A small minority of the nation's wealthiest taxpayers is getting the lion's share of the benefits from $84 billion in special tax preferences, credits and other tax breaks now in the tax code, a new study confirmed yesterday.

Prepared by the Treasury for Sen. Edmund S. Muskie (D-Maine), the survey shows that almost half the $84 billion in special tax breaks for individuals in fiscal 1977 went to persons with incomes of $30,000 a year or more - about 5 percent of all taxpayers.

About 31 per cent of the total went to those in the $50,000-a-year-and-up brackets - a minuscule 1.4 percent of the taxpaying public. By contrast, taxpayers earning $10,000 or less - who make up 52 percent of the nation's taxpayers - got 12 percent of the breaks.

The study was made public by the Senate Budget Committee, of which Muskie is chairman.He said in a statement that the findings "support the Carter administration's contention that its tax reform proposals would improve the progressive nature of the tax system."

At the same time, the figures showed many of the administration's proposed "reforms" would take away breaks that go primarily to those whose incomes range from $20,000 to $50,000 ayear. These include ending deductions for state gasoline taxes and limiting writeoffs for medical costs.

Publication of the study came at a time when Muskie and Sen. Russell B. Long (D-La.), chairman of the Senate Finance Committee, appear headed for another confrontation over "tax reform" legislation. Long's panel, which has jurisdiction over the internal revenue code, has recommended undoing several key provisions of the 1976 Tax Reform Act.

The Treasury study showed which taxpayers benefited the most last year from 69 special tax breaks that affect individuals - exemptions, deductions, credits, exclusions and deferrals. It did not deal with $28 billion in breaks that affect corporations.

Of the 69 special breaks and preferences, there were 21 in which more than half the benefits went to taxpayers in the $50,000-and-up brackets In seven more categories, half the benefits or more went to tax payers with incomes of $30,000 or higher.

Individuals in the $10,000 to $20,000 range, who accounted for 32 percent of the nation's taxpayers last year, got about 20 percent of the tax breaks. And those in the $20,000 to $30,000 category - 11.3 percent of all taxpayers - got 19 percent of the breaks.

Predictably, the study's breakdown showed some tax breaks skewed almost entirely toward the rich - in part because wealthy persons pay proportionately higher taxes and therefore reap bigger deductions, and partly because income from their investments is taxed at more favorable rates than earnings from wages and salaries.

For example, the $6.9 billion in tax breaks from the present special tax treatment of capital gains goes almost entirely to upper-income taxpayers. About 68 percent benefits those in the $50,000-and-up brackets. When the $30,000 category is included, the figure rises to 83 percent.

Capital gains are the profits realized from the sale of stocks or other property. Only half a capital gain is subject to taxation, compared with full taxation of ordinary income such as wages. The capital gains preference has been a major target of tax "reformers."

Those in the $50,000-and-up brackets also received 85 percent of the $1.7 billion in breaks from the current tax exemption for interest on state and municipal bonds. Carter has proposed giving localities a subsidy if the drop that tax-free status.

Other big tax breaks going largely to taxpayers in the $50,000-and-up brakets include the tax shelter for percentage depletion on oil, gas and hard minerals; accelerated depreciation on housing investments, and deductions for charitable contributions to education.

The list contained one surprise: $51 million of the $1.5 billion in Tax savings from deductions for unemployment benefits went to taxpayers who had incomes of $50,000 or higher - suggesting that many upper-income taxpayers are receiving jobless benefits. Carter has proposed ending that.

On the other hand, some of the tax breaks listed went primarily to low or middle-income taxpayers:

Of the $4.5 billion in tax savings from deductions allowed for interest paid on home mortgages, for example, 78 percent went to taxpayers in the $15,000 to $50,000 ranges, with another 10 percent going to those earning $15,000 or less. There was a similar pattern on deductions for real estate taxes.

78 percent of the $2.2 billion in savings from deductions for medical expenses went to taxpayers in the $10,000 to $50,000 range. Carter has proposed cutting this back sharply by denying most deductions except in case of catastrophic, long-term illnesses.

76 percent of the $685 million in savings from deductions for state gasoline taxes - another that Carter has proposed eliminating - went to taxpayers with incomes between $15,000 and $50,000 a year. Those earning less than $15,000 got 14 percent of the breaks.

56 percent of the tax break for retired workers receiving Social Security went to taxpayers earning less than $10,000 a year. That income group also accounted for 63 percent of those benefiting from a deduction for veteran's benefits under the GI bill.

The size of these tax breaks grows each year with the economy. The $112 billion in total tax expenditures for individuals and corporations in fiscal 1977 is expected to swell to $136 billion in fiscal 1979, which begins Oct. 1 - $102 billion for individuals and $34 billion for corporations.