineralization shows a mosaic texture of individual sphalerite, galena, and chalcopyrite grains. Stage 2 mineralization consists of fine- to medium-grained sphalerite, bornite, chalcocite, betekhtinite, and digenite, with lesser galena in symplectic and vermicular intergrowths. Iron content in Stage 1 sphalerite was determined by X-ray analysis to be approximately 4.0-5.0 mole%, and in Stage 2 sphalerite to be approximately 0.5-1.5 mole%. Gold is more concentrated in the upper portions of ore shoots. At the U.S. Treasury deposit, silver and gold values are zoned around the wrench-fault interaction, as well as with elevation (Fig. 6 ). FLUID INCLUSION STUDIES

Primary fluid inclusions in quartz, totaling 656, were analyzed for homogenization temperature and salinities from each stage of mineralization and in poorly to non-mineralized vein material outside the ore shoots. Bulk analysis of inclusion volatiles were performed on 74 samples, and 6D of inclusion waters was measured on 6 samples (Behr, 1988). All data and sample locations are given in Norman ( 1991 ). Others have studied fluid inclusions from the St. Cloud deposit (Reynolds, 1983; Loucks, 1984; and M. Wilson, pers. commun., 1982); however, the respective stage of mineralization is not clear in these studies and they are not included here.

Microthermometry Microthermometry measurements were performed on a Linkam TH 600 stage (MacDonald and Spooner, 1981) with a precision of _+0.1°C for T<200°C and +_1.0°C for T> 200°C. Calibration to 0°C was checked daily by measuring the melting point of pure water to insure the repeatability of Tm (melting point ) to +_0. l ° C . Fluid inclusions were found to consist of liquid dominant and rarer vapor dominant types. The later type was observed in seven samples from the St. Cloud deposit and in three samples from the U.S. Treasury deposit, and from both stages of mineralization. A Th value of 259°C on one vapor-rich inclusion and a Th of 253°C on an adjacent liquid-rich inclusion suggest that the vapor-rich inclusion was trapped under boiling conditions. It is therefore assumed that Th values represent trapping temperatures (Tt). The few secondary inclusions that were encountered were not measured. Microthermometry data for inclusions from the two mines are considered together. However, because of gross differences between Stage l and Stage 2 mineralization, inclusion data from each stage are considered separately. Our microthermometry studies indicate that Th and salinity values for both stages of minerali. zation are remarkably similar. Th values range between 211 and 294°C and Tm between 0.0 and - 4 . 4 ° C (0.0-3.4 eq.wt.% NaCl, using the equation in

73

G E O L O G Y AND G E O C H E M I S T R Y O F M I N E R A L I Z I N G FLUIDS, C H L O R I D E MIN IN G DISTRICT. NM

Stage 2

Stage 1 Salinity

Th

Th

Salinity

120

120

120

60

6O

6O

60

30

30

3O

30

0

0

0

~% 120 O

=

r.~

160 240

320

(c*)

0

1

2

3

240

eq.wt.~. Nael

350

300 250 I ' ' ' " "

160

4

300g'-.

3%

0

""'..~

gg

,,,

,

go 706030 I 0

,

ol

I

Percent

gg

I I II tO/ 2001- I 8 t I I I go 705030 10

Percent

I

9g

go 7 ~ 0 5 0 10

Percent

1 2 3 4

2 % I

200__,

'

0

eq.wt.NNaCl

(12°)

2, ,,%.. 250[

~

320

%ee~

o g9 / go , 7r~0.I0 , , , % 1o

Percent

Fig. 7. Histograms and cumulative frequency diagrams for Th and salinity values for both stages of mineralization. Double peaks and the skewed distribution of data suggest multiple populations of data for both stages. Non-linear or "S" curves on cumulative frequency diagrams indicate several populations of data (Sinclair, 1986). Because 7", and salinity are paired variables, cluster analysis rather than Sinclars (1986) methods were used to determine the means of the populations.

Potter et al., 1978 ). Inclusions from non-ore samples yielded somewhat lower 7', and salinity values than those from ore samples. When plotted on salinity-Th diagrams (Fig. 7 ), the fluid inclusion data indicate a much larger sample to sample variation than can be explained by the imprecision of measurements. Random variation in T;; and salinity seems unlikely, therefore a systematic variation is considered. Using a statistical approach suggested by Beane and Hayes (1987) and detailed in Sinclair ( 1986 ), three populations are quite evident in each stage of mineralization for the St. Cloud-U.S. Treasury fluid inclusion data (Fig. 7 ). These populations are also apparent in histogram plots of the data (Fig. 7 ). Clu~ter analysis of the data (Burford, 1968) was used to further define the these populations as: high TABLE 2 Mean values of the three populations of fluid inclusion data determined by cluster analysis. These populations are evident in cumulative frequency plots ( Fig. 7 ) Population:

A

B

C

Stage ! 7". (°C) Salinity (eq.wt.% NaCI)

275 2.2

208 0.84

229 0.56

Stage 2 Th (°C) Salinity (eq.wt.% NaCI)

268 1.8

269 0.66

221 0.66

-0.001

260 0.67

0.00069 0.00009 0.016 0.0048 n.d. 0.0002 0.044 0.0097 0.043 n.d. 99.84

I LS.'I. Stage 1 SC4F

-0.002

269 1.35

0.00089 0.0005 0.0085 0.020 n.d. n.d. 0.033 0.038 0.029 n.d. 99.76

St. Cl'd Stage I SC58C

-0.0005

285 1.42

0.0017 0.0003 0.095 0.019 n.d. 0.00002 0.18 0. i 5 0.30 n.d. 99.39

U.S.T. Stage 2 SCUI4B

-0.001

259 0.44

0.00053 0.00004 0.020 0.0083 0.016 0.00004 0.044 0.017 0.1 n.d. 99.82

St. Ci'd Stage 2 RH2

-0.0004

253 0.61

0.00021 0.00002 0.0017 0.0055 0.0072 0.00014 0.019 0.0056 0.10 n.d. 99.87

Barren Vein SI0

n.d. = none detected, detection lim;:s a~e about 0.00005 mol.% for most gas species. n.m. - not measured. ( I. ) methane concentration included in C,H,,.

y

AverageTh (~C) Salinity (eq.wt.% NaC! )

H: ( mol.% ) He CH4 N: CO Ar H,S C,jH,, CO2 SO: H20

Sample:

Location:

-0.0003

237 0.91

0.00023 0.00016 0.10 0.016 n.d. 0.00009 0.0020 0.11 0.086 n.d. 99.78

Barren Vein SC63

0.003

n.m. n.m.

0.90036 n.d. 0.21 0.56 0.093 0.0005 n.d. 0.56 !.38 n.d. 99.20

St. Cl'd Surface SCI0

O.0015

n.m. n.m.

0.0004 n.d. 0.015 0.16 0.019 n.d. n.d. 0.41 0.80 n.d. 98.60

U.S.T. Surface SC3.5

n.m.

n.m. n.m.

0.0004 n.d. (1.) 0.74 n.m. n.d. 0.009 0.76 1.14 0.21 97.14

Channel Sample Over Ore OI0

Representative analyses of fluid inclusion volatiles. Hydrogen concentrations have been corrected for spurious H2 formed during thermal decrepitation(Norman, unpubl., described in Behr, 1988)

TABLE 3

....d

o.to,~ soldturs ooeJ.mS "~ONH .~o IDH jo uo.~mlOS %Og e Joql.xo u! pol!Oq uoql pug spunodtuo:~ o~.ue~ao onotuoJ ol uo~.mios HOnN loq '~uoJls e u! poueolo pug 'po~o.~d pugq 'oz.~s-qsotu 017+ ' 0 I - ol poqsnao uoq~ 'sluotu~eaj poz[s -u~.~-~ ~noqp. ol poqsn.~3 ,~!os.~eo3 ~,s.n~O.mA~so|dtu~ S "( L861 ) s u p I ~ S pu~ uetu -JON u! p~q.uosop se a~,otuoJloods ssgtu o|odn.~pgnb e gu!sn soldtues ~Jnseoal "S'fl-pnolD "IS no potuaopod o.~:, sol.neloa uo!snlou! p!n Ujo sos~leue ~IInlt

ss ilOU "( 8 "g!_q) D pue fl suop, elndod tuoaj ~|~Jp, u~ ~,sotule suo!snlou! u!eluo3 ol punoj oao~ seo.~e poz!le~u!tu ~p.~smo tuo.U s~ldtueS "uop,ez.qeaou!tu jo seoae iie tuoaj soldtues u! Jnooo suo.~e.lndod ~.~q~, I|V "(D) ,~!U!leS ~oI 'o.mleaodtuol qg!q pue '( fl ) ,~l!u!les n~ol 'oJnlejodtuol ~ol :(V) ,~l!U!leS qg!q 'o.m~,eaodtuol "ou!i luo3efpe oql tuoJj ")~ l'0JO ,u,z u! oouoJojj!p oanleJadtuol g sI3oUOJ elep j o aU!l tl3ea pue D~ I'0 su agels uo!snl3u ! p!niJ oql j o uo!s!oaJd oql :luatua.ms -eatu jo p e j l J e ug aJe e l e p j o s,~eJJe aeau!-I "pole3!pu! s! (~ alqe I ) pole!30ssg s! uo!sni3u ! tpva q3[q~ ol UO!l~lndod Otll 'slootls a.lo ol lua3e.fpg uuon zlJenb uoJaeq tuoJj pue 'uo!lezqeaau!tu jo sa~els o~al aql tuoJj suo!snl3u ! Joj ~l!u!!es paleln31e3 sosaan oanleaadtual uo!lez!ua~otuo H "8 "~!a

(Do) I l l OCj£

D dn°aD fl d n o a D V dnoaD -

n

b

0

oo~

og~

oo£

OgL 0 O0

Cv~N

v o ®

I,d,

pua~aq

r,I.TaA uaaaeE[

~N I

(3o) q.I, og£ !

00£

OG~

do dooO

~

I

I

(3 o) q±

00~

~ '

OgL i

vvv 0 0 0 ~ v w DO 0 O0 u~mm,lp~v

0 •

Og£

00£

Og~

OOg

ogt

' 0 ° O'U~' ' (]D ( ~ C~NII)~' VV

o v v

~

o

U2

V

(U~Ov Otu~J

|

v w

v

l=,d°

t

v

OOQ @ 0 @

v Q

Q~ ~ ~ S

s® e

Q

CD

Q

o •

aN IS ,

~/_

,

@.

•

£

,

,

6

® ®o I one28 ® oO i

t

•

~ N

dlnk

II,

,

i

i

~N "IDIHISIG DNINIIAI3GlSOqH,3 'SGlfiqJ 9NIZlqVM:INIlAIJO ASISIN.:ltt30_:19 QNV h9010-:i9

76

D.I. NORMAN ET AL.

0.6-

o) bJ .J t-

0.4-

J 0 > O0 U. 0 O.l" Ld J 0

0

'

~

.2s

m STAGEr

Nz

CO z

INCLUSION GASEOUS SPECIE ---STAGE ;[

BARREN VEIN

I I SURFACE

Fig. 9. Comparison of average concentrations of 3 gaseous species in bulk analyses of fluid inclusion volatiles. Surface samples are grab samples from near the center of the outcropping vein and above ore shoots.

then boiled in pure water, whereas ore samples were further cleaned in electrolytic cells. Inclusions were opened by ~hermal decrepitation at 450°C in quartz furnaces. Ore samples were leached after gas analysis and the leachate was analyzed for major anions and cations, and some metals (Behr, 1988). Organic species contained in the gas fraction were cryogenically separated and analyzed by gas chromatography/mass spectrometry (GO/MS). Gaseous species detected are CO2, C,,H,,, H2S, N2, and CH4 in decreasing order of abundance, with lesser H2, CO, He, and Ar (Table 3 ). SO2 was detected in some analyses of surface samples. Organic species were observed in mass spectrometer analyses of fluid inclusions from both the St. Cloud and U.S. Treasury deposits. Relatively lower concentrations were also found in non-mineralized vein material. The gas analysis facility was not calibrated for various isomers of the organic species detected, hence the results are reported only as C,,H,,. Differentiation of hydrocarbon species was achieved via G e l MS after combining splits of the fluid inclusion volatiles from several analyses. Hydrocarbons measured were principally C3 to C5 alkenes, propylene, and butene that were generally in ratios of about 2:2: I. Minor pentene was also detected, but none occurred in samples from outside of mineralized areas. The CO2 measured is probably spurious because equilibrium calculations in-

GEOLOGYANDGEOCHEMISTRYOF MINERALIZINGFLUIDS,CHLORIDEMININGDISTRICT,NM

77

dicate that it should not be present at the concentrations detected. Values of CO correlate positively with C,H,, which suggests that CO values resulted from pyrolysis of organic compounds. The concentration of gaseous species in fluid inclusions shows a variation with sample location and stage of mineralization (Fig. 9). Stage 2 inclusion fluids have. greater amounts of H2S, CO2, and total gaseous species than Stage 1 inclusion fluids (Table 3, Fig. 9). Inclusion fluids from non-mineralized samples adjacent to ore shoots have low concentrations of all gases. In general, concentrations of all gaseous species increase with elevation (Fig. 10). The greatest measured concentrations of most gaseous species are from surface samples that overlie or are immediately peripheral to sulfide mineraliSt.

7000

U.S. Treasury

Cloud I

I

I

B

On

O~" O r~

7000

[]

6800

6800 -(DO

6600

6600

'o

'

O 6400 6200

0 0

6400

°oO

CO2

" r

62O0 - A

CO2

.

6000

6000 I i

0.0

i

,

05

I

I

,

1.0

J

1.5

2.0

I

Mol. % CO2 I

7000 _[-

"

"

i

0.5

0.0

I

1.0

,

15

,

1.

2.0

Mol. % C0 2

I

•

!

•

!

i

7000

'

6800 ~ 0

g oooo

6600

i6400~ o o

6400

ooo[, ,,

O

O

I @

6200

H~S 6000 I-

HaS

6000 I

0.00

~0

0.05

0.10

,

I

,

0.15

•

0.20

0.00

Mol. % H2S

® = Stage

I

0.0,5

t

I

I

0.10

0.15

,

0.20

Mol. % H2S Legend 1 o

A = Non-0re

o

= Stage 2 = Surface

Fig. 10. Concer;tration of CO2 and H2S versus eJevation for inclusions from both deposits. Concentrations of~he principal gaseous species measured in inclusion including N2 and CH4 increase with elevatic,n in a similar fashion as CO2. The exception is H2Sthat shows a decrease in elevation; many samples from near or at the surface of the mines have no detectable ( < 0.0001 moi.% ) H2S in the inclusions.

78

D.i. NORMAN ET AL.

zation. N:, CO2 and CH4 are in significantly higher concentrations in these samples, whereas H2S and C,,H,, are not. In fact, H2S shows a strong inverse relationship with elevation. Remarkably, H2S was detected in only 2 of 14 surface samples collected over ore shoots, while there were measurable concentrations of H2S in all analyzed inclusions from within the ore bodies. In addition to the H 2 S anomaly, gas/water ratio, CO2 concentration, N2 concentration, and N2/CO2 exhibit anomalies over the St. Cloud deposit, and to a lesser extent over the U.S. Treasury deposit. Figure I l shows some of these anomalies as concentrations plotted against distance along the vein. Boiling and excess gas

Inclusions from samples collected at depth within ore bodies generally contain CO_, concentrations from 0.02 to 0.56 mol.%. On Giggenbach's (1980) K ''2 diagram, these measurements plot near the vapor-liquid equilibration curve with y values (steam fraction) of + 0.0005 to - 0 . 0 0 l, similar to analyses of New Zealand geothermal waters. This indicates that the inclusions contained slight additions of vapor (positive y values), or that the inclusions formed from liquids that were slightly depleted in volatiles due to boiling (negative y values). Surface samples containing CO_, concentrations >0.5 tool.% and levels of N2 and CH4> 0.05 tool.% also have y values of +0.0015 or greater (Table 3 ). This suggests that volatiles exsolving from mineralizing fluid during boiling are preferentially trapped in fluid inclusions: as ore fluids boil, vapor bubbles nucleate on fluid-solid interfaces and are trapped along the walls of inclusions, and result in high gas content in bulk analyses of inclusions (Norman and Sawkins, 1987). Thus, the St. Cloud-U.S. Treasury inclusion samples with high-gas contents indicate that exsolved volatiles have segregated from the mineralizing fluid, risen buoyantly, and were trapped in fluid inclusions that formed over ore mineralization. As discussed in a later section, this phenomena can be important in mineral exploration. Enrichment of N2 in respect to CO, in inclusions from surface samples over ore shoots (Fig. I l ) provides further evidence for this contention, because N2 has a higher partition coefficient from liquid to vapor than does CO, (Giggenbach, 1980). Fig. 11. Bar graphs showing the concentration CO,, HzS, and N2 and N2/CO2 ratio for analyses of channel samples from surface exposures of the U.S. Treasury-St. Cloud vein. The horizontal line is a three-point running average. The approximate positions of ore shoots are illustrated. N2. CO2 and the N2/CO 2 ratio indicate anomalies over the blind St. Cloud ore shoots, and to a lessor extent over the U.S. Treasury ore shoo t that outcrops. H2S, on the other hand, exhibits lower values over the ore shoots.

GEOLOGY AND GEOCHEMISTRY OF MINERALIZING FLUIDS, CHLORIDE MINING DISTRICT, NM

4.0 ,mm=m

ore

ore

3.¢

m

N 2.0

NW .10 J

ore

ore .OB

~n'06 -

;olr

/ $E

NW 1.0

.B

I N"

6

I

n

,I

O

£.4 .2

SE

NW

L

ore

ore

N"6' Z

~.4

/ ,J, NW

/

!

$J0

m

lOO 200 •

me'£ers

J

79

80

D.I. NORMAN ET AL.

DISCUSSION

Two similar events The geological data are quite clear that there were two stages of mineralization at the St. Cloud-U.S. Treasury deposit as well as throughout the Chloride district. The fluid inclusion thermometric data for the two stages, on the other hand, are quite similar and each is statistically divided into three groups (Table 2 ) that are themselves are nearly alike. Only the gas data shows some differences between inclusions f.om the different stages of mineralization. Temperatures of geothermal fluids as they approach the surface are, to a large extent, controlled by boiling, which in turn is related to the hydrostatic pressure (assuming an open system) and vapor pressure of the fluid (Henley, 1985; Henley and Brown, 1985 ). Since the data indicate that boiling occurred during both stages of mineralization, the similar Th values for the fluids indicates that the fluids had a roughly similar gas chemistry, and mineralization occurred at depths that did not differ by more than 30%. The similar nature of the fluid inclusion data can be attributed to events that occurred not far different in time and controlled by the same hydrology and structures. Assuming this, it is not surprising that two mineralization events were in many ways similar even though the chemistry of the fluids was sufficiently dissimilar as to result in the precipitation of different assemblages of ore minerals. There is no geological way to determine whether barren quartz formed during Stage 1 or 2, or was a separate event. Because the fluid inclusion data for barren quartz is indistinguishable from B and C groups of data for both stages, it is presumed that barren quartz formed during both stages of mineralization.

Gas chemistry of the mineralizing fluids Inclusions peripheral to ore shoots contain relatively low amounts of gaseous species and H2S concentrations of about 0.02 to 0.0001 mol.%. Microthermometry indicates that these inclusions belong almost exclusively to populations B and C discussed earlier (Table 2 ). Inclusions from within ore shoots contain all 3 populations and bulk analyses of their contained volatiles show much greater levels of H2S and CO2. These relationships suggest that population A inclusions for both stages have greater concentrations of gaseous species than populations B and C. This is further indicated for Stage 2 when H2S is plotted against salinity (Fig. 12 ), for which a correlation coefficient of 0.34 is determined. This is a surprising good correlation when it is considered that boiling should have depleted gases in some inclusions and added excess amounts of gases to other inclusions. Both salinity and H2S have an inverse relationship with delta D (~ie former is illustrated in Fig. 12 ) which indicates

GEOLOGY AND GEOCHEMISTRY OF MINERALIZING FLUIDS, CHLORIDE MINING DISTRICT, NM -5O

81

o

L

.=,-t

Stage 2

J~ - 6 0 t~

o

\

-70

o

-80

\ o

-90

0.20 .0.15 0 ~-'0.10

0.05

•

o

@

@ @

0.00

•

0.0

~e

®e

0.4 o.~ ~.2 '

S a l i n i t y

i .....

(eq.

i

wlL. %

i

1.6

NaCI)

Fig. 12. Plots of O'D and concentration of H2S versus salinity for Stage 2 fluids. Hydrogen isotopes were only measured on four samples of Stage 2 fluid inclusion waters. These data suggest that lower salinity waters have isotopic compositions near that of Tertiary meteoric waters (Taylor, 1974), whereas the more saline waters where from a different source. The lower plot indicates that H2S correlates with salinity. Regression analysis for the salinity-HzS analyses of Sta~e 2 inclusions yielded an R2=0.34 and an intercept of - 0 . 0 1 eq.wt.% NaCi.

that both these quantities are associated with an isotopically light water, and that two fluids were involved in the Stage 2 mineralization process. The gas composition of ore fluids is estimated by selectively averaging the bulk analyses that have the greatest measured CO2 and H2S (Table 4). These estimated gas contents must be considered as minimum values since the microthermometry data indicates the presence of some gas-poor populations B and C inclusions in all samples analyzed. Sulfur fugacities calculated from these averaged analyses agrees well with the sulfur fugacities for Stage 1 and 2 as calculated from iron contents in sphalerite. The higher sulfur fugacities indicated for Stage 2 fluids allowed for the precipitation of bornite rather than chalcopyrite. It is interpreted that the source of organic species in the St. Cloud-U.S. Treasury ore fluids was from deep Paleozoic limestones and/or channel sandstones of the Abo Formation. Hydrothermal fluid flow through the Abo Formation would explain the observed alteration and loss of copper of these rocks, and the copper-rich nature of the mineralization, especially Stage 2. it is further interpreted that the low concentrations of C,H, species in fluid inclu-

82

D.I. NORMAN ET AL.

TABLE 4 Average chemistry of the ore fluids and the fluid with which they mixed, and calculated metal solubility. See text for details of the calculations Stage 1

Average co:::~osition ( mol,% ) Hz 0.0011 He 0.00009 CH4 0.013 N_, 0.0078 Ar 0.00015 HzS 0.029 C,,H,, 0.019 CO,_ 0.049 Iog.fo., I Iog.L.,log.d,, ~ 1~ (°C) Salinity (eq.wt.% NaCi )

Metal solubility 4 (ppm ) Zn Pb Ag Cu At= Fe l~rt,ssure s (bar) .lll:~ ,/~,~,:

./~: ./c'lt,

./h:s Total Depth ( m )

-- 36.7 - 11.4 - 11.7 to - 1 !.8 275 2.2

3.6 0.16 0.18 0.24 0.048 0.062

59.4 2.6 1.7 1.2 0.3 65.2 780

Stage 2

Vein fluid

0.0008 0.0001 0.058 0.048 0.0099 0.20 0.066 0.26 -37.0 -9.6 - 9.4 tO

0.00016 0.000050 0.026 0.0008 0.00078 0.OO081 0.01 ! 0.092 -40.6 -14.3

-

268 1.8

23.4 0.69 0.65 0.19 2.9 0.0002

10.4

225 0.61

0. l I 0.001 0.002 0.0004 0.0003 0.0006

53.3 bars 13.8 1.2 6.0 2.0 76.3 930

tCalculated assuming CH4-CO2-H20 equilibrium, data from Robie et al. ( 1978 ). -'Calculated using gas analytical data and thermodynamic data from Robie et al. ( 1978 ). ~Calculated from iron content of sphalerite (Barton and Skinner, 1979 ). 4Calculated using the computer program GEOMOD t Norman, unpubl.) that assumes equilibrium between solutions with the indicated gas compositions, T~, and salinity, with major ore minerals including bornite. SCalculated from Henry's Law equations in Drummond ( 1981 ), steam tables in Henley et al. ( 1984 ), and density formulas in Haas ( 1971 ).

sions adjacent to ore shoots indicates that these fluids (populations B and C inclusions) are samples of shallow-circulating, steam-heated waters that never penetrated to significant depths to encounter the Paleozoic units.

GEOLOGY AND GEOCHEMISTRY OF MINERALIZING FLUIDS. CHLORIDE MINING DISTRICT. NM

83

Ore fluids and the depositional process From all considerations, fluids represented by population A inclusions for both stages are considered to be representatives of the two ore solutions. They have the greatest temperatures, salinities, and H2S contents, all of which would better serve ~thetransport of Ag and Cu~ the principal economic metals in the St. Cloud and U.S. Treasury deposits. Using the modeling program G~OMOD (Norman, unpubl. ), the solubilities of chalcopyrite, bornite, pyrite, sphalerite, galena, acanthite, and gold were calculated for the general chemistry of mineralizing fluids (Table 4). These calculated values strongly indicate that population A fluids alone had the capability to transport significant quantities of Cu, Zn and Ag to form the St. Cloud-U.S. Treasury ore deposits. All of our fluid inclusion data, as well as the lack of sulfide mineralization outside of ore shoots, suggests that ore fluids (population A fluids) were strictly confined to the structurally controlled ore shoots. Many lines of evidence indicate that ore fluids upwelled from depth along the wrench-faultnormal-fault intersections to form the ore shoots: sulfide mineralization occurs only along or immediately adjacent to these intersections; the rakes of ore bodies follow the plunges of these intersections (Fig. 4); and mineralization is zoned around these intersections (Fig. 5 ). Population B and C inclusions are interpreted as shallow-circulating, steamheated waters. Population C inclusions have Th values similar to those of steam-heated waters found in active geothermal fields (Moore et al., 1990); Behr (1988) determined that the principal anion present in these inclusions is bicarbonate for both stages o¢ mineralization, which is also typical of steamheated waters. Population B and C fluids differ only in Th, therefore it is inferred that population B fluids represent steam-heated waters (population C fluids) that became further heated by wall rocks as they approached the sites of ore deposition. Mechanisms of ore deposition at the St. Cloud and U.S. Treasury deposits appear to have been very complicated and to have involved both fluid mixing and boiling processes. Excursions of relatively less dense (cooler and less saline) population B and C fluids into the structural intersections would serve to cap the upwelling flow of population A fluids (during both stages of mineralization). The mixing of these fluids alone could alter their chemistry enough to be a prime control on ore deposition. However, our mixing model involving Stage 2 ore solutions and population C fluids indicate that the amounts and grade of metals existing in the St. Cloud-U.S. Treasury deposits could only form if extreme (70%) mixing occurred, and zinc values would increase upward, which is not true (Fig. 13). The additional process of boiling seems to be required in order to obtain the observed mineral deposits. The occurrence of vapor-filled inclusions and calculated positive y values for most gas analyses of surface samples indicate

84

D.I. NORMAN

Mixing

Boiling 500

ET AL.

, ---,--

100

-

230 600

"~\Ro

\\

700

o

,'~ o

Q

aoo

250 60

z~ o

\

\

•

900

240

80

\\

A

2 6 0 o(_~ ° ~,,,q

0

X

270

40

1000

280

/

1100 -

1300 -1

' 0

290 ' l

Log(grade) t

["

= Au =

0 -1

I

i

0

1

300

Log(grade)

oz./ton

o

=

Cu

%

Ag o z . / t o n

® o

= Pb =Zn%

%

Fig. 13. Models for Stage 2 mineral deposition calculated using computer program GEOMOD (Norman, unpubl. ). Grades are calculated by dividing the decrease in metal solubility by the decrease in quartz solubility as the process of mixing or boiling continues. The mixing model is for solutions like S*~age2 (Table 4 ) combining with fluids like those represented by population C inclusions, except it was assumed more extreme condition as suggested by the fluid inclusion data, that is, the primary solutions at depth beneath the ore deposit had a temperature of 300~C and salinity 3. eq.wt.% NaCI. The boiling model is for continuous open system boiling. Basemetal concentrations were assumed to be no more than about half their solubility in 300°C waters, gold concentrations were assumed not to exceed about 100 ppb and mineral depositions did not occur until the fluids were supersaturated. Calculated grades are similar to the St. CloudU.S. Treasury ore that typically ran 3% combined Pb and Zn, 2% Cu, and 15 oz. Ag/ton. Differences between the models and actual ore grades may be explained by small errors in the very idealized calculations, errors in the thermodynamic data, or because kinetics were not considered. Overall, the agreement between the boiling model and the ore deposit is not bad considering the numerous assumptions that were made in the model, including equilibrium.

that the fluids were boiling, at least in the upper portions of the deposits. Our boiling model predicts somewhat similar grades as those observed and the proper metal zonation with gold values high in the system. Further, the nearly vertical distribution of data in Figure 7 could be explained by loss of gases, particularly CO2, from mineralizing fluids as the result of boiling (Hedenquist and Henley, 1985 ). However, from our gas analyses the indicated levels of COx in the ore fluids are much too low to account for the measured differences in salinity if boiling was the only process involved in ore deposition. Therefore, a combination of boiling and fluid-mixing processes is envisioned as responsible for ore deposition at the St. Cloud and U.S. Treasury deposits,

GEOLOGY AND GEOCHEMISTRY OF MINERALIZING FLUIDS, CHLORIDE MINING DISTRICT, NM

85

with fluid mixing dominant in the lower portions of the deposits and boiling dominant in the upper portions. The curious lack of detectable H2S in inclusion gases from most samples collected over ore shoots and the measurable occurrence of this gas in inclusions collected away from ore shoots suggests that the loss of this species occurred in association with mineral deposition. The amount of H2S indicated in the ore fluids is far in excess of the calculated amounts of metals in the ore fluids, hence the lack of H2S cannot be explained by simple deposition of sulfides. A possible explanation is that oxidation of H2S took place in response to an increase in fluid f02, either from loss of H2S to the boiling process, or from mixing of near-surface oxygenated waters, or both. Since sulfide minerals cannot precipitate without reduced sulfur, this could explain why deposition of ore minerals cease( rather abruptly at the top of ore shoots. USE OF FLUID INCLUSIONS FOR MINERAL EXPLORATION

Gas analysis This study indicates that analysis of fluid inclusion volatiles has direct application to mineral exploration. The data show that gases dissolved in ore solutions are released by boiling and are trapped in fluid inclusion assemblages above the site of mineral deposition. Boiling is indicated to have occurred principally in areas where ore fluids were fluxing with non-ore fluids. Thus, bulk analyses of fluid inclusions may delineate blind ore bodies through the presence of anomalous gas contents that partitioned into the vapor phase. For example, the blind ore bodies at the St. Cloud mine could have been found by the analyses illustrated in Figure I I. Combined with microthermometry-determined temperatures and salinities, analyses of inclusion gases provide significant chemical data to estimate the metal-carrying capacity of paleogeothermal fluids and the nature of mineral deposition. Despite complications presented by the occurrence of multiple fluids, calculations of metals in solution for the St. Cloud-U.S. Treasury deposits successfully indicated that mineralizing fluids had the ability "to transport significant base metals, silver, and gold, and that Stage 2 mineralization should be richer in zinc, silver, and gold, relative to Stage 1. The data also indicates that ore solutions may be modified by loss of H: to boiling :a the upper portions of deposits. Unfortunately, metal-solubility calculations for surface samples overlying capped mineral deposits are probably not effective in delineating the existence of ore.

Microthermometry Our microthermometry information has increased the knowledge of Chloride district mineralization and can be useful in future exploration in this

86

D.I. NORMAN ET AL.

area. Characteristics of ore fluids as solutions bearing 2% or more dissolved salts and having temperatures of about 270°C has been borne out for other mineral deposits in the district (Loucks, 1984). Steam-heated waters with a distinctively lower salinity of abeut 0.6 eq.wt.% NaCl are indicated to have deposited barren quartz and are probably responsible for the regional propylitic alteration. Therefore, fluid inclusion microthermometry can be used to determine if a given quartz occurrence was formed from fluids with the desired characteristics and might be related to economic mineralization.

Model for the Chloride district Understanding the processes by which a mineral deposit is formed is important in exploration. Our fluid inclusion data verify and improve aspects of previous models for the Chloride district. Harrison ( 1988b, 1989a) has stressed the importance of open-space development along wrench-fault intersections in localizing mineralization. The data presented here indicate why these intersection are so important. Ore fluids were basically upweiling along these intersections, which became the sites for fluid boiling and mixing. Fluid inclusion data strengthen the idea that Stage l and Stage 2 mineralizations were different events separated by as much as 2.5 m.y. The analytical data indicate that these stages differed chemically and that trapping pressures also differed between the two stages (Table 4). The most straightforward explanation for the pressure difference, since P- T conditions were near boiling for both stages, is that the thickness of overlying strata increased from time of Stage l deposition to time of Stage 2 deposition. Assuming hydrostatic conditions, the respective depths beneath the water table for Stages l and 2 would have been about 780 and 930 m (calculated from the density equations in Haas, 1971 ). These calculated depths imply that Stage l deposition occurred immediately after eruption of late Oligocene intermediate volcanism (basaltic andesite of Poverty Creek in Fig. 4) and that Stage 2 deposition occurred after eruptions of silicic volcanic rocks that added approximately 150-200 m to the Chloride district stratigraphic section. This is in good agreement with age dates of mineralization and the volcanic rocks. CONCLUSIONS

The St. Cloud and U.S. Treasury deposits were deposited in two stages at paleodepths of about 780 and 930 m, respectively. Deposition occurred immediately after two episodes of volcanism from fluids with similar temperatures, but different chemistry. Stage l fluids had a salinity of about 2.2 eq.wt.% NaC1 and less than 0.1 tool.% dissolved gases; Stage 2 fluids had a salinity of about 1.8 eq.wt.% NaCl and about 0.5 mol.% dissolved gases. H2S in Stage 2 fluids was an order of magnitude greater than in Stage l fluids, and the sulfur

GEOLOGY AND GEOCHEMISTRY OF MINERALIZING FLUIDS, CHLORIDE MINING DISTRICT. NM

87

fugacity for Stage 2 fluids was two orders of magnitude greater than for Stage 1 fluids. Thus, Stage 2 fluids deposited bornite and iron-rich sphalerite in contrast to Stage 1 fluids that deposited chalcopyrite and iron-poor sphalerite. During deposition, fluids of both stages were boiling and mixing with nearsurface, steam-heated waters with lower salinity and gas contents. Fluids of both stages utilized deeply penetrative wrench faults intersections as conduits. Positive anomalies in CO2, N2, and N2/CO2 in inclusions from surface samples over mineralization are indicative of boiling. Gas analyses of fluid inclusions would have identified the blind ore shoots at the St. Cloud mine, and present an effective and inexpensive tool in delineating such blind deposits. Calculated metal solubilities based on inclusion data agree generally with observed mineralization at the St. Cloud and U.S. Treasury deposits. Similar calculations may be applied to exit t :,ration to determine ifpaleofluids had the capability of transporting significant quantities of metal to form an ore deposit. In the Chloride district, fluid inclusion salinities of 2 eq.wt.% or greater should be an effective guide to :'aineralized veins and ore shoots within these veins. ACKNOWLEDGEM ENTS

This research was supported in part by N.S.F. Grant EAR 8419081 (D.N.), Mineral Institute Grant G l 194135 (D.N.), and a dissertation grant from the New Mexico Bureau of Mines and Mineral Resources (R.H.). We thank the St. Cloud Mining Company for their cooperation and field support for C.B.A., and First Miss Gold, Inc. for their cooperation. Stable isotope measurements were done by Andrew Campbell at the laboratories of Danny Rye, Yale University. Appreciation is also extended to Jose Manrique who helped with statistical calculations, and to Kevin Clower and Christopher Wilkowske who did the drafting.

REFERENCES Albinson, T.F., 1988. Geological reconstruction of the paleosurfaces of the Sombrerete, Colorado, and Fresnilio districts, Zacatecas, Mexico. Econ. Geol., 83:1647-1667. Aldrich, M.J., Chapin, M.E. and Laughlin, A,W., i 986. Stress history and tectonic development of the Rio Grande rift, NM. J. Geophys. Res., 91 (B6): 6199-6211. Barton, P.B., Jr. and Skinncr, B.J., 1979. Sulfide mineral stabilities. In: H.L. Barnes (Editor), Geochemistry of Hydrothermal Deposits. Wiley-lnterscience, New York, NY, pp. 278-403. Beane, R.E. and Haynes, F.M., 1987. Analyses of fluid inclusion data from hydrothermal mineral deposits using cumulative frequency plots. Geol, Soc. Am., Abstr. Programs, 19(7): 583. Behr, C.B., 1988. Geochemical analyses of ore fluids from the St. Cioud-U.S. Treasury vein system, Chloride mining district, NM. M.S. thesis, NM Inst. Mining & Tech., Socorro, NM, 125 pp. (unpubl.).

88

D.I. NORMAN ET AL.

Burford, K.L., 1968. Statistics: A Computer Approach. Charles E. Merrill, Columbus, OH, 397 p~'. Chapin, C.E., 1983. An overview of Laramide wrench faulting in the southern Rocky Mountains with emphasis on petroleum exploration. In: J.D. Lowell (Editor), Rocky Mountains Fore~and Basins and Uplifts. Rocky Mtn. Assoc. Geol., Denver, CO, pp. 169-179. Chapin, C.E. and Cather, S.M., 195 I. Eocene tectonics and sedimentation in the Colorado Plateat:-Rocky MountaLn area. Ariz. Geol. Soc. Digest., 14:173-198. Drummond, S.E., 1981. Boiling and mixing ofhydrothermal fluids: Chemical effects on lailjerai precipitation. Ph.D. Dissertation, Penn. St. Univ., Univ. Park, PA. Univ. Micro. Int'l. Ann Ar~aor, MI, 380 pp. Eggle~ton, T.L. and Norman, D.I., 1986. Alteration associated with the rhyolite porphyry of K]ine Mountain, Black Range, New Mexico. N.M. Geol. Soc. Guideb., 37:16-17. Eggleston, T.L. and Norman, D.I., 1987. Rb-Sr dating of the Taylor Creek rhyolites. Isochron. West, 48: 22-25. Ericksen, G.E., Wedon, Jr., H., Eaton, G.P. and Leland, G.R., 1970. Mineral resources of the Black Range Primitive Area, Grant, Sierra and Carton Counties, NM. U.S. Geol. Surv., Bull. 1319-E, 162 pp. Giggenbach, W.F., 1980. Geothermal gas equilibria. Geochim. Cosmochim. Acta, 45: 20212032. Haas, J.L., Jr., 1971. The effect of salinity on the maximum thermal gradient ofa hydrothermal system at hydrostatic pressure. Econ. Geol. 66: 940-946. Harrison, R.W., 1986. General geology of Chloride mining district, Sierra and Carton counties, NM. N.M. Geol. Soc. Guideb., 37th Field Conference, pp. 265-272. Harrison, R.W., 1988a. Mineral paragene~is, structure, and "ore shoot" geometry at the U.S. Trcasury mine, Chloride mining district, NM. N.M. Geol., 10: IO-I l, 15-16. Harrison, R.W., 1988b. Primary structural control of epithermal mineralized shoots in southeastern Chloride mining (~,istrict, NM. N.M. Geol., 10:80-8 I. Harrison, R,W., 1989a. A regional study of precious-metal mineralization, Chloride mining district, NM. In: A.E. Torma and I.H. Gundiler (Editors), Precious and Rare Metal Technologies. Process Metallurgy 5. Elsevier, Amsterdam, pp. 51-67. Harrison, R.W., 1989b. Exotic blocks within the early Tertiary Rubio Peak Formation in the north-central Black Range, NM, occurrence: insights into post-emplacement tectonic activity, economic implications and emplacement hypothesis. N.M. Geol. Soc. Guideb., 40th Field Conference, pp. 99-106. Harrison, R.W., 1989c. Early Tertiary wrench-fault tectonics along the eastern flank of the Black Range, southwestern New Mexico. Geol. Soc. Am., Abstr. Programs, 21: 203. Harrison, R.W. and Chapin, C.E., ! 990. A NNE-trending, dextral wrench-fault zone of Laramidc age in Southern New Mexico. Geol. Soc. Am., Abstr. Programs, 22(7 ): A327. Hedenquist, J.W. and Henley, R.W., 1985. The importance of CO, on freezing point measurements of fluid inclusions: Evidence from active geothermal systems and implications for epithermal ore deposition. Econ. Geol., 80:1379-1406. Henley, R.W., 1985. The geothermal framework dor epithermal deposits. Soc. Econ. Geol., Rev. Econ. Geol., 2: 1-24. Henley, R.W. and Brown, K.L., 1985. A practical guide to the thermodynamics of geothermal fluids and hydrothermal ore deposits. Soc. Econ, Geol., Rev. Econ. Geol., 2: 24-44. Henley, R.W., Truesdeil, A.H. and Barton, P.B., Jr., 1984. Fluid-mineral equilibria in hydrothermal systems. Soc. Econ. Geol., Rev. Econ. Geol. 1,267 pp. J icha, H.L., Jr., 1954. Geology and mineral deposits of the Lake Valley quadrangle, Grant, Luna, and Sierra Counties, New Mexico. N.M. Bur. Mines Min. Resour., Bull. 37, 93 pp. Loucks, R.R., 1984. Investigations in the Chloride mining district, Sierra County, NM: Parts If, Ill, IV. Unpubl. Report for First Miss Gold, inc., with permission.

GEOLOGY AND GEOCHEMISTRY OF MINERALIZING FLUIDS, CHLORIDE MINING DISTRICT, NM

89

MacDonald, A.J. and Spooner, E.T.C., 1981, Calibration of the Linkham TH600 programmable heating-cooling stage for microthermometric examination of fluid inclusions. Econ. Geol., 76: 1248-1258. Maxwell, C.H. and Heyi, A.V., 1976. Preliminary geologic map of the Winston quadrangle, Sierra County, NM. U.S. Geol. Surv., Open-fih: Rep. 76-858, 1 sheet. Mclntosh, W.C., 1989. Ages and distribution of ignimbrites in the Mogollon-Datil volcanic field, southwest New Mexico: a stratigraphic framework using 4°Ar/39Ar dating and paleomagnetism. PhD Dissertation, N.M. Inst. Min. Technol., Socorro, NM, 324 pp. (unpubl.) Moore, J.M., Lemieux, M.M. and Adams, M.C., 1991. Occurrence of CO2-enriched fluids in active geothermal systems: Data from fluid inclusions. Proc. 15th Workshop on Reservoir Engineering, Stanford University (in press). Norman, D.I., ! 991. Fluid inclusion and other geochemical data for samples from the Chloride District, NM. N.M. Bur. Mines Mineral. Resour., Open File Rep. 376. Norman, D.I. and Sawkins, F.J., 1987. Analysis of volatiles in fluid inclusions by mass spectrometry. Chem. Geol., 61: 1-10. Potter, R.W., Clynne, M.A. and Brown, D.L., 1978. Freezing point depression of aqueous sodium chloride solutions. Econ. Geol., 73: 284-285. Reynolds, T.J., 1983. Fluid inclusion studies of the Silver Bar prospect, New Mexico. Report for First Miss Gold, Inc., with permission, 9 pp. ( unpubl. ). Robie, R.A., Hemingway, B.S. and Fisher, J.B., Jr., 1979. Thermodynamic properties of minerals and related substances at 298.15K and I Bar pressure and at higher Temperatures. U.S. Geol. Surv., Bull. 1492, 456 pp. Sinclair, A.J., 1985. Statistical interpretation of soil geochemical data. In: W.K. Fletcher, S.J. Hoffman, M.B. Mehrtens, A.J. Sinclair and l. Thomson (Editors), Exploration Geochemistry: Design and Interpretation of Soil Surveys. Soc. Econ. Geol., Rev. Econ. Geol., 3: 97116. Skinner, B.J., 1966. The system Cu-Ag-S. Econ. Geol., 61: 1-36. Taylor, H.P., 1974. The application of oxygen and hydrogen isotope studies to problems of hydrothermal alteration and ore deposition. Econ. Geol., 69: 843-883.